JPH0545520A - Polarizing plate and liquid crystal display device - Google Patents

Abstract

PURPOSE:To obtain the polarizing plate which is hardly changed in polarization performance by inclination by compensating the change, in the transmission axis of a polarizer by the inclination and to obtain the liquid crystal display device which exhibits a good contrast and is extremely wide in a visual field angle. CONSTITUTION:This polarizing plate 4 is constituted by adhering a lamination type sealing film 1 consisting of the laminate of double refractive films varying in the wavelength dispersion characteristics of double refractions on at least one side of a polarizer 3 and has the delay phase axis of the films having the small wavelength dispersion of the double refractions in this laminated sealing film in such a state that this axis intersects orthogonally with the delay phase axis of the film having the large wavelength dispersion of the double refractions and intersects with the absorption axis of the polarizer. This liquid crystal display device is constituted by disposing such polarizing plate on at least one side of the liquid crystal cell.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarizing plate having a small change in polarization performance depending on the viewing angle, and a liquid crystal display device using the polarizing plate and having a wide viewing angle.

[0002]

2. Description of the Related Art Polarization for realizing a liquid crystal display device having a wide viewing angle in a liquid crystal display whose performance is remarkably improved due to a large screen and a high density display, and a narrow viewing angle is still a matter of concern. It's been a long time since I was asked for a board. Conventionally, as a polarizing plate, there has been known one in which an isotropic sealing film made of a biaxially stretched triacetyl cellulose film or the like, that is, a sealing film showing almost no birefringence is adhered to a polarizer. The sealing film is for preventing the intrusion of water and improving the durability of the polarizer. However, as described above, there is a problem that the obtained liquid crystal display device has a narrow viewing angle.

[0003]

An object of the present invention is to develop a polarizing plate capable of obtaining a liquid crystal display device having a wide viewing angle. In view of the above, the inventors of the present invention have diligently studied, and have clarified that the transmission axis of the polarizing plate changes depending on the viewing angle, which causes the viewing angle of the liquid crystal display device to be narrowed. Further research has led to the present invention.

[0004]

According to the present invention, a laminated encapsulating film comprising a laminate of birefringent films having different birefringence wavelength dispersion characteristics is adhered to at least one side of a polarizer. The slow axis of the film having a small wavelength dispersion of birefringence in the laminated encapsulating film is orthogonal to the slow axis of the film having a large wavelength dispersion of birefringence and is orthogonal to the absorption axis of the polarizer. Characteristic polarizing plate,
And a liquid crystal display device characterized by arranging the polarizing plate on at least one side of a liquid crystal cell.

[0005]

With the above structure, the fact that the fast axis of the birefringent film changes depending on the tilt angle allows the change of the transmission axis of the polarizer due to the tilt (azimuth angle) to the phase difference of the birefringent film. Based on another birefringent film having different wavelength dispersion characteristics of birefringence, the deviation from linearly polarized light due to the wavelength dispersion of the phase difference generated at that time is compensated.

[0006]

EXAMPLE A polarizing plate of the present invention is one in which a laminated encapsulating film is adhered to a polarizer. Examples thereof are shown in FIGS. 1 and 3. Reference numeral 1 is a laminated sealing film, 2 is an adhesive layer, and 3 is a polarizer. As is apparent from FIGS. 1 and 3, the laminated encapsulating film 1 is provided on at least one side of the polarizer 3 and, if necessary, on both sides.

The laminated encapsulating film used in the present invention is a birefringent film having different wavelength dispersion characteristics of birefringence so that the slow axis of a film having a small wavelength dispersion of birefringence and that of a large film are orthogonal to each other. It is laminated on. The number of layers of the birefringent film is arbitrary, but it is preferable that it is less than the transparency due to suppression of reflection loss,
Generally, it is a laminated body of about 2 to 3 sheets. Figure 2
In the above, the laminated encapsulating film 1 in which two birefringent films 11 and 12 having different wavelength dispersion characteristics of birefringence are laminated with an adhesive layer 2 is illustrated.

In the present invention, as the birefringent film on the polarizer side, a film made of a polymer having a poor sealing ability such as a polyvinyl alcohol film can be used. Is not particularly limited. A polymer having excellent transparency is preferably used. The birefringent film can be obtained by, for example, a method in which a polymer film is uniaxially or biaxially stretched.

Examples of general polymers for forming a birefringent film having excellent sealing ability include polycarbonate, triacetyl cellulose, polymethyl methacrylate, polyether sulfone, polyethylene terephthalate, polyarylate, and polyimide. ..

The birefringent film can be laminated using, for example, a transparent adhesive or pressure-sensitive adhesive. There is no particular limitation on the type of the adhesive or the like. From the viewpoint of preventing changes in the optical properties of the birefringent film, those that do not require a high temperature process during curing or drying are preferable, and those that do not require a long curing treatment or drying time are desirable. The adhesive or pressure-sensitive adhesive is also preferably used for bonding the laminated encapsulating film and the polarizer.

The laminated encapsulating film preferably used in the present invention has a birefringent film having refractive indices of n _x , n _y and n in the slow axis direction, the fast axis direction and the thickness direction, respectively.
when is _{z, wherein: Q = (n x -n z} ) / (n x -n y)
A film having a small wavelength dispersion of birefringence, which has a Q value (same below) calculated from 0.5 to 0.9 and a Q value of 0.1 to 0.1.
5 is a laminate of films having a large wavelength dispersion of birefringence. This makes it possible to accurately compensate for the deviation of the transmission axis of the polarizer due to the inclination.

The formation of the birefringent film exhibiting the above Q value is carried out, for example, by applying an electric field in the thickness direction of a polymer such as polycarbonate having a positive birefringence in which a slow axis appears in the orientation direction of molecules. It can be performed by a method in which the film is cured while controlling the orientation, and the film is stretched.

Incidentally, in the case of a film made of a polymer exhibiting positive birefringence, n _y and n _z are equal to each other in the case of perfect uniaxial orientation, and the Q value is 1, and in the case of biaxial orientation, the Q value is 1. Is greater than 1. On the other hand, in the case of a film made of a polymer having a negative birefringence in which a fast axis appears in the orientation direction of molecules such as polystyrene, n _x and _{nz in} the case of perfect uniaxial orientation.
Become equal and the Q value becomes 0. In the case of biaxial orientation, Q
The value becomes negative. Therefore, in any case, it is difficult for the monolayer film to exhibit the compensation effect.

That is, in the above description, in the polarizer arranged in crossed Nicols, the transmission axis thereof is the tilt axis (rotation axis).
However, in the positive uniaxially oriented film of the positive birefringence system described above, the change of the fast axis becomes the same direction as the change of the transmission axis of the polarizer, and the compensation effect by the birefringence is obtained. It does not appear. Further, in the biaxially oriented film of positive birefringence system, the change of the fast axis is faster than the change of the transmission axis of the polarizer, and the birefringence has the opposite effect. On the other hand, negative birefringence perfect uniaxially oriented film or biaxially oriented film has its slow axis changed in the direction opposite to the transmission axis of the polarizer and is visually affected by the phase difference including wavelength dispersion. It is easy to reduce the sex. In the case of a film having a Q value of 0.5, the slow axis and the fast axis hardly change due to the inclination, and in such a case, a sufficient compensation effect cannot be obtained.

In the laminated encapsulating film more preferably used in the present invention, the birefringence is larger on the longer wavelength side. This makes it possible to obtain a laminated encapsulating film which is excellent in compensation for wavelength dispersion of birefringence. Formation of a laminated encapsulating film exhibiting such characteristics is advantageous, for example, by combining a film having a small wavelength dispersion of birefringence and a large retardation with a film having a large wavelength dispersion of birefringence and a small retardation. It can be carried out.

That is, the optimum compensation is achieved when the phase difference is ½ of the wavelength at all wavelengths. However, in the ordinary birefringent film, the wavelength dispersion of birefringence is larger on the shorter wavelength side. Have the opposite relationship. Therefore, it is difficult to compensate the wavelength dispersion of birefringence with a single birefringent film. As described above, by stacking a film having a small wavelength dispersion of birefringence and a large retardation and a film having a large wavelength dispersion of birefringence and a small retardation with their slow axes orthogonal to each other, the birefringence can be increased. The wavelength side can be made larger and the influence of chromatic dispersion can be reduced by using it. From the viewpoint of dealing with the visible light region, it is preferable to use a laminated encapsulating film having an in-plane retardation of 180 to 370 nm.

In the above description, the retardation is the difference in refractive index (Δ) between the slow axis direction and the fast axis direction in the birefringent film.
n) and the thickness (d) of the birefringent film (Δn ·
It is defined in d). Further, the in-plane retardation is defined by superimposing or adjusting the retardation in each birefringent film forming the laminated encapsulating film.

In the present invention, an appropriate polarizer can be used without any particular limitation. Generally, a film made of a hydrophilic polymer such as polyvinyl alcohol is treated with a dichroic dye such as iodine and stretched, or a film made of a plastic film such as polyvinyl chloride and oriented with polyene is used. Is used.

In the polarizing plate of the present invention, the slow axis of the film having small wavelength dispersion of birefringence in the laminated encapsulating film is
It is bonded so as to be orthogonal to the absorption axis of the polarizer. The orthogonal state of the slow axis and the absorption axis is similar to the orthogonal state between the slow axes of the birefringent film in the laminated encapsulating film, and does not mean a completely orthogonal state in terms of working accuracy and the like. However, it is preferable that the angle is closer to 90 degrees from the viewpoint of compensation effect. In this case, the slow axis of the birefringent film or the absorption axis of the polarizer is based on the front (azimuth: 0).

In the liquid crystal display device of the present invention, the above polarizing plate is arranged on one side or both sides of the liquid crystal cell. Such a liquid crystal display device is illustrated in FIG. Reference numeral 4 is a polarizing plate, and 5 is a liquid crystal cell. The liquid crystal cell used is arbitrary. For example,
Examples include an active matrix drive type represented by a thin film transistor type, and a simple matrix drive type represented by a twist nematic type or a super twist nematic type.

Example 1 A uniaxially stretched polyvinyl alcohol film (n _x : 1.494, n _y : 1.487, n _z : 1) which was cured by applying an electric field of 15 kv and then stretched 100% at 120 ° C. .48
9, Q value: 0.76, retardation: 982 nm) and a uniaxially stretched polycarbonate film (n _x : 1.x) which was cured by applying an electric field of 20 kv and stretched at 10% at 155 ° C.
_{591, n y: 1.577, n} z: 1.587, Q value: 0.
24, phase difference: 708 nm) with their slow axis (stretching axis)
Were made orthogonal to each other and laminated via an acrylic pressure-sensitive adhesive to form a laminated sealing film (in-plane retardation: 274 nm). Next, the above-mentioned laminated encapsulating film was adhered to both sides of a polarizer obtained by dyeing a polyvinyl alcohol film with iodine and then stretching it, to obtain a polarizing plate. The slow axis of the uniaxially stretched polyvinyl alcohol film was arranged so as to be orthogonal to the absorption axis of the polarizer.

Comparative Example 1 Instead of the laminated sealing film, a biaxially stretched triacetyl cellulose film (n _x : 1.503, n _y : 1.502,
A polarizing plate was obtained in the same manner as in Example 1 except that _nz : 1.501, Q value: 8.00, retardation: 10 nm) was used as the sealing film.

Comparative Example 2 A uniaxially stretched polycarbonate film (n _x : 1.588, n _y : 15% stretched at 155 ° C. after being cured while applying an electric field of 10 kv instead of the laminated encapsulating film.
1.583, _nz : 1.584, Q value: 0.74, phase difference:
A polarizing plate was obtained according to Example 1 except that (278 nm) was used for the sealing film.

Evaluation Test Wavelength Dispersion of Retardation The wavelength dispersion characteristics of retardation of the sealing films used in Example 1 and Comparative Example are shown in FIG.

Wavelength Dispersion of Transmittance When the absorption axes of the polarizing plates obtained in Example 1 and Comparative Example are tilted by 45 degrees with respect to the tilt axis, and the polarizing plates are tilted by 60 degrees with respect to the horizontal plane. , Initial (when not inclined)
Placed in crossed Nicols with respect to the polarizing plate of, ie, 1
The transmittance of the analyzer placed at 35 degrees was measured for each wavelength, and the results are shown in FIG. It should be noted that the smaller the value, the greater the compensation effect for the change in the transmission axis of the polarizing plate.

Viewing angle: The polarizing plate obtained in Example 1 or Comparative Example is adhered to both sides of the twisted nematic liquid crystal cell to form a display device, and the contrast ratio in the horizontal direction and the vertical direction is 10: 1 or more. I checked the range. The results are shown in Table 1.

[Table 1]

[0027]

According to the present invention, it is possible to obtain a polarizing plate which can compensate for a change in the transmission axis of a polarizer due to tilting and whose polarization performance is unlikely to change due to tilting. As a result, by applying such a polarizing plate to a liquid crystal cell, it is possible to obtain a liquid crystal display device exhibiting a good contrast and having a wide viewing angle.

[Brief description of drawings]

FIG. 1 is a sectional view of an example of a polarizing plate.

FIG. 2 is a cross-sectional view illustrating a laminated sealing film.

FIG. 3 is a sectional view of another embodiment of the polarizing plate.

FIG. 4 is a sectional view of an embodiment of a liquid crystal display device.

FIG. 5 is a graph showing chromatic dispersion of phase difference.

FIG. 6 is a graph showing wavelength dispersion of transmittance.

[Explanation of symbols]

 1: Laminated sealing film 11, 12: Birefringent film 3: Polarizer 4: Polarizing plate 5: Liquid crystal cell

Claims (5)

[Claims] 1. A laminated encapsulating film comprising a laminate of birefringent films having different birefringence wavelength dispersion characteristics bonded to at least one side of a polarizer, and the birefringence in the laminated encapsulating film. A polarizing plate characterized in that the slow axis of the film having a small wavelength dispersion of is orthogonal to the slow axis of the film having a large wavelength dispersion of birefringence and is also orthogonal to the absorption axis of the polarizer. 2. The birefringent film has refractive indices n _x , n _y , and n _{z in} the slow axis direction, the fast axis direction, and the thickness direction, respectively.
And a film having a small wavelength dispersion of birefringence having a Q value of 0.5 to 0.9 calculated by the formula: Q = (n _x −n _z ) / (n _x −n _y ), The polarizing plate according to claim 1, which comprises a laminated encapsulating film made of a laminated body of films having a large wavelength dispersion of birefringence and having a Q value of 0.1 to 0.5. 3. The polarizing plate according to claim 1, wherein a laminated encapsulating film having a birefringence that increases toward the long wavelength side is used. 4. The polarizing plate according to claim 1, which comprises a laminated encapsulating film having an in-plane retardation of 180 to 370 nm. 5. A liquid crystal display device comprising the polarizing plate according to claim 1 arranged on at least one side of a liquid crystal cell.