JPH07333437A - Production of optically anisotropic element and liquid crystal display element formed by using the same - Google Patents

Abstract

PURPOSE:To produce optically anisotropic elements which have an excellent contrast and visual field angle characteristic and have less unequal colors and unequal contrasts continuously over a long period of time by specifying the surface roughness of a film before passing the film through stages. CONSTITUTION:This process for production of the optically anisotropic elements has a stage for applying distortion deformation to the film consisting of a thermoplastic resin and having light transmissivity by holding the film between two pieces of rolls varying in circumferential speeds and applying shearing force in both directions of the film and a transverse uniaxial stretching stage. The surface roughness (Ra) of the film before passing the film to these stages is specified to a range of 1nm<=Ra<=20nm. In such a case, shaped or unshaped particles are kneaded into the resin or are applied together with a binder on the film in order to obtain the surface roughness (Ra) before passing the films through the stages. The optical characteristics uniform over the entire surface free from unequalness and defects are obtd. continuously over a long period of time by specifying the roughness of the raw film in such a manner.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention, when used in a liquid crystal display device, provides a method for producing an optically anisotropic element capable of uniformly improving the viewing angle characteristics of display contrast and display color over the entire display screen, and a liquid crystal display device using the same. Regarding

[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 twisted angle of 160 ° or more in the alignment of liquid crystal molecules and has steep electro-optical characteristics. Therefore, a simple matrix without active elements (thin film transistor or diode). A large-capacity display can be obtained by time-division driving even with a striped electrode structure. But,
The response speed is slow (hundreds of milliseconds), and gradation display is difficult, and a liquid crystal display element (TFT-
It does not exceed the display performance of LCD and MIM-LCD.

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. The phase difference appears when tilted, and the phase difference that appears in the liquid crystal cell should be 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, the LCD has a negative uniaxial property and the optical axis is neither perpendicular nor parallel to the film surface, and by using an optical anisotropic element inclined by 10 to 40 degrees from the film normal, A patent application was filed (Japanese Patent Application No. 4-308377) to find out that the viewing angle can be greatly expanded. In addition, in order to continuously manufacture a film having such optical characteristics, the film has a negative uniaxial property by continuously providing a difference in shearing force on both sides of the film, and also has an optical anisotropy with an inclined optical axis. He found that the device can be manufactured and filed a patent application (Japanese Patent Application No. 4-324116).

[0007]

However, when this method is used for continuous production for a long period of time, some birefringence unevenness occurs, and when it is used in a liquid crystal display device, there is a problem that the display image has uneven color and uneven contrast. It was Therefore, an object of the present invention is to propose a method for continuously producing an optical anisotropic element having excellent contrast and viewing angle characteristics and having little color unevenness and contrast unevenness for a long period of time.

[0008]

Means for Solving the Problems The above-mentioned problems are as follows. (1) A film made of a thermoplastic resin and having a light-transmitting property is sandwiched between two rolls having different peripheral speeds, and a shearing force is applied in the surface direction of the film. In the method for producing an optically anisotropic element having a step of giving strain deformation to the film by adding the film and a lateral uniaxial stretching step, the surface roughness (Ra) of the film before passing through the step is represented by the formula (1). A method for manufacturing an optical anisotropic element characterized by being in a range. Formula (1) 1 nm ≤ Ra ≤ 20 nm (2) Characteristic that regular or amorphous particles are kneaded into a resin or coated with a binder in order to obtain the surface roughness (Ra) of the film before the step. The method for producing an optical anisotropic element according to (1) above. (3) The method for producing an optically anisotropic element as described in (1) or (2) above, wherein the optical axis of the optically anisotropic element is inclined from the film normal direction and has negative uniaxiality. (4) TN with a twist angle of about 90 ° between two electrode substrates
A liquid crystal cell sandwiching a type liquid crystal, two polarizing elements disposed on both sides of the liquid crystal cell, and between the liquid crystal cell and the polarizing element,
This is achieved by a liquid crystal display device characterized in that at least one optically anisotropic element manufactured by the method described in (1) to (5) above is arranged.

Next, the present invention will be described in detail. The film having a light-transmitting property made of a thermoplastic resin in the present invention means that the film or sheet made of a thermoplastic resin has a light transmittance of 70% or more, preferably 85% or more and a haze of 2% or less, preferably 1. Anything less than 5% is targeted. Specifically, polycarbonate, polyarylate, polysulfone, polyethylene terephthalate,
Polyethylene naphthalate, polyether sulfone, polyphenylene sulfide, polyphenylene oxide, polyallyl sulfone, polyvinyl alcohol, polyamide, polyimide, polyolefin, polyvinyl chloride, cellulosic polymer, polyacrylonitrile, polystyrene, also binary system, Various ternary copolymers, graft copolymers, blends and the like are preferably used. It is preferable to knead regular or amorphous particles into these thermoplastic resins or to apply them together with a binder in order to impart surface roughness. At this time, the particles to be used are not particularly limited in composition and may be a mixture of two or more kinds, but in order to maintain the roughness of the film surface even when sandwiched between two high temperature rolls, Inorganic substances are preferred. For example, barium sulfate, manganese colloid, titanium dioxide,
There are fine powders of inorganic substances such as strontium barium sulfate and silicon dioxide, and further examples thereof include synthetic silica obtained by a wet method or gelation of silicic acid.

The film used in the present invention is basically preferably free of plane orientation. For example, when the main in-plane refractive indices are nx and ny and the refractive index in the thickness direction is nz, {(nx + ny) / 2−nz} × d is 100 nm or less, preferably 30 nm or less. If this value is large, it is difficult to control the retardation value of the optically anisotropic element obtained by strain deformation due to shear in a small region. However, if the film before shearing is optically isotropic, the optical characteristics after shear strain will be slightly deviated from the negative uniaxiality, so to adjust it,
It is preferable to add a step of 0.5% to 20% width uniaxial stretching or unbalanced biaxial stretching before or after shearing. 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.

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 100 kg /
cm or more. Further, as a method of applying a shearing force to the film surface, (Tg-50) degrees or more of the film and (T
Under a temperature condition of g + 10) degrees or less, 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. The fact that the main refractive index can be inclined by the shearing force is considered to be due to the strain as shown in Fig. 4 being applied inside the film. In FIG. 4, the assumed cube A inside the film is deformed by the difference in the peripheral speeds of the two rolls, deformed like a solid B, and is further sent out as a solid C. At this time, it is considered that the molecules inside the cube are also tilted.

By the way, when the film is continuously manufactured under the above conditions, there is a problem that birefringence unevenness gradually occurs and wrinkles gradually increase. It was found that when the film comes into contact with the roll, a minute amount is generated, which gradually accumulates and expands. Further, as a result of further investigation of the phenomenon, it was found that the roughness of the roll surface, the surface roughness of the original film, the temperature difference between the original film and the roll, and the variations in the film tension are deeply involved. In particular,
Due to the temperature difference between the original film and the roll, thermal expansion occurs when the film contacts the roll, and wrinkles occur due to variations in the film tension in the width direction, but there is sufficient slippage between the film and the roll. It was found that the wrinkle disappeared.

However, even if the temperature difference between the film and the roll and the variation in the tension are very slight, they cause delicate wrinkles and are difficult to control. Therefore, it is preferable to improve the slip property of the film roll. However, in order to make the surface of the roll or the surface of the raw film rough to reduce the friction coefficient of the contact surface, if it is made too rough, the haze after processing becomes high and it becomes a visible defect, which is not preferable.
In particular, roughening the roll surface is not a preferable means because it significantly increases the haze. Based on the above, as a result of thorough examination, the roughness of the original film is expressed by the formula (1) 1 nm ≦ R.
It has been found that by setting the range of a ≦ 20 nm, uniform optical characteristics can be continuously obtained for a long time on the entire surface without unevenness and defects, and the present invention has been completed.

Next, the operation of the optical 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. 1 and 2 show polarization states of light propagating in a liquid crystal cell when a sufficient voltage equal to or higher than a 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. FIG. 2 is a diagram showing a polarization state of light when light is vertically incident on the liquid crystal cell. When the natural light L0 is vertically incident on the polarizing plate A having the polarization axis PA, the light transmitted through the polarizing plate PA becomes the linearly polarized light L1, so that the polarizing plate B almost completely blocks L1.

When a sufficient voltage is applied to the TN type liquid crystal cell, the alignment state of the liquid crystal molecules is schematically shown as a model with one liquid crystal molecule, which is represented by LC 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 is 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 intends 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 is significantly increased as the viewing angle is increased, resulting in a sharp decrease 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 of the normal line 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. If the liquid crystal cell can be regarded as a positive uniaxial optical anisotropic body with an inclined optical axis, in order to compensate for it, a negative uniaxial optical anisotropic body with the optical axis inclined in a direction orthogonal to each other is used. The body is preferred. 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. .

In the present invention, the negative uniaxial property in which the optical axis is tilted has a relationship of N3 <N2 = N1 among the refractive indexes N1, N2 and N3 of the sheet having optical anisotropy. It is a thing. 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 N1 and N2 are exactly equal, and it is sufficient if they are almost equal. Specifically, if | N2−N1 | / | N2−N3 | ≦ 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 N3 is inclined by 10 to 40 degrees from the normal line direction of the sheet surface. More preferably, the degree is from 35 degrees. Also, {(N2 + N1) / 2-N3} × d is 100
It is preferably ˜400 nm.

[0021]

Embodiments will be described in detail below with reference to embodiments. Example 1 Polycarbonate film manufactured by Mitsubishi Gas Chemical Co., Inc., IU
PILON (FB2000B), 50 μm (F-1) is uniaxially stretched by a tenter and the film (F-
2) was obtained. The stretching conditions are as follows. Stretching temperature: 175 degrees Stretching ratio: 17% Film feeding speed: 2 m / min

Next, (F-2) was slit into a width of 20 cm and sandwiched between rolls having different peripheral speeds shown in FIG. 5 to produce a film (F-3) in a roll shape of 200 m. In FIG. 5, roll R1 is a feed roll, and R2 and R3 are nip rolls and residual heat rolls having no drive system. R4 and R5 are rolls each having a drive system, and can control the peripheral speed difference 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. R2 to R
In No. 5, a heater was incorporated inside the roll, and a temperature sensor was attached to the roll surface. The sensor temperature was fed back to the heater to control the temperature with an accuracy of ± 1 degree by PID control. Also, the material of each roll is hard chrome plated and then polished to obtain surface roughness, roundness,
The cylindricity was 0.1S, 15 μm, and 15 μm, respectively.

The molding conditions of (F-3) in the apparatus of FIG. 5 are as follows. Peripheral speed of R4 and R5: 2.01 m / min, 2.00
m / min Surface temperature of R4 and R5: 130 degrees Force applied to the film sandwiched between R4 and R5: 3000K
Roll diameter of g R4 and R5: 150mm

Comparative Example 1 Polycarbonate film manufactured by Mitsubishi Gas Chemical Co., Inc., IU
PILON (FB2000B: the amount of silica kneaded in is 1.5 times that of Example 1), 50 μm
In the same manner as in Example 1, (F-4) was subjected to tenter transverse uniaxial stretching and then sandwiched between rolls having different peripheral speeds to prepare a film (F-5) in a roll shape of 200 m.

Comparative Example 2 A polycarbonate having a styrene-equivalent weight average molecular weight of 30,000 obtained by condensation of phosgene and bisphenol A was dissolved in methylene chloride to prepare a 20% solution. This was cast on a stainless drum, continuously stripped, and dried to obtain a film (F-6). Then, in the same manner as in Example 1, the tenter was transversely uniaxially stretched, and then sandwiched between rolls having different peripheral speeds to prepare a film (F-7) having a roll shape of 200 m.

<Evaluation of Film Used for Manufacturing Optically Anisotropic Element> F-1, F-2, F- in Examples and Comparative Examples
A sample of 15 cm × 30 cm was cut out from the film of No. 4 and F-6, 36 points were uniformly selected, and an Ellipsometer AEP-100 manufactured by Shimadzu Corporation was used in a transmission mode.
The Re 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 refractive index of the film in the three axial directions (N1 to N3),
The retardation (Rth) of the plane orientation was calculated. FIG. 7 shows the relationship of the triaxial refractive index calculated. Here, the largest refractive index is N1, the smallest refractive index is N3, and the N1 is
The other main refractive index orthogonal to N3 is N2. Also,
Surface roughness and haze are TOPO-3 manufactured by WYKO.
D, measured with a Nippon Denshoku Industries haze meter NDH.
The results are shown in Table 1.

<Evaluation of Optically Anisotropic Elements> From the optically anisotropic elements F-3, F-5, and F-7 in Examples and Comparative Examples,
A 15 cm × 30 cm sample was cut out, and the film triaxial refractive index (N1 to N3) and plane orientation retardation (Rth) were determined as described above. Further, the inclination angle (β) from N3 and the film normal direction was calculated. The inclination angle variation (Δβ) is the value (expressed as a percentage) obtained by dividing the difference between the maximum and minimum values at the 36 points by the average value, and the Re value variation (ΔRth) is the measurement at the adjacent measurement point. The maximum value among the absolute values of the difference between the values divided by the average value. The results are shown in Table 2. 30 Hz rectangular wave when the optical anisotropic elements of Examples and Comparative Examples F-3, F-5, and F-7 as the optically anisotropic element shown in FIG. 1 were used in the liquid crystal cell and the film was not arranged. The viewing angle characteristics of 0V / 5V contrast in the above were measured by LCD-5000 manufactured by Otsuka Electronics. 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 visual contrast and color unevenness are shown in Table 2. The product of the difference in refractive index between the extraordinary ray and the ordinary ray 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.
In addition, the polarization axis of the polarizing plate of the TN liquid crystal cell in this measurement,
The directions of the rubbing axis of the liquid crystal cell and the optical axis of the optical compensation sheet are shown in FIG.

[0028]

[Table 1]

[0029]

[Table 2]

According to the present invention, the viewing angle characteristics and contrast of the TN type liquid crystal display device are improved, and the color unevenness of the display screen due to the birefringence unevenness is reduced, and the liquid crystal display of high quality display excellent in visibility is obtained. An element can be provided. Needless to say, even if the present invention is applied to an active matrix liquid crystal display element using a three-terminal or two-terminal element such as TFT or MIM, excellent effects can be obtained.

[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 diagram showing a mechanism of film deformation due to shearing.

FIG. 5 is a view of a different peripheral speed roll device used in the present invention.

FIG. 6 is a configuration diagram of a liquid display element used in this example.

[Explanation of symbols]

A, B: Polarizing plates PA, PB: Polarizing axes F: Film LCS: TN type liquid crystal cell RF: Optical anisotropic element LO: Incident light L1: Linearly polarized light passing through the polarizing plate L2: Passing TN type liquid crystal cell Polarized light (mainly elliptically polarized light) LC: A model representation of the liquid crystal in the TN liquid crystal cell. R1 to R6: Roll

Claims (4)

[Claims] 1. A film made of a thermoplastic resin and having a light-transmitting property is sandwiched between two rolls having different peripheral speeds, and a shearing force is applied in the surface direction of the film to give strain deformation to the film. In the method for producing an optically anisotropic element having a step and a lateral uniaxial stretching step, the surface roughness (Ra) of the film before passing through the step is in the range of formula (1). Device manufacturing method. Formula (1) 1 nm ≦ Ra ≦ 20 nm 2. The resin according to claim 1, wherein the particles of a regular shape or an irregular shape are kneaded with a resin or coated with a binder to give a surface roughness (Ra) before the film is processed. Manufacturing method of optically anisotropic element. 3. The method for producing an optical anisotropic element according to claim 1, wherein an optical axis of the optical anisotropic element is inclined from the film normal direction and has negative uniaxiality. 4. The twist angle between the two electrode substrates is approximately 90.
4. A liquid crystal cell sandwiching a TN type liquid crystal of 2 °, two polarizing elements disposed on both sides of the liquid crystal cell, and the liquid crystal cell and the polarizing element are manufactured by the method according to claim 1. A liquid crystal display element characterized by arranging at least one optically anisotropic element.