JP2003075634A - Method for manufacturing optical film, and optical film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical film without impairing the high contrast characteristics of a liquid crystal display device. SOLUTION: The retardation of the film can be decreased, then the optical anisotropy, which impairs the high contrast characteristics of a liquid crystal display device, can be decreased by applying a magnetic field on the film material in the process of manufacturing the optical film consisting of cellulose acetate by a flow cast method, or by applying a magnetic field on the optical film consisting of cellulose acetate manufactured by a flow cast method while heating the film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is useful for constructing an elliptically polarizing film when used together with an optical protective film, particularly a protective film for a polarizing film used in a liquid crystal display device or a retardation film. The present invention relates to a method for producing an optical film, a method for treating the optical film, an optical film obtained by these methods, a polarizing film obtained by using the optical film, and an elliptically polarizing film.

[0002]

2. Description of the Related Art In recent years, polarizing films have been used in liquid crystal display devices for which demand is expanding. The polarizing film is
A polarizing element is formed by adsorbing and orienting iodine and a dichroic dye on polyvinyl alcohol, and the polarizing element is covered with a protective film. The protective film used for such a purpose is preferably made of a material having a small birefringence and a small optical anisotropy (hereinafter simply referred to as anisotropy). For this reason, a cellulose triacetate film having a smaller optical anisotropy than that of polyester or polycarbonate is used as a protective film. Further, a cellulose acetate film formed by a solution casting method has been used as a protective film because it can form a thin film having excellent flatness.

However, even a cellulose triacetate film having a small optical anisotropy and excellent flatness,
Since the surface orientation of the raw material occurs in the thickness direction of the film in the drying step during the production by the solution casting method, the retardation value (RTH) due to this contribution increases, and therefore,
When this cellulose triacetate film is used in a liquid crystal display device, it is possible to avoid influencing the visual characteristics of a high contrast liquid crystal display device such as a TFT type or gradation display FSTN type liquid crystal display device that cannot be ignored. Can not.

When the optical film is produced by the solution casting method, the film is oriented in the plane because shrinking stress is generated in the lengthwise and widthwise directions of the film during the drying process. In order to eliminate the effect of even a little, pulling tension is applied in the longitudinal direction so as to cancel the contraction stress, and the both ends in the width direction are gripped so that the material does not contract. Further, in order to improve the productivity of the film, it is necessary to shorten the drying time of the solution casting method when trying to increase the film-forming line speed of the cellulose acetate ester film. This is achieved, for example, by increasing the drying temperature to speed up the evaporation of the solvent. However, when the evaporation speed of the solvent is increased, the flatness of the film is impaired and, at the same time, a remarkable plane orientation in the thickness direction of the film is generated in the drying step, so that the retardation value (RTH) due to this contribution becomes large.

On the other hand, there is a demand for a polarizing film and an elliptically polarizing film which can be applied without substantially impairing the high contrast of the liquid crystal cell. In order to achieve this purpose, it is thought that drying in a state where no contraction stress is applied in the drying step of the solution casting method, or a force that structurally cancels this even if stress is applied may be applied. Further, it is conceivable to treat the film-formed cellulose acetate ester film from the outside so as to reduce the retardation value (RTH), but with the cellulose acetate ester film, the control is not easy and the production suitability is sufficient. Not really.

[0006]

The problem to be solved by the present invention is to manufacture an optical film which is excellent in productivity without substantially impairing the high contrast characteristics of a liquid crystal display device which realizes high contrast. The object is to provide a method, a processing method, an optical film obtained by these methods, a polarizing film formed by using the optical film, and an elliptical polarizing film.

[0007]

The present inventor has found that when a magnetic field is applied to a film in the step of producing an optical film made of cellulose acetate by a solution casting method, or when an optical film produced by a solution casting method is used. It has been found that the optical anisotropy of the optical film can be controlled by applying a temperature and a magnetic field to the film for use, and the present invention is to solve the above problems by utilizing this principle.

A first means for solving the problems of the present invention is to remove the acetic acid cellulose ester dope solution from a casting machine onto a casting support and solidify it, then peel it from the casting support and dry it to remove acetic acid. In a method for producing an optical film made of cellulose ester, solidification, peeling of the film,
An optical film for controlling the optical anisotropy of the film by applying a magnetic field to the film surface at a magnetic flux density of 0.002 T (tesla) or more and 7.0 T or less in at least one of the steps of drying. In the manufacturing method.

In the method for producing an optical film of the present invention, the magnetic flux density applied to the surface of the film is 0.00
It can be 2T or more and 0.1T or less.

The second means for solving the problems of the present invention is to provide an optical film (Tg-30) made of cellulose acetate ester.
(° C) or higher and a heat treatment temperature of Tg or lower, a magnetic field is applied to the surface of the film at a magnetic flux density of 0.002 T (tesla) or higher and 7.0 T or lower, and the film is treated for 30 minutes or more and 500 hours or less for optical use A method for controlling the anisotropy of an optical film, which comprises controlling the optical anisotropy of a film (where Tg represents the glass transition temperature (° C) of the optical film of cellulose acetate ester). It is in.

In the method for controlling the anisotropy of the optical film according to the second means for solving the problems of the present invention, the magnetic flux density is 0.
It can be set to 002T or more and 0.1T or less, and particularly to 0.
It is preferable to set it to 002T or more and 0.02T or less.

A third means for solving the problems of the present invention is to provide an optical film made of cellulose acetate with 0.002 T (tesla) on the surface of the optical film at a heat treatment temperature below Tg (Tm-10 ° C.). ) The above is for controlling the anisotropy of the optical film by controlling the anisotropy of the optical film by applying a magnetic field with a magnetic flux density of 7.0 T or less for 30 seconds or more and 30 minutes or less. Method (where
Tg represents the glass transition temperature (° C) of the optical film of cellulose acetate ester, and Tm represents its melting point (° C). )It is in.

In the optical film anisotropy control method according to the third means for solving the problems of the present invention, the magnetic flux density is 0.
It can be set to 002T or more and 0.1T or less, and in particular, 0.
The heat treatment time is preferably 002T or more and 0.02T or less, and the heat treatment time is preferably 90 seconds or less.

A fourth problem solving means of the present invention is an optical film manufactured or processed by any one of the first to third problem solving means, and is defined by the following (formula i). Retardation value (RTH) is 0 nm <RTH ≦ 5
It is in an optical film characterized by being in the range of 0 nm. RTH = | {(nMD + nTD) / 2−nTH} | × d (Formula i) [where nMD is the film refractive index in the direction parallel to the casting direction, and nTD is the film refractive index in the direction perpendicular to the casting direction. rate,
nTH is the film refractive index in the thickness direction, and d is the film thickness (nm). ]

A fifth problem solving means of the present invention comprises a polarizing element and a protective film coated on at least one of the polarizing elements, and the protective film is any one of the first to third problem solving means. A polarizing film, which is an optical film produced or treated by the method described in 1.

A sixth means for solving the problems of the present invention comprises a polarizing element and a protective film coated on at least one of the polarizing elements. This protective film is an optical film according to the fourth means for solving the problems. It is in a polarizing film characterized by that.

A seventh means for solving the problems of the present invention is characterized by comprising the polarizing film according to the fifth or the sixth means for solving the problems and a retardation film laminated on the polarizing film. Located on an elliptically polarized film.

First, the retardation value (RTH) represented by the following (formula i) will be explained in relation to the cellulose acetate ester film of the present invention. RTH = | {(nMD + nTD) / 2-nTH} | × d (formula i) [where nMD is the film refractive index in the direction parallel to the casting direction, and nTD is the film refractive index in the direction perpendicular to the casting direction. rate,
nTH is the film refractive index in the thickness direction, and d is the film thickness (nm). ]

In general, when light is incident on a substance having optically uniaxial anisotropy, two linearly polarized lights having different phase velocities propagate, and two refracted lights appear. Such a phenomenon is called birefringence, and the magnitude of birefringence depends on the difference between the refractive index (np) in the direction parallel to the optical axis and the refractive index (nv) in the direction perpendicular to the optical axis. Δn = np-nv (formula ii) is called birefringence. The following (equation iii) Re = Δn × d (equation iii), which is the product of this birefringence (Δn) and thickness (d), is called the retardation value (Re), which represents the optical anisotropy of the substance. Used as an index.

Generally, as the retardation value of a film, as shown in FIG. 1, the absolute value of the difference between the two refractive indexes orthogonal to each other in the film surface when measured from the direction perpendicular to the film surface (hereinafter referred to as "front surface"). It is expressed as the following (formula iv) based on "birefringence". Hereinafter, this is referred to as the front retardation value (RMT). RMT = | nMD-nTD | × d (formula iv)

Among cellulose acetate ester films, cellulose triacetate film is known to have an extremely low front retardation value,
There is great demand for a polarizing plate protective film used in a liquid crystal display device. However, when used as a polarizing plate of a liquid crystal display device, not only the optical anisotropy in the direction perpendicular to the display surface of the liquid crystal display device but also the optical anisotropy at an arbitrary angle with respect to the display surface is considered. There is a need. In this case, in addition to the two refractive indexes (nMD, nTD) orthogonal to each other in the film plane, it is necessary to consider the contribution of the refractive index (nTH) in the direction perpendicular to the film plane. For this reason, the optical anisotropy of the film needs to take into consideration the retardation value (RTH) shown in (Formula i) (hereinafter referred to as "thickness direction retardation").

As described above, the retardation value (RTH) is as defined by the equation (i), and in order to reduce the retardation value (RTH), the refractive index term | {(nMD + nTD ) / 2-nTH} | and the film thickness d must both be reduced. In order to maintain the mechanical strength of the cellulose acetate ester film, it is necessary to increase the film thickness, but there is a limit naturally due to the demand for film thinning. Therefore, to reduce the retardation value (RTH), | {(nMD
It is necessary to minimize + nTD) / 2-nTH} | to minimize the optical anisotropy.

As described above, in order to increase the film-forming line speed of the casting method in order to improve the productivity of the film, the drying time of the solution casting method must be shortened. This means, for example, reducing the amount of solvent,
It is achieved by drying with a high-concentration dope cast or by increasing the drying temperature to speed up the evaporation of the solvent.
As already mentioned, when the evaporation speed of the solvent is increased, the flatness is impaired, and at the same time, a remarkable plane orientation occurs in the film thickness direction during the drying process, resulting in a retardation value (RT
Since H) becomes large, the high contrast performance of the liquid crystal cell is impaired. Therefore, there is a trade-off between improving productivity by speeding up the film production line and minimizing the retardation value (RTH), and it is difficult to achieve both at the same time.

The means for solving the above-mentioned problems of the present invention reduces the retardation value (RTH) by appropriately controlling the optical anisotropy of the film even when an optical film made of cellulose acetate is formed at a high line speed. There is something you can do. As described above, according to the present invention, 0.002T (Tesla) is applied to the film surface during at least one of solidification, peeling and drying of the film in the process of producing a film made of cellulose acetate by the solution casting method.
A magnetic field is applied with a magnetic flux density of 7.0 T or less to control the optical anisotropy of the film, or a similar magnetic flux density is applied to the film surface while heating an optical film made of cellulose acetate having optical anisotropy. The magnetic anisotropy is applied to control the optical anisotropy.

As shown in FIG. 2, generally in the film formation by the solution casting method, the film thickness becomes 1/5 to 1/10 of the original casting as the solvent is removed in the drying step,
The orientation of the polymer orthogonal to the thickness direction of the film progresses, and the refractive index (nTH) in the thickness direction becomes as small as (n'TH). Therefore, the retardation value (RTH) in the thickness direction becomes large. The retardation value (RTH) of a commercially available cellulose triacetate film prepared by a usual film forming method is 6 although it depends on the kind of solvent used during film formation.
0 ~ 150nm, Polycarbonate film 220n
There is also m. On the other hand, the retardation value (RTH) of the cellulose acetate film produced or treated by the method of the present invention can be appropriately controlled by applying a magnetic field or a heating magnetic field, and its absolute value is 0 to 50 n.
It is possible to obtain an optical film having a small value of m, and thus having a very small optical anisotropy. Moreover, the absolute value of the difference between the in-plane retardation value represented by (Formula iv) and the thickness direction retardation value represented by (Formula i) is 0 to 50 nm, and an optical film having little optical anisotropy as a whole film is obtained. be able to.

The cellulose acetate used in the present invention is not particularly limited, and those commonly used such as cellulose nitrate ester and cellulose acetate ester can be used. Among these, cellulose triacetate of cellulose acetate type is preferably used as the polarizing plate of the liquid crystal display plate.

Cellulose acetate is generally produced through a two-step reaction of esterification of cellulose and hydrolysis of the produced ester, and high-purity wood pulp or cotton linter is used as the raw material cellulose. Acetic anhydride to cellulose, acetic acid and sulfuric acid as a catalyst to carry out an acetic acid esterification reaction to produce cellulose triacetate in which almost all hydroxyl groups are esterified, and subsequently in the aging step, water is added to the produced cellulose triacetate, It has been practiced to partially hydrolyze the ester group to obtain a cellulose acetate having a desired degree of acetylation. Water is added to the solution that has undergone the aging step to precipitate and decompose the cellulose acetate, the residual solvent and impurities are removed in the next washing step, and the cellulose acetate is obtained by drying. Cellulose acetate having different properties is obtained depending on the degree of esterification (degree of substitution), and the degree of substitution is represented by an index of acetylation degree.
Cellulose acetate, which is widely used industrially, is roughly classified into two types, cellulose diacetate and cellulose triacetate, and the acetylation degree of each is about 55% (degree of substitution 2.4) and 61% (degree of substitution). 2.
9). As the cellulose acetate ester film used for the optical film, any commercially available product can be used, and cellulose ester having a single average acetylation degree (average substitution degree) may be used alone, or the acetylation degree ( A plurality of cellulose esters having different degrees of substitution) may be contained.
The degree of acetylation is not particularly limited, but the degree of acetylation is about 55%.
A film having a degree of substitution of 2.4 to 61% (degree of substitution of 2.9) is suitable for an optical film.

In order to improve the mechanical properties of the formed film or to increase the solvent drying rate, the film raw material is provided with a plasticizer such as triphenyl phosphate (TPP),
Phosphate ester compounds such as tricresyl phosphate (TCP), dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DBP),
Phthalic acid ester compounds such as dioctyl phthalate (DOP) and diethylhexyl phthalate (DEHP), fatty acid ester compounds such as butyl oleate, methylacetyl ricinoleate and dibutyl sebacate, acetyltriethyl citrate (OACTE), acetyltributyl citrate ( A citric acid ester compound such as OACTB) or various trimellitic acid ester compounds may be added.

Further, the cellulose acetate ester film has excellent properties such as high light transmittance, but has no property of absorbing ultraviolet rays, so that the liquid crystal of the liquid crystal display device is prevented from deterioration by ultraviolet rays. It is preferable to add an ultraviolet absorber to the film. In a preferred embodiment of the present invention, the compound represented by the following general formula (1) is preferably contained in the film in an amount of 0.1 to 10% by weight as an ultraviolet absorber, but the present invention is not limited thereto. Not a thing.

[0030]

[Formula i]

The compound having the general formula (1) not only has the effect of preventing ultraviolet rays, but also improves the slipperiness of the film surface. Slidability is required when a polarizing film is manufactured from a polarizing element and a protective film, the film is saponified (hydrophilic treatment), the polarizing element and the protective film are bonded with an adhesive, and the protective film is applied onto the protective film. Hard coat coating process, and transport work for performing these processes.If the scratch resistance of the film surface is not sufficient in these processes and work, the protective film surface may be scratched during the work. Therefore, the liquid crystal display device in which the polarizing film using such a protective film is incorporated shows a fatal display defect, and the slipperiness of the film prevents the surface of the film from being scratched. Required.

In addition to the above compounds, inorganic or organic fine particles may be contained in order to impart slipperiness. Examples of inorganic compounds include silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, calcium carbonate,
There are talc, clay, calcined kaolin, calcined calcium silicate, hydrated calcium silicate, aluminum silicate, magnesium silicate and calcium phosphate. Examples of organic compounds (polymers) include silicone resins, fluororesins, and acrylic resins.

In the general formula (1), R 1 is generally a hydrogen atom, an alkyl group having 3 to 5 carbon atoms or an α-dimethylbenzyl group, and a hydrogen atom or an alkyl group having 4 or 5 carbon atoms. Are preferred, especially hydrogen atom or t
-Butyl groups are more preferred, and hydrogen atoms are most preferred. R2 is generally a methyl group, an alkyl group having 4 or 5 carbon atoms or an octaloyloxyethyl group, and preferably a methyl group or an alkyl group having 4 or 5 carbon atoms, particularly a methyl group or t-butyl. More preferred are groups, with t-butyl being most preferred. As X,
A hydrogen atom or a chlorine atom is preferable, and a hydrogen atom is particularly preferable.

The cellulose acetate ester film containing such an ultraviolet absorber preferably exhibits a transmittance of light having a wavelength of 400 nm of 65% or more, and particularly 80
% Or more is preferable. The transmittance of light having a wavelength of 370 nm is preferably 3% or less, and particularly preferably 1% or less. Further, in order to improve the storability of the molded film, it is preferable to contain a deterioration inhibitor in the film. Specifically, it is a peroxide decomposer, a radical inhibitor, a metal deactivator, an acid scavenger or the like.

[0035]

BEST MODE FOR CARRYING OUT THE INVENTION A method for producing an optical film for controlling film anisotropy in the present invention, a method for treating the optical film, and an optical film obtained by these methods will be described below. Are not necessarily limited to these embodiments.

The first embodiment of the present invention is a method for producing an optical film for controlling film anisotropy. This method comprises converting cellulose acetate acetate into methylene chloride,
Chloroform and other chlorine-based hydrocarbons, acetone, methyl ethyl ketone and other ketones, methyl acetate, ethyl formate and other esters, 1,4-dioxane, 1,3
-Dissolved in a solvent in which dioxolane and other ethers, methanol, ethanol and other lower alcohols are arbitrarily mixed, and the cellulose acetate and the solid content such as a plasticizer, an ultraviolet absorber, a slip agent, and a deterioration inhibitor. Was prepared so that the total weight thereof was 18 to 35 wt% in the dope to give a cellulose acetate solution, and the solution was cast on a casting support, for example, a drum or a band, which was finished in a mirror state by a casting machine. After extrusion and solidification, the cellulose acetate ester film is peeled from the casting support and dried to form a cellulose acetate film, and the film surface can be formed over any or all of the steps of solidification, peeling and drying after the casting. In addition, the optical anisotropy of the film is controlled by applying a magnetic field with a magnetic flux density of 0.002 T (tesla) or more and 7.0 T or less.

The strength of the magnetic field applied to the surface of the film depends on the temperature and time of heat treatment, but the maximum magnetic flux density is about 7.0 T (Tesla), preferably 0.0.
02-7.0T (Tesla), most preferably 0.00
It is 2 T or more and 0.1 T (tesla) or less, and the processing time is about 30 seconds to 30 minutes. The magnetic field may be applied at any stage of the manufacturing process such as solidification, peeling and drying after the casting. That is, the voltage may be applied in all steps or in a part of the steps, and the combination thereof is arbitrary. The direction in which the magnetic field is applied may be perpendicular to the film surface, or may be oblique or nearly horizontal, but the optical anisotropy of the cellulose acetate ester film is appropriately adjusted as a result of applying the magnetic field. It goes without saying that the adjustment is made so that it can be controlled within the range. However, in the casting method for molding the cellulose acetate ester film,
For ease of manufacture, it is desirable that the magnetic field lines be perpendicular to the film surface in the direction in which the magnetic field is applied. The reason why the magnetic flux density is preferably 0.1 T (tesla) or less is 0.1
Retardation value (RTH) even if it exceeds T (Tesla)
However, the degree of decrease is smaller than the degree of increase in the magnitude of the magnetic flux density.

Regarding the casting and drying methods in the solvent casting method, US Pat. No. 2,336,310 is used.
No. 2367603, No. 2492078, No. 24
92977, 2492978, 2607704.
No. 2739069, No. 2739070, British Patent No. 640731, No. 736892, Japanese Patent Publication Nos. 45-4554, 49-5614, and JP-A-60-.
No. 176834, No. 60-203430, No. 62-1
It is described in detail in each publication of No. 15035. Further, the thickness of the molded film in the present invention is 5 to 500 μ.
m, preferably 20 to 200 μm, more preferably 60 to 120 μm. This range is preferable in terms of transparency and film strength.

The second embodiment of the present invention is a method for treating an optical film made of a cellulose acetate produced by a casting method, which is an optical film (Tg-30 ° C.). As described above, while heating at a heat treatment temperature of Tg or less, a magnetic field is applied to the surface of the film with a magnetic flux density of 0.002 T (tesla) or more and 7.0 T or less, and the film is anisotropic for 30 minutes to 500 hours and is anisotropic. Is a method of controlling.

The strength of the magnetic field applied to the surface of the film depends on the temperature and time of heat treatment, but the magnitude of the magnetic flux density is 0.002 to 7.0 T (tesla), preferably
0.002T or more and 0.1T (tesla) or less, most preferably 0.002 to 0.02T (tesla), and the heat treatment time depends on the heat treatment temperature and the applied magnetic flux density, but is 30 minutes or more. 500 hours or less. The object can be achieved even if the heat treatment time is 500 hours or more, but the effect converges when the heat treatment time at the industrial level is 500 hours, and there is no effect even if the heat treatment is performed over 500 hours. . When heat treatment is applied to a roll of cellulose acetate ester film while applying a magnetic field, magnetic force lines are not applied to each part of the roll surface at a constant angle, so it is difficult to control the film anisotropy of each part of the film to the same level, Not preferable. The magnitude of magnetic flux density is 0.002T
The reason why 0.1 T (tesla) or less, particularly 0.02 T or less is preferable is that the retardation value (RTH) is reduced even when it exceeds 0.02 T (tesla). This is because it is small compared to the degree of increase in size.

In the second embodiment, the optical film made of cellulose acetate may be applied from the roll while being heated and a magnetic field may be applied to the surface of the film. However, the optical film made of cellulose acetate is used. It is also possible to perform processing by applying a magnetic field to the surface of the film while heating the sheet while keeping it stationary or moving it. The direction of applying the magnetic field to the surface of the film may be perpendicular to the surface of the film, or may be oblique or nearly horizontal, but as a result of applying the magnetic field, the optical properties of the cellulose acetate ester film may be increased. It goes without saying that the anisotropy is adjusted so that it can be controlled within an appropriate range.

The third embodiment of the present invention is also a method for treating an optical film made of a cellulose acetate produced by a casting method, the optical film exceeding the Tg (Tm- While heating at a heat treatment temperature of 10 ° C. or less, a magnetic field is applied to the surface of the film with a magnetic flux density of 0.002 T (tesla) or more and 7.0 T (tesla) or less, and the film is processed for 30 seconds or more and 30 minutes or less. This is a method of controlling optical anisotropy.

The strength of the magnetic field applied to the surface of the film depends on the heat treatment temperature and time, but the magnitude of the magnetic flux density is
0.002-7.0T (Tesla), preferably 0.0
02T or more and 0.1T (tesla) or less, and most preferably 0.002T or more and 0.02T (tesla) or less. The heat treatment time is 30 seconds or more and 30 minutes or less.
The object can be achieved even if the heat treatment time exceeds 30 minutes, but the effect converges when the heat treatment time at the industrial level is 30 minutes, and even if the heat treatment exceeds 30 minutes, the effect is not so great. Absent. The most preferable heat treatment time is 9
It is 0 second or less, and the reason is that the reduction of the retardation value (RTH) is large when the time is 90 seconds or less, but the reduction rate decreases when the time exceeds 90 seconds. When heat treatment is applied to a roll of cellulose acetate ester film while applying a magnetic field, magnetic field lines are not applied to each part of the roll surface at a constant angle in the same manner as above, so that the optical anisotropy of each part of the film surface is at the same level. However, there is a problem that the heat transfer from the outer layer to the inner layer of the roll is delayed. This causes a problem that the inside of the roll of the long film cannot reach the target temperature in a short time of 30 seconds or more and 30 minutes or less. Further, since the end portion of the film is not gripped, the heat treatment temperature is sufficiently higher than the glass transition temperature, so that the film is softened and the shape of the film is destroyed, which is not preferable.

In the third embodiment, the optical film is manufactured by applying a magnetic field to the surface of the film while heating the optical film while feeding it out from a roll, or in a state in which a sheet made of an optical film is left stationary or moved. It is preferable to apply a magnetic field to the surface of the film while heating the film while it is being manufactured. When applying a magnetic field to the film surface while heating the optical film in a stationary state or in a moved state, it is preferable to carry out while grasping both ends of the film sheet. The direction of applying the magnetic field to the surface of the film may be perpendicular to the surface of the film, or may be oblique or nearly horizontal, but as a result of applying the magnetic field, the optical properties of the cellulose acetate ester film may be increased. It goes without saying that the anisotropy is adjusted so that it can be controlled within an appropriate range.

As a fourth embodiment, the present invention is directed to the optical film obtained by the above-mentioned first to third embodiments. This optical film has the following (formula i). Retardation value (RTH) defined by is 0
An optical film made of cellulose acetate having a thickness in the range of nm <RTH ≦ 50 nm. RTH = | {(nMD + nTD) / 2−nTH} | × d (formula i) [nMD is the film refractive index in the direction parallel to the casting direction, n
TD is the film refractive index in the direction perpendicular to the casting direction, nTH is the film refractive index in the thickness direction, and d is the film thickness (n
m). The cellulose acetate ester film having a thickness direction retardation value (RTH) in the range of (formula i) exhibits extremely excellent high contrast performance of a liquid crystal cell when used in a polarizing plate of a liquid crystal display device.

The magnetic field generating means may be a permanent magnet as well as a coil represented by a solenoid coil.
Further, it may be a means for generating a magnetic field intermittently (pulse magnetic field generating means).

The method for evaluating the physical properties of the optical film made of cellulose acetate according to the present invention will be described below.

The glass transition temperature (Tg) of the cellulose acetate ester film was measured by using a differential scanning calorimeter (DSC), and when 10 mg of the sample film was heated in a nitrogen stream at a heating rate of 10 ° C./min, the base temperature was measured. The arithmetic mean of the temperature that begins to be eccentric from the line and the temperature that returns to the new baseline,
Alternatively, when an endothermic peak appears at Tg, the temperature at which this endothermic peak has a maximum value is defined as Tg (° C).

The melting point (Tm) of the cellulose acetate ester film is 10 mg of the sample film using a differential scanning calorimeter (DSC) as in the measurement of the glass transition temperature (Tg).
When the temperature was raised in a nitrogen stream at a heating rate of 10 ° C / min,
The temperature showing the maximum value of the endothermic peak of melting is defined as Tm (° C).

[0050]

EXAMPLES Some examples of the present invention will be described below, but the present invention is not limited thereto. (Examples 1 to 5, Comparative Examples 1 and 2) The following dope compositions were prepared. Cellulose triacetate (acetylation degree 61.0%) 100 parts by weight Triphenyl phosphate 9 parts by weight 2- (2'-hydroxy-3 ', 5'-di-t-butylphenyl) benzotriazole 8 parts by weight Methylene chloride 430 parts by weight Parts Methanol 90 parts by weight This composition was placed in a closed container, kept at 80 ° C. under pressure and completely dissolved while stirring.

The dope is then filtered, cooled to 23 ° C.
, The film is uniformly cast on a stainless steel band to form a film, and magnetic fields of various magnetic flux densities [unit: T (Tesla)] shown in Table 1 are perpendicular to the surface of the film. The solvent was evaporated and the film was peeled from the stainless band while it was allowed to peel from the top for 3 minutes. Then, the drying was completed while being conveyed by a number of rolls to obtain a cellulose triacetate film having a film thickness of 80 μm. The glass transition temperature (Tg) and melting point (Tm) of this film are 151 ° C. and 2 respectively.
It was 87 ° C.

Table 1 shows the retardation values of the films formed according to the examples and comparative examples. Here, Comparative Example 1 is exactly the same as Examples 1-5 and Comparative Example 2 except that no magnetic field is applied. In Table 1, RMT represents the film front retardation value, and RTH represents the retardation value in the film thickness direction. Retardation value is ellipsometer (AEP of Shimadzu Corporation-
100). The retardation value (RTH) in the film thickness direction was determined by calculating nTH from the measurement value of the angle dependence of the retardation value.

[0053]

[Table 1]

The cellulose triacetate film produced by the above method is punched out to form two films, a polyurethane adhesive is applied to one side of each of these films, and the films are treated with polyvinyl alcohol and a dichroic dye. A polarizing film was completed by adhering it to both sides of a polarizing element (30 μm) composed of.

This polarizing film has a magnetic flux density of 0.00
It contains a cellulose triacetate film formed by a casting method while applying a magnetic field of 2 T (tesla) or more and 7.0 T or less, and as seen from Table 1, its front retardation (RMT) value is 3.7 nm to 5.4 nm, which is almost constant, while the thickness direction retardation (RT
All H) values are 50 nm or less. Therefore, the difference | RMT-RTH | between the two values is smaller as the value of the retardation value (RTH) in the thickness direction is smaller, and the results are shown in Examples 1 to 5
In the case of, the difference value | RMT-RTH |
And an optical film excellent in optical anisotropy was obtained. The smaller the | RMT-RTH | value of the cellulose triacetate film, the better the polarizing film is for the high contrast performance of the liquid crystal cell.
The polarizing film using the above optical film satisfies this requirement. The magnetic flux density is 0.1T
(Tesla) or less is preferable, and the reason is that the retardation value (RTH) decreases even if it exceeds 0.1 T (Tesla), but the degree of decrease is smaller than the degree of magnetic flux density, Therefore, 0.002-0.1T is preferable.

On the other hand, when an optical film of cellulose triacetate was formed without applying a magnetic field as in Comparative Example 1 or a magnetic field was applied as in Comparative Example 2, the magnetic flux density was more than 0.002T. The optical film of cellulose triacetate formed by applying a magnetic field with a small magnetic flux density of 0.001 T has a retardation value (R
TH) is large, and therefore the | RMT-RTH | value of the film is significantly large at 69.8 and 57.4, respectively, and the polarizing films using the optical films of Comparative Examples 1 and 2 are
This adversely affected the high contrast performance of the liquid crystal cell.

(Examples 6 to 10, Comparative Examples 1, 3 to 5) Samples of Comparative Example 1 obtained above (optical film made of cellulose triacetate formed without any treatment)
Was subjected to a magnetic field treatment in the air oven while restraining both the vertical and horizontal directions at the heat treatment temperature and treatment time shown in Table 2. In Examples 6 to 10 and Comparative Examples 3 and 5, a magnetic field was applied to the film perpendicularly to the thickness direction, and in Comparative Example 4, the film was heat-treated at a temperature below the glass transition temperature (Tg) without applying a magnetic field. The glass transition temperature (Tg) of the cellulose triacetate film of Comparative Example 1 was 151 ° C., and in Examples 6 to 9 and Comparative Examples 3 to 4, this glass transition temperature (Tg).
g), in Example 10, (Tg−30 = 121) ° C.
Comparative Example 5 was heat-treated at (Tg-35 = 116) ° C. In addition, the retardation value (RMT) of Table 2,
(RTH) is the same evaluation value as in Table 1.

[0058]

[Table 2]

The cellulose triacetate film treated by the above method was processed in the same manner as in Examples 1 to 5 and Comparative Examples 1 and 2 to complete a polarizing film. Examples 6 to 1
0 and Comparative Examples 3 and 5 have a magnetic flux density of 7.0 T (Tesla) on the cellulose triacetate film (untreated) of Comparative Example 1.
Is an example in which the heat treatment temperature and the treatment time are changed while applying the magnetic field of Comparative Example 4, and Comparative Example 4 performs only the heat treatment without applying a magnetic field to the cellulose triacetate film of Comparative Example 1. It is an example. From this result, while applying the magnetic flux density of 7.0 T (Tesla), (Tg
The cellulose triacetate film subjected to a treatment time of 30 minutes to 500 hours in the heat treatment temperature range of −30) ° C. to Tg ° C. has a retardation value (RT
H) is 50 nm or less, and | RMT-RTH |
The value is as small as 0 to 50 nm, and the polarizing film containing the optical film having such excellent optical isotropy has obtained satisfactory satisfactory results in the high contrast performance of the liquid crystal cell.

On the other hand, a magnetic field was applied as in the optical film of cellulose triacetate formed without applying a magnetic field as in Comparative Example 1 and Comparative Examples 3 and 4, but the processing time was 15 minutes. Shorter than the prescribed minimum processing time of 30 minutes,
Alternatively, the heat treatment temperature is the lowest heat treatment temperature (Tg-30).
The optical film of cellulose triacetate treated at 116 ° C., which is lower than 0 ° C., has a particularly large retardation value (RTH) in the thickness direction, and therefore the film's | RMT-
The RTH | values were also significantly large at 60.4 to 69.8, and the polarizing films using the optical films of Comparative Examples 1 and 3 to 5 adversely affected the high contrast performance of the liquid crystal cell.

(Examples 11-15, Comparative Examples 1, 6-8)
The sample of Comparative Example 1 obtained previously (optical film of cellulose triacetate formed without any treatment) was restrained in both vertical and horizontal directions in an air oven to give a magnetic flux density of 0.1 T (Tesla). While applying a magnetic field perpendicularly to the surface of the film, a magnetic field was applied at the heat treatment temperature and treatment time shown in Table 3. The glass transition temperature (Tg) and the melting point (Tm) of the cellulose triacetate film of Comparative Example 1 were 151 ° C. and 287 ° C., respectively.
The heat treatment was carried out for 30 seconds to 30 minutes at a heat treatment time of 152 ° C to (Tm-10 = 277) ° C exceeding g. In Comparative Example 6,
At a heat treatment temperature of (Tm-10 = 277) ° C. for 20 seconds,
In Comparative Example 7, the heat treatment temperature was (Tm-7 = 280) ° C. for 30 seconds, and in Comparative Example 8, the magnetic flux density was 2 without applying.
It processed at the heat processing temperature of 30 degreeC for 8 minutes. Note that the letter values (RMT) and (RTH) in the letter in Table 3 are the same evaluation values as in Tables 1 and 2.

[0062]

[Table 3]

The cellulose triacetate film treated by the above method was processed in the same manner as in Examples 1 to 10 and Comparative Examples 1 to 5 to complete a polarizing film. Example 1
1 to 15 and Comparative Examples 6 and 7 have a magnetic flux density of 0.1 T in the cellulose triacetate film (untreated) of Comparative Example 1.
(Tesla), the heat treatment temperature and the treatment time were changed, and Comparative Example 8 was heat-treated without applying magnetic flux density to the cellulose triacetate film of Comparative Example 1 (untreated). Here is an example. Comparative Example 7 seems to be because the heat treatment temperature is high near the melting point (Tm).
The film shape could not be maintained because it could not stand the heat treatment. From these results, the cellulose triacetate film subjected to a magnetic flux density of 0.1 T (tesla) at a heat treatment temperature of (Tm-10) ° C. or higher over Tg ° C. for a treatment time of 30 seconds to 30 minutes has a thickness direction of Retardation (RTH) value is 50n
A polarizing film containing an optical film having an optical isotropy of less than m and a small | RMT-RTH | value of 0 to 50 nm has a good and satisfactory result in high contrast performance of a liquid crystal cell. was gotten.

On the other hand, the optical film of cellulose triacetate formed without applying a magnetic field as in Comparative Examples 1 and 8 and the magnetic field as in Comparative Example 6 had a processing time of 20 seconds. The optical film of cellulose triacetate treated for a time shorter than the predetermined minimum treatment time of 30 seconds has a large retardation value (RTH) in the thickness direction, and therefore the film's | RMT-RTH | value is 54.4 to respectively. 69.8, which is remarkably large, and Comparative Example 1,
The polarizing films using the optical films 6 and 8 adversely affect the high contrast performance of the liquid crystal cell.

(Examples 16 to 20, Comparative Examples 1 and 9) The sample of Comparative Example 1 obtained previously (optical film of cellulose triacetate film-formed without any treatment) was vertically placed in an air oven. While applying the magnetic field while restraining both lateral directions and changing the magnetic flux density in the thickness direction, the magnetic field was applied at the heat treatment temperature below the glass transition temperature shown in Table 4 while changing the treatment time. As described above, the glass transition temperature (Tg) of the cellulose triacetate film of Comparative Example 1
Is 151 ° C., and Examples 16 to 20 are 0.002T.
With the above, a magnetic field is applied with a magnetic flux density of 7.0 T (tesla) or less, and at a heat treatment temperature of (Tg-15 = 136) ° C., 30
Treated for minutes to 500 hours. In Comparative Example 9, while applying a magnetic field with a magnetic flux density of 0.001 T (Tesla), which is smaller than that in Example, at a heat treatment temperature of (Tg−15 = 136) ° C.,
It was heat-treated for 30 minutes. Note that the letter values (RMT) and (RTH) in the letter in Table 4 are the same evaluation values as in Tables 1 to 3.

[0066]

[Table 4]

The cellulose triacetate film treated by the above method was processed in the same manner as in Examples 1 to 10 and Comparative Examples 1 to 5 to complete a polarizing film. Example 16
20 to Comparative Example 9, the cellulose triacetate film of Comparative Example 1 (untreated) has a magnetic flux density of 0.001 to 7.
The heat treatment was performed at 0 ° C. (Tesla) at a heat treatment temperature of 136 ° C. and the treatment time was changed from 30 minutes to 500 hours. From this result, at the heat treatment temperature of (Tg-15 = 136) ° C., in the magnetic flux density range of 0.002 to 7.0 T (Tesla), the cellulose triacetate film subjected to the treatment time of 30 minutes to 500 hours had a thickness Direction retardation (RTH) value is 50nm or less, | R
The MT-RTH | value was as small as 0 to 50 nm, and the polarizing film containing the optical film having such excellent optical isotropy gave good and satisfactory results in the high contrast performance of the liquid crystal cell.

On the other hand, when an optical film of cellulose triacetate was formed without applying a magnetic field as in Comparative Example 1 or a magnetic field was applied as in Comparative Example 9, the magnetic flux density was the minimum density of 0. The optical film of cellulose triacetate, which has been subjected to a magnetic field treatment at 0.001T lower than 0.002T (terrace), has a particularly large retardation value (RTH) in the thickness direction, and therefore the film's | RMT-R
The TH | values were also significantly large, 69.8 and 57.7, respectively, and the polarizing films using the optical films of Comparative Examples 1 and 9 adversely affected the high contrast performance of the liquid crystal cell.

(Examples 21 to 25, Comparative Examples 1 and 10) Samples of Comparative Example 1 obtained above (optical film made of cellulose triacetate formed without any treatment)
While restraining both longitudinal and lateral directions in an air oven and applying a magnetic field by changing the magnetic flux density in the thickness direction, the treatment time was changed at a heat treatment temperature exceeding the glass transition temperature (Tg) shown in Table 5. Processed. As described above, the glass transition temperature (T
Since g) is 151 ° C., Examples 21 to 25 and Comparative Example 10 are heat-treated at a temperature of (Tg + 100 = 251) ° C. for 30 seconds to 30 minutes. Table 5
Table 1 shows the retardation values (RMT) and (RTH) of
Through are the same evaluation values as in Table 4.

[0070]

[Table 5]

The cellulose triacetate film treated by the above method was processed in the same manner as in Examples 1 to 15 and Comparative Examples 1 to 9 to complete a polarizing film. Example 21
25, Comparative Example 10 has a magnetic flux density of 0.001 to the cellulose triacetate film (untreated) of Comparative Example 1.
Heat treatment temperature of 251 ° C over 7.0T (Tesla)
The processing time was changed from 30 seconds to 30 minutes. From this result, Tg ° C was exceeded (Tg + 100
= 251) at a heat treatment temperature of 0.002 to 7.0 T
The cellulose triacetate film subjected to a magnetic field treatment with a magnetic flux density of (Tesla) and a treatment time of 30 seconds to 30 minutes has a retardation (RTH) value in the thickness direction of 50 nm or less, and the | RMT-RTH | value is 0-50
The polarizing film containing an optical film having a small size of nm and excellent in optical isotropy gave good and satisfactory results in the high contrast performance of the liquid crystal cell.

On the other hand, when an optical film of cellulose triacetate was formed without applying a magnetic field as in Comparative Example 1 or a magnetic field was applied as in Comparative Example 10, the magnetic flux density was the minimum density of 0. The optical film of cellulose triacetate subjected to the magnetic field treatment at 0.001T lower than 0.002T (terrace) has a large retardation value (RTH) in the thickness direction, and therefore the film's | RMT-
The RTH | values were also significantly large at 69.8 and 57.7, respectively, and the polarizing films using the optical films of Comparative Examples 1 and 10 adversely affected the high contrast performance of the liquid crystal cell.

(Examples 26 to 33, Comparative Examples 1, 11 to 1)
3) The sample of Comparative Example 1 (optical film of cellulose triacetate formed into a film without any treatment) obtained above was constrained in both vertical and horizontal directions in an air oven to give a magnetic flux density of 5 in the thickness direction. The heat treatment temperature and the treatment time shown in Table 6 were changed while applying a magnetic field of 0.0 T (tesla). As described above, the glass transition temperature (Tg) of the cellulose triacetate film of Comparative Example 1 was 151.
Since the melting point (Tm) is 287 ° C., (Tg-30
= 121) ° C. or higher and Tg (151) ° C. or lower at a heat treatment temperature of 30 minutes to 500 hours while applying a magnetic field having a magnetic flux density of 5.0 T (tesla) (Examples 26 to 28), and Beyond Tg (151) ° C (Tm-10
= 277) ° C., heat treatment was performed for 30 seconds to 30 minutes while applying a magnetic field of 5.0 T (tesla) (Examples 29 to 33). The retardation values (RMT) and (RTH) in Table 6 are the same evaluation values as in Tables 1 to 5.

[0074]

[Table 6]

The cellulose triacetate film treated by the above method was processed in the same manner as in Examples 1 to 25 and Comparative Examples 1 to 10 to complete a polarizing film. Example 2
Nos. 6 to 28 applied a magnetic flux density of 5.0 T (tesla) to the cellulose triacetate film (untreated) of Comparative Example 1 to obtain a heat treatment temperature of 121 to 151 ° C, that is, (Tg-
30) is an example of heat treatment for 30 minutes to 500 hours in the temperature range of C to Tg ° C., Examples 29 to 33 are the cellulose triacetate film of Comparative Example 1 (untreated) with a magnetic flux density of 5.0 T (Tesla). And heat treatment temperature 1
52-277 ° C, that is, exceeding Tg ° C (Tm-1
This is an example of heat treatment for 30 seconds to 30 minutes in a temperature range of 0) ° C. In Comparative Example 14, the heat treatment temperature was the melting point (T
Although it is considered to be high because it is close to m), the film shape could not be maintained because it could not withstand the heat treatment. From this result, the cellulose triacetate film and T which were subjected to the magnetic field treatment in the temperature range of (Tg−30) ° C. to Tg ° C. for 30 minutes to 500 hours were heated.
In the temperature range of more than g ° C to (Tm-10) ° C, 30
The cellulose triacetate film subjected to a magnetic field while being heated for 30 seconds from 30 seconds has a retardation (RTH) value in the thickness direction of 50 nm or less.
The RMT-RTH | value was as small as 0 to 50 nm, and the polarizing film containing the optical film having such excellent optical isotropy gave good and satisfactory results in the high contrast performance of the liquid crystal cell.

On the other hand, the optical film of cellulose triacetate formed without applying a magnetic field as in Comparative Example 1 and the magnetic field as in Comparative Examples 12 and 13 were processed with the minimum processing time. The optical film of cellulose triacetate treated for 20 seconds, which is shorter than 30 seconds, has a particularly large retardation value (RTH) in the thickness direction, and therefore the film's | RMT-RTH | value is 6 respectively.
9.8, 57.7, 52.3, which is large, and Comparative Example 1,
The polarizing film using the optical films 12 and 13 is
This adversely affected the high contrast performance of the liquid crystal cell.

[0077]

INDUSTRIAL APPLICABILITY The optical film made of the cellulose acetate of the present invention has a small birefringence (RTH) in the thickness direction and an extremely small optical anisotropy, and is therefore suitable for a polarizing film used in a liquid crystal cell. .

[Brief description of drawings]

FIG. 1 is a perspective view of an optical film showing the film refractive index and thickness.

2A and 2B show polymer alignment states before and after drying of an optical film. FIG. 2A is a perspective view before drying, and FIG. 2B is a perspective view after drying.

Claims (13)

[Claims] 1. An optical film made of cellulose acetate is produced by extruding a cellulose acetate ester dope solution from a casting machine onto a casting support to solidify it, peeling it from the casting support and drying. 7. In the above method, 0.002 T (tesla) or more on the film surface in at least one of the steps of solidifying, peeling and drying the film.
A method for producing an optical film, which comprises applying a magnetic field with a magnetic flux density of 0 T or less to control the optical anisotropy of the film. 2. The method for manufacturing an optical film according to claim 1, wherein the magnetic flux density applied to the surface of the film is 0.002 T or more and 0.1 T or less. Production method. 3. An optical film made of cellulose acetate is heated at a heat treatment temperature of (Tg−30 ° C.) or higher and Tg or lower to 0.002T on the surface of the film.
Anisotropic optical film characterized by controlling the optical anisotropy of the optical film by applying a magnetic field with a magnetic flux density of (Tesla) or more and 7.0 T or less for 30 minutes or more and 500 hours or less. To control sex (where Tg is
The glass transition temperature (° C.) of the optical film of cellulose acetate is shown. 4. The anisotropic method of an optical film according to claim 3, wherein the magnetic flux density is 0.002 T or more and 0.1 T or less. Sex control method. 5. The anisotropic method of an optical film according to claim 4, wherein the magnetic flux density is 0.002 T or more and 0.02 T or less. Sex control method. 6. An optical film made of cellulose acetate is heated to a temperature above Tg (Tm-10 ° C.) at a heat treatment temperature of not more than 0.002 T (tesla) and not more than 7.0 T on the surface of the optical film. A method for controlling the anisotropy of an optical film, wherein the anisotropy of the optical film is controlled by applying a magnetization at a magnetic flux density for a time of 30 seconds or more and 30 minutes or less (where Tg is , Glass transition temperature (℃) of cellulose acetate optical film
Tm represents the melting point (° C.). ). 7. The anisotropic method of an optical film according to claim 6, wherein the magnetic flux density is 0.002 T or more and 0.1 T or less. Sex control method. 8. The method for controlling anisotropy of an optical film according to claim 7, wherein the magnetic flux density is 0.002 T or more and 0.02 T or less. Sex control method. 9. The method for controlling the anisotropy of an optical film according to claim 6, wherein the heat treatment time is 90 seconds or less. Directional control method. 10. An optical film produced or treated by the method according to claim 1.
An optical film having a retardation value (RTH) defined by the following (formula i) in a range of 0 nm <RTH ≦ 50 nm. RTH = | {(nMD + nTD) / 2−nTH} | × d (Formula i) [where nMD is the film refractive index in the direction parallel to the casting direction, and nTD is the film refractive index in the direction perpendicular to the casting direction. rate,
nTH is the film refractive index in the thickness direction, and d is the film thickness (nm). ] 11. An optical element comprising a polarizing element and a protective film coated on at least one of the polarizing elements, the protective film being produced or treated by the method according to any one of claims 1 to 9. A polarizing film characterized by being a film. 12. A polarizing film comprising a polarizing element and a protective film coated on at least one of the polarizing elements, wherein the protective film is the optical film according to claim 10. 13. An elliptically polarizing film comprising the polarizing film according to claim 11 and a retardation film laminated on the polarizing film.