JP4730503B2 - Method for producing cellulose ester film - Google Patents

Description

ここで、Ｔ_１は冷却ドラムの表面温度、Ｋ_１はフィルム剥離前にベルトが冷却ドラムに接する時間、Ｔ_２はフィルム剥離時の雰囲気露点、Ｋ_２はフィルム剥離後にベルトが流延位置に達するまでの時間である。
【００８３】
さて、上記条件にて製造した試料全てについて表面を子細に観察し、そのコンデンス状態すなわち曇り具合を評価した。結果は次のとおりである。

ここで、〇は曇りがなく極めて良好な状態、△は極めて僅かながら曇りが認められるが許容できる状態、×は全面が曇っており許容できない状態であることをそれぞれ意味する。
【００８４】
【発明の効果】
本発明によれば、膜厚が２０〜２００μｍのセルロースエステルフィルムを製造するに際して、曇りの発生が効果的に抑えられる。
【図面の簡単な説明】
【図１】本発明の実施形態を説明するための参考図
【図２】ベルト流延方式の装置の概略図
【符号の説明】
１ フィルム
２ 冷却ドラム
３ ベルト
４ 流延ダイ[0001]
BACKGROUND OF THE INVENTION
The present invention is used in various functional films such as a protective film for polarizing plates, a retardation film, a viewing angle widening film, an antireflection film used in a plasma display, an organic EL display, etc. The present invention relates to a method for producing a cellulose ester film that can be used for various functional films.
[0002]
[Problems to be solved by the invention]
Liquid crystal display devices (LCDs) are widely used as display devices for word processors, personal computers, etc. because they can be directly connected to IC circuits with low voltage and low power consumption, and can be particularly thin. Yes. The basic configuration of this LCD is, for example, that polarizing plates are provided on both sides of a liquid crystal cell.
[0003]
This polarizing plate passes only light having a polarization plane in a certain direction. Therefore, the LCD plays an important role of visualizing the change in the alignment of the liquid crystal due to the electric field. That is, the performance of the LCD greatly depends on the performance of the polarizing plate. The polarizing plate generally has a structure in which a polarizing plate protective film is laminated on both sides of the polarizer. An LCD is configured by laminating a polarizing plate having such a structure on a liquid crystal cell.
[0004]
The polarizer is obtained by, for example, adsorbing and stretching iodine or the like on a polymer film. That is, a solution called H ink containing a dichroic substance (iodine) is wet-adsorbed on a polyvinyl alcohol film, and then the film is uniaxially stretched to orient the dichroic substance in one direction. It is. As the polarizing plate protective film, a cellulose resin, particularly cellulose triacetate is used.
[0005]
The polarizing plate protective film is manufactured using a belt casting apparatus shown in FIG. In the figure, 1 is a film, 2 is a cooling drum, 3 is a casting belt, 4 is a casting die, 5 is a peeling roll, 6 is a roll drying section, and 7 is a winding roll. When such an apparatus is used, after the dope is cast from the casting die 4 to the casting belt 3, the film 1 is peeled off by the peeling roll 5 provided facing the cooling drum 2, and further roll-dried. After completely drying at the part 6, the film 1 is wound up by the winding roll 7.
[0006]
Now, a cellulose ester film having a film thickness of 20 to 200 μm, which is used as an optical film in this way, has been produced, but the resulting film sometimes had cloudiness.
[0007]
Therefore, the problem to be solved by the present invention is to provide a technique for suppressing the occurrence of fogging when producing a cellulose ester film having a film thickness of 20 to 200 μm.
[0008]
[Means for Solving the Problems]
As a result of diligent investigations to solve the above problems, the occurrence of fogging is caused by the cooling state in the cooling drum, particularly the surface temperature of the cooling drum at a position where the film is separated from the cooling drum, and the film cooling. It has been found that it greatly depends on the time from contact with the drum until peeling.
[0009]
That is, by controlling the surface temperature of the cooling drum at a position where the film is separated from the cooling drum to a specific temperature, and controlling the time from when the film contacts the cooling drum until it is peeled off to a specific time, It was found that clouding can be suppressed.
[0010]
  The present invention has been achieved based on such findings,
  A dope composition containing a cellulose ester is cast on the belt of a film forming apparatus having a belt and a cooling drum, and a film of the cast dope composition is peeled from the belt at a position on the cooling drum. This is a method for producing a cellulose ester film having a thickness of 20 to 200 μm,
  The surface temperature of the cooling drum at a position where the film on the belt is separated from the cooling drum is 6 ° C. or more,
  The film is peeled off from the belt within 30 to 900 seconds after the dope composition is cast on the belt, and peeled off within 0.5 to 20 seconds after the film is in contact with the cooling drum. Be done
This is solved by the method for producing a cellulose ester film.
[0011]
Furthermore, by controlling the dew point of the atmosphere when the film is peeled from the belt to be 7 ° C. or less, the above problem can be solved more effectively.
[0012]
In addition, the above problem can be solved by controlling the time until the belt from which the film has been peeled (exactly a part thereof) reaches the casting site of the dope composition to be 0.5 to 30 seconds. It is solved more effectively.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
In the method for producing a cellulose ester film according to the present invention, a dope composition containing a cellulose ester is cast on the belt of a film forming apparatus having a belt and a cooling drum, and the film of the cast dope composition is the above-described film. A method of producing a cellulose ester film having a thickness of 20 to 200 μm (particularly 40 μm or more and 100 μm or less) by peeling from the belt at a position on a cooling drum, wherein the film on the belt is cooled The surface temperature of the cooling drum at a position away from the drum is 6 ° C. or more (particularly 7 ° C. or more and 15 ° C. or less), and the time from the contact of the film with the cooling drum to the peeling is zero. 5 to 20 seconds (especially 1.5 seconds or more and 12 seconds or less). And the dew point of the atmosphere when peeling a film from a belt is 7 degrees C or less (especially 5 degrees C or less. A lower limit is not prescribed | regulated). In addition, the time required for (a part of) the belt from which the film has been peeled to reach the casting portion of the dope composition is 0.5 to 30 seconds (particularly, 1 second to 10 seconds). This will be described in detail below.
[0014]
The method for producing a cellulose ester film according to the present invention can also be carried out using a conventional belt casting apparatus shown in FIG. Therefore, a detailed description of the device itself is omitted. “The surface temperature of the cooling drum at a position where the film on the belt is separated from the cooling drum”, “the time from the contact of the film with the cooling drum until the film is peeled”, “the belt from which the film has been peeled is doped. The meaning of “time to reach the casting point of the composition” is as follows (see FIG. 1).
[0015]
First, since the film 1 comes into contact with the cooling drum 2 when the belt 3 comes into contact with the cooling drum 2, the time (position) at which the film 1 comes into contact with the cooling drum 2 is point A in FIG. Since the film 1 is peeled from the belt 3 at a position on the cooling drum 2, the point (position) at which the film 1 on the belt 3 is separated from the cooling drum 2 is point B in FIG. Furthermore, the casting point is point C.
[0016]
Therefore, the “surface temperature of the cooling drum at a position where the film on the belt is separated from the cooling drum” is the surface temperature of the cooling drum 2 at the point B. The “time from when the film comes into contact with the cooling drum until it is peeled off” is the time required to move from point A to point B. Further, the “time until the belt from which the film has been peeled off reaches the casting portion of the dope composition” is the time required for movement from point B to point C. The point C may be on the cooling drum 2 or may be on a path followed after the film (web) 1 has passed through the cooling drum 2.
[0017]
The film forming process employed in the method for producing a cellulose ester film of the present invention includes a dissolution process, a casting process, a solvent evaporation process, a peeling process, a drying process, and a winding process, which will be described in detail below.
[0018]
[Dissolution process]
In this specification, the cellulose ester solution is referred to as cellulose ester dope or simply dope. This step is a step of forming a dope by dissolving the flakes in cellulose ester flakes in an organic solvent mainly composed of a good solvent described below while stirring in the dissolution vessel. In this invention, it is preferable to adjust solid content concentration in dope to 15 mass% or more, and 18-35 mass% is the especially preferable range.
[0019]
However, if the solid content concentration in the dope is too high, the viscosity of the dope becomes too high, and a sharkskin or the like may occur during casting to deteriorate the film flatness. desirable. The dope viscosity is preferably adjusted in the range of 10 to 50 Pa · s.
[0020]
For dissolution, a method carried out at normal pressure, a method carried out below the boiling point of the preferred organic solvent (ie good solvent), a method carried out under pressure above the boiling point of this good solvent, a method carried out by the cooling dissolution method, a method carried out at high pressure, etc. There are various dissolution methods. When using a method of dissolving at a temperature higher than the boiling point of a good solvent and applying a pressure that does not boil, pressurization at 40.4 to 120 ° C. to 0.11 to 1.50 MPa suppresses foaming and shortens Can be dissolved in time.
[0021]
As the cellulose ester used in the present invention, a lower fatty acid ester of cellulose is preferably used. The lower fatty acid in the lower fatty acid ester of cellulose ester means a fatty acid having 6 or less carbon atoms, such as cellulose acetate, cellulose propionate, cellulose butyrate, etc., and JP-A-10-45804 and 8- Specific examples of cellulose lower fatty acid esters include mixed fatty acid esters such as cellulose acetate propionate and cellulose acetate butyrate described in US Pat. No. 2,176,61, US Pat. No. 2,319,052, and the like. Incidentally, the substitution degree measurement of the acyl group of cellulose ester can be carried out according to ASTM-D-817-96.
[0022]
By the way, among the above fatty acids, cellulose acetate and cellulose acetate propionate are often used as preferable ones, but in the case of the cellulose ester film obtained in the present invention, the degree of polymerization is particularly 250 to 400 from the viewpoint of film strength. Are preferred.
[0023]
The cellulose ester film obtained in the present invention is preferably used as a cellulose ester having a total substitution degree of 2.5 to 3.0, and a cellulose ester having a total substitution degree of 2.55 to 2.85 is particularly preferable. When the total substitution degree is 2.55 or more, the mechanical strength of the film increases. On the other hand, when the total substitution degree is 2.85 or less, the solubility of cellulose ester is improved and the generation of foreign matters is further reduced.
[0024]
As cellulose acetate propionate, when the acetyl group substitution degree is X and the propionyl group substitution degree is Y,
2.55 ≦ X + Y ≦ 2.85
1.5 ≦ X ≦ 2.4
Those within the range are preferably used.
[0025]
As the cellulose ester, a cellulose ester synthesized from cotton linter, a cellulose ester synthesized from wood pulp, or a cellulose ester synthesized from other raw materials can be used alone or in combination.
[0026]
The solvent used in preparing the dope is not particularly limited as long as it is a solvent that can dissolve the cellulose ester, but even a solvent that cannot dissolve it alone can be dissolved by mixing with another solvent. Anything can be used. In general, a mixed solvent composed of a good solvent, methylene chloride and a poor solvent of cellulose ester, is used, and the mixed solvent preferably contains 4 to 30% by mass of the poor solvent.
[0027]
Other good solvents that can be used include, for example, methylene chloride, methyl acetate, ethyl acetate, amyl acetate, acetone, tetrahydrofuran, 1,3-dioxane, 1,4-dioxane, cyclohexanone, ethyl formate, 2,2,2- Trifluoroethanol, 2,2,3,3-tetrafluoro-1-propanol, 1,3-difluoro-2-propanol, 1,1,1,3,3,3-hexafluoro-2-methyl-2- Examples include propanol, 1,1,1,3,3,3-hexafluoro-2-propanol, 2,2,3,3,3-pentafluoro-1-propanol, nitroethane, and the like. However, organic halogen compounds such as methylene chloride, dioxane derivatives, methyl acetate, ethyl acetate, acetone, and the like are also preferable organic solvents (that is, good solvents). The use of methyl acetate is particularly preferred because the resulting film has less curl.
[0028]
Examples of poor solvents for cellulose esters include alcohols having 1 to 8 carbon atoms such as methanol, ethanol, n-propanol, iso-propanol, n-butanol, sec-butanol, tert-butanol, methyl ethyl ketone, methyl isobutyl ketone, and acetic acid. Examples include ethyl, propyl acetate, monochlorobenzene, benzene, cyclohexane, tetrahydrofuran, methyl cellosolve, and ethylene glycol monomethyl ether. These poor solvents can be used alone or in combination of two or more.
[0029]
In the method for producing a cellulose ester film of the present invention, it is also preferable to use a cooling dissolution method when dissolving the cellulose ester. As this cooling dissolution method, for example, the techniques described in JP-A-9-95538, 9-95544 and 9-95557 can be used. Moreover, the high-pressure dissolution method described in JP-A-11-21379 can also be preferably used.
[0030]
After dissolution, the cellulose ester solution (dope) is preferably filtered through a filter medium, defoamed, and sent to the next step with a pump. In this case, the dope contains a plasticizer, an antioxidant, and an ultraviolet absorber. Agents, dyes, fine particles and the like are preferably added. These additives may be added together with the cellulose ester and the solvent during the preparation of the cellulose ester solution, or may be added during or after the solution preparation.
[0031]
A plasticizer can be added to the cellulose ester film produced by the technique of the present invention. Examples of the plasticizer that can be used for this purpose include phosphate ester plasticizers, phthalate ester plasticizers, and citrate ester plasticizers, but of course not limited thereto. Among these, phosphate ester types include triphenyl phosphate, tricresyl phosphate, cresyl diphenyl phosphate, octyl diphenyl phosphate, diphenyl biphenyl phosphate, trioctyl phosphate, tributyl phosphate, etc., and phthalate ester types include diethyl phthalate, Dimethoxyethyl phthalate, dimethyl phthalate, dioctyl phthalate, dibutyl phthalate, di-2-ethylhexyl phthalate, butyl benzyl phthalate, dibenzyl phthalate, butyl phthalyl butyl glycolate, ethyl phthalyl ethyl glycolate, methyl phthalyl ethyl glycolate, etc. In addition, citric acid ester plasticizers include triethyl citrate, tri-n-butyl citrate, acetyl trie Rushitoreto, acetyl tri-n-butyl citrate, can be preferably used acetyl tri-n-(2-ethylhexyl) citrate and the like.
[0032]
These are preferably used alone or in combination, and two or more of these plasticizers may be used in combination as required. The plasticizer is preferably used in an amount of 1 to 30% by weight based on the cellulose ester, more preferably 2 to 25% by weight, particularly preferably 2 to 15% by weight, and most preferably. Is 3-12% by mass.
[0033]
The cellulose ester film produced by using the technique of the present invention may contain an additive exhibiting the same action as the plasticizer in addition to the plasticizer. Such an additive may be any low molecular organic compound that can plasticize the cellulose ester film.
[0034]
It is preferable that an ultraviolet absorber is added to the cellulose ester film produced using the technique of the present invention. For example, from the viewpoint of preventing deterioration of the liquid crystal, those that are excellent in the ability to absorb ultraviolet rays having a wavelength of 370 nm or less and that absorb less visible light having a wavelength of 400 nm or more are preferably used from the viewpoint of good liquid crystal display properties. The cellulose ester film particularly preferably has a transmittance at a wavelength of 380 nm of 10% or less, more preferably 5% or less, and still more preferably 1% or less.
[0035]
In carrying out the present invention, for example, a blue dye may be used as an additive in order to adjust the color of the film. Preferable dyes include anthraquinone dyes. This anthraquinone dye may have an arbitrary substituent at positions 1 to 8 of anthraquinone. Preferable substituents include an anilino group, a hydroxyl group, an amino group, a nitro group, or a hydrogen atom that may be substituted. The amount of these dyes added to the film is 0.1 to 1000 μg / m in order to maintain the transparency of the film.², Preferably 10-100 μg / m²It is.
[0036]
In addition, it is preferable to add fine particles as a matting agent to the cellulose ester film in order to impart slipperiness. These fine particles 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. Note that the larger the average diameter of the fine particles, the greater the mat effect, while the smaller the average diameter, the better the transparency. For this reason, the average diameter of the primary particles of the fine particles is preferably 5 to 50 nm, more preferably 7 to 20 nm.
[0037]
The type of the fine particles is not particularly limited, and examples thereof include AEROSIL200, 200V, 300, R972, R972V, R974, R202, R812, OX50, TT600 manufactured by Nippon Aerosil Co., and particularly preferable are AEROSIL200V, R972V, and the like. . These fine particles usually form secondary particles having an average particle diameter of 0.01 to 1.0 μm, and are present as aggregates of primary particles in the film, and 0.01 to 1.0 μm on the film surface. Produce irregularities. Therefore, the content of these fine particles is preferably 0.005 to 0.3% by mass with respect to the cellulose ester. In particular, the cellulose ester film preferably contains fine particles of silicon dioxide. By casting the cellulose ester film using a cellulose ester solution containing fine particles of silicon dioxide, the cellulose ester film is caused by aggregation of fine particles in the cellulose ester solution. The foreign matter failure is significantly reduced.
[0038]
Examples of the method for preparing the fine particle dispersion include the following three methods.
[0039]
[Preparation Method A]
After stirring and mixing the solvent and the fine particles, dispersion is performed with a disperser. This is a fine particle dispersion. The fine particle dispersion is added to the dope solution and stirred.
[0040]
[Preparation Method B]
After stirring and mixing the solvent and the fine particles, dispersion is performed with a disperser. This is a fine particle dispersion. Separately, a small amount of cellulose ester is added to the solvent and dissolved by stirring. The fine particle dispersion previously obtained is added to this and stirred. This is a fine particle addition solution. And this fine particle addition liquid is fully mixed with dope liquid with an in-line mixer.
[0041]
[Preparation Method C]
Add a small amount of cellulose ester to the solvent and dissolve with stirring. Fine particles are added to this and dispersed by a disperser. This is a fine particle addition solution. And this fine particle addition liquid is fully mixed with dope liquid with an in-line mixer.
[0042]
Among these, the preparation method A is excellent in the dispersibility of the silicon dioxide fine particles, and the preparation method C is excellent in that the silicon dioxide fine particles hardly reaggregate. Especially, the said preparation method B is a preferable preparation method excellent in both the dispersibility of silicon dioxide microparticles | fine-particles, and the point which silicon dioxide microparticles | fine-particles cannot reaggregate further.
[0043]
[Distribution method]
The concentration of silicon dioxide when the silicon dioxide fine particles are mixed and dispersed with a solvent or the like is preferably 5 to 30% by mass, more preferably 10 to 25% by mass, and most preferably 15 to 20% by mass. A higher dispersion concentration is preferable because liquid turbidity with respect to the added amount tends to be low, and haze and aggregates are improved.
[0044]
The solvent used is preferably lower alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, and butyl alcohol. Solvents other than the lower alcohol are not particularly limited, but it is preferable to use a solvent used at the time of film formation of the cellulose ester.
[0045]
Using the dope thus obtained, a cellulose ester film can be obtained through a casting process described below.
[0046]
[Casting process]
In this step, the dope is fed to the pressure die through the pressure type quantitative gear pump, and the dope is cast from the pressure die onto the endless metal belt that is infinitely transferred at the casting position. Incidentally, the surface of the belt for casting is a mirror surface.
[0047]
Other casting methods include a doctor blade method in which the film thickness of the cast dope film is adjusted with a blade, or a reverse roll coater method in which the film is adjusted with a reverse rotating roll. A pressure die that can be prepared and is easy to make the film thickness uniform is preferable. Examples of the pressure die include a coat hanger die and a T die, and any of them is preferably used.
[0048]
In order to increase the film forming speed, two or more pressure dies may be provided on the belt for casting, and the dope amount may be divided and stacked. Alternatively, the inside of the die can be divided by slits, and a plurality of dope solutions having different compositions can be cast at the same time (also referred to as co-casting) to obtain a laminated structure cellulose ester film.
[0049]
The dope thus obtained is cast on a belt to form a film.
[0050]
[Solvent evaporation step]
This step is a step of heating a web (in this specification, a dope film formed by casting a dope on a casting belt is called a web) on the casting belt to evaporate the solvent. . Methods for evaporating the solvent include a method in which air is blown from the web side and / or a method in which heat is transferred from the back surface of the belt by liquid, a method in which heat is transferred from the front and back by radiant heat, and the like. From the point of view, it is preferable. A method of combining them is also preferable. Furthermore, the web on the belt after casting is preferably dried on the belt in an atmosphere of 40 to 100 ° C. In order to maintain an atmosphere of 40 to 100 ° C., it is preferable to apply hot air at this temperature to the upper surface of the web or heat by means such as infrared rays.
[0051]
When producing a cellulose ester film by the technique of the present invention, it is desirable to peel the web from the belt within 30 to 900 seconds after casting. If it peels in less than 30 seconds, not only the surface quality of a film will fall but it is unpreferable also in terms of moisture permeability. Further, drying for more than 900 seconds is not preferable because surface quality is deteriorated due to deterioration of peelability and strong curling occurs in the film.
[0052]
[Peeling process]
This step is a step of peeling the web from which the solvent has evaporated on the belt from the belt at the peeling position. The peeled film (web) is sent to the next step. However, if the residual solvent amount of the web at the time of peeling is too large, it is difficult to peel off, or conversely, if it is peeled off after sufficiently drying on the belt, part of the web is peeled off.
[0053]
About the temperature of the atmosphere in a peeling position, Preferably it is 15-50 degreeC, More preferably, it is 18-35 degreeC. And it is important that the surface temperature of this cooling drum in the position which a film separates from a cooling drum is 6 degreeC or more. Preferably, they are 7 degreeC or more and 15 degrees C or less.
[0054]
Further, it is important that the time from the contact of the film with the cooling drum to the peeling is 0.5 to 20 seconds. Preferably, it is 12 seconds or more and 15 seconds or less. In addition, the dew point of the atmosphere when peeling the film from the belt is preferably 7 ° C. or less. More preferably, it is 5 degrees C or less (a lower limit is not prescribed | regulated in particular).
[0055]
It is preferable that the residual solvent amount of the web in the said peeling position is 25-120 mass%, More preferably, it is 40-100 mass%. This residual solvent amount is defined by the following equation.
[0056]
Residual solvent amount (%) = [(mass before heat treatment of web−mass after heat treatment of web) / (mass after heat treatment of web)] × 100
Note that the heat treatment for measuring the residual solvent amount means performing heat treatment at 115 ° C. for 1 hour.
[0057]
In order to adjust the residual solvent amount at the time of peeling as described above, the temperature at the peeling position on the belt is controlled so that the surface temperature of the belt after casting is controlled and the organic solvent can be efficiently evaporated from the web. Is preferably set in the above range. In order to control the belt temperature, it is preferable to use a method having good heat transfer efficiency, and for example, a back surface heat transfer method using liquid is preferable.
[0058]
The heat transfer method using radiant heat or hot air is difficult to control the belt temperature and cannot be said to be a preferable method. However, in a belt machine, a belt with gentle wind is used to control the temperature of the belt where the belt to be transported has come down. The temperature can be adjusted. The belt temperature can be partially changed by dividing the heating means, and the casting position, drying section, peeling position, etc. of the casting belt can be set to different temperatures.
[0059]
As a method for increasing the film forming speed (the film forming speed can be increased because the film is peeled off while the amount of residual solvent is as large as possible), there is a gel casting method (gel casting) that can peel even if there is a large amount of residual solvent. For this, a poor solvent for the cellulose ester is added to the dope, and after the dope is cast, a method of gelation, a method of gelling by lowering the belt temperature, or the like can be used. Further, there is a method of adding a metal salt in the dope.
[0060]
When practicing the present invention, gelation can be performed on a belt, and the film can be strengthened to accelerate peeling and increase the film forming speed.
[0061]
When peeling at a time when the amount of residual solvent is larger, if the web is too soft, the flatness will be impaired at the time of peeling, and slippage and vertical streaks are likely to occur due to the peeling tension.Therefore, peeling remains due to the balance between economic speed and quality. The amount of solvent is determined. The peeling tension at the time of peeling the belt and the film is usually 196 to 245 N / m, but when wrinkles are likely to occur at the time of peeling, it is preferable to peel at 190 N / m or less, and further, the lowest peelable Peeling is preferred at a tension of ~ 166.6 N / m and then at a minimum tension of ~ 137.2 N / m, but most preferred is a peel at a minimum tension of ~ 100 N / m. In particular, the lower the release tension, the better the in-plane retardation Ro can be kept low. The in-plane retardation Ro is preferably less than 20 nm, more preferably less than 10 nm, and then preferably less than 5 nm, most preferably 0 to 1 nm.
[0062]
In the present invention, the in-plane retardation Ro is measured with a three-dimensional refractive index at a wavelength of 590 nm using an automatic birefringence meter KOBRA-21ADH (manufactured by Oji Scientific Instruments), and the obtained refractive indexes Nx and Ny. , Nz. A retardation value Rt in the film thickness direction is 0 to 300 nm, more preferably 0 to 150 nm, and more preferably 0 to 70 nm depending on the application. here,
Ro = (Nx−Ny) · d
Rt = [(Nx−Ny) / 2−Nz] · d
It is.
[0063]
In the cellulose ester film, the angle θ formed between the slow axis direction and the film forming direction and the retardation Ro in the in-plane direction are in the relationship of the following formula, and are particularly preferably used as an optical film such as a protective film for a polarizing plate.
[0064]
P ≦ 1-sin²(2θ) · sin²(ΠRo / λ)
P = 0.9999
Here, Nx is the refractive index in the slow axis direction in the film plane, Ny is the refractive index in the fast axis direction in the film plane, Nz is the refractive index in the film thickness direction, and d is the film thickness (nm). It is. θ is an angle (radian) formed between the slow axis direction in the film plane and the film forming direction (the straight direction of the film), and λ is the light in the three-dimensional refractive index measurement for obtaining the above Nx, Ny, Nz, and θ. The wavelength of 590 nm, that is, π is the circumference.
[0065]
[Drying process]
In this process, the film is dried while holding the width using a drying device and / or a tenter device that holds and transports both ends of the film with clips or pins, and transports the film alternately through rolls arranged in a staggered pattern. It is a process to do. It is preferable to keep the conveyance tension in the drying step as low as possible, since Ro can be kept low, and it is preferably 190 N / m or less. Further, it is preferably 170 N / m or less, particularly 140 N / m or less. Most preferred is 100 to 130 N / m. Moreover, it is effective to maintain below the said conveyance tension until the amount of residual solvents in a film will be at least 5 mass% or less.
[0066]
As a drying means, hot air is generally blown on both sides of the film, but there is also a means of heating by applying microwaves instead of air flow. However, too rapid drying tends to impair the flatness of the film. Incidentally, it is better to dry at a high temperature when the residual solvent is about 8% by mass or less. The drying is generally performed at 40 to 250 ° C. throughout, but it is particularly preferable to dry at 40 to 160 ° C.
[0067]
In the drying process after peeling from the belt surface, the film tends to shrink in the width direction by evaporation of the solvent. Of course, the shrinkage increases as the temperature rapidly dries. Drying while suppressing this shrinkage as much as possible is preferable for improving the flatness of the finished film.
[0068]
From this point of view, for example, as shown in Japanese Patent Application Laid-Open No. 62-46625, in the entire drying process or a part of the process, the film is dried while holding the width ends of the film with clips or pins in the width direction (tenter). Among them, a tenter method using a clip and a pin tenter method using a pin are preferably used.
[0069]
At this time, the draw ratio in the width direction is preferably 0 to 100%. When used as a polarizing plate protective film, 5 to 20% is more preferable, and 8 to 15% is most preferable. On the other hand, when used as a retardation film, 10 to 40% is more preferable, and 20 to 30% is most preferable. Ro can be controlled by the draw ratio, and a higher draw ratio is preferable because the flatness of the finished film is excellent.
[0070]
When the tenter is used, the residual solvent amount of the film is preferably 20 to 100% by mass at the start of the tenter, and drying can be performed while the tenter is applied until the residual solvent amount of the film becomes 10% by mass or less. Preferably, it is 5 mass% or less more preferably. 30-150 degreeC is preferable, as for the drying temperature in the case of performing a tenter, 50-120 degreeC is more preferable, and 70-100 degreeC is the most preferable. The lower the drying temperature, the less transpiration of the UV absorber, plasticizer, etc., and the process contamination can be reduced. On the other hand, the higher the drying temperature, the better the flatness of the film.
[0071]
In the film drying step, the film peeled off from the belt is further dried, and the residual solvent amount is preferably 0.5% by mass or less, more preferably 0.1% by mass or less, and most preferably. Is 0 to 0.01% by mass or less.
[0072]
In the film drying step, generally, a method of drying while transporting the film by a roll suspension method or the pin tenter method as described above is adopted. The means for drying the film is not particularly limited, and is usually performed with hot air, infrared rays, a heating roll, microwaves, or the like. From the point of simplicity, it is preferable to carry out with hot air. The drying temperature is preferably divided into 3 to 5 steps in the range of 40 to 150 ° C., and is increased stepwise, and the drying temperature is more preferably in the range of 80 to 140 ° C. in order to improve dimensional stability.
[0073]
In the process from immediately after casting through the solution casting film-forming method to drying, the atmosphere in the drying apparatus may be air, but may be performed in an inert gas atmosphere such as nitrogen gas, carbon dioxide gas, or argon. . However, the danger related to the explosion limit of the evaporating solvent in the dry atmosphere must always be considered.
[0074]
[Winding process]
This step is a step of winding up as a cellulose ester film after the residual solvent amount becomes 2% by mass or less, and obtaining a film having good dimensional stability by making the residual solvent amount 0.4% by mass or less. Can do. The winding method may be a commonly used one. Specifically, there are a constant torque method, a constant tension method, a taper tension method, a program tension control method with a constant internal stress, etc. Just do it.
[0075]
Although the order of the explanation is mixed, in further detail, in carrying out the present invention, the dope concentration, the pumping amount, the slit gap of the die base, It is good to control the extrusion pressure, the speed of the casting belt, etc. Further, as a means for making the film thickness uniform, it is preferable to use a method in which the film thickness detection means is used to feed back the programmed feedback information to each of the above apparatuses.
[0076]
The film thickness of the cellulose ester film varies depending on the intended use, but that of the finished film is usually in the range of 20 to 200 μm, more preferably in the range of 40 to 120 μm, particularly in the range of 40 to 80 μm as a film for a liquid crystal image display device. Is used.
[0077]
The polarizing plate can be produced by a general method. For this, for example, a method in which a cellulose ester film is subjected to alkali saponification treatment, and a polyvinyl alcohol film is immersed and stretched in an iodine solution, and bonded to both surfaces of the polarizing film using a completely saponified polyvinyl alcohol aqueous solution. There is. The alkali saponification treatment refers to a treatment of immersing the cellulose ester film in a high-temperature strong alkaline solution in order to improve the wetness of the water-based adhesive and improve the adhesiveness.
[0078]
Cellulose ester films include hard coat layers, antiglare layers, antireflection layers, antifouling layers, antistatic layers, conductive layers, optically anisotropic layers, liquid crystal layers, alignment layers, adhesive layers, adhesive layers, undercoats. Various functional layers such as a layer can be provided. These functional layers can be provided by a method such as coating or vapor deposition, sputtering, plasma CVD, or atmospheric pressure plasma treatment. The polarizing plate thus obtained is provided on one side or both sides of a liquid crystal cell, and a liquid crystal display device is obtained using this.
[0079]
By using a protective film for a polarizing plate made of a cellulose ester film obtained by the technique of the present invention, it is possible to provide a polarizing plate excellent in durability, dimensional stability, and optical isotropy as well as in a thin film. . Furthermore, a liquid crystal display device using such a polarizing plate or retardation film can maintain stable display performance over a long period of time. The cellulose ester film obtained by the technique of the present invention can also be used as a base material for an antireflection film or an optical compensation film.
[0080]
【Example】
Under the following conditions, a cellulose ester film sample was actually produced using the technique according to the present invention (Examples 1 to 3). For comparison, a sample of a cellulose ester film was also produced by setting the temperature condition and time condition to values outside the range defined in the present invention (Comparative Example).
[0081]
Film transport tension: 100 to 120 N / m
Dope component: cellulose triacetate: 100 parts by mass
Methylene chloride: 350 parts by mass
Ethanol: 12 parts by mass
Triphenyl phosphate: 12 parts by mass
Tinuvin 326 (Ciba Specialty Chemicals): 0.5 parts by mass
Aloesil 200V (manufactured by Nihon Aloesil): 0.1 parts by mass
Dry film thickness: 40 μm
* The above is common to Examples 1 to 3 and Comparative Example
[0082]

Where T₁Is the surface temperature of the cooling drum, K₁Is the time for the belt to contact the cooling drum before the film is peeled off, T₂Is the atmospheric dew point when the film is peeled, K₂Is the time until the belt reaches the casting position after film peeling.
[0083]
Now, the surface of all the samples manufactured under the above conditions was closely observed, and the condensed state, that is, the cloudiness was evaluated. The results are as follows.

Here, “◯” indicates a very good state with no fogging, “Δ” indicates that a slight fogging is recognized but acceptable, and “×” indicates that the entire surface is cloudy and cannot be permitted.
[0084]
【The invention's effect】
According to the present invention, when a cellulose ester film having a film thickness of 20 to 200 μm is produced, occurrence of fogging is effectively suppressed.
[Brief description of the drawings]
FIG. 1 is a reference diagram for explaining an embodiment of the present invention.
FIG. 2 is a schematic view of a belt casting apparatus.
[Explanation of symbols]
1 film
2 Cooling drum
3 Belt
4 Casting die

Claims (3)

A dope composition containing a cellulose ester is cast on the belt of a film forming apparatus having a belt and a cooling drum, and a film of the cast dope composition is peeled from the belt at a position on the cooling drum. This is a method for producing a cellulose ester film having a thickness of 20 to 200 μm,
The surface temperature of the cooling drum at a position where the film on the belt is separated from the cooling drum is 6 ° C. or more,
The film is peeled off from the belt within 30 to 900 seconds after the dope composition is cast on the belt, and peeled off within 0.5 to 20 seconds after the film is in contact with the cooling drum. A method for producing a cellulose ester film, wherein:   The method for producing a cellulose ester film according to claim 1, wherein the dew point of the atmosphere when peeling the film from the belt is 7 ° C or less.   The method for producing a cellulose ester film according to claim 1 or 2, wherein the time until the belt from which the film has been peeled reaches the casting site of the dope composition is 0.5 to 30 seconds.

Images