JP2009061618A - Solution casting method and equipment - Google Patents

Abstract

[PROBLEMS] To easily detect the formation of a precoat having a predetermined strength in an auxiliary filtration system.
A filter aid is dispersed in a raw material dope and filtered by a first filter or a second filter. A filter aid is deposited on the filter medium support 60 in the filters 47 and 48, and the material dope 41 is efficiently filtered using the filter medium support 60 and the filter medium 63 composed of the deposited layer. When the filtration pressure becomes high, the filters 47 and 48 are switched, and the washing liquid is supplied to the used filter for washing. After the cleaning, the precoat liquid 61 is circulated so that the filter aid is uniformly deposited on the filter medium support 60 to form a precoat 62a having a predetermined strength. The total amount of filter aid that can provide a predetermined precoat strength is determined in advance, and this total amount of filter aid is dispersed in the precoat solution. When the outputs of the turbidimeters 69a and 69b are below a certain value, a precoat having a predetermined strength is obtained.
[Selection] Figure 3

Description

  The present invention relates to a solution casting method and equipment for efficiently filtering a dope.

  Various display devices such as liquid crystal displays use various polymer films including a polarizing plate protective film and a viewing angle widening film. As a method for producing such a polymer film for optical use, there are a melt film forming method and a solution film forming method. In the solution casting method, a dope containing a polymer and a solvent is cast on a traveling support to form a casting film, and then the casting film is peeled off from the support and dried to form a film. There is no problem of thermal damage as in the melt film-forming method. Therefore, it is optimal as a method for producing a polymer film that requires high transparency and optical characteristics.

  By the way, the dope contains foreign substances that are insoluble in the solvent of the dope and originally contained in the dope raw material, and impurities such as dust and dirt mixed in preparing the dope. . However, when a dope containing impurities is used, impurities are deposited on the support as dirt, and it becomes difficult to peel off the casting film from the support, and the finished film causes light scattering at the impurities, etc. Cause problems. For this reason, it is necessary to remove impurities in the dope as much as possible before being subjected to casting.

  Therefore, in the solution casting method, the dope before casting is usually filtered with a porous filter medium for the purpose of removing impurities in the dope. As the filter medium, a filter paper, a metal filter, a filter cloth or the like is used. However, each of the filter media has a problem that the passage hole is blocked as time passes from the start of filtration, and the filtration time is prolonged, or the filtration efficiency increases due to an increase in filtration pressure or a reduction in filtration flow rate. For this reason, when using a metal filter, measures are taken to clean and regenerate the metal filter by supplying a cleaning liquid to the metal filter in the direction opposite to the filtration direction and circulating it. However, even if these measures are taken, they are temporary, and the present situation is that the filtration efficiency has not been fundamentally improved.

Moreover, it is difficult to remove impurities that are hardly soluble in a solvent only by using a filter medium such as a filter paper, a metal filter, or a filter cloth. Thus, for example, Patent Document 1 proposes an auxiliary filtration method that removes hardly soluble impurities by using a filter aid in addition to the filter medium. The filter aid is an inert particle or powder, such as silicon dioxide (SiO ₂ ). This filter aid is used by being randomly deposited on a filter medium support such as a wire mesh filter. When the dope is passed through the filter medium on which such a deposited layer is formed, impurities can be adsorbed and recovered by the filter aid regardless of whether it is poorly soluble or not, and thus a highly clear filtrate can be obtained. It is done. In addition, if a filter aid is used, clogging of the filter medium can be suppressed, so that productivity can be improved.
JP 2004-107629 A

  By the way, in solution casting, in order to increase the production rate, it is important that the casting membrane by casting dope has self-supporting property at an early stage. For this purpose, it is desirable to increase the dope concentration, but when the above-mentioned auxiliary filtration is performed on a high viscosity dope, the pressure loss becomes large. In order to suppress this pressure loss, it is necessary to use a filter aid having a large particle size and capable of retaining the voids to some extent.

  In the auxiliary filtration method, in order to obtain a predetermined filtration accuracy, a pre-coating operation is required in which a certain amount of the filter aid is deposited on a filter medium support such as a wire mesh filter. In this pre-coating operation, it is efficient to use the same liquid as that for filtration without the need to exchange the liquid. However, in the case of a high-viscosity liquid such as a dope in solution casting, there is a problem that since the viscosity is high, the pressure loss is large and the flow rate cannot be increased. When the flow rate cannot be increased, a long time is required for the precoat operation, and as a result, an efficient filtration operation cannot be performed. On the other hand, instead of the actually used filtrate, for example, it may be possible to use only the solvent for forming the dope as the precoat liquid. In this case, however, the viscosity of the precoat liquid is reduced, and the filter aid is precipitated. A new problem arises that the performance increases. When the sedimentation property increases, a non-uniform concentration distribution is generated in the filter aid in the filter housing, and it becomes difficult to ensure the uniformity of the precoat.

  Therefore, it is conceivable to use a diluted dope solution obtained by diluting the dope in the middle, but this solution has a property that it is easy to dry, and if the process operation is not properly performed, a skin layer is formed on the surface. Is formed immediately, and the micropores on the surface of the filter aid are blocked, resulting in a new problem that filtration cannot be performed properly. In addition, when the precoat operation is performed with a hermetic filter, there is also a problem that it is difficult to determine whether or not the precoat has been reliably formed.

  The present invention has been made in view of the above problems, and can reliably form a uniform precoat so that the dope can be efficiently filtered, and can detect completion of precoat formation while the filter is sealed. It is an object of the present invention to provide a solution casting method and equipment that can be used.

  The present invention uses a plurality of filter devices having a filter medium formed by forming a precoat in which a filter aid is deposited on a filter medium support, and adds the filter aid to a dope containing a polymer and a solvent and performs filtration. The filtered dope is cast on a support that runs endlessly to form a cast film, and after the cast film has self-supporting properties, it is peeled off from the support and dried to form a film. In the solution casting method, for the purpose of forming the precoat, a washing step of stopping the supply of the dope to the filter having a reduced filtration ability and removing the filter aid as a slurry after washing, and the precoat A precoat liquid is circulated and supplied to the filter after the washing step to form the precoat, and a precoat forming step of the dope inlet side of the filter in the precoat forming step. Detecting the turbidity of the coating liquid and detecting the completion of precoat formation when the turbidity is below a certain value, and removing the precoat liquid from the filter after completion of the formation of the precoat, The dope is added to the filter after completing the precoat liquid extraction step before performing the precoat liquid extraction step of replenishing the filter with a saturated gas of the solvent according to the extracted precoat liquid content and the washing step. And a filter switching step of switching the filter.

  The present invention is based on the relationship between the strength of the precoat and the amount of filter aid at that time, and the filter aid in the circulating precoat liquid so that the precoat has strength to withstand the liquid draining step and the filtration step. A total amount is determined, and a precoat solution is prepared based on the total amount of the filter aid. In the precoat liquid extraction step, the present invention allows the precoat liquid to be extracted by its own weight so that the saturated gas of the solvent is replenished between the tank in which the precoat liquid is extracted and stored and the filter. It is characterized by using a communication pipe.

The present invention is characterized in that the precoat liquid is formed by dispersing the filter aid in a diluted dope obtained by diluting the dope with the solvent. Further, the present invention is characterized in that a precoat liquid extraction flow rate in the precoat liquid extraction step is 1 × 10 ⁻³ m / sec or less with respect to the surface of the precoat layer. As the filter aid, SiO ₂ having an average particle size of 20 to 50 μm is used, the polymer is cellulose acylate, the filter aid concentration of the precoat liquid is 0.25 to 5% by weight, and the cellulose concentration is 0. 0.5 to 5% by weight.

  The present invention uses a plurality of filter devices having a filter medium formed by forming a precoat in which a filter aid is deposited on a filter medium support, and adds the filter aid to a dope containing a polymer and a solvent and performs filtration. The filtered dope is cast on a support that runs endlessly to form a cast film, and after the cast film has self-supporting properties, it is peeled off from the support and dried to form a film. In the solution casting apparatus, the supply of the dope to the filter having a reduced filtration ability is stopped, and the washing agent is removed after removing the filter aid as a slurry, and the precoat is formed. The precoat liquid is circulated and supplied to the filter after the washing, and the precoat forming part for forming the precoat, and the precoat formation by the precoat forming part, the dope of the filter is included. A turbidimeter that detects the turbidity of the precoat liquid on the side, a precoat formation completion determination unit that determines that precoat formation is complete when the turbidity value detected by the turbidimeter is equal to or less than a predetermined value, and the precoat formation After completion of precoat formation by the completion determination unit, the precoat liquid is extracted from the filter, and the filter is cleaned by the precoat liquid extraction unit for replenishing the saturated gas of the solvent corresponding to the extracted precoat liquid and the cleaning unit. In this case, the apparatus has a switching unit that sends the dope to the filter that has finished extracting the precoat liquid and switches the filter.

In addition, the precoat forming section is configured to circulate the precoat liquid so that the precoat has a strength capable of withstanding the liquid draining and the dope filtration based on the relationship between the strength of the precoat and the amount of the filter aid at that time. The total amount of the filter aid is determined, and a precoat solution is prepared based on the total amount of the filter aid. A precoat liquid preparation tank is provided that forms the precoat liquid by dispersing the filter aid in a diluted dope obtained by diluting the dope with the solvent. In addition, the precoat liquid withdrawal unit includes a precoat liquid tank that stores the precoat liquid, a return piping system that is connected to an outlet side of the filter and returns the precoat liquid to the precoat liquid tank, and a solvent for the precoat liquid tank And a solvent saturated gas communication pipe for supplying saturated gas to the inlet side of the filter, and the precoat liquid extraction flow rate in the precoat liquid extraction apparatus is 1 × 10 ⁻³ m / sec with respect to the surface of the precoat layer. It is characterized as follows. As the filter aid, SiO ₂ having an average particle diameter of 20 to 50 μm is used, the polymer is cellulose acylate, the precoat liquid has a filter aid concentration of 0.25 to 5% by weight, and a cellulose concentration. Is 0.5 to 5% by weight.

  According to the present invention, the turbidity of the precoat liquid on the dope inlet side of the filter in the precoat forming step is detected, and when the turbidity is below a certain value, the completion of precoat formation is detected. Therefore, it is not necessary to confirm the formation of the precoat, and the formation of the precoat can be easily detected. Further, the precoat forming section is based on the relationship between the strength of the precoat and the amount of the filter aid at that time, so that the precoat has a strength capable of withstanding the liquid draining and filtration of the dope. The total amount of filter aid is determined and a precoat solution is prepared based on the total amount of filter aid. Therefore, the formation of precoat is detected based on the turbidity detection value of the precoat solution on the dope inlet side of the filter, and at the same time, the predetermined strength A precoat having

  Moreover, since the precoat liquid is extracted from the filter by its own weight after the precoat is formed, the skin layer is not formed on the surface of the precoat as compared with the method of pressurizing and extruding with dry air or dry nitrogen. Inability to filter or decrease in filtration efficiency due to formation is eliminated. In addition, since the solvent saturated gas corresponding to the extracted precoat liquid is replenished to the filter, even if the precoat liquid is extracted after the precoat is formed, the precoat is not dried and a skin layer is not formed, and the precoat is reliably maintained. can do. Moreover, even when a precoat solution in which a filter aid is added to a diluted dope is used, a skin layer is not formed.

By forming the precoat liquid by dispersing the filter aid in a diluted dope obtained by diluting the dope with a solvent, an appropriate sedimentation rate can be ensured and a filter layer having a uniform thickness by the filter aid can be formed. Moreover, collapse of the formed precoat can be suppressed by setting the precoat liquid extraction flow rate in the precoat liquid extraction step to 1 × 10 ⁻³ m / sec or less with respect to the surface of the precoat layer. In particular, SiO ₂ having an average particle diameter of 20 to 50 μm is used as a filter aid, the polymer is cellulose acylate, the filter aid concentration of the precoat liquid is 0.25 to 5% by weight, and the cellulose concentration is 0.00. By adjusting the content to 5 to 5% by weight, it is possible to reliably form the precoat and to extract the precoat liquid with reduced collapse, and to reliably filter the dope when the cellulose acylate film is formed into a solution. Can do.

  A solution casting apparatus 10 shown in FIG. 1 includes a raw material dope preparation unit 11, a filtration unit 12, and a film forming unit 13.

  The raw material dope preparation unit 11 includes a meter 14, a solvent tank 15, an additive tank 16, a dissolution tank 17, and a storage tank 18. A polymer 20 is placed in the meter 14, and the polymer 20 is weighed and put into the dissolution tank 17. A solvent 21 is stored in the solvent tank 15, and the input amount to the dissolution tank 17 is adjusted by controlling the open / close valve 23. An additive 22 is stored in the additive tank 16, and the input amount to the dissolution tank 17 is adjusted by controlling the open / close valve 24.

  The polymer 20 according to the present invention is not particularly limited as long as it is applicable to the solution casting method. Among these, if cellulose acylate is used, a film having high transparency and excellent optical properties can be obtained, and therefore, it is suitable for optical applications such as a protective film for polarizing plates and an optical compensation film. Among them, when cellulose acetate is used, and particularly cellulose triacetate having an average degree of acetylation of 57.5% to 62.5% is used, a film having excellent optical properties can be obtained. The above-mentioned degree of acetylation means the amount of bound acetic acid per unit weight of cellulose, and can be determined according to the measurement and calculation of the degree of acetylation in ASTM: D-817-91 (test method for cellulose acetate and the like). In this embodiment, granular cellulose triacetate is used. In addition, when using a granular polymer, it is preferable that 90 weight% or more is a particle size of 0.1-4 mm from a compatible viewpoint with a solvent, More preferably, a particle size is 1-4 mm. It is.

  The solvent 21 is preferably a halogenated hydrocarbon, an ester, a ketone, an ether, an alcohol, or the like, but is not particularly limited, and may be appropriately selected in consideration of solubility with the polymer to be used. The solvent 21 may be one type of compound or a mixed solvent in which a plurality of compounds are mixed. Specifically, halogenated hydrocarbons (eg, dichloromethane, etc.), esters (eg, methyl acetate, methyl formate, ethyl acetate, amyl acetate, butyl acetate, etc.), ketones (eg, acetone, methyl ethyl ketone, cyclohexanone, etc.) ), Ethers (eg, dioxane, dioxolane, tetrahydrofuran, diethyl ether, methyl-t-butyl ether, etc.), alcohols (eg, methanol, ethanol, etc.) and the like.

  The additive 22 may be appropriately selected according to the desired characteristics of the film 19. For example, a plasticizer, an ultraviolet absorber, a peeling accelerator, a fluorine surfactant, and the like can be given. Among these, as the plasticizer, phosphate ester type (for example, triphenyl phosphate (hereinafter referred to as TPP), tricresyl phosphate, cresyl diphenyl phosphate, octyl diphenyl phosphate, diphenyl biphenyl phosphate (hereinafter referred to as BDP). ), Trioctyl phosphate, tributyl phosphate, etc.), phthalate esters (eg, diethyl phthalate, dimethoxyethyl phthalate, dimethyl phthalate, dioctyl phthalate, etc.), glycolate esters (eg, triacetin, tributyrin, butyl phthalyl butyl glycolate) Ethyl phthalyl ethyl glycolate, methyl phthalyl ethyl glycolate, butyl phthalyl butyl glycolate, etc.). Among these, TPP is particularly preferable for making cellulose acylate into a film. In addition, a well-known thing can be used for a plasticizer besides the above, and it does not specifically limit. Further, as the ultraviolet absorber, for example, oxybenzophenone compounds, benzotriazole compounds, salicylic acid ester compounds, benzophenone compounds, cyanoacrylate compounds, nickel complex compounds are preferable, and among them, benzotriazole compounds and benzophenone compounds are preferable. Compounds are particularly preferred.

  The dissolution tank 17 includes a stirring blade 27 that is rotated by a motor 26. As the stirring blade 27 rotates, the polymer 20, the solvent 21, and the additive 22 in the dissolution tank 15 are stirred. By this stirring, a crude solution 30 in which the solute such as the polymer 20 is not completely dissolved in the solvent is obtained.

  The crude solution 30 in the dissolution tank 17 is temporarily stored in the storage tank 18. As a result, the dissolution tank 17 is emptied, and a continuous batch system in which the crude dissolution solution 30 is repeatedly formed becomes possible. The storage tank 18 is also provided with a stirring blade 32 that is rotated by a motor 31. By rotating the stirring blade 32, the crude solution 30 is stirred and made uniform.

  The crude solution 30 in the storage tank 18 is sent to the heater 40 via the pump 35 and the pipe 36. As the heater 40, an in-line mixer such as a multi-tube heat exchanger or a static mixer is used. The crude solution 30 is heated by the heater 40. The heating temperature is preferably 50 to 120 ° C., and the heating time is preferably 5 to 30 minutes. By this heating, the solute such as the polymer 20 necessary for solution film formation is completely dissolved without modification, and the raw material dope 41 is prepared. Thus, the raw material dope 41 prepared is made into 14 to 22 weight% as solid content concentration of a cellulose ester. In addition, you may concentrate the raw material dope 41 by the flash concentration method etc. as needed.

  The raw material dope 41 heated by the heater 40 is sent to the cooler 42. The cooler 42 cools the material dope 41 to the boiling point of the main solvent or less. The cooled raw material dope 41 is sent to the body feed tank 45 of the filtration unit 12 by the pump 43.

  The filtration unit 12 includes a body feed tank 45, a filter aid tank 46, a first filter 47, a second filter 48, a precoat liquid circulation unit 49, a cleaning liquid circulation unit 50, and a casting dope storage tank 51. The raw material dope 41 is filtered using the filter aid 44 to produce a casting dope 52. In addition, valves V1 to V7 are attached to the pipes connecting the tanks 45, 46 and 51 and the filters 47 and 48, and the pipes are switched by opening and closing the valves V1 to V7. Switching of the devices 47 and 48, cleaning processing of the used filters 47 and 48, pre-coating processing, etc. are performed. Thereby, the raw material dope 41 is continuously filtered, and the casting dope 52 is obtained. The valves V1 to V7 installed in the filtration unit 12 are not limited to this embodiment, and may be increased or decreased as necessary. Moreover, the pump which is not illustrated is arrange | positioned suitably at the location as needed.

A filter aid solution 56 is stored in the filter aid tank 46. The filter aid solution 56 is sent to the body feed tank 45 via the pump 57 and the valve 58. The filter aid solution 56 is obtained by dispersing a desired filter aid 44 in a solvent in advance, and is used for the purpose of improving the trapping efficiency of impurities contained in the raw material dope 41. The filter aid 44 is not particularly limited. For example, granular diatomaceous earth (SiO ₂ ) or a derivative derived from a cellulosic compound is preferably used. Moreover, it is preferable that said solvent contains at least 1 or more types of the same solvent as the solvent contained in the raw material dope 41 from a compatible viewpoint with dope. The addition amount of the filter aid 44 to the raw material dope 41 is 0.01 to 10% by weight, preferably 0.05 to 5% by weight, and more preferably 0.1 to 2% by weight. The type, composition, average particle diameter, and bulk density of the filter aid 44 are described in detail in JP-A-2004-107629, and this description can also be applied to the present invention.

  A raw material dope 41 and a filter aid solution 56 are placed in the body feed tank 45. The body feed tank 45 includes a stirring blade 54 that is rotated by a motor 53. By rotating the stirring blade 54, a certain ratio of the filter aid solution 56 can be uniformly dispersed in the raw material dope 41.

  When auxiliary filtration is performed using the first filter 47, the line is switched so that the body feed tank 45 is connected to the first filter 47 by opening and closing the valves V1 to V6. The raw material dope 41 mixed with the filter aid 44 is sent to the first filter 47. In the first filter 47, as shown in FIG. 2, a filter aid 44 is randomly deposited on a filter medium support 60 made of a wire mesh filter to form a deposited layer 62. These filter medium supports 60 and A filter medium 63 is composed of the deposited layer 62.

  In addition, after the cleaning process of the first filter 47, only the filter medium support 60 is present, and in this state, proper auxiliary filtration cannot be performed. Therefore, the deposited layer 62 having a constant thickness is formed on the filter medium support 60. Form. This initial deposited layer 62 is a precoat 62 a, and this precoat 62 a is formed by circulating the precoat liquid 61 (see FIG. 3) through the first filter 47 for a predetermined time by the precoat liquid circulation section 49.

  As shown in FIG. 2, in the first filter 47, only the raw material dope 41 passes through the filter medium 63, and the filter aid 44 is randomly deposited on the filter medium 63 to form a deposited layer 62. When the raw material dope 41 passes through the filter medium 63 composed of the deposited layer 62 and the filter medium support 60, the impurities 64 are adsorbed and collected by the filter aid 44, and also due to the numerous voids formed in the deposited layer 62. A relatively large impurity is captured. Therefore, when the raw material dope 41 passes through the filter medium 63, the impurities 64, undissolved substances, and the like are filtered, and a filtrate with high clarity is obtained. This filtrate is supplied to the film forming unit 13 as a casting dope 52, and a high-quality film 19 free from impurities is manufactured.

  The second filter 48 is configured in the same manner as the first filter 47. Then, the filtration is performed by one filter 47, the washing is performed by the other filter 48, and the pre-coating process is subsequently performed. And continuous filtration is attained by performing washing | cleaning / precoat process and filtration process alternately. Although two filters 47 and 48 are used, the number of filters 47 and 48 is not limited to this, and may be three or more. And it filters with one filter, for example, the 1st filter 47, and when the filtration pressure becomes high, it switches to the other 2nd filter 48, and performs continuous filtration. Then, the first filter 47 that has been switched due to an increase in the filtration pressure is washed by removing the filter aid 44 and the filtered material as slurry (see FIG. 4) by the washing liquid circulation unit 50. After the cleaning, the precoat liquid circulating unit 49 circulates the precoat liquid 61 to the first filter 47 to form a precoat 62a as shown in FIG. After the precoat 62a is formed, it enters a standby state for the next switching. In addition to arranging a plurality of filters 47 and 48 in parallel, they may be arranged in series. In this case, the efficiency of collecting impurities by filtration can be increased.

  Next, the precoat process and the cleaning process of the filters 47 and 48 will be described. As shown in FIG. 3, the precoat liquid circulation unit 49 includes a precoat liquid preparation tank 65, a precoat liquid storage tank 66, valves 65a and 67, a pump 68, turbidimeters 69a and 69b, and a controller 72. I have. In addition to a predetermined amount of the raw material dope 41 containing the filter aid 44 being sent from the body feed tank 45 via the pump 45a to the precoat liquid preparation tank 65, a diluting solvent 70 is sent from the solvent tank 71 via the valve 71a. A predetermined amount is sent to form a precoat liquid 61 in which the raw material dope 41 containing the filter aid 44 is diluted to a constant concentration. The precoat liquid 61 is stirred and uniformed by a stirring blade 65c that is rotationally driven by a motor 65b.

The precoat liquid 61 has a solid content concentration of cellulose ester of 0.25 to 7% by weight, and the addition amount of the filter aid 44 to the precoat liquid 61 is 0.01 to 10% by weight. In addition, Preferably the solid content concentration of a cellulose ester is 0.5 to 5 weight%, and the addition amount of the filter aid 44 is 0.25 to 5 weight%. More preferably, the solid content concentration of the cellulose ester is 2 to 4% by weight, and the addition amount of the filter aid 44 is 0.7 to 2% by weight. In addition, when the solid content concentration of the cellulose ester with respect to the precoat liquid 61 is less than 0.25% by weight or when the addition amount of the filter aid 44 is less than 0.01% by weight, the viscosity becomes low, and the filter aid. As a result, the sedimentation property of the precoat is increased and the uniformity of the precoat cannot be ensured. In addition, when the solid content concentration of the cellulose ester exceeds 7% by weight or when the additive of the filter aid exceeds 10% by weight, the viscosity increases, the pressure loss increases, and the flow rate can be increased. Disappear. The filter aid 44 is made of SiO ₂ having an average particle size of 10 to 70 μm. More preferably, an average particle diameter of 20 to 50 μm is good. The filter medium support is a 350 mesh wire mesh made of SUS.

  As shown in FIG. 3, in the precoating process, first, the precoat liquid 61 is sent from the precoat liquid storage tank 66 to the first filter 47, passes through the filter medium support 60, and then returns to the precoat liquid storage tank 66. Circulated. When passing through the filter medium support 60, as shown in FIG. 2, the filter aid 44 in the precoat liquid 61 gradually deposited on the filter medium support 60, and the deposited layer 62 had a predetermined thickness. In some cases, it is determined that the formation of the precoat 62a is completed, and the precoat process is ended. In FIG. 3, the precoat liquid preparation tank 65 and the precoat liquid storage tank 66 are provided, but the precoat liquid storage tank 66 is omitted, and the precoat liquid preparation tank 65 performs preparation and storage, and the precoat liquid 61 is added. It may be circulated.

  Opening the filter 47 and checking the formation thickness of the precoat 62a is not preferable because it requires labor, and there is a possibility that the surface of the precoat 62a may be crushed due to contact with air. Therefore, a first turbidity meter 69 a is provided in a connecting pipe 74 between the outlet side of the precoat liquid storage tank 66 and the inlet side of the filter 47, and a second pipe 73 is provided between the precoat liquid storage tank 66 and the filter 47. A turbidimeter 69b is provided, and the controller 72 determines whether or not the formation of the precoat 62a is complete based on the outputs of the turbidimeters 69a and 69b. That is, when the precoat 62a is formed, the filtration performance is stabilized, and most of the filter aid 44 in the precoat liquid is captured by the filter medium 63, so that the filter aid in the precoat liquid 61 from the outlet of the filter 47 is obtained. Agent 44 is drastically reduced. The decrease of the filter aid 44 is monitored by the controller 72 via the turbidimeters 69a and 69b, and when the output of the turbidimeters 69a and 69b is equal to or lower than a predetermined value, it is determined that the formation of the precoat 62a is completed. To do. After determining the completion of the precoat formation, the process proceeds to the next precoat liquid extraction step. Although the first and second turbidimeters 69a and 69b are provided, the completion of the formation of the precoat 62a may be detected only by the first turbidimeter 69a. However, by using the second turbidity meter 69b as well, it is possible to reliably detect the completion of the formation of the precoat 62a based on the turbidity on the inlet side and outlet side of the filtrate of the filter 47.

  The turbidimeters 69a and 69b are only required to be able to detect the amount of the filter aid 44 of the precoat liquid 61, and the detection method is not limited. For example, an absorptiometric method or a laser scattered light method that detects turbidity is used.

  The total amount of filter aid in the precoat solution in the precoat formation step is determined in advance based on the relationship between the precoat and the amount of filter aid in the precoat. That is, the precoat having a certain strength and the amount of filter aid at that time are obtained by experiments and the like, and when the precoat is formed using a certain amount of filter aid, a certain strength is obtained. Shall. For the sake of safety, it is preferable that the amount of circulating filter aid is preliminarily determined by experiments or the like, for example, several% to more than 10%. Therefore, when the turbidimeter output becomes a certain value or less and a clarified liquid is obtained, it is determined that most of the filter aid has been used for precoat formation, and the completion of formation of a precoat having a predetermined strength can be known. it can.

  FIG. 6 is a graph showing the relationship between the total amount of filter aid in the precoat liquid and the thickness and strength of the precoat formed at this time, and the thickness of the precoat increases as the total amount of filter aid increases. A predetermined strength can be obtained.

  After determining the completion of the formation of the precoat 62a, the precoat liquid 61 in the filter 47 is extracted by its own weight. As described above, due to the drainage due to its own weight, it becomes a mild condition in which the drainage rate is lowered as compared with the method of pressurizing and pushing using dry air or dry nitrogen, and a skin layer is formed on the surface of the deposited layer 62. Disappear. Further, at the time of extraction by its own weight, the valve V7 of the communication pipe 75 that communicates the precoat liquid storage tank 66 and the filter 47 is opened and communicated, so that the solvent saturated gas 76 is removed from the precoat liquid at the time of liquid removal by its own weight. The filter 47 can be replenished from the storage tank 66. By substituting the inside of the filter 47 with the solvent saturated gas 76, there is no formation of a burrs of the deposited layer 62 due to solvent drying or a skin layer in which the burrs spread over a wide range, and the next filtration. Processing can be performed stably.

  Next, the cleaning process will be described. When any one of the filters 47 and 48 is used and the deposition layer 62 becomes a certain thickness or more and the filtration pressure increases, the filters 47 and 48 are switched. For example, when the filtration is performed by the first filter 47 and the filtration pressure of the first filter 47 increases and the switching time comes, the valves V1 to V6 are switched and the second filter 47 is switched from the first filter 47 to the second filter. The flow path is switched so that the raw material dope 41 flows to the vessel 48, and continuous filtration is performed. After switching to the second filter 48, the first filter 47 is drained and washed, and then the above-described precoat treatment is performed. Note that this switching is preferably performed by gradually increasing or decreasing the flow rate of each other.

  As shown in FIG. 4, the cleaning liquid circulation unit 50 includes a cleaning liquid tank 80, a cleaning tank 81, a recovery tank 82, a supply pipe 83, a recovery pipe 84, pumps 78 a, 79 a, 85, 92 a, and a heater 86. And a separator 87 and a drying device 88 (see FIG. 5). The supply pipe 83 is connected to the solvent outlet of the cleaning tank 81 and the dope outlet of the first filter 47, and the supply pipe 83 is provided with a pump 85 and a heater 86. The recovery pipe 84 is connected to the solvent inlet of the cleaning tank 81 and the dope inlet of the first filter 47. A cleaning liquid 89 is stored in the cleaning liquid tank 80, and a predetermined amount of the cleaning liquid 89 is sent into the cleaning tank 81 by opening / closing control of the valve 80a. The cleaning liquid 89 is not particularly limited as long as it can clean the filter medium 63 of the filters 47 and 48, but the solvent is at least one of the solvents constituting the dope solvent. The total solvent constituting the dope solvent is more preferable.

  A multi-tube heat exchanger is used as the heater 86, and the cleaning liquid 89 is heated by the heater 86. The heating temperature is set to a temperature equal to or higher than a value 20 ° C. lower than the boiling point of the cleaning liquid 89 at normal pressure under the condition that the cleaning liquid 89 does not boil. By heating the cleaning liquid 89 and supplying it to the filter 47 in this way, the cleaning effect of the filter 47 can be improved.

  The cleaning liquid 89 sent to the filter 47 through the supply pipe 83 passes through the filter medium 63 in the direction opposite to that of the raw material dope 41 during the filtration process, and is returned to the cleaning tank 81 through the recovery pipe 84. As a result, the cleaning liquid 89 is circulated and supplied to the filter 47, and the deposited layer 62 on the filter medium support 60 is peeled off from the filter medium support 60. The peeled deposited layer (cake) 62 is dispersed in the cleaning liquid 89 to become a slurry 90, discharged from the filter 47 and returned to the cleaning tank 81. The turbidity of the slurry 90 is measured using a turbidimeter 84a. When the turbidity of the slurry 90 reaches a target value, a part (or all) of the cleaning tank 81 is discharged from the discharge pipe 78 and the pump 78a. To the recovery tank 82. When the discharge of the slurry 90 to the recovery tank 82 is completed, a new cleaning liquid 89 is replenished from the cleaning liquid tank 80 to the cleaning tank 81. Then, when the predetermined cleaning time is reached and the turbidity of the slurry 90 becomes a predetermined value or less, the circulation of the cleaning liquid 89 is stopped. After this stop, the cleaning liquid 89 is extracted from the first filter 47. This extraction is performed reliably and quickly by sending the solvent saturated gas 76, nitrogen gas, or the like of the cleaning liquid into the first filter 47.

  The slurry 90 sent to the recovery tank 82 is sent to a separator 87 via a pipe 91, and is separated into a residue 90a solution 90b by the separator 87. A circulation pipe 79 between the recovery tank 82 and the washing tank 81 is provided with a pump 79a and a viscometer 79b.

  The viscometer 79b constantly measures the viscosity of the recovered slurry 90. Then, the cleaning liquid 89 is charged into the cleaning tank 81 so that the viscosity of the slurry 90 is always within a certain range based on the viscosity measured by the viscometer 79b. Thereby, for example, after the viscosity of the slurry 90 is set to 200 mPa · s or less, the slurry 90 can be sent to the separator 87. Here, when the viscosity of the slurry 90 is 200 mPa · s or less, separation and recovery can be performed while securing a flow rate. However, since the slurry 90 having a viscosity exceeding 200 mPa · s has a high viscosity, workability by the separator 87 is lowered.

  In the separator 87, the slurry 90 is separated into the residue 90a and the solution 90b by using a strainer 95 formed into a cylindrical shape by a 350 mesh metal mesh made of SUS. Also in this separator 87, the residue 90a acts as a filter aid, and the residue 90a and the solution 90b can be reliably separated. It should be noted that immediately after the separation, there is no precoat formed by the residue 90a, and the separation cannot be performed reliably. For this reason, the slurry 90 is circulated between the separator 87 and the recovery tank 82 using the circulation pipes 92 and 93 at the start of separation so that a precoat having a predetermined thickness is formed. The circulation pipes 92 and 93 have a pump 92a and valves 92b and 93b. Then, when a precoat is formed in the same manner as in FIG. 2 by circulation of the slurry 90, a separation effect is obtained. Therefore, the valve 93b is switched to change from the circulation treatment to the separation treatment, and the solution 90b after the separation is recovered. Send to tank 94. The recovered solution 90b can be reused for dope preparation and cleaning.

  The separator 87 is provided with a pressure gauge 96 to detect the filtration pressure in the separator 87. When the filtration pressure value reaches a predetermined value, it is determined that the thickness of the separation filter medium has reached the use limit, and the separation process ends. In this case, the strainer 95 is taken out from the separator case 87a, and the residue 90a is dried by a drying device 88 as shown in FIG.

  The drying device 88 includes a drying chamber 120, a solvent gas recovery device 121, and a drying air circulation device 122. In the drying chamber 120, strainers 95 are arranged side by side. The drying air 123 is sent into the drying chamber 120 by the drying air circulation device 122. In addition to volatilization of the solution 90b from the residue 90a, impurities 64 are combusted by the dry air. The drying method may be performed by a direct heating method using a burner or a heater in addition to circulation of the drying air 123.

  The drying air 123 subjected to drying is in a state containing a solvent gas. For this reason, the drying air 123 is once recovered by the solvent gas recovery device 121, and the solvent 21 is removed here. Thereafter, the drying air 123 from which the solvent 21 has been removed is heated to a predetermined temperature by the heating unit in the drying air circulation device 122 and then supplied again into the drying chamber 120 as the drying air 123. The residue 90a after drying can be reused as a filter aid. In addition, when reusing, it may be used as it is or may be mixed with a new filter aid 44 at an appropriate ratio.

The slurry 90 is fed from the recovery tank 82 to the separator 87 by the pump 79a, or a pressure feeding method using N ₂ gas by a pressurizing device (not shown) or a method using its own weight may be used. In order to ensure a smooth flow of the slurry 90, the concentration of the slurry 90 is preferably 0.15 wt% or more and 25 wt% or less. The concentration of the slurry 90 is the ratio of the residue 90a contained in the slurry 90. In the case of the slurry 90 in which this concentration exceeds 25% by weight, it is difficult to send the slurry 90, and it is particularly difficult to send the slurry using its own weight.

  As described above, the pre-coat operation is performed on the filter 47 after the cleaning process by the pre-coat liquid circulation unit 49 to form the pre-coat 62a on the filter medium support 60 as shown in FIG. Although the cleaning process and the precoat process have been described only for the first filter 47, the same cleaning process and the precoat process are also performed for the second filter 48.

  In the cleaning liquid circulation unit 50, the cleaning tank 81, the recovery tank 82, and the separator 87 are used. However, the recovery tank 82 is omitted, and a separator 87 is provided instead of the recovery tank 82, so that the slurry 90 Recovery and separation may be performed.

  As shown in FIG. 7, the film forming unit 13 includes a casting chamber 100, a transfer portion 101, a tenter 102, a drying chamber 103, and a winder 104, and the film 106 is formed using the casting dope 52. Made. In the casting chamber 100, a casting die 107 in which a discharge port for the casting dope 52 is formed, a casting drum 108 acting as a support, and a peeling roller 109 are arranged.

  The casting dope 52 from which impurities have been removed is cast on a casting drum 108 that is rotating endlessly through a casting die 107, whereby a casting film 111 is formed. The surface temperature of the casting drum 108 is preferably substantially constant within a range of −10 ° C. to 10 ° C. If the dope is cast on such a casting drum 108, the dope is quickly cooled, so that a gel-like casting film 111 is formed within a short time. As the casting drum 108 rotates, gelation of the casting film 111 proceeds, and the casting film 111 having self-supporting properties is peeled off from the casting drum 108 as a wet film 113 while being supported by the peeling roller 109.

  In the crossover part 101, the wet film 113 is supported by a large number of rollers, and drying proceeds while being transported. In the tenter 102, both end portions of the wet film 113 are held by holding means such as pins, and then dried while being transported to form a film 106. Thereafter, the film 106 is wound up into a roll shape by the winding roller 105.

  A filter 114 is installed on the upstream side of the casting die 107, and the dope before being used for casting is filtered. This further removes impurities in the casting dope. In this embodiment, an apparatus including a metal filter is used, but the filtration method is not particularly limited, and filter paper is also preferably used. Here, the pores of the filter preferably have an average pore diameter of 100 μm or less in order to remove even fine impurities. If the average pore size is too small, the time required for filtration becomes longer, so the filtration efficiency is lowered. On the other hand, if the average pore diameter is too large, it is difficult to capture fine impurities in the casting dope 52. A filter may be appropriately selected in consideration of productivity and the like.

  From casting die, decompression chamber, support structure, co-casting, peeling method, stretching, drying conditions for each step, handling method, curling, winding method after flatness correction, solvent recovery method, film recovery method Up to this point, the details are described in paragraphs [0617] to [0889] of JP-A-2005-104148, and these descriptions can also be applied to the present invention.

  The film obtained by the present invention has high transparency and retardation value and low humidity dependency. Therefore, although it can use suitably as a retardation film of a polarizing plate especially, it can utilize also as a protective film for protecting the surface of a polarizing plate. Regarding specific uses of the cellulose ester film of the present invention, in Japanese Patent Application Laid-Open No. 2005-104148, for example, from [1088] paragraph to [1265] paragraph, as a liquid crystal display device, TN type, STN type, VA type, The OCB type, the reflection type, and other examples are described in detail, and this description can also be applied to the present invention.

  Examples of the present invention and comparative examples will be shown below to specifically describe the present invention. However, the present invention is not limited to these examples and comparative examples.

  A material dope 41 was prepared by mixing the following various dope materials. In this example, as the solvent 21, a mixed solvent in which dichloromethane, methanol, and 1-butanol were mixed was used.

[Dope raw material]
Cellulose triacetate 100 parts by weight Dichloromethane 320 parts by weight Methanol 83 parts by weight 1-butanol 3 parts by weight Plasticizer A 7.6 parts by weight Plasticizer B 3.8 parts by weight UV agent a 0.7 part by weight UV agent b 0.3 part by weight Part citrate ester mixture 0.006 parts by weight fine particles 0.05 parts by weight

  The cellulose triacetate has a substitution degree of 2.84, a viscosity average polymerization degree of 306, a water content of 0.2% by weight, a viscosity of 6% by weight in a dichloromethane solution of 315 mPa · s, an average particle diameter of 1.5 mm, and a standard A powder having a deviation of 0.5 mm, plasticizer A is triphenyl phosphate, plasticizer B is diphenyl phosphate, and UV agent a is 2 (2'-hydroxy-3 ', 5' -Di-tert-butylphenyl) benzotriazole, UV agent b is 2 (2'-hydroxy-3 ', 5'-di-tert-amylphenyl) -5-chlorobenzotriazole, citrate The compound is a mixture of citric acid, monoethyl ester, diethyl ester and triethyl ester, and the fine particles have an average particle size of 15 nm and a Mohs hardness of about 7 It is silicon dioxide. Moreover, at the time of preparation of the raw material dope 41, a retardation control agent (N-N-di-m-toluyl-N-P-methoxyphenyl-1,3,5-triazine-2,4,6-triamine) was used as a film. It added so that it might become 4.0 weight% with respect to the total weight at the time.

  Next, in the solution casting apparatus 10 shown in FIG. 1, the raw material dope 41 was filtered using the filter 47 in the filtration unit 12. At this time, diatomaceous earth having an average particle diameter of 35 μm was used as a filter aid, and a precoat formation treatment was performed before the raw material dope 41 was filtered in advance, and after the precoat was formed, the precoat solution was extracted.

In the precoat solution, diatomaceous earth having an average particle size of 35 μm as a filter aid, a dope solution containing 20% by weight of cellulose triacetate, and a solvent for dilution are placed in a precoat solution preparation tank, and the filter aid concentration of the precoat solution is 2. It was prepared such that the cellulose concentration was 0.0% by weight and the cellulose concentration was 3.5% by weight, and was stored in the precoat liquid storage tank after preparation. The precoat liquid was circulated between the filter 47 and the precoat liquid storage tank 66 at a flow rate of 40 L (liter) / min / m ² to form a precoat on the filter medium support in the first filter. As the filter medium support 60, a 350 mesh wire mesh made of SUS was used.

And the filter aid density | concentration was detected from the turbidimeter output using the photoelectric sensor (F71RAN) by Takenaka Electronics Co., Ltd. as the 1st and 2nd turbidimeters 69a and 69b. Since the second turbidity meter 69b is disposed in the outlet side pipe 73 of the filter 47, it became 0% by weight 3 minutes after the start of circulation of the precoat liquid. Further, the first turbidity meter 69a arranged in the inlet side piping 74 of the filter 47 gradually decreases from the initial value of 2.0% by weight when the circulation of the precoat liquid is started, and becomes 0% by weight after 30 minutes. became. At this point, it was determined that precoat formation was complete. The total amount of filter aid in the circulating precoat liquid is determined from the total amount of filter aid when a predetermined strength is obtained. In this embodiment, the amount of filter aid is 37.5 kg / m ² (filtration area 1 m ² Per filter aid). The amount of the filter aid is an amount that forms an average thickness of 3 mm with respect to the total filtration area of the filter medium support.

After the formation of the precoat 62a is completed, the precoat liquid 61 is extracted from the first filter 47 by its own weight. At this time, the valve V7 of the communication pipe 75 is opened, the precoat liquid storage tank 66 and the first filter 47 are communicated, and the solvent corresponding to the amount of the precoat liquid 61 extracted in conjunction with the extraction of the precoat liquid 61 due to its own weight. A saturated gas 76 is replenished in the first filter 47. Thus, unlike the conventional case where the formed precoat 62a is drained by applying pressure with dry air or dry nitrogen, a skin layer is not formed on the surface of the precoat 62a. Further, by adjusting the opening degree of the valve V7 of the communication pipe 75, the extraction flow rate of the precoat liquid 61 can be set to 1 × 10 ⁻³ m / s or less with respect to the surface of the precoat layer. By setting the extraction speed of the precoat liquid 61 to 1 × 10 ⁻³ m / s or less, a precoat 62a having no collapse was obtained. At this time, when the filter 47 was opened and the inside was confirmed, it was confirmed that a precoat 62a having a uniform predetermined thickness was obtained. When the precoat formation treatment was performed again under the same conditions, the same results were obtained.

[Comparative Example 1]
The configuration was the same as Example 1 except that the amount of the filter aid in the precoat solution was 6.3 kg / m ² . With this amount of filter aid, a precoat thickness of 0.5 mm was obtained. With this precoat thickness, a predetermined strength could not be obtained, and a part of the liquid drainage step collapsed, making filtration impossible.

[Comparative Example 2]
The configuration was the same as in Example 1 until the formation of the precoat 62a was completed. After completion of the precoat 62a, nitrogen gas was sent to the first filter 47 and drained. After this drainage, burrs were generated on the surface of the precoat 62a, the pressure loss increased, and filtration became impossible.

[Comparative Example 3]
Example 1 was the same as Example 1 except that the degree of opening of the valve of the communication pipe was controlled so that the self-drainage speed in Example 1 was 2 × 10 ⁻³ m / s. Since the self-drainage speed was high, the precoat formed at the corner flowed, and as a result, the precoat could not be formed.

Based on the above results, by preparing the precoat liquid so that the amount of the filter aid in the circulating precoat liquid is 37.5 kg / m ² , the thickness of the precoat can be reduced to 3 mm. And strength that did not collapse in the filtration process was obtained.

In addition, the precoat liquid can be formed and maintained by setting the speed at the time of draining the precoat liquid to 1 × 10 ⁻³ m / s or less with respect to the surface of the precoat layer. It can be seen that by replenishing the gas into the filter, it is possible to prevent the occurrence of burrs on the surface of the precoat and to form a precoat having an excellent filtering ability.

It is the schematic which shows an example of the solution casting apparatus which implemented this invention. It is sectional drawing which expands and shows the filter medium and precoat in a filter. It is the schematic which shows a precoat liquid circulation part. It is the schematic which shows a washing | cleaning-liquid circulation part. It is the schematic which shows a drying apparatus. It is a diagram which shows an example of the relationship between the filter aid total amount in the circulating precoat liquid, and the intensity | strength of the precoat at this time. It is the schematic which shows a film forming unit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Solution casting apparatus 11 Raw material dope preparation unit 12 Filtration unit 13 Film forming unit 14 Meter 19 Film 30 Coarse solution 41 Raw material dope 44 Filter aid 45 Body feed tank 46 Filter aid tank 47 First filter 48 First filter 48 2 Filter 49 Precoat liquid circulation part 50 Cleaning liquid circulation part 51 Casting dope storage tank 52 Casting dope 56 Filter aid solution 60 Filter medium support 61 Precoat liquid 62 Deposited layer 62a Precoat 63 Filter medium 64 Impurity 65 Precoat liquid preparation tank 66 Precoat Liquid storage tank 69 Turbidimeter 70 Solvent for dilution 71 Solvent tank 72 Controller 75 Communication pipe 76 Solvent saturated gas

Claims (11)

Using a plurality of filter units having a filter medium formed by forming a precoat in which a filter aid is deposited on a filter medium support, the filter aid is added to the dope containing a polymer and a solvent, and the filtered dope is filtered. Is cast on an endless running support to form a cast film, and after the cast film has self-supporting properties, it is peeled off from the support and dried to form a film. In
A washing step of stopping the dope supply to the filter having a reduced filtration ability and washing the filter aid after removing the filter aid as a slurry.
A precoat liquid for forming the precoat is circulated and supplied to the filter after the washing step, and a precoat formation step for forming the precoat,
Detecting the turbidity of the precoat liquid on the dope inlet side of the filter in the precoat formation step, and detecting the completion of precoat formation when the turbidity is below a certain value,
A precoat liquid extraction step of replenishing the filter with a saturated gas of the solvent according to the extracted precoat liquid content while extracting the precoat liquid from the filter after completion of the formation of the precoat;
A solution casting method comprising: a filter switching step of sending the dope to the filter that has completed the precoat liquid extraction step and switching the filter before performing the cleaning step.   Based on the relationship between the strength of the precoat and the amount of filter aid at that time, the total amount of filter aid in the circulating precoat solution is determined so that the precoat has strength to withstand the liquid draining step and the filtration step. 2. The solution casting method according to claim 1, wherein a precoat solution is prepared based on the total amount of the filter aid. In the precoat liquid extraction step, the precoat liquid is extracted by its own weight,
3. The solution casting method according to claim 1, wherein replenishment of the saturated gas of the solvent is performed by a communication pipe communicating between the tank in which the precoat liquid is extracted and stored and the filter. .   The solution casting method according to any one of claims 1 to 3, wherein the precoat liquid is formed by dispersing the filter aid in a diluted dope obtained by diluting the dope with the solvent. 5. The solution casting method according to claim 1, wherein the precoat liquid extraction flow rate in the precoat liquid extraction step is 1 × 10 ⁻³ m / sec or less with respect to the surface of the precoat layer. . As the filter aid, SiO ₂ having an average particle size of 20 to 50 μm is used, the polymer is cellulose acylate, the filter aid concentration of the precoat liquid is 0.25 to 5% by weight, and the cellulose concentration is 0. The solution casting method according to any one of claims 1 to 5, wherein the content is 5 to 5% by weight. Using a plurality of filter units having a filter medium formed by forming a precoat in which a filter aid is deposited on a filter medium support, the filter aid is added to the dope containing a polymer and a solvent, and the filtered dope is filtered. Is cast on a support that runs endlessly to form a cast film, and after the cast film has self-supporting properties, it is peeled off from the support and dried to form a film. In
A cleaning unit that stops the supply of the dope to the filter having a reduced filtering ability and removes the filter aid as a slurry after washing,
A precoat liquid for forming the precoat is circulated and supplied to the filter after the washing, and a precoat forming section for forming the precoat,
A turbidimeter that detects the turbidity of the precoat solution on the dope inlet side of the filter during precoat formation by the precoat formation unit,
A precoat formation completion determination unit that determines the completion of precoat formation when the turbidity value detected by the turbidimeter is equal to or less than a predetermined value;
After completion of precoat formation by the precoat formation completion determination unit, a precoat liquid is extracted from the filter, and a precoat liquid extraction unit for replenishing the saturated gas of the solvent in the extracted precoat liquid;
A solution casting apparatus, comprising: a switching unit that sends the dope to the filter that has finished extracting the precoat liquid and switches the filter when the filter is cleaned by the cleaning unit.   The precoat forming section is based on the relationship between the strength of the precoat and the amount of filter aid at that time, so that the precoat has a strength that can withstand the liquid draining and filtration of the dope. 8. The solution casting apparatus according to claim 7, wherein a total amount of the filter aid is determined, and a precoat solution is prepared based on the total amount of the filter aid.   9. The solution casting apparatus according to claim 8, further comprising a precoat liquid preparation tank that forms the precoat liquid by dispersing the filter aid in a diluted dope obtained by diluting the dope with the solvent. The precoat liquid extraction unit includes a precoat liquid tank that stores the precoat liquid, a return piping system that is connected to an outlet side of the filter and returns the precoat liquid to the precoat liquid tank, and a solvent saturated gas of the precoat liquid tank And a solvent saturated gas communication pipe for supplying the precoat liquid to the inlet side of the filter, and the precoat liquid extraction flow rate in the precoat liquid extraction apparatus is 1 × 10 ⁻³ m / sec or less with respect to the surface of the precoat layer. The solution casting apparatus according to any one of claims 7 to 9, wherein As the filter aid, SiO ₂ having an average particle size of 20 to 50 μm is used, the polymer is cellulose acylate, the filter aid concentration of the precoat liquid is 0.25 to 5% by weight, and the cellulose concentration is 0. The solution casting apparatus according to any one of claims 7 to 10, wherein the content is 5 to 5% by weight.

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