JPH04265906A - Phase difference film and liquid crystal display device using this film - Google Patents

Abstract

PURPOSE:To improve the heat resistance of a phase difference film and the visual field angle of the liquid crystal display device formed by using this film. CONSTITUTION:This liquid crystal display device consists of a liquid crystal element formed by sandwiching a nematic liquid crystal twist-oriented between two sheets of electrode substrates facing each other, at least two sheets of double refractive films, and a pair of polarizing plates disposed on both sides of these films. The above-mentioned double refractive films consist of at least one sheet of uniaxially stretched films having a positive intrinsic double refraction value and at least one sheet of uniaxially stretched films having a negative intrinsic double refraction value. In addition, the film having the negative intrinsic double refraction value is formed by uniaxially stretching a styrene copolymer film at the glass transition temp. thereof or at a high temp. which is not higher by 10 deg.C than this temp.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device using a retardation film and twisted nematic liquid crystal or cholesteric liquid crystal. More specifically, the present invention relates to a retardation film used for correcting coloration and increasing the viewing angle of the liquid crystal device, and a liquid crystal display device using the same.

[0002] Liquid crystal display devices have low voltage, low power consumption, and
It has many features such as being able to be directly connected to a C circuit, having a variety of display functions, and being able to achieve high productivity and light weight, and its applications have been expanding. Twisted nematic liquid crystal display devices (hereinafter referred to as STN-LCDs), in which the twist angle of liquid crystal molecules is 160 degrees or more, are currently in practical use as dot matrix type liquid crystal display devices used in OA-related equipment such as word processors and personal computers, and have become mainstream. There is. STN-LCD is a twisted nematic liquid crystal display device (TN
-LCD), it is possible to maintain high contrast even during high multiplex driving.

[0003] STN cannot make the external appearance of a liquid crystal display device white, and exhibits a green to yellow-red color, making it unsuitable for use as a display device. In order to solve this problem, a method has been proposed in which one or more three layers of optically anisotropic material are provided between a pair of polarizing plates. In this case, when the linearly polarized light that has passed through one of the pair of polarizing plates passes through the liquid crystal layer of the liquid crystal element and the optically anisotropic body, elliptically polarized light whose major axes are substantially aligned in the wavelength range from about 400 nm to about 700 nm is obtained. As a result, when the light passes through the other polarizing plate, no specific wavelength range is blocked, and the light becomes white.

[0004] There are also patent applications for a single retardation plate used for color removal in STN-LCDs. For example, JP-A-63-189804 relates to a polycarbonate film uniaxially stretched so that the retardation (product of birefringence value and film thickness) measured by a polarizing microscope is 200 to 350 nm or 475 to 625 nm. be. Furthermore, JP-A-63-167304 relates to a film laminate in which two or more uniaxially stretched birefringent films or sheets are stacked so that their optical principal axes are perpendicular to each other. . In the above invention, when two birefringent films (respective retardation values R1 and R2) are stacked orthogonally, the retardation of the laminate becomes |R1, R2.
By utilizing the fact that a retardation film of | can be obtained, R1
, R2 have large retardation values, |R1, R2 | is 90 to 180 nm, 200 to 35
This is aimed at the effect that it can be adjusted to a range of 0 nm, 475 to 625 nm, etc.

[0005] All of the above inventions are aimed at removing coloring from STN-LCDs, and in this respect they have been greatly improved and a display close to white/black has been obtained. In addition, a method using a polymeric birefringent film (hereinafter referred to as a retardation film) has cost advantages, and demand is rapidly increasing.

[0006]

[Problems to be Solved by the Invention] However, although this retardation film can almost eliminate coloring when the liquid crystal display is viewed from the front, when the display is viewed from an angle, coloring due to slight changes in angle can be removed. STN-LC that the displayed content on the screen disappears
The reality is that the problems with viewing angle characteristics seen in all D types have not been resolved. This problem is also a serious issue for STN-LCDs. The present invention significantly improves the viewing angle characteristics of the above-mentioned STN-LCD, and provides a novel retardation film and a liquid crystal display device using the same. The present invention also provides a retardation film in which the birefringence value changes little due to high heat applied during the process of incorporating the retardation film into a liquid crystal device, and therefore the viewing angle improvement effect is less reduced, and a liquid crystal display device using the same. .

[0007]

[Means for Solving the Problems] The objects of the present invention were achieved by the following method. (1) A film made of a styrene-based copolymer is uniaxially stretched at the glass transition temperature of the copolymer or at a temperature not exceeding 10°C. Retardation film. (2) The styrenic copolymer is a styrene derivative/acrylonitrile copolymer, a styrene derivative/methacrylonitrile copolymer, a styrene derivative/acrylic ester copolymer, or a styrene derivative/methacrylic ester copolymer, and The retardation film having negative intrinsic birefringence according to (1) above, characterized in that the styrene derivative is present in the copolymer in a proportion of 85% to 50% by weight. (3) The retardation film having a negative intrinsic birefringence value as described in (1) above, wherein the glass transition temperature of the film is 105° C. or higher. (4) A liquid crystal element comprising twisted oriented nematic liquid crystal sandwiched between two opposing electrode substrates, at least two birefringent films, and a pair of polarizing plates placed on both sides sandwiching them. at least one uniaxially stretched film, the birefringent film having a positive intrinsic birefringence value;
A liquid crystal display device comprising at least one uniaxially stretched film having a negative intrinsic birefringence value, characterized in that the film having a negative intrinsic birefringence is the styrenic copolymer described in (1) above. LCD display device.

The present invention was achieved by examining whether the viewing angle problem of STN-LCDs could be improved by changing the refractive index of the retardation film in three-dimensional directions. Specifically, it was found that there is a close relationship between the viewing angle dependence of retardation (Re), which is defined as the product of the film's birefringence value (Δn) and thickness (d), and the viewing angle of the LCD. As a result of repeated studies on the viewing angle dependence of dation, we found that a film with an optical axis substantially in the normal direction of the film, specifically a biaxially stretched film with a negative intrinsic birefringence value, and a biaxially stretched film with a positive intrinsic birefringence value. It was discovered that the viewing angle could be significantly improved by inserting a laminated film with a uniaxially stretched film having
No. 92). However, as a result of intensive research, it was discovered that although there was a significant overall improvement in the viewing angle, there were still areas where the viewing angle was insufficient in certain directions. It was discovered that the viewing angle characteristics of a liquid crystal display device can be significantly improved by inserting a laminate of a uniaxially stretched polymer film with a uniaxially stretched polymer having a negative intrinsic birefringence value and a uniaxially stretched polymer film with a negative intrinsic birefringence value between a liquid crystal cell and a polarizing plate ( (Patent Application No. 2-242982).

All film materials currently used as retardation films have positive birefringence values. If the refractive index in the direction of the stretching axis of a longitudinally uniaxially stretched film of a polymer with a positive intrinsic birefringence value is nMD, the refractive index in the direction perpendicular to the stretching axis is nTD, and the refractive index in the direction normal to the film surface is nND, each The magnitude relationship of the refractive index is nMD>nTD≧nND. Therefore, when the incident light enters the film plane perpendicularly, R
e=(nMD-nTD)d. Next, when the incident light passes through a plane perpendicular to the stretching direction, the birefringence value changes from △n=nMD-nTD to △n=nMD-nND as the incident angle changes.
Varies within the range of . Here nMD-nTD≦nMD-n
Since it is ND, Δn does not change or increases due to oblique incidence. On the other hand, since the optical path length increases due to oblique incidence, Re
=Δn·d increases rapidly with oblique incidence. In addition, when the incident light is incident at an angle from the normal direction of the film to the direction of the stretching axis, △n changes from nMD-nND to nND-n
Since there is a sudden change up to TD, the decrease cannot be compensated for even by increasing the optical path length, and with oblique incidence, Re=△n・
d decreases rapidly. In principle, the uniaxially stretched film with the smallest rate of change in retardation has nMD>nTD=
In the case of nND, Re also changes greatly due to an increase in the optical path length due to oblique incidence.

By the way, in the present invention, in the case of a laminate of a uniaxially stretched film formed from a polymer having a positive intrinsic birefringence value and a uniaxially stretched film formed from a polymer having a negative intrinsic birefringence value, the film method is used. The retardation in the linear direction does not add up and disappear, and can be freely controlled depending on the purpose, such as films with extremely small changes in retardation or films with moderate changes in retardation in response to oblique incidence in all directions. It turned out to be effective. In particular, these effects are particularly noticeable when a uniaxially stretched polymer film with a positive intrinsic birefringence value and a uniaxially stretched polymer film with a negative intrinsic birefringence value are laminated such that their stretching axes are orthogonal to each other. It turned out to be the case. A film laminate with a similar effect, that is, a small retardation change in all directions, is obtained by forming a uniaxially stretched film made of a polymer having a positive intrinsic birefringence value and a polymer having a positive intrinsic birefringence value. In an orthogonal laminate of a uniaxially stretched film formed from a polymer having a negative intrinsic birefringence value and a uniaxially stretched film formed from a polymer having a negative intrinsic birefringence value, Both cannot be realized, and can only be realized by the configuration of Japanese Patent Application No. 2-242982.

Now, in a laminate of a uniaxially stretched film formed from a polymer having a positive intrinsic birefringence orthogonal to a uniaxially stretched film formed from a polymer having a negative intrinsic birefringence value, each uniaxially stretched film is By controlling the orientation level of the molecules by stretching, etc., it is possible to freely control the viewing angle dependence of the laminate's retardation, either almost eliminating it or making a moderate change. STN-LC
It was found that when a retardation film was disposed between the polarizing plate and the liquid crystal cell in D, the viewing angle of the STN-LCD could be significantly expanded.

More specifically, the present invention is directed to coloring caused by the birefringence of a liquid crystal cell in a liquid crystal display device using twisted nematic liquid crystal or cholesteric liquid crystal having a twist angle of 90° or more, particularly from 180° to 330°. This invention relates to a liquid crystal display device that eliminates this phenomenon and enables expansion of the viewing angle and high contrast region.Regarding the retardation in the normal direction of the film, the film is made of a polymer with a positive intrinsic birefringence value. The sum of the retardation in uniaxial stretching and the retardation in uniaxial stretching of a film formed from a polymer having a negative intrinsic birefringence value is obtained. however,
If the stretching axes of the uniaxially stretched film of the polymer having positive and negative intrinsic birefringence values coincide, retardation is canceled out, which is not preferable. Therefore, it is preferable that the stretching axes of the film laminate are arranged substantially perpendicular to each other. Specifically, the relative angle is most preferably 70° to 110°.

However, this is not the case when the films having positive and negative intrinsic birefringence values are arranged via a liquid crystal cell. That is, 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 particular, when used as a polarizing plate protective film, it has the advantage of widening the viewing angle and reducing costs. Further, the film in the present invention includes not only a film as generally considered, but also a film-like material coated on a certain base material. Moreover, the uniaxially stretched film 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, and is subject to the present invention. Therefore, it includes horizontal uniaxial stretching using a tenter method, longitudinal uniaxial stretching using the difference in circumferential speed between rolls, and in this case, cases in which natural shrinkage during width direction stretching is performed or limited.

In the present invention, the film having a positive intrinsic birefringence value preferably has a light transmittance of 70% or more and is achromatic, more preferably a light transmittance of 90% or more.
% or more, it is achromatic. Here, the intrinsic birefringence value (△n°)
is the birefringence value when molecules are ideally oriented in one direction, and is approximately expressed by the following formula (1).

[0015]

[Math 1]

There are no restrictions on the polymer used for the film having a positive intrinsic birefringence value, but specific examples include polycarbonate, polyarylate, polyethylene terephthalate, polyether sulfone, polyphenylene sulfide, polyphenylene oxide, polyallyl sulfone, and polyamide. Imides, polyimides, polyolefins, polyolefins, polyvinyl chloride, cellulose, polyester polymers, etc. are preferred, and in particular polycarbonate polymers, polyarylate polymers, polyester polymers, etc., which have a large intrinsic birefringence value and can be used by solution film formation. Polymers that can easily form a planar, homogeneous film 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 a negative intrinsic birefringence value is preferably an achromatic film with a light transmittance of 70% or more, and more preferably an achromatic film with a light transmittance of 90% or more. be. By the way, even if the absolute value of the intrinsic birefringence value is small, it can be fully utilized by increasing the thickness or increasing the stretching ratio, but in order to avoid these restrictions, the intrinsic birefringence value should preferably be an absolute value. The value is 0.02 or more, more preferably 0.04 or more. In addition, in order to prevent molecules once oriented by stretching from relaxing their orientation due to temperature rise during the LCD manufacturing process or display, the glass transition temperature (Tg) of the styrenic copolymer of the present invention is set to 100.
℃ or higher, preferably 105℃ or higher, preferably 110℃ or higher
℃ or higher.

The thickness of the uniaxially stretched film having a birefringence value is not particularly limited, but is preferably in the range of 10 μm to 1 mm. The molecular weight of the copolymer film is not particularly limited unless it is particularly small, but the weight average molecular weight is preferably from 200,000 to 900,000, particularly preferably from 350,000 to 700,000. Although any commonly used polymerization method can be used, copolymers polymerized by emulsion polymerization are particularly preferred because their molecular weight can be easily increased.

Here, the styrene derivatives include styrene and α
-Methylstyrene, o-methylstyrene, p-methylstyrene, p-chlorostyrene, p-nitrostyrene, p
- Styrene derivatives such as aminostyrene, p-carboxystyrene, p-phenylstyrene, and 2,5-dichlorostyrene. There are no particular restrictions on the copolymer component that can be used here, but specific examples include acrylic esters such as methyl acrylate, ethyl acrylate, and propyl acrylate, methyl methacrylate, ethyl methacrylate, and methacrylate. Methacrylic acid esters such as butyl acid, acrylic acid, methacrylic acid, acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, butadiene, isoprene, maleic anhydride, vinyl acetate, ethylene, propylene, etc., and these ingredients may be used singly or in combination. It may consist of ingredients. The weight ratio of styrene derivative and copolymer component is 85/15 to 5.
Preferably 0/50, from 75/25 to 60/
40 is particularly preferred. If this ratio is high, the heat resistance will be low, and if this ratio is low, the development of birefringence will be low, both of which are undesirable. In order to increase heat resistance, it is preferable to use acrylonitrile, methacrylonitrile, acrylic esters, or methacrylic esters in the above weight ratio as a copolymer component.

[0020] If a large amount of unreacted monomers or low-molecular oligomers remain during the polymerization process, the heat resistance of the film will be impaired and birefringence will tend to decrease due to heat, which is undesirable. The amount of residual monomer should be less than 1% by weight, preferably less than 0.2% by weight. The styrenic copolymer of the present invention may be used in combination of two or more copolymer components having different compositions. It may also be used in combination with polymers other than styrenic copolymers. At this time,
When the composition ratio of the styrene component to the whole becomes less than 50% as a result of mixing, the expression of birefringence properties decreases,
Care must be taken as this may not be practical. Further explanation will be given below according to examples.

[0021]

[Example] Example 1 A copolymer of styrene/acrylonitrile (molecular weight approximately 400,000) with a weight ratio of 65/35 synthesized by an emulsion polymerization method was dissolved in methylene chloride, and then cast onto a glass plate to produce approximately A transparent film of 120 μm was obtained. The glass transition temperature of this copolymer was 112°C. This film was stretched by approximately 40% by varying the stretching temperature between 100°C and 120°C.
After uniaxial stretching, the birefringence value was measured. Further, this stretched film was heat-treated in an oven at 90° C. for 4 hours, and then the birefringence was measured once again to determine the rate of decrease in birefringence after the heat treatment. The measurement results are shown in Table 1. This result shows that the higher the stretching temperature, the smaller the birefringence, and the film is useless as a retardation film. On the other hand, the stretching temperature is 1
It can be seen that when the temperature is lower than 05°C, the rate of decrease in birefringence due to heat treatment increases, and the suitability of the film for incorporation into a liquid crystal display device decreases. Furthermore, if the stretching temperature is lower than the glass transition temperature, uneven stretching tends to occur. Therefore, the practical stretching temperature is 110°C, which is around the glass transition temperature of this copolymer.
and 120°C.

[0022]

[Table 1]

Example 2 A copolymer of styrene/methyl methacrylate (molecular weight approximately 400,000) with a weight ratio of 65/35 synthesized by emulsion polymerization was dissolved in methylene chloride and then cast onto a glass plate. A transparent film of about 130 μm was obtained. The glass transition temperature of this copolymer was 108°C. This film was stretched at a stretching temperature of 100°C to 120°C, and
After 0% uniaxial stretching, the birefringence value was measured. Further, this stretched film was heat-treated in an oven at 90° C. for 4 hours, and then the birefringence was measured once again to determine the rate of decrease in birefringence after the heat treatment. The measurement results are shown in Table 2. From this result, it can be seen that the higher the stretching temperature, the smaller the birefringence, and the film is useless as a retardation film. On the other hand, it can be seen that when the stretching temperature is lower than 105° C., the rate of decrease in birefringence due to heat treatment increases, and the suitability of the film for incorporation into a liquid crystal display device decreases. Furthermore, if the stretching temperature is lower than the glass transition temperature, uneven stretching tends to occur. Therefore, the practical stretching temperature is between 105°C and 115°C, which is around the glass transition temperature of the present copolymer.

[0024]

[Table 2]

Example 3 Polycarbonate having a weight average molecular weight of 100,000 was dissolved in methylene chloride, cast on a stainless steel belt, continuously peeled off and dried to obtain a polycarbonate film. The film was longitudinally uniaxially stretched between rolls with different circumferential bundles at a temperature of 170°C, and the birefringence value was +2.9 x 10-3.
A polycarbonate uniaxially stretched film having a thickness of 105 μm was obtained. This film has a birefringence value of -2.
A laminate film was produced by stacking a 3×10 −3 styrene/acrylonitrile copolymer uniaxially stretched film with the stretching axis directions perpendicular to each other. Next, personal word processor PWP-LQX (manufacturing number 02) manufactured by Epson Corporation.
G0000515) was removed, and the film laminate produced above was placed between the liquid crystal cell and the polarizing plate so that the stretching axis of the polycarbonate film was oriented up and down and the polycarbonate film was on the liquid crystal cell side. When the display characteristics were examined, a clear black and white display was obtained, and a wide viewing angle of more than 110 degrees in total vertically and more than 100 degrees in left and right directions was obtained.

Example 4 Polycarbonate having a weight average molecular weight of 100,000 was dissolved in methylene chloride, cast on a stainless steel belt, continuously peeled off and dried to obtain a polycarbonate film. The film was longitudinally uniaxially stretched between rolls with different circumferential bundles at a temperature of 170°C, and the birefringence value was +2.9 × 10-3,
A polycarbonate uniaxially stretched film having a thickness of 105 μm was obtained. This film has a birefringence value of -2.1 as shown in Table 2.
A laminate film was produced by stacking a x10-3 styrene/methacrylic acid ester copolymer uniaxially stretched film with the stretching axis directions perpendicular to each other. Next, Epson Corporation personal word processor PWP-LQX (serial number 0
2G0000515) was removed, and the film laminate produced above was placed between the liquid crystal cell and the polarizing plate so that the polycarbonate film stretching axis of the film laminate was oriented up and down and the polycarbonate film was on the liquid crystal cell side. When the display characteristics were examined, a clear black and white display was obtained, and a wide viewing angle of more than 110 degrees in total vertically and more than 100 degrees in left and right directions was obtained.

Comparative Example 1 When examining the display characteristics of the personal word processor PWP-LQX (purchased stage) used in Examples 3 and 4, it was found that although a black and white display was obtained, the viewing angle was very narrow, with a total of 50° in the upper and lower directions. , the left and right sides totaled 45 degrees.

[0028]

Effects of the Invention: The uniaxially stretched styrenic copolymer film of the present invention having a negative intrinsic birefringence value shows little change in birefringence value even under high heat, and has a positive intrinsic birefringence value compared to the film of the present invention. By stacking the films with the film with the stretching axes perpendicular or almost perpendicular to each other and placing them between the liquid crystal cell and the polarizing plate, a high-quality black and white display with a wide viewing angle can be obtained. Its optical properties remain stable even when subjected to

Claims (4)

[Claims] Claim 1: A film made of a styrene copolymer is prepared at a temperature at or above the glass transition temperature of the copolymer.
A retardation film having negative intrinsic birefringence characterized by being uniaxially stretched under high temperature conditions not exceeding ℃. 2. The styrene copolymer is a styrene derivative/acrylonitrile copolymer, a styrene derivative/methacrylonitrile copolymer, a styrene derivative/acrylic ester copolymer, or a styrene derivative/methacrylic ester copolymer. 2. The retardation film having negative intrinsic birefringence according to claim 1, characterized in that the styrene derivative is present in the copolymer in an amount of 85% to 50% by weight. 3. The copolymer has a glass transition temperature of 105
2. The retardation film having a negative intrinsic birefringence value according to claim 1, wherein the retardation film has a negative intrinsic birefringence value of at least .degree. 4. A liquid crystal element comprising twisted oriented nematic liquid crystal sandwiched between two opposing electrode substrates, at least two birefringent films, and a pair of polarized lights disposed on both sides sandwiching them. plate, wherein the birefringent film comprises at least one uniaxially stretched film having a positive intrinsic birefringence value and at least one uniaxially stretched film having a negative intrinsic birefringence value. 2. A liquid crystal display device, wherein the film having negative intrinsic birefringence is the styrenic copolymer according to claim 1.