JP2001226495A - Optical film, method for producing optical film, polarizing plate by using the same optical film and vertical alignment(va) type liquid crystal-displaying device by using the same polarizing plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical film having higher upper limit value of retardation (Rt value) in thickness direction than that of a conventional cellulose ester film, capable of making it as a thin film and used for a polarizing plate- protecting film, a method for producing the same film by using a non-chlorine- based solvent and a vertical alignment(VA) type liquid crystal-displaying device. SOLUTION: This optical film is characterized by using the cellulose ester having >=2.75 and <=2.86 acetyl substitution degree.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an optical film,
The present invention relates to a manufacturing method thereof, a polarizing plate using the optical film as a protective film for a polarizer, and a vertical alignment (VA) type liquid crystal display device using the polarizing plate.

[0002]

2. Description of the Related Art In recent years, the demand for optical films, especially for polarizing plate protective films for liquid crystal display devices, has been increasing due to the increase in the number of photograph shots and the explosive spread of mobile phones and notebook computers. Transparency and flatness are indispensable for optical films. Cellulose-based films (usually dissolved in an organic solvent, and the solution is cast to form a film) require solubility in the solvent. If the solubility is poor, the transparency of the film is deteriorated, and the planarity is deteriorated due to insoluble matter. In recent years, a material having a short dissolution time and good solubility has been demanded due to an increase in demand. Further, the organic solvent, methylene chloride, which is most frequently used for dissolution at present, has a tendency to be significantly regulated as having a great effect on the environment because it is a chlorine-based organic solvent. Under such circumstances, there is a demand for an optical film which is soluble in a non-chlorine solvent and satisfies the above-mentioned required characteristics. Further, among optical films, a polarizing plate protective film for a liquid crystal display device used for a mobile phone, a notebook personal computer, etc. has high transparency, excellent heat resistance, excellent scratch resistance, and dimensional change in addition to the above-mentioned required characteristics. It is demanded that the amount is small and the number of bright spot foreign matters is small.

A liquid crystal display device has a low voltage and low power consumption.
Since it can be directly connected to an IC circuit and can be made particularly thin, it is widely used as a display device of a word processor, a personal computer, or the like.

The basic configuration of this liquid crystal display device is that a polarizing plate is provided on both sides of a liquid crystal cell. The polarizing plate passes only light having a polarization plane in a certain direction. Therefore, in the liquid crystal display device, it plays an important role of visualizing the change in the orientation of the liquid crystal due to the electric field. That is, the performance of the liquid crystal display device largely depends on the performance of the polarizing plate. The general configuration of the polarizing plate will be described with reference to the drawings.
FIG. 1 is a schematic sectional view of a polarizing plate. In the figure, reference numeral 1 denotes a polarizer, and protective films (referred to as polarizer protective films) 2 are laminated on both sides of the polarizer 1. A liquid crystal display device is formed by laminating a polarizing plate having such a configuration on a liquid crystal cell.

The polarizer is obtained by adsorbing and stretching iodine or the like on a polymer film. That is, a solution called an H ink containing a dichroic substance (iodine) is wet-adsorbed to a polyvinyl alcohol film, and then the film is uniaxially stretched to orient the dichroic substance in one direction. It is.

The polarizing plate protective film is provided for the purpose of improving the durability of the polarizer. Conventionally, as the polarizing plate protective film, cellulose resin, polycarbonate resin, polyarylate resin, polysulfone resin, polyether sulfone resin, Norbornene resin, polystyrene resin, polyacrylate resin, polyester resin and the like are known, and among these resins, cellulose triacetate (TAC) is particularly preferably used. In such a liquid crystal display device, recently, from the viewpoint of contrast and the like, a liquid crystal display device using a twisted nematic (TN) type liquid crystal having a twist angle of 90 degrees has been replaced with a super twisted nematic (STN) having a twist angle of 160 degrees or more. The current situation is a shift to a liquid crystal display device using a type liquid crystal.

However, a liquid crystal display device using an STN type liquid crystal utilizes the birefringence of the liquid crystal.
In a normally white liquid crystal display device using an N-type liquid crystal, there is a problem that a white background is colored blue or yellow. Therefore, in a black and white display, the contrast, the viewing angle are narrow, and it is difficult to colorize. There is a problem.

To solve this problem, that is, as a method of compensating for the birefringence, a technique using a retardation plate below the above-mentioned polarizing plate has been proposed. According to this technique, the problem of coloring is solved, but the viewing angle is hardly improved.

In order to solve the problem of the viewing angle, a technique of producing a birefringent film whose refractive index in the thickness direction is larger than the refractive index in the direction perpendicular to the birefringent optical axis and using this as a retardation plate has been proposed. was suggested. Further, a technique has been proposed in which films each having a positive and negative intrinsic birefringence value are used one by one, or a laminated film is used as a retardation plate. Also, JP-A-7-21
As disclosed in Japanese Patent No. 8724, a polarizing plate has been proposed in which at least one protective film of a polarizer is a plastic film composed of triacetyl cellulose having an in-plane retardation value of 30 to 70 nm measured with light having a wavelength of 590 nm. .

[0010] By these proposed techniques, the change in contrast due to the viewing angle is reduced, and the viewing angle characteristics are improved. By the way, low voltage, low power consumption, the biggest problem in the liquid crystal display device, which has a great feature not found in other display devices on the thinner, that is, the demand for improvement of the problem of narrow viewing angle has been increasingly stronger, Further research is underway.

As one of such research and development, a liquid crystal of a type different from the TN or STN type liquid crystal has been proposed. That is, the liquid crystal cell of the TN or STN type is a type in which the liquid crystal molecules are parallel to the polarizing plate when the voltage is turned off, and the liquid crystal molecules are vertically aligned with the polarizing plate when the voltage is turned on.
A so-called vertical alignment (VA) type liquid crystal cell using a liquid crystal molecule in which liquid crystal molecules are perpendicular to the polarizing plate when the voltage is turned off and parallel when the voltage is turned on, for example, using a negative liquid crystal having a negative dielectric anisotropy is developed. Reached.

This vertical alignment (VA) type liquid crystal display device (hereinafter also referred to as VA type liquid crystal display device)
When the voltage is turned off, the liquid crystal molecules are perpendicular to the polarizing plate, and when the voltage is turned on, the liquid crystal molecules are aligned in a vertical alignment mode in which the liquid crystal molecules are aligned in parallel with the polarizing plate. The viewing angle is relatively wide as compared with the STN type liquid crystal display device. However, as the size of the liquid crystal screen increases, there is an increasing demand for a polarizing plate that further widens the viewing angle.

Among the functions required of a protective film for a polarizing plate used for a polarizing plate used in a VA liquid crystal display device, physical properties include dimensional stability, transparency and elastic modulus, and optical properties include Requires retardation, which is an index of the refractive index. In particular, a high retardation value (Rt value) in the thickness direction is required among retardations.

The retardation value in the thickness direction is expressed by the following (1)
It is a value defined by an expression. (1) Formula Rt value = ((nx + ny) / 2−nz) × d nx is the refractive index of the film in the casting direction of the film, ny is the refractive index of the film in the direction (width direction) perpendicular to the casting direction,
nz represents the refractive index of the film in the thickness direction, and d represents the thickness (nm) of the film.

As a film satisfying these functions, a cellulose ester film has been conventionally used because of its excellent optical isotropy and transparency. However, generally used cellulose ester films are made from a cellulose ester having a degree of acetyl substitution of 2.88 or more. According to the study by the authors, in the range of substitution in this range, the retardation value in the thickness direction (Rt value) is obtained. There was also an upper limit to increasing.

Further, in order to obtain a larger retardation value (Rt value) in the thickness direction in the conventional cellulose ester film, it was necessary to increase the thickness of the film. However, portability of liquid crystal display devices has recently been demanded, and miniaturization, particularly, thinness of the liquid crystal display devices has been demanded.

As an organic solvent of a cellulose acetate solution used for producing a cellulose acetate film, a chlorinated hydrocarbon such as methylene chloride (boiling point: 41 ° C.) is used. However, methylene chloride tends to be significantly regulated due to problems with the human body and the global environment. In addition to methylene chloride, solvents known to have solubility in cellulose acetate include acetone (boiling point: 56 ° C.), methyl acetate (boiling point: 5 ° C.).
6.3 ° C.), tetrahydrofuran (boiling point 65.4 ° C.),
1,3-dioxolan (boiling point 75 ° C.), nitromethane (boiling point 101 ° C.), 1,4-dioxane (boiling point 110
° C).

In the above-mentioned solvent, acetone having a low boiling point only swells cellulose acetate in a usual method, but does not dissolve it. In recent years, attempts have been made to dissolve cellulose acetate in acetone to produce fibers and films.

J. M. G. FIG. Cowie et al., Die M
akromolekulare Chemie, 143
Vol., P105-114 (1971), cellulose acetate (acetyl substitution degree 2.80-2.90) was cooled from -80 ° C to -70 ° C in acetone, and then heated to 0.5 to 5 ° C. It reports that a dilute solution with a mass% dissolution was obtained.

Also, Kenji Ude et al., "Dry spinning of cellulose triacetate from acetone solution" (Journal of the Fiber Society of Japan, 34
Vol., P57-61 1981).
Is reported to have been obtained. JP-A-9-95544 and JP-A-9-95557 disclose cellulose acetate by a cooling dissolution method using an organic solvent substantially consisting of acetone or using acetone and another organic solvent in common with the above technology. It is proposed to apply to film production. JP-A-9-95538 discloses ethers other than acetone (eg, dioxane, dioxolan, THF, diethyl ether, etc.), ketones (eg, acetone, methyl ethyl ketone, cyclohexanone, etc.) or esters (eg, methyl acetate, ethyl acetate, butyl acetate). ,
Cellulose acetate is dissolved by a cooling dissolution method using an organic solvent selected from the group consisting of amyl acetate) to produce a film. However, the dope obtained by dissolving the cellulose ester having a high degree of acetyl substitution in a high range as described above by a special method such as the cooling dissolution method has poor stagnation stability, and the production efficiency cannot be increased without increasing the concentration. There was a problem.

[0021]

Therefore, the first aspect of the present invention
The problem is that the upper limit of the thickness direction retardation (Rt value) is higher than that of a conventional cellulose ester film,
An object of the present invention is to provide an optical film which can be thinned and which is used for a protective film for a polarizing plate, and a method for producing the same using a non-chlorine solvent.

A second object of the present invention is to provide a polarizing plate using the optical film and a vertical alignment (VA) type liquid crystal display device using the polarizing plate.

[0023]

Means for Solving the Problems The present inventor has proposed that a VA type liquid crystal display device uses a film whose retardation value in an in-plane direction is controlled as disclosed in Japanese Patent Application Laid-Open No. Hei 7-218724. But its effect is low,
Conventionally, when using a film in which the retardation value in the thickness direction, which is a value representing the anisotropy in the plane direction and the thickness direction, is used instead of the in-plane retardation value as studied, the VA-type liquid crystal display device The present inventors have found that the viewing angle is significantly wide, and have studied the method of further increasing the retardation value in the thickness direction. As a result, the present invention has been reached.

Furthermore, by using the cellulose ester of the present invention, a high concentration can be obtained even in a cooling solvent method, a cooling dissolution method, and a high pressure dissolution method using a non-halogen organic solvent, which has been conventionally poor in stagnant stability and whose concentration cannot be increased. It was found that a dope having good stagnation stability was obtained by the method of the present invention, which led to the present invention. Specific means for achieving the present invention will be described below.

1) The degree of acetyl substitution is 2.75 or more;
An optical film using a cellulose ester of 86 or less.

2) The optical film as described in 1) above, wherein the degree of acetyl substitution is 2.84 or less.

3) The optical film according to 1) or 2), wherein the retardation value in the thickness direction (Rt value) defined by the above formula (1) is 90 to 200 nm.

(4) The optical film as described in any one of (1) to (3), wherein the film has a thickness of 75 to 165 μm.

5) The degree of acetyl substitution is 2.75 or more;
Dissolving a cellulose ester of 86 or less in an organic solvent, casting the solution on a rotating belt or drum, peeling the residual solvent from the belt or drum at 5-200%, and further drying to evaporate the organic solvent A method for producing an optical film, comprising:

6) The method for producing an optical film according to 5), wherein a cellulose ester having a degree of acetyl substitution of 2.84 or less is used.

7) The degree of acetyl substitution is 2.75 or more;
86 or less cellulose ester, -100 ℃ ~ -10
A belt that rotates a cellulose ester solution obtained by performing at least once a step of mixing and swelling with a substantially halogen-free organic solvent cooled to 0 ° C. and heating to 0 ° C. to 120 ° C. Or cast on a drum and then remove the residual solvent amount from the belt or drum by 5 to 2
A method for producing an optical film, wherein the organic solvent is peeled off at 00%, and further dried to evaporate an organic solvent.

8) The method for producing an optical film according to 7), wherein a cellulose ester having a degree of acetyl substitution of 2.84 or less is used.

9) The degree of acetyl substitution is 2.75 or more;
A cellulose ester of 86 or less is mixed with an organic solvent containing substantially no halogen, and swelled.
The cellulose ester solution obtained by performing the step of cooling to 0 ° C. to 120 ° C. as one cycle at least once is cast on a rotating belt or drum, and then from the belt or drum. A method for producing an optical film, characterized in that the amount of residual solvent at the time of peeling is 5 to 200%, the organic solvent is evaporated after drying.

10) The method for producing an optical film as described in 9) above, wherein a cellulose ester having a degree of acetyl substitution of 2.84 or less is used.

11) The degree of acetyl substitution is 2.75 or more,
2.86 or less of a cellulose ester is mixed with an organic solvent and swelled to allow the mixture to swell from 5 × 9.8 × 10 ⁵ to 2 × 9.8 × 10 ^7.
The cellulose ester solution obtained by carrying out at least once the step of pressurizing to Pa and the step of releasing as one cycle is cast on a rotating belt or drum, and then the residual solvent when peeling off from the belt or drum 5 to 5
A method for producing an optical film, comprising peeling off at 200%, further drying and evaporating an organic solvent.

12) A cellulose ester having a degree of acetyl substitution of 2.84 or less is used.
3. The method for producing an optical film according to item 1.

13) A polarizing plate, wherein the optical film described in any one of 1) to 4) is bonded to at least one surface of a polarizer.

14) A protective film for a polarizing plate, comprising an optical film produced by the production method according to any one of 5) to 12).

15) A vertical alignment (VA) type liquid crystal display device comprising a polarizing plate using the optical film according to any one of 1) to 4).

Hereinafter, the present invention will be described in detail. The cellulose ester used for the optical film of the present invention has an acetyl substitution degree of 2.75 or more and 2.86 or less, and preferably has an acetyl substitution degree of 2.84 or less. As the substituent, an acyl group having 3 carbon atoms may be 0.14 to 0.25.
May be included. The cellulose ester of an acyl group having 2 to 3 carbon atoms is an acetyl group or a propionyl group obtained by substituting a hydroxyl group of cellulose with a predetermined degree of substitution by a conventional method. The acetyl substitution degree of the present invention includes those acyl groups. Specific examples include cellulose triacetate, cellulose acetate propionate, cellulose propionate, and the like. Among them, cellulose triacetate and cellulose acetate propionate are preferred, and cellulose triacetate is particularly preferred. The method for measuring the degree of substitution of the acetyl group is ASTM-
It can be measured according to D817-96. The basic principle of the method for synthesizing cellulose fatty acid esters is Marusawa,
It is described in a number of documents such as Uda, cellulose resin (Nikkan Kogyo Shimbun, 1970), Migita et al., And wood chemistry (Kyoritsu Shuppan, 1968).

The number average molecular weight of the cellulose ester used in the optical film of the present invention is preferably from 70,000 to 30,000 in order to obtain preferable mechanical strength as the optical film.
0000 is preferable, and 80,000 to 200,000 is more preferable. By setting the degree of substitution in this range, the retardation value in the thickness direction can be increased, and a polarizing plate having excellent viewing angle characteristics can be obtained by using the optical film as a protective film for a polarizing plate.

In recent years, from the viewpoint of environmental problems, chlorine-based solvents have been significantly regulated. JP-A-9-9554
No. 4 and No. 9-95557 propose that cellulose acetate is dissolved by a cooling dissolution method using an organic solvent substantially consisting of acetone or using acetone and another organic solvent in common, and is applied to film production. are doing. JP-A-9-95538 discloses ethers other than acetone (eg, dioxane, dioxolan, tetrahydrofuran, diethyl ether, etc.), ketones (eg, acetone, methyl ethyl ketone, cyclohexanone, etc.) or esters (eg, methyl acetate, ethyl acetate, butyl acetate). , Amyl acetate, etc.) to dissolve cellulose acetate by a cooling dissolution method using an organic solvent selected from the group consisting of an organic solvent and a film. However, there is a problem that the dope dissolved by a special method such as the cooling dissolution method has poor stagnation stability.

The concentration of the cellulose ester used in the optical film of the present invention can be increased even by the special dissolution method using a non-chlorine solvent as described above, and the stagnation stability is remarkably improved. Films made from a dope dissolved in a non-chlorine solvent are also transparent, with good flatness for optical films, especially when used as protective films for polarizing plates,
A protective film for a polarizing plate having high retardation in the thickness direction and excellent in viewing angle characteristics can be obtained.

As the cellulose ester used in the optical film of the present invention, either a cellulose ester synthesized from cotton linter or a cellulose ester synthesized from wood pulp can be used alone or in combination. It is preferable to use a large amount of cellulose ester synthesized from a cotton linter having good releasability from a belt or a drum because productivity efficiency is high. When the proportion of the cellulose ester synthesized from the cotton linter is 60% by mass or more, the effect of the releasability becomes remarkable, the ratio is preferably 60% by mass or more, more preferably 85% by mass or more, and furthermore, it can be used alone. Most preferred.

The optical film of the present invention preferably contains a plasticizer. The plasticizer that can be used is not particularly limited, but in the case of a phosphate ester system, phthalic acid such as triphenyl phosphate, tricresyl phosphate, cresyl diphenyl phosphate, octyl diphenyl phosphate, diphenyl biphenyl phosphate, trioctyl phosphate, tributyl phosphate, etc. For acid ester type, diethyl phthalate, dimethoxyethyl phthalate, dimethyl phthalate, dioctyl phthalate, dibutyl phthalate, di-2-ethylhexyl phthalate, etc., and for glycolate type, triacetin, tributyrin, butyl phthalyl butyl glycolate, ethyl phthalyl ethyl It is preferable to use glycolate, methylphthalylethyl glycolate, butylphthalylbutylglycolate alone or in combination. There. If necessary, two or more plasticizers may be used in combination. When used for a cellulose ester, the use ratio of a phosphate ester-based plasticizer is preferably 50% or less because hydrolysis of the cellulose ester film is hardly caused and durability is excellent. It is more preferable that the ratio of the phosphate plasticizer is smaller, and it is particularly preferable to use only the phthalate or glycolate plasticizer.

It is preferable that the optical film of the present invention contains an ultraviolet absorber. As the ultraviolet absorber, from the viewpoint of preventing deterioration of the liquid crystal, the liquid crystal is excellent in the ability to absorb ultraviolet light having a wavelength of 370 nm or less and has good liquid crystallinity. From the viewpoint of displayability, those having absorption of visible light having a wavelength of 400 nm or more as small as possible are preferably used. In particular, the transmittance at a wavelength of 370 nm needs to be 10% or less, preferably 5% or less, and more preferably 2% or less.

Examples of commonly used compounds include, for example, oxybenzophenone compounds, benzotriazole compounds, salicylic acid ester compounds, benzophenone compounds, cyanoacrylate compounds, nickel complex salt compounds, and the like. Examples will be given below, but of course the present invention is not limited to these.

2- (2'-hydroxy-5'-methylphenyl) benzotriazole, 2- (2'-hydroxy-3 ', 5'-di-tert-butylphenyl) benzotriazole, 2- (2'- Hydroxy-3'-ter-
Butyl-5'-methylphenyl) benzotriazole,
2- (2'-hydroxy-3 ', 5'-di-tert-butylphenyl) -5-chlorobenzotriazole, 2-
(2'-hydroxy-3 '-(3 ", 4", 5 ", 6"
-Tetrahydrophthalimidomethyl) -5'-methylphenyl) benzotriazole, 2,2-methylenebis (4- (1,1,3,3-tetramethylbutyl) -6-
(2H-benzotriazol-2-yl) phenol), UV-7: 2,4-dihydroxybenzophenone, 2,2'-dihydroxy-4-methoxybenzophenone, 2-hydroxy-4-methoxy-5-sulfobenzophenone, bis (2-methoxy-4-hydroxy-5
-Benzoylphenylmethane) and the like.

In the present invention, one of these ultraviolet absorbers
It is preferable to use at least two kinds of UV absorbers, and two or more different UV absorbers may be contained. As a method of adding the ultraviolet absorber, the ultraviolet absorber may be dissolved in an organic solvent such as alcohol, methylene chloride, or dioxolan and then added to the dope, or may be added directly to the dope composition.
It is known that inorganic powders such as inorganic powders which are not dissolved in an organic solvent are dispersed in an organic solvent and a cellulose ester using a dissolver or a sand mill and then added to a dope.

The amount of the ultraviolet absorber used in the present invention is 0.1 to 2.5% by mass, preferably 0.5 to 2.0% by mass, more preferably 0.8 to 2.0% by mass based on the cellulose ester. 2.0% by mass. If the amount of the ultraviolet absorber is more than 2.5% by mass, the transparency tends to deteriorate, which is not preferable.

The cellulose ester film of the present invention may be, for example, silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, calcium carbonate, kaolin, talc, calcined calcium silicate or hydrated calcium silicate in order to improve handleability. , Aluminum silicate, magnesium silicate, calcium phosphate and the like, and a matting agent such as a crosslinked polymer.
Among them, silicon dioxide is preferable because the haze of the film can be reduced. The fine particles have an average secondary particle size of 0.
The content is preferably from 0.01 to 1.0 μm and the content is preferably from 0.005 to 0.3% by mass based on the cellulose ester.

Fine particles such as silicon dioxide are preferably surface-treated with an organic substance because the haze of the film can be reduced. Preferred organic substances for the surface treatment include halosilanes, alkoxysilanes, silazanes, siloxanes and the like. The larger the average diameter of the fine particles, the greater the matting effect, and the smaller the average diameter, the better the transparency. Therefore, the average diameter of the primary particles of the fine particles is preferably 5 to 50 nm, more preferably 7 to 14 nm. These fine particles are usually present in the film as an aggregate, and it is preferable to form irregularities of 0.01 to 1.0 nm on the film surface. AEROSIL 200, 3 manufactured by Aerosil Co., Ltd.
00, R972, R974, R202, R812, OX
50, TT600, etc., preferably AERO
SILR972, R974, R202, R812 and the like.

In the present invention, the vertical alignment (V
An A) type liquid crystal display device uses a liquid crystal cell of a vertical alignment mode in which liquid crystal molecules are perpendicular to a polarizing plate when voltage is turned off and liquid crystal molecules are aligned in parallel with the polarizing plate when voltage is turned on. Refers to a liquid crystal display device using a polarizing plate used for protecting a polarizer.

As the polarizer having the polarizing plate protective film provided on the surface, conventionally known polarizers can be used. For example, a film made of a hydrophilic polymer such as polyvinyl alcohol is stretched by treating it with a color dye such as iodine, or a plastic film such as vinyl chloride is treated to produce a polyene (a series of conjugated double bonds). Are used. And the polarizing plate is
The polarizing plate protective film is formed as a laminate obtained by laminating at least one side of a polarizer. The polarizing plate thus obtained is provided on one side or both sides of a vertical alignment (VA) type liquid crystal cell to obtain a VA type liquid crystal display device.

The optical film of the present invention and the protective film for a polarizing plate using the film have an absolute value of the retardation value (Rt value) in the thickness direction of the formula (1) of 9
It is preferably from 0 to 200 nm. More preferably, it is 100 to 175 nm. By using a polarizing plate using an optical film having a thickness direction retardation value (Rt value) within the above range as a polarizing plate protective film, V
Good viewing angle characteristics in an A-type liquid crystal display device can be obtained.

The retardation value (Rt value) in the thickness direction is measured using an automatic birefringence meter KOBRA-21ADH (manufactured by Oji Scientific Instruments) at a wavelength of 590 nm in an environment of 23 ° C. and 55% RH. , The three-dimensional refractive index is measured, and the refractive indexes nx, ny, and nz are obtained. The polarizing plate protective film using the optical film of the present invention preferably has a thickness of 75 to 165 μm. More preferably, it is 85 to 125 μm. In a conventional protective film for a polarizing plate made of a cellulose ester film, a thickness of at least 130 μm or more was required to obtain a retardation value (Rt value) in the thickness direction of, for example, 150 nm.

However, the protective film for a polarizing plate using the optical film of the present invention can obtain a high retardation value in the thickness direction (Rt value) while having a smaller thickness than a conventional protective film for a polarizing plate. Become.

Next, a method for producing the optical film of the present invention will be described. First, the outline of a cooling dissolution method and a pressure dissolution method, which are methods for dissolving cellulose acetate, will be described.

<Cooling dissolution method> The acetyl-substituted cellulose ester of the present invention (hereinafter referred to as cellulose ester)
Is a method of dissolving and adjusting a cellulose ester in a mixed organic solvent containing an organic solvent A and acetone by a cooling dissolution method. First, at room temperature in acetone or organic solvent A, or acetone and organic solvent A
, A cellulose ester is gradually added thereto with stirring. At this stage, the cellulose ester is in a state of being swollen in the solvent. next,
The mixture is cooled. The cooling temperature may be a temperature equal to or higher than the freezing point of the solvent, and is in a temperature range from -100 ° C to -10 ° C. Next, when this cooled product is heated to a temperature of 0 ° C. or more and 50 ° C. or less, the cellulose ester is dissolved in the solvent, and a uniform solution (dope) is obtained. In addition, in order to accelerate dissolution, the operation of cooling and heating may be repeated. Whether or not the dissolution is sufficient is determined by visually observing the appearance of the solution.

<Pressurized dissolution method> The pressurized dissolution method is a method in which a mixture of organic solvent A, a mixed organic solvent containing acetone and cellulose ester is kept under high pressure, and then the pressure is released and the mixture is released under normal pressure. It is a method of adjusting the cellulose ester solution (dope) by keeping it near. Until a mixture is obtained, the same procedure as in the above-mentioned cooling dissolution method is carried out. First, a cellulose ester is gradually added to acetone, an organic solvent A, or a mixed solvent of acetone and the organic solvent A while stirring at room temperature. At this stage, the cellulose ester is in a state of swelling in the solvent. The mixture is then kept under high pressure. Pressure is 5 ×
The effect is recognized from 9.8 × 10 ⁵ Pa or more, and the dissolution time can be shortened as it is higher. However, if it is too high, the equipment becomes too large, and the effect of shortening the dissolution time gradually becomes saturated. If it is at least 0.8 × 10 ⁷ Pa, a sufficient effect can be obtained. The pressure was then released and the mixture was 9.8 ×
By maintaining the pressure at 10 ³ Pa or more and 9.8 × 10 ⁵ Pa or less, the cellulose ester is dissolved in the solvent and a uniform solution can be obtained. In order to accelerate dissolution, pressurization,
The release operation may be repeated. Whether or not the dissolution is sufficient is determined by visually observing the appearance of the solution.

Next, a method for producing the optical film of the present invention using the above cellulose ester solution will be described.
The dope is formed by dissolving the cellulose ester of the present invention having a degree of acetyl substitution of 2.75 or more and 2.86 or less in an organic solvent. The concentration of cellulose ester in the dope is 10
It is about 35% by mass. From the viewpoint of productivity, the concentration is preferably as high as possible.

As the organic solvent, methyl acetate, ethyl acetate, amyl acetate, acetone, tetrahydrofuran,
3-dioxolan, 1,4-dioxane, cyclohexanone, ethyl formate, 2,2,2-trifluoroethanol, 2,2,3,3-hexafluoro-1-propanol, 1,3-difluoro-2-propanol, 1,1,
1,3,3,3-hexafluoro-2-methyl-2-propanol, 1,1,1,3,3,3-hexafluoro-2-propanol, 2,2,3,3,3-pentafluoro Non-chlorine organic solvents such as -1-propanol and nitroethane are preferred. Of course, methylene chloride can also be used. Among them, methyl acetate and acetone can be preferably used. It is preferable to use a lower alcohol such as methanol, ethanol, or butanol in combination because the solubility of the cellulose ester in an organic solvent can be improved and the viscosity of the dope can be reduced. Particularly, ethanol having a low boiling point and low toxicity is preferable.

In the dope, a plasticizer such as a phthalic acid ester and a phosphoric acid ester, and an additive such as an ultraviolet absorber and a matting agent may be added. Then, the obtained dope is cast on a rotating belt or drum support, dried until peelable, and then separated from the support. The peeled raw dry film is further dried to almost completely evaporate the organic solvent in the film. The content of the organic solvent in the film is preferably 2% by mass or less, more preferably 0.4% by mass or less in order to obtain good dimensional stability of the film.

In producing a film, the following method is useful and preferable in obtaining a high retardation value in the thickness direction.

(1) In the process until the film is peeled off from the belt or drum, the retardation value (Rt value) increases when the amount of residual solvent at the time of peeling is reduced, and decreases when the amount is increased. In this case, the amount of the residual solvent at the time of peeling is preferably 5 to 200%, more preferably 5 to 10%.
0%, more preferably 10 to 50%.

The amount of the residual solvent in the film is represented by the following equation. Residual solvent amount = mass of remaining volatiles / mass of film after heat treatment x 100%.
This is a value obtained by subtracting the film mass after the heat treatment from the film mass before the heat treatment when the heat treatment is performed at 1 ° C. for 1 hour.

(2) The retardation value (Rt value) decreases when the tension at the time of peeling and the tension at the time of transport in the drying zone increase, and increases when the tension decreases. As a preferable peeling tension, 5 × 9.8 to 40 × 9.8 N
/ M, more preferably 10 × 9.8 to 30 × 9.8 N
/ M, more preferably 10 × 9.8 to 25 × 9.8 N
/ M. In addition, the transport tension in the drying zone is preferably 5 × 9.8 to 20 × 9.8 N / m, more preferably 8 × 9.8 to 15 × 9.8 N / m, and still more preferably 8 × 9.8 to 15 × 9.8 N / m. It is 9.8 to 12 × 9.8 N / m. (3) Alternatively, in the drying process after peeling the film from the belt or drum, when the film is dried while being stretched by the pin tenter method or the clip tenter method, the retardation value (Rt value) increases as the stretching magnification increases. , Smaller when smaller. The preferred stretching ratio is 2 to 50%, more preferably 5 to 40%, and still more preferably 10 to 30%.

[0068]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.

Example 1 1. Preparation of Cellulose Triacetate (TAC) Film Sample Nos. 1 to 15 were produced by the following method.

1) Sample No. Preparation of Samples Nos. 1 and 6 Nos. 1 and 6 were produced.

(Dope Composition A) Cellulose triacetate having a degree of acetyl substitution of 2.76 100 parts by mass 2- (2'-hydroxy-3 ', 5'-di-t-butylphenyl) benzotriazole 1 part by mass ethylphthalyl Ethyl glycolate 4 parts by mass Methylene chloride 475 parts by mass Ethanol 50 parts by mass The above composition was charged into a closed vessel, and completely dissolved while keeping the temperature at 80 ° C and stirring under pressure. Next, the dope was filtered, cooled and maintained at 33 ° C., and was uniformly cast on a rotating endless stainless steel band of 6 m (effective length 5.5 m) stretched between two drums, and peeled. When the solvent is evaporated until the residual solvent amount becomes 70%, the solvent is peeled off from the stainless steel band at a peeling tension of 15 × 9.8 N / m, and is transported by a number of rolls at a transporting tension of 13 × 9.8 N / m. After drying, the sample No. having a thickness of 150 μm was dried. 1 and film thickness 12
0 μm sample No. 6 was obtained.

2) Sample No. Preparation of Samples 2-5 and 7-15 Sample N was prepared using cellulose triacetates having different degrees of substitution as shown in Table 1 except that the amount of residual solvent and the film thickness were changed.
o. Sample Nos. 1 and 6 were used. 2-5 and 7-15
Was prepared.

2. Preparation of Liquid Crystal Display Sample Liquid crystal display device sample N using 1 to 15
o. 101-115 were produced by the method shown below.

1) Liquid crystal display device sample No. 101, 10
Preparation of Sample No. 6 1 and 6 were alkali-treated with a 2.5 mol / L aqueous sodium hydroxide solution at 40 ° C. for 60 seconds, and washed with water for 3 minutes to form a saponified layer to obtain an alkali-treated film. Next, a polyvinyl alcohol film having a thickness of 120 μm was immersed in an aqueous solution 10 containing 1 part by mass of iodine and 4 parts by mass of boric acid.
It was immersed in 0 parts by mass and stretched 4 times at 50 ° C. to form a polarizing film. The above-mentioned alkali-treated sample films were adhered to both sides of this polarizing film using a completely saponified polyvinyl alcohol 5% aqueous solution as an adhesive to prepare polarizing plates, which were provided on both sides of a VA type liquid crystal cell. . 10
1,106 were obtained.

2) Liquid crystal display device sample No. 102-10
Production of Sample Nos. 5 and 107 to 115 Using the liquid crystal display device sample Nos. Liquid crystal display device sample Nos. 102 to 105 and 107 to 115 were produced.

The TAC film sample No. Table 1 shows the measurement results of the retardation values (Rt values) of 1 to 15.

[0077]

[Table 1]

The liquid crystal display device sample No. 101-11
Table 2 shows the measurement results of the viewing angle characteristics of No. 5.

[0079]

[Table 2]

Evaluation method (1) Retardation value (Rt value) Using an automatic birefringence meter KOBRA-21ADH (manufactured by Oji Scientific Instruments) under an environment of 23 ° C. and 55% RH.
At a wavelength of 590 nm, a three-dimensional refractive index measurement is performed,
Refractive indices nx, ny and nz were obtained, and a retardation value (Rt value) was calculated according to the above equation.

(2) Evaluation of Viewing Angle Characteristics The obtained liquid crystal display device was provided with VG3 manufactured by AMT Corporation.
White display with 65N video pattern generator,
It performs black display and 8-gradation display of gray, and the contrast ratio at the time of white / black display is LCD-70 manufactured by Otsuka Electronics Co., Ltd.
At 00, the measurement was performed in a range of 60 degrees vertically and horizontally. An angle indicating a contrast ratio ≧ 10 was defined as a viewing angle.

(3) Comprehensive Evaluation The evaluation was made according to the following criteria.

◎: Very excellent viewing angle characteristics, no production problems, no workability, no defects as optical film products ○: Excellent viewing angle characteristics but inferior to ◎ level. No problems in production, processability, and no defects as optical film products. △: Excellent viewing angle characteristics, but problems in production, processability,
There are some defects as optical film products. X: Manufacturing problems, processability, fatal defects as optical film products. As shown in the results of Tables 1 and 2, the cellulose ester film of the present invention has a viewing angle characteristic. It is possible to reduce the film thickness if the film is peeled with the same residual solvent amount as the conventional TAC film, and it is possible to peel the film with a high residual solvent amount if the film thickness is approximately the same. Therefore, it was confirmed that productivity could be greatly increased.

On the other hand, if the degree of acetyl substitution is too small, the wet heat resistance is inferior, and there is a concern that decomposition may occur in the saponification treatment step or in the high-temperature and high-humidity state after the product is formed. did. When the amount of the residual solvent exceeds 200%, the retardation in the thickness direction does not increase, and when the amount of the residual solvent is too low, the film is hardly peeled off from the stainless steel plate, and the film is broken, and deformation and scratches are conspicuous. could not. Regarding the film thickness, if the thickness is smaller than 75 μm, it is difficult to produce a high retardation in the thickness direction. If the film thickness is larger than 165 μm, it is hard to cut and difficult to handle, and the workability is poor.
And

Embodiment 2 1. Preparation of Cellulose Triacetate (TAC) Film Sample Nos. 16 to 30 were produced by the following method.

1) Sample No. Preparation of Sample No. 16 by the method shown below. 16 were produced. (Dope composition B) 100 parts by mass of cellulose triacetate having a degree of acetyl substitution of 2.76 2- (2'-hydroxy-3 ', 5'-di-t-butylphenyl) benzotriazole 1 part by mass ethylphthalylethyl glycolate 4 parts by mass Methyl acetate 534 parts by mass Ethanol 63 parts by mass Of the above composition, a mixed solution of a methyl acetate / ethanol mixed solvent (9/1 mass ratio) and ethylphthalylethyl glycolate was cooled to -70 ° C. Then, TAC having a degree of acetyl substitution of 2.76 is added thereto with good stirring. After about 20 minutes, the TAC had swelled, so it was left for 1 hour while maintaining the temperature at -70 ° C with slow stirring.
Thereafter, the cooled mixture was slowly stirred at 50 ° C.
For 30 minutes. The cycle of cooling and heating the mixture was repeated twice to obtain a transparent and uniform cellulose ester dope. This solution was allowed to stand at 23 ° C. for 30 days, and the state of the dope was observed.

The dope immediately after dissolution was rotated in a length of 6 m (effectively 5.5 mm) stretched on two drums in the same manner as in Example 1.
m) Evenly cast on the endless stainless steel band,
At the time when the solvent was evaporated until the amount of the remaining solvent after peeling reached 100%, the peeling tension was 15 × 9.8 N from the stainless steel band.
/ M, and the transport tension is 13 × 9.8N with many rolls.
/ M and dried while transported at a thickness of 120 μm.
C film 16 was obtained.

2) Sample No. Preparation of Samples 17 to 19 Sample N was prepared except that the degree of acetyl substitution was changed as shown in Table 5.
o. Sample No. 16 in the same manner as Sample No. 16 17 to 19 were produced. In addition, sample No. 18 and 19 dope for 24 hours
After the stagnation at 3 ° C., a gel-like dope was formed.

3) TAC film sample No. Preparation of Sample No. 20 except that the amount of the residual solvent peeled off from the belt was set to 50%. A film was produced in the same manner as in Example 16.

4) Sample No. Preparation of TAC film sample No. 21 by the method described below. 21 was produced.

(Dope Composition B) 100 parts by mass of cellulose triacetate having a degree of acetyl substitution of 2.76 100 parts by mass of 2- (2′-hydroxy-3 ′, 5′-di-t-butylphenyl) benzotriazole 1 part by mass ethylphthalyl Ethyl glycolate 4 parts by mass Methyl acetate 534 parts by mass Ethanol 63 parts by mass The above-mentioned composition was mixed at room temperature and allowed to stand with gentle stirring for 30 minutes. After confirming that the TAC has swollen,
And left for 1 hour. Thereafter, the cooled mixture was heated at 50 ° C. for 30 minutes with slow stirring. The cycle of cooling and heating the mixture was repeated twice to obtain a transparent and uniform cellulose ester dope. This solution is kept at 23 ° C
When the state of the dope was observed by standing for 30 days, it was found that the dope remained unchanged as a transparent and uniform dope.

The dope immediately after dissolution was rotated in a length of 6 m (effectively 5.5 mm) stretched on two drums in the same manner as in Example 1.
m) Evenly cast on the endless stainless steel band,
At the time when the solvent was evaporated until the amount of the remaining solvent after peeling reached 100%, the peeling tension was 15 × 9.8 N from the stainless steel band.
/ M, and the transport tension is 13 × 9.8N with many rolls.
/ M and dried while transported at a thickness of 120 μm.
C film 21 was obtained.

5) Sample No. Preparation of Sample No. 22 TAC having a degree of acetyl substitution of 2.89 as 22
Except that Sample No. was used. As in the case of 21, a TAC solution (dope) was prepared, and a film sample was prepared. Immediately after dissolution, some insoluble matter could be confirmed, but 24 h
r. After the stagnation, the dope became gel-like.

6) TAC film sample No. 23, 24
Preparation of TAC film sample No. 23, 24
Was prepared.

(Dope Composition C) Cellulose triacetate having a degree of acetyl substitution of 2.84 100 parts by mass 2- (2'-hydroxy-3 ', 5'-di-t-butylphenyl) benzotriazole 1 part by mass ethylphthalyl Ethyl glycolate 4 parts by mass Methyl acetate 534 parts by mass Ethanol 63 parts by mass
Let stand for minutes. After confirming that the TAC had swelled, the mixture was filled in a 100-μm-thick aluminum container, covered with aluminum foil to prevent air from entering, and sealed by caulking. This sealed container was packed in a rubber bag, and after lightly degassing, the rubber bag was sealed. The rubber bag is set in a ceramic molding type rubber hydrostatic pressure press (made by Kobe Steel) and pressurized at a pressure of 9.8 × 10 ⁷ Pa for 2 hours while maintaining the temperature at 20 ° C. Then slowly return to atmospheric pressure for 30 minutes.
Let stand for a minute. This pressure-release cycle was repeated three times to obtain a transparent and uniform TAC dope. This solution is kept at 23 ° C
24 hr. Observation of the state of the dope by standing still showed that the dope remained transparent and uniform.

The dope immediately after the dissolution was rotated in a length of 6 m (effectively 5.5 mm) stretched on two drums in the same manner as in Example 1.
m) Evenly cast on the endless stainless steel band,
At the time when the solvent was evaporated until the amount of the remaining solvent after peeling reached 100%, the peeling tension was 15 × 9.8 N from the stainless steel band.
And dried while being transported by a number of rolls at a transport tension of 13 × 9.8 N. 23 was obtained.

7) Sample No. Preparation of Sample No. 24 TAC having an acetyl substitution degree of 2.89 as 24
Except that Sample No. was used. 23, a TAC solution (dope) was prepared. 24
Was prepared. Immediately after dissolution, however, some insoluble matter could be confirmed, but 24 hr. After the stagnation, the dope became gel-like.

8) TAC film sample No. 25-28
Preparation of acetyl substitution degree 2.80, cooling temperature, heating temperature Table 3
TAC film sample no. 21
It was produced in the same manner as described above.

[0099]

[Table 3]

Sample No. In Nos. 26 and 27, even when the cooling-heating cycle was repeated, insolubles did not disappear, and films could not be produced.

Sample No. In No. 28, a uniform dope was obtained immediately after dissolution, but after several hours, gelation started and the dope was slightly colored.

9) Sample No. Sample No. 29 to 30 except that the acetyl substitution degree was 2.80 and the pressure was as shown in Table 4. 23.

[0103]

[Table 4]

Sample No. In No. 29, the insoluble material did not disappear even when the pressure-opening cycle was repeated, and a film could not be produced. Sample No. In No. 30, a decrease in viscosity and coloring were observed when a cycle of pressurization and release was repeated, and thus a film could not be produced.

2. Preparation of Liquid Crystal Display Sample No. TAC film sample No. A polarizing plate was prepared in the same manner as in Example 1 using Sample Nos. 16 to 25 and 28, provided on both sides of a VA liquid crystal cell. 216-
225 and 228 were obtained.

The above TAC film sample No. 16-30
Table 5 shows the measurement results of the retardation values (Rt values).

[0107]

[Table 5]

The liquid crystal display device sample No. 216-22
Table 6 shows the measurement results of the viewing angle characteristics of Nos. 5 and 228.

[0109]

[Table 6]

Evaluation method (1) Evaluation of dissolved state Sensory evaluation was performed according to the following criteria.

；: Transparent and uniform dope ；: Some insolubles were confirmed X: A large amount of insolubles was present or in a gel state (1) Retardation value (Rt value) It evaluated similarly.

(3) Evaluation of Viewing Angle Characteristics Evaluation was performed in the same manner as in Example 1.

(4) Comprehensive Evaluation The evaluation was made according to the following criteria.

A: Very excellent viewing angle characteristics, no production problems, processability, and no defects as optical film products. A: Excellent viewing angle characteristics, but inferior to level. No problems in production, processability, and no defects as optical film products. △: Excellent viewing angle characteristics, but problems in production, processability,
Sample No. having some defect as optical film product x: Manufacturing problem, processability, fatal defect as optical film product 25 has excellent viewing angle characteristics, but even if the cooling temperature is lowered to -100 ° C or lower, the dissolution effect is already saturated and does not change much. Since the evaporation was fast and inefficient and the productivity was poor, the overall evaluation was rated as △.

Sample No. 26, sample no. 27, sample N
o. The sample No. 29 was evaluated as x because the insolubles did not disappear even after the cooling-heating cycle or the pressure-opening cycle was repeated and a film could not be produced.

Sample No. In No. 28, a uniform dope can be obtained immediately after dissolution, but after several hours, gelation starts and the dope starts to be slightly colored.
And

Sample No. In No. 30, since a decrease in viscosity and coloring were observed after repeated cycles of pressurization and release and decomposition began to occur, a film could not be produced, and the overall evaluation was evaluated as x.

Accordingly, it can be seen from Table 6 that the cellulose ester of the present invention has excellent solubility and stagnation stability even with the special dissolution method using a non-chlorine solvent, and also has good viewing angle characteristics.

[0119]

By using the cellulose acetate of the present invention, an optical film having good transparency, flatness and retardation value (Rt value) in the thickness direction can be obtained.
In a VA liquid crystal display device using a polarizing plate used as a protective film for a polarizing plate using the film, a favorable viewing angle widening effect could be obtained even though it was thinner than a conventional product. In addition, even when a non-chlorine solvent is used, it is possible to obtain a solution having good solubility and stagnant stability, and the optical film obtained by casting and casting is the same as a case using a chlorine solvent. Similar performance could be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a polarizing plate.

[Explanation of symbols]

 1 polarizer 2 protective film

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) B29L 7:00 B29L 7:00 F term (Reference) 2H091 FA08X FA08Z FB02 FC23 GA06 HA18 KA02 LA19 4F071 AA09 AF31Y AG22 AG23 AG28 AG29 AG34 AG36 AH19 BA02 BA03 BB02 BC01 BC02 BC12 4F205 AA01A AG01 AH42 GA07 GB02 GC02 GC07 GE22 GF24 GN22 GN29 4J002 AB021 FD010 FD020 FD050 GG02 GP00

Claims (15)

[Claims] 1. The acetyl substitution degree is 2.75 or more and 2.8.
An optical film using 6 or less cellulose esters. 2. The optical film according to claim 1, wherein the degree of acetyl substitution is 2.84 or less. 3. The optical film according to claim 1, wherein a retardation value (Rt value) in a thickness direction defined by the formula (1) is 90 to 200 nm. (1) Formula Rt value = ((nx + ny) / 2−nz) × d (where nx is the refractive index of the film in the casting direction of the film, and ny is the film in the direction (width direction) perpendicular to the casting direction). , Nz represents the refractive index of the film in the thickness direction, and d represents the thickness (nm) of the film.) 4. The optical film according to claim 1, wherein the film has a thickness of 75 to 165 μm. 5. The acetyl substitution degree is 2.75 or more and 2.8.
Dissolving up to 6 cellulose esters in an organic solvent, casting it on a rotating belt or drum, peeling off the residual solvent amount from the belt or drum at 5 to 200%, and further drying to evaporate the organic solvent A method for producing an optical film, comprising: 6. The method for producing an optical film according to claim 5, wherein a cellulose ester having a degree of acetyl substitution of 2.84 or less is used. 7. The acetyl substitution degree is 2.75 or more and 2.8.
6 or less cellulose ester, -100 ℃ ~ -10 ℃
Mixed with a substantially halogen-free organic solvent cooled to
The cellulose ester solution obtained by swelling and heating at a temperature of 0 ° C. to 120 ° C. as one cycle is performed at least once and cast on a rotating belt or drum, and then the residual solvent is removed from the belt or drum. 5-20
A method for producing an optical film, comprising separating at 0%, further drying and evaporating an organic solvent. 8. The method for producing an optical film according to claim 7, wherein a cellulose ester having a degree of acetyl substitution of 2.84 or less is used. 9. The acetyl substitution degree is 2.75 or more and 2.8.
At least a step of mixing a cellulose ester of 6 or less with an organic solvent substantially free of halogen, swelling, cooling to −100 ° C. to −10 ° C., and heating to 0 ° C. to 120 ° C. as one cycle The cellulose ester solution obtained by performing once is cast on a rotating belt or drum, and then the amount of residual solvent when peeling from the belt or drum is removed at 5 to 200%, and further dried to remove the organic solvent. A method for producing an optical film, comprising evaporating. 10. A cellulose ester having a degree of acetyl substitution of not more than 2.84.
3. The method for producing an optical film according to item 1. 11. The acetyl substitution degree is 2.75 or more.
86 or less of a cellulose ester is mixed with an organic solvent, swelled and pressurized to 5 × 9.8 × 10 ⁵ to 2 × 9.8 × 10 ⁷ Pa, and the step of releasing is defined as at least one cycle. The cellulose ester solution obtained by performing the process is cast on a rotating belt or drum, and then the amount of residual solvent when peeling from the belt or drum is 5 to 200.
%, Followed by drying and evaporating the organic solvent to produce an optical film. 12. A cellulose ester having a degree of acetyl substitution of 2.84 or less.
2. The method for producing an optical film according to item 1. 13. A polarizing plate, wherein the optical film according to claim 1 is adhered to at least one surface of a polarizer. 14. A protective film for a polarizing plate, comprising an optical film produced by the production method according to claim 5. Description: 15. A vertical alignment (VA) type liquid crystal display device, comprising a polarizing plate using the optical film according to claim 1. Description: