JPH0667023A - Manufacture of birefringent film, and liquid crystal display device - Google Patents

Abstract

PURPOSE:To manufacture a birefringent film, which improves coloration and visual angle characteristics of STN-LCD, at a low cost with high productivity by a simple process. CONSTITUTION:A plastic film formed of thermoplastic resin is shrunk in a transverse direction by passing a roller which imparts compressive strain in the transverse direction to a carrying direction and then the refractive index in the thickness direction is made large. The manufacture of the birefringent film uses a roller consisting of, for example, a shaft 1, rubber 2, a bearing 3 and an angle adjuster 4 as the roller.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a birefringent film, and a liquid crystal display device including at least the birefringent film, a polarizing plate and a liquid crystal cell.

[0002]

2. Description of the Related Art Liquid crystal display devices have many features such as low voltage and low power consumption, which can be directly connected to an IC circuit, various display functions, and weight reduction. It is widely used as a display device for word processors and personal computers. Among them, a twisted nematic liquid crystal display device (hereinafter STN-LCD) having a twist angle of liquid crystal molecules of 160 ° or more has a larger capacity than a conventional twisted nematic liquid crystal display device (TN-LCD) having a twist angle of 90 °. Since it is capable of high speed response and is excellent in high-speed response, it is currently the mainstream of liquid crystal display devices.

However, the STN-LCD has a problem that the displayed image is colored in blue or yellow (blue mode or yellow mode). For this reason, the contrast and visibility are low in black and white display, and colorization is extremely difficult. there were. So to compensate for this coloring,
A two-layer liquid crystal black-and-white or color display using a reverse twisted STN liquid crystal cell has been proposed. However, since a plurality of liquid crystal cells are used, the weight and volume of the display device increase, or the cost increases. There is another problem that the viewing angle characteristic is deteriorated such that the contrast sharply decreases with a slight change in the viewing angle or the background color changes.

In order to solve this problem, Japanese Patent Laid-Open No. 63-
No. 167303, No. 63-167304, No. 63-1
89804, 63-261302, 63-14.
9624, JP-A-1-201607 and 1-201.
608, 1-105217, JP-A-2-2853.
No. 03, No. 2-59702, No. 2-24406, No. 2-146002, No. 2-257103, JP-A-3.
-23404, 3-126012, 3-181.
As described in the specifications of No. 905 and No. 3-194503, there has been proposed a method of using a retardation plate instead of the reverse twist STN liquid crystal.

According to these methods, the coloring of the STN-LCD is significantly improved, the weight and volume of the display device itself is significantly reduced, and the cost is reduced, but the STN-LCD is reduced.
The viewing angle characteristics of were hardly improved.

Therefore, in order to improve this viewing angle characteristic,
JP-A-2-285303 discloses a method in which a birefringent film having a refractive index in the thickness direction larger than that in the direction perpendicular to the optical axis of the birefringence is prepared by electric field orientation and used as a retardation plate. was suggested. According to this method, the change in contrast depending on the viewing angle is reduced and the viewing angle characteristics are improved, but the effect is still small, and it is necessary to apply a high voltage to the molten polycarbonate for a long time, and the manufacturing process is complicated. Therefore, it was difficult to increase the productivity and reduce the cost. Further, Japanese Patent Laid-Open No. 2-160204 proposes a method in which a rod-shaped polycarbonate obtained by extrusion molding is cut into a plate shape and polished and used as a retardation plate. It was extremely difficult to produce a board at low cost. In addition, EP-0482620
A method in which a heat-shrinkable film is attached to the surface of a polycarbonate film, stretched, and then the polycarbonate film obtained by peeling off the heat-shrinkable film is used as a retardation plate for A2 is proposed. In addition to the film, a heat-shrinkable film and an adhesive are required, which makes it difficult to produce at low cost.

Further, JP-A-2-256023, JP-A-3-141303, JP-A-3-14122, and JP-A-3-24.
In Japanese Patent No. 502, there has been proposed a method in which one film each having positive and negative intrinsic birefringence or a laminated film is used as a retardation plate. According to this method, the birefringence of the two films can be adjusted according to the characteristics of the liquid crystal cell, so that the viewing angle characteristics can be improved more precisely, but two or more birefringent films prepared separately are used. It is necessary to use it, and the cost increases accordingly.

[0008]

The object of the present invention is to
An object of the present invention is to propose a manufacturing method for manufacturing a birefringent film capable of greatly improving the coloring and viewing angle characteristics of N-LCD by a simple process at low cost and high productivity. Another object of the present invention is to provide a liquid crystal display device in which a displayed image is less colored, has excellent contrast and visibility, and has good viewing angle characteristics.

[0009]

SUMMARY OF THE INVENTION (1) A birefringent film characterized in that (1) a plastic film made of a thermoplastic resin is passed through a roller which gives a compressive strain in the width direction with respect to the conveying direction. Manufacturing method (2)
(1) A method for producing a birefringent film, comprising a step of uniaxially stretching the plastic film, (3) a pair of polarizing plates arranged on both sides of a liquid crystal cell. And a birefringent film obtained by the manufacturing method according to (1) or (2) is provided on at least one of the liquid crystal cell and the polarizing plate.

The thermoplastic resin of the present invention is not particularly limited, but the molecules oriented in the birefringent film of the present invention are LC
In order to prevent orientation relaxation due to the manufacturing process of D or heating, the glass transition point of the polymer used in the birefringent film of the present invention is preferably 90 ° C or higher, more preferably 11 ° C.
There is no particular limitation as long as it is 0 ° C. or higher and the molecular weight is not particularly small, but the weight average molecular weight is preferably in the range of 10,000 to 1,000,000, particularly preferably 30,000 to 7
It is in the range of 0,000. As a method for polymerizing this polymer, a commonly used method can be applied.

The thermoplastic resin used in the birefringent film of the present invention is preferably one having a birefringence-expressing property,
Its intrinsic birefringence value may be positive or negative. As specific examples of the polymer having a positive intrinsic birefringence value, polycarbonate, polyarylate, polyethylene terephthalate, polyamide, polyimide, polysulfone, polyether sulfone, polyether ketone, polyethylene naphthalate, etc. are preferable, and a polycarbonate polymer is particularly preferable. , Polyarylate-based polymers, polyester-based polymers, polysulfones, and the like, and polymers having a large intrinsic birefringence value and capable of easily forming a planar homogeneous film by solution casting are preferable. Further, the above-mentioned polymer may be not only a homopolymer but also a copolymer, a derivative thereof, a blended product and the like.

Specific examples of polymers having a negative intrinsic birefringence value include styrene polymers, acrylic acid ester polymers, methacrylic acid ester polymers, acrylonitrile polymers and methacrylonitrile polymers. Preferably, the polystyrene-based polymer has three aspects:
It is most preferable because the absolute value of the intrinsic birefringence value is large, the transparency is excellent, there is no coloring, and solution film formation is possible.

Here, the styrene-based polymer means a homopolymer of styrene and a styrene derivative, a copolymer of styrene and a styrene derivative, a polymer having at least an unsaturated double bond in a main chain or a side chain, and at least styrene or styrene. It includes a graft polymer obtained by addition-polymerizing a monomer containing a derivative, and a blended product. The styrene derivative is, for example, α-methylstyrene, o-methylstyrene, p-methylstyrene, p-chlorostyrene, p-nitrostyrene, p-aminostyrene, p-carboxystyrene, p-phenylstyrene, 2,5-.
Examples thereof include dichlorostyrene, but are not limited to the above.

Copolymers and blends of styrene and styrene derivatives (hereinafter abbreviated as ST) are not particularly limited as long as they have an appropriate film forming property with ST.
Even if it has a phase-separated structure, it is an object of the present invention as long as the transparency and the like are not impaired. For example, as the copolymer, 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 / N-phenylmaleimide, ST /
Itaconic acid, ST / vinyl carbazole, ST / n-phenyl acrylamide, ST / vinyl pyridine, ST /
Vinyl naphthalene, α-methylstyrene / acrylonitrile, α-methylstyrene / methacrylonitrile, ST
/ Vinyl acetate copolymers and styrene / styrene derivative copolymers. Of course, in addition to the binary copolymers listed above, use of ternary or higher copomary such as ST / α-methylstyrene / acrylonitrile, ST / N-phenylmaleimide / acrylonitrile, ST / α-methylstyrene / methylmethacrylate, etc. Can be done.

A graft polymer obtained by addition-polymerizing a monomer having at least styrene or a styrene derivative to a polymer having at least an unsaturated double bond in the main chain or side chain is a rubber such as SBR, SBS or BR. A graft polymer obtained by addition-polymerizing a monomer containing styrene or a styrene derivative can be mentioned.
As a specific example of this graft polymer, a graft polymer obtained by grafting styrene-acrylonitrile on SBR,
Examples thereof include a graft polymer obtained by grafting styrene-acrylonitrile on BR and a graft polymer obtained by grafting styrene on SBS.

The blend is not only a blend of the above-mentioned styrene homopolymer, styrene derivative homopolymer, styrene and styrene derivative copolymer and graft polymer, but also a polymer comprising styrene and a styrene derivative (hereinafter abbreviated as PST), Blends with polymers without PST can also be used. These blends are, for example, PST / butyl cellulose, PST /
There is coumarone resin. As the polymer having negative intrinsic birefringence in the present invention, a graft polymer obtained by addition-polymerizing a monomer containing at least styrene or a styrene derivative to the main chain or side chain is preferable from the viewpoint of brittleness.

Using these thermoplastic resins, a plastic film is first produced by a melt extrusion method, a melt casting method, a calender method or the like. When this plastic film is further uniaxially stretched, the stretched direction (X direction)
In the width direction (Y direction) and the thickness direction (Z direction) perpendicular to the polymer main chain, the orientation of the polymer chain in the Y direction and the Z direction decreases with the orientation of the polymer in the X direction. As a result, a polymer having a positive intrinsic birefringence gives a film whose principal refractive index (nx) in the stretching direction is larger than principal refractive indexes ny and nz in the direction perpendicular thereto, and in this case, the principal refractive index in the thickness direction is generally obtained. The refractive index nz is smaller than both the main refractive indices nx and ny parallel to the surface. In the case of a polymer having a negative intrinsic birefringence, the principal refractive index nz in the thickness direction is generally either the principal refractive index nx or ny parallel to the plane due to the orientation of the polymer main chain in the stretching direction. Will be larger than.

However, the above plastic film is
A polymer having a positive intrinsic birefringence value can be obtained by passing it through a roller that gives a compressive strain in the width direction with respect to the transport direction of the film before or after the axial stretching or after the stretching. Has a larger main refractive index nz than at least one of the refractive indexes parallel to the surface, and has a negative intrinsic birefringence, the main refractive index nz in the thickness direction is at least the refractive index parallel to the surface. It was found to be smaller than either one. That is, the fact that the refractive index in the thickness direction of the birefringent film can be controlled by the present invention is considered as follows. By passing a plastic film made of a thermoplastic resin through a roller that applies a compressive strain in the width direction with respect to the transport direction, an in-plane compressive force is applied to the film, and as a result, the thickness of the film increases and the thickness direction increases. Since the polymer main chain is oriented, the polymer having a positive intrinsic birefringence value has a large refractive index in the thickness direction, and the polymer having a negative intrinsic birefringence value has a small refractive index in the thickness direction.

The roller used in the present invention for applying compressive strain in the width direction with respect to the conveying direction is Japanese Patent Publication No. 2-56.
53, Japanese Patent Publication No. 2-5655, JP-A-59-20983.
No. 0, the expander roller described in JP-A-57-51573 and the like are preferable, and there is no particular limitation. FIG. 1 illustrates the shapes of main expander rollers. It is preferable to use the film so that the film is taken in from the curved convex surface of the roller and carried out on the concave surface side as shown in FIG. Figure 1
In the rollers shown in (1) to (4), the angle of rotation shown in FIG. 3 is preferably 0.5 degrees or more. Further, a plurality of the above rollers may be arranged in series to convey the film continuously. Further, the temperature at which the film passes through the roller is preferably equal to or higher than the temperature at which the molecules constituting the birefringent film can move.

The birefringent film of the present invention preferably has a light transmittance of 70% or more, is substantially transparent and has an achromatic color, and further has a light transmittance of 90% or more and is substantially It is preferably transparent and achromatic. Here, the intrinsic birefringence (Δn) means a birefringence value when molecules are ideally oriented in one direction, and is approximately represented by the following mathematical expression (1).

[0021]

[Equation 1]

The birefringent film of the present invention is a birefringent film whose main refractive index in the thickness direction is greater than at least one of the main refractive indexes parallel to the surface, or the birefringent film is further uniaxially stretched to give a thickness A birefringent film having a main refractive index in the direction smaller than at least one of the main refractive indexes parallel to the surface and larger than the other one. Further, the birefringent film of the present invention may form a uniaxially stretched film such as a birefringent film of the same kind or another kind and another polarizing film or a protective film, or a laminate of biaxially stretched films, if necessary. By laminating the same kind or another birefringent film, it is possible to control the wavelength characteristic of the birefringence index in each birefringent film, and it becomes possible to highly compensate the retardation in the birefringent liquid crystal cell. FIG. 4 shows an example in which birefringent films are laminated. 11 is a birefringent film, 12
Is an adhesive layer, and 13 is a birefringent film of the same kind or another kind. The wavelength characteristic of birefringence can be changed by the combination of the birefringent films to be laminated, the number of laminated layers, the crossing angle of the optical axes, and the like. Therefore, the number of stacked layers and the like may be appropriately determined according to the wavelength characteristics of the required birefringence, the phase difference to be compensated, and the like. However, from the viewpoint of suppressing the deterioration of the transmittance and the visibility due to the absorption loss and the reflection loss at the laminated interface, the smaller number of laminated layers is advantageous.

The liquid crystal panel of the present invention has a birefringent film whose main refractive index in the thickness direction is larger than at least one of main refractive indexes parallel to the surface, or a birefringent film whose refractive index in the thickness direction is on the surface. A retardation plate composed of a birefringent film having a refractive index smaller than one refractive index in the parallel direction and larger than the other refractive index is arranged on one side or both sides of a birefringent liquid crystal cell. The structure is illustrated in FIG. 5 (type in which retardation plates are provided on both sides). 14 is a polarizing plate, 1
Reference numeral 5 is a birefringent film, and 16 is a birefringent liquid crystal cell. The upper part of the liquid crystal panel shown in FIG. 5 is the viewing side.

The birefringent film used for forming the liquid crystal panel is for compensating the phase difference caused by the birefringent liquid crystal cell. As a result, a black-and-white display and a color display in which coloring is prevented and contrast and viewing angle characteristics are improved are achieved. In that case, a retardation plate that achieves a high degree of compensation including the wavelength characteristics of the birefringence of the liquid crystal cell is preferably used. Hereinafter, the invention will be described in detail with reference to Examples.

[0025]

Example Preparation of Polycarbonate Film (PC-F1) 17% by weight of polycarbonate having a weight average molecular weight of 100,000
A methylene chloride solution is cast on a stainless steel band, dried to a residual volatile content of 3%, peeled off, and dried to give a polycarbonate film having a residual volatile content of 1% or less and a thickness of 100 μm. (PC-F
1) was prepared.

Preparation of polystyrene-based graft polymer film (PSt-F1) 12.5 wt% methylene of a graft polymer (PSt-K1) having a weight average molecular weight of 380,000 and having SBR grafted with styrene-acrylonitrile as shown below. The chloride solution was cast on a stainless steel band and the residual volatile content was reduced to 3
% To a state of 100%, the residual volatile content was 1% or less, and a polystyrene-based copolymer film (PSt-F1) having a thickness of 100 μm was prepared.

[0027]

[Chemical 1]

Example 1 Preparation of Birefringent Film (CN-1) A polycarbonate film (PC-F1) was expanded at 158 ° C. with an expander roller having a rotation angle of 2 ° (FIG. 1: EX-
The birefringent film (CN-1) was obtained by taking in from the curved convex surface of R1), carrying it out on the concave side, and further uniaxially stretching it by 20% at 158 ° C. in the carrying direction.

Example 2 Preparation of Birefringent Film (DN-1) Polystyrene-based graft polymer film (PSt-F)
The birefringent film (1) was taken in from the curved convex surface of an expander roller (EX-R1) having a rotation angle of 3 ° at 115 ° C. and carried out on the concave side, and further uniaxially stretched 120% at 115 ° C. in the carrying direction ( DN-1) was obtained. Example 3 Preparation of birefringent film (CN-2) Polycarbonate film (PC-F1) expander roller (EX-R2) having a rotation angle of 3 ° at 165 ° C.
Take in from the convex side of the curve and carry out on the concave side.
A birefringent film (CN-2) was obtained by uniaxially stretching 20% in the transport direction at 8 ° C.

Example 4 Preparation of Birefringent Film (DN-2) Polystyrene-based graft polymer film (PSt-F)
Double refraction by taking in 1) from the curved convex surface of an expander roller (EX-R2) with a rotation angle of 2.5 ° at 110 ° C. and carrying it out on the concave side, and further uniaxially stretching 120% in the conveying direction at 110 ° C. A film (DN-2) was obtained.

Comparative Example 1 Preparation of Birefringent Film (CO-1) A polycarbonate film (PC-F1) was uniaxially stretched at 158 ° C. by 20% in the conveying direction to obtain a birefringent film (CO-1).

Comparative Example 2 Preparation of Birefringent Film (DO-1) Polystyrene-based graft polymer film (PSt-F)
A birefringent film (DO-1) was obtained by uniaxially stretching 1) at 115 ° C in the transport direction by 120%.

[0033]

[Table 1]

Evaluation of Birefringence The birefringent films (CN-1), (CN-2),
(DN-1), (DN-2), (CO-1), and (D
The refractive index (nx) in the optical axis direction for light having a wavelength of 633 μm in O-1), the refractive index (ny) in the direction perpendicular to the optical axis in the plane, and the refractive index (nz) in the thickness direction of the film are Ten points are measured at equal intervals in the width direction (using an Abbe refractometer and ellipsometry), and the average value nx,
Table 2 shows ny and nz.

[0035]

[Table 2]

In the relation of nx, ny and nz, (CN
-1) and (CN-2) are nx>nz> ny, and (DN-1) and (DN-2) are ny>nz>.
It can be seen that it is nx. On the other hand, (CO-1),
And (DO-1), nx>ny> nz.

Evaluation of Viewing Angle Dependence in Liquid Crystal Panel A liquid crystal display mounted on a commercially available word processor (Toshiba: Rupo) was disassembled, and a polarizing plate was used instead of the optical compensation film on the viewing side of the STN liquid crystal cell. A liquid crystal panel for a black and white display was produced by applying the adhesive to a refraction film with the same optical axis configuration as the product. Table 3 shows the obtained viewing angles in the up / down directions in which the contrast ratio in the driven state and the non-driven state of the liquid crystal panel was 10: 1 or more.

[0038]

[Table 3]

From the above, it can be seen that the liquid crystal panel compensated using the retardation plate made of the birefringent film of the present invention has a small change in contrast with viewing angle and has a large viewing angle.

[Brief description of drawings]

FIG. 1 shows a specific example of a roller used in the present invention.

FIG. 2 shows a method of transporting a film on a convex surface of a roller.

FIG. 3 shows a rotation angle of a roller.

FIG. 4 is a laminate of birefringent films of the present invention.

FIG. 5 shows a specific example of the liquid crystal display device of the present invention.

[Explanation of symbols]

 1: Axis 2: Rubber 3: Bearing 4: Angle adjuster 5: Roller 6: Small gap 11: Birefringent film 12: Adhesive layer 13: Birefringent film of the same kind as or different from 11: 14 Polarizing film 15: Birefringent film Refraction film 16: birefringent liquid crystal cell

Claims (3)

[Claims] 1. A method for producing a birefringent film, which comprises a step of passing a plastic film made of a thermoplastic resin through a roller which applies a compressive strain in a width direction with respect to a conveying direction. 2. The method for producing a birefringent film according to claim 1, comprising a step of uniaxially stretching the plastic film. 3. A pair of polarizing plates arranged on both sides of the liquid crystal cell sandwiching the liquid crystal cell, and at least one between the liquid crystal cell and the polarizing plate, the compound obtained by the manufacturing method according to claim 1 or 2. A liquid crystal display device comprising a refraction film.