JP2008170716A - Method of manufacturing retardation film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a retardation film made of an amorphous thermoplastic resin that is sequentially and biaxially stretched longitudinally and laterally and having superior optical quality although it is a thin film. <P>SOLUTION: The retardation film having a slow axis in a width direction is made by stretching the long amorphous thermoplastic resin film with controlled specific thickness quality by two times or more in terms of the width direction after stretching it by 1.1 times or more in terms of the longitudinal direction, wherein the conditions of (1) 0 nm≤R<SB>0</SB>≤200 nm, (2) 50 nm≤Rth≤300 nm, (3) d≤50 μm, (4) film length direction R<SB>0</SB>variation ≤10 nm/10 m, (5) film length direction R<SB>0</SB>variation ≤1 nm/5 mm are satisfied, where R<SB>0</SB>is the retardation in the film surface, Rth is the retardation in the film thickness direction, and d is the film thickness. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a retardation film that is incorporated in, for example, a liquid crystal display device and used to improve the contrast of the liquid crystal display device and expand the viewing angle.

In recent years, liquid crystal display devices have been widely used in applications such as personal computer display devices and liquid crystal televisions. As one of liquid crystal display devices, there is a TN (twisted nematic) mode liquid crystal display device using a light application effect. However, the TN mode liquid crystal display device has a problem that the viewing angle is narrow and the response speed is slow.
In order to solve this problem, a VA (Vertically Aligned) mode liquid crystal display device using a birefringence effect has been proposed. Further, as a VA mode liquid crystal display device, an MVA (Multi-domain Vertical Alignment) mode liquid crystal display device with improved viewing angle dependency has been proposed. In the MVA mode liquid crystal display device, a domain regulating means including a projection having an inclined surface is provided on the inner surface of the substrate constituting the liquid crystal cell. By dividing the alignment direction of the liquid crystal molecules into two or more directions by this domain regulating means and uniformizing the amount of light passing through the liquid crystal cell, the viewing angle dependency, which greatly varies the display luminance depending on the expected angle, is improved. Yes.

However, even in the MVA mode liquid crystal display device with improved viewing angle dependency, there is still a problem that the contrast is lowered when the liquid crystal display surface is viewed at an angle of 45 degrees. In order to solve such a problem, a retardation film is used in the liquid crystal display device.
As the retardation film, a synthetic resin film excellent in transparency and heat resistance, such as polycarbonate and polysulfone, is often used. In addition to these properties, cyclic olefin resin films having excellent properties such as photoelastic coefficient, wavelength dispersibility, and water vapor transmission rate have been used.

By the way, a biaxial retardation film in which the refractive index in the length direction and the width direction is larger than the refractive index in the thickness direction is effective in improving the viewing angle of the VA mode or MVA mode liquid crystal display device. It is known that there is.
In order to fully exhibit the function as a retardation film, it is necessary to combine with the birefringence of the liquid crystal panel, and the retardation film is strongly required to have an appropriate retardation. Retardation is controlled by the refractive index and thickness of the film.

As a method for producing the biaxial retardation film, the following Patent Document 1 proposes a sequential biaxial stretching method in which a transverse stretching step using a tenter stretching machine or the like is performed after the longitudinal stretching step.
However, no method has been shown for obtaining a quality sufficient to improve the viewing angle of the VA mode or MVA mode liquid crystal display device. In particular, in recent years, liquid crystal display devices have been required to be compact and thin, and accordingly, retardation films have also been required to be thin. However, when the retardation film is thinned, the retardation also decreases as the thickness decreases, and there is a problem in that the desired retardation value cannot be reached. Further, in the retardation film, retardation variation and retardation unevenness are problematic because the display quality deteriorates and color unevenness occurs. As a cause of this, it has been conventionally considered that one of the causes is an inaccurate thickness of the pre-stretched film. In the retardation compensation film obtained by stretching, the retardation unevenness due to the thickness unevenness occurs.

On the other hand, Patent Document 2 proposes a method in which the thickness accuracy in the non-stretched film stage is defined and uniaxially stretched to reduce the variation in retardation.
However, the present inventors have experienced a tendency that retardation variation and retardation unevenness tend to be larger in the sequential biaxial stretching method than in the uniaxial stretching method. It was not done.
JP 2002-148438 A Japanese Patent No. 2841377

An object of the present invention is to provide a retardation film made of an amorphous thermoplastic resin that is excellent in optical properties while being a thin film, and a method for producing the same, in view of the above-described problems.

The method for producing a retardation film according to the present invention includes:
A long amorphous thermoplastic resin film having an average thickness of 100 μm or more, a thickness variation in the length direction of 1 μm or less per 10 m, and a thickness unevenness of 5 μm in the length direction of 0.2 μm or less is represented by the following (1 ) To (5), the film is stretched 1.1 times or more in the length direction, and then stretched 2 times or more in the width direction so that the slow axis substantially coincides with the width direction. .
(1) 0 nm ≦ R ₀ ≦ 200 nm
(2) 50 nm ≦ Rth ≦ 300 nm
(3) d ≦ 50 μm
(4) Film length direction R ₀ variation ≦ 10 nm / 10 m
(5) Film length direction R ₀ unevenness ≦ 1 nm / 5 mm
Moreover, the amorphous thermoplastic resin film is formed by a melt extrusion method.
R ₀ represents retardation in the film plane, Rth represents retardation in the film thickness direction, and d represents film thickness.

  An amorphous thermoplastic resin film having an average thickness of 100 μm or more is stretched 1.1 times or more in the length direction and then stretched twice or more in the width direction, thereby having a desired retardation. And it can be set as the phase difference film which is a thin film and has a slow axis in the width direction. Furthermore, when the thickness variation and thickness variation in the length direction of the pre-stretch film are in a specific range, the retardation variation and variation in the length direction of the obtained retardation film can be accurately controlled within a small range. The resulting retardation film is sufficiently effective to improve the viewing angle of the VA mode or MVA mode liquid crystal display device, and does not cause optical quality deterioration such as color unevenness. Further, since the thickness is 50 μm or less, it can contribute to thinning of a liquid crystal television or the like.

Details of the present invention will be described below.
As the amorphous thermoplastic resin constituting the amorphous thermoplastic resin film used in the present invention, those having excellent transparency are preferable. For example, polycarbonate-based, polysulfone-based, polyethersulfone-based, polystyrene-based, cyclic olefin Polymer, polyvinyl alcohol, cellulose acetate, polyvinyl chloride, polyacrylic such as polymethylmethacrylate, polyarylate, polyamide and the like. These polymers may be used alone or in combination of two or more.

Among the above-mentioned cyclic olefin polymers, norbornene resins are preferable because they have a small photoelastic coefficient and excellent retardation stability against external stress.
Examples of the norbornene resin include ring-opening (co) polymers of norbornene monomers, addition copolymers of norbornene monomers and olefin monomers, addition (co) polymers of norbornene monomers and derivatives thereof Etc. Norbornene-type resin may be used independently and 2 or more types may be used together.
Of these, unsaturated bonds necessarily remain in (co) polymers with ring opening, and even in addition (co) polymers, unsaturated bonds may remain depending on the type of monomer. . In such a case, it is preferable to hydrogenate these unsaturated bonds from the viewpoint of emphasizing durability such as oxidative degradation due to thermal history and discoloration due to ultraviolet rays.

  In the above-mentioned amorphous thermoplastic resin, the deterioration of the amorphous thermoplastic resin during molding and the heat resistance, ultraviolet resistance, smoothness, etc. of the retardation film, as long as the function of the retardation film is not impaired. In order to improve the resistance, phenol-based, phosphorus-based antioxidants; lactone-based thermal degradation inhibitors; benzophenone-based, benzotriazole-based, acrylonitrile-based UV absorbers; aliphatic alcohol ester-based, polyvalent Various additives such as alcohol partial ester-based and partial ether-based lubricants; amine-based antistatic agents and the like may be added. An additive may be used independently and 2 or more types may be used together.

  In the present invention, a long film before stretching having an average thickness of 100 μm or more, a thickness variation in the length direction of 1 μm or less per 10 m, and a thickness unevenness of 5 mm in the length direction of 0.2 μm or less is used. There is a need.

  As a method of forming a substantially non-stretched amorphous thermoplastic resin film using the amorphous thermoplastic resin, a conventionally used method is used. Specifically, after casting a solvent solution capable of dissolving the amorphous thermoplastic resin on an appropriate support, the solvent is dried and peeled off from the support to obtain a film, or an amorphous A melt extrusion method that supplies a thermoplastic thermoplastic resin to an extruder, melts and kneads it, and extrudes it into a film form from a die attached to the tip of the extruder, resulting in low cost and low environmental impact Therefore, the latter method is preferably used.

In the present invention, a melt-extruded film is preferably used as the amorphous thermoplastic resin film before stretching. FIG. 1 is a schematic configuration diagram showing an example of an apparatus for forming an amorphous thermoplastic resin film by melt extrusion.
As shown in FIG. 1, when forming an amorphous thermoplastic resin film, first, a molten amorphous thermoplastic resin is extruded from the extruder 1 and supplied to the mold 2. A semi-molten amorphous thermoplastic resin is discharged from the mold 2 in the form of a film and is cooled and solidified by contacting the cooling roll 4. In addition, 5 is a touch roll and is provided in order to adhere the film 3 to the cooling roll 4. The film 3 thus solidified is wound up through rolls 6 and 7.

  Here, the distance from the outlet of the mold 2 to the contact point where the film 3 contacts the cooling roll 4, that is, the air gap is preferably short. The shorter the air gap, the more the thickness variation due to disturbance can be reduced. That is, a film having an appropriate thickness profile can be stably produced. A preferable air gap is 70 mm or less.

Further, when the film 3 comes into contact with the cooling roll 4, it is desirable that air does not enter between the cooling roll 4 and the film 3, and it is desirable that the cooling rate is uniform over the entire surface. Therefore, it is desirable to press the film 3 toward the cooling roll 4 by pressing means such as the touch roll 5 in the vicinity of the downstream side of the contact.
The pressing means is not limited to the touch roll 5, and an air knife or electrostatic pinning may be used. However, it is desirable to use a touch roll having a surface made of an elastic material because it is excellent in stability and the film 3 can be uniformly pressed against the cooling roll 4.

The temperature of the cooling roll 4 varies depending on the type of resin constituting the film 3, but is preferably in the range of Tg-10 ° C to Tg-100 ° C when the glass transition temperature Tg of the resin used is used. . In addition, the glass transition temperature Tg used throughout this specification means what is measured with a differential scanning calorimeter.
In order to ensure the smoothness and transparency of the film, the surface roughness of the cooling roll 4 is preferably 0.5 μm or less, more preferably 0.3 μm or less in terms of the Ry value defined in JIS B 0601. preferable. Although the said cooling roll 4 can be comprised with various materials, Preferably it consists of a metal, for example, what is comprised by carbon steel, stainless steel, etc. is used suitably. When the cooling roll 4 made of metal is used, the temperature of the cooling roll 4 can be quickly maintained at a constant temperature, and the film 3 can be efficiently cooled.
Further, in order to increase the thickness accuracy in the length direction of the film, it is desirable that the roll has a small eccentric runout. Specifically, it is preferably 30 μm or less, and more preferably 10 μm or less.

Since the fluidity of the resin changes when the temperature of the mold 2 varies, it is desirable that the temperature of the mold 2 is stable. Preferably, the temperature of the portion that contacts the molten resin constituting the film 3 of the mold 2 is desirably kept within a set temperature ± 0.5 ° C, more preferably within the set temperature ± 0.2 ° C. .
In general, the roll temperature greatly affects the solidification point of the resin. Therefore, it is desirable to have a structure in which a temperature adjusting mechanism is connected to or built in the shaft core portion of the cooling roll 4 so that the temperature of the cooling roll 4 can be adjusted to various temperatures. As a preferred temperature adjusting means, a temperature adjusting means of an electric heating system in which a sheathed heater is incorporated in the shaft core portion and heated so as to set the cooling roll 4 to an appropriate temperature, or a temperature adjusting means by electromagnetic induction action by an induction heating coil. Further, temperature adjusting means such as a heat medium circulation heating method in which a heat medium for temperature control is circulated through a flow path provided in the shaft core portion to heat the cooling roll to a set temperature can be used. Particularly preferred is a heat medium circulation heating method, and a gas or a liquid such as water or oil may be used as the heat medium. In particular, it is desirable to use a liquid such as water or oil having a large heat capacity. Preferable examples of such a heat medium flow path include those having a two-row spiral structure or a four-row spiral structure inside.

In order to produce a retardation film using the above-described long amorphous thermoplastic resin film, a polymer molecule is predetermined by stretching the amorphous thermoplastic resin film at a temperature near its glass transition temperature Tg. What is necessary is just to orientate in a direction.
Below, the method of extending | stretching an amorphous thermoplastic resin film to a film length direction (henceforth a longitudinal direction hereafter) first in the temperature range of the glass transition temperature Tg vicinity is demonstrated in detail.

  As a method of stretching a long amorphous thermoplastic resin film unwound from a film roll in the longitudinal direction, a neck-in-stretch method between rolls, a proximity roll stretching method, etc. can be applied, but the retardation is controlled. It is desirable to adopt a roll-to-roll neck-in stretching method that has the advantage that it is easy to perform and that defects such as scratches and wrinkles are less likely to occur in the film. Neck-in stretching between rolls means that the film being transported is sandwiched between two pairs of nip rolls located across a stretching zone that is sufficiently longer than the film width, and downstream of the peripheral speed of the upstream nip roll. This is a method of obtaining a desired draw ratio by increasing the peripheral speed of the side nip roll. At this time, both end portions in the width direction of the film are free ends that are not restrained, and exhibit a neck-in phenomenon in the width direction along with stretching in the longitudinal direction.

  The temperature of the film when the amorphous thermoplastic resin film is stretched in the longitudinal direction is appropriately adjusted depending on the amount of compensation retardation to be applied to the retardation film, but if low, the film may be broken during stretching. On the other hand, if it is high, it may be difficult to obtain a desired retardation. Therefore, (glass transition temperature Tg-20 ° C. of unstretched film) to (glass transition temperature Tg + 50 ° C. of unstretched film) is preferable, (Glass transition temperature Tg-10 ° C. of unstretched film) to (Glass transition temperature Tg + 40 ° C. of unstretched film) is more preferable.

Further, if the draw ratio at this time is too small, the increase in Rth is restricted by the subsequent transverse stretching (stretching in the width direction), and therefore, it is preferably 1.1 times or more. On the other hand, if it is too large, it is necessary to make the transverse stretching ratio very large in order to obtain the desired R _0, and therefore it is preferably 1.5 times or less.

The longitudinally uniaxially stretched film obtained in the above-described manner is less than the glass transition temperature Tg of the amorphous thermoplastic resin and 5% from the stretching temperature in order to prevent a decrease in the front retardation _R0 due to thermal relaxation. It is cooled and fixed at a temperature lower than 0 ° C. and wound up in a roll or conveyed to the next step.

Next, a method for stretching the obtained longitudinally uniaxially stretched film in the width direction of the film (hereinafter also referred to as a transverse direction) will be described in detail. At this time, the longitudinally uniaxially stretched film may be transported to the lateral stretching process as it is, or after being wound up into a roll shape, it may be unwound again and transported to the lateral stretching process.
As a method of stretching the longitudinally uniaxially stretched film conveyed to the transverse stretching step in the lateral direction, both ends in the width direction of the film are gripped by arbitrary gripping means such as a tenter clip, and the gripping means are separated from each other. It is desirable to use a tenter stretching machine that runs while being gradually displaced in the direction.

  If the draw ratio when the film is stretched in the transverse direction is too low, the slow axis direction oriented in the machine direction cannot be made transverse, the desired film thickness cannot be achieved, or the necessary slow phase Axis accuracy cannot be secured and display quality may be deteriorated. On the other hand, if it is too high, the film is liable to break, so it is 2 times or more and 5 times or less, preferably 4 times or less. More preferably, it is 3.5 times or less.

  Further, the temperature of the film when stretched in the transverse direction is appropriately adjusted depending on the amount of compensation retardation to be imparted to the retardation film, but if low, the film may be broken during stretching, whereas if high, Since it may be difficult to obtain a desired frontal retardation, the temperature is basically not higher than that in the longitudinal stretching step, and a glass transition temperature Tg-20 ° C to a glass transition temperature Tg + 35 ° C is preferable. The temperature Tg-10 ° C to the glass transition temperature Tg + 25 ° C is more preferable.

In the above-described manner, the retardation film obtained by stretching the film after the longitudinal stretching in the transverse direction is subjected to a glass transition in order to prevent a decrease in the front retardation R ₀ and the thickness direction retardation Rth due to thermal relaxation. The temperature is lower than the temperature Tg, and is cooled and fixed at a temperature 5 ° C. or more lower than the transverse stretching temperature.

  Since the portion sandwiched between the tenter clips at both ends in the width direction of the laterally stretched film thus obtained is not stretched, both ends of the film including this portion are removed by slitting to obtain a desired width position. A phase difference film is obtained.

In addition, the measurement of the thickness variation in the length direction of the amorphous thermoplastic resin film and the retardation variation in the length direction of the retardation film after stretching was performed by measuring a range of 10 m in length at 50 mm intervals in the center of the film, The maximum value is obtained from the minimum value.
In addition, the measurement of the thickness variation in the length direction of the amorphous thermoplastic resin film and the retardation variation in the length direction of the retardation film after stretching is performed by measuring a range of 1 m in length at 1 mm intervals in the center of the film, The maximum value-minimum value in the 5 mm section is regarded as unevenness, and it is obtained from the maximum thickness unevenness in all sections.

  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.

A saturated norbornene-based resin (trade name “ZEONOR 1420” manufactured by Nippon Zeon Co., Ltd.) is used as an amorphous thermoplastic resin, which is supplied to a single screw extruder, kneaded and melted, and attached to the tip of the single screw extruder. Was subjected to melt extrusion to obtain a long film having an average thickness of 120 μm. The thickness variation in the length direction of the film was 0.8 μm, and the thickness unevenness was 0.15 μm. In addition, it was 135.5 degreeC when the glass transition temperature Tg of resin was measured with the differential scanning calorimeter (The Seiko Electronics Co., Ltd. make, brand name "DSC220C").
The obtained long roll-shaped thermoplastic saturated norbornene resin film was continuously unwound and stretched in the length direction (longitudinal direction) using an inter-roll neck-in stretching machine. The film is continuously heated to 145 ° C., stretched at this temperature so that the stretch ratio is 1.45 times, gradually cooled to 23 ° C. after stretching, and the longitudinally uniaxially stretched film is wound into a roll It was.
Next, the long roll-shaped longitudinally uniaxially stretched film was continuously unwound and stretched in the width direction (lateral direction) using a tenter stretching machine. The film is continuously heated to 145 ° C. and stretched at this temperature so that the stretching ratio is 2.60 times, and then gradually cooled to 23 ° C. and continuously wound up in a roll shape, and the longitudinal and transverse sequential stretching positions A phase difference film was obtained. The thickness of the obtained retardation film was 37.2 μm.

(Comparative Example 1)
A retardation film was obtained in the same manner as in Example 1 except that the thickness variation of the used film before stretching was 1.5 μm and the thickness unevenness was 0.3 μm. The thickness of the obtained retardation film was 37.3 μm.

(Comparative Example 2)
A retardation film was obtained in the same manner as in Comparative Example 1 except that the draw ratio of Comparative Example 1 was 1.60. The thickness of the obtained retardation film was 61.3 μm.

(Comparative Example 3)
A retardation film was obtained in the same manner as in Example 1 except that the transverse stretching was not performed. The thickness of the obtained retardation film was 99.7 μm.
The retardation of the retardation film obtained in the above Examples and Comparative Examples was measured using an automatic birefringence measuring apparatus (manufactured by Oji Scientific Instruments, trade name “KOBRA-21ADH”). The thickness of the film was measured using a film thickness measuring instrument (contact type thickness measuring instrument manufactured by Seiko EM, trade name “Millitron 1240”). The results are shown in Table 1.

It is a schematic diagram which shows the melt extrusion apparatus for manufacturing the amorphous thermoplastic resin film used for this invention method.

Explanation of symbols

1 Extruder 2 Mold 3 Film 4 Cooling Roll 5 Touch Roll 6 and 7 Roll

Claims (2)

A long amorphous thermoplastic resin film having an average thickness of 100 μm or more, a thickness variation in the length direction of 1 μm or less per 10 m, and a thickness unevenness of 5 μm in the length direction of 0.2 μm or less is represented by the following (1 ) To (5), the film is stretched 1.1 times or more in the length direction and then stretched twice or more in the width direction to produce a retardation film in which the slow axis substantially coincides with the width direction. Method.
(1) 0 nm ≦ R ₀ ≦ 200 nm
(2) 50 nm ≦ Rth ≦ 300 nm
(3) d ≦ 50 μm
(4) Film length direction R ₀ variation ≦ 10 nm / 10 m
(5) Film length direction R ₀ unevenness ≦ 1 nm / 5 mm
(However, R ₀ is retardation in the film plane, Rth is retardation in the film thickness direction, and d is the thickness of the film.)   The method for producing a retardation film according to claim 1, wherein the amorphous thermoplastic resin film is formed by a melt extrusion method.

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