JPH03279903A - Laminated phase difference plate and liquid crystal panel - Google Patents

Abstract

PURPOSE:To make improvement in contrast and visibility by laminating >=2 sheets of stretched plastic films which are substantially the same in refractive index characteristics in such a manner that the optical axes thereof intersect with each other and averaging the refractive indices in respective bearings within the plane of the laminate. CONSTITUTION:The refractive indices of the respective bearings within the plane of the laminate are averaged by adhering >=2 sheets of the transparent stretched plastic films 1 which are substantially the same in the refractive index characteristic via transparent adhesive layers 2. The light transmitted diagonally through the laminate has the phase differences varying according the azimuth thereof as the vertical and horizontal route components vary with the azimuth if there is the anisotropy in the refractive index in the respective bearings within the plane. The averaging the refractive indices of the respective bearings within the plane by lamination signifies that the refractive indices are approximated to the isotropy and, therefore, the refractive indices are theoretically nearer the isotropy as intersection angle of the optical axes is smaller. The liquid crystal panel having an excellent visual field angle and contrast and high visibility is obtd. in this way.

Description

Detailed Description of the Invention: Industrial Field of Application The present invention relates to a laminated retardation plate formed by laminating two or more stretched plastic films so that the refractive index in the in-plane direction is averaged, and a viewing angle compensation plate formed from the same. This invention relates to a liquid crystal panel with excellent visibility.

Conventional technology and issues LCD displays are used in clocks, calculators, game consoles, televisions,
It is also used in a wide range of fields such as computers, and various practical applications are being considered in new fields. Liquid crystal displays are classified into various types depending on the liquid crystal type, arrangement method, driving method, etc., such as TN type, STN type, super homeotropic type, simple matrix type, and active matrix type, but in general, the viewing angle is lower than that of CRT. It has been pointed out that it is narrow and lacks contrast, resulting in poor visibility.

Conventionally, a method of attaching a stretched plastic film to a liquid crystal panel has been proposed to overcome this drawback, but no satisfactory solution has yet been found.

The inventors of the present invention conducted extensive research to develop new means for improving visibility, and discovered that the main cause of poor visibility was the birefringence of liquid crystals.To overcome this, the refractive index in each direction within the plane We obtained new knowledge that a compensator plate with the same refractive index in the thickness direction but different from that in the plane is effective, and various attempts were made to develop viewing angle compensators with such characteristics. For example, biaxial stretching may be performed to obtain a uniform stretching ratio, or multiaxial compression orientation may be performed using a press method, etc.
Alternatively, it is a plastic film obtained by precision inflation molding.

However, it is difficult to control the refractive index of the plastic film both in the in-plane direction and in the thickness direction, in addition to the fact that it takes a long time to manufacture and it is difficult to form a large size. There were some difficulties in imparting satisfactory characteristics.

Means for Solving the Problems The present invention overcomes the above problems by laminating stretched plastic films.

That is, the present invention laminates two or more stretched plastic films having substantially the same refractive index characteristics so that their optical axes intersect, and averages the refractive index in each direction within the plane of the laminate. , a laminated retardation plate formed such that the refractive index in the thickness direction is different from the refractive index in the plane, and a viewing angle compensation plate made of the laminated retardation plate on at least one side of the liquid crystal cell. The present invention provides a liquid crystal panel characterized in that:

By laminating stretched plastic films, it is possible to superimpose or adjust the phase difference of birefringent light due to each stretched plastic film, and it is possible to have an effect equivalent to controlling the refractive index in the laminate. In that case, by laminating the stretched plastic films so that their optical axes intersect, the refractive index in each direction within the plane of the laminate can be averaged. If there is anisotropy in the refractive index in each direction within the plane, the light that passes obliquely through the laminate will have different vertical and horizontal path components depending on the azimuth, for example, as shown in Figure 4 (, However, in the case of isotropy, it is not affected by the vertical and horizontal path components, so as shown in Figure 3, the phase difference will be the same in each direction in the plane. The reason why a difference in phase difference appears as either the upper or lower curve is that the stretched plastic film has a fast axis and a slow axis for birefringent light.Also, the phase difference is caused by the transmission angle of obliquely transmitted light. The difference is that the refractive index in the thickness direction of the laminate acts.In the above, averaging the refractive index in each direction in the plane by laminating stretched plastic films does not approach isotropy. Therefore, theoretically, the smaller the angle of intersection of the optical axes, the closer to isotropy. This averaging can be achieved by stacking layers such that the angle of intersection of the optical axes is constant, and it is possible to avoid the need to stack layers while changing the angle of intersection of the optical axes depending on the difference in refractive index.

EXAMPLE The laminated retardation plate of the present invention is composed of a laminate of two or more stretched plastic films having substantially the same refractive index characteristics. An example is shown in Figure 1. This is made by laminating four sheets of transparent stretched plastic film 1 by bonding them together via a transparent adhesive layer 2.

Stretched plastic films having substantially the same refractive index characteristics can be formed, for example, by processing plastic films under the same stretching conditions. The stretching conditions may be determined as appropriate, such as uniaxial or biaxial. A stretched plastic film having precisely the same refractive index characteristics can be obtained, for example, by cutting two stretched plastic films into predetermined dimensions.

There are no particular limitations on the type of plastic film used. Among those having excellent transparency, those having a light transmittance of 80% or more in the visible light region are preferred.

Examples include films made of plastics such as cellulose acetate, polycarbonate, polyvinyl alcohol, polystyrene, polymethyl methacrylate, polyethersulfone, and modified products thereof. The thickness of the stretched plastic films to be laminated is generally 300 mm or less, but is not limited to this and may be determined as appropriate depending on the number of laminated layers, light transmittance, etc.

The number of stretched plastic films laminated is an arbitrary number of two or more. As mentioned above, from the point of view of the degree of averaging or accuracy of the refractive index in each direction within the plane, it is advantageous to increase the number of laminated layers, but from the point of view of light transmittance, it is advantageous to have a smaller number of laminated layers. be.

Therefore, the number of layers to be laminated is appropriately determined based on the required degree of averaging of the refractive index, accuracy, and light transmittance.

Generally, in the case of a film with a large difference in refractive index in each direction in the plane, such as a -axially stretched film, an even number of 4 to 20 films are laminated, and in the case of a film that is simultaneously biaxially stretched, refraction in orthogonal directions in the plane is stacked. 2 if the rates are substantially the same.
~20 sheets are stacked.

The lamination should be performed so that the refractive index in each direction within the plane of the formed laminate is averaged (the optical axes of each stretched plastic film intersect with each other. The intersecting angle of the optical axes is determined as appropriate depending on the number of laminated layers, etc.) be done.

In general, in the case of objects with large differences in refractive index in each direction within the plane, the intersection angle (
ψ), and when the refractive indices in orthogonal directions within the plane are substantially the same, the intersection angle is set to satisfy the formula: ψ=90/M. Note that M in the formula is the number of stretched plastic films to be laminated.

A preferred method for laminating stretched plastic films is as follows:
As illustrated in FIG. 2, a pair panel 3 is made by laminating two stretched plastic films 1 so that their optical axes (arrows) are perpendicular to each other.
This is a method in which the paired panels 3 are stacked at a predetermined crossing angle based on an arbitrary optical axis. When stacking paired panels, the intersection angle (ψ) of their leading axes is calculated using the formula: ψ
-90/N (where N is the number of paired panels to be used) is preferably laminated.

The above formula applies to a film with large differences in refractive index in each direction within the plane, such as a -axially stretched film, and when the refractive index in orthogonal directions in the plane is substantially the same, such as a simultaneously biaxially stretched film. It is preferable that the layers are stacked so as to satisfy the formula: ψ=45/N.

Adhesives or pressure-sensitive adhesives are usually used for laminating stretched plastic films. In that case, a material with good transparency such as, but not limited to, an acrylic adhesive or pressure-sensitive adhesive is preferably used. In addition, from the viewpoint of preventing changes in the optical properties of the stretched plastic film, those that do not require high-temperature processes during curing, drying, etc. are preferably used.

A method of transferring an adhesive layer provided on a separator is also preferable from the viewpoint of preventing changes in optical properties.

The refractive index in the thickness direction of a laminated retardation plate can be controlled by methods based on the thickness and refractive index of the stretched plastic film used, as well as the thickness and refractive index of the adhesive used for laminating the films. can also be done.

In the liquid crystal panel of the present invention, a viewing angle compensation plate made of the above-mentioned laminated retardation plate is arranged on one side or both sides of a liquid crystal cell. FIG. 5 shows an example of its configuration. This is a type that has a viewing angle compensation plate only on one side, with polarizing plates 4 and 7 and 5
is a viewing angle compensation plate, and 6 is a liquid crystal cell. Note that 51 is a transparent adhesive layer, and the polarizing plate 4 side is the viewing side.

The viewing angle compensator used in forming a liquid crystal panel compensates for the shift in the phase difference of birefringent light in a liquid crystal cell due to the azimuth angle (see FIG. 4). This improves the narrowness of the viewing angle and compensates for the reduction in contrast caused by changes in the viewing angle.

Note that there are no particular limitations on the liquid crystal panel to be used. For example, nematic (TN) type, super twisted nematic (STN) type, super homeotropic type, simple matrix type, active matrix type, etc.
It can be used for liquid crystal panels of various liquid crystals, arrangement systems, drive systems, etc.

Example 1 A polycarbonate film with a thickness of 50 μI was axially stretched by 20% at 160°C to obtain a stretched plastic film (-C).
nx: 1.585, ny: 1.578, nz: 1
．． 578, where nXXny is the refractive index in the in-plane vertical or horizontal direction, and Hz is the refractive index in the thickness direction. (same below) 2
A pair of panels is formed by laminating the films using an acrylic adhesive with their optical axes perpendicular to each other, and two of the paired panels are laminated using an acrylic adhesive so that the optical axes of the films intersect at 45 degrees. Further lamination was performed to obtain a laminated retardation plate having a refractive index of 1.578 in the thickness direction.

Example 2 Uniaxially stretched polycarbonate film with a stretching ratio of 30% (
nx: 1.589, ny: 1.576, nz: 1
．． A laminated retardation plate having a refractive index in the thickness direction of 1.576 was obtained in the same manner as in Example 1 except that 576) was used.

Example 3 Three of the paired panels obtained in Example 1 were laminated via an acrylic adhesive so that the optical axes of arbitrary films intersected at 30 degrees, and the laminated layer had a refractive index of 1.578 in the thickness direction. A retardation plate was obtained.

Example 4 Three of the paired panels obtained in Example 1 were laminated via an acrylic adhesive so that the optical axes of arbitrary films intersected at 60 degrees, and the lamination position had a refractive index of 1.578 in the thickness direction. A retardation plate was obtained.

Example 5 100 pieces of polyvinyl alcohol film with a thickness of 80 μm
Stretched plastic film obtained by axially stretching 50% at °C (nx: 1.561, ny: 1.555, nz
A laminated retardation plate having a refractive index of 1.555 in the thickness direction was obtained in the same manner as in Example 1, except that 1.555) was used.

Example 6 Polystyrene film with a thickness of 75μ■ is heated to 50% at 85℃
- Stretched plastic film formed by axial stretching (nx: 1
．． 584, ny: 1.592, nz: 1.592) was used, except that the refractive index in the thickness direction was 1.5.
92 laminated retardation plates were obtained.

Example 7 One polymethyl methacrylate film with a thickness of 100 μ-
Stretched plastic film obtained by simultaneous biaxial stretching of 50% at 10°C (nx: 1.4900, ny: 1.4900
, l"12:1.4905) were laminated with their optical axes perpendicular to each other via an acrylic adhesive to obtain a laminated retardation plate with a refractive index of 1.4905 in the thickness direction. .

Comparative Example 1 The uniaxially stretched polycarbonate film obtained in Example 1 was used as it was as a retardation plate.

Comparative Example 2 The simultaneously biaxially stretched polymethyl methacrylate film obtained in Example 7 was used as it was as a retardation plate.

For the (laminated) retardation plates obtained in Evaluation Test Examples 1 to 7 and Comparative Examples 1 and 2, the azimuth angle was 90 with respect to the optical axis of an arbitrary film (0 degrees) under the conditions of a transmission angle of 30 degrees and a wavelength of 550 ng+. We investigated the phase difference between degrees (angle perpendicular to the reference optical axis in the plane). The smaller the variation in the phase difference, the higher the degree of uniformity or averaging of the refractive index within the plane.

The results are shown in the table.

(Unit: nm) Example 8 A viewing angle compensation plate made of the laminated retardation plate obtained in Example 1 was adhered to one side of an active matrix type nematic liquid crystal panel to obtain a liquid crystal panel.

In the case of the above-mentioned display, the displayed color did not change when viewed from the front, but the contrast or visibility improved when viewed from an oblique direction, and the displayed content could be determined in all directions even from an approximately 60 degree oblique direction.

Comparative Example 3 A liquid crystal panel was obtained according to Example 8, except that the retardation plate (-axis stretched polycarbonate film) obtained in Comparative Example 1 was used as the viewing angle compensator.

The above-mentioned film shows no improvement effect when viewed from an oblique direction in a direction other than the optical axis direction of the film, and its contrast and visibility are poor, especially in directions deviated from the optical axis by about 30 degrees and about 60 degrees. In this case, the oblique angle at which the display contents could be distinguished was about 20 degrees.

Example 9 A viewing angle compensation plate made of the laminated retardation plate obtained in Example 6 was adhered to one side of a super twisted nematic type liquid crystal panel to obtain a liquid crystal display device.

The above-mentioned display has improved contrast or visibility from an oblique direction, and the display contents could be distinguished from an oblique direction of approximately 50 degrees in all directions.

Example 10 A viewing angle compensation plate made of the laminated retardation plate obtained in Example 5 was adhered to one side of an active matrix nematic liquid crystal panel to obtain a liquid crystal panel.

In the case of the above-mentioned display, the displayed color did not change when viewed from the front, but the contrast or visibility improved when viewed from an oblique direction, and the displayed content could be determined in all directions even from an approximately 60 degree oblique direction.

Effects of the Invention According to the present invention, it is possible to easily obtain a large-sized laminated retardation plate with small variations in refractive index in each direction within the plane, and by using a viewing angle compensator made of the same, the viewing angle and contrast can be adjusted. A liquid crystal panel with excellent high visibility can be obtained.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional view illustrating a laminated retardation plate, Fig. 2 is a perspective view illustrating the forming members of the laminated retardation plate, and Figs. 3 and 4 show the relationship between the transmission angle, phase difference, and azimuth angle. The graph shown in FIG. 5 is a cross-sectional view illustrating a liquid crystal panel. 1: Stretched plastic film 2: Adhesive layer 3: Bare panel 4.7: Polarizing plate 5: Viewing angle compensation plate 6: Liquid crystal cell

Claims (1)

[Claims] 1. Two or more stretched plastic films having substantially the same refractive index characteristics are laminated so that their optical axes intersect, and the refractive index in each direction within the plane of the laminate is averaged. A laminated retardation plate characterized in that the retardation plate is formed so that the refractive index in the thickness direction is different from the refractive index in the plane. 2. A liquid crystal panel characterized in that a viewing angle compensation plate made of the laminated retardation plate according to claim 1 is disposed on at least one side of a liquid crystal cell.