JP2857889B2 - Liquid crystal display - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a liquid crystal display device using a nematic liquid crystal, a cholesteric liquid crystal, or a smectic liquid crystal.

[Conventional technology]

Liquid crystal display devices have many features, such as being able to connect directly to IC circuits with low voltage and low power consumption, having various display functions, and being able to achieve high productivity and light weight. Its uses have expanded.

However, on the other hand, there is also a field where the expansion of applications has been delayed due to poor display quality. The biggest problems in liquid crystal display using nematic liquid crystal or cholesteric liquid crystal are that the display screen is colored and the viewing angle is narrow.

Regarding the problem of coloring, there is a strong need for black and white display, as well as removal of coloring can be a necessary condition for color display of a liquid crystal display, and a two-layer liquid crystal system has been devised. However, the use of a retardation film having a birefringence property obtained by stretching a single polymer film has begun to attract attention in order to eliminate the cost increase associated with two liquid crystal layers.

[Problems to be solved by the invention]

However, in this retardation film, coloring can be almost completely removed in a direction perpendicular to the surface of the liquid crystal display, but when the display is viewed from an oblique direction, coloring due to a slight change in angle and display contents on the screen disappear. The problem of the viewing angle characteristic has become apparent and has become a serious problem regarding the use of a phase difference film.

[Means for solving the problem]

The present invention has been completed as a result of repeated studies to eliminate the problems of the above retardation film and to provide a novel liquid crystal display device. The present invention focuses on that the cause of the above problem is the viewing angle dependency of the retardation defined as the product of the film's birefringence value and thickness, and the optical path length and the birefringence value in the film due to the change in viewing angle are As a result of repeated studies based on the presumption that the viewing angle dependence of the retardation is eliminated by using a plurality of films in an inversely proportional relationship, the optical axis or the light beam is substantially in the normal direction of the film. A liquid crystal display device comprising a light transmissive film having an axis and a negative intrinsic refraction value and a uniaxially stretched polymer film having a positive intrinsic birefringence value and light transmissivity inserted between a liquid crystal cell and a polarizing plate. It has been found that the viewing angle dependency in the above can be almost completely removed, and the present invention has been completed. That is, the present invention resides in a liquid crystal display device having the following features.

(1) It has at least one optical axis or ray axis within 45 ° around the normal direction of the film, or has a refractive index in the normal direction of the film of η _TH and a refractive index in the longitudinal direction of η _MD. , When the refractive index in the width direction is η _TD At least one light-transmissive film (A) having a negative intrinsic refractive index,
A liquid crystal display device comprising at least one uniaxially stretched polymer film (B) having a positive intrinsic birefringence value and having light transmittance inserted between a liquid crystal cell and a polarizing plate.

(2) The film (A) wherein the film (A) is a film having a negative intrinsic birefringence value, and the polymer constituting the film is substantially plane-oriented.
The liquid crystal display device as described in the above.

(3) The film (A), wherein the film (A) is a biaxially oriented film having a negative intrinsic birefringence value.
The liquid crystal display device according to (2).

(4) The film according to (1) to (2), wherein the film (A) is a film in which two uniaxially oriented films having a negative intrinsic birefringence value are combined so that their orientation directions are orthogonal to each other. Liquid phase display device.

(5) The liquid crystal display device according to (1) or (2), wherein the film (A) is a polymer solution film having a negative intrinsic birefringence value.

(6) The film according to (1) to (1), wherein the film (A) has a negative intrinsic birefringence value and liquid crystal molecules are plane-aligned.
The liquid crystal display device according to (2).

(7) The liquid crystal display device according to (1) to (5), wherein the film (A) is formed from a polystyrene-based polymer or an acrylate-based polymer.

(8) The liquid crystal display device according to the above (1), wherein the film (A) has a positive intrinsic birefringence value, and molecules are substantially oriented in a normal direction of the film surface.

(9) The film according to (1) or (8), wherein the film (A) has a positive intrinsic birefringence value, and the liquid crystal molecules are substantially oriented in the normal direction of the film surface. Liquid crystal display.

(10) At least one of the films (A) is provided in advance as a protective film on a liquid crystal cell side of a polarizing plate used in a liquid crystal display device.
The liquid crystal display device according to (9).

About.

In general, even birefringent films obtained by uniaxially stretching a polymer film having a positive intrinsic birefringence value or films having a negative intrinsic birefringence value have large morphological birefringence depending on the orientation, and consequently positive birefringence. In the film having
When the incident beam passes through a plane orthogonal to the stretching direction, the birefringence value takes a value close to constant or increases independently of the angle of incidence. Therefore, in a polymer uniaxially stretched film having a positive intrinsic birefringence value, the retardation further increases with an increase in the optical path in the film due to an increase in the angle between the incident angle and the normal to the film surface. The viewing angle becomes narrow. Also, when the incident beam is inclined from the normal direction to the stretching axis direction, the molecular arrangement is randomized in the cross section perpendicular to the stretching axis, so that the birefringence value increases with the increase in the angle between the incident beam and the normal. Decreases sharply. Further, in this case, the viewing angle is narrowed because a sharp decrease in the retardation cannot be avoided even if the optical path in the film increases with an increase in the oblique incident angle.

By the way, in the present invention, a film or a film having an optical axis or a ray axis in the direction perpendicular to the surface means that the birefringence value is nearly zero in the direction perpendicular to the surface, that is, the retardation is almost zero, The film exhibits refraction and changes the retardation. The film in the present invention may be any film having an optical axis or a light axis substantially in a direction perpendicular to the surface. More specifically, from the direction normal to the surface
It is only necessary that the optical axis has at least one optical axis or ray axis within 45 °, and accordingly, those having a vertical retardation other than zero are also included. Even if the optical axis or ray axis is not within 45 ° of the circumference, the refractive index in the plane direction of the film is η _TH , the refractive index in the longitudinal direction of the film is η _MD , and the refractive index in the width direction of the film is η _TD . When When the condition is satisfied, the object of the present invention is obtained. Now, in the laminate of the film and the uniaxially stretched film having a positive intrinsic birefringence value, the incident monochromatic light beam is obliquely incident by being inclined from the normal direction of the film surface in a direction perpendicular to the stretching axis of the uniaxially stretched film. In this case, the increase in the retardation due to the increase in the optical path for oblique incidence is suppressed and kept constant, and in the incidence from the normal direction to the stretching direction, a sharp decrease in the retardation is prevented. It has been found that there is a surprising effect of keeping the temperature constant, and that the viewing angle is greatly increased when incorporated in a liquid crystal display device.

More specifically, the present invention relates to a nematic liquid crystal,
The present invention relates to a liquid crystal display device that uses a cholesteric liquid crystal or smectic liquid crystal and eliminates the coloring phenomenon caused by the birefringence of the liquid crystal cell and enables the viewing angle and the high contrast region to be expanded. Compensation of the retardation in the vertical direction of the liquid crystal cell is made possible by at least one uniaxially stretched film having a refractive value and a light transmitting property. The compensation of the retardation at oblique incidence is made by a synergistic effect of the uniaxially stretched film and the film having an optical axis or a light axis in the normal direction of the film. There is no particular limitation on the relative positional relationship regarding the order of lamination of these films, and they may be disposed between the liquid crystal cell and the polarizing plate.
It may be placed on either side of the liquid crystal cell, and a plurality of films may be arranged so as to sandwich the liquid crystal. In addition, a uniaxially stretched film having a positive intrinsic birefringence value and / or a film having an optical axis or a light axis in a normal direction having a negative intrinsic birefringence value are used as substitutes for the protective film on the liquid crystal side of the polarizing plate. By using this, it is possible to realize the function of expanding the viewing angle and to reduce the cost.

The film in the present invention is generally considered, and includes not only a film but also a film-like material applied to a certain base material.

In addition, the uniaxially stretched film includes not only a pure uniaxial film but also a biaxially imparted film. That is, it refers to a compound having birefringence due to anisotropy in molecular orientation and having a function of compensating for at least a phase difference in a vertical direction of a liquid crystal cell. Therefore, horizontal uniaxial stretching by the tenter method, vertical uniaxial stretching using the difference in peripheral speed between rolls, and in this case, both cases where shrinkage in the width direction, ie, necking, are permitted and limited. Furthermore, in biaxial stretching, there is no limitation on the stretching method such as when there is a difference in the stretching ratio in the orthogonal direction, but a preferred method is to set the ratio of the distance between rolls / film width to 3 or more, more preferably 5 or more. This is longitudinal uniaxial stretching allowing 10% or more netting or horizontal uniaxial stretching by a lanta method. In longitudinal uniaxial stretching using a difference in peripheral speed between rolls, as is known in stretching of PVA (polyvinyl alcohol) used for a polarizing plate and the like, if the interval between rolls is reduced, stretching unevenness is likely to occur. Also, limiting the netting extremely is not an optimal mode because there is a possibility that the compensation effect of the film having the optical axis in the normal direction may be slightly reduced.

A polymer film having a positive intrinsic birefringence value that compensates for the retardation of a liquid crystal cell has a light transmittance of 70%.
It is preferable that there is no special restriction,
Among them, polycarbonate, polyarylate, polyethylene terephthalate, polyethersulfone, polyphenylene sulfide, polyphenylene oxide, polyallylsulfone, polyamideimide, polyimide, polyolefin, polyacrylonitrile, cellulose, polyester, and the like are preferable, and polycarbonate-based polymers are particularly preferable. Is preferred.

Here, even if the intrinsic birefringence value is negative, since the value is small, morphological birefringence is increased by stretching, and as a result, a material having a positive birefringence value is also included. Further, the above materials include not only homopolymers but also copolymers, derivatives thereof, blends and the like.

The polymer used as a material for producing a film having a negative intrinsic birefringence value in the present invention is not particularly limited, but may be a polystyrene-based polymer, an acrylate-based polymer, or a methacrylate-based polymer. And acrylonitrile-based polymers and methacrylonitrile-based polymers are preferred. A polystyrene-based polymer film is most preferred because it has two viewpoints, namely, a large absolute value of intrinsic birefringence and excellent transparency.

Here, the styrene polymer is a homopolymer of styrene and a styrene derivative, a copolymer and a blend of styrene and a styrene derivative.

Styrene derivatives include, for example, α-methylstyrene, o-
Examples include methylstyrene, p-methylstyrene, p-chlorostyrene, p-phenylstyrene, 2,5-dichlorostyrene and the like. Styrene and styrene derivatives (hereinafter ST)
Abbreviated to), ST is not particularly limited as long as it has good film forming property, transparency, water resistance, heat resistance, clear cut property, workability with ST. , As copolymers, ST / acrylonitrile, ST / methacrylonitrile, ST / methyl methacrylate, ST / ethyl methacrylate, ST / α-chloroacrylonitrile, ST / methyl acrylate, ST / ethyl acrylate, ST / acryl Butyl acid, ST / acrylic acid, ST / methacrylic acid, ST / butadiene, ST / isoprene, ST / maleic anhydride, ST / vinyl acetate, copolymer and styrene /
Styrene derivative copolymers and the like can be mentioned. Of course, tertiary or higher copolymers can be used in addition to the above-mentioned binary copolymers. In addition, the blended product includes not only the above-mentioned styrene homopolymer, styrene derivative homopolymer and a blend between styrene and styrene derivative copolymer, but also a polymer composed of styrene and styrene derivative (hereinafter abbreviated as PST) and a polymer not containing PST. Can also be used. These blends are, by way of example, PST / butyl cellulose, PST / coumarone resin.

The plane orientation of the film as referred to in the present invention is an orientation parameter in which the molecular arrangement when the film plane is viewed from a direction perpendicular to the plane is defined by 1/2 (3 cos ² θ-1). A film having a value near zero and having an orientation parameter larger than zero when viewed from the direction of the cut surface of the film.

These plane orientations are caused by the thickness shrinkage in the biaxial stretching process or the thickness shrinkage in the solvent evaporation process in the solution casting. These films have an optical axis substantially in the normal direction of the film, and have a function of expanding the viewing angle of a liquid crystal display. It was also found that a function equivalent to these can be obtained by orthogonally crossing two uniaxially stretched films having a negative intrinsic birefringence value. In this case, the uniaxially stretched films need not always be used in a stacked state, and there is no restriction on the arrangement such as inserting a uniaxially stretched film having a positive intrinsic birefringence value between the two uniaxially stretched films. In the above embodiment, the film having a negative intrinsic birefringence value in which the thickness is shrunk by solvent evaporation to obtain the plane orientation is different from the forced stretching, and the plane orientation of the polymer constituting the film is uniform, It is the most excellent in that it does not cause a target unevenness.

Films can also be obtained by orienting molecules having a positive intrinsic birefringence value in the direction normal to the film surface so as to exhibit an optical axis or ray axis substantially in the direction normal to the film. In the method of orientation, in the case of a polymer film, electrodes are provided on both sides of the film in the process of forming a film by melt extrusion, and a high voltage is applied to perform orientation. However, this method requires a high electric field of 20 MV / m or more, and in some cases, dielectric breakdown may occur. Therefore, a preferred method is to align the liquid crystal monomer and fix it later. For example, a method in which a compound polymerizable by ultraviolet light, visible light, or the like is mixed with a liquid crystal monomer, and polymerization is advanced and fixed while maintaining the orientation of the liquid crystal monomer in an electric field is preferable. Further, the liquid crystalline monomer itself may have photopolymerizability.

That is, the idea of the present invention resides in that a film having an optical axis or a light axis substantially in the normal direction of the film is used in combination with the longitudinally uniaxially stretched film, and the specific means is not limited.

〔Example〕

 Hereinafter, the present invention will be described in detail with reference to examples.

Example 1 Polycarbonate having a molecular weight of 80,000 and an intrinsic birefringence of 0.104, obtained by condensation of phosgene and bisphenol A, was dissolved in methylene dichloride to form a 10% solution. The solution was cast on a steel drum and continuously stripped to a thickness of 90 μm and a width of 90 μm.
A transparent polycarbonate film (PC film) of 500 mm was obtained. The film was stretched 33% by a tenter at a temperature of 170 ° C. to obtain a retardation film having a thickness of 68 μm and a ration of 560 nm.

The film and a polystyrene biaxially stretched film GSS15 (150 μm) manufactured by Dainippon Ink Co., Ltd. were superimposed, and the viewing angle dependence of the retardation was measured using a monochromatic light having a wavelength of 632.8 nm using a birefringence meter AEP-100 manufactured by Shimadzu Corporation. As a result, the retardation became almost independent of the angle as shown in Table 1. Also, when the above two films are inserted between the STN liquid crystal cell and the polarizer on the analyzer side, the viewing angle range is greatly improved regardless of the order of insertion and the relative angle of superposition, and the film is tilted by 50 ° or more. And the display screen could be seen clearly.

When the refractive index was measured with an Atsube refractometer, the polystyrene film was found to have η _TH = 1.555, η _MD = 1.543, η _TD = 1.54
2 And

Comparative Example 1 The angle dependence of the retardation of the polycarbonate film having a retardation of 560 nm in Example 1 was measured in the same manner as in Example 1. The viewing angle was 20 ° or less in combination with the liquid crystal cell. Also, η _TH
= 1.574, η _MD = 1.591, η _TD = 1.582 It was.

Comparative Example 2 The optical properties of the biaxially stretched GSS15 in Example 1 were measured in the same manner as in Example 1. The results are shown in Table 1. The film alone cannot be used as a film for compensating the phase difference of the liquid crystal because the normal direction retardation is close to zero.

Example 2 The polycarbonate film formed in Example 1 was subjected to longitudinal stretching at a temperature of 170 ° C. and a stretching ratio of 29% using rollers having different peripheral speeds without fixing both sides of the film.

At this time, the interval between the rolls is 5m, the netting rate is 13%,
Film feed speed is 2m / min, film take-up speed is 2.
It was 6m / min.

The obtained film was superimposed on a biaxially oriented polystyrene film OPS-50 manufactured by Mitsubishi Monsanto Kasei Co., Ltd., and the retardation was measured in the same manner as in Example 1. As a result, the angle dependence of the retardation was small.

In addition, when the polycarbonate film is used as a protective film on the liquid crystal cell side of the analyzer-side polarizing plate, and a biaxially stretched polystyrene film is inserted between the STN liquid crystal cell and the analyzer, the viewing angle is greatly increased.て も The screen could be seen clearly even when tilted more than once.

In addition, η _{TH of} polystyrene film = 1.556, η _MD =
1.543, η _TD = 1.542 It was.

Comparative Example 3 Table 1 shows the results of measuring the angle dependence of the retardation of the polycarbonate film obtained in Example 2.

When used alone as a retardation film, the viewing angle is
It was less than 30 ゜.

Comparative Example 4 Table 1 shows the optical characteristics of the biaxially oriented polystyrene OPS-50 in Example 2. With this film alone, the retardation in the normal direction was close to zero, so that it could not be used as a film for compensating the phase difference of the liquid crystal.

Example 3 Polystyrene electrified styrene MW-1 manufactured by Denki Kagaku KK
Was dissolved in a 1: 1 mixed solvent of toluene and MEK (methyl ethyl ketone), and a solution film was formed in the same manner as in the polycarbonate film of Example 1 to obtain a polystyrene film having a thickness of 100 μm. When the two films and the polycarbonate obtained in Example 2 were laminated and inserted between a liquid crystal cell and an analyzer, the image was clear and the viewing angle was greatly increased. Further, the polystyrene film was not biaxially stretched, and a plane orientation was formed by thickness shrinkage in the solvent evaporation process, so that there was almost no unevenness corresponding to local unevenness of birefringence, and high quality image was obtained.
Also in this case, the image was clear even at an inclination of 50 °, and the optical characteristics of the film laminate were good as shown in Table 1.

Η _{TH of} polystyrene film = 1.551, η _MD = 1.548,
η _TD = 1.548 It was.

Comparative Example 5 When the optical characteristics of the polystyrene film obtained in Example 3 were examined, the results were as shown in Table 1. Also in this case, the polystyrene film alone could not compensate for the phase difference of the STN liquid crystal cell.

Example 4 The polystyrene film obtained in Example 3 was subjected to 100% longitudinal uniaxial stretching at a temperature of 120 ° C. The two films were orthogonally inserted between the STN liquid crystal cell and the analyzer. Further, the polycarbonate film obtained in Example 1 was inserted between a liquid crystal cell and a polarizer. Also in this case, a clear image was obtained.
Table 1 shows the optical characteristics of the two polystyrene films crossed orthogonally and the polymer carbonate laminated.

Polystyrene film has η _TH = 1.553, η _MD = 1.556,
η _TD = 1.539 It was.

Comparative Example 6 Table 1 shows the optical characteristics of a laminate in which two uniaxially stretched polystyrene films obtained in Example 4 were crossed. In addition, the film alone could not remove the coloring of the STN liquid crystal cell, and was unsuitable as an optical compensation film.

Example 5 A polyarylate U-polymer AX-1500 manufactured by Sumitomo Chemical was dissolved in methylene dichloride to obtain an 8% solution. The solution was cast on a steel drum and continuously stripped to a thickness of 80 μm and a width of 80 μm.
A 500 nm transparent polyarylate film was obtained.

The film was stretched at a draw ratio of 35% at a temperature of 195 ° C. using rollers having different peripheral speeds without fixing both sides of the film. At this time, the netting rate was 11%. The interval between the rolls was 3 m, and the film feed speed was 4 m / min. The obtained film and two polystyrene films obtained in Example 3 were laminated and interposed between the STN liquid crystal cell and the analyzer. The viewing angle range was greatly improved, and the display screen could be seen clearly even at an angle of 40 ° or more. Table 1 shows the optical characteristics of the laminated film.

Comparative Example 7 When the polyarylate film obtained in Example 5 alone was used as a retardation film, the viewing angle was 30 ° or less.

 Table 1 shows the optical characteristics of the film.

Comparative Example 8 When an image was displayed on the STN liquid crystal cell alone used in Examples 1 to 5, the screen was reddish purple, the viewing angle was narrow, and the image was unclear at 20 ° or more.

[Effect of the Invention] Whether the optical film or the optical axis is substantially in the normal direction of the film surface The combination of a film satisfying the above condition and a uniaxially stretched film having a positive intrinsic birefringence value significantly improves the viewing angle dependence of the retardation of the uniaxially stretched film alone, and provides a retardation to the nematic, cholesteric or smectic liquid crystal cell. The viewing angle is remarkably improved when used as a film.

Claims (3)

(57) [Claims] 1. The film has at least one optical axis or ray axis within 45 ° around the normal direction of the film, or has a refractive index in the normal direction of the film of η _TH and a refractive index in the longitudinal direction of the film. When η _MD and the refractive index in the width direction are η _TD At least one light transmissive film (A) having a negative intrinsic refraction value, which satisfies any of the following conditions:
A liquid crystal display device comprising at least one uniaxially stretched polymer film (B) having a positive intrinsic birefringence value and having light transmittance inserted between a liquid crystal cell and a polarizing plate. 2. The liquid crystal display device according to claim 1, wherein the film (A) is a film in which a polymer has a substantially plane orientation. 3. The liquid crystal display device according to claim 1, wherein the film (A) is a biaxially oriented film.