KR19980031817A - Retardation film for liquid crystal display devices and its manufacturing method - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a retardation film for a liquid crystal display device and a method for manufacturing the same, and more particularly, to a retardation film capable of improving a viewing angle in a twisted nematic liquid crystal display device and a method for manufacturing the same. In the general form of the retardation film, the refractive anisotropy has a vertical or parallel direction with respect to the surface. The present invention relates to a method of making the left and right and the upper and lower viewing angles independent and simultaneously improving by inclining the direction of the refractive index anisotropy of the retardation film with the surface.

Description

Retardation film for liquid crystal display devices and its manufacturing method

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a retardation film for a liquid crystal display device and a method for manufacturing the same, and more particularly, to a method for manufacturing a retardation film capable of improving a viewing angle in a twisted nematic liquid crystal display device.

When an external magnetic field is applied to a twisted nematic liquid crystal display device in a normally black mode, liquid crystal molecules are arranged in response to an electric field, and light transmission is generated based on the following equation.

I = I _o sin ² [θ (1 + u ² ) ^1/2 ] u = πR / θλ, R = Δn

Where I is intensity of transmitted light, I _o is intensity of incident light, Δn is birefringence, d is liquid crystal cell thickness, λ is wavelength of transmitted light, θ is twist angle of twisted nematic liquid crystal, and R is phase difference.

As shown in the above equation, the phase difference is closely related to the viewing angle. Therefore, in order to improve the viewing angle, compensation of the phase difference is desirable.

A uniaxial refractive index anisotropic body and a biaxial refractive index anisotropic body are used for the retardation film provided between a liquid crystal substrate and a polarizing plate for retardation compensation. As shown in Fig. 1, uniaxiality and biaxiality are determined depending on whether n _x and n _y are equal when the refractive indices in the x, y and z directions of the Cartesian coordinate system are n _x , n _y , n _z , respectively. . That is, as shown in FIG. 1A, when the refractive indices of the two directions are the same and the magnitude thereof is different from the other direction, it is called uniaxiality. 1B and 1C, when all three directions have refractive indices of different sizes, it is referred to as biaxiality.

The retardation film using the uniaxial refractive index anisotropic body generally used is arrange | positioned so that the long axis of an ellipsoid may be parallel or perpendicular to a film surface.

Then, the manufacturing method of the conventional retardation film is demonstrated in detail with reference to attached drawing.

FIG. 2 is a cross-sectional view showing a conventional retardation film, and as shown in FIG. 2, the film surface 2 and the surface formed by the x- and y-axes of the negative uniaxial refractive index anisotropic body 4 are formed so as to be horizontal to each other. Anisotropy is symmetrical about the z axis direction. Since the negative refractive anisotropic body 4 has the same refractive index in the x-axis direction and the y-axis direction, and the z-axis direction has a smaller refractive index value, the long axis is the x-axis or y-axis direction and the short axis is the z-axis direction. As a general retardation film, this negative writing rate anisotropy is used. On the other hand, a positive refractive anisotropic body refers to an anisotropic body having a larger refractive index in the z-axis direction than the refractive index in the x-axis or y-axis direction. Both forms show symmetry about the z axis, so the refractive anisotropy depends only on the path through which light passes and the angle between the z axis.

FIG. 3 shows the birefringence rates experienced in the path 1 and the path 2 of light having the same azimuth angle φ with respect to the z axis shown in FIG. 2. As shown in FIG. 3, it can be seen that the same birefringence is observed in both paths.

In the twisted nematic liquid crystal display device, the viewing angle graph according to the phase difference change is shown in FIG. The horizontal axis of the graph represents the left and right viewing angles, and the vertical axis represents the up and down viewing angles.

4A is a graph in which a viewing angle is measured without using a retardation film, and FIGS. 4B to 4D are graphs in which a retardation value is changed using a retardation film.

4B to 4D, when the phase difference is larger, the left and right viewing angles are wider, and thus the widest viewing angle is shown in FIG. 4D. On the other hand, as the phase difference increases, the upper viewing angle is improved, but the lower viewing angle is narrowed.

Therefore, in order to have a wide viewing angle in the up, down, left, and right sides, the retardation film should have a larger phase difference in FIG. 4D showing the widest upper viewing angle on the upper side and a smaller phase difference value in FIG. 4A showing the widest lower viewing angle on the lower side.

However, in the conventional twisted nematic liquid crystal display using a uniaxial anisotropic retardation film, the liquid crystal is symmetrical in the left and right viewing angle direction and asymmetric in the vertical direction, whereas in the case of the retardation film, both the vertical and horizontal directions are symmetrical. Has Therefore, there is a problem that it is not possible to properly compensate for the viewing angle in the vertical direction using the retardation film.

Therefore, the present invention is to improve the independent and at the same time the viewing angle by independent phase difference in the up, down, left and right.

1 is a view showing the refractive anisotropic ellipsoid of the retardation film,

2 is a cross-sectional view showing a conventional retardation film,

3 shows the birefringence rate experienced in path 1 and the birefringence rate experienced in path 2,

FIG. 4 is a graph showing a change in viewing angle characteristic according to a phase difference change in a twisted nematic liquid crystal display device.

5 is a cross-sectional view illustrating a method of manufacturing a phase difference film according to a first embodiment of the present invention;

6 is a cross-sectional view showing a retardation film according to a first embodiment of the present invention,

7 shows the birefringence rate experienced in path 1 and the birefringence rate experienced in path 2,

8 is a cross-sectional view illustrating a method of manufacturing a phase difference film according to a second embodiment of the present invention.

9 is a cross-sectional view illustrating a method of manufacturing a phase difference film according to a third embodiment of the present invention.

Explanation of symbols on the main parts of the drawings

2: retardation film surface 4: refractive index ellipsoid 6: polymer

8: extending direction 10: cutting direction

The retardation film for liquid crystal display devices which concerns on this invention for achieving such a subject is characterized by the inclination of the long axis of a uniaxial anisotropic body with respect to a film surface.

A first manufacturing method of such a retardation film for a liquid crystal display device comprising the steps of preparing a polymer film, stretching the polymer film in an oblique direction, and rotating the polymer film by 90 ° about an axis perpendicular to the polymer film plane. And stretching in the plane direction of the film, and the second manufacturing method includes producing a polymer film, stretching the polymer film in the plane direction, and cutting the polymer film inclined with respect to the polymer film plane. In the third production method, a plurality of thin polymer films are prepared, a polymer film is stretched in a plane direction, a polymer film is laminated, and the laminated polymer film is inclinedly cut with respect to the polymer film surface. It includes.

In the method of manufacturing the retardation film according to the present invention, the retardation film experiences different retardation according to the path of the light so as to control the viewing angle independently up and down and left and right.

Then, embodiments of the retardation film manufacturing method according to the present invention with reference to the accompanying drawings will be described in detail to be easily carried out by those skilled in the art.

5 is a cross-sectional view illustrating a method of manufacturing a phase difference film according to a first embodiment of the present invention.

As shown in FIG. 5A, first, a uniaxial or biaxial thin polymer film 6 is produced, stretched 8 in a direction inclined to the film 6, and the major axis of the refractive anisotropic body is directed to the film 6 surface. Be inclined.

As shown in Fig. 5B, the film 6 is rotated 90 degrees about an axis perpendicular to the film plane, and then stretched 8 in the plane direction of the film 6 to complete the retardation film.

On the other hand, among the materials that become a liquid crystal phase at high temperature, there are those having a disk-like molecular structure, using such a liquid crystal material can be obtained the same retardation film as described above. That is, the liquid crystal film may be made such that the disc surface of the discotic liquid crystal molecules is inclined at a predetermined angle with respect to the film surface. In order to make such a film, the disk surface of the discotic liquid crystal molecules is arranged inclined with respect to the surface of the film, and then the temperature is lowered to room temperature to solidify, and the liquid crystal material dissolved in the solvent is also used for the film surface of the molecules. There is a method of arranging inclined and then heating and drying.

The refractive index anisotropy of the refractive index ellipsoid of the retardation film produced by this method along the light path is as follows. FIG. 6A shows a retardation film in which the major axis of the film surface 2 and the refractive index anisotropic body are inclined, and FIG. 6B is a cross-sectional view when the film is rotated 90 degrees about an axis perpendicular to the film plane.

As shown in Fig. 6A, when the negative refractive anisotropic body 4 is inclined, the light beam experiences at path 1 and path 2 which form the same azimuth angle φ with respect to the z _o axis in which the direction of light travels perpendicular to the film surface. The refractive indices are shown in FIGS. 7A and 7B, respectively. In path 1, the refractive indices of the major and minor axes are the same, and the birefringence is zero. On the other hand, it can be seen that anisotropy of refractive index is observed in path 2.

90. In the case of a rotation by the film is shown in Figure 6B Figure 6A path of the path that has the same advancing direction to the first path 23 and the path 4, so also obtain the same azimuth angle (φ) made symmetrical to each other with respect to the axis z _o The birefringence observed is the same as in Fig. 7C, which is the median value of the birefringence experienced in path 1 and path 2. This is a suitable condition for forming the symmetry of the viewing angle which is one of the problems when the retardation film is attached to the liquid crystal display device.

8 is a cross-sectional view illustrating a method of manufacturing a phase difference film according to a second embodiment of the present invention.

As shown in FIG. 8, the polymer film 6 having a uniaxial or biaxial thickness is made thick, and then stretched 8 in the plane direction to have the largest birefringence in the direction parallel to the surface.

The stretched film 6 is cut 10 at an angle to the surface to complete the retardation film.

9 is a cross-sectional view illustrating a method of manufacturing a phase difference film according to a third embodiment of the present invention.

As shown in Fig. 9, a plurality of thin polymer films 6 are produced, each polymer film 6 is stretched 8 in the plane direction, and laminated.

When the laminated polymer film 6 is cut 10 diagonally, the film 6 is completed.

Unlike the conventional method, the refractive index anisotropy is observed independently without being symmetrical about one axis, thereby effectively improving the viewing angle characteristic of a twisted nematic liquid crystal display device having vertical and asymmetrical values.

Therefore, the method of manufacturing the retardation film according to the present invention has the effect of simultaneously controlling and simultaneously improving the viewing angle in the twisted nematic liquid crystal display device.

Claims (10)

As retardation film for liquid crystal display devices, The long axis of the uniaxial anisotropic body is inclined with respect to the said film surface, The retardation film for liquid crystal display devices characterized by the above-mentioned. The retardation film of claim 1, wherein the retardation film is made of a polymer. Manufacturing a polymer film, Stretching the polymer film in an oblique direction, Stretching the polymer film by 90 ° about an axis perpendicular to the polymer film plane and stretching in the plane direction of the film The manufacturing method of the retardation film for liquid crystal display devices containing these. The method of claim 3, wherein the polymer film has a uniaxial property. Manufacturing a polymer film, Stretching the polymer film in a plane direction, Cutting the polymer film inclined with respect to the polymer film surface Method for producing a retardation film comprising a. The method of claim 5, wherein the polymer film has a uniaxial property. Making several polymer films, Stretching each of the polymer films in a plane direction, Laminating the polymer film, Cutting the laminated polymer film inclined with respect to the polymer film surface The manufacturing method of the retardation film for liquid crystal display devices containing these. The method of claim 7, wherein the polymer film has a uniaxial property. Arranging a disk-shaped liquid crystal material layer having a liquid crystal phase at a high temperature such that the disk surface of the liquid crystal molecules is inclined to the layer surface, and Lowering the temperature of the liquid crystal material layer to solidify The manufacturing method of the retardation film for liquid crystal display devices containing these. Preparing a solution in which the discotic liquid crystal material is dissolved in a solvent, Arranging the disk surface of the liquid crystal molecules to be inclined to the layer surface in the solution layer formed of the solution, and Heating and drying the solution layer The manufacturing method of the retardation film for liquid crystal display devices containing these.