JP2009196116A - Solution film forming method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a precoat using a used precoat liquid to extend filter life in an aid filtration system. <P>SOLUTION: A filter aid solution 56 is dispersed in a raw material dope 41, and filtered with a first filter 47 or a second filter 48. A filter aid is deposited on a filter medium support 60 in the filter 47, and the raw material dope 41 is filtered out by using a filtering medium 63 composed of the filter medium support 60 and a deposition layer. When a filtration pressure becomes high, a plurality of filters 47, 48 are switched, and a cleaning liquid is flowed in a used filter to be cleaned. After being cleaned, the precoat liquid 61 is circulated, the filter aid is deposited on the filter medium support 60 to form the precoat 62a. The used precoat liquid has high clarification degree and the following precoat 62a is formed by utilizing the used precoat to reduce the deposition of impurities into the filtering medium 63, and the filter life can be extended. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a solution casting method and equipment using a filtration device for filtering a polymer solution with a filter aid.

  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 polymer solution containing a polymer and a solvent (hereinafter referred to as a dope) is cast on a traveling support to form a cast film, and then the cast film is peeled off from the support. This is a method of drying into a film, and there is no problem of heat 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, all the filter media have problems that the liquid passage hole is blocked as time passes from the start of filtration, and that the filtration time is prolonged, the filtration pressure is increased, the filtration flow rate is reduced, and the filtration efficiency is lowered. 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. As a filter aid, for example, inert particles or powder such as silicon dioxide (SiO ₂ ) is used. 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 the filtration method using a filter aid, it is generally performed to prepare a precoat solution for each precoat formation treatment. This is because the prescription can be stably performed and there is a merit that the amount is stable.

  However, since it is necessary to prepare a precoat solution for each precoat forming process, there is a problem that an efficient continuous filtration process cannot be performed.

  The present invention has been made in view of the above problems. In the auxiliary filtration method, the precoat liquid is efficiently prepared during the precoat formation treatment, and further, the filtration life can be easily increased, and the film is efficiently formed. An object of the present invention is to provide a solution casting method and equipment that can be used.

  In order to achieve the above object, the present invention uses a plurality of filters having a filter medium formed with 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. The filtered dope is cast on an endless running support to form a cast film, and the cast film is self-supporting and then peeled off from the support. In the solution casting method of taking and drying to form a film, the washing step of washing the filter medium support by stopping the supply of the dope to the filter having a reduced filtration ability in the filter, and the washing A precoat liquid for forming the precoat is circulated and supplied to the filter after the process, and the precoat liquid is extracted from the filter after the precoat is formed. A pre-coating liquid extraction step, and a filter switching step of switching the filter by sending the dope to the filter that has finished extracting the pre-coating liquid when the filter is washed in the washing step, The filter switching step, the washing step, the precoat forming step, and the precoat liquid extraction step are periodically repeated sequentially, and the dope is continuously filtered using the plurality of filters selectively, In the precoat forming step, the precoat liquid is formed by dispersing at least the filter aid in the filtered dope or the precoat liquid used for precoat formation.

  In addition, the present invention provides a filter using a plurality of filters having a filter medium formed by forming a precoat in which a filter aid is deposited on a filter medium support, and adding the filter aid to a dope containing a polymer and a solvent. 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, the film is peeled off from the support and dried. In the solution casting apparatus, the supply of the dope to the filter having a reduced filtering ability is stopped, and the cleaning unit for cleaning the filter medium support and the filtering after the cleaning step are completed. A precoat liquid for forming the precoat is circulated and supplied to a container, and a precoat liquid is formed, and a precoat liquid drainage part for extracting the precoat liquid from the filter after the precoat is formed. A filter switching unit that sends the dope to the filter that has finished extracting the precoat liquid, and switches the filter; a cleaning unit; the precoat formation unit; the precoat liquid extraction unit; and a filter switching unit. And at least the filter aid is dispersed in a control unit for continuously filtering the dope using the plurality of filters sequentially, and the filtered dope or precoat liquid used for precoat formation. And a precoat liquid forming section for forming a precoat liquid.

In the present invention, the viscosity of the precoat liquid is 0.5 to 200 mPas. Moreover, the terminal sedimentation speed | rate of the filter aid in the said precoat formation process is controlled to 10 < ^-4 > -1cm / sec. Furthermore, the flow rate of the precoat liquid with respect to the filter medium support in the precoat formation step is set to 3.3 to 80 L / m ² / min. Moreover, the extraction flow rate of the precoat liquid in the precoat liquid extraction step is set to 1 × 10 ⁻³ m / sec with respect to the surface of the precoat layer. Further, SiO ₂ having an average particle size of 20 to 50 μm as the filter aid is used, the polymer is cellulose acylate, and the filter aid concentration of the precoat liquid is 0.25 to 5.0% by weight, The cellulose concentration is 0.5 to 5.0% by weight. In the precoat liquid extraction step, the filter is replenished with the saturated gas of the solvent in accordance with the extracted precoat liquid while extracting the precoat liquid from the filter.

  According to the present invention, in the precoat forming step, a precoat liquid having a high clarity can be obtained by forming a precoat liquid by dispersing at least a filter aid in a filtered dope or a precoat liquid used for precoat formation. . By forming a precoat using this precoat liquid, impurities and the like during the formation of the precoat are not captured by the precoat being formed as compared to the case of using a normal precoat liquid. Accordingly, since a precoat with less clogging is formed, the filtration life is extended by about 20%. Accordingly, the frequency of switching the filter is reduced, and efficient solution film formation becomes possible.

  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 solvent tank 17 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 24 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 forming unit 49, a washing unit 50, and a casting dope storage tank 51. The raw material dope 41 is filtered using the agent 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 a precoat liquid 61 (see FIG. 3) through the first filter 47 for a predetermined time by the precoat forming unit 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. In addition, the first filter 47 after switching is cleaned by removing the filter aid 44 and the filtered material as the slurry 90 (see FIG. 4) by the cleaning unit 50. After the cleaning, the precoat liquid 61 is circulated to the first filter 47 by the precoat forming unit 49 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 forming unit 49 includes a precoat liquid preparation tank 65, a precoat liquid storage tank 66, valves 65a, 67, 68a, a pump 68, turbidimeters 69a, 69b, a controller 72, It has.

  The precoat liquid preparation tank 65 prepares a precoat liquid. The precoat solution is prepared by different methods at the start of film formation and during continuous film formation. At the start of film formation, the filtered dope is not formed. In this state, the film is formed as follows by the first method. First, a predetermined amount of the raw material dope 41 containing the filter aid 44 is sent from the body feed tank 45 to the precoat liquid preparation tank 65 via the pump 45a, and the dilution solvent 70 is sent from the solvent tank 71 via the valve 71a. Then, a predetermined amount is fed 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. The viscosity of the precoat liquid at this time is 0.5 to 200 mPas. 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 5.0 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. As the filter aid 44, SiO ₂ having an average particle diameter of 10 to 70 μm is used, and an average particle diameter of 20 to 50 μm is more preferable. 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.

The flow rate of the precoat liquid with respect to the filter medium support 60 in the precoat forming step is 3.3 to 80 L / m ² / min, preferably 20 to 60 L / m ² / min. When the flow rate exceeds 80 L / m ² / min, the deposited layer 62 cannot be formed on the filter medium support 60. Further, if the flow rate is less than 3.3 L / m ² / min, the strong precoat 62a cannot be formed.

The terminal sedimentation rate of the filter aid in the precoat forming step is controlled to 10 ^{−4 to 1} cm / sec. This terminal sedimentation rate is preferably 10 ⁻³ to 10 ⁻² cm / sec. In order to obtain a terminal sedimentation velocity within this range, the liquid viscosity and the auxiliary particle size are changed. When the terminal sedimentation rate is less than 10 ⁻⁴ cm / sec, the pressure loss is large and the flow rate cannot flow. When it exceeds 1 cm / sec, a uniform precoat cannot be formed.

  The concentration of the filter aid in the precoat solution is in the range of 0.01 to 10.0% by weight, preferably 0.1 to 2.0% by weight. If the filter aid concentration exceeds 6.0% by weight, interference sedimentation occurs and a uniform precoat cannot be formed. Further, if the filter aid concentration is less than 0.1% by weight, it takes a long time to form the precoat and the efficiency is lowered.

  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 the filter aid at that time are obtained by experiments and the like, and when the precoat is formed using a certain amount of the 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 can be 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. 4 is a graph showing the relationship between the total amount of the 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 the 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.

  When the precoat formation process is performed in this manner, the precoat residual liquid used for the precoat is filtered by the precoat 62a, and thus becomes a liquid with high clarity. In precoat formation for the second and subsequent times, a precoat liquid is formed by adding a filter aid to the precoat residual liquid using the precoat liquid preparation tank 65. In this case, the precoat residual liquid in the precoat liquid storage tank 66 is sent to the precoat liquid preparation tank 65 by switching the switching valve 68a. Since the second and subsequent precoat liquids are liquids having a high degree of clarification by filtration at the time of precoat formation, the precoat formed with this liquid is an impurity that becomes a clogging factor than the precoat liquid at the start of film formation. Less is. Therefore, an increase in filtration life of about 20% is recognized compared to the first precoat.

  The precoat liquid is determined to be a total amount that can be reused as the precoat liquid about 3 to 5 times. In each precoat liquid formation, a new precoat liquid is prepared by adding a certain amount of filter aid A1 (kg) to the used precoat liquid. The amount A1 of the filter aid is an amount necessary and sufficient to form a precoat having a required strength, and is determined in advance according to the size and ability of the filter. In addition, you may add a solvent so that the filter aid density | concentration of a precoat liquid may become in a fixed range as needed. When used as a precoat liquid about 3 to 5 times, the precoat liquid is diluted by the cleaning liquid remaining on the inner wall of the pipe or tank, and the polymer concentration cannot be maintained at a predetermined value. In this case, the viscosity of the precoat solution is lowered, the sedimentation property of the filter aid is increased, and the uniformity of the precoat cannot be ensured. In response to this, in the precoat solution preparation tank, a dope is added in addition to the filter aid and the solvent so that the concentration is within a certain range again. Add and prepare. This additional preparation is performed when the used precoat liquid deviates from a predetermined prescription value and when the precoat liquid becomes a certain amount or less.

  As shown in FIG. 5, in the additional preparation, the input amount B1 (kg) of the dope 130 and the input amount C (kg) of the solvent 131 are obtained, and the obtained input amounts B1, C and the filter aid 132 are supplied. The amount A1 is charged into the precoat solution preparation tank 65. The precoat liquid preparation tank 65 contains a precoat liquid (used precoat liquid) 133 after the precoat is formed. When this used precoat liquid amount is D (kg) and the precoat liquid completion amount E (kg), the difference (ED) is defined as a preparation amount F (kg). When the target polymer concentration of the completed precoat solution 134 is G (%), the polymer concentration of the used precoat solution 133 is H (%), and the polymer concentration of the additional dope 130 is L (%). The amount B1 (kg) of the dope 130 is obtained from B1 = (E × GD × H) / L. Then, by subtracting the dope amount B1 (kg) and the filter aid amount A1 (kg) from the preparation amount F (kg), the additional input amount C (= F−A1-B1) of the solvent 131 can be obtained. The completed amount E (kg) of the precoat liquid 134 and the target polymer concentration G are set values, and the used precoat liquid amount D (kg) is obtained by a tank weight meter (level meter) or the like. Further, the polymer concentration H (%) of the used precoat liquid 133 can be obtained using, for example, a viscometer.

  In addition to using the precoat residual liquid as the precoat liquid, the cast dope 52 stored in the cast dope storage tank 51 may be used. In this case, the pump 77 is used to send the casting dope 52 to the precoat liquid preparation tank 65, dilute with a solvent in the precoat liquid preparation tank 65, and add a filter aid to obtain a predetermined concentration. The precoat liquid can be formed. As described above, by forming a precoat using the filtered casting dope 52 from the beginning of the solution film formation, the filtration life is extended to about 20% as in the case of using the precoat residual liquid. Can do.

  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. 6, the cleaning 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 78a, 79a, 85, 92a, and a heater 86. , A separator 87, and a strainer drying section (drying gas circulation section) 88 (see FIG. 7). 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 deposited layer (cake) 62 after being peeled 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 has been reached and the turbidity of the slurry 90 has become below a certain value, 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 and a 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 poured 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 deteriorated.

  The separator 87 has a bottomed cylindrical separator body 87a, a lid 87b attached to the separator body 87a via a hinge portion 87c so as to be opened and closed, and an air vent pipe 87d attached to the lid 87b. Further, a strainer storage portion 87e is formed in the separator main body 87a, and a strainer 95 is stored vertically in the strainer storage portion 87e. By placing the strainer 95 vertically, the flow of the slurry 90 in the separator 87 is vertically downward, and an efficient separation operation can be performed. The separator main body 87a is provided with a slurry supply port 87f and a dry air discharge port 87g at the top thereof.

  The strainer 95 is formed in a bottomed cylindrical shape by a SUS wire mesh, and separates the slurry 90 into a residue 90a and a solution 90b. The inner diameter D of the strainer 95 is smaller than the length L of the strainer 95. Thus, by making the inner diameter D smaller than the length L, the cake thickness can be reduced, the resistance of the liquid at the time of recovery is small, and the amount of processing liquid can be increased. Moreover, the drying load after the cake recovery can be reduced.

  The mesh of the wire mesh is 50 to 500, preferably 60 to 400 mesh, and more preferably 80 to 360 mesh. If it is less than 50 mesh, it becomes impossible to ensure the clarity of the filtered solution 90b, and if it exceeds 500 mesh, the pressure loss increases and both are inferior in practicality. The wire mesh has a tatami woven structure, and is preferably a flat woven structure or a twill woven structure. Further, it is preferable to reinforce the strainer 95 with punching metal or the like as necessary. In this case, the opening ratio of the reinforcing material such as punching metal is preferably 30% or more.

Further, instead of the wire mesh strainer 95, a filter cloth strainer may be used. Similarly to the wire mesh strainer, the filter cloth strainer is also formed in a bottomed cylindrical shape. The filter cloth has an air permeability of 0.3 to 50 cc / cm ² · sec, preferably 0.5 to 40 cc / cm ² · sec, and more preferably 1 to 20 cc / cm ² · sec. When the air permeability is less than 0.3 cc / cm ² · sec, the pressure loss increases, and when the air permeability exceeds 50 cc / cm ² · sec, it becomes impossible to ensure clarity, and both are practical. Inferior.

  In this embodiment, one strainer 95 is disposed for each separator, but a plurality of strainers 95 may be disposed for each separator. In this case, a plurality of strainers are provided. As a result, the amount of the separation liquid can be increased, and the filter aid and the filtrate can be separated efficiently. A plurality of separators each provided with one strainer may be arranged.

  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. At the start of separation, the air in the separator 87 is extracted from the air vent pipe 87d. By performing this air venting, the slurry 90 is reliably filled in the separator 87, and reliable filtration can be performed.

The flow rate of the slurry 90 to the separator 87 is 50 to 1500 L / min per unit filtration area (1 m ² ). When the flow rate is less than 50 L, the treatment time becomes long, which is not preferable. On the other hand, when the flow rate exceeds 1500 L, the apparatus becomes large and the equipment cost increases, which is not preferable.

  The circulation pipes 92 and 93 have a pump 92a and valves 92b and 93b. When the precoat is formed in the same manner as in FIG. 2 due to the circulation of the slurry 90, the separation effect is obtained. Therefore, the valve 93b is switched to change 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. The final deposition thickness of the residue (filter aid) 90a in the separator 87 is preferably 0.5 to 50 cm. If this final deposition thickness is less than 0.5 cm, the filtration area of the separator 87 is required, which disadvantageously increases the size of the apparatus. On the other hand, if the final deposition thickness exceeds 50 cm, the pressure loss for separation increases, and flow restriction and pressure resistance restriction occur, which is not preferable.

  After the separation processing, the drying air is sent to the residue 90a in the strainer 95 by the strainer drying unit 88 shown in FIG.

  The strainer drying unit 88 includes a drying air circulation pipe 120, valves 121 and 122, a solvent gas concentration sensor 123, a solvent gas recovery device 124, a drying air circulation device 125, and a drying unit controller 126. During the drying process, the valves 93b, 121, and 123 are opened and closed, the slurry circulation pipes 92 and 93 are closed, and the drying air circulation pipe 120 is opened. Then, the drying air 97 is sent into the separator 87 by the drying air circulation device 125. By this dry air 97, the solution 90b is volatilized from the residue 90a. In addition to the circulation of the drying air 97, the drying method may be performed by a direct heating method using a burner, a heater, or a jacket method, or may be used in combination. The drying air 97 may use a gas other than air. Further, the residue 90 a may be dried by sending steam to the separator 87 instead of the drying air 97.

  The drying air 97 used for drying is in a state containing a solvent gas. For this reason, the solvent 21 is recovered and removed from the drying air 97 by the solvent gas recovery device 124. Thereafter, the drying air 97 from which the solvent 21 has been removed is heated to a predetermined temperature by the heating unit in the drying air circulation device 125 and then supplied again into the separator 87 as the drying air 97.

  As the drying proceeds, the solvent gas concentration in the drying air circulation pipe 120 decreases. This is detected by the drying unit controller 126 based on the output value of the solvent gas concentration sensor 123, and when the concentration sensor 123 reaches a certain value, it is determined that the drying is finished. If it determines with completion | finish of drying, the drying part controller 126 will lower | hang the drying wind preset temperature of the strainer drying part 88 as needed, and will send this to the separator 87 as cooling air, and the strainer 95 and residue 90a in the separator 87 will be sent. Cool down. After they have cooled to near room temperature, alarm 126a informs the operator that the drying is complete. After the drying is completed, the operator opens the lid 87b (shown by a two-dot chain line in FIG. 7) of the separator 87 and takes out the residue 90a from the strainer 95. The residue 90a is taken out by replacing the strainer 95 with an empty one, and the residue 90a may be directly sucked from the strainer 95 in the separator main body 87a by a vacuum type suction device (not shown).

  In addition to opening the separator 87 and taking out the residue 90a after drying, it may be performed fully automatically. In this case, a suction nozzle that can be raised and lowered is arranged on the lid 87b of the separator main body 87a, and descends after the residue 90a is dried, so that the residue in the strainer 95 is automatically sucked. 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 a pump 79a, and a pressure feeding method using a carrier gas such as nitrogen gas by a pressurizing device (not shown) or a method using a dead weight due to a drop is used. May be. 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 feed the slurry 90, and it is particularly difficult to feed the slurry 90 using its own weight. On the other hand, when the slurry concentration is less than 0.15% by weight, the collection time of the residue 90a becomes long, and the collection efficiency is remarkably lowered.

  As described above, the pre-coat operation is performed on the filter 47 after the cleaning process by the pre-coat forming unit 49 to form a 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 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 to recover the slurry 90. And separation may be performed.

  As shown in FIG. 8, 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.

[Comparative example]
A material dope 41 was prepared by mixing the following various dope materials. In the comparative 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 Powder with a deviation of 0.5 mm, plasticizer A is triphenyl phosphate, plasticizer B is diphenyl phosphate, 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, and the citrate compound is It is a mixture of citric acid, monoethyl ester, diethyl ester and triethyl ester. The fine particles have an average particle diameter of 15 nm and a Mohs hardness of about 7. It is of silicon. 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 the precoat solution preparation tank 65, and the precoat solution has a filter aid concentration of This was prepared so that the cellulose concentration was 3.0 wt% and the cellulose concentration was 3.5 wt%, and stored in the precoat liquid storage tank 66 after the preparation. This precoat liquid was circulated between the filter 47 and the precoat liquid storage tank 66 at a flow rate of 20 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 0.75 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.

Moreover, the terminal sedimentation rate of the filter aid at the time of precoat formation was 10 ^<-3 > cm / sec. The final sedimentation velocity of the filter aid was measured by sedimentation distance measurement and calculation using the Stokes equation. The time required for forming the precoat 62a was 1 hour.

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 is 1 × 10 ⁻⁴ m / s or more and 1 × 10 ⁻³ m / s or less with respect to the surface of the precoat layer. can do. By setting the extraction rate of the precoat liquid 61 to 1 × 10 ⁻³ m / s or less, a precoat 62a without collapsing 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. In addition, a precoat liquid cannot be extracted efficiently as the extraction speed is less than 1 × 10 ⁻⁴ m / s.

  The raw material dope 41 was filtered using the precoat 62a formed as described above, and the filtration differential pressure increase period until the filtration differential pressure reached 0.2 MPa to 0.8 MPa was determined. Then, when the filtration differential pressure reached 0.8 MPa, it was determined that filtration was impossible, and the filtration unit was switched. The filtration differential pressure increase period at this time is 5 days.

Thereafter, the washing liquid was circulated through a filter having a used filter aid and recovered by a separator 87. The strainer 95 set in the separator 87 was made of a bottomed cylindrical wire mesh having an inner diameter of 400 mm and a length of 800 mm. This strainer 95 has a filtration area of 1 m ² , and a wire mesh with a 100-mesh plain woven weave was used. The viscosity of the slurry 90 was 10 mPa · s, and the concentration of the slurry 90 was 3%. The slurry 90 was supplied from the upper part of the strainer 95, the filtrate was taken out from the lower part, and the filter aid was collected in the strainer 95.

  In the initial stage of the recovery operation, the air inside the separator 87 was vented, and the air venting time was 1.5 minutes. The slurry 90 was pumped to the separator 87 using 0.1 MPa nitrogen gas, and the flow rate of the slurry 90 was 160 L / min. The cake residue thickness in the strainer 95 after the recovery was 100 mm. Since the residue slightly leaked from the strainer 95 at the initial stage of the collection operation, it was returned to the liquid supply source tank as an initial circulation, and after confirming the clarification, it was taken out as a collected liquid.

  After separating the residue 90a and the solution 90b by the separator 87, the solution 90b in the strainer 95 is driven out by air, and 40 ° C. drying air is circulated in the separator 87 by the strainer drying unit 88 as a drying air circulation unit. I let you. 48 hours after circulating and supplying the dry air, the solvent content in the residue 90a was 0.2% or less. A highly clear solution 90b could be obtained.

[Example 1]
In the comparative example, 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 the precoat solution preparation tank 65, and the filter aid concentration of the precoat solution is set. Was 3.0 wt% and the cellulose concentration was 3.5 wt%. In Example 1, the used precoat liquid used in the comparative example was used, and a filter aid was added thereto. The same as the comparative example, except that the precoat was formed using a filter aid concentration of 3.0 wt% as the precoat liquid. In Example 1, since the used precoat liquid was used, the clarification degree of the liquid was high, and the filtration differential pressure increase period at this time was 6 days, and the period was extended by about 20%.

[Example 2]
In Example 1 above, the precoat solution was prepared using the used precoat solution, but instead, a precoat solution was formed using the filtered dope 52 that had been filtered. Also in this case, since a liquid with high clarity was used, the filtration differential pressure increase period was 5.8 days, which was extended by about 20% as in Example 1.

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 formation part. 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 explanatory drawing of additional preparation. It is the schematic which shows a washing | cleaning part. It is the schematic which shows a strainer drying part. It is the schematic which shows a film forming unit.

Explanation of symbols

41 Raw material dope 44 Filter aid 47 First filter 48 Second filter 49 Precoat forming part 50 Washing part 60 Filter medium support 61 Precoat liquid 62 Deposited layer 62a Precoat 63 Filter medium 64 Impurity 87 Separator 87e Strainer storage part 88 Strainer drying Part 90 Slurry 90a Residue 90b Solution 95 Strainer 120 Drying air circulation piping

Claims (9)

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 washing the filter medium support by stopping the supply of the dope to the filter having a reduced filtration capacity among the filters;
A precoat forming step of circulatingly supplying a precoat liquid for forming the precoat to the filter after the washing step, and forming the precoat,
A precoat liquid extraction step of extracting the precoat liquid from the filter after the formation of the precoat;
When washing the filter in the washing step, the dope is sent to the filter that has finished extracting the precoat liquid, and the filter switching step for switching the filter,
The filter switching step, the washing step, the precoat formation step, and the precoat liquid extraction step are periodically repeated in order, and the dope is continuously filtered using the plurality of filters selectively,
In the precoat forming step, the precoat liquid is formed by dispersing at least the filter aid in the filtered dope or the precoat liquid used for precoat formation.   The pre-coat formation process is performed by adding the target pre-coat liquid amount and the pre-set value when the pre-coat forming used pre-coat liquid decreases or deviates from the pre-set value in the pre-coat forming step. Solution casting method.   The solution casting method according to claim 1 or 2, wherein the viscosity of the precoat liquid is 0.5 to 200 mPas. The solution casting method according to claim 3, wherein the terminal sedimentation rate of the filter aid in the precoat forming step is controlled to 10 ^{−4 to 1} cm / sec. The solution casting method according to claim 4, wherein a flow rate of the precoat liquid with respect to the filter medium support in the precoat forming step is set to 3.3 to 80 L / m ² / min. The solution casting method according to claim 5, wherein a precoat liquid extraction flow rate in the precoat liquid extraction step is set to 1 × 10 ⁻³ m / sec or less with respect to the surface of the precoat layer. SiO ₂ having an average particle diameter of 20 to 50 μm is used as the filter aid, the polymer is cellulose acylate, the filter aid concentration of the precoat liquid is 0.25 to 5.0% by weight, and the cellulose concentration The solution casting method according to any one of claims 1 to 6, wherein the content is 0.5 to 5.0% by weight.   The said precoat liquid extraction process replenishes the said filter with the saturated gas of the said solvent according to this extracted precoat liquid part, extracting the precoat liquid from the said filter. 2. The solution casting method according to item 1. 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
Among the filters, a cleaning unit that stops the supply of the dope to the filter having a reduced filtering capacity and cleans the filter medium support;
Circulatingly supplying a precoat liquid for forming the precoat to the filter after the washing step, and forming a precoat,
A precoat liquid extraction part for extracting the precoat liquid from the filter after the formation of the precoat;
A filter switching unit that sends the dope to the filter that has finished extracting the precoat liquid, and switches the filter;
A controller for controlling the cleaning unit, the precoat forming unit, the precoat liquid extraction unit, and a filter switching unit, and continuously filtering the dope using the plurality of filters in sequence;
A solution casting apparatus comprising: a precoat liquid forming unit that forms a precoat liquid by dispersing at least the filter aid in the filtered dope or the precoat liquid used for precoat formation.

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