JPH07151915A - Optical anisotropic element, its production and liquid crystal display element using the same - Google Patents

Abstract

PURPOSE:To improve view field characteristics and irregular colors in a TN liquid crystal display element and to provide a liquid crystal display element of high quality display excellent in visibility. CONSTITUTION:The optical anisotropic element consists of a thermoplastic resin film having transmitting property for light and optical anisotropy. Re value is defined as {(na+nb)divided by 2-nc}Xd, wherein na, nb and nc are three optical elastic axes of the film and nb is the smallest, and the (d) is the film thickness. The average Re in the plane satisfies 50<Re<400 nm. The angle beta is defined as an angle between nb and the normal line of the film, and the average betain the plane satisfies 10<=beta<=50. The coeffts. of variation Cbeta, CPe for ' and Re of the optical anisotropic film are Cbeta<=0.1 and CRe<=0.1, respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical anisotropic element capable of improving the display contrast and viewing angle characteristics of a display color of a liquid crystal display element, a method for producing the same, and a liquid crystal display element using the same.

[0002]

2. Description of the Related Art CRTs, which are the mainstream of display devices for OA equipment such as Japanese word processors and desktop personal computers
Have been converted into liquid crystal display elements which have the great advantages of thinness, light weight, and low power consumption. Most of the liquid crystal display elements (hereinafter, referred to as LCDs) that are currently popular use twisted nematic liquid crystals. The display method using such a liquid crystal can be roughly classified into a birefringence mode and an optical rotation mode.

An LCD using a birefringence mode has a liquid crystal molecule array twisted by 160 ° or more and has steep electro-optical characteristics. Therefore, a simple matrix electrode without active elements (thin film transistor or diode) is used. Even with the structure, a large capacity display can be obtained by time-division driving. However, it has a drawback that the response speed is slow (several hundreds of milliseconds) and gradation display is difficult, and a liquid crystal display element (TFT-LCD) using an active element is used.
, MIM-LCD, etc.) is not reached.

For the TFT-LCD and MIM-LCD, there is used a display system (TN type liquid crystal display element) of optical rotation mode in which the alignment state of liquid crystal molecules is twisted by 90 °. This display method is the most effective method as compared with other LCDs because it has a high response speed (several + milliseconds), can easily obtain a black and white display, and has a high display contrast. However, since the twisted nematic liquid crystal is used, there is a viewing angle characteristic that the display color and the display contrast change depending on the viewing direction in view of the principle of the display system, and the display performance of the CRT cannot be exceeded.

Japanese Unexamined Patent Publication Nos. 4-229828 and 4-2.
As disclosed in Japanese Patent No. 58923, a method has been proposed in which an optical anisotropic element is arranged between a pair of polarizing plates and a TN type liquid crystal cell to increase the viewing angle.

The optical anisotropic element proposed in the above publication has a phase difference of almost zero in the direction perpendicular to the surface of the liquid crystal cell and exerts no optical action from the front. A phase difference appears when tilted, and the phase difference that appears in the liquid crystal cell is compensated. However, even with these methods, the viewing angle of LCD is still insufficient, and further improvement is desired. In particular, when considered as a vehicle-mounted type or as a substitute for a CRT, the current viewing angle cannot cope with the situation. In order to solve the above problems, an optical anisotropic element having negative uniaxiality in the liquid crystal display element and an optical axis which is neither perpendicular nor parallel to the film surface, and which is inclined by 10 to 30 degrees from the film normal is used. It was found that the viewing angle can be greatly expanded by applying a patent application (Japanese Patent Application No. 4-308377). Further, in order to realize a film having such optical characteristics at low cost and with high productivity, by continuously applying a shearing force to both surfaces of the film, the film has a negative uniaxial property and an optical property. The inventors have found that an optical anisotropic element having an inclined axis can be continuously manufactured with high productivity and applied for a patent (Japanese Patent Application No. 4-324116).

[0007]

However, the film produced by the above-mentioned method is liable to exhibit birefringence unevenness, which causes problems such as color unevenness and contrast unevenness of the displayed image which are considered to be caused by the birefringence unevenness. Therefore, an object of the present invention is to propose an optical anisotropic element having excellent viewing angle characteristics and less uneven color and uneven contrast, and a method for manufacturing the same at low cost.

[0008]

The above-mentioned problems are (1) a film having a light-transmitting property and an optical anisotropy, which is made of a thermoplastic resin, and has three optical elastic axes of the film having the optical anisotropy. , Na, nb, nc, the smallest value is nb, the angle between the film normal and nb is β, and the thickness is d, the average β in the film plane is
10 ≦ β ≦ 50 degrees and {(na + nb) / 2−n
The average value of Re values defined by c} × d is 50 ≦ R
e ≦ 400 nm, the variation coefficients Cβ and C _{Re of} the β and Re value variations are Cβ ≦ 0.1 and C _Re ≦, respectively.
An optical anisotropic element characterized by being 0.1. (2) The optical anisotropic element according to (1), which has an optical negative uniaxial property and has an optical axis inclined from the film normal direction. (3) The optical anisotropic element according to (1) or (2), wherein the thermoplastic resin has a positive intrinsic birefringence value. (4) An optical anisotropy including a step of applying a shearing force to a film made of a thermoplastic resin and having a light-transmitting property between rolls having different peripheral speeds and applying a shearing force to the film in the surface direction. In the method for producing a rectangular element, the surface roughness, roundness, and cylindricity of the roll are 0.15 S or less, 20 μm or less, and 20 μm or less, respectively. (5) An optical anisotropy including a step of applying a shearing force to a film made of a thermoplastic resin and having a light transmissive property between rolls having different peripheral speeds and applying a shearing force in the surface direction of the film. In the method for manufacturing a rectangular element, the ratio of the peripheral speed Vh of a roll having a high speed and the peripheral speed Vl of a roll having a low speed of the two rolls having different peripheral speed ratios is 1.001 to 1.03. And a method for manufacturing an optical anisotropic element. (6) A step of uniaxially stretching in the width direction of the film is provided before or after the step of sandwiching the film between rolls having different peripheral speeds and imparting shear deformation to both sides of the film to give strain deformation. Features (4)
(5) A method for manufacturing an optically anisotropic element according to (5). (7) T with a twist angle of approximately 90 ° between the two electrode substrates
A liquid crystal cell sandwiching an N-type liquid crystal, two polarizing elements arranged on both sides of the liquid crystal cell, and the optical anisotropic element according to (1) or (2) between the liquid crystal cell and the polarizing element. (3)
A liquid crystal display device characterized in that at least one optically anisotropic element manufactured by the method described in (6) is arranged.
Achieved by

Next, the present invention will be described in detail. The light-transmitting film made of a thermoplastic resin in the present invention has a light transmittance of 70% or more in a film or sheet shape made of a thermoplastic resin, and more preferably 85.
% Or more are all targeted, and a resin having a positive intrinsic birefringence value is more preferable. Specifically, polycarbonate, polyarylate, polysulfone, polyethylene terephthalate, polyethylene naphthalate, polyether sulfone, polyphenylene sulfide, polyphenylene oxide, polyallyl sulfone, polyvinyl alcohol,
Polyamide, polyimide, polyolefin, polyvinyl chloride, cellulose-based polymer, various binary and ternary copolymers, graft copolymers, blends and the like are preferably used.

In the present invention, at least two rolls are used to apply the shearing force to the film. A shearing force is applied to the film by forcing the roll speed difference between these rolls. The force with which the roll nips the film is preferably capable of preventing the phenomenon that the film slips and the shearing force is not transmitted, and the reduction force (the force with which the roll pushes the film) is 50 kg per unit length in the width direction of the film. / Cm or more, more preferably 150 kg /
cm or more.

The triaxial refractive index characteristic of the film before shearing is not particularly limited, and may or may not be optically isotropic. However, in the case where the film before being subjected to shearing is optically isotropic, in order to develop negative uniaxiality, 0.
It is preferable to add a step of 5% to 20% width direction uniaxial stretching or unbalanced biaxial stretching. The unbalanced biaxial stretching is biaxial stretching in which the stretching ratios in the longitudinal direction and the width direction are different, and in this case, the stretching ratio in the width direction is 0.5 in the longitudinal direction and the stretching ratio in the width direction. % To 20% is preferable.

As a method of applying a shearing force to the film surface, the film is (Tg-50) degrees or more and (Tg +
The temperature may be 30 ° C. or less, and the film may be sandwiched between two rolls having different peripheral speeds or rotating so as to advance the film in the opposite direction, and the film may be pulled out. Regarding the fact that the main refractive index could be inclined by the shearing force, it is considered that the strain as shown in FIG. 5 is applied inside the film. In FIG. 5, the assumed cube a inside the film is deformed by the peripheral speed difference between the two rolls, deformed like a solid b, and is further sent out as a solid c. At this time, the molecules inside the cubic are also considered to be tilted.

Although the present invention has an advantage that a negative uniaxial film having an inclined optical axis can be continuously manufactured at low cost, it has a problem that birefringence unevenness is likely to occur. As a result of diligent study on this, C
It has been found that when β and C _Re exceed 0.1, the display characteristics of the liquid crystal display device are significantly adversely affected. So Cβ, C _Re
As a result of pursuing the cause of increasing, the birefringence unevenness has periodicity, and the period corresponds to the length of one round of the roll, and the unevenness, roundness, and cylindricity of the roll surface It turned out to be deeply involved. That is, the contact pressure between the film and the roll varies depending on the location of the film due to the unevenness of the surface of the roll, the roundness, and the cylindricity, which results in a partial variation in the shearing force and uneven birefringence. presume.

Ideally, these irregularities should have surface roughness, circularity, and cylindricity as close to zero as possible, but in reality, this is impossible, and practically 0.15 each.
S, 20 μm, preferably 20 μm or less, 0.1 S, 1
It was found that 0 μm and 10 μm or less are more preferable.
Here, the surface roughness in the present invention refers to JIS. B0
It is the maximum height (Rmax) defined by 601. The roundness and cylindricity are defined in JIS. These are the values of circularity and cylindricity defined by B0621. There are no particular restrictions on the roll manufacturing method and polishing method for achieving the above-mentioned surface roughness, roundness, and cylindricity, but metal rolls and ceramics rolls that are resistant to deformation by pressure are preferred.

Further, it has been found that if the peripheral speed ratio is made too large, the roll and the film slip irregularly, and the birefringence unevenness occurs due to the excessive deformation of the film. Therefore, it was found that in order to produce an optical compensation film with little unevenness of birefringence, there is an optimum place in a region where the peripheral speed ratio is considerably small. Specifically, the peripheral speed ratio is preferably 1.001 to 1.03, and 1.002 to 1.0
1 is more preferable. This condition range is a completely different region from the generally recognized different peripheral speed rolling, and is rather close to a constant speed. In order to stabilize at such a low peripheral speed ratio, it is necessary to thoroughly eliminate drive unevenness in the motor and the speed reducer, and the vector inverter as the motor and the speed reducer as
A planetary roll reducer or the like is preferable. Further, the fact that the optical axis is tilted and can be controlled in the range of such a low peripheral speed ratio is completely unpredictable by the conventional rolling technology.

Next, the operation of the optically anisotropic element manufactured by the manufacturing method of the present invention will be described by taking a TN type liquid crystal display element as an example with reference to the drawings. FIG. 2 shows the polarization state of light propagating in the liquid crystal cell when a sufficient voltage equal to or higher than the threshold voltage is applied to the liquid crystal cell. Since the transmittance characteristic of light particularly when a voltage is applied greatly contributes to the viewing angle characteristic of the contrast, a case where a voltage is applied will be described as an example. Figure 2
FIG. 3 is a diagram showing a polarization state of light when light is vertically incident on a liquid crystal cell. Polarizing plate A in which natural light L0 has a polarization axis PA
The light transmitted through the polarizing plate PA becomes a linearly polarized light L1 when it is vertically incident on the polarizing plate B.
1 is cut off.

When the arrangement state of the liquid crystal molecules when a sufficient voltage is applied to the TN type liquid crystal cell is schematically shown as a model with one liquid crystal molecule, LC is shown in the schematic diagram. When the molecular long axis of the liquid crystal molecule LC in the liquid crystal cell is parallel to the path of light,
Since there is no difference in the refractive index on the incident surface (in the plane perpendicular to the light path), there is no phase difference between the ordinary and extraordinary rays propagating in the liquid crystal cell, and the linearly polarized light passing through the LC cell passes through the liquid crystal cell. Even when transmitted, it propagates as linearly polarized light. Polarization axis PB of polarizing plate B
Is set to be perpendicular to the polarization axis PA of the polarizing plate A, the linearly polarized light that has passed through the liquid crystal cell cannot pass through the polarizing plate B, resulting in a dark state.

FIG. 3 is a diagram showing the polarization state of light when the light obliquely enters the liquid crystal cell. Natural light of incident light L
When 0 is obliquely incident, the polarized light L1 transmitted through the polarizing plate A becomes almost linearly polarized light. (In the actual case, it becomes elliptically polarized due to the characteristics of the polarizing plate). In this case, the refractive index anisotropy of the liquid crystal causes a difference in the refractive index on the incident surface of the liquid crystal cell, and the light L2 transmitted through the liquid crystal cell is elliptically polarized and not blocked by the polarizing plate B. As described above, in the case of oblique incidence, blocking of light in a dark state becomes insufficient, resulting in a large decrease in contrast, which is not preferable.

The present invention is intended to prevent the deterioration of the contrast due to the oblique incidence and improve the viewing angle characteristics uniformly in any part of the display screen. FIG. 1 shows an example of the configuration according to the present invention. An optical anisotropic element RF having an optical axis tilted from the normal direction of the liquid crystal cell of light and having improved birefringence unevenness is arranged between the polarizing plate B and the liquid crystal cell. The optically anisotropic element RF is a birefringent body that polarizes more as the angle of incidence of light with respect to the optical axis increases. As in the case of FIG. 2, the elliptically polarized light L which is obliquely incident on the liquid crystal display device having such a structure and is transmitted through the liquid crystal cell is transmitted.
No. 2 is a good liquid crystal that does not depend on the viewing angle because the elliptically polarized light is modulated into the original linearly polarized light by the phase delay effect when transmitting through the optically anisotropic element RF, and the same transmittance is obtained even at various oblique incidences. A display device was realized.

It is presumed as follows that the viewing angle of the liquid crystal display device can be greatly improved by the present invention. Most TN-LCDs adopt a normally white mode. With respect to the viewing angle characteristics in this mode, the transmittance of light from the black display portion remarkably increases as the viewing angle increases, resulting in a sharp drop in contrast. The black display is the state when a voltage is applied, but at this time, the TN type liquid crystal cell can be regarded as a positive uniaxial optical anisotropic body in which the optical axis is slightly inclined from the direction normal to the cell surface. Also, in the case of intermediate gradation, the optical axis seems to further tilt from the normal direction of the LC cell.

When the optical axis of the liquid crystal cell is tilted from the direction normal to the surface of the liquid crystal cell, it is expected that the optical anisotropic body having the optical axis in the normal direction will be insufficiently compensated. Further, if the liquid crystal cell can be regarded as a positive uniaxial optical anisotropic body, in order to compensate for it, a negative uniaxial optical anisotropic body having an inclined optical axis is preferable as shown in FIG. For these reasons, it is presumed that the viewing angle characteristics are significantly improved by the optical anisotropic body having the optical axis inclined from the normal direction in the present invention, more preferably the optical anisotropic body having negative uniaxiality. .

Negative uniaxiality in which the optical axis is inclined in the present invention means nα, nβ, nγ when the triaxial refractive index of a sheet having optical anisotropy is set in ascending order.
<Nβ = nγ. Therefore, it has a characteristic that the refractive index in the optical axis direction is the smallest. However, it is not necessary that the values of nβ and nγ be exactly equal, and it is sufficient if they are almost equal. Specifically, if | nβ−nγ | / | nβ−nα | ≦ 0.2, there is no practical problem. Further, as a condition for greatly improving the viewing angle characteristics of the TFT and TN liquid crystal cell, it is preferable that the optical axis, that is, the direction of the refractive index nα is tilted by 10 ° to 50 ° from the normal to the sheet surface. More preferably, the degree is 40 degrees. Further, when the sheet thickness is D, 50 ≦ (nβ−nα) × D
It is preferable to satisfy the condition of ≦ 400 nm, and 70 ≦
It is more preferable to satisfy the condition of (nβ-nα) × D ≦ 250 nm.

[0023]

Embodiments will be described in detail below with reference to embodiments. Example 1 Polycarbonate having a styrene-equivalent weight average molecular weight of 30,000 obtained by condensation of phosgene and bisphenol A was dissolved in methylene dichloride to prepare a 20% solution. This was cast on a steel drum, continuously stripped off, and dried to obtain a film (F-1) having a width of 15 cm and a thickness of 100 μm. The film (FS-1 to FS-5) is obtained by sandwiching the film between rolls having different peripheral speeds shown in FIG.
Was manufactured in a roll shape for 200 m.

In FIG. 6, roll R1 is a feed roll, and R2 and R3 are nip rolls and residual heat rolls having no drive system. R4 and R5 each have a drive system,
Both rolls have a diameter of 150 mm and the peripheral speed difference can be controlled arbitrarily. Further, the structure is such that the pressure between R4 and R5 can be controlled by hydraulic pressure. R6 is a winding roll having a drive system, and the winding speed is controlled by tension control. Each of R2 to R5 has a heater inside the roll, and a temperature sensor is attached to the roll surface. The sensor temperature is fed back to the heater to control the temperature with an accuracy of ± 1 degree by PID control. The material of each roll was hard chrome plated and then polished, and the surface roughness, roundness and cylindricity were measured. Table 1 shows the molding conditions, surface roughness, roundness, and cylindricity of FS-1 to FS-5 in the apparatus of FIG.

[0025]

[Table 1]

Next, FS-1 to FS-5 were transversely uniaxially stretched by a tenter to obtain films RF-1 to RF-5. The stretching conditions of the film are shown in Table 2.

[0027]

[Table 2]

<Measurement of Optical Properties of Optical Compensation Sheet> FS-1 to FS-5 and RF-1 to RF- in Example 1.
From the optically anisotropic element of No. 5, a sample of 15 cm × 60 cm was cut out, 36 points were uniformly selected, and an Ellipsometer AEP-100 manufactured by Shimadzu Corporation was used in a transmission mode.
The value and its oblique incident angle dependence were determined. The refractive index in the width direction and the film thickness were measured with an Abbe refractometer and a micrometer, respectively. From the average value of these measured values, the triaxial direction refractive index of the film and the tilt angle of the main refractive index axis were calculated. FIG. 7 shows the relationship of the triaxial refractive index calculated. Here, the smallest refractive index is n1, the widthwise refractive index is n2, the other main refractive index orthogonal to n1 and n2 is n3, and the angle at which n1 is inclined from the film normal direction is β. The variation of the inclination angle (Cβ) is the value (expressed as a percentage) obtained by dividing the difference between the maximum value and the minimum value of the 36 points by the average value, and the variation of the Re value (CRe) is the value of the measurement value of the adjacent measurement point. Absolute value of difference divided by average value (expressed as a percentage)
Is the maximum value among the values obtained by dividing by the measurement point interval (cm). The results are shown in Tables 3 and 4. Where F-1 is n
1, n2 and n3 were 1.583, respectively, and were optically isotropic.

[0029]

[Table 3]

[0030]

[Table 4]

Example 2 <Evaluation of viewing angle characteristics and color unevenness> When optically anisotropic elements RF-1 to RF-5 shown in FIG. 3 were used in a liquid crystal cell and when no film was arranged. About the viewing angle characteristics and the front contrast of 0 V / 5 V contrast in a 30 Hz rectangular wave, LCD-manufactured by Otsuka Electronics
It was measured by 5000. The position of the contrast 10 is defined as the viewing angle, and the results of the viewing angle characteristics in the up, down, left, and right directions and the color unevenness by visual observation are shown in Table 4. The product of the difference in refractive index between the extraordinary light and the ordinary light of the liquid crystal used in the liquid crystal cell used here and the gap size of the liquid crystal cell is 450 nm, and the twist angle is 90 degrees. The directions of the polarization axis of the polarizing plate of the TN liquid crystal cell, the rubbing axis of the liquid crystal cell, and the optical axis of the optical compensation sheet in this measurement are shown in FIG.

[0032]

[Table 5]

According to the present invention, a viewing angle characteristic of a TN type liquid crystal display device is improved, color unevenness of a display screen due to uneven birefringence is reduced, and a liquid crystal display device of high quality display excellent in visibility is provided. Can be provided. In addition, the present invention
It goes without saying that excellent effects can be obtained even when applied to an active matrix liquid crystal display element using a 3-terminal or 2-terminal element such as T or MIM.

[0034]

[Brief description of drawings]

FIG. 1 is a diagram illustrating an example of a configuration of a liquid crystal display element of the present invention. Specifically, it is a diagram illustrating the arrangement of a negative uniaxial optically anisotropic body whose optical axis is inclined from the normal direction and a liquid crystal cell.

FIG. 2 is a diagram illustrating a configuration of a conventional TN type liquid crystal display element and a diagram for explaining a light transmission state when light is perpendicularly incident on a display surface.

FIG. 3 is a diagram illustrating a configuration of a conventional TN type liquid crystal display element and a light transmission state when light obliquely enters a display surface.

FIG. 4 is a conceptual diagram of optical compensation in the present invention.

FIG. 5 is a diagram showing a mechanism of film deformation due to shearing.

FIG. 6 is a view of a different peripheral speed roll device used in the present invention.

FIG. 7 is a diagram showing triaxial refractive index characteristics.

FIG. 8 is a configuration diagram of a liquid display element used in this example.

[Explanation of symbols]

A, B: Polarizing plates PA, PB: Polarizing axes LCS: TN type liquid crystal cell RF: Optical anisotropic element LO: Incident light L1: Linearly polarized light passing through the polarizing plate A L2: Polarized light passing through the TN type liquid crystal cell ( Mainly elliptically polarized light) LC: A model representation of liquid crystal in a TN liquid crystal cell. θ: incident angle of linearly polarized light. R1 to R6: rolls n1 to n3: main refractive index of film

Claims (7)

[Claims] 1. A film made of a thermoplastic resin having optical transparency and optical anisotropy, the smallest value among three optical elastic axes of the film having optical anisotropy, na, nb and nc. Is nb, the angle between the film normal and nb is β, and the thickness is d, the average β in the film plane is 10 ≦ β ≦ 50 degrees and {(n
a + nb) ÷ 2-nc} × d, the in-plane average value of the Re values is 50 ≦ Re ≦ 400 nm, and the variation coefficient Cβ, C of the variation of β and Re values of the optically anisotropic film.
An optical anisotropic element characterized in that _Re is respectively Cβ ≦ 0.1 and C _Re ≦ 0.1. 2. The optical anisotropic element according to claim 1, which exhibits an optically negative uniaxial property and has an optical axis inclined from the film normal direction. 3. The optical anisotropic element according to claim 1, wherein the thermoplastic resin has a positive intrinsic birefringence value. 4. A step of applying a shearing force to a film made of a thermoplastic resin and having a light-transmitting property between rolls having different peripheral speeds and applying a shearing force in the surface direction of the film, thereby providing a strain deformation. In the method for producing an optical anisotropic element, the surface roughness, circularity, and cylindricity of the roll are 0.15 S or less, 20 μm or less, and 20 μm or less, respectively. 5. A step of applying a shearing force to a film made of a thermoplastic resin and having a light transmissive property between rolls having different peripheral speeds and applying a shearing force in the surface direction of the film, thereby providing the film with strain deformation. In the method of manufacturing an optical anisotropic element, the peripheral speed Vh of a roll having a high speed and the peripheral speed V of a roll having a low speed of the two rolls having different peripheral speed ratios.
A method of manufacturing an optical anisotropic element, wherein the ratio of 1 is 1.001 to 1.03. 6. A step of uniaxially stretching in the width direction of the film is provided before or after the step of sandwiching the film between rolls having different peripheral speeds and applying a shearing force difference to both sides of the film to give strain deformation. 6. The method for manufacturing an optical anisotropic element according to claim 4, wherein 7. The twist angle between the two electrode substrates is approximately 90.
3. A liquid crystal cell formed by sandwiching a TN type liquid crystal of 2 °, two polarizing elements arranged on both sides of the liquid crystal cell, and the optical anisotropic element according to claim 1 or 2 between the liquid crystal cell and the polarizing element. A liquid crystal display device comprising at least one optically anisotropic element manufactured by the method according to claim 3.