JPH1086208A - Manufacture of optical film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a heat resistant optical film which is excellent in film smoothness and has a low double refraction index, i.e., a small optical phase difference, has a very high uniformity of an optical principal axis of an index ellipsoid in the inside of the film face and in the thickness direction, and can be used for a liquid crystal display panel. SOLUTION: When a thermoplastic resin having a glass transition point of 190 deg.C or higher is to be formed into a film by extrusion method, a zone of a lip tip is brought under pressure reduction lower than about 1.013×10<5> pa and an atmospheric temperature is kept within about 100 deg.C from a resin temperature at a tip of a die slip. Further, the film is treated with heat continuously in between rolls having dryers by a fixed tension in the flowing direction of the film.

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 an optical film, and more particularly, to a method for producing a transparent electrode substrate for a flexible liquid crystal display element requiring heat resistance, which has a small optical retardation and an optical principal axis. The present invention relates to the production of a uniform heat-resistant optical film.

[0002]

2. Description of the Related Art Conventionally, a glass substrate has been used as a transparent electrode substrate for a liquid crystal display element. However, in a liquid crystal display element using a glass substrate, the thickness of the liquid crystal display element body is reduced because the glass substrate itself is thick. It is difficult and has the drawback that it is difficult to reduce the weight, and further, there are problems in terms of flexibility and impact resistance.

As a method of improving the disadvantages of the glass substrate liquid crystal display element, a liquid crystal panel is continuously manufactured using a plastic film, thereby reducing the cost of processing the liquid crystal panel, mass-producing the liquid crystal panel, and reducing the weight. Improvement in impact resistance has been studied. For example, Japanese Unexamined Patent Publication No.
68099 and JP-A-54-126559 show that a liquid crystal display panel is manufactured continuously using a long polyester film on which a conductive metal oxide material is deposited instead of a glass substrate. However, it was hard to say that the heat resistance in the cell processing step and the optical characteristics when mounted were excellent.

In order to solve this problem, research was conducted to apply a thermoplastic resin film having excellent heat resistance and optical isotropy to these applications. It has been found that an increase in the birefringence of the film and an increase in the amplitude of the optical principal axis of the refractive index ellipsoid in the plane of the film and in the thickness direction are serious drawbacks. For example, in a TN type liquid crystal display device, the incident light linearly polarized by the polarizing plate becomes elliptically polarized light which is partially different from the birefringence of the transparent electrode film and the deviation in the film plane, so that the contrast is reduced and the display is reduced. This causes unevenness. STN (Super Twisted Ne)
matic) type liquid crystal display devices cannot not only obtain high-definition display than TN type liquid crystal display devices due to the optical retardation developed from the birefringence of the transparent electrode film, but also have retardation generated from liquid crystal molecules. Therefore, optimization of a combination of a liquid crystal cell with a polarizing plate, a retardation plate, and a transparent electrode film for compensating an optical phase difference is very complicated.

Further, as a liquid crystal cell using a thermoplastic resin film is put to practical use, the display area becomes large, and the electrodes are kept at a uniform interval.
The base material is required to be hardly deformed, and the thickness of the base material has increased from the initial thickness of 100 μm to more than 300 μm. At this time, the birefringence in the thickness direction becomes more important,
In particular, a reflection type color mode, which will become the mainstream in the future, causes color misregistration, so that an immediate improvement is desired.

As a method for improving the retardation of an optical material, a method using a special polycarbonate having a special dihydric phenol as a structural unit (JP-A-63-108024) is known. Method of blending the materials of (T. Inoue et al., Journal of Polymer Scienc
e, Part B, 25, 1629 (1987).), a method of graft copolymerizing a polycarbonate having a positive intrinsic birefringence and a negative polystyrene (Nikkei New Materials, September 26, 1988, pp. 60-62). ), A method using a norbornene-based resin having a polar group (functional materials, January 1993,
Articles on pages 40 to 52) have been proposed, but it is hard to say that both have excellent heat resistance and optical properties.
The optical main axis has not been made uniform.

[0007] In a short time from the moment when the film extruded by the extrusion method is cooled and solidified by the cooling rolls and wound up, the shear stress generated in the die and the stretching of the resin from the die slip are caused. Due to the temperature distribution occurring in the flow direction and the thickness direction, it was not possible to avoid the molecular orientation in the plane and the thickness direction generated in the film. This tendency has the property of becoming more pronounced as the film thickness increases, and has become a serious drawback as the display screen becomes larger.

[0008]

The object of the present invention is to provide a film having excellent smoothness and a small birefringence, that is, a film having a small optical retardation, and a refractive index in the film plane and in the thickness direction. It is an object of the present invention to provide a method for producing a heat-resistant optical film which has a very high optical axis uniformity of an ellipsoid and can be used for a liquid crystal display panel.

[0009]

As a result of intensive studies in view of this point, the present invention has found that the temperature distribution in the thickness direction is minimized and the solidification rate at the molecular level is slowed down and uniformized. It has been found that the molecular orientation of is suppressed.

That is, when a thermoplastic resin having a glass transition point of 190 ° C. or more is formed into a film by the extrusion method, the zone at the tip of the lip is reduced under a reduced pressure of less than 1.013 × 10 ⁵ Pa, and the atmospheric temperature is reduced. This is a method for producing an optical film that is formed by keeping the temperature of the resin at the tip of the die slip within −100 ° C., and a more preferred embodiment is to apply a constant tension in the flow direction between the two rolls of the extruded film. When heat treatment, the interval between the two rolls is larger than the width of the film, and heat treatment is performed at a temperature lower than the glass transition point of the film from the temperature at which the linear expansion coefficient of the film increases,
The film after heat treatment has an in-plane birefringence of 1 × 10 ⁻⁴ or less, and the heat treatment film has a refractive index ellipsoid with an optical principal axis swing of ± 10 ° or less. The birefringence in the thickness direction is 1 × 10 ⁻³ or less, and the deflection width of the optical main axis of the refractive index ellipsoid in the film thickness direction after the heat treatment is ± 10 ° or less, and the thermoplastic resin is polysulfone or polyether. This is a method for producing an optical film, which is at least one selected from the group consisting of sulfone, polyetherimide, and polyarylate.

[0011]

More specifically, the inside of this section is kept under a reduced pressure of less than 1.013 × 10 ⁵ Pa except for the gap where the film flows, from the tip of the die slip to the cooling roll, and the inside of this section is reduced. Molding while keeping the ambient temperature within -100 ° C of the die-slip tip resin temperature increases the degree of freedom of molecular motion in the thickness direction and increases the heat-retaining effect, so that stress relaxation can be obtained before contacting the cooling roll. It is what came to be.

Although the die lip tip zone in the normal melt extrusion method is heated by radiant heat, it is basically extruded into the atmosphere, so that a rapid temperature drop occurs due to heat dissipation using air as a medium, and the film surface is heated. And a temperature distribution inside the film. Further, the molecular orientation is completely solidified by contact with the cooling roll. Therefore, since the molecular orientation in the film immediately before the molecular orientation is completely completed is drawn out in a state due to this temperature distribution, it is ideal to maintain the resin temperature in the die lip until it contacts the cooling roll. . For this reason, it is indispensable to keep the lip tip zone warm by a heat insulating material and to heat it from the outside in order to reduce the birefringence in the thickness direction. At this time, although it depends on the characteristics of the molten resin, the temperature of the resin at the tip of the lip is -100C.
-50 ° C
Is most favorable for birefringence.

As a specific method for this, a section from the tip of the lip to the cooling roll is covered with a heat insulating material except for a gap in which the film flows.
Install the insulation surface some distance away. Furthermore, it has been found that it is the most economical and practical method for keeping the temperature low by evacuating the inside with a vacuum pump and setting a pressure reduction zone of less than 1.013 × 10 ⁵ Pa. 1.013 ×
In an atmosphere exceeding 10 ⁵ Pa, heating from the outside is required, and there is a drawback that the apparatus becomes large. On the other hand, when there is a temperature drop exceeding -100 ° C, the effect of reducing the birefringence in the thickness direction is small, and the object is not achieved.

In order to mainly remove the molecular orientation in the in-plane direction, the film in which the molecular orientation in the thickness direction is suppressed as much as possible is continuously heat-treated using a roll-to-roll heat treatment machine provided with a dryer. At this time, the roll interval is set to L
_MD , when the film width is L _TD , L _MD should be used so that the film heat-treated between the rolls is not given biaxial stretchability.
/ L _TD > 1.0. In addition, in order to heat-treat the film so that the appearance of the film is not impaired, it is necessary to drive the roll so that an appropriate tension is applied to the film. Molecular chain starts to micro-Brownian motion which constitutes the film, the linear expansion coefficient begins to rise temperature (hereinafter Tg ^- hereinafter) of the film at a temperature that is below the glass transition point (hereinafter referred to as Tg) from 10 preferably above Tg ℃ -30
By heat-treating the extruded film at a low temperature, the appearance is good and the in-plane birefringence of the film is 1 × 10 ^-4.
Or less, the deflection of the optical principal axis is ± 10 ° or less, the birefringence in the thickness direction of the film is 1 × 10 ⁻³ or less, and the deflection of the optical principal axis of the refractive index ellipsoid in the thickness direction. Is ± 10 ° or less. As a definition of birefringence, when the three components of the refractive index have a relationship of Nx>Ny> Nz, the in-plane birefringence is Nx−
Ny and the thickness direction birefringence are Nx-Nz.

When the film is heat-treated under the condition of L _MD / L _TD ≦ 1.0, shrinkage in the film width direction can be restricted due to the short roll interval, and simultaneous biaxial tensile stress occurs in the film plane. I do. Therefore, the ideal uniaxial orientation of the polymer chain is suppressed, and the orientation angle distribution of the polymer chain is developed in the width direction of the film. Since the optical principal axis of the refractive index ellipsoid greatly depends on the molecular chain orientation angle, the film should have an L _MD /
When the heat treatment is performed under the condition of L _TD ≦ 1.0, the degree of freedom of the molecular chain orientation is not controlled in one direction, so that the optical principal axis cannot be made uniform.

When the extruded film is continuously heat-treated under the condition of L _MD / L _TD > 1.0, if the heat treatment is performed at a high temperature of Tg or higher, the polymer chains constituting the film have a high fluidity, so that the film has high fluidity. Although the molecular chain orientation in the flow direction easily occurs and the optical principal axis is made uniform, the birefringence increases due to film stretching. The heat treatment temperature Tg of the film ^- even too than low, molecular chains micro Brownian motion is frozen, since it is only a local molecular vibrational, hardly occur molecular chain orientation by heat treatment, the optical main axis In some cases, uniformity is not achieved.

The fluctuation width of the optical principal axis indicates a variation in molecular chain orientation, and is ± 10 ° or less, preferably ± 5 ° or less. If the deviation of the optical principal axis is too large, when the linearly polarized light passes through the film, a deviation occurs in the polarization state of the transmitted light, and partial light leakage occurs. as a result,
It is not preferable because a light shutter function important for TN and STN type liquid crystal display elements is impaired by the electrode substrate film.

Examples of the thermoplastic resin having a Tg of 190 ° C. or higher in the present invention include polysulfone, polyethersulfone, polyetherimide, polyarylate, and resins blended with these. The resin of the present invention may contain a small amount of a stabilizer, a lubricant, a dye, or the like as an additive.

The thickness of the heat-resistant optical film according to the present invention is 10 μm to 500 μm, and more preferably 50 μm to 400 μm.
m is preferable in terms of workability and flexibility. The surface roughness of the film in the present invention is preferably 0.5 μm or less, and more preferably 0.1 μm or less. When the surface roughness is larger than 0.5 μm, in the STN type liquid crystal display device, display unevenness caused by a change in the liquid crystal cell gap due to unevenness of the electrode film surface is more remarkably observed than display unevenness caused by the optical phase difference of the transparent electrode film. Is done.

The optical properties of the film of the present invention were measured by the following methods. (1) Birefringence Birefringence at a wavelength of 550 nm by measuring the optical phase difference at a wavelength of 550 nm using a polarizing microscope BH2 manufactured by Olympus Optical Co., Ltd. and a Berek compensator, and further measuring the thickness of the film. The rate was measured. (2) Optical principal axis After measuring the photoelastic sensitivity with a photoelasticity measuring device, the optical principal axis was measured by examining the phase increase axis and the slow axis of the refractive index ellipsoid using a Berek compensator.

[0021]

The present invention will be described below with reference to examples and comparative examples. << Example 1 >> Polyethersulfone resin of Sumitomo Chemical Co., Ltd .: Victrex PES4100G (Tg = 22)
6 ° C.) by a melt extrusion method to shield the die lip tip zone except for the gap where the film flows, and 0.8 × 10 ⁵ Pa
, And formed into a film at an ambient temperature of -45 ° C from the resin temperature of the die lip tip (290 ° C). Table 1 shows the properties of the obtained film. This film is called L _MD / L _TD
= 11, processing temperature of 206 ° C, line speed of 1.0 M /
min, the film tension is 1.0 kg / width (film width:
630 mm). Table 1 shows the properties of the polyethersulfone film after the heat treatment.

Example 2 A film was formed under the reduced pressure of 0.9 × 10 ⁵ Pa in the same manner as in Example 1 at an atmosphere temperature of −90 ° C. from the resin temperature of the die lip tip (290 ° C.). Table 1 shows the properties of the obtained film. _LMD / _LTD = 11, processing temperature of 216 ° C., line speed of 1.0 M / min, film tension of 1.2 kg
/ Width (film width: 630 mm). Table 1 shows the properties of the polyethersulfone film after the heat treatment.

Example 3 A film was formed under the reduced pressure of 0.6 × 10 ⁵ Pa in the same manner as in Example 1 and at an ambient temperature of -90 ° C. from the resin temperature of the die lip tip (290 ° C.). Table 1 shows the properties of the obtained film. _LMD / _LTD = 23, processing temperature of 216 ° C., line speed of 0.4 M / min, film tension of 1.2 kg
/ Width (film width: 300 mm). Table 1 shows the properties of the polyethersulfone film after the continuous heat treatment.

Comparative Example 1 Materials of the same lot as in Example 1 were used, but the die lip tip zone was not reduced under reduced pressure.
The film was formed without controlling the ambient temperature. Table 1 shows the properties of the obtained film. This film is _LMD / L
_TD = 23, process temperature 226 ° C, line speed 0.4M
/ Min at a film tension of 1.2 kg / width (film width: 300 mm). The properties of the polyethersulfone film after the continuous heat treatment are shown in Table 1. The optical retardation was confirmed, and the liquid crystal display panel using the obtained film as a transparent electrode did not have sufficient contrast and good visibility. Did not.

COMPARATIVE EXAMPLE 2 A film was formed in the same manner as in Example 1 under a reduced pressure of 0.8 × 10 ⁵ Pa at an ambient temperature of -45 ° C. from the die lip tip resin temperature (290 ° C.). Table 1 shows the properties of the obtained film. This film was subjected to L _MD / L _TD = 1, processing temperature of 206 ° C., line speed of 1.0 M / min and film tension of 1.0 kg / min.
Continuous heat treatment was performed under the condition of a width (film width: 630 mm). Table 1 shows the properties of the polyethersulfone film after the continuous heat treatment, but no effect of the heat treatment was observed. In a liquid crystal display panel in which the obtained film was mounted as a transparent electrode, a deviation occurred in the polarization state of transmitted light, and the contrast was insufficient and the visibility was not good.

[0026]

[Table 1]

[0027]

According to the present invention, a heat-resistant optical film having a small optical anisotropy and a uniform optical principal axis of a refractive index ellipsoid can be continuously produced. Can be used effectively to mass-produce panels. In addition, when the heat-resistant optical film obtained by the present invention was mounted on a liquid crystal display panel as a transparent electrode film for a flexible liquid crystal display element, a high-definition display without display unevenness was exhibited.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI // B29K 81:00 B29L 7:00

Claims (5)

[Claims] When a thermoplastic resin having a glass transition point of 190 ° C. or more is formed into a film by an extrusion method, the zone at the tip of the lip is reduced under a reduced pressure of less than 1.013 × 10 ⁵ Pa, and the atmospheric temperature is reduced. From the resin temperature at the tip of the die slip
A method for producing an optical film, wherein molding is performed while keeping the temperature within 100 ° C. 2. Extruded film is heat-treated between two rolls by applying a constant tension in the flow direction.
2. The optical film production according to claim 1, wherein the heat treatment is performed within a temperature range from the temperature at which the distance between the rolls of the book is greater than the width of the film and the linear expansion coefficient of the film is lower than the glass transition point of the film. Method. 3. The film after heat treatment has an in-plane birefringence of 1 × 10 ⁻⁴ or less, and the width of the optical principal axis of the refractive index ellipsoid in the film after heat treatment has a fluctuation of ± 10 ° or less. 3. The method for producing an optical film according to claim 1, wherein: 4. The method according to claim 1, wherein the birefringence in the film thickness direction after the heat treatment is 1 × 10 ⁻³ or less, and the fluctuation width of the optical principal axis of the refractive index ellipsoid in the film thickness direction after the heat treatment is ± 10 ° or less. The method for producing an optical film according to claim 1, wherein: 5. The optical element according to claim 1, wherein the thermoplastic resin is at least one selected from the group consisting of polysulfone, polyethersulfone, polyetherimide, and polyarylate. Film production method.