JP4576968B2 - Method for producing cellulose ester film - Google Patents

Description

  The present invention is useful for a cellulose ester film used for an optical film useful for various display devices such as a liquid crystal display device or an organic electroluminescence display, particularly for a protective film for a polarizing plate and a retardation film used for these display devices. The present invention relates to a method for producing a cellulose ester film.

  Conventionally, a cellulose ester film as an optical film is generally produced by a solution casting film forming method. In this method, first, a cellulose ester is dissolved in a mixed solvent obtained by adding a good solvent for a cellulose ester such as methylene chloride and a poor solvent for a cellulose ester such as methanol, ethanol, butanol, or cyclohexane. A cellulose ester solution (hereinafter also referred to as a dope) is prepared by adding an agent or an ultraviolet absorber, and the dope is transferred to an endless metal support having a mirror-finished surface (for example, a belt or a drum, hereinafter, Cast uniformly from the casting die on the substrate), evaporate the solvent on the substrate, solidify the dope film (hereinafter also referred to as web), peel it off with a peeling roll, It is transferred by a transfer roll and further dried through a drying device or a tenter. It is intended to obtain.

  By the way, cellulose ester (hereinafter referred to as vinegar cotton), which is a raw material for the cellulose ester film, is made from pulp and cotton, both of which are susceptible to variations in quality due to the influence of weather conditions and the like. Moreover, the component contained in the crystal | crystallization of vinegared cotton is changing also under the influence of the additive and water quality which are used in the reaction process to vinegared cotton. Among the trace components contained in vinegared cotton, calcium and magnesium are particularly abundant, and these elements are considered to exist as sulfates and oxalates in vinegared cotton.

  However, when a cellulose ester film is produced by the conventional solution casting film forming method using such a cellulose ester raw material, deposits (dirt) accumulate on the surface of the support, and the web (dope from the support) The peelability of the film) is extremely deteriorated, and the haze of the film base itself is greatly increased, which makes it unusable as a film product. Further, when the deposit (dirt) deposited and accumulated on the surface of the support is to be wiped off by cleaning, there is a problem that the surface of the support is scratched by the deposited crystals of the deposit.

  As a result of intensive research in view of the above points, the present inventor found that the compound of these elements contained in the vinegar cotton, which is the raw material for the cellulose ester film, contained a large amount of the above calcium and magnesium element compounds. It has been found that the dope is deposited on the surface of the support on which the dope is cast and accumulates as a deposit (dirt).

Here, prior patent documents related to calcium and magnesium contained in a cellulose ester film used for a conventional protective film for polarizing plate include the following.
JP, 2000-313766, A The present applicant previously proposed the invention of the cellulose ester film which specified the quantity of calcium and magnesium contained in a cellulose ester film. JP, 2000-314811, A The applicant of the present invention is an invention of an optical film including a cellulose ester film suitable for a protective film for a polarizing plate such as a liquid crystal display, and includes calcium and magnesium contained in the optical film. We proposed an invention that defined the amount of.

  However, the invention described in the above-mentioned Patent Document 1 defines the amount of calcium and magnesium contained in the cellulose ester film, thereby enabling the bright spot foreign matter of the cellulose ester film used for the protective film for the polarizing plate, particularly the polarizing plate. The foreign matter called bright spot foreign matter observed under the orthogonal state (crossed Nicol state) is reduced.

  In addition, the invention described in the above-mentioned Patent Document 2 improves the surface quality and optical isotropy problems of the film by regulating the amount of calcium and magnesium contained in the optical film, and processes the film. The improvement of the property, especially the cutting property was improved.

  However, even if the amount of calcium and magnesium contained in the cellulose ester film is specified in this way, the contents of calcium and magnesium in the vinegar cotton are very small. There has been a problem that reduction of elements is extremely difficult.

  Heretofore, in the production of a cellulose ester film by a solution casting film forming method, there is no technique that defines a solvent for dissolving a cellulose ester by the solubility of calcium and magnesium contained in a trace amount in the cellulose ester.

  An object of the present invention is to solve the above-mentioned problems of the prior art, and in the method of producing a cellulose ester film by a solution casting film forming method, precipitation of calcium and magnesium compounds on the casting support surface and accumulation thereof. An object of the present invention is to provide a method for producing a cellulose ester film, which can prevent the occurrence of deposits (dirt) and can produce a high-quality cellulose ester film suitable for a protective film for a polarizing plate with extremely high productivity. There is.

In order to achieve the above object, a method for producing a cellulose ester film according to the first aspect of the present invention comprises mixing a cellulose ester containing either 5 or 100 ppm of the total weight of cellulose ester with one or both of calcium and magnesium with a solvent. After heating to a temperature 10 ° C. or more higher than the boiling point of the solvent, dissolving the cellulose ester in the solvent to produce a dope, and then cooling the dope to a temperature lower than 5 ° C. below the boiling point of the solvent, Cast on an endless support (belt or drum, hereinafter also referred to as support) having a mirror-finished surface, evaporate the solvent on the support, and dope film (hereinafter also referred to as web) Is solidified, then peeled off by a peeling roll, transferred by a transfer roll, and further dried. By the method for producing a cellulose ester film, after preparation of the dope, the dope was cast dope in the below 5 ℃ lower temperature than the solvent boiling point on the support within 24 hours, endless infinite transition support Wind controlled to a lower water dew point in the range of 2 ° C. to 10 ° C. than the temperature of the support immediately after peeling the web in the space near the support surface from the web peeling point to the casting point where the dope is cast again. And after the web is peeled from the support, the time until the dope is cast again on the support is set to 1 to 5 seconds.

According to the invention of claim 1, in the method for producing a cellulose ester film by a solution casting film forming method, after the dope is produced, the dope is brought to a temperature lower than the boiling point of the solvent by 5 ° C. or less on the support within 24 hours. Since the dope is casted on the surface, precipitation of calcium and magnesium compounds on the surface of the support and generation of deposits (dirt) accumulated even when using vinegar containing a large amount of calcium and magnesium is prevented. it is Ru can. Then, in the space in the vicinity of the support surface from the web peeling point of the support to the casting point where the dope is cast again, the water is lower in the range of 2 ° C. to 10 ° C. than the temperature of the support immediately after peeling the web. By supplying a controlled wind to the dew point, condensation of water molecules on the support surface can be prevented, thereby preventing precipitation of calcium and magnesium. In addition, since the time until casting the dope on the support again is 1 to 5 seconds after peeling the web from the support, it does not give time for water to condense on the support surface, preventing condensation, Thereby, precipitation of calcium and magnesium can be suppressed. Thus eventually precipitation and deposits these accumulated calcium and magnesium compounds to the support surface can be further prevent (dirt), good cellulose ester film quality suitable for a protective film of a polarizing plate The effect is that it can be manufactured with high productivity .

  Next, embodiments of the present invention will be described, but the present invention is not limited thereto.

  As the cellulose ester used in the method for producing a cellulose ester film of the present invention, cellulose triacetate, cellulose diacetate, cellulose acetate butyrate, cellulose acetate propionate, cellulose acetate propionate butyrate and the like are preferably used. In the case of cellulose triacetate, cellulose triacetate having a degree of polymerization of 250 to 400 and a bound acetic acid amount of 54 to 62.5% is particularly preferable, and a bound acetic acid amount of 58 to 62.5% is more preferable because of its strong base strength. Particularly preferred is a cellulose ester having a total acyl group substitution degree of less than 2.85.

  In the case of cellulose acetate propionate, an acetyl group substitution degree of 1.75 to 2.15 and a propionyl group substitution degree of 0.60 to 0.80 are preferred. Such cellulose acetate propionate can be obtained by substituting the acetyl group and / or propionyl group within the above-mentioned range by acetic anhydride and / or propionic anhydride for the hydroxyl group of cellulose by a conventional reaction. The measuring method of the substitution degree of an acetyl group and / or a propionyl group can be measured by ASTM-D817-96. Preferable ranges of the substitution degree of the acetyl group and the propionyl group are from 1.75 to 1.95 and from 0.61 to 0.76, respectively.

  Cellulosic ester can be used either alone or as a mixture of cellulose ester synthesized from cotton linter and cellulose ester synthesized from wood pulp.

  About the specific manufacturing method of the cellulose ester of this invention, it is compoundable by the method described in Unexamined-Japanese-Patent No. 10-45804, for example.

  In the present invention, if the number average molecular weight of the cellulose ester is too low, the strength will be low, and if it is too high, the viscosity of the solution may be too high, so it is preferably 70000-300000, more preferably 70000-250,000.

  It is preferable to use a large amount of cellulose ester synthesized from a cotton linter that has good releasability from an endless belt or drum because of high productivity efficiency. The ratio of cellulose ester synthesized from cotton linter is 60% by mass or more, and the effect of releasability becomes remarkable, so 60% by mass or more is preferable, more preferably 85% by mass or more, and further, it can be used alone. Most preferred.

  In the present invention, it is preferable to add a plasticizer from the viewpoints of mechanical strength, dimensional stability, and the like. As the addition amount, cellulose ester film or cellulose is acylated with an acetyl group and an acyl group having 3 to 4 carbon atoms. It is preferably 3 to 30% by mass, more preferably 10 to 30% by mass, and particularly preferably 15 to 25% by mass relative to the converted cellulose ester film. In general, when the amount of plasticizer added increases, the dimensional change easily occurs. However, according to the present invention, the dimensional change rate can be remarkably reduced.

  Although it does not specifically limit as a plasticizer which can be used by this invention, Phosphate ester plasticizer, phthalate ester plasticizer, trimellitic ester plasticizer, pyromellitic acid plasticizer, glycolate plasticizer Citric acid ester plasticizers, polyester plasticizers, and the like can be preferably used.

  For phosphate esters, triphenyl phosphate, tricresyl phosphate, cresyl diphenyl phosphate, octyl diphenyl phosphate, diphenyl biphenyl phosphate, trioctyl phosphate, tributyl phosphate, etc.For phthalate esters, diethyl phthalate, dimethoxyethyl phthalate, dimethyl Trimellitic plasticizers such as phthalate, dioctyl phthalate, dibutyl phthalate, di-2-ethylhexyl phthalate, butyl benzyl phthalate, etc. Tributyl trimellitate, triphenyl trimellitate, triethyl trimellitate, pyromellitic ester Examples of plasticizers include tetrabutyl pyromellitate, tetraphenyl pyromellitate, tetraethyl pyromellitate, In acid ester type, triacetin, tributyrin, ethyl phthalyl ethyl glycolate, methyl phthalyl ethyl glycolate, butyl phthalyl butyl glycolate, etc., citrate ester plasticizers such as triethyl citrate, tri-n-butyl citrate Acetyl triethyl citrate, acetyl tri-n-butyl citrate, acetyl tri-n- (2-ethylhexyl) citrate and the like can be preferably used. As the polyester plasticizer, a copolymer of a dibasic acid and a glycol such as an aliphatic dibasic acid, an alicyclic dibasic acid, or an aromatic dibasic acid can be used. The aliphatic dibasic acid is not particularly limited, and adipic acid, sebacic acid, phthalic acid, terephthalic acid, 1,4-cyclohexyl dicarboxylic acid and the like can be used.

  As the glycol, ethylene glycol, diethylene glycol, 1,3-propylene glycol, 1,2-propylene glycol, 1,4-butylene glycol, 1,3-butylene glycol, 1,2-butylene glycol and the like can be used. These dibasic acids and glycols may be used alone or in combination of two or more. The molecular weight of the polyester is preferably in the range of 500 to 2000 in terms of weight average molecular weight from the viewpoint of compatibility with the cellulose resin.

  In the present invention, it is particularly preferable to use a plasticizer having a vapor pressure of less than 1333 Pa at 200 ° C., more preferably a compound having a vapor pressure of 666 Pa or less, and more preferably 1 to 133 Pa. The non-volatile plasticizer is not particularly limited, and examples thereof include arylene bis (diaryl phosphate) ester, tricresyl phosphate, trimellitic acid tri (2-ethylhexyl), and the like.

  These plasticizers can be used alone or in combination of two or more.

  In the present invention, it is preferable to use an ultraviolet absorber for the cellulose ester film for the purpose of protecting the liquid crystal material, and the ultraviolet absorber has an excellent ability to absorb ultraviolet rays having a wavelength of 370 nm or less from the viewpoint of preventing deterioration of the liquid crystal. In addition, from the viewpoint of good liquid crystal display properties, those that absorb as little visible light as possible with a wavelength of 400 nm or more are preferably used.

  In the present invention, in a film having a film thickness of 20 to 250 μm, a preferable polarizing plate can be provided without deteriorating the durability of the polarizing plate by setting the transmittance at a wavelength of 370 nm to 10% or less. The transmittance at a wavelength of 370 nm is more preferably 5% or less, and particularly preferably 2% or less.

  In the present invention, the ultraviolet absorber added to the cellulose ester film is particularly preferably an ultraviolet absorber having two or more aromatic rings in the molecule.

  Examples of commonly used compounds include, but are not limited to, oxybenzophenone compounds, benzotriazole compounds, salicylic acid ester compounds, benzophenone compounds, cyanoacrylate compounds, nickel complex compounds, and the like. Alternatively, polymer ultraviolet absorbers described in JP-A-6-148430 and JP-A-2000-273437 can also be preferably used.

  In the present invention, these ultraviolet absorbers may be used alone or in a mixture of two or more different types.

  The ultraviolet absorber preferably used in the present invention is a benzotriazole ultraviolet absorber, a benzophenone ultraviolet absorber, or the like. An embodiment in which a benzotriazole-based ultraviolet absorber with less unnecessary coloring is added to the cellulose ester film is particularly preferable.

  The ultraviolet absorber may be added to the dope after dissolving the ultraviolet absorber in an organic solvent such as alcohol, methylene chloride, dioxolane, or directly into the dope composition. For an inorganic powder that does not dissolve in an organic solvent, a dissolver or a sand mill is used in the organic solvent and cellulose ester to disperse and then added to the dope.

  The amount of the ultraviolet absorber preferably used in the present invention is preferably 0.1 to 5% by mass, more preferably 0.5% by mass with respect to the cellulose ester from the viewpoint of ultraviolet absorption effect and transparency. It is -2.5 mass%, More preferably, it is 0.8-2.0 mass%.

In the present invention, the cellulose ester film preferably contains a benzotriazole ultraviolet absorber represented by the following general formula (1).

In the formula, R ₁ , R ₂ , R ₃ , R ₄ and R ₅ may be the same or different, and are a hydrogen atom, a halogen atom (each atom of chlorine, bromine, iodine, fluorine, etc.), a nitro group, a hydroxyl group Alkyl groups (eg, methyl, ethyl, n-propyl, iso-propyl, aminopropyl, n-butyl, sec-butyl, tert-butyl, chlorobutyl, n-amyl, iso-amyl, hexyl, octyl, nonyl, stearyl Each group such as amidobutyl, decyl, dodecyl, pentadecyl, hexadecyl, cyclohexyl, benzyl, phenylethyl, phenylpropyl), an alkenyl group (for example, each group such as vinyl, allyl, methallyl, dodecenyl, tridecenyl, tetradecenyl, octadecenyl), Aryl groups (eg phenyl, 4-methylphenyl, 4 Each group such as ethoxyphenyl, 2-hexoxyphenyl, 3-hexoxyphenyl), alkoxy group (for example, each group such as methoxy, ethoxy, propoxy, butoxy, chlorobutoxy, decoxy, diaminophenoxy, ethoxy, pentadecoxy, octadecoxy) ), Oxycarbonyl groups (for example, carbomethoxy, carbobutoxy, carbohexoxy, carbopentadecoxy, etc.), aryloxy groups (for example, phenoxy, 4-methylphenoxy, 2-propylphenoxy, 3-amylphenoxy, etc.) Each group), alkylthio group (for example, each group such as methylthio, ethylthio, t-butylthio, t-octylthio, benzylthio, etc.), arylthio group (for example, phenylthio, methylphenylthio, ethylphenylthio, methoxy) Phenylthio, ethoxyphenylthio, naphthylthio, etc.), mono- or dialkylamino groups (for example, N-ethylamino, Nt-octylamino, N, N-diethylamino, N, N-di-t-butylamino, etc.) Each group), an acylamino group (for example, each group such as acetylamino, benzoylamino, methanesulfonylamino, etc.), a 5- or 6-membered heterocyclic residue containing an oxygen atom or nitrogen atom (for example, piperidino, morpholino, pyrrolidino, A heterocyclic residue such as piperazino), R ₄ together with R ₅ may form a 5- or 6-membered ring consisting of carbon atoms.

In the above general formula (1), the substituent represented by R _{1 to} R ₅ preferably has 5 to 36 carbon atoms, and the alkyl group preferably has 1 to 18 carbon atoms.

  Next, examples of the compound represented by the above general formula are shown, but the present invention is not limited thereto.

(1-1) 2- (2'-hydroxy-5'-t-butylphenyl) -benzotriazole (1-2) 2- (2'-hydroxy-3 ', 5'-di-t-butylphenyl) -Benzotriazole (1-3) 2- (2'-hydroxy-3'-t-butyl-5'-methylphenyl) -5-chlorobenzotriazole (1-4) 2- (2'-hydroxy-3 ' , 5'-di-t-butylphenyl) -5-chlorobenzotriazole (1-5) 2- (2'-hydroxy-5'-isooctylphenyl) -benzotriazole (1-6) 2- (2 '-Hydroxy-5'-n-octylphenyl) -benzotriazole (1-7) 2- (2'-hydroxy-3 ', 5'-di-t-amylphenyl) -benzotriazole (1-8) 2- (2'-hydroxy- 5'-dodecylphenyl) -benzotriazole (1-9) 2- (2'-hydroxy-5'-hexadecylphenyl) -benzotriazole (1-10) 2- (2'-hydroxy-3'-t- (Amyl-5'-benzophenyl) -benzotriazole In the present invention, examples of the compounds described in JP-A-60-128434, pages 10 to 12, including the above-mentioned compounds, are included in the present invention. (IV-1) to (IV-39) can be used. The above-mentioned benzotriazole-based compound used in the present invention can be easily synthesized by, for example, the method described in Japanese Patent Publication No. 44-29620 or a method analogous thereto.

  In the present invention, the cellulose ester film can be added with fine particles such as silicon oxide as a matting agent as required. It is preferable that fine particles such as silicon oxide are surface-treated with an organic substance because the haze of the film can be reduced. Preferred organic materials for the surface treatment include halosilanes, alkoxysilanes, silazanes, siloxanes, and the like. The larger the average diameter of the fine particles, the greater the mat 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.

  Examples of the fine particles of silicon oxide include AEROSIL 200, 200V, 300, R972, R972V, R974, R202, R812, OX50, and TT600 manufactured by Aerosil Co., preferably AEROSIL 200, 200V, R972, R972V, R974, R202, R812 and the like can be mentioned.

  In the present invention, the production of the cellulose ester film by the solution casting film forming method is not particularly limited, and may be a method generally used in the art. For example, U.S. Pat. Nos. 2,492,978, 2,739,070, 2,739,069, 2,492,977, 2,336,310, 2, No. 367,603, No. 2,607,704, British Patent No. 64,071, No. 735,892, No. 45-9074, No. 49-4554, No. 49-5614 Reference can be made to the methods described in Japanese Patent Publication Nos. 60-27562, 61-39890, 62-4208, and the like.

  In the present invention, either a single solvent or a mixed solvent may be used for dissolving the cellulose ester. However, it is preferable to use a mixture of a good solvent and a poor solvent of the cellulose ester in terms of production efficiency, and there are many good solvents. Is preferable from the viewpoint of solubility of the cellulose ester. The preferable range of the mixing ratio of the good solvent and the poor solvent is 70 to 98% by mass for the good solvent and 30 to 2% by mass for the poor solvent.

  With the good solvent and the poor solvent used in the present invention, those that dissolve the cellulose ester used alone are defined as good solvents, and those that swell or do not dissolve alone are defined as poor solvents. Therefore, depending on the average degree of acetylation of the cellulose ester, the good solvent and the poor solvent change. For example, when acetone is used as the solvent, the cellulose acetate becomes a good solvent when the bound acetic acid amount is 55%, and the poor solvent when the bound acetic acid amount is 60%. End up.

  The good solvent used in the present invention is not particularly limited, but examples include organic halogen compounds such as methylene chloride and dioxolanes in the case of cellulose triacetate, and methylene chloride, acetone, and methyl acetate in the case of cellulose acetate propionate. It is done.

  Moreover, it is although it does not specifically limit as a poor solvent used for this invention, For example, methanol, ethanol, i-propyl alcohol, n-butanol, cyclohexane, acetone, cyclohexanone etc. are used preferably.

  As a method for dissolving the cellulose ester in preparing the dope, a method performed at normal pressure, a method performed below the boiling point of the main solvent, a method performed under pressure above the boiling point of the main solvent, and JP-A-9-95544 There are various dissolution methods such as a method using a cooling dissolution method as described in JP-A-9-95557 or 9-95538, a method using a high pressure as described in JP-A-11-21379, and the like. .

  Heating at a temperature above the boiling point of the solvent at normal pressure and under a pressure range where the solvent does not boil and dissolving while stirring prevents the generation of massive undissolved material called gel or mamako. preferable. Further, a method in which cellulose ester is mixed with a poor solvent and wetted or swollen, and then mixed with a good solvent and dissolved is also preferably used.

  Here, in the solution casting film forming method for producing a cellulose ester film, the mechanism by which the support surface is soiled over time is not well understood, but when cellulose ester is mixed with a solvent and dissolved by heating, cellulose Calcium and magnesium salts that are minutely contained in the ester are also dissolved, and then, when the dope temperature is cooled to 5 ° C. or lower from the boiling point of the solvent, the salt having low solubility in the organic solvent becomes supersaturated. In this state, it seems that it will be very easy to precipitate after 24 hours.

  The type of the pressure vessel for dissolving the dope is not particularly limited, as long as it can withstand a predetermined pressure and can be heated and stirred under pressure. In addition to the pressure vessel, other instruments such as a pressure gauge and a thermometer are appropriately disposed. The pressurization may be performed by a method of injecting an inert gas such as nitrogen gas or by increasing the vapor pressure of the solvent by heating. Heating is preferably performed from the outside. For example, a jacket type is preferable because temperature control is easy.

  The heating temperature with the addition of the solvent is preferably a temperature in the range where the solvent used is at or above the normal pressure and the solvent does not boil from the viewpoint of the solubility of the cellulose ester, but is necessary if the heating temperature is too high. Increased pressure increases productivity. The range of preferable heating temperature is 45-120 degreeC, 60-110 degreeC is more preferable, and the range of 70-105 degreeC is further more preferable. Further, the pressure is adjusted at a set temperature so that the solvent does not boil.

  In addition to cellulose ester and solvent, additives such as necessary plasticizers and UV absorbers are mixed with the solvent in advance, dissolved or dispersed, and then added to the solvent before dissolving the cellulose ester. You may throw into dope.

  The cellulose ester is dissolved in the dissolution kettle while stirring, and the dope is prepared. After dissolution, the dope is filtered through a filter medium, defoamed, and sent to the next step with a pump.

  The dope is fed to the casting die through a pressurized metering gear pump, and the dope is cast from the extrusion casting die onto the support at the casting position.

  A stainless steel endless belt or drum having a mirror-finished surface is used as a support for casting the dope.

  The belt temperature during film formation can be cast at a temperature in the general temperature range of 0 ° C. to less than the boiling point of the solvent, but is preferably 5 to 10 ° C. lower than the solvent boiling point.

  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 facilitates uniform film thickness is preferred. Examples of the pressure die include a coat hanger die and a T die, and any of them is preferably used.

  In order to increase the film forming speed, two or more pressure dies may be provided on the casting support, and the dope amount may be divided to form a multilayer film.

  The web (casting film) on the support is dried by applying a dope whose temperature is controlled to be lower than the boiling point of the low boiling solvent of the solvent used and the mixed solvent from the casting die to a substantially uniform film thickness. When the residual solvent amount in the web is 200% or more by weight of the solid content, the web temperature is less than the solvent boiling point, and in the range of 100 to 200%, the solvent boiling point is + 10 ° C. or less, from 100% or less to peeling. The web is dried with warm air, warm water or an infrared heater so that the boiling point of the solvent is in the range of + 20 ° C. or lower.

  On the support, it is desirable that the amount of residual solvent in the web is dried to 150% or less, and further, 120% or less is from the viewpoint of reducing the occurrence of residual peeling of the web on the support and film deformation at the time of peeling. preferable.

  The web temperature when the web is peeled from the support is preferably 0 to 30 ° C. in that a large web strength is obtained, and 5 to 30 ° C. is more preferable in order to prevent condensation of water droplets in the air.

In the method for producing a cellulose ester film of the present invention, from the web peeling point of an endless metal support that moves infinitely, to the space near the support surface from the casting point where the dope is cast again on the support, By supplying a wind whose water dew point is controlled to be lower within the range of 2 ° C. to 10 ° C. than the temperature of the support immediately after the web is peeled off, condensation of water molecules on the surface of the support is prevented, whereby calcium and magnesium Precipitation can be prevented . If the dew point of water is 2 ° C. or less from the support temperature, the effect of preventing condensation can be obtained sufficiently. Lowering the dew point to 10 ° C. or less increases the equipment cost and supports the web. When peeling from the body, it tends to spark due to electrification and increases the risk of ignition, which is not preferable.

Furthermore, the time until the dope is cast on the support again after peeling the web from the support is set to 1 to 5 seconds, so that the time for condensing water on the support surface is not given, and the condensation is performed. It prevents, thereby inhibiting the precipitation of calcium and magnesium. When the time between peeling and re-casting is less than 1 second, because the equipment is in close proximity from the peeling point to the casting point, and there is a problem that the edge of the web tears during peeling In addition, there is no time to take safety measures to raise the dice. Also, if the time between stripping and recasting exceeds 5 seconds, the stripping point is very far from the casting point, and the amount of low dew point gas (wind) supplied to this space increases. This is not desirable because the equipment cost for supplying dry air is greatly increased.

  The gap between the casting die slit and the support surface is preferably in the range of 0.3 to 2 mm. In addition, when the support transport speed is 10 m / min or more, the web coming out from the casting die slit is depressurized to mix air and flutter the casting ribbon (create a horizontal streak in the width direction of the film). It is desirable to suppress it. The pressure reduction in the decompression chamber varies depending on the film thickness of the web and the support conveyance speed, but a range of about 50 to 800 Pa is practical.

  The gap between the lower end of the decompression chamber and the support does not increase the amount of suction air (if it becomes too large, a dry film of the dope at the end of the casting die slit can be formed). About 5-5 mm is preferable, and 0.5-3 mm is more preferable when depressurizing the decompression chamber to -100 Pa or more.

  In the post-drying step of the film, the film peeled off from the support is further dried, and the residual solvent amount during winding is 3% by mass or less, preferably 1% by mass or less, more preferably 0.5% by mass or less. It is preferable in terms of film dimensional stability and environment.

  In the film drying process, generally, a roll suspension system, a pin tenter system, or a clip tenter system is used for drying while transporting the film. For the liquid crystal display member, it is preferable to dry while maintaining the width by a tenter method in order to improve flatness and dimensional stability. In particular, holding the width in a large amount of residual solvent immediately after peeling from the support exhibits a more dimensional stability improving effect.

  In particular, in the drying process after peeling from the support, the film tends to shrink in the width direction by evaporation of the solvent. Shrinkage increases with drying at higher temperatures. Drying while suppressing this shrinkage as much as possible is preferable for improving the flatness of the finished film. From this point, for example, a method / tenter method in which all or part of the drying process as shown in JP-A-62-46625 is performed while holding the width at both ends of the web with a clip in the width direction. Is preferred.

  When the residual solvent amount is 10 to 100% by mass, and / or when the residual solvent amount is 5 to 10% by mass and the temperature is 110 to 150 ° C. If the stretching is about 1 to 20%, the effect of improving the flatness of the cellulose ester film is particularly preferable.

  The means for drying the film is not particularly limited, and is generally performed with hot air, infrared rays, a heating roll, microwaves, or the like. It is preferable to dry with hot air in terms of simplicity. The drying temperature is preferably divided into 3 to 5 stages in the range of 40 to 150 ° C., and gradually increased. Performing in the range of 80 to 150 ° C. further improves the flatness and dimensional stability. preferable.

  These steps from casting to post-drying may be performed in an air atmosphere or an inert gas atmosphere such as nitrogen gas. It goes without saying that the dry atmosphere is carried out in consideration of the explosion limit concentration of the solvent.

  The winding machine relating to the production of the cellulose ester film of the present invention may be generally used, and winding methods such as a constant tension method, a constant torque method, a taper tension method, a program tension control method with a constant internal stress, etc. Can be rolled up.

  In the present invention, the thickness of the cellulose ester film is 20 to 200 [mu] m, preferably 25 to 150 [mu] m, in order to reduce the thickness and weight of the polarizing plate used in the LCD. If it is thinner than this, the stiffness of the film is lowered, so that troubles such as wrinkles and film breakage are likely to occur in the polarizing plate preparation process. If it is thicker than this, the contribution to thinning of the LCD is small.

  In the present invention, it is preferable that the cellulose ester film has less foreign matter. In particular, it is preferable that there are few foreign substances recognized in the polarization crossed Nicol state.

  Foreign matter recognized in the polarization crossed Nicol state means that two polarizing plates are placed in a direct (crossed Nicol) state, a cellulose ester film is placed between them, light from the light source is applied from the opposite side, and the observed bright spot is Say. Since such foreign matters are observed as bright spots when light from the light source from the opposite side leaks only at the location of the foreign matters, the size and number of the foreign matters can be easily identified.

As for the number of foreign matters, it is preferable that there are 200 or less foreign matters having a size of 5 to 50 μm recognized in a polarization crossed Nicol state and substantially zero foreign matters having a size of 50 μm or more per 250 mm ² area. More preferably, the number of foreign matters of 5 to 50 μm is 100 or less, more preferably 50 or less.

  In order to obtain a cellulose ester film having few foreign substances as described above, any means can be used. For example, it can be achieved by filtering a dope composition in which cellulose ester is dissolved in a solvent using the following filter paper.

  Here, as the filter medium, it is preferable that the absolute filtration accuracy is small in order to remove insoluble matters, but there is a problem that the filter medium is likely to be clogged if the absolute filtration accuracy is too small. Therefore, a filter medium having an absolute filtration accuracy of 0.008 mm or less is preferable, a filter medium in the range of 0.001 to 0.008 mm is more preferable, and a filter medium in the range of 0.003 to 0.006 mm is more preferable.

  The material of the filter medium is not particularly limited, and a normal filter medium can be used. However, a plastic filter medium such as polypropylene and Teflon (registered trademark) and a metal filter medium such as stainless steel are preferable because fibers do not fall off. Moreover, it is more preferable to use two or more of the above filter papers.

  The dope solution can be filtered by a normal method, but the method of filtering while heating at a temperature not lower than the boiling point of the solvent at normal pressure and in a range where the solvent does not boil is the difference between before and after the filter medium. The increase in pressure (hereinafter sometimes referred to as filtration pressure) is small and preferable. A preferable temperature range is 45 to 120 ° C, more preferably 45 to 70 ° C, and still more preferably 45 to 55 ° C.

A smaller filtration pressure is preferred. The filtration pressure is preferably 1.6 × 10 ⁶ Pa or less, more preferably 1.2 × 10 ⁶ Pa or less, and further preferably 1.0 × 10 ⁶ Pa or less. The filtration pressure can be controlled by appropriately selecting the filtration flow rate and the filtration area.

  The cellulose ester film produced by the method of the present invention can be used as a member of an optical element or a display device. This member is a member used in a liquid crystal display device, for example, for a polarizing plate or a polarizing plate. Examples thereof include a protective film, a retardation plate, a reflecting plate, a viewing angle widening film, an optical compensation film, an antiglare film, an antireflective film, and an antistatic film.

  Among them, it is more preferable to apply the cellulose ester film produced by the method of the present invention in a polarizing plate, a polarizing plate protective film, a retardation plate, and a viewing angle widening film that have strict requirements for dimensional stability.

  The production method of the polarizing plate using the cellulose ester film produced by the method of the present invention is not particularly limited, and can be produced by a general method. For example, there is a method in which a cellulose triester film is treated with an alkali and bonded to both surfaces of a polarizing film produced by immersing and stretching in an iodine solution using a completely saponified polyvinyl alcohol aqueous solution. Instead of the alkali treatment, for example, a method for improving adhesiveness as described in JP-A-6-94915 and JP-A-6-118232 may be used.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated concretely, this invention is not limited to these.

Examples 1-3
Vinegar cotton, the raw material of cellulose ester film, uses cellulose ester synthesized from cotton linter, and prepares three kinds of vinegar with different calcium and magnesium contents by changing the number of times of water washing during synthesis of vinegar cotton. And used.

  The dope was prepared by completely dissolving the vinegar cotton together with the following raw materials while stirring up to 70 ° C. in a sealed kettle. Within 24 hours after the dope is made 5 ° C. lower than the solvent boiling point after the preparation of the dope, the dope is passed through a casting die in which the temperature is kept at 30 ° C., and the film thickness is about 400 μm on a stainless steel endless belt. The dope flow was controlled so that the film was cast, hot air was applied from the front and back of the belt to dry the cast film, and the film was peeled from the support when the residual solvent amount of the film reached about 100%. At this time, the support temperature was 10 ° C., and there was no film peeling residue on the belt surface after peeling, and the film was peeled cleanly. In this state, continuous operation was performed for 12 hours.

  The film was dried at 100 ° C. to finally obtain a film having a thickness of 80 μm. Moreover, the boiling point of the mixed solvent used here was 42 degreeC.

  Calcium and magnesium contents in vinegared cotton in Examples 1 to 3, time (hour) required from casting the dope to 37 ° C. or less, casting to air conditioning dew point (° C.) of casting point, and The time (seconds) from peeling to casting is summarized in Table 1 above.

(Dope composition)
Cellulose ester Cellulose acetate propionate 100 parts by weight Acetyl substitution degree 1.9, propionyl substitution degree 0.73
Mn = 70000, Mw = 220,000, Mw / Mn = 3.14
2- (2'-hydroxy-3 ', 5'-di-t-
Butylphenyl) benzotriazole 1 part by weight Triphenyl phosphate 8.5 parts by weight Ethylphthalyl ethyl glycolate 2 parts by weight AEROSIL 200V 0.1 part by weight Methylene chloride 420 parts by weight Ethanol 30 parts by weight Comparative Examples 1-9
For comparison, it is carried out in the same manner as in Examples 1 to 3 above, but as shown in Table 1 below, the calcium and magnesium contents in vinegar cotton and the dope are set to 37 ° C. or lower to casting. takes time (hours), air dew point of the separation point-casting point (° C.), as well as various varied the conditions of time until the peeling-casting (s), 12 hours no line casting of the dope, cellulose acetate Propionate film was produced.

Evaluation method About the cellulose acetate propionate films obtained in Examples 1 to 3 and Comparative Examples 1 to 9, the surface of the belt was observed, the calcium and magnesium were quantified, and the haze of the film was measured. Are summarized in Table 2 below.

<Observation of dirt on belt surface>
The degree of contamination of the film was visually observed on the belt surface after stopping casting under illumination with a halogen lamp.

<Quantification of calcium and magnesium>
Quantification of calcium and magnesium adhering to vinegar cotton and the belt surface was performed by a method of elemental analysis using ICP-AES after pretreatment with a microwave wet decomposition apparatus (sulfuric acid decomposition). Further, the deposit on the belt surface was sampled by wiping the belt surface 10 m ² with a nonwoven fabric soaked with pure water.

<Haze of film>
The film after drying at the portion where the surface of the belt was soiled was measured using a haze meter (1001DP type, manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS K-6714.

  As is apparent from the results in Table 2, in Examples 1 to 3 of the present invention, the adhesion of dirt due to calcium and magnesium compounds was not substantially confirmed, and the belt surface was kept clean. In Comparative Examples 1 to 9, dirt was observed even in a film formation time as short as 12 hours, and the obtained film had a haze portion with a haze exceeding 1 on the entire surface or partly in a group. Stood out.

Claims (1)

After mixing cellulose ester containing either 5 or 100 ppm of the total weight of calcium and magnesium with a solvent, the mixture is heated to a temperature 10 ° C. higher than the boiling point of the solvent to dissolve the cellulose ester in the solvent. The dope is prepared, and then the dope is cooled to a temperature lower than 5 ° C. below the boiling point of the solvent, and then an endless transition support (belt or drum, hereinafter referred to as support) having a mirror-finished surface. (Also referred to as a web), the solvent is evaporated on the support, and the dope film (hereinafter also referred to as a web) is solidified, then peeled off by a peeling roll, transferred by a transfer roll, and further dried. In the method for producing a cellulose ester film by a solution casting film-forming method comprising: Ri and casting the dope on a support 5 ° C. after the following low temperature within 24 hours, the support from the web separation point of the support of the endless infinite transition, until the casting point casting a dope again The air near the surface is supplied with a controlled air dew point in the range of 2 ° C. to 10 ° C. below the temperature of the support immediately after the web is peeled, and after the web is peeled from the support, The method for producing a cellulose ester film is characterized in that the time until casting the dope is 1 to 5 seconds .