JPH0497322A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To improve a contrast and to obtain a bright black/white display by disposing a uniaxially stretched film of a polymer having a positive intrinsic double refractive value, a uniaxially stretched film of a polymer having a negative intrinsic double refractive value, and a layer having optical activity between a polarizing plate and a liquid crystal cell. CONSTITUTION:The uniaxially stretched film consisting of the polymer having the positive intrinsic double refractive value and light transmittability, the uniaxially stretched film consisting of the polymer having the negative intrinsic double refractive value and light transmittability and the film having the optical activity are inserted between the liquid crystal cell and the polarizing plate. The retardations in the normal direction of the film of the laminated, more preferably orthogonally laminated body of the longitudinally uniaxially stretched film formed of the polymer having the positive intrinsic double refractive value and the longitudinally uniaxially stretched film formed of the polymer having the negative intrinsic double refractive value are added to each other and are made incident in all bearings without being eliminated, by which the change in the retardation is obviated. The contrast is improved in this way and the brighter black/white display is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid crystal display device using leistid nematic liquid crystal, cholesteric liquid crystal or smectic liquid crystal.

[Conventional technology]

Liquid crystal display devices have many features such as low voltage, low power consumption, direct connection to IC circuits, a variety of display functions, high productivity and light weight. Its uses have expanded.

However, on the other hand, there are some fields where the expansion of applications is delayed due to the constraint of poor display quality.

The biggest problems with liquid crystal displays using nematic liquid crystals or cholesteric liquid crystals are the coloring of the display screen and the narrow viewing angle.

Regarding the problem of coloring, not only is removing coloration a necessary condition for color display on a liquid crystal display, but there is also a strong need for black and white display, and a two-layer liquid crystal system has been devised. However, in order to eliminate the high costs associated with stacking two layers of liquid crystal, attention is beginning to be focused on the use of retardation films in which birefringence is imparted by stretching a single polymer film.

[Problem to be solved by the invention]

However, although this retardation film can considerably remove coloring in the direction perpendicular to the plane of the liquid crystal display, further improvements are required.

Furthermore, when the display is viewed from an angle, problems with viewing angle characteristics such as coloring due to slight changes in angle or disappearance of the displayed content on the screen become apparent, which is an important issue regarding the use of retardation films.

[Means to solve the problem]

The present invention was completed as a result of repeated research in order to eliminate the problems of the above-mentioned retardation film and provide a new liquid crystal display device.

That is, the present invention is as follows.

(1) At least one uniaxially stretched film made of a polymer that has a positive intrinsic birefringence value and optical transparency; and at least one film made of a polymer that has a negative intrinsic birefringence value and optical transparency. A liquid crystal display device comprising a uniaxially stretched film and at least one optically active film inserted between a liquid crystal cell and a polarizing plate.

(2) The stretching axes of the uniaxially stretched film made of a polymer with a positive intrinsic birefringence value and optical transparency and the uniaxially stretched film made of a polymer with a negative intrinsic birefringence value and optical transparency are orthogonal to each other. The liquid crystal display device according to (1) above, characterized in that:

(3) The liquid crystal display device according to (1) or (2) above, wherein the optically active film is formed of chiral liquid crystal.

The present invention was completed as a result of repeated studies, focusing on the fact that the problems of the prior art are due to two causes.

The reason why the viewing angle at the second point is narrow is that the retardation defined as the product of birefringence and thickness in the liquid crystal cell and the retardation film changes greatly depending on the incident angle of light. Regarding this point, we conducted repeated studies based on the assumption that the dependence of retardation on viewing angle would be eliminated by creating a multiple film configuration in which the optical path length within the film and the birefringence value are inversely proportional to each other as the viewing angle changes. As a result, a film having an optical axis substantially in the normal direction of the film, specifically a laminated film of a biaxially stretched film having a negative intrinsic birefringence value and a uniaxially stretched film having a positive intrinsic birefringence value. It was discovered that the viewing angle could be significantly improved by inserting the device between the liquid crystal cell and the polarizing plate, and a patent application was filed (Japanese Patent Application No. 63278592).

In addition, as a result of further research, we have discovered that by inserting a laminate of a uniaxially stretched polymer film with a positive intrinsic birefringence value and a uniaxially stretched polymer film with a negative intrinsic birefringence value between the liquid crystal cell and the polarizing plate, the liquid crystal We found that viewing angle dependence in display devices could be almost completely eliminated and filed a patent application (Japanese Patent Application No. 1281247).

In general, in a birefringent film obtained by uniaxially stretching a polymer film having a positive intrinsic birefringence value, when the incident beam passes through a plane perpendicular to the stretching direction, the birefringence value does not depend much on the incident angle and is close to a constant value. take or increase. Therefore, in a uniaxially stretched film made of a polymer with a positive intrinsic birefringence value, retardation increases as the angle between the incident angle and the normal to the film surface increases, which increases the optical path within the film. However, the viewing angle becomes narrower.

Furthermore, when the incident beam is tilted from the normal direction toward the stretching axis, the molecular arrangement becomes random in the cross section perpendicular to the stretching axis, so birefringence increases as the angle between the incident beam and the normal increases. Decrease rapidly. Furthermore, in this case, the viewing angle becomes narrower because a sudden decrease in retardation cannot be avoided due to an increase in the optical path within the film as the oblique incidence angle increases.

By the way, in the present invention, lamination, preferably orthogonal lamination, of a longitudinally stretched film formed from a polymer having a positive intrinsic birefringence value and a longitudinally stretched film formed from a polymer having a negative intrinsic birefringence value In the case of a film, the retardation in the normal direction of the film is added to each other and does not disappear, and can be freely controlled depending on the purpose, such as films with no change in retardation or films with moderate retardation changes in response to omnidirectional incidence. It has been found that this has an excellent effect.

Furthermore, when incorporated into a liquid crystal display device, the viewing angle was found to be significantly expanded.

Regarding the second point, further removal of coloring in the vertical direction of the liquid crystal cell, that is, achieving clearer white/blackness, one of the causes is optical rotation and optical rotation dispersion due to twisting of the liquid crystal in the liquid crystal cell. As a result of intensive investigation, it was found that an optically active film was placed between the liquid crystal cell and the polarizing plate to improve contrast and reduce tint.

More specifically, the present invention eliminates the coloring phenomenon caused by the birefringence of the liquid crystal cell in a liquid crystal display device using twisted nematic liquid crystal, cholesteric liquid crystal, or smectic liquid crystal having a twist angle of 90 degrees or more, and improves the visual field. Regarding the retardation in the film normal direction, the retardation in the longitudinal-axial stretching of a film made of a polymer having a positive intrinsic birefringence value A countable value of the retardation in longitudinal-axial stretching of a film formed from a polymer having a negative intrinsic birefringence value and a negative intrinsic birefringence value is obtained. However, if the stretching axes of the uniaxially stretched polymer films having positive and negative intrinsic birefringence values coincide, the retardation will be canceled and the objective will not be achieved. Therefore, it is preferable that the stretching axes of the film laminate do not coincide but are arranged at a certain angle, and more preferably that they are arranged orthogonally to each other. Orthogonality as used in the present invention does not necessarily have to be completely 90°, but only needs to be a relative angle in the range of 70° to 110°.

Further, the films do not have to be always used in a laminated manner, but may be placed on both sides of the liquid crystal cell, or may also serve as a protective film on the liquid crystal cell side of the polarizing plate. In this case, there is an advantage that the viewing angle can be expanded and the cost can be reduced.

In addition, the term "optically active film" refers to any film that has optical rotation, and there are no limitations to its content, such as films made of polymers with a helical structure such as polypeptides, and polymers in which chiral liquid crystals have optical transparency. This includes those dispersed in microscopic sizes. Regarding the levorotation and dextrorotation of optical activity, it is preferable that the optical activity is opposite to the optical rotation of the liquid crystal of the liquid crystal cell used. Further, the film may be disposed between the polarizing plate and the liquid crystal, and there is no limitation on the relative positional relationship with the polymer-axially stretched film. The light transmittance of the film is preferably 90% or more, and the thickness is preferably several 100 microns or less.

The uniaxially stretched film in the present invention is not only a pure uniaxial film, but also a film that essentially functions as a uniaxial film even if it is imparted with some biaxiality.

Therefore, it includes transverse uniaxial stretching by a tenter method, longitudinal-axial stretching using the difference in circumferential speed between rolls, and in this case, cases in which natural shrinkage is performed during width direction stretching are also limited.

Now, in the present invention, the film having light transmittance and a positive intrinsic birefringence value preferably has a light transmittance of 70% or more and is achromatic, and more preferably has a light transmittance of 90%. The above is an achromatic color. There are no restrictions on the polymer used for the film, but specifically polycarbonate,
Polyacrylate polyethylene terephthalate, polyether sulfone, polyphenylene sulfide, polyphenylene oxide, polyallyl sulfone, polyamideimide, polyimide, polyolefin, polyvinyl chloride,
Cellulose, polyester polymers, etc. are preferred, particularly polycarbonate polymers, polyarylate polymers,
Polymers such as polyester polymers, which have a large intrinsic birefringence value and can easily be formed into a planar and homogeneous film by solution casting, are preferred.

Moreover, the above-mentioned polymer may be not only a homopolymer but also a copolymer, a derivative thereof, a blend thereof, and the like.

In the present invention, the film having light transmittance and a negative intrinsic birefringence value preferably has a light transmittance of 70% or more and is achromatic, and more preferably has a light transmittance of 90%. The above is an achromatic color.

Are there any restrictions on the polymers used for the film? Specifically, styrene polymers, acrylic ester polymers,
Methacrylic acid ester polymers, acrylonitrile polymers, methacrylonitrile polymers, vinylnaphthalene polymers, vinylpyridine polymers, hinylcarbazole polymers, and phenyl acrylamide polymers are preferred, and polystyrene polymers are preferred. is the most preferable from three viewpoints, namely, the absolute value of the intrinsic birefringence value is large, the transparency is excellent and there is no coloration, and solution film formation is possible.

Here, the styrenic polymer includes a homopolymer of styrene and a styrene derivative, a copolymer of styrene and a styrene derivative, a blend, and the like.

Examples of styrene derivatives include α-methylstyrene, 0-
Examples include, but are not limited to, methylstyrene, p-methylstyrene, p-chlorostyrene, O-nitrostyrene, p-aminostyrene, p-carboxylstyrene, p-phenylstyrene, 2,5-dichlorostyrene, etc. .

Copolymers and blends of styrene and styrene derivatives (hereinafter abbreviated as ST) are not particularly limited as long as they have suitable film-forming properties with ST, and even if they have a phase-separated structure, they have transparency etc. For example, copolymers include ST/acrylonitrile, ST/methacrylonitrile, ST/methyl methacrylate, ST/ethyl methacrylate, ST/α-chloroacrylonitrile. , ST/methyl acrylate, ST/ethyl acrylate, ST/butyl acrylate, ST/acrylic acid, ST/methacrylic acid, ST/butadiene, ST/isoprene, ST/maleic anhydride, ST/itaconic acid, ST/ Vinylcarbazole, ST/n-phenylacrylamide, ST/vinylpyridine, ST/vinylnaphthalene, α-methylstyrene/acrylonitrile, α-methylstyrene/methacrylonitrile, ST/vinyl acetate copolymer and styrene/styrene derivative copolymer, etc. Can be mentioned. Of course, in addition to the binary copolymers listed above, ST/α-methylstyrene/acrylonitrile, ST/α-methylstyrene/
Ternary or higher copolymers such as methyl methacrylate can also be used. Blends include not only the above-mentioned styrene homopolymers, styrene derivative homopolymers, and styrene and styrene derivative copolymers, but also polymers made of styrene and styrene derivatives (hereinafter abbreviated as PST) and polymers that do not contain PST. Blends of can also be used. Examples of these blends are PST/butylcellulose and PST/Chroman resin.

Chiral liquid crystals in the present invention typically include cholesteric liquid crystals and nematic liquid crystals in which a chiral agent is mixed. Furthermore, these liquid crystals include those with positive dielectric anisotropy and those with negative dielectric anisotropy.

In the present invention, the cholesteric liquid crystal, nematic liquid crystal, and chiral agent are not particularly limited, but typical ones are shown below.

Cholesteric liquid crystals have an optically active group in a rigid path or a tortuous path, and are as follows.

cholesteryl benzoate methylbutyl)-4' cyanobiphenyl CH.

■Azo type, azoxy type liquid crystal■ N=N] -N=N ↓ N=N ↓ N=N -N=N- ■Nematic liquid crystal ■Schiff base type nematic liquid crystal■ ■■Phenylcyclohexane type biphenylcyclohexane type liquid crystal C ,H.

C.N. C.H.

C.N. C, H7 C,8,0 ■Pyrimidine-based dioxane-based liquid crystal ■Biphenyl/terphenyl liquid crystal C,H.

5H C,H,, - C,H,, - C,H,, - 1oH2 C,H,0- C5H,, O- C,H,, O- C,H,, O- C,oH2,0 - ■ Benzoic acid ester liquid crystal ■ Cyclohexylcarboxylic acid ester liquid crystal C, H,
-7H15 C,H C,H,OC,H,0 C2H,OC,H,,0 C2H,OC,H,,0 CN CN CN CN etc.

Chiral agent: Representative chiral agents are shown below.

Now, in the present invention, if the chiral liquid crystal is a polymer such as a polypeptide, it can be applied as it is to a uniaxially stretched film, a glass substrate of a liquid crystal cell, the liquid crystal side of a polarizing plate protective film, etc. to form a film-like material. be.

If it is a monomer or liquid, it is difficult to form a single film, so it is preferable to use a film-like material in which a light-transmitting polymer or the like is used as a matrix and dispersed in the form of fine particles. In this case, there are no limitations on the method of forming the matrix polymer or the liquid crystal fine particles.

〔Example〕

This will be explained in detail below using examples.

Example 1 A polycarbonate having a molecular weight of about 200,000 was dissolved in a methylene dichloride solvent to obtain a 10% solution and a 20% solution. The solution was cast onto a steel drum and continuously peeled off to obtain a transparent polycarbonate film (PC film) with a thickness of 90 μm and a width of 500 mm. The film was stretched by 16% by longitudinal-axial stretching using rolls having different circumferential speeds at a temperature of 170°C to obtain film (a).

Next, polystyrene having a molecular weight of about 200,000 was dissolved in methylene dichloride to form a 25% solution. The solution was cast onto a steam drum and continuously peeled off to a thickness of 90 μm and a width of 5
A 00 mm polystyrene film (PSt film) was obtained. The film was stretched by 17% by longitudinal-axial stretching using rolls having different circumferential speeds at a temperature of 90°C to obtain a film (b). Film (a) and film (b) are laminated so as to be perpendicular to each other, and the stretching axis direction of film (a) is set to θ.
= 0, and the stretching axis direction of film (b) is 090°, θ = 0, θ = 45 from the normal direction of the film laminate.
The retardation Re (40) and the retardation R (0) in the normal direction when the incidence is obliquely 40 degrees in the directions of θ and θ = 90 degrees are measured, and the rate of change in retardation Re (0) −Re (40) l:lRe (0) was calculated. However, AEP-100 manufactured by Shimabara Manufacturing Co., Ltd. was used to measure the retardation. The light source used had a wavelength of 632.8 nm, and the measurement results are shown in Table 1. Next, a chiral liquid crystal film was coated on the polycarbonate film of the laminate. The film forming recipe and method are shown below.

The mixture of the above formulation was stirred with a homogenizer at 40°C for 5 minutes at a rotation speed of 15 (100 rpm) to obtain an emulsion.The emulsion was coated on the polycarbonate of the laminate with a doctor blade, and A film-like substance was obtained.

In addition, the film laminate has a cell gap size of 6 μm and a liquid crystal twist angle of 270O1△nXd (birefringence value x thickness).
PS between 0.68 μm liquid crystal cell and polarizing plate (observation side)
When the T film (b) was inserted so that it was facing the liquid crystal cell side, and the display characteristics as a liquid crystal display device were investigated, good black and white display and high contrast were obtained, and the viewing angle was greatly improved, and the left and right A wide viewing angle of 60° or more was obtained for each, and 50° or more for the top and bottom. Further, although the viewing angle in the vertical and diagonal directions was conventionally narrow, according to the present invention, a wide viewing angle of 50' or more in the vertical and diagonal directions can be obtained. Here, the contrast 5 is defined as the boundary line for the viewing angle range.

The results are shown in Table-1.

Comparative Example 1 In Example 1, the characteristics were evaluated under the same conditions as Example 1 without applying the film containing chiral liquid crystal. The viewing angle was the same as in Example-1, but the contrast was lower than in Example-1.

Comparative Example 2 The polycarbonate film obtained in Example 1 was stretched by 33% at a temperature of 175° C. using a longitudinal-axial stretching device using rolls with different circumferential speeds to obtain a birefringent film. The optical properties of the film are shown in Table 1. In addition, a chiral liquid crystal similar to that in Example 1 was coated on the film, and the display characteristics of the liquid crystal cell were examined by inserting it between the liquid crystal cell of Example 1 and a polarizing plate (observation side). In the observation, although a good black and white display was achieved, it became colored under oblique incidence, and the contrast was 20 degrees each in the upper and lower directions, and 25 degrees each in the left and right directions, resulting in a narrow field of view.

Comparative Example-3 Only the film (a) obtained in Example 1 was evaluated for characteristics under the same conditions as in Example 1. The results are shown in Table-1.

Comparative Example-4 Only the film (b') obtained in Example 1 was evaluated for characteristics under the same conditions as in Example I. The results are shown in Table-1.

〔Effect of the invention〕

As can be seen from the Examples and Comparative Examples, in the present invention, a wide viewing angle can be achieved by laminating a uniaxially stretched polymer film with a positive intrinsic birefringence value and a uniaxially stretched polymer film with a negative intrinsic birefringence value, and optical By arranging an active layer between the polarizing plate and the liquid crystal cell, the contrast is also greatly improved and clear black/white display can be obtained.

Claims (3)

[Claims] (1) At least one uniaxially stretched film made of a polymer that has a positive intrinsic birefringence value and optical transparency; and at least one film made of a polymer that has a negative intrinsic birefringence value and optical transparency. A liquid crystal display device comprising a uniaxially stretched film and at least one optically active film inserted between a liquid crystal cell and a polarizing plate. (2) The stretching axes of the uniaxially stretched film made of a polymer with a positive intrinsic birefringence value and optical transparency and the uniaxially stretched film made of a polymer with a negative intrinsic birefringence value and optical transparency are orthogonal to each other. The liquid crystal display device according to claim 1, characterized in that the liquid crystal display device comprises: (3) Claims (1) to (2) characterized in that the optically active film is formed from a chiral liquid crystal.
The liquid crystal display device described.