JPH04343303A - Polarizing plate and liquid crystal display device - Google Patents

Abstract

PURPOSE:To obtain a polarizing plate which hardly changes its polarization performance due to tilting and to obtain a liquid crystal display device showing good contrast and excellent view angle range by compensating changes of transmitting axis of the polarizer for wavelength due to azimuth angle. CONSTITUTION:A laminar sealing film 1 comprising a laminated body of double refraction films having different wavelength dispersion characteristics of double refraction is adhered to at least one side of a polarizer 3. In a polarizing plate 4, the axis for the late phase of the film having smaller wavelength dispersion of double refraction in the laminar film 1 is perpendicular to the axis of the late phase axis of the film having larger dispersion. Also, the former axis is parallel to the absorption axis of the polarizer 3. The liquid display device contains this polarizing plate 4 at least one side of a 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 whose polarization performance hardly changes due to azimuth or wavelength, and a liquid crystal display device using the same which has an excellent wide viewing angle.

0002

[Background Art] Although the performance of liquid crystal displays has been significantly improved due to larger screens and higher display densities, the narrow viewing angle remains a concern.Polarized light is used to realize liquid crystal display devices with wide viewing angles. It's been a long time since I've been looking for a board. BACKGROUND ART Conventionally, as a polarizing plate, one in which an isotropic sealing film made of a biaxially stretched triacetyl cellulose film or the like, that is, exhibiting almost no birefringence, is adhered to a polarizer has been known. The sealing film is for preventing moisture from entering and improving the durability of the polarizer. However, as mentioned above, there was a problem in that the viewing angle of the resulting liquid crystal display device was narrow.

0003

SUMMARY OF THE INVENTION An object of the present invention is to develop a polarizing plate that can provide a liquid crystal display device with a wide viewing angle. In view of the above, the present inventors conducted extensive research and found that the transmission axis of a polarizing plate changes depending on the viewing angle, especially its azimuth angle, and also for each wavelength, and this is the cause of narrowing the viewing angle of a liquid crystal display device. In order to overcome this problem, we conducted further research and came up with the present invention.

0004

[Means for Solving the Problems] The present invention has a laminate type sealing film made of a laminate of birefringent films having different birefringent wavelength dispersion characteristics adhered to at least one side of a polarizer. In the laminated sealing film, the slow axis of the film with small wavelength dispersion of birefringence is perpendicular to the slow axis of the film with large wavelength dispersion of birefringence, and is parallel to the absorption axis of the polarizer. The present invention provides a polarizing plate according to the present invention, and a liquid crystal display device characterized in that the polarizing plate is disposed on at least one side of a liquid crystal cell.

[0005]

[Operation] With the above configuration, by utilizing the fact that the fast axis of the birefringent film changes depending on the azimuth angle (tilt angle),
Changes in the transmission axis of the polarizer for each wavelength due to the azimuth angle can be compensated based on the effect of the retardation that differs for each wavelength of the laminated sealing film.

[0006]

[Example] The polarizing plate of the present invention is obtained by adhering a laminated sealing film to a polarizer. Examples thereof are illustrated in FIGS. 1 and 3. 1 is a laminated sealing film, 2 is an adhesive layer, 3
is a polarizer. As is clear from FIGS. 1 and 3, the laminated sealing film 1 is provided at least on one side of the polarizer 3, and if necessary, on both sides.

[0007] The laminated sealing film used in the present invention consists of birefringent films having different wavelength dispersion characteristics of birefringence, such that the slow axes of the films with small and large birefringent wavelength dispersions are perpendicular to each other. It is a layered structure. The number of laminated birefringent films is arbitrary, but it is preferable to have as few as possible from the viewpoint of transparency due to suppression of reflection loss, etc., and generally a laminated body of about two to three films is used. In Figure 2,
A laminated sealing film 1 is illustrated in which two birefringent films 11 and 12 having different birefringent wavelength dispersion characteristics are laminated with an adhesive layer 2 interposed therebetween.

In the present invention, as the birefringent film on the polarizer side, a film made of a polymer having poor sealing ability, such as a polyvinyl alcohol film, can also be used. There are no particular limitations. Polymers with excellent transparency are preferably used. The birefringent film can be obtained, for example, by uniaxially or biaxially stretching a polymer film.

Examples of common polymers that form birefringent films with excellent sealing ability include polycarbonate, triacetyl cellulose, polymethyl methacrylate, polyether sulfone, polyethylene terephthalate, polyarylate, and polyimide. .

[0010] The birefringent films can be laminated using, for example, a transparent adhesive or adhesive. There are no particular limitations on the type of adhesive or the like. From the viewpoint of preventing changes in the optical properties of the birefringent film, it is preferable to use a film that does not require high-temperature processes during curing or drying, and desirably a film that does not require a long curing treatment or drying time. Note that such an adhesive or pressure-sensitive adhesive is also preferably used for adhering the laminated sealing film and the polarizer.

The laminated sealing film preferably used in the present invention has refractive indices of nx, ny, and ny in the slow axis direction, fast axis direction, and thickness direction of the birefringent film, respectively.
nz, the formula: Q=(nx-nz)/(nx-
Q value (the same applies hereafter) calculated by ny) is 0.5 to 0.9
A film with a small wavelength dispersion of birefringence and a Q value of 0
．． It is made of a laminate of films having a large wavelength dispersion of birefringence of 1 to 0.5. Thereby, it is possible to accurately compensate for a shift in the transmission axis of the polarizer due to the azimuth angle.

[0012] A birefringent film exhibiting the above-mentioned Q value can be formed by applying an electric field in the thickness direction of a polymer such as polycarbonate, which has a slow axis in the molecular orientation direction and exhibits positive birefringence. This can be carried out by, for example, curing the film while controlling its orientation, and then stretching the film.

Incidentally, in a film made of a polymer exhibiting positive birefringence, ny and nz are completely uniaxially oriented.
are equal and the Q value is 1, and in the case of biaxial orientation, Q
The value becomes greater than 1. On the other hand, in a film made of a polymer such as polystyrene, which exhibits negative birefringence with a fast axis appearing in the molecular orientation direction, in the case of complete uniaxial orientation, nx and nz are equal and the Q value is 0. In the case of biaxial orientation, the Q value is negative. Therefore, in any case, it is difficult to produce a compensation effect with a single layer film.

In other words, in the polarizer arranged in crossed nicols, its transmission axis changes in a direction perpendicular to the tilt axis (the tilt angle from the vertical plane), but in the case of the positive birefringence system described above, In a completely uniaxially oriented film, the change in its fast axis is in the same direction as the change in the transmission axis of the polarizer, which tends to reduce the compensation effect due to birefringence. Furthermore, in a biaxially oriented film with positive birefringence, the change in its fast axis is faster than the change in the transmission axis of the polarizer, so birefringence tends to have the opposite effect, making it difficult to develop a compensation effect. On the other hand, in a completely uniaxially oriented film or a biaxially oriented film with a negative birefringence system, the slow axis changes in the opposite direction to the transmission axis of the polarizer, and visibility is affected by the retardation including wavelength dispersion. It is easy to reduce the sex. As a result, in either case, a single layer film is difficult to exhibit a compensatory effect effective in widening the viewing angle with excellent visibility. Note that in the case of a film with a Q value of 0.5, the slow axis and the fast axis hardly change due to the tilt.

[0015] A laminated sealing film more preferably used in the present invention has a birefringence that increases toward longer wavelengths. This makes it possible to obtain a laminated sealing film that is excellent in compensating for wavelength dispersion due to birefringence. Formation of a laminated sealing film exhibiting such characteristics is advantageous, for example, by combining a film with a small wavelength dispersion of birefringence and a large retardation and a film with a large wavelength dispersion of birefringence and a small retardation. It can be carried out.

In other words, optimal compensation is achieved at all wavelengths when the retardation is 1/2 of the wavelength, but in a normal birefringent film, the wavelength dispersion of birefringence is larger on the shorter wavelength side. have the opposite relationship. Therefore, it is difficult to compensate for the wavelength dispersion of birefringence with a single birefringent film. As mentioned above, by laminating a film with a small wavelength dispersion of birefringence and a large retardation and a film with a large wavelength dispersion of birefringence and a small retardation with their slow axes perpendicular to each other, the birefringence can be lengthened. It can be made larger toward the wavelength side, and by using this, the influence of wavelength dispersion can be reduced. Note that from the standpoint of dealing with the visible light region, it is preferable to use a laminated sealing film with an in-plane retardation of 180 to 370 nm.

[0017] In the above, the retardation is the difference in refractive index (△
n) and the thickness (d) of the birefringent film (△n・
d). Further, the in-plane retardation is defined by the superposition or adjustment of the retardations in each birefringent film forming the laminated sealing film.

[0018] In the present invention, any suitable polarizer can be used, and there is no particular limitation. In general, films made of hydrophilic polymers such as polyvinyl alcohol are treated with dichroic dyes such as iodine and stretched, or plastic films such as polyvinyl chloride are treated to orient polyene. A polarizing film is used.

In the polarizing plate of the present invention, the slow axis of the film with small birefringence wavelength dispersion in the laminated sealing film is
It is attached so that it is parallel to the absorption axis of the polarizer. The above-mentioned parallel state of the slow axis and absorption axis means a completely parallel or orthogonal state from the point of view of work accuracy, similar to the orthogonal state between the slow axes of the birefringent film in the laminated sealing film. However, from the point of view of compensation effect, it is preferable that the intersection angle be smaller or closer to 90 degrees. In this case, the slow axis of the birefringent film or the absorption axis of the polarizer is based on the front (azimuth angle: 0).

In the liquid crystal display device of the present invention, the above-mentioned polarizing plate is arranged on one side or both sides of a liquid crystal cell. An example of such a liquid crystal display device is shown in FIG. 4 is a polarizing plate, and 5 is a liquid crystal cell. Any liquid crystal cell may be used. for example,
Examples include active matrix drive type, typified by thin film transistor type, and simple matrix drive type, typified by twisted nematic type and super twisted nematic type.

Example 1 After curing while applying an electric field of 15 kV, the temperature was 120°C.
Uniaxially stretched polyvinyl alcohol film (nx: 1.494, ny: 1.487, nz:
1.489, Q value: 0.76, phase difference: 982 nm), and after curing while applying an electric field of 20 kV, 155
A uniaxially stretched polycarbonate film (nx: 1.591, ny: 1.577, nz: 1.
587, Q value: 0.24, retardation: 708 nm) are laminated via an acrylic adhesive with their slow axes (stretching axes) perpendicular to each other to form a laminated sealing film (in-plane retardation: 274).
nm) was formed. Next, the above laminated sealing film was adhered to both sides of a polarizer made by dyeing a polyvinyl alcohol film with iodine and then stretching it, via an acrylic adhesive, to obtain a polarizing plate. The uniaxially stretched polyvinyl alcohol film was arranged so that its slow axis was parallel to the absorption axis of the polarizer.

Comparative Example 1 Instead of the laminated sealing film, a biaxially stretched triacetyl cellulose film (nx: 1.503, ny: 1.50
2, nz: 1.501, Q value: 8.00, phase difference: 10
A polarizing plate was obtained in the same manner as in Example 1, except that a polarizing film (in-plane retardation: 10 nm) was used for the sealing film.

Comparative Example 2 Instead of a laminated sealing film, a uniaxially stretched polycarbonate film (nx: 1.588, n
y: 1.583, nz: 1.584, Q value: 0.74,
A polarizing plate was obtained according to Example 1, except that a retardation film (retardation: 278 nm, in-plane retardation: 278 nm) was used for the sealing film.

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

Wavelength dispersion of transmittance Regarding the case where the absorption axis of the polarizing plate obtained in Example 1 and Comparative Example is tilted at 45 degrees with respect to the tilt axis, the case where the polarizing plate is tilted at 60 degrees with respect to the horizontal plane , initial (when not tilted)
arranged in a crossed nicol manner with respect to the polarizing plate of 1
The transmittance with respect to an analyzer placed at 35 degrees was measured for each wavelength, and the results are shown in FIG. Note that the smaller the value, the greater the compensation effect for changes in the transmission axis of the polarizing plate.

A display device is formed by adhering the polarizing plates obtained in Example 1 or Comparative Example to both sides of the viewing angle twisted nematic type liquid crystal cell, and the contrast ratio in the horizontal and vertical directions is 10:1 or more. I checked the range. The above results are shown in Table 1.

[Table 1]

[0027]

According to the present invention, it is possible to compensate for the change in the transmission axis of the polarizer for each wavelength due to the azimuth angle, and it is possible to obtain a polarizing plate whose deflection performance is not easily changed by 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 that exhibits good contrast and has a wide viewing angle.

[Brief explanation of the drawing]

FIG. 1 is a cross-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 example of a polarizing plate.

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

FIG. 5 is a graph showing wavelength 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 sealing film made of a laminate of birefringent films having different birefringent wavelength dispersion characteristics is adhered to at least one side of a polarizer, and the birefringence of the laminated sealing film is A polarizing plate characterized in that a slow axis of a film having a small wavelength dispersion of birefringence is perpendicular to a slow axis of a film having a large wavelength dispersion of birefringence and parallel to an absorption axis of a polarizer. 2. The refractive indices of the birefringent film in the slow axis direction, fast axis direction, and thickness direction are respectively nx, ny,
nz, the formula: Q=(nx-nz)/(nx-
A film with a small birefringent wavelength dispersion whose Q value calculated by ny) is 0.5 to 0.9, and a film whose Q value is 0.1 to 0.
．． 2. The polarizing plate according to claim 1, comprising a laminate type sealing film made of a laminate of films having a large wavelength dispersion of birefringence of 5. 3. The polarizing plate according to claim 1, comprising a laminated sealing film in which the birefringence becomes larger on the longer wavelength side. 4. The polarizing plate according to claim 1, comprising a laminated sealing 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 disposed on at least one side of a liquid crystal cell.