JPH1010321A - Optical film and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical film which is excellent in thickness accuracy and in optical quality, and low in equipment and running costs without requiring the use of a solvent and a process for producing the same. SOLUTION: The optical film used for a liquid crystal display element is <=20nm in the double refraction phase difference in the intra-surface direction measured with a light source of a wavelength 589nm when the film thickness is converted to 100μm equiv. In addition, the variation in the double refraction phase difference within the range usable as the image of the liquid crystal display element is <=±5nm regardless of the film thickness in the transverse direction and machine feed direction of the film. In addition, peeling patterns do not substantially exist on the film surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an optical film and a method for producing the same.

[0002]

2. Description of the Related Art A liquid crystal display device incorporating a retardation compensator (hereinafter simply referred to as "retarder") comprising a stretched optical film has been used. This retardation plate is usually formed by stretching a polymer film (for example, an unstretched film (including a sheet) such as a polycarbonate (PC) film or a polysulfone (PSf) film) as a raw material and orienting the film. A desired phase difference is obtained. Various methods have been proposed for producing an unstretched film used as a raw material of an optical film as follows. However, each of these conventional methods has disadvantages and is not always satisfactory.

(1) As a method for obtaining a film having good optical quality, there has been proposed a solvent casting method in which a resin is dissolved in a solvent, cast on an endless belt or a base film, dried, and then peeled off ( JP-A-4-301415
No.). However, according to this method, equipment costs and running costs are high, and the working environment tends to be poor.

(2) As a method using an extruder, there has been proposed a method in which a resin extruded from a T-die is pressed between rolls (JP-A-2-61899). But,
The film obtained by this method has uneven thickness, die lines, and gear marks, and has a large residual phase difference. Therefore, the quality of the film used for optical applications is not sufficient.

(3) In recent years, a method has been proposed in which a film-shaped resin extruded in a molten state from a T-die is pressed in an arc shape between a cast drum and an endless metal belt as a mirror-surface molding method of polypropylene (PP). (JP-A-6-17)
No. 0919). This is a manufacturing method in which a resin cast from a T-die is cooled and compressed into a sheet shape by a metal roll and a metal endless belt. However, this
Although a film having no uneven thickness, no die line, and no gear mark can be manufactured, the occurrence of the residual retardation cannot be sufficiently eliminated.

When the above film is stretched as a raw material to form a retardation film, the stretching treatment is difficult to be performed uniformly due to the remaining retardation, and it is necessary to sufficiently perform an annealing treatment as a pre-process of stretching. Therefore, a long preheating zone is required, and equipment costs and running costs increase.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical film and a method for manufacturing the same, which do not require the use of a solvent, have low equipment costs and running costs, are excellent in thickness accuracy and optical quality. It is in.

[0008]

Means for Solving the Problems The optical film of the present invention 1 (the invention of claim 1) is an optical film used for a liquid crystal display device. The in-plane birefringence phase difference (hereinafter simply referred to as “phase difference”) measured with a light source of 589 nm is 20 nm or less, and the variation of the phase difference within a range usable as an image of a liquid crystal display element is as follows. Regardless of the film thickness in the film width direction and the machine feeding direction, it is ± 5 nm or less, and substantially no peeling pattern is present on the film surface.

Since the retardation depends on the wavelength and the thickness of the film, in the present invention 1, the measurement wavelength is determined by the d-line spectrum (589 nm) of Na atom,
The phase difference in the in-plane direction when converted to the equivalent of 00 μm is 20 n
m. When the phase difference becomes large, unevenness in preheating during post-processing such as stretching (eg, unevenness of hot air) causes a phenomenon such as a change in stress relaxation at each temperature, and remains in the film immediately before stretching. Inconveniences such as uneven applied stress occur. The residual stress is caused by the orientation in the machine feeding direction (MD direction), but when performing transverse stretching in which the film is stretched in the film width direction, the orientation in the MD direction is changed to the film width direction (perpendicular to MD). In addition, the film must be rearranged, which not only imposes a burden on the film, but also makes it difficult to orient to the set retardation value.

In the first aspect of the present invention, “variation” refers to an average value of the phase difference of a film in a range actually used (for example, a portion actually displayed as a screen when used in a liquid crystal display) as a center value. It indicates how much the measured value of each point deviates from the center value, and indicates the maximum value of the deviation on the plus side and the minus side with respect to the center value. If dust or foreign matter is mixed during the measurement, the abnormal value shall be removed. Furthermore, when using a film whose end portion shows a significantly large or small value so that the center value is clearly changed, the average value at the end portion or a portion excluding a portion that can be regarded as a singular point is taken as the center value, and The target is also a measured value in the range excluding the end. And, even if the variation of the phase difference within the range that can be used as an image of the liquid crystal display element, regardless of the film thickness in the film width direction and the machine feeding direction, even after stretching the extruded raw material when it exceeds ± 5 nm as an actually measured value, The variation in stress is maintained in the stretched film.

If there is a variation in stress in the phase difference plate, optical distortion of the liquid crystal cannot be completely compensated, so that unintended color development is confirmed. The spots that should be red turn yellow and the spots that should turn blue turn green. In a black-and-white display liquid crystal, yellow or blue may be seen. In severe cases, it cannot be used. The phase difference is not only proportional to the degree of orientation in the film, but also proportional to the thickness. When the thickness is different with the same degree of orientation, the larger the thickness, the larger the phase difference. Therefore, it is necessary to make the thickness as equal as possible. For this reason, in order to perform optical compensation using a phase difference plate, it is necessary to obtain a phase difference equal to the set value as much as possible over the entire surface and to make the thickness equal.

In the present invention 1, the phrase "substantially no release pattern is present on the film surface" means that there is no release pattern generated on the film surface when the release from the cooling roll is uneven. Such a peeling pattern is a pattern that usually extends linearly in the width direction on the film surface, and as described above, is formed by uneven / unstable peeling, but is not caused by a thickness change or a material change. Although they are slightly observed visually, they are extremely difficult to quantitatively evaluate.

According to a second aspect of the present invention, there is provided a method of manufacturing an optical film, wherein a film-shaped thermoplastic resin extruded in a molten state from a T-die is cooled by a cooling roll and a plurality of pressure-controlled rolls. A method of manufacturing an optical film in which an arc-shaped narrow pressure is applied between an endless belt and a tension applied so as not to be loosened, and then the cooled resin film is peeled from a cooling roll by a peeling means. Of the two or more rolls to be applied, at least the distance relationship between the rotation axis of the most downstream roll that is narrowing the thermoplastic resin and the rotation axis of the cooling roll is represented by “radius of cooling roll + most downstream roll”. This method satisfies the requirement of “radius + thickness of endless belt ≦ distance between rotation axes ≦ radius of cooling roll + radius of the most downstream roll + thickness of endless belt + thickness of resin film”. If the distance between the rotation axes is out of the above range, the dispersion of the phase difference of the obtained resin film becomes large.

In the second aspect of the present invention, the thickness of the resin film is defined as the thickness of the thermoplastic resin on a line connecting the center of the rotation axis of the cooling roll for pressing the thermoplastic resin and the most downstream roll viewed from the longitudinal direction. Point to. Further, at that time, the thermoplastic resin has a thickness distribution in an equal width direction in which an end portion becomes thicker, and a thickness of a region showing the most uniform thickness in the width direction is defined as a resin film thickness.

The thickness of the resin film may be measured by using a commercially available β-ray thickness measuring instrument, and a mechanism capable of feeding back to a lip gap control method such as a heat block method, a lip heater method, or a robot method may be introduced. In the lip control based on human experience, the control is about ± 5%, and even a skilled worker has a limit of ± 3%. Therefore ±
In order to achieve 2% or less, it is easy to introduce an automatic control mechanism including measurement.

In the second aspect of the present invention, a metal roll having a high thermal conductivity and a high-precision mirror finish is used as the cooling roll. The material of the cooling drive roll that presses the endless belt may be metal, or if the peeling point of the endless belt from the thermoplastic resin is on a straight line in the width direction, the roll surface is soft like silicon rubber. The material may be used. In this case, if the silicon rubber layer is not deformed and does not recover, the gap between the endless belt and the cooling roll may be set to zero.

The molding speed is preferably 2 m / min or more. If the molding speed is lower than this, the motors that rotate the rolls do not rotate stably, which is not preferable.

In the method for producing an optical film according to the second aspect of the present invention,
Extruding a film-like thermoplastic resin in a molten state from a T-die,
The extruded film-like thermoplastic resin is cooled between a cooling roll whose temperature is adjusted to a temperature lower than the melting temperature of the thermoplastic resin and an endless belt that is tensioned so as not to be loosened by a plurality of pressure-controlled rolls. By narrowing the pressure in an arc shape, the thermoplastic resin can be cooled in a state where the stress is not left as much as possible, so that the phase difference can be reduced.

The distance relationship between the rotation axis of the most downstream roll that is narrowing the thermoplastic resin and the rotation axis of the cooling roll is described as “radius of cooling roll + radius of the most downstream roll + thickness of endless belt”. ≦ distance between rotating shafts ≦ radius of cooling roll + radius of the most downstream roll + thickness of endless belt + thickness of resin film ”, the pressure is sandwiched between the cooling roll and the most downstream roll via the endless belt. Since the squeezing pressure applied to the thermoplastic resin can be made uniform, the dispersion of the phase difference in the width direction and the machine feed direction can be reduced, and the position where the endless belt peels off from the thermoplastic resin surface is always kept. Since it can be on the straight line in the width direction at the lowermost stream, peel marks are drastically reduced.

As a result, an unstretched film (sheet) which can be used as a raw material for a retardation plate using an engineering plastic such as PC can be obtained. That is,
By improving the manufacturing method of forming a sheet by cooling the resin cast from a conventional T-die while narrowing it with a metal roll and a metal endless belt, the book as a high-precision phase difference plate can be produced at low cost. The optical film of the invention 1 can be obtained, and a high-precision retardation film can be obtained by stretching the raw material obtained by this method.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure of an apparatus used in the manufacturing method of the present invention will be described with reference to FIG. A thermoplastic resin 2 extruded into a film in a molten state from a T-die 1 is guided on a cooling roll 3 and cooled. In the initial state, the endless belt 9 is supported by the cooling drive roll 4 and the belt pulling roll 6, and the most downstream roll (hereinafter, referred to as a belt pressing roll) 7 for narrowing the pressure of the thermoplastic resin 2 includes the cooling drive roll 4 and the belt It is located almost at the midpoint of the pulling roll 6 and is not in contact with the endless belt 9. At this time, the cooling drive roll 4 is approached from above the cooling roll 3 to clamp the thermoplastic resin 2.

Next, the belt pressing roll 7 is driven, and the rotation axis of the belt pressing roll 7 and the cooling roll 3 are moved.
And the distance relationship between the rotation axes is expressed as “radius of cooling roll 3 +
Radius of belt pressing roll 7 + thickness of endless belt 9 ≦
By setting “the distance between the rotation axes ≦ the radius of the cooling roll 3 + the radius of the belt pressing roll 7 + the thickness of the endless belt 9 + the thickness of the resin film”, there is little variation in the phase difference in the width direction of the thermoplastic resin and the machine feeding direction. At the same time, the peeling pattern generated on the resin film surface is drastically reduced due to the uniformization of the peeling point, and an optical film substantially free of non-mirror patterns generated on the resin film surface is obtained.

The method of measuring the resin thickness between the cooling roll 3 and the cooling driving roll 4 is such that the position of the roll to be moved for clamping is measured in advance by a position sensor or the like. If the distance between the cores is gradually reduced while visually observing the resin, a change occurs in the surface state of the resin. The specularity of the film surface on the side where cooling is newly performed is improved, or an optical change when sandwiched between two polarizing plates can be confirmed. The resin thickness is determined from the value read from the position sensor at the moment when this change occurs. In order to further improve the measurement accuracy, the position sensor may be corrected by sandwiching several types of films whose film thickness is clearly known. Even if the sufficiently solidified film is crushed with a roll, it can be measured by this method, since it does not clearly change in comparison with the original dimensions when it is performed at room temperature.

[0024]

The present invention will be described in more detail with reference to examples.

Example 1 Polycarbonate (trade name "Panlite K-128")
5 ", Tg140 ° C, manufactured by Teijin Chemicals Ltd.) was dried in a dryer with a dehumidifier at 120 ° C for 5 hours, and then supplied to an extruder (single-screw extruder, full flight type) and extruded from a T-die. The extrusion temperature was set to a maximum temperature of 320 ° C.

The obtained molten resin was supplied between the cooling roll 3 and the endless belt 9 each controlled at 140 ° C. using the apparatus shown in FIG. The cooling roll 3 has a roll diameter of 90
A double-cell iron roll of 0 mm, surface length of 1000 mm and hard chromium plating, and a cooling rubber roll 4 used was a silicon rubber roll having a roll diameter of 900 mm and a surface length of 1000 mm. The endless belt is made of stainless steel with a belt width of 10
00 mm.

The molding speed was 20 m / min, and the driving at the time of squeezing was mainly a cooling drive roll 4, and the cooling roll 3 was a corotating rotation constituted by a pulley with a clutch.

Further, the belt pressing roll 7 has a diameter of 60.
0 mm, a surface length of 1000 mm, and hard chrome plating were used.

The distance between the axis of the cooling roll 3 and the cooling drive roll 4 = the radius of the cooling roll 3 + the thickness of the endless belt 9 + the radius of the cooling drive roll 4 + the rotation axis of the cooling roll 3 and the cooling drive roll 4. The thickness of the linear resin sheet connecting the axes = 450.0 + 1.0 + 450.0 + 0.8 (m
m) = 901.8 (mm), distance between the axial centers of the cooling roll 3 and the belt pressing roll 7 = radius of the cooling roll 3 + thickness of the endless belt 9 + radius of the belt pressing roll 7 + cooling roll 3 and the belt pressing roll Thickness of linear resin sheet connecting axes of rotation shafts of No. 7 = 450.0 + 1.0
+ 300.0 + 0.7 (mm) = 751.7 (mm) to prepare an optical film.

Embodiment 2 The belt pressing roll 7 is a silicon rubber roll, and the distance between the rotation axes of the cooling roll 3 and the belt pressing roll 7 = radius of the cooling roll 3 + thickness of the endless belt 9 + radius of the belt pressing roll 7 = 450. 0.0 + 1.0 + 300.
An optical film was prepared in the same manner as in Example 1 except that 0 (mm) was set to 751.0 (mm).

Comparative Example 1 Distance between the rotation axes of the cooling roll 3 and the belt pressing roll 7 = radius of the cooling roll 3 + thickness of the endless belt 9 + radius of the belt pressing roll 7 + 10 (mm) = 450.0 +
1.0 + 300.0 + 10 (mm) = 761.0 (m
An optical film was prepared in the same manner as in Example 1 except that m) was used.

Comparative Example 2 Distance between the rotation axes of the cooling roll 3 and the belt pressing roll 7 = radius of the cooling roll 3 + thickness of the endless belt 9 + radius of the belt pressing roll 7-0.5 (mm) = 450.0
+ 1.0 + 300.0-0.5 (mm) = 750.5
An optical film was prepared in the same manner as in Example 2 except that the thickness was changed to (mm).

Comparative Example 3 An optical film was prepared in the same manner as in Example 1 except that the take-off device was changed to a polishing roll (three) type and the following conditions were used. Cooling roll: Hard chrome plating, roll diameter 400 mm,
Surface length 900 mm Roll temperature: 150 ° C, 140 ° C, 130 ° C from upstream Forming speed: 20 mm / min

Comparative Example 4 Polycarbonate "Panlite K-1285" was converted to CH
₂ Cl ₂ , a resin concentration of 25% by weight, and a polyethylene terephthalate film (trade name “Grade OX”,
(Coating width: 700 mm, coating thickness: 300 μm), and drying temperatures of 40 ° C., 80 ° C., and 1 ° C.
The film was passed through a drying oven having a length of 2 m at 20 ° C. and a feeding speed of 1 m / min to obtain a film having a thickness of 75 μm after drying.

COMPARATIVE EXAMPLE 5 Three slit nozzles each having a width of 10 mm were arranged in parallel in the machine feed direction at 50 mm intervals on the opposite side to the resin sheet as viewed from the cooling roll 3, and the cooling roll 3 after the endless belt 9 was separated.
An optical film was prepared in the same manner as in Example 1, except that air was sprayed on the above resin at 0.5 second intervals.

Comparative Example 6 An optical film was prepared in the same manner as in Comparative Example 5, except that the slit nozzle was arranged in the sheet width direction and air was continuously blown.

Evaluation retardation The optical films obtained in Examples 1 and 2 and Comparative Examples 1 to 6 were manufactured by Otsuka Electronics Co., Ltd.
The phase difference was measured by "D series", the film thickness was measured with a micrometer (manufactured by Tokyo Seimitsu Co., Ltd.), and the phase difference was converted to a value of 100 μm per film thickness. In addition, the measurement was 10
The measurement was performed at mm intervals.

Peeled Pattern The presence or absence and strength of the peeled pattern of the optical films obtained in each of Examples and Comparative Examples were visually evaluated according to the following criteria. [Judgment Criteria] ○: No peeling pattern △: Weak peeling pattern ×: Strong peeling pattern

[0039]

[Table 1]

[0040]

As described above, the optical film of the present invention 1 has a small birefringence retardation and its variation and has substantially no peeling pattern on its surface. Used to form a good film. Further, the method for producing an optical film of the second aspect of the present invention has a lower equipment cost and a lower running cost than the solvent casting method, has a small birefringence retardation and its variation, and has substantially no peeling pattern. An optical film can be obtained.

[0041]

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a manufacturing method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 T die 2 Resin 3 Cooling roll 4 Cooling drive roll 5 Peeling roll 6 Belt pulling roll 7 Belt pressing roll 8 Roll 9 Endless belt

Claims (2)

[Claims] 1. An optical film used for a liquid crystal display device, wherein when the film thickness is converted to a value equivalent to 100 μm, the in-plane birefringence phase difference measured by a light source having a wavelength of 589 nm is 20 nm or less; In addition, the variation of the birefringence phase difference within a range usable as an image of the liquid crystal display element is ± 5 nm or less regardless of the film thickness in the film width direction and the machine feeding direction, and a peeling pattern is substantially formed on the film surface. An optical film characterized by the fact that it does not exist. 2. A film-shaped thermoplastic resin extruded in a molten state from a T-die is formed into an arc shape between a cooling roll and an endless belt tensioned by a plurality of pressure-controlled rolls so as not to be loosened. A method for producing an optical film in which a cooled resin film is peeled from a cooling roll by a peeling means, wherein at least a thermoplastic resin is narrowed among two or more rolls for applying tension to an endless belt. The distance relationship between the rotation axis of the most downstream roll and the rotation axis of the above-mentioned cooling roll is expressed as “radius of cooling roll + radius of most downstream roll + thickness of endless belt ≦ distance between rotation axes ≦ radius of cooling roll”. A method for producing an optical film, which satisfies the requirement of “+ the most downstream roll radius + the thickness of the endless belt + the thickness of the resin film”.