JP2005316094A - Birefringent film, production method of birefringent film, elliptically polarizing plate, polarizing plate and liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a birefringent film which does not require excess supplementary materials, will not give rise to troublesome production processes and has a controlled refractive index in the thickness direction. <P>SOLUTION: The birefringent film is obtained, by subjecting a polymer film consisting of thermoplastic resin to longitudinally uniaxial stretching with a free end; and if refractive index in the stretching direction in the film surface is nx, the refractive index in a direction orthogonal to the stretching direction in the film surface is ny and the refractive index in the film thickness direction is nz, the following relation is satisfied: 0<(nx-nz)/(nx-ny)<1. In the production method of the birefringent film, when subjecting the thermoplastic resin film to the longitudinally uniaxial stretching with a free end, stretching is performed without applying a constraint force inside a heating furnace, and cooling is performed after the stretching. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a birefringent film used for an optical device such as a liquid crystal display device and a method for producing the same, and more specifically, a method for obtaining a birefringent film by uniaxial stretching of a polymer film made of a thermoplastic resin, and The present invention relates to a birefringent film and a polarizing plate, an elliptically polarizing plate and a liquid crystal display device using the birefringent film.

  A liquid crystal display device has many features such as being able to be driven at a low voltage, low power consumption, being able to be directly connected to an IC circuit, a variety of display functions, and easy weight reduction. Yes. Therefore, liquid crystal display devices are widely used as display devices such as personal computers and word processors.

  Various modes of liquid crystal used in liquid crystal display devices have been proposed and used. As one type of this type of liquid crystal display device, a twisted nematic liquid crystal display device (hereinafter referred to as STN-LCD) in which the twist angle of liquid crystal molecules is 160 ° or more is known.

  The STN-LCD has a problem that the displayed image tends to be colored blue or yellow. Therefore, when performing monochrome display, contrast and visibility tend to be lowered, and color display is extremely difficult.

  Therefore, in order to alleviate the problems caused by the blue or yellow coloring, a structure in which a retardation film obtained by stretching a thermoplastic resin film is laminated on a liquid crystal display cell, that is, a structure called an FTN method is used. Proposed.

  In the FTN method, the retardation film is laminated on the STN-LCD, and the problem due to blue or yellow coloring is reduced. However, when the retardation film is laminated, there is a problem that the contrast is drastically lowered or the background color is changed only by a slight change in the viewing angle. That is, the viewing angle characteristics tend to deteriorate.

  Therefore, various methods for controlling the refractive index in the thickness direction of the retardation film have been proposed in order to suppress the deterioration in viewing angle characteristics when such a retardation film is used.

  For example, Patent Document 1 below proposes a method in which a polycarbonate film whose refractive index in the thickness direction is larger than the refractive index in the direction perpendicular to the birefringent optical axis is used as a retardation plate by electric field orientation. ing. According to this method, a change in contrast due to a change in viewing angle is reduced, and the viewing angle dependency of contrast is improved. However, the effect is not sufficient, and it is necessary to apply a high voltage to the molten polycarbonate for a long time, so that the manufacturing process is complicated and the cost has to be increased.

  On the other hand, Patent Documents 2 to 4 disclose a method in which a film having a positive intrinsic birefringence index and a film having a negative intrinsic birefringence are used one by one or a laminate of these films as a retardation plate. Yes. According to this method, the birefringence can be adjusted by adjusting the usage amount of the film having a positive intrinsic birefringence and a negative film in accordance with the characteristics of the liquid crystal display cell. Therefore, the viewing angle characteristic can be improved more precisely. However, two types of birefringent films produced separately have to be prepared, and the cost has been inevitably high.

  On the other hand, in Patent Document 5 below, when a uniaxially stretched film made of a thermoplastic resin is contracted in the stretching direction while being heated, it is brought into contact with a member made of a roll, a belt, a flat plate, or the like, and the direction perpendicular to the stretching direction is set. A method for preventing dimensional changes is disclosed. According to this method, since the dimensional change in the direction orthogonal to the stretching direction is prevented, the viewing angle dependency when used as a phase difference plate can be improved. However, since it has a step of being fixed in contact with the roll, belt or flat plate, the surface state of these members is transferred to the surface of the obtained retardation film, resulting in poor appearance of the retardation film or optical quality. Tended to decline.

  In Patent Document 6 below, a heat-shrinkable film is bonded to a polymer film made of a thermoplastic resin to obtain a laminated film. A method is disclosed in which a contraction force due to heating is applied to contract in the width direction and then stretched while being widened in the width direction. In this method, a heat shrinkable film is further required, and a process for bonding or peeling the heat shrinkable film is increased, resulting in a problem that productivity is greatly reduced.

In any of the methods described in Patent Documents 1 to 6 described above, some force is applied in a direction other than the stretching direction in order to control the refractive index in the thickness direction of the film. This is because, as described in the following Patent Document 7, simply by stretching, the refractive index in the stretching direction is nx, the birefringence in the direction orthogonal to the stretching direction in the film plane is ny, and the film thickness direction In the case of a resin film showing positive birefringence when the refractive index of nz is nz, when uniaxially stretched, (nx-nz) / (nx-ny) = 1, and in a resin film showing negative birefringence, In the case of uniaxial stretching, (nx−nz) / (nx−ny) = 0. That is, a birefringent film satisfying 0 <(nx−nz) / (nx−ny) <1 can be obtained even when the resin film exhibiting positive or negative birefringence is uniaxially stretched. Was thought to be impossible.
JP-A-2-285303 JP-A-2-256603 Japanese Patent Laid-Open No. 3-141303 Japanese Patent Laid-Open No. 3-24502 JP-A-6-300916 JP 2000-304924 A JP-A-5-157911

  In view of the current state of the prior art described above, the present invention provides a birefringent film in which the refractive index in the thickness direction is controlled without requiring extra auxiliary materials and without complicating the manufacturing process. It is an object of the present invention to provide a manufacturing method thereof, and the birefringent film, an elliptically polarizing plate, a polarizing plate and a liquid crystal display device which are constituted using the birefringent film.

  The birefringent film according to the present invention is a birefringent film obtained by stretching a polymer film made of a thermoplastic resin by free-end longitudinal uniaxial stretching, and the refractive index in the stretching direction in the film plane is nx, Birefringence where 0 <(nx−nz) / (nx−ny) <1, where ny is the birefringence in the direction perpendicular to the stretching direction in the film plane and nz is the refractive index in the film thickness direction. Features an adhesive film.

The method for producing a birefringent film according to the present invention is a method for producing a birefringent film by longitudinally uniaxially stretching a thermoplastic resin film, and starts stretching when the thermoplastic resin film is longitudinally uniaxially stretched. Thereafter, free end longitudinal uniaxial stretching is performed without applying a restraining force until the orientation is fixed by cooling.

  The elliptically polarizing plate according to the present invention is characterized by comprising a laminate in which the birefringent film of the present invention and a polarizing plate are bonded together.

  The polarizing plate according to the present invention is characterized in that it has a polarizer protective film on at least one surface, and the polarizer protective film on at least one surface is composed of the birefringent film of the present invention.

  The liquid crystal display device according to the present invention has a structure in which a liquid crystal cell and a birefringent film constructed according to the present invention are laminated.

  Details of the present invention will be described below.

  The thermoplastic resin used in the present invention is not particularly limited, and examples thereof include polycarbonate, polyvinyl alcohol, cellulose resin, norbornene resin, polyarylate, polyimide, polysulfone, and polyethersulfone. . Polycarbonate and norbornene resins are preferred because of excellent optical properties such as transparency.

  The method for producing the thermoplastic resin film used in the present invention is not particularly limited, and various molding methods that are conventionally used for molding a film can be used, such as a melt extrusion method and a solution casting method. And calendar method.

  The free end longitudinal uniaxial stretching in the present invention refers to a stretching method in which a force that restricts deformation in a direction other than the stretched direction is not applied. Regulating deformation in a direction other than the stretching direction is to hold the film with a jig or the like, suppress shrinkage along the direction orthogonal to the stretching direction, or heat-stretch by bonding with a heat-shrinkable film, and the stretching direction. It means that the shrinkage force of the heat shrink film is applied in the orthogonal direction.

  In the method for producing a birefringent film according to the present invention, when a thermoplastic resin film is heated and stretched by free longitudinal uniaxial stretching, stretching is performed without applying a restraining force to the film until the orientation is fixed by cooling after stretching starts. And the film is cooled after stretching. In this case, it is preferable to cool immediately after stretching, and it is desirable that the cooling is started without applying a restraining force to the film, and the orientation state obtained by stretching is fixed. Therefore, it is preferable that a heating furnace for heating at the time of stretching and a cooling furnace for cooling are made continuous.

  The heating furnace refers to a section for heating in order to stretch the film, and the cooling furnace refers to a film temperature equal to or lower than Tg (glass transition temperature) in order to fix the molecular orientation in the stretched film. It shall mean a section for lowering. In the part where the heating furnace and cooling furnace are connected, an intermediate furnace is arranged to prevent the atmosphere in the heating furnace and the air in the cooling furnace from mixing and to control the temperature in each furnace. May be.

  Furthermore, in order to prevent the atmosphere of the heating furnace and the air in the cooling furnace from mixing, an appropriate air blower or exhaust device may be disposed between the heating furnace and the cooling furnace.

Applying restraining force to the film means that the film is mechanically restrained by placing a nip roll, a pinch roll or the like in a heating furnace, for example, in stretching.
Inserting a film with a nip roll 6 disposed in the heating furnace 3 of the manufacturing apparatus of FIG. 2 used in a comparative example to be described later corresponds to an operation for giving a binding force.

  In the production method of the present invention, free end longitudinal uniaxial stretching is performed in the stretching step without applying the above-described restraining force, whereby birefringence in which 0 <(nx−nz) / (nx−ny) <1. A film can be obtained. The reason for this is not clear. This is considered to be transmitted to the upstream film. The cooling rate of the film is considered to be high in the surface portion, low in the central portion in the film thickness direction, and inclined in the thickness direction. Therefore, it is considered that the shrinkage stress is inclined in the thickness direction. Therefore, since the shrinkage stress is transmitted to the film in the heating furnace, orientation occurs in the film thickness direction, and it is considered that 0 <(nx−nz) / (nx−ny) <1.

  In addition, when a restraint force is applied to the film in the heating furnace for stretching or between the heating furnace and the cooling furnace, the shrinkage stress is not transmitted to the film in the heating furnace, and normal longitudinal uniaxial stretching and Similarly, it is considered that nx> ny = nz.

  Although the thickness of the thermoplastic resin film used for the birefringent film is not particularly limited, for example, when it is used for compensating a phase difference in a liquid crystal display device, a thickness of about 40 to 300 μm is used.

  In the birefringent film of the present invention, the refractive indexes nx, ny and nz in the stretching direction, the direction orthogonal to the stretching direction in the plane, and the thickness direction have a relationship of nx> nz> ny. By using it as a retardation film for compensating refraction, it is possible to increase the viewing angle and improve the contrast. When considering a refractive index ellipsoid having three-dimensional (x axis, y axis, z axis) refractive indexes of nx, ny, and nz, the direction of the line of sight is changed from the normal direction (z axis direction) of the refractive index ellipsoid. When tilted in the y-axis direction, when there is a relationship of nx> nz> ny, the change in phase difference with respect to the direction of the line of sight as compared with the case of nx> ny = nz as obtained by normal uniaxial stretching. Becomes smaller.

  Such a birefringent film can be formed into an elliptically polarizing plate by laminating with a polarizing film.

  The polarizing film may be made of an appropriate material and is not particularly limited. For example, an iodine polarizing film or dye mainly composed of a polymer having a hydrophilic group such as a polyvinyl alcohol polymer. A polarizing film or a polyene polarizing film is used. Further, the polarizing plate may be a laminate in which a transparent polarizer protective film made of the birefringent film of the present invention is laminated on at least one surface thereof.

  When laminating a birefringent film and a polarizing film, an appropriate method can be used and is not particularly limited. Examples of the laminating method include a method using an adhesive having high light transmittance.

  The polarizing plate according to the present invention is preferably disposed on at least one side of the liquid crystal display cell in the liquid crystal display device.

The liquid crystal display device is composed of a liquid crystal cell, a birefringent film for optical compensation, a polarizing plate if necessary, an illumination system, and the like. A liquid crystal display device with improved viewing angle characteristics and contrast can be obtained simply by disposing at least one side of the liquid crystal cell as a film or a polarizing plate.

  Any liquid crystal cell can be used as the liquid crystal display device, for example, a twisted nematic (TN) type, a super twisted nematic (STN) type, a vertical alignment (VA) type, or an in-plane switching (IPS) type. A liquid crystal cell such as can be used.

  The birefringent film according to the present invention is obtained by stretching a thermoplastic resin film uniaxially at the free end, and the refractive index relationship is 0 <(nx−nz) / (nx−ny) <1, that is, nx. Since> nz> ny, it is excellent in birefringence and suitable for applications that compensate the birefringence of the liquid crystal display cell. Therefore, by using the birefringent film of the present invention as an optical film, it is possible to provide a suitable optical film as a phase difference plate such as an STN-LCD, and the viewing angle of a liquid crystal display device can be increased, or contrast can be increased. It becomes possible to improve.

  In the method for producing a birefringent film according to the present invention, when a polymer film made of a thermoplastic resin is stretched uniaxially at the free end, a restraining force is given to the film halfway from the start of stretching until the orientation is fixed by cooling. There is no stretching, and the stretching is performed in a section in which a heating furnace for stretching and a cooling furnace for cooling after stretching are continuous. Therefore, as described above, a birefringent film having (nx−nz) / (nx−ny) larger than 0 and smaller than 1 can be obtained. Therefore, when the birefringent film obtained by the present invention is used for the retardation film of the liquid crystal display cell, the birefringence of the liquid crystal display cell can be compensated, and the viewing angle characteristics and the contrast are improved. Etc.

  In particular, when used in an STN-LCD, the above-described problems due to blue and yellow coloring can be reduced without deteriorating viewing angle characteristics and the like.

  Since the elliptically polarizing plate, the polarizing plate and the liquid crystal display device according to the present invention use the birefringent film of the present invention, the birefringence is compensated for without increasing the cost as described above. In addition, it is possible to provide an elliptically polarizing plate and a polarizing plate that are suitably used for the liquid crystal display, and it is possible to provide a liquid crystal display device with improved viewing angle characteristics and contrast.

  Hereinafter, the present invention will be clarified by describing specific examples of the present invention. In addition, this invention is not limited to a following example.

Example 1
A norbornene-based resin (manufactured by Nippon Zeon Co., Ltd., product number: ZEONEX) was molded by a melt casting method to obtain a thermoplastic resin film having a thickness of 50 μm and a width of 500 mm. The glass transition temperature Tg obtained from the peak temperature of the loss modulus of elasticity of this thermoplastic resin film was 131.4 ° C., and the refractive index was nx = ny = nx = 1.525.

The thermoplastic resin film was stretched uniaxially at the free end using the apparatus shown in FIG. 1 so that the line speed was 2.0 m / sec and the stretching ratio was 1.1 times.
In the apparatus shown in FIG. 1, a heating furnace 3 for stretching and a cooling furnace 5 for cooling in order to fix the orientation after stretching are continuously arranged via an intermediate furnace 4. In addition, nip rolls 1 and 1 are arranged in the front stage of the heating furnace 3, and nip rolls 2 and 2 for taking up the obtained birefringent film are arranged on the downstream side of the cooling furnace 5. The temperature of the heating furnace 3 was 137 ° C., and the lengthwise dimension of the heating furnace was about 100 cm. The temperature in the cooling furnace 5 is 1
It was 20 ° C., and the lengthwise dimension was about 100 cm. The length of the intermediate furnace 4 was 20 cm, and the intermediate furnace 4 was configured such that air was sent and exhausted from both the heating furnace 3 and the cooling furnace 5.

  The thermoplastic resin film F was guided by the nip rolls 1 and 1 in a state preheated at 120 ° C. and supplied to the heating furnace 3.

  Heating and stretching were performed in the heating furnace 3 under the above conditions, cooled in the cooling furnace 5 and taken up by the nip rolls 2 and 2.

  As described above, a birefringent film having a width of 470 mm was obtained.

(Example 2)
Except that the draw ratio was 1.3 times, the film was stretched in the same manner as in Example 1 to obtain a birefringent film having a width of 440 mm after stretching.

(Comparative Example 1)
Except that the apparatus shown in FIG. 2 was used, stretching was performed in the same manner as in Example 1 to obtain a birefringent film having a width of 475 mm. The apparatus shown in FIG. 2 is configured in the same manner as the apparatus shown in FIG. 1 except that a pair of nip rolls 6 and 6 are arranged in the heating furnace 3. Here, since the film F is sandwiched between the nip rolls 6 and 6, the film F is given a restraining force by the nip rolls 6 and 6 at the time of heating and stretching.

(Comparative Example 2)
Except that the draw ratio was 1.3 times, the film was stretched in the same manner as in Comparative Example 1 to obtain a birefringent film.

(Evaluation of Examples and Comparative Examples)
About each birefringent film obtained by the Example and the comparative example, the phase difference was measured using the phase difference measuring apparatus (the Oji Scientific Instruments company make, KOBRA21ADH), and nx, ny, and nz were calculated | required. The results are shown in Table 1 below.

  As is clear from Table 1, in the birefringent films obtained in Comparative Examples 1 and 2, (nx−nz) / (nx−ny) was about 1, whereas Examples 1 and 2 Then, it can be seen that (nx−nz) / (nx−ny) is smaller than 1.

1 is a schematic configuration diagram used for stretching a thermoplastic resin film in Example 1. FIG. The schematic block diagram used in extending the thermoplastic resin film in the comparative example 1. FIG.

Explanation of symbols

F ... thermoplastic resin film 1 ... nip roll 2 ... nip roll 3 ... heating furnace 4 ... intermediate furnace 5 ... cooling furnace 6 ... nip roll

Claims (5)

  A birefringent film obtained by stretching a polymer film made of a thermoplastic resin by uniaxial stretching at the free end, wherein the refractive index in the stretching direction in the film plane is nx, and is orthogonal to the stretching direction in the film plane A birefringent film in which 0 <(nx−nz) / (nx−ny) <1, where ny is the birefringence in the direction and nz is the refractive index in the film thickness direction. A method for producing a birefringent film by longitudinally uniaxially stretching a thermoplastic resin film,
2. The birefringent film according to claim 1, wherein, at the time of longitudinal uniaxial stretching of the thermoplastic resin film, free end longitudinal uniaxial stretching is performed without applying a restraining force until the orientation is fixed by cooling after stretching is started. Manufacturing method.   An elliptically polarizing plate comprising a laminate in which the birefringent film according to claim 1 and a polarizing plate are bonded together.   A polarizing plate having a polarizer protective film provided on at least one surface thereof, wherein the polarizer protective film is the birefringent film according to claim 1. A liquid crystal display device comprising: a liquid crystal cell; and the birefringent film according to claim 1, which is laminated on at least one surface of the liquid crystal cell.

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