JPH06230368A - Phase difference plate and liquid crystal display device formed by using the same - Google Patents

Abstract

PURPOSE:To provide the liquid crystal display device which has a wide visual angle and is less colored by using such phase difference plates that make a double refractive index zero at a certain wavelength of visible light. CONSTITUTION:This liquid crystal display device is constituted of an upper polarizing plate 1, the first phase difference plate 2, the second phase difference plate 3, a liquid crystal cell 4, a lower polarizing plate 5 and an upper substrate 6, lower substrate 7, transparent electrodes 8 and nematic liquid crystal 9 of the liquid crystal cell. A PC (polycarbonate) film of 2.00mum retardation to 589nm light is used for the first phase difference plate 2 and a PMMA (polymethyl methacrylate) film likewise of 2.02mum is used for the second phase difference plate 3. The PC film is higher in the dependency of the refractive index on wavelength than the PMMA film. The phase difference plate which makes the retardation zero exactly at 550nm is obtd. if both are laminated by unifying uniaxial stretching axes. The PC is positive in photoelastic constant and conversely the PMMA is negative. This combination is more advantageous in terms of visual angle characteristic than the combination of positive and positive or negative and negative.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a retardation plate and a liquid crystal display device using the same.

[0002]

2. Description of the Related Art Coloring of super twisted nematic (STN) liquid crystal display is disclosed in Japanese Patent Publication No. 50249/1993.
It was greatly improved by the compensation method using the retardation film disclosed in the publication. To enhance the color compensation effect,
A means for controlling the wavelength dependence of n is also disclosed in JP-A-2-12080.
No. 4, JP-A-3-13917, and the like.

[0003]

However, such a compensation method using a retardation plate results in the deterioration of the wide viewing angle characteristic inherent to STN. Means for increasing the viewing angle by increasing the refractive index of the retardation film in the film thickness direction is also disclosed in Japanese Patent Application Laid-Open No. HEI-2.
However, it has not been a fundamental solution.

The present invention solves such a problem, and its purpose is to provide Δn at a certain wavelength of visible light.
A new phase difference plate is provided so as to reduce the liquid crystal to zero, and by using this, a liquid crystal display device having a wide viewing angle and little coloring is provided.

[0005]

The retardation plate of the present invention is characterized in that Δn becomes zero at at least one wavelength of visible light. The wavelength is preferably near 550 nm, which has high visibility.

Further, it is characterized in that stretched films of at least two kinds of polymers are laminated, or a mixture or copolymer of at least two kinds of polymers is stretched.

The polymer is a combination of polymers having different Δn wavelength dependences caused by stretching.

The polymer is a combination of a polymer having a positive photoelastic constant and a polymer having a negative photoelastic constant.

Further, the liquid crystal display device of the present invention includes the retardation plate, a liquid crystal cell in which twisted and aligned nematic liquid crystal is sandwiched between two opposing substrates, and a pair of liquid crystal cells sandwiching the liquid crystal cell on both sides. And a polarizing plate of.

[0010]

The STN liquid crystal has excellent viewing angle characteristics by nature. However, the retardation plate provided for color compensation rather deteriorates the viewing angle characteristics. Therefore, in the present invention, by using a retardation plate in which Δn becomes zero at a certain wavelength of visible light, it is devised to have the same characteristics as STN at least at that wavelength. If the wavelength is in the vicinity of 550 nm where the visibility is high, the viewing angle characteristics of white light can be expected to be substantially the same as STN.

In order to obtain a retardation plate in which Δn becomes zero at a certain wavelength, there is a method of laminating a plurality of polymer stretched films as disclosed in, for example, Japanese Patent Laid-Open No. 13917/1993. . In FIG. 7, curves 41 and 42 show the wavelength dependence of the retardation of the polycarbonate (PC) uniaxially stretched film and the polymethylmethacrylate (PMMA) uniaxially stretched film, respectively. However, the retardation was defined by the product of the birefringence Δn and the film thickness d. The PC film has a greater wavelength dependence of Δn than the PMMA film. Therefore, when the two are laminated with their axes aligned so that their retardations cancel each other, the curve 43
, The retardation becomes zero at a certain wavelength,
That is, a film having Δn of zero is obtained. As a combination of a plurality of polymers, it is advantageous not only that the wavelength dependence of Δn is different, but also that the photoelastic constants are positive and negative in terms of viewing angle characteristics. The PC has a positive photoelastic constant and the PMMA has a negative photoelastic constant. In addition to the method of laminating films as described above, JP-A-3-2
As disclosed in 9921, there is also a method of stretching a mixture of a plurality of polymers or a copolymer.

Now, in order to eliminate STN coloration with the above retardation film, compensation conditions different from the conventional ones are required. FIG. 8 shows the optimum retardation condition of the retardation plate when performing STN color compensation with one retardation plate. However, the axial direction is fixed to the optimum value. 5 in this
The curve of 3 is a condition that has been widely used from the past.
Other than this, the optimum conditions are periodically present at 51, 52, 54 and 55, and it has been found that the same color compensation effect is obtained under any of the conditions. Therefore, using the condition of 51,
With the retardation plate of the present invention, color compensation equivalent to the conventional one can be performed.

[0013]

【Example】

(Example 1) As shown in FIG. 1, the liquid crystal display device in Example 1 of the present invention comprises an upper polarizing plate 1, a first retardation plate 2, a second retardation plate 3, a liquid crystal cell 4, and a lower liquid crystal cell 4. It is composed of a side polarizing plate 5, an upper substrate 6 of a liquid crystal cell, a lower substrate 7, a transparent electrode 8 and a nematic liquid crystal 9.

A PC film having a retardation of 2.00 μm for a light of 589 nm is used for the first retardation plate 2, and a PM film of 2.02 μm is similarly used for the second retardation plate 3.
MA film was used. The PC film has a greater wavelength dependence of Δn than the PMMA film. When both are laminated with their uniaxial stretching axes aligned, as shown in FIG.
A retardation plate having a retardation of zero at 50 nm was obtained. Also, PC has a positive photoelastic constant, and conversely PMM.
A is negative. This combination is more advantageous in terms of viewing angle characteristics than the combination of positive and positive or negative and negative.

As the nematic liquid crystal 8, a liquid crystal ZLI-4620 manufactured by Merck & Co., Ltd. was added with 0.8% by weight of a chiral dopant S-811 also manufactured by Merck. ZLI
The birefringence Δn of −4620 was 0.1327, and the cell gap d was 6.5 μm, so Δn × d was 0.86.
It becomes μm.

FIG. 2 is a diagram showing the relationship of each axis of the liquid crystal display device in the first embodiment. The polarization axis (absorption axis) direction of the upper polarizing plate 1 is 11, and the stretching axis direction of the first retardation plate 2 is 1.
2, the stretching axis direction of the second retardation plate 3 is 13, the rubbing direction of the upper substrate 6 of the liquid crystal cell is 14, the rubbing direction of the lower substrate 7 of the liquid crystal cell is 15, the polarization axis direction of the lower polarizing plate 5 is 16
Then, the angle 21 formed by 11 and 12 is 45 degrees to the right, 1
The angle 22 formed by 2 with 14 is 90 degrees, the angle 23 formed by 13 with 14 is 90 degrees, the twist angle 24 of the nematic liquid crystal 9 is 240 degrees on the left, and the angle 25 formed by 15 with 16 is 45 degrees on the right.

FIG. 3 is a diagram showing the viewing angle characteristics of the liquid crystal display device according to the first embodiment. Here, the center of the figure is the direction of the substrate normal, and the six concentric circles surrounding it are the inclination angles from the substrate normal of 10 degrees, 20 degrees, 30 degrees, 40 degrees, and 50 in order from the inside.
The direction of 60 degrees is shown. Also 31, 32, 33
Are contrast ratios of 1: 1, 1: 3 and 1:10, respectively.
Is an isocontrast curve. This viewing angle characteristic is symmetrical and has a narrow so-called reversal region where the contrast ratio is 1: 1 or less, and is an excellent characteristic almost equivalent to STN that does not use a retardation plate. Further, the display color was a white achromatic color on a black background, and the change of the display color was small even if the viewing angle direction was changed.

If polysulfone, polyester naphthalate, or the like having a greater wavelength dependence of Δn is used instead of PC, a retardation plate having a smaller retardation can provide the same characteristics. Further, a stretched film of polystyrene (PSt) having a negative photoelastic constant on a retardation plate having a large wavelength dependence of Δn, and a polyvinyl alcohol (PVA) having a positive photoelastic constant on a retardation plate having a small wavelength dependence of Δn. Equivalent characteristics can be obtained even in a combination of positive and negative reversals such as the stretched film of.

(Example 2) As shown in FIG. 4, the liquid crystal display device in Example 2 of the present invention comprises an upper polarizing plate 1, a retardation plate 2, a liquid crystal cell 4, a lower polarizing plate 5, and a liquid crystal cell. It is composed of an upper substrate 6, a lower substrate 7, a transparent electrode 8 and a nematic liquid crystal 9.

The phase difference plate 2 is composed of PC and PMMA 1: 1.
A film mixed at a ratio of 2 was uniaxially stretched to produce a film, and the retardation was set to zero at exactly 550 nm. As the nematic liquid crystal 8, liquid crystal ZLI-4620 manufactured by Merck Ltd. was used as in Example 1, and Δn × d was used.
Of 0.86 μm.

FIG. 5 is a diagram showing the relationship of each axis of the liquid crystal display device in the second embodiment. The angle 21 formed by 11 and 12 is 45 degrees to the right, the angle 22 formed by 12 and 14 is 90 degrees, the twist angle 24 of the nematic liquid crystal 9 is 240 degrees left, and the angle 25 formed by 16 is 15 degrees 45 degrees right.

The viewing angle characteristics of this liquid crystal display device are as shown in Example 1.
As with the left and right, it was an excellent one with a narrow inversion area. Moreover, the display color was also a white achromatic color on a black background, and the change in the display color was small even when the viewing angle direction was changed.

Even if a retardation plate obtained by copolymerization and stretching instead of mixing is used, the same characteristics can be obtained.

(Comparative Example) The structure and axial relationship of the liquid crystal display device in the comparative example are the same as those in Example 2, except that a PC uniaxially stretched film is used as the retardation plate 2. The retardation is 0.57 nm for light of 589 nm, and Δn for light of any wavelength.
Is never zero.

FIG. 6 is a diagram showing the viewing angle characteristics of the liquid crystal display device in Comparative Example 1. This viewing angle characteristic has poor symmetry,
Inversion is also likely to occur. The display color on the front side is a white achromatic color on a black background, but a large color change from green to orange occurs depending on the viewing angle direction, and the display is very difficult to see.

In the above embodiments, examples in which the retardation plate of the present invention is combined with STN liquid crystal are given.
Even when combined with liquid crystals such as ECB, FLC, GH, and PDLC, a new effect different from the conventional one is obtained.

[0027]

As described above, according to the present invention, there is newly provided a retardation plate in which Δn becomes zero at a certain wavelength of visible light. By using this retardation plate, a wide viewing angle and coloring can be achieved. A small number of liquid crystal display devices can be provided.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a liquid crystal display device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an axial relationship of the liquid crystal display device in Embodiment 1 of the present invention.

FIG. 3 is a diagram showing viewing angle characteristics of the liquid crystal display device according to the first embodiment of the present invention.

FIG. 4 is a cross-sectional view of a liquid crystal display device in Example 2 of the present invention and a comparative example.

FIG. 5 is a diagram showing an axial relationship of liquid crystal display devices in Example 2 of the present invention and a comparative example.

FIG. 6 is a diagram showing viewing angle characteristics of a liquid crystal display device in a comparative example.

FIG. 7 is a diagram illustrating characteristics of the retardation plate of the present invention.

FIG. 8 is a diagram showing optimum compensation conditions for a phase difference plate for STN.

[Explanation of symbols]

1 Upper Polarizing Plate 2 (First) Phase Difference Plate 3 Second Phase Difference Plate 4 Liquid Crystal Cell 5 Lower Polarizing Plate 6 Upper Substrate of Liquid Crystal Cell 4 7 Lower Substrate of Liquid Crystal Cell 8 Transparent Electrode 9 Nematic Liquid Crystal 11 Upper Side Polarization axis (absorption axis) direction of polarizing plate 12 Stretching axis direction of (first) retardation plate 2 13 Stretching axis direction of second retardation plate 3 14 Rubbing direction of upper substrate 6 of liquid crystal cell 15 Liquid crystal cell Rubbing direction of lower substrate 7 16 Polarization axis (absorption axis) direction of lower polarization plate 21 Angle formed by 11 11 12 22 Angle formed by 12 23 13 Angle formed by 14 24 Twist angle of nematic liquid crystal 9 25 Angle formed by 16 and 15 31 Contrast curve with contrast ratio 1: 1 32 Contrast curve with contrast ratio 1: 3 33 Contrast curve with contrast ratio 1:10 41 Rita PC film Wavelength dependency of the retardation 42 wavelength dependency of the retardation of the PMMA film 43 wavelength dependency of the retardation of the PC film / PMMA film laminate 51 primary compensation condition of the retardation plate for STN 52 2 of the retardation plate for STN Next compensation condition 53 Third-order compensation condition of STN retardation plate 54 Fourth-order compensation condition of STN retardation plate 55 55th-order compensation condition of STN retardation plate

Claims (6)

[Claims] 1. A retardation plate having a birefringence Δn of zero at least at one wavelength of visible light. 2. The retardation plate according to claim 1, wherein at least two kinds of polymer stretched films are laminated. 3. A stretched mixture or copolymer of at least two types of polymers.
The retardation plate described. 4. The Δn produced by stretching the polymer.
4. The retardation plate according to claim 2, wherein the retardation plate is a combination of polymers having different wavelength dependences. 5. The polymer according to claim 2, wherein the polymer is a combination of a polymer having a positive photoelastic constant and a polymer having a negative photoelastic constant.
The retardation plate described. 6. A retardation plate, a liquid crystal cell in which twisted and aligned nematic liquid crystal is sandwiched between two opposing substrates, and a pair of polarizing plates disposed on both sides of the liquid crystal cell. A liquid crystal display device characterized by the above.