JP2009092760A - Method for manufacturing retardation film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a retardation film made of noncrystalline thermoplastic resin and having excellent optical characteristics irrespective of thin film. <P>SOLUTION: In this manufacturing method, the noncrystalline thermoplastic resin film having a slow axis in the longitudinal direction of the long film and having specific thickness and an in-plane retardation value (Re) is used and extended in the transverse direction while contracting it in the longitudinal direction in the atmosphere having glass transition temperature of the resin film of Tg to Tg+25°C. <P>COPYRIGHT: (C)2009,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. One liquid crystal display device is a TN (twisted nematic) mode liquid crystal display device using an optical rotation 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 brightness 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.
For the retardation film, a synthetic resin film excellent in transparency and heat resistance, such as polycarbonate, 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 required to have an appropriate retardation value, particularly in recent years. As liquid crystal display devices typified by mobile phones and the like have become more compact and thinner, biaxial retardation films have been required to be thinner.

  The specific thickness of the biaxial retardation film used in those liquid crystal display devices is required to be 30 μm or less, and as the optimum retardation value, the in-plane retardation value Re is 100 to 160 nm, It is known that the film thickness direction retardation value Rth is 100 to 200 nm.

  As a method for producing the biaxial retardation film, the following Patent Document 1 proposes a stretching method in which a transverse stretching process using a tenter stretching machine or the like is performed. However, there is a problem that it may be difficult to achieve the desired relationship between Re and Rth only in the transverse stretching step.

On the other hand, Patent Document 2 below proposes a sequential biaxial stretching method in which a transverse stretching process using a tenter stretching machine or the like is performed after a longitudinal stretching process. According to this method, it is said that the Re variation in the width direction of the film is suppressed to be small in a wide range, but the draw ratio in the width direction is only 1.5 times, and the thickness of the obtained retardation film is also thick. Further, the retardation value Re is only less than 60 nm, and there is a problem that the use is limited.
JP 2001-215332 A JP 2002-148438 A

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

The method for producing a retardation film according to the present invention has a slow axis in the longitudinal direction of a long film, and substantially has a refractive index of nx> ny = nz (where nx and ny are main in-plane directions). A non-crystalline thermoplastic resin film having a thickness of 60 to 100 μm and an in-plane retardation value (Re) of 100 to 250 nm satisfying the relationship of refractive index, nz represents the main refractive index in the thickness direction) In the atmosphere of the glass transition temperature Tg to Tg + 25 ° C. of the resin film, the length is 15 to 30 μm, Re is 100 to 160 nm, and the film thickness direction retardation value (Rth) is 100 to 200 nm. The film is stretched by 2.0 to 4.0 times in the lateral direction while being contracted by 0.4 to 3.5% in the direction.

  According to the method for producing a retardation film of the present invention, although it is a thin film, it has a sufficient film in-plane retardation value Re and a film thickness direction retardation value Rth, and has a slow axis in the short direction of the film. It can be set as a retardation film. The obtained retardation film is sufficiently effective for improving the viewing angle of the VA mode or MVA mode liquid crystal display device and has a thickness of 30 μm or less. Can contribute to the reduction of film thickness.

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

Among the polymers listed above, a norbornene resin, which is a kind of cyclic olefin resin, is preferable because it has a small photoelastic coefficient and is excellent in stability of the retardation value 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 saturate these unsaturated bonds by hydrogenation from the viewpoint of emphasizing durability such as oxidative degradation due to thermal history and discoloration due to ultraviolet rays.

  The non-crystalline thermoplastic resin prevents deterioration of the non-crystalline thermoplastic resin during molding and improves the heat resistance, UV resistance, smoothness, etc. of the retardation film as long as it does not interfere with the retardation film function. For this purpose, various additives such as an antioxidant, a thermal deterioration inhibitor, an ultraviolet absorber, a lubricant, and an antistatic agent may be added. An additive may be used independently and 2 or more types may be used together.

In the present invention, the long film has a slow axis in the longitudinal direction, and is substantially nx> ny = nz (where nx and ny are the main refractive index in the film plane, and nz is the main refractive index in the thickness direction). It is necessary to use an amorphous thermoplastic resin film having a thickness of 60 to 100 μm and an in-plane retardation value (Re) of 100 to 250 nm that satisfies the relationship of (refractive index).
In order to obtain the amorphous thermoplastic resin film, it is necessary to first form a substantially non-oriented long film and then longitudinally stretch the film in the longitudinal direction.

  First, a conventionally used method is used as a method for forming a substantially unstretched amorphous thermoplastic resin film. 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 There is a melt extrusion method in which a thermoplastic resin is supplied to an extruder, melted and kneaded, and extruded from a mold attached to the tip of the extruder into a film to obtain a film. Any manufacturing method may be adopted as long as a substantially non-oriented film is obtained.

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 longitudinal 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 the long amorphous thermoplastic resin film unwound from the original film roll in the longitudinal direction, an inter-roll neck-in stretching method, a proximity roll stretching method, etc. can be applied. It is desirable to employ a roll-to-roll neck-in stretching method that has the advantage that the film is easily controlled and 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 retardation value desired to be imparted to the retardation film, but if low, the film may break during stretching. On the other hand, if it is high, it may be difficult to obtain a desired retardation value. Therefore, the glass transition temperature Tg-20 ° C to Tg + 50 ° C of the film is preferable, and the glass transition temperature Tg-10 ° C to Tg + 40 ° C is more preferable. preferable.

  In addition, if the draw ratio at this time is too small, the increase in Rth is restricted by the subsequent transverse stretching (stretching in the short direction), and therefore, it is preferably 1.1 times or more. On the other hand, if it is too large, it will be necessary to make the transverse stretching ratio very large in order to obtain the desired Re.

The longitudinally uniaxially stretched film obtained as described above is lower than the glass transition temperature Tg of the film and 5 ° C. or more lower than the stretching temperature in order to prevent a decrease in the in-plane retardation value Re due to thermal relaxation. It is cooled and fixed to the temperature and wound up in a roll or conveyed to the next process.
Thus, having a slow axis in the longitudinal direction of the long film and substantially satisfying the relationship of nx> ny = nz, having a thickness of 60 to 100 μm and an in-plane retardation value Re of 100 to 250 nm An amorphous thermoplastic resin film is obtained.

Next, the method of extending | stretching the obtained longitudinally uniaxially stretched film to the transversal direction (henceforth a horizontal direction) of a film is demonstrated in detail.
As a method of stretching the longitudinally uniaxially stretched film conveyed to the transverse stretching process in the lateral direction, both ends in the short 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 travels while gradually displacing in the direction of movement.

  FIG. 1 is a schematic configuration diagram showing an example of an apparatus for stretching an amorphous thermoplastic resin film in a transverse direction by a tenter stretching machine. As shown in FIG. 1, by gripping both lateral ends of the film with the tenter clip 1, first pass through the preheating zone 2, and then pass through the stretching zone 3 where the widening angle 5 of the tenter clip rail is widened, The film is stretched in the transverse direction. Thereafter, it is cooled and solidified by passing through the cooling zone 4. The solidified film is then wound up.

  If the temperature of the preheating zone 2 is low, the film may break in the next stretching zone 3, while if it is high, the retardation value in the longitudinal direction is too low, so that the glass transition temperature Tg-20 ° C to glass Transition temperature Tg + 60 ° C. is preferable, and glass transition temperature Tg−10 ° C. to glass transition temperature Tg + 40 ° C. is more preferable.

  Next, the temperature of the stretching zone 3 is appropriately adjusted depending on the retardation value desired to be imparted to the retardation film. If the temperature is low, the film may be broken during stretching, whereas if it is high, the desired in-plane retardation value is obtained. Since it may be difficult to obtain Re, the temperature is basically not higher than that in the longitudinal stretching step, and is preferably a glass transition temperature Tg−5 ° C. to a glass transition temperature Tg + 25 ° C., and a glass transition temperature Tg− A glass transition temperature Tg + 15 ° C. is more preferable.

  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 The axial accuracy cannot be ensured and the display quality may be deteriorated. On the contrary, if it is too high, the film is liable to break, so it is 2 times or more and 4 times or less, more preferably 3 times or less. Is done.

In the present invention, the film is contracted by 0.5 to 3.5% in the longitudinal direction when stretched in the transverse direction. Specifically, for example, using a tenter stretching machine, the tenter clip 1 that grips both ends of the film in the short direction is run at a constant speed on a tenter rail having a widening angle of θ. can do. At this time, the contraction amount in the longitudinal direction is expressed by the following equation.
Longitudinal shrinkage (%) = (1−cos θ) × 100
However, the amount of contraction in the longitudinal direction can be adjusted by appropriately setting the widening angle θ and the tenter clip travel speed. In that case, it is not always necessary to keep it constant from the start to the end of stretching.

If the amount of shrinkage in the longitudinal direction is too small, the stretch ratio becomes low and the desired film thickness and phase difference cannot be achieved, the stretch zone length becomes long and the equipment becomes unreasonable, or the input film width is small. The width of the resulting retardation film may not reach the desired width.
On the other hand, if the amount of shrinkage in the longitudinal direction is too large, the stretching speed increases and the film tends to break, or a large input film width is required, making it difficult to cope with the equipment.
Therefore, the amount of shrinkage in the longitudinal direction is preferably 0.4 to 3.5%, more preferably 0.5 to 3.0%.

  And the retardation film obtained by extending | stretching the film after completion | finish of longitudinal stretching in the horizontal direction in the above-mentioned way is for preventing the fall of the in-plane retardation value Re and thickness direction retardation value Rth by thermal relaxation. In addition, the cooling zone 4 is cooled and fixed at a temperature lower than the glass transition temperature Tg and 5 ° C. or more lower than the transverse stretching temperature.

  Since the portions sandwiched by the tenter clips 1 at both ends in the width direction of the laterally stretched film thus obtained are not stretched, the film both ends including this portion are removed by slitting, and a desired width is obtained. A retardation film is obtained.

  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 melt-extruded and wound up into a roll to obtain a long film having an average thickness of 90 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-based resin film was continuously unwound and stretched in the longitudinal direction (longitudinal direction) using an inter-roll neck-in stretching machine. The film is continuously heated to 150 ° C., stretched at this temperature so that the stretch ratio is 1.4 times, gradually cooled to 23 ° C. after stretching, and both ends of the film are slit and removed. A vertically uniaxially stretched film having a width of 1000 mm was wound into a roll.

  The in-plane retardation value Re of the obtained longitudinally stretched film was measured using an automatic birefringence measuring apparatus (trade name “KOBRA-21ADH” manufactured by Oji Scientific Instruments). 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”). As a result of measurement, Re was 165 nm and the thickness of the film was 75 μm.

  Next, the long roll-shaped longitudinally uniaxially stretched film was continuously unwound, and the film central portion 800 mm was stretched in the short direction (lateral direction) using a tenter stretching machine. The film was continuously heated to 145 ° C. and stretched so that the stretching ratio was 3.0 times and the longitudinal shrinkage was 0.85%. At this time, the widening angle of the tenter stretching machine was fixed at 7.5 degrees, and the traveling speed with respect to the traveling direction of the tenter clip was constant. Thereafter, the film was gradually cooled to 23 ° C. and continuously wound into a roll to obtain a longitudinally and transversely stretched retardation film. About the obtained retardation film, the retardation values Re and Rth, the optical axis deviation from the short direction, and the thickness were measured at intervals of 10 cm from the central part 1000 mm in the short direction. Re, Rth and thickness are average values, and the optical axis deviation is the maximum absolute value.

A longitudinally and transversely stretched retardation film was obtained in the same manner as in Example 1 except that the widening angle of the tenter stretching machine was 12 degrees and the longitudinal shrinkage was 2.2%. The results are shown in Table 1.
[Comparative Example 1]

The width of the longitudinally uniaxially stretched film is about half, the distance between the opposing tenter clips is 400 mm, the widening angle of the tenter stretching machine is 2.1 degrees (the same as the example of Patent Document 2), and the longitudinal shrinkage is 0. A longitudinal and transverse sequential stretch retardation film was obtained in the same manner as in Example 1 except that the content was 07%. The results are shown in Table 1. In addition, the travel distance of the tenter clip during transverse stretching required about 1.8 times that in the case of Example 1.
[Comparative Example 2]

  A longitudinally and transversely stretched retardation film was obtained in the same manner as in Example 1 except that the widening angle of the tenter stretching machine was 15 degrees and the longitudinal shrinkage was 3.5%. The results are shown in Table 1.

It is a schematic block diagram explaining the tenter drawing machine which can be used for this invention.

Explanation of symbols

1 Tenter Clip 2 Preheating Zone 3 Stretching Zone 4 Cooling Zone 5 Widening Angle θ

Claims (1)

The long film has a slow axis in the longitudinal direction, and the refractive index is substantially nx> ny = nz (where nx and ny are the main refractive index in the film plane, and nz is the main refractive in the thickness direction). A non-crystalline thermoplastic resin film having a thickness of 60 to 100 μm and an in-plane retardation value (Re) of 100 to 250 nm, and a thickness of 15 to 30 μm and a Re of 100 The resin film is shrunk by 0.4 to 3.5% in the longitudinal direction in an atmosphere of glass transition temperature Tg to Tg + 25 ° C. so that the retardation value (Rth) in the film thickness direction is 100 to 200 nm. A method for producing a retardation film, wherein the film is stretched 2.0 to 4.0 times in the short direction.

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