JP2733371B2 - New optical sheet and liquid crystal display device having the sheet - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical sheet having a unique birefringence characteristic which can be favorably used as a retardation compensation sheet, and a liquid crystal display device using the same.

[Conventional technology]

In order to realize black and white display in an STN (Super Twisted Nematic) liquid crystal display, it is necessary to remove coloring due to the birefringence of the STN liquid crystal. As this method, a two-layer cell system in which a liquid crystal cell for optical compensation is further stacked to eliminate the coloring of the liquid crystal has been put to practical use. It has disadvantages such as a decrease in transmittance and a need for a stronger light source.

As a method of solving such a disadvantage of the two-layer cell system,
There is a method in which a polymer sheet having the same optical characteristics as a liquid crystal cell for optical compensation is replaced. However, a polymer sheet having such optical characteristics (hereinafter referred to as a retardation compensation sheet) is used for a liquid crystal. It is necessary that the sheet has high birefringence and combined birefringence characteristics.

Furthermore, recently, the visibility of a liquid crystal display (hereinafter abbreviated as LCD) when viewed obliquely, that is, the coloring of the display and the contrast ratio when viewed obliquely have become a problem. Has been requested.

When one phase difference compensation sheet is used for the STN liquid crystal, a preferable retardation value (hereinafter referred to as R value or Δn) required for the phase difference compensation sheet including the viewing angle dependency.
・ Abbreviated as d) was reported in the spring, 1990, at the 37th JSAP, 30a-D-10.

That is, according to the report, as shown in FIG. 3, when viewing the same place at different viewing angles, the R value required for each viewing angle is not symmetric. In FIG. 3 (3-1), when a certain point 2 of the phase difference compensating sheet 1 is viewed by changing the viewing angle to the inclination angle θ and the circumferential angle φ (1-1), a request for the sheet 1 is given. The R value to be obtained is shown in a circle in nm units (3-2).

A phase sheet having such a complicated R value is difficult with a phase sheet generally used up to now, and is difficult with a simple uniaxially stretched sheet or a biaxially stretched sheet, and has a more improved phase difference compensation. A sheet is required.

[Problems to be solved by the invention]

According to the present invention, when the phase sheet is viewed at a certain angle with respect to the vertical line of the sheet surface as in a conventional uniaxially stretched sheet or biaxially stretched sheet, the R value is a value symmetrical about the vertical line Instead, the present invention provides an optical sheet such as a phase sheet which is most preferable for STN-LC and the like, having an asymmetrical R value.

[Means and actions for solving the problem]

 The present invention solves the above problems.

 That is, the present invention is as follows.

(1) When the polyvinylidene fluoride sheet is heated and placed in a high electromagnetic field, the direction perpendicular to the sheet surface and the direction of the electromagnetic field are from 1 degree to 89 degrees.
An optical sheet that is polarized at a degree angle.

(2) The optical sheet according to (1), wherein the direction perpendicular to the sheet surface and the direction of the electromagnetic field are set at an angle of 10 to 80 degrees.

(3) A liquid crystal display device having the optical sheet of (1) or (2).

The sheet of the present invention will be described with reference to FIG. In FIG.
The average polarization direction of the polymer chains constituting the sheet 3 is in the direction indicated by 4. (Slanted by α degrees with the seat surface direction). When the sheet 3 has an R value from the vertical direction 5,
The R values from 6 and 7 in the diagonal direction from θ show different values.

On the other hand, as shown in FIG. 2, in the stretched synthetic resin sheet 8 conventionally generally used as a retardation compensation sheet, the average orientation direction of the polymer chains is in a direction 9 parallel to the sheet surface,
The R value of the sheet from the vertical direction 10 and the R value from 11 and 12 inclined from the vertical direction 10 by θ degrees are equal to 11 and 12.

The change of the R value at each angle of the conventional sheet varies depending on the kind of resin of the sheet, uniaxial stretching or biaxial stretching, stretching ratio, etc., but becomes a symmetrical R value around the vertical direction 10.

On the other hand, in the sheet of the present invention shown in FIG. 1, the change of the R value according to each angle does not become a symmetric R value about the vertical direction 5. The R value from each direction of the sheet of the present invention differs depending on the type of the resin, the degree of polarization, the inclination angle α of the polarization direction, and the like. By appropriately selecting the factors that influence the R value, a phase sheet having an R value distribution close to the R value shown in FIG. 3-2 can be obtained.

The thickness of the optical sheet described in the present invention is the thickness of a commonly used sheet or film, preferably 10μ
m to 1000 μm, more preferably 50 μm to 500 μm.

The certain angle described in the present invention means that the perpendicular direction of the sheet surface and the polarization direction exist at an angle that can be detected in the sheet of the present invention, and preferably 1 degree to 89 degrees.
Degrees, more preferably from 2 to 88 degrees, most preferably from 10 to 80 degrees.

The optical sheet of the present invention can be formed by placing a polarizable polyvinylidene fluoride (hereinafter, abbreviated as PVdF) sheet obliquely in a high electromagnetic field in a heated state.

PVdF shows various crystal forms depending on molding conditions, and includes II type (or α type), I type (or β type) and the like.

In an I-type (β-type) crystal, the dipole moment of CF ₂ is oriented in the same direction at right angles to the molecular chain, causing spontaneous polarization. β
When a DC high voltage is applied to the sheet containing the type crystal at a high temperature, polarization occurs, and then the sheet is cooled to room temperature. When a high DC voltage is applied, if the voltage is applied obliquely at a fixed angle from the direction perpendicular to the PVdF sheet surface, the optical sheet of the present invention polarized in the applied direction can be obtained.

Regarding the crystal form of PVdF, polarization by applying high DC voltage, etc., refer to the functional fluorine-containing polymer, (Nikkan Kogyo Shimbun, Showa 57
Years), pp. 172 to 175, Polymer, 26 , June 853 (198
5), Applied Physics Letter, 24 , No. 10, 15 (1974), etc., and these methods can be used.

Heretofore, the application of the PVdF sheet has been performed in a direction perpendicular to the sheet surface, and, of course, the polarization direction has also been performed in a direction perpendicular to the sheet surface.

The sheet of the present invention in which the polarization direction of the sheet is oblique is different in R value depending on the viewing angle as described in FIG. 1, and the R value does not show a symmetric value with respect to a vertical line of the sheet surface.

The optical sheet of the present invention is favorably used as a retardation compensation film for an LCD device. Especially, it can be used well for STN-LCD. This will be described with reference to FIG.

A general STN liquid crystal-LCD basically includes a light source unit 13, a polarizing film 14, an STN-LCD unit 13 and a polarizing film 16 shown in FIG.

In the above STN-LCD, since the refractive index of the STN liquid crystal varies depending on the wavelength of light, linearly polarized light becomes elliptically polarized light 17 with the STN liquid crystal. In order to convert this elliptically polarized light into linearly polarized light, a conventional phase difference compensation sheet
18 are used.

The conventional phase difference compensating sheet has a structure as shown in FIG. 2, and has a problem as described above.

The liquid crystal display device of the present invention uses the optical sheet of the present invention for the retardation compensation sheet 18. The phase difference compensation sheet 18 may be one sheet or two or three sheets. Although the optical sheet of the present invention may be used as a single sheet, it is most preferable to obtain the most preferable R value by using the optical sheet in combination with a conventional retardation compensation sheet, thereby obtaining a liquid crystal display device having a wider viewing angle. be able to.

〔The invention's effect〕

The present invention is as described above.By using the optical sheet of the present invention, for example, for an LCD using STN liquid crystal, the difference between when the LCD is viewed directly from the front and when it is viewed obliquely is small. As a result, a reduction in the coloring of the display, an improvement in the contrast ratio, and the like can be obtained, and the LCD can be more easily viewed. Furthermore, the present invention is used not only for STN-LCD but also for other types of LCD.

[Brief description of the drawings]

1 and 2 show that the R value of the optical sheet of the present invention and the R value of the conventional phase difference compensating sheet are different depending on the viewing angle. FIG. 3 shows preferable R values required for the phase difference compensation sheet of STN-LC. FIG. 4 shows an example of use of the optical sheet of the present invention.

Claims (3)

(57) [Claims] 1. An optical sheet obtained by polarizing a polyvinylidene fluoride sheet in a high electromagnetic field in a heated state with a perpendicular direction of the sheet surface and an electromagnetic field direction at an angle of 1 to 89 degrees. 2. The optical sheet according to claim 1, wherein the direction perpendicular to the sheet surface and the direction of the electromagnetic field are set at an angle of 10 to 80 degrees. 3. A liquid crystal display device having the optical sheet according to claim 1.