Patent Publication Number: US-7896541-B2

Title: Mixer and mixing method of mixing polymer dope, and solution casting apparatus and process

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a mixer and mixing method of mixing polymer dope, and a solution casting apparatus and process. More particularly, the present invention relates to a mixer and mixing method of mixing polymer dope with high efficiency even in a simple structure, and a solution casting apparatus and process. 
     2. Description Related to the Prior Art 
     Cellulose acylates are used as a support of polymer film contained in photosensitive materials, such as photographic films, owing to advantageous characteristics, for example rigidity, non-flammability, and the like. A typical example of cellulose acylate is cellulose triacetate (TAC) having an average acetylation degree of 57.5-62.5%. Also, the polymer film of the cellulose triacetate (TAC) is used as a protection film of a polarizing element, or an optical compensation film (view angle enlarging film or the like), any of those being incorporated in a liquid crystal display (LCD) panel. This is effective because of optically utilizing the highly isotropic property of the polymer film. 
     Typical examples of producing method for the polymer film include extrusion and solution casting. In the extrusion, polymer is heated and melted, and extruded by an extruder to produce the polymer film. The extrusion is characterized in high productivity and a low manufacturing cost. However, a thickness of the polymer film is difficult to adjust in the extrusion, which is not suitable for producing optical film due to occurrence of die lines on the polymer film. In the solution casting, polymer dope is used and constituted by polymer and a solvent. The polymer dope is cast on a casting support, and stripped from the support when a self-supporting property develops. The stripped self-supporting cast film is dried, and wound as the polymer film. The solution casting is capable of producing the polymer film with high isotropic property, with regularity in the thickness, and without foreign material in comparison with the extrusion. The solution casting is utilized specifically for producing the polymer film of optical use. 
     In general, additives are used additionally in the polymer dope for the solution casting and mixed with the polymer and the solvent. The use of the additives in the polymer dope is effective, because retardation control agents can adjust the optical performance. Flame retardants impart such important property to the polymer film as non-flammable property as important property. Release agents can be used for raising productivity in the course of manufacture. 
     To prepare the polymer dope for casting, liquid additive having the additives is mixed with the polymer dope as solution of the polymer in the solvent. Examples of mixing methods include in-line mixing and batch mixing or tank mixing. In the in-line mixing, the polymer dope is supplied through a flow line continuously to mix the additives with the polymer dope. In the batch mixing or tank mixing, the additives are mixed with the polymer dope stored in a tank. The in-line mixing is generally used in the solution casting for mixing the additives with the polymer dope. 
     Examples of mixers for the in-line mixing of liquid additive with the polymer dope include a dynamic mixer and a static mixer. The dynamic mixer includes a stirring blade for rotating. The static mixer mixes fluid without using such a movable element. In the dynamic mixer, a shaft for the stirring blade must be kept rotatable smoothly with suitable lubricant property. Also, the dynamic mixer requires a sealing property to prevent leakage of the polymer dope or the additives through a gap near to the shaft. It is known to use oil or lubricant as sealant for requirements of both of the lubricant property and sealing property. However, an ideal structure of satisfying the requirements at the same is extremely difficult to create. When the dynamic mixer is used, unwanted mixture of the lubricant is likely to occur with the polymer dope as foreign material at the time of stir. The contamination will result in lowering the suitability of the polymer film for shaping, and lowering the optical performance. In contrast, the static mixer can operate without such problem of contamination in the dynamic mixer, because of lack of a movable element. 
     Accordingly, the static mixer is used for mixing the additives with the polymer dope. U.S. Pat. Pub. No. 2008/056064 (corresponding to JP-A 2006-076280) discloses an example of the static mixer. A twisting type of static mixer includes twisted plates for mixing fluid by a flow in a curved passage in a flow line. A Sulzer mixer includes plural crossed plates combined to cross one another alternately, to split the fluid in plural flows in the flow line. The flowing fluid is split and moved in reverse in a repeated manner, to mix the additives with the polymer dope. 
     Recent development of the liquid crystal display panel is remarkable so that there are many new types of liquid crystal display panels, such as TN type, VA type and the like. The polymer film must be produced to have specifics suitable for the various types of liquid crystal display panels. Improvement related to the efficiency in producing the polymer film is an important concern to manufacturers of the polymer film. 
     However, a great number of the static mixer must be optimized at each time of a change in the polymer dope or composition of the additives typically in a system for manufacturing various types of polymer films. Efficiency in the manufacture of the polymer film will be lower due to complexity in operation for the optimization of mixing. 
     Various compounds are used as additive components in the liquid additive because of the increase in the number of types of optical films. An amount of addition of the additive components has been increased recently in comparison with the former period. It is very likely that the viscosity ratio between the dope and the liquid additive is higher, as the viscosity of the liquid additive is lower than that of conventional liquid additive. The liquid additive cannot be mixed sufficiently with the dope at a short time according the twisting or splitting of a conventional static mixer, due to the high viscosity ratio. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems, an object of the present invention is to provide a mixer and mixing method of mixing polymer dope with high efficiency even in a simple structure, and a solution casting apparatus and process. 
     In order to achieve the above and other objects and advantages of this invention, a mixer for mixing liquid additive with polymer dope constituted by polymer and solvent includes a mixing conduit for flow of the polymer dope. A supply conduit ejects the liquid additive into the polymer dope in the mixing conduit. A rotor hub is positioned downstream from the supply conduit, contained in the mixing conduit rotatably about an axis directed in a flow direction of the polymer dope, and has a diameter decreasing end on an upstream side. A driving device rotates the rotor hub by electromagnetic induction through a chamber inside the mixing conduit. 
     Furthermore, a support is disposed within the mixing conduit to extend transversely to the flow direction, for supporting the rotor hub rotatably. A flow opening is formed in the support, for passing the polymer dope in the flow direction about the rotor hub. 
     The supply conduit has a distribution channel, opposed to the diameter decreasing end in the flow direction, for spreading the liquid additive transversely to the flow direction, to eject the liquid additive into the polymer dope. 
     Furthermore, an element for stir is formed with a peripheral surface of the rotor hub, and constituted by a groove or blade. 
     The driving device includes plural magnets incorporated in the rotor hub, and arranged with predetermined polarity relative to a circumferential direction. An electromagnet assembly is secured outside the mixing conduit and around the rotor hub, for generating a magnetic field, to rotate the rotor hub with the plural magnets. 
     The mixing conduit includes a jacket for maintaining temperature of the polymer dope at a level equal to or lower than a boiling point thereof. 
     The rotor hub is constituted by plural rotor hubs arranged serially along the mixing conduit in the flow direction, and the driving device is positioned to correspond to each of the rotor hubs. 
     A ratio ηd/ηt of viscosity ηd of the polymer dope to viscosity ηt of the liquid additive is equal to or more than 1 and equal to or less than 1×10 5 . 
     A ratio D 1 /D 2  of an outer diameter D 1  of the rotor hub to an inner diameter D 2  of the mixing conduit is equal to or more than 0.1 and equal to or less than 0.95 as viewed on a plane perpendicular to the flow direction. 
     A ratio V 1 /V 2  of a flow speed V 1  of the polymer dope to a flow speed V 2  of the liquid additive is equal to or more than 1 and equal to or less than 5 on the upstream side from the rotor hub and on a downstream side from the distribution channel. 
     A distance between an outlet of the supply conduit and the diameter decreasing end is equal to or more than 1 mm and equal to or less than 200 mm. 
     The rotor hub is driven to rotate at such a peripheral speed V 3  as to set a Reynolds value Re higher than 0.02, where the Reynolds value Re is defined by an equation of:
 
 Re=Δd·V 3· ρk/ηk  
 
     where ηk is viscosity of the polymer dope supplied with the liquid additive; 
     ρk is density of the polymer dope; 
     Δd is a size of a gap between an inside of the mixing conduit and a peripheral surface of the rotor hub. 
     Also, a solution casting apparatus includes a mixing conduit for flow of polymer dope constituted by polymer and solvent. A supply conduit ejects liquid additive into the polymer dope in the mixing conduit. A rotor hub is positioned downstream from the supply conduit, contained in the mixing conduit rotatably about an axis directed in a flow direction of the polymer dope, and having a diameter decreasing end on an upstream side. A driving device rotates the rotor hub by electromagnetic induction through a chamber inside the mixing conduit. A casting die ejects casting dope obtained from the rotor hub by mixing the polymer dope and the liquid additive. A support moves continuously and forming cast film upon casting the casting dope thereon. A dryer dries the cast film stripped from the support. 
     Also, a solution casting process includes a step of feeding polymer dope constituted by polymer and solvent to flow through a mixing conduit. Liquid additive is ejected through a supply conduit into the polymer dope in the mixing conduit. The liquid additive is mixed with the polymer dope to obtain casting dope, by use of a rotor hub positioned downstream from the supply conduit, contained in the mixing conduit, having a diameter decreasing end on an upstream side, and rotated by electromagnetic induction about an axis directed in a flow direction of the polymer dope. The casting dope is cast on a support moving continuously, to form cast film. The cast film stripped from the support is dried. 
     The supply conduit has a distribution channel, opposed to the diameter decreasing end in the flow direction, for spreading the liquid additive transversely to the flow direction, to eject the liquid additive into the polymer dope. 
     Also, a mixing method of mixing liquid additive with polymer dope constituted by polymer and solvent is provided. The liquid additive is ejected through a supply conduit into the polymer dope in a mixing conduit. The liquid additive is mixed with the polymer dope by use of a rotor hub positioned downstream from the supply conduit, contained in the mixing conduit, having a diameter decreasing end on an upstream side, and rotated by electromagnetic induction about an axis directed in a flow direction of the polymer dope. 
     Consequently, polymer dope can be mixed with high efficiency even in a simple structure, because the hub shaped with a decreasing diameter can operate for tubularly directing and collecting the polymer dope with the liquid additive directly downstream from the supply conduit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects and advantages of the present invention will become more apparent from the following detailed description when read in connection with the accompanying drawings, in which: 
         FIG. 1  is an explanatory view in flow diagram illustrating a solution casting system; 
         FIG. 2  is an exploded perspective view illustrating a in-line mixer; 
         FIG. 3  is a vertical section taken on line III-III in  FIG. 4  illustrating the in-line mixer; 
         FIG. 4  is a cross section taken on line IV-IV in  FIG. 3  illustrating the in-line mixer; 
         FIG. 5  is a vertical section illustrating another in-line mixer with a rotor hub having a diameter decreasing pointed end with a curve; 
         FIG. 6  is a vertical section illustrating one in-line mixer with an impeller hub; 
         FIG. 7  is a vertical section illustrating still another in-line mixer with a rotor hub having a downstream pointed end; 
         FIG. 8  is an exploded perspective view illustrating another preferred in-line mixer; 
         FIG. 9  is a vertical section illustrating the in-line mixer; 
         FIG. 10  is a vertical section illustrating one in-line mixer with a hub having a downstream end in a concave form; 
         FIG. 11  is a vertical section illustrating another in-line mixer with a hub having a downstream pointed end; 
         FIG. 12  is a vertical section illustrating one in-line mixer in which an area of a flow space about a hub decreases in a flow direction; 
         FIG. 13  is a vertical section illustrating a multi-stage mixer including plural hubs; 
         FIG. 14  is a vertical section illustrating a combination of two in-line mixers in series. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE PRESENT INVENTION 
     In  FIG. 1 , a solution casting apparatus or system  10  is illustrated. The solution casting apparatus or system  10  includes a storage tank  11 , a casting chamber  12 , a pin tentering machine  13 , a clip tentering machine  14 , a dryer  15  with a drying chamber, a cooler  16  with a cooling chamber, and a winding chamber  17 . 
     The storage tank  11  includes a stirring blade  11   b , a motor  11   a  for rotating the stirring blade  11   b , and a jacket  11   c . Polymer dope  21  is contained in the storage tank  11 , and is constituted by solution of a polymer in solvent. The polymer is material to produce polymer film  20 . The polymer dope  21  in the storage tank  11  is conditioned for a predetermined temperature by the jacket  11   c . Rotation of the stirring blade  11   b  keeps quality of the polymer dope  21  uniform by suppressing aggregation of the polymer. A flow line  22  or conduit for the polymer dope  21  is installed to extend from the storage tank  11 . 
     Various elements reside in the flow line  22 , including a gear pump  23 , a filtration device  24 , and an in-line mixer  25  as rotor/stator device in a sequence of the flow. A liquid additive supply line  26  or conduit is disposed to join into the flow line  22  at a portion downstream from the filtration device  24 . An additive feeder  28  is connected with the liquid additive supply line  26 , and supplies liquid additive  27  to the flow line  22 . A casting dope  29  is obtained from the polymer dope  21  and the liquid additive  27  by the in-line mixer  25 . A casting die  30  is supplied with the casting dope  29 . 
     The casting chamber  12  has the casting die  30 , a casting drum  32 , a stripping roller  34 , temperature adjusters  35  and  36 , and a decompressor  37  with a decompression chamber. The casting drum  32  is rotatable about a drum shaft  32   a . A driving mechanism (not shown) drives the casting drum  32  to rotate in the direction Z 1 . The casting chamber  12  and the casting drum  32  are conditioned so as to facilitate gelation of a cast film  33  by adjustment with the temperature adjusters  35  and  36 . 
     A support surface  32   b  of the casting drum  32  in rotation receives the polymer dope  21  ejected by the casting die  30 . The cast film  33  is formed from the polymer dope  21  on the support surface  32   b . While the casting drum  32  makes approximately a three fourths rotation, self-supporting property develops in the cast film  33  by gelation. A self-supporting cast film  38  is obtained when the stripping roller  34  strips the cast film  33  from the casting drum  32 . 
     The decompressor  37  is disposed upstream from the casting die  30  with reference to the direction Z 1 . A rear surface of the bead of the polymer dope  21  is decompressed at a predetermined pressure, the rear surface being positioned to contact the support surface  32   b  of the casting drum  32 . This is effective in reducing influence of entrained flow of gas in rotation of the casting drum  32 . The cast film  33  with reduced unevenness can be formed by forming the bead between the casting die  30  and the casting drum  32 . 
     A material for the casting die  30  is a substance having low thermal expansion and high resistance to corrosion in a liquid such as electrolytic aqueous solution, dichloromethane, methanol and other mixed solution. For precision of finish of the contact surface of the casting die  30  on the liquid, the contact surface preferably has a surface roughness equal to or less than 1 micron, and a straightness equal to or less than 1 micron per meter in each of various directions. 
     The support surface  32   b  of the casting drum  32  is plated with chrome plating and has sufficient strength and resistance to corrosion. The temperature adjuster  36  circulates a heat exchange medium in the casting drum  32  to keep the temperature of the support surface  32   b  at a predetermined level. The heat exchange medium is conditioned at the predetermined level of temperature, and flows in the flow path in the casting drum  32  to control the support surface  32   b  thermally. 
     The width of the casting drum  32  is not limited, but is preferably 1.1-2.0 times as great as a casting width of the dope. The material of the casting drum  32  is stainless steel, and preferably SUS  316  steel with sufficient strength and resistance to corrosion. The support surface  32   b  of the casting drum  32  is plated with chrome plating, preferably hard chrome plating with Vickers hardness Hv of 700 or more and a thickness of 2 microns or more. 
     A condenser  39  and a recovery device  40  are contained in the casting chamber  12 . The condenser  39  condenses solvent gas into solvent of the liquid phase. The recovery device  40  withdraws and recovers the solvent condensed by the condenser  39 . After the recovery, the solvent is reused to prepare dope. 
     A transition region  41  extends from the casting chamber  12 , and is arranged with the pin tentering machine  13  and the clip tentering machine  14  in a sequence of the flow. Transport rollers  42  in the transition region  41  transport the self-supporting cast film  38  into the pin tentering machine  13 . A great number of pin plates are incorporated in the pin tentering machine  13 ; are pierced in web edges of the self-supporting cast film  38  for support, and travel on their paths. Evaporative gas is blown to the self-supporting cast film  38  moved by the pin plates, so that the self-supporting cast film  38  is dried to become the polymer film  20 . 
     The clip tentering machine  14  has a great number of tenter clips for clamping web edges of the polymer film  20  and for traveling on their paths for stretch. Evaporative gas is applied to the polymer film  20  transported by the tenter clips, for drying in the course of tentering in the transverse direction. 
     Edge slitters  43   a  and  43   b  are positioned downstream respectively from the pin tentering machine  13  and the clip tentering machine  14 . The edge slitters  43   a  and  43   b  slit web edge portions of the polymer film  20 . The web edge portions are blown with air toward crushers  44   a  and  44   b , and crushed for reuse as raw material of dope. 
     A great number of dryer rollers  47  are arranged in the dryer  15 , and transport the polymer film  20  in contact. An air conditioner (not shown) adjusts the temperature, humidity and the like of the atmosphere in the dryer  15 , so that the polymer film  20  is dried by passing the dryer  15  in a conditioned state. An adsorption solvent recovery device  48  is connected with the dryer  15 , and recovers the solvent from the polymer film  20  by adsorption. 
     The cooler  16  is positioned downstream from the dryer  15 , and cools the polymer film  20  down to the room temperature. A static eliminator  49  or elimination bar is disposed downstream from the cooler  16 , and eliminates static charge from the polymer film  20 . Also, a knurling roller  50  is disposed downstream from the static eliminator  49 , and knurls web edges of the polymer film  20 . A winder  51  is contained in the winding chamber  17 , and has a spindle and a press roller  52 . The polymer film  20  is wound about the spindle in a roll form. 
     In  FIGS. 2 and 3 , the in-line mixer  25  is illustrated. The in-line mixer  25  includes a first mixing conduit  71  as stator, a second mixing conduit  72  as stator, a rotor hub  73  or shear head, and a support  74 . 
     An intermediate portion  71   a  and a cylindrical wall  71   b  are included in the first mixing conduit  71  and arranged in a flow direction X 1  of the polymer dope  21 . The intermediate portion  71   a  is connected with the flow line  22  by way of an inlet port. A nozzle shaped supply conduit  77  is disposed to extend through a side of the flow line  22 , and is supplied by the additive feeder  28  with additives through the liquid additive supply line  26 . An inner diameter of the cylindrical wall  71   b  is greater than that of the flow line  22 . The intermediate portion  71   a  has a first inner diameter at its upstream end and equal to that of the flow line  22 , and a second inner diameter at its downstream end and equal to that of the cylindrical wall  71   b . The inner diameter of the intermediate portion  71   a  gradually increases in the flow direction toward the cylindrical wall  71   b.    
     An inner diameter of the second mixing conduit  72  is set nearly equal to an inner diameter of the flow line  22 . A flange  72   a  is formed at an upstream end of the second mixing conduit  72 . An outer diameter of the flange  72   a  is set equal to an outer diameter of the cylindrical wall  71   b  of the first mixing conduit  71 . 
     A downstream side  71   c  of the cylindrical wall  71   b  is set in contact with an upstream side  72   b  of the flange  72   a  to secure the second mixing conduit  72  to the first mixing conduit  71 . Thus, a mixing chamber  80  is defined inside the mixing conduits  71  and  72 . 
     A support flange  74   a  or disk projects from the support  74  at its downstream end. An outer diameter of the support flange  74   a  is slightly smaller than an inner diameter of the cylindrical wall  71   b  of the first mixing conduit  71 . A downstream side  74   b  of the support flange  74   a  is disposed in contact with the upstream side  72   b  of the flange  72   a . Four support projections  74   c  are formed on the support  74  to extend in the upstream direction. Flow openings  74   d  or outlet port is defined between the support projections  74   c  and is formed to communicate with the flow line  22 . 
     The rotor hub  73  includes a hub body  73   a  and an inclined portion  73   b . An outer diameter of the hub body  73   a  is smaller than an inner diameter of the cylindrical wall  71   b , and is greater than an inner diameter of an upstream end of the support  74 . The inclined portion  73   b  has a conical shape, and projects from the hub body  73   a  in the upstream direction. A pointed end  73   c  is formed with the inclined portion  73   b  and directed in the upstream direction X 1 . The rotor hub  73  is contained in the mixing chamber  80 . A downstream end face  73   e  of the rotor hub  73  is supported by the support projections  74   c . A peripheral surface  73   d  of the hub body  73   a  and the inclined portion  73   b  are kept spaced from an inner surface  71   d  of the first mixing conduit  71 . 
     A flow space  85  is defined between the cylindrical wall  71   b  and the rotor hub  73  to communicate from the flow line  22  to the flow openings  74   d . As viewed in a cross section on a plane vertical to the direction X 1 , the flow space  85  is in a ring shape. The flow space  85  has such a form that its cross sectional area S 2  is smaller than a cross sectional area S 1  of the flow line  22 , and gradually decreases in the direction X 1  between the first mixing conduit  71  and the rotor hub  73 . 
     A conduit end portion is formed on the supply conduit  77 . The conduit end portion extends flatly in the transverse direction of the flow line  22 . A nozzle shaped distribution channel  77   a  is formed in the conduit end portion. As an outlet, the nozzle shaped distribution channel  77   a  is shaped to open as a slot directed in the transverse direction of the flow line  22 . 
     The shape of the nozzle shaped distribution channel  77   a  is quadrilateral as viewed in a cross section. A size of the nozzle shaped distribution channel  77   a  is not limited and should be so large as to form a laminar flow of the liquid additive. In the embodiment, the nozzle shaped distribution channel  77   a  is so formed relative to the supply conduit  77  that its size in the vertical direction is equal to or smaller than that of the supply conduit  77 , and that a size of the nozzle shaped distribution channel  77   a  in a horizontal direction transverse to the flow direction is greater than that of the supply conduit  77 . 
     In  FIG. 4 , a magnet assembly  90  is incorporated in the hub body  73   a . A plurality of magnets with different poles are arranged in the magnet assembly  90  and alternately in the circumferential direction of the peripheral surface  73   d . A jacket  95  is disposed about the first mixing conduit  71 . Heat exchange medium flows in the jacket  95  in a state conditioned at a predetermined temperature, to keep the polymer dope  21  and the liquid additive  27  maintained in a predetermined range of the temperature through the mixing chamber  80 . 
     An electromagnet assembly  98  includes a great number of electromagnets arranged about the jacket  95 . Each of the electromagnets includes a core of iron, windings positioned about the core, and a pair of lead wires. A controller  99  is connected by the lead wires to the windings in the electromagnet assembly  98 . The controller  99  causes a current to flow in a predetermined direction in the lead wires in a conditioned manner, to generate a magnetic field with poles of the electromagnet assembly  98  about the jacket  95 . 
     A preferable material of the first mixing conduit  71 , the second mixing conduit  72  and the rotor hub  73  is stainless steel, with sufficient strength and resistance to corrosion in view of contact with the polymer dope  21  and the liquid additive  27 . 
     A material for the support  74  is preferably a substance which does not require lubricant, and also with which no powder as dust will occur in friction with the rotor hub  73  in operation. Examples of the material for the support  74  are Teflon (trade name for tetrafluoroethylene), and various polymers. 
     A method of producing the polymer film  20  in the solution casting apparatus or system  10  is described now by referring to  FIG. 1 . In the storage tank  11 , heat exchange medium is caused to flow in the jacket  11   c  to adjust the temperature of the polymer dope  21  at 25-35 deg. C. The stirring blade  11   b  rotates to stir the polymer dope  21  for a uniform state. The polymer dope  21  of a predetermined amount is supplied from the storage tank  11  by the gear pump  23  to the filtration device  24 , and filtrated to remove impurity. 
     The additive feeder  28  supplies the polymer dope  21  with the liquid additive  27  after the filtration. The in-line mixer  25  obtains the casting dope  29  from the polymer dope  21  and the liquid additive  27 . Thus, the casting dope  29  is supplied to the casting die  30 . 
     An inner temperature of the casting chamber  12  is conditioned by the temperature adjuster  35  at a constant level in a range of 10-57 deg. C. Solvent gas, formed by gasifying from the polymer dope  21  or the cast film  33 , disperses in the casting chamber  12 . In the embodiment, the solvent gas is condensed and liquefied by the condenser  39  and recovered by the recovery device  40 . The recovered solvent is refined by a refiner and reused as solvent for dope. 
     The casting drum  32  is caused to rotate about the drum shaft  32   a  by a driving mechanism. The support surface  32   b  moves in the direction Z 1  at a constant speed of 50-200 meters per minute. 
     The polymer dope  21  is conditioned at a constant level of the temperature equal to or more than 30 deg. C. and equal to or less than 35 deg. C. The casting die  30  casts the polymer dope  21  on the support surface  32   b  of the casting drum  32 , to form the cast film  33 . The temperature adjuster  36  adjusts the support surface  32   b  at a constant level of the temperature equal to or more than −10 deg. C. and equal to or less than 10 deg. C. Thus, the cast film  33  on the support surface  32   b  is cooled and gelled, and comes to have a self-supporting property. In the course of cooling, gelation of the cast film  33  is promoted owing to forming crosslinking points as a basis of crystallization. 
     The stripping roller  34  strips the cast film  33  with self-supporting property from the casting drum  32  to obtain the self-supporting cast film  38 . The transport rollers  42  transport the self-supporting cast film  38  to the pin tentering machine  13 . 
     In the pin tentering machine  13 , numerous pins operate to pierce in web edges of the self-supporting cast film  38 . While the self-supporting cast film  38  is run, drying is promoted to obtain the polymer film  20 . The polymer film  20 , with solvent before complete evaporation, is transported into the clip tentering machine  14 . In the clip tentering machine  14 , web edges of the polymer film  20  are clamped by numerous tenter clips, and are dried and stretched. 
     The polymer film  20  transported out of the pin tentering machine  13  and the clip tentering machine  14  is slitted by the edge slitters  43   a  and  43   b  to remove web edges. The polymer film  20  runs through the dryer  15  and the cooler  16 , and becomes wound by the winder  51  in the winding chamber  17 . The web edges removed by the edge slitters  43   a  and  43   b  are crushed by the crushers  44   a  and  44   b , and are reused as chips for preparing dope. 
     A web length of the polymer film  20  can be equal to or more than 100 meters in the casting direction. The polymer film  20  has a width equal to or more than 600 mm, and preferably can have a width equal to or more than 1,400 mm and equal to or less than 2,500 mm. The feature of the invention is effective also if the width is over 2,500 mm. The thickness of the polymer film  20  as a final product is not limited, but can be equal to or more than 20 microns and equal to or less than 80 microns. 
     Details of the in-line mixer  25  are described now. In  FIG. 1 , the gear pump  23  causes the polymer dope  21  to flow in the flow line  22  before the first mixing conduit  71  at a predetermined flow speed V 1  downstream from the nozzle shaped distribution channel  77   a . A pump (not shown) resides in the liquid additive supply line  26  or the additive feeder  28 . The liquid additive  27  is supplied to the polymer dope  21  from the nozzle shaped distribution channel  77   a  of the supply conduit  77  at a flow speed V 2  controlled by the pump. Premix solution  100  or first solution, obtained from the polymer dope  21  and the liquid additive  27 , flows into the mixing chamber  80  through the flow line  22 . The flow speed V 1  is preferably equal to or more than 0.002 m/s and equal to or less than 0.06 m/s, and desirably equal to or more than 0.01 m/s and equal to or less than 0.05 m/s. The flow speed V 2  is preferably equal to or more than 0.0003 m/s and equal to or less than 0.01 m/s, and desirably equal to or more than 0.0004 m/s and equal to or less than 0.002 m/s. 
     In  FIGS. 3 and 4 , the rotor hub  73  rotates about the axis AX while supported by the support  74  with attraction and repulsion between the pole of the magnet assembly  90  and the pole of the electromagnet assembly  98  controlled by the controller  99 . The peripheral surface  73   d  on the rotor hub  73  moves round at the speed V 3 . 
     The rotor hub  73  is supported by the support projections  74   c  while rotated about the axis AX. The first solution  100  in the mixing chamber  80  passes the flow space  85  and the flow openings  74   d  and comes to flow in a downstream portion of the flow line  22  next to the second mixing conduit  72 . The pointed end  73   c  of the inclined portion  73   b  on the rotor hub  73  divides or directs the first solution  100  in a radial direction from the center of the cross section as viewed in the direction X 1 . The first solution  100  from the pointed end  73   c  flows in the flow space  85  between the rotor hub  73  and the inner surface  71   d . The peripheral surface  73   d  moves round to cause a shear flow of the first solution  100  in the flow space  85 . The first solution  100  passed through the flow space  85  is collected together, and passes the flow openings  74   d.    
     In the embodiment, the cross sectional area S 2  of the flow space  85  decreases in the flow direction. The shear strain rate of the first solution  100  passing the flow space  85  increases, so that viscosity of the polymer dope  21  and the liquid additive  27  decreases in the flow space  85 . Mixing of the liquid additive  27  with the polymer dope  21  is facilitated by shear flow of the first solution  100  in rotation of the peripheral surface  73   d  in the flow space  85 . Also, the speed V 3  of the peripheral surface  73   d  can be adjusted by the control of the controller  99  specifically if a characteristic of the polymer dope  21  or the liquid additive  27  changes according to a change in the composition. Consequently, it is possible in the invention to optimize the mixing condition according to the viscosity of the polymer dope  21  and the liquid additive  27  to facilitate preparation of the casting dope  29  of a uniform state. 
     According to the feature of the invention, there is no need of a shaft or other driving members requiring lubricant oil. A dynamic mixer can operate in easy optimization of mixing of the first solution  100 . Unwanted mixing of lubricant or the like in the solution can be prevented, to eliminate a cause of degradation in view of optical performance of the polymer film. 
     It is preferable to condition the temperature of the first solution  100  in the flow space  85  in a predetermined range equal to or lower than the boiling point of the polymer dope  21  by use of the heat exchange medium for flow in the jacket  95 . Foaming of the polymer dope  21  is suppressed in lowering the viscosity of the first solution  100  within the flow space  85 . Thus, preparation of the casting dope  29  is further facilitated as uniform mixture of the polymer dope  21  and the liquid additive  27 . 
     An angle defined by the shape of the pointed end  73   c  is preferably an acute angle for suppressing a loss in the pressure in the flow of the first solution  100 . An angle θ of the pointed end  73   c  as viewed in the section on a plane containing the axis AX of the hub body  73   a  is preferably less than 80 degrees, in particular preferably 40-50 degrees, and desirably 45 degrees. 
     Let ηk be the viscosity of the first solution  100 . Let ρk be density of the first solution  100 . Let Δd be a clearance of the flow space  85 . It is preferable that V 1 /V 2  is equal to or more than 1 and equal to or less than 5, and a Reynolds value Re of the first solution  100  in the flow space  85  is equal to or more than 0.02. The Reynolds value Re of the first solution  100  in the flow space  85  can be desirably equal to or more than 0.1. Note that the Reynolds value Re is defined by the following equation.
 
 Re=Δd·V 3· ρk/ηk  
 
     It is possible to mix the liquid additive  27  with the polymer dope  21  effectively only in case of satisfying the condition of the Reynolds value Re, irrespective of a laminar flow, turbulent flow or other states of flow of the first solution  100 . 
     Note that viscosity, density and other characteristics of the first solution  100 , the polymer dope  21  and the liquid additive  27  can be determined according to JIS K 7117, JIS K 7112 and the like. 
     In the in-line mixer  25 , a ratio ηd/ηt of the viscosity ηd of the polymer dope  21  to the viscosity ηt of the liquid additive  27  may be set as desired without limitation, but can be preferably in a range equal to or more than 1 and equal to or less than 1×10 5 . 
     A distance L 1  from the pointed end  73   c  to the nozzle shaped distribution channel  77   a  of the supply conduit  77  is preferably equal to or more than 1 mm and equal to or less than 200 mm. Should the distance L 1  be less than 1 mm, the rotor hub  73  is likely to interfere with the supply conduit  77  in an unwanted manner. Should the distance L 1  be more than 200 mm, the liquid additive  27  from the supply conduit  77  is difficult to introduce to the pointed end  73   c.    
     In the above embodiment, the shape of each of the mixing chamber  80  and the rotor hub  73  is defined with a greater width in its middle as viewed in the flow direction. However, the mixing chamber  80  and the rotor hub  73  according to the invention may have any suitable shape which can satisfy the condition of the cross sectional area S 2  decreasing in the flow direction. A ratio S 1 /S 2  of the cross sectional area S 1  to the cross sectional area S 2  is preferably equal to or more than 1.05 and equal to or less than 2. Should the ratio S 1 /S 2  be more than 2, the shear strain rate will increase extraordinarily due to excessive loss of the pressure or influence of flow of extension, to cause difficulty in supply of the polymer dope  21  to the casting die  30 . Should the ratio S 1 /S 2  be less than 1.05, there occurs no drop in the viscosity of the polymer dope  21  and the liquid additive  27  in passage of the flow space  85 . 
     Let D 1  be an outer diameter of the hub body  73   a  as viewed on a plane perpendicular to the direction X 1 . Let D 2  be an inner diameter of the inner surface  71   d  of the mixing chamber  80  as viewed on the same plane. It is preferable that the ratio D 1 /D 2  is equal to or more than 0.1 and equal to or less than 0-95. Should the ratio D 1 /D 2  be less than 0.1, sufficient stress of shear cannot be generated in the first solution  100 . Should the ratio D 1 /D 2  be more than 0.95, problems are likely to occur in that the rotor hub  73  may interfere with the inner surface  71   d  and unwanted lumps containing the additives may be created. Note that the value D 1  may be defined to be equal to an outer diameter of the inclined portion  73   b  instead of the outer diameter of the hub body  73   a.    
     In the above embodiment, the in-line mixer  25  is single. However, a plurality of in-line mixers  25  may be arranged serially in the flow direction X 1  in the flow line  22 . This is effective in mixing the liquid additive  27  with the polymer dope  21  to prepare the casting dope  29  in a reliably uniform manner. 
     In the above embodiment, the inner diameter of the cylindrical wall  71   b  is greater than that of the flow line  22 . However, that inner diameter of the cylindrical wall  71   b  may be equal to or less than that of the flow line  22 . The cross sectional area S 2  of the flow space  85  should decrease gradually in the downstream direction inside the cylindrical wall  71   b.    
     In the above embodiment, the axis AX of the rotor hub  73  extends in the flow direction X 1 . However, the axis AX may be directed eccentrically, or differently from the flow direction X 1  in a condition of increasing the shear strain rate of the first solution  100 . It is still preferable that the axis AX coincides with the flow direction X 1  in view of mixing the whole of the first solution  100  adequately. 
     In the above embodiment, the axis of the form of the hub body  73   a  extends to coincide with the rotational axis of the rotor hub  73 . However, those may not coincide with one another and can be eccentric. For example, the axis of the form of the hub body  73   a  may be different from and parallel with the rotational axis of the rotor hub  73 . 
     Mixing of the liquid additive  27  with the polymer dope  21  is promoted specifically locally at a point of a decrease in the cross sectional area of the flow space  85 , namely a decrease of the viscosity. Thus, it is preferable that a portion of the flow space  85  surrounded by the peripheral surface  73   d  and the inner surface  71   d  has a form extending long in the flow direction X 1  of the first solution  100 . For example, a portion of the flow space  85  surrounded by the peripheral surface  73   d  and the inner surface  71   d  can be preferably longer than a size of the inclined portion  73   b.    
     In the above embodiment, the downstream end face  73   e  of the rotor hub  73  is supported by the support projections  74   c . However, ring-shaped grooves or projections may be formed on the downstream end face  73   e  for guiding the support projections  74   c.    
     Other preferred embodiments are hereinafter described. Elements similar to those of the above embodiment are designated with identical reference numerals. 
     Various modifications of the structure for directing and supplying the polymer dope  21  toward the flow space  85  are possible in manners different from the use of the rotor hub  73  having the pointed end  73   c . For example, a rotor hub can have a pointed end of an obtuse angle. In  FIG. 5 , one preferred rotor hub  123  is illustrated, including an inclined portion  123   b  positioned upstream from the hub body  73   a , and a leading end  123   c  of the inclined portion  123   b  with a curved form. 
     In the above embodiment, the peripheral surface  73   d  rotates locally for a shear flow of the first solution  100 . In  FIG. 6 , another preferred embodiment is illustrated. An impeller hub  153  or rotor hub or shear head is incorporated in an in-line mixer. Impeller blades  150  project from the peripheral surface  73   d , and rotate to stir the first solution  100  passing the flow space  85 . The liquid additive  27  can be mixed with the polymer dope  21  reliably by use of the impeller blades  150 . Any type of various elements for stir can be formed on the peripheral surface  73   d  or the inner surface  71   d  in place of the impeller blades  150 , the elements including rotor grooves, projections shaped in a twisted plate of a quadrilateral, projections in combination of plural plates crossed with one another, and the like. The form, number, pitch, arrangement and other condition of the impeller blades  150 , projections or rotor grooves are not limited and can be determined suitably for the purpose. 
     In  FIG. 7 , one preferred rotor hub  173  is illustrated. An inclined portion  173   g  is disposed on a downstream side of the hub body  73   a  and projects with a pointed end  173   h . The auxiliary pointed end  173   h  on the inclined portion  173   g  can have a form of an acute angle or obtuse angle. Also, a top of the end  173   h  may have a curved surface. 
     In the above embodiment, the magnet assembly  90  is disposed outside the jacket  95 . However, a peripheral surface of the first mixing conduit  71  may be provided with the magnet assembly  90 , and the jacket  95  may be disposed about the magnet assembly  90 . 
     In the above embodiment, the rotor hub  73  is used dynamically to mix the liquid additive  27  with the polymer dope  21 . Furthermore, it is possible in the invention to mix the liquid additive  27  with the polymer dope  21  statically. An in-line mixer  225  as static mixer is hereafter described. 
     In  FIGS. 8 and 9 , the in-line mixer  225  includes a first mixing conduit  271 , a second mixing conduit  272 , a hub  273  or shear head, and supports  274   a  and  274   b . The first mixing conduit  271  has a tubular shape, and includes an intermediate portion  271   a  and a cylindrical wall  271   b  arranged in the flow direction X 1 . The intermediate portion  271   a  is connected with the flow line  22  by way of an inlet port. The supply conduit  77  is positioned inside a portion of the flow line  22  upstream from the intermediate portion  271   a.    
     An inner diameter of the cylindrical wall  271   b  is greater than that of the flow line  22 . The intermediate portion  271   a  has a first inner diameter at its upstream end and equal to that of the flow line  22 , and a second inner diameter at its downstream end and equal to that of the cylindrical wall  271   b . The inner diameter of the intermediate portion  271   a  gradually increases in the flow direction toward the cylindrical wall  271   b.    
     The second mixing conduit  272  has a tubular shape, and includes a cylindrical wall  272   a  and an intermediate portion  272   b  arranged in the flow direction X 1 . The intermediate portion  272   b  is connected with the flow line  22 . An inner diameter of the cylindrical wall  272   a  is set equal to that of the cylindrical wall  271   b . The intermediate portion  272   b  has a first inner diameter at its upstream end and equal to that of the cylindrical wall  272   a , and a second inner diameter at its downstream end and equal to that of the flow line  22 . The inner diameter of the intermediate portion  272   b  gradually decreases in the flow direction from the cylindrical wall  272   a  toward the flow line  22 . 
     A mixing chamber  280  is defined inside the mixing conduits  271  and  272  by their connection in contact of the cylindrical wall  271   b  with the cylindrical wall  272   a . The hub  273  is contained in the mixing chamber  280  inside the first mixing conduit  271 . A collection flow path  282  is defined in the mixing chamber  280  inside the second mixing conduit  272 . 
     The hub  273  includes a hub body  273   a  and an inclined portion  273   b . An outer diameter of the hub body  273   a  is smaller than an inner diameter of the cylindrical wall  271   b . The inclined portion  273   b  is formed to project from the hub body  273   a . A pointed end  273   c  of the inclined portion  273   b  is in a conical shape, pointed with an acute angle, and directed upstream in the flow direction X 1 . A downstream end face  273   e  of the hub body  273   a  is oriented substantially perpendicular to the axis of the hub body  273   a . A peripheral surface  273   d  of the hub  273  extends to the downstream end face  273   e  with a bend. 
     The supports  274   a  and  274   b  are four supports. An inner surface  271   d  of the first mixing conduit  271  is connected fixedly with the peripheral surface  273   d  by the supports  274   a  and  274   b  which extend from the peripheral surface  273   d . The supports  274   a  and  274   b  keep the peripheral surface  273   d  spaced from the inner surface  271   d  in setting in the mixing chamber  280 . A flow space  285 , communicating from the flow line  22  to the collection flow path  282 , is defined between the inner surface  271   d  and the hub  273  in a ring shape as viewed in a cross section on a plane perpendicular to the flow direction X 1 . A cross sectional area S 4  of the flow space  285  on a plane perpendicular to the flow direction X 1  is set equal to a cross sectional area S 3  of the flow line  22 . Also, the jacket  95  is disposed about the mixing conduits  271  and  272 . 
     Details of the in-line mixer  225  are described now. The polymer dope  21  flows in the flow line  22  at the predetermined flow speed V 1  by use of the gear pump  23  of  FIG. 2 . The additive feeder  28  supplies the liquid additive  27  to the polymer dope  21  at the flow speed V 2  from the nozzle shaped distribution channel  77   a  in a manner spread in the transverse direction of the flow line  22 . 
     The first solution  100  containing the polymer dope  21  and the liquid additive  27  is supplied by the flow line  22  to the mixing chamber  280 . As the hub  273  is supported by the supports  274   a  and  274   b  with a gap inside the inner surface  271   d , the first solution  100  in the mixing chamber  280  flows past the in-line mixer  225  through the flow space  285  and the collection flow path  282 . 
     The pointed end  273   c  of the inclined portion  273   b  positioned upstream from the hub  273  directs the first solution  100  in the mixing chamber  280  in a radial direction from the center to the periphery as viewed in the cross section. The first solution  100  having passed the pointed end  273   c  flows through the flow space  285  in contact with the inner surface  271   d  and the peripheral surface  273   d . The first solution  100  from the flow space  285  flows in the collection flow path  282 . The collection flow path  282  collects the first solution  100  after being directed tubularly by the pointed end  273   c . It is possible in the in-line mixer  225  to mix the liquid additive  27  in the polymer dope  21  through the tubular flow space of the first solution  100 . In comparison with a well-known static mixer, the in-line mixer  225  can mix the liquid additive  27  efficiently with the polymer dope  21 . The total number of a plurality of the in-line mixers  225  required for the polymer dope  21  in a uniform manner can be moderately low. 
     The form of the inclined portion  273   b  with the pointed end  273   c  is effective in suppressing a loss in the pressure to supply the flow space  285  with the first solution  100 . Also, the cross sectional area S 4  of the flow space  285  on a plane perpendicular to the flow direction X 1  is equal to the cross sectional area S 1  of the flow line  22 , so that the first solution  100  can be directed and supplied in suppressing a loss in the pressure. A vortex can be created readily in the first solution  100  through the collection flow path  282 , as the downstream end face  273   e  extends to the peripheral surface  273   d  with a curve. The vortex can cause ensured mixing of the liquid additive  27  with the polymer dope  21 . Examples of the vortices are a twin vortex, Karman vortex and the like. 
     The temperature of the first solution  100  in the collection flow path  282  is adjusted and set at a constant level in a range equal to or lower than the boiling point of the polymer dope  21  by use of the heat exchange medium through the jacket  95 . Mixing and preparation of the casting dope  29  from the polymer dope  21  and the liquid additive  27  can be further facilitated, because the viscosity of the first solution  100  through the collection flow path  282  can be lowered in suppressing foaming of the polymer dope  21 . 
     In order to reduce the loss in the pressure, the angle θ of the pointed end  273   c  is determined preferably in an equal manner to that of the pointed end  73   c  of the in-line mixer  25 . A viscosity ratio ηd/ηt between the polymer dope  21  and the liquid additive  27  is preferably equal to or more than 1 and equal to or less than 1×10 5 , where ηd is a viscosity of the polymer dope  21  and ηt is a viscosity of the liquid additive  27 . 
     A distance L 2  from the pointed end  273   c  to the nozzle shaped distribution channel  77   a  of the supply conduit  77  is preferably equal to or more than 1 mm and equal to or less than 200 mm. Should the distance L 2  be less than 1 mm, the rotor hub  73  may interfere with the supply conduit  77  to cause a problem. Should the distance L 2  be more than 200 mm, difficulty arises in supply of the liquid additive  27  from the supply conduit  77  to the pointed end  73   c.    
     Let L 3  be a length from the pointed end  273   c  to the downstream end face  273   e . Let L 4  be a length of the collection flow path  282  between its upstream and downstream ends. The length L 4  is preferably equal to or more than the length L 3  for the purpose of ensuring mixing of the first solution  100  in the collection flow path  282 . 
     In the above embodiment, the second mixing conduit  272  is originally separate from the first mixing conduit  271 . However, a single mixing conduit constituted by the mixing conduits  271  and  272  may be used as one piece of metal. 
     In the above embodiment, collection of tubularly directed dope occurs in the collection flow path  282  in the second mixing conduit  272 . However, the collection flow path  282  may be defined in a region downstream from the hub  273 . The flow space  285  can be defined as a region directly between an inner surface of the mixing chamber  280  and the hub  273 . 
     A hub according to the invention may be structures differently from the hub  273  for the purpose of creating a vortex in the first solution  100  in the flow space  285  in contact with the peripheral surface  273   d  or the inner surface  271   d . In  FIG. 10 , another preferred hub  313  or shear head is illustrated. There is a hub body  313   a , on which a downstream end face  313   d  of the hub  313  has a concave form. In  FIG. 11 , one preferred hub  323  or shear head has a downstream end face  323   d  formed on the hub body  273   a  with a pointed end. A peripheral surface of the hub body  273   a  may extend to a downstream end face of a hub with a curve continuously, for example, a partially spherical surface. 
     In  FIG. 12 , another preferred in-line mixer  335  as static mixer is illustrated. A hub  333  or shear head is contained in the in-line mixer  335 . A flow space  336 , defined between the hub  333  and the first mixing conduit  271 , has a cross sectional area gradually decreasing in comparison with the flow line  22 . The liquid additive  27  can be mixed with the polymer dope  21  efficiently, because viscosity of the polymer dope  21  or the liquid additive  27  is lowered in passage of the flow space  336  by an increase in the shear strain rate. The hub  333  includes a pointed end  333   c , a downstream end face  333   d , and an inclined surface  333   g . The inclined surface  333   g  guides the first solution  100  toward the downstream end face  333   d . Preferably, the inclined surface  333   g  is formed with a suitable curve in connection with the downstream end face  333   d  in the hub  333 . Note that a hub lateral surface of the hub  333  contains straight lines from the pointed end  333   c  to the downstream end face  333   d  and also a plurality of curved lines on the pointed end  333   c  and the downstream end face  333   d . The hub  333  with the inclined surface  333   g  as hub lateral surface is an element of directing the fluid according to the invention. 
     In the above embodiment, the hub is single. However, a plurality of mixers or hubs may be combined serially in the flow direction. In  FIG. 13 , a multi-stage mixer  355  as static mixer has first, second and third hubs  353   a ,  353   b  and  353   c  or shear heads each of which is structurally the same as the hub  333 . Rod-shaped supports  357   a  and  357   b  support the second hub  353   b  on the first hub  353   a  and the third hub  353   c  on the second hub  353   b . Also, the number of the hubs in the multi-stage mixer  355  may be two or four or more. 
     Furthermore, different types of the mixers may be combined and arranged particularly in a serial arrangement of the mixers or plural rotor hubs. For example, the in-line mixer  225  may be positioned upstream from the in-line mixer  25  as illustrated in  FIG. 14 . This is effective in splitting the first solution  100  before mixing in the in-line mixer  25  at the same time as mixing the liquid additive  27  with the polymer dope  21 . A loss in the pressure of the pointed end  73   c  of the rotor hub  73  can be suppressed, to ensure mixing of the liquid additive  27  with the polymer dope  21 . Also, let L 3  be a distance from the pointed end  273   c  to the downstream end face  273   e . Let L 5  be a distance from the downstream end face  273   e  to the pointed end  73   c  located downstream from the hub  273 . It is preferable that L 5  is equal to or more than L 3  in order to ensure mixing of the first solution  100  in the collection flow path  282 . 
     In the above embodiment, the casting drum  32  is used. However, a support for casting may be an endless casting belt. Although the solution casting of the above embodiments is a method of a type of rapid casting, a polymer film producing process of the invention may be a solution casting method of a drying type, melt casting method, and the like. 
     Materials for preparing the polymer dope  21  in the invention, such as polymer solvent and additives, are hereinafter described. 
     [Polymer] 
     For polymer of the dope, cellulose esters can be preferably used, such as cellulose acylates. Specifically, triacetyl cellulose (TAC) is desirable. Preferably, 90 wt. % or more of the entirety of TAC should be particles of 0.1-4 mm. 
     Preferable examples of cellulose acylates satisfy all of the conditions I-III as follows for the purpose of high transparency:
 
2.5 ≦A+B≦ 3.0  I
 
0 ≦A≦ 3.0  II
 
0 ≦B≦ 2.9  III
 
     where A and B represent a degree of substitution of an acyl group (—CO—R) formed by substituting hydroxy groups in cellulose. A represents a degree of substitution of an acetyl group (—CO—CH 3 ) formed by substituting hydroxy groups in cellulose. B represents a total degree of substitution of acyl groups having 3-22 carbon atoms. 
     The cellulose is constructed by glucose units making a beta-1,4 bond, and each glucose unit has a liberated hydroxy group at 2, 3 and 6-positions. Cellulose acylate is a polymer in which part or whole of the hydroxy groups are esterified so that the hydrogen is substituted by acyl groups having two or more carbon atoms. The degree of substitution for the acyl groups in cellulose acylate is a degree of esterification at 2, 3 or 6-position in cellulose. Accordingly, when 100% of the hydroxy group at the same position is substituted, the degree of substitution at this position is 1. 
     The total degree of substitution DS 2 +DS 3 +DS 6  for the acyl groups at the second, third or sixth positions is in the range of 2.00-3.00, preferably 2.22-2.90, and in particular preferably 2.40-2.88. Further, a ratio DS 6 /(DS 2 +DS 3 +DS 6 ) is preferably 0.28 or more, and particularly 0.30 or more, and especially in the range of 0.31-0.34. The sign DS 2  is a degree of substitution for the acyl groups at 2-position in hydroxy groups in the glucose unit. The signs DS 3  and DS 6  are degrees of substitution for the acyl groups at respectively 3 and 6-positions in hydroxy groups in the glucose unit. 
     An acyl group of only one example may be contained in the cellulose acylate of the invention. However, cellulose acylate may contain acyl groups of two or more examples. If two or more acyl groups are contained, one of the plural acyl groups should be preferably an acetyl group. Let DSA be a total degree of substitution for the acetyl groups. Let DSB be a total degree of substitution for other acyl groups at 2, 3 and 6-positions than the acetyl groups. The value DSA+DSB is preferably in the range of 2.22-2.90, and particularly in the range of 2.40-2.88. Further, the DSB is preferably at least 0.30, and especially at least 0.70. Furthermore, the percentage of a substituent at 6-position in the DSB is preferably at least 20%, preferably at least 25%, especially at least 30% and most especially at least 33% Further, the value DSA+DSB at 6-position is at least 0.75, preferably at least 0.80, and especially at least 0.85. Cellulose acylate satisfying the above conditions can be used to prepare a solution or polymer dope having a preferable solubility. Especially, chlorine-free type organic solvent can be preferably used to prepare adequate polymer dope. Also, the polymer dope can be prepared to have a low viscosity, high solubility, and the suitability for filtration becomes higher. 
     To obtain cellulose to produce cellulose acylates, any one of linter cotton and pulp cotton may be used. 
     Examples of acyl groups in cellulose acylates having two or more carbon atoms can be aliphatic groups, aryl groups, and the like. For example, cellulose acylates may be alkyl carbonyl esters, alkenyl carbonyl esters, aromatic carbonyl esters, aromatic alkyl carbonyl esters, and the like of cellulose, and can further contain a substitution group. Preferable examples of groups include: propionyl, butanoyl, pentanoyl, hexanoyl, octanoyl, decanoyl, dodecanoyl, tridecanoyl, tetradecanoyl, hexadecanoyl, octadecanoyl, iso-butanoyl, tert-butanoyl, cyclohexane carbonyl, oleoyl, benzoyl, naphthyl carbonyl, and cinnamoyl. Among those, particularly preferable groups are propionyl, butanoyl, dodecanoyl, octadecanoyl, tert-butanoyl, oleoyl, benzoyl, naphthyl carbonyl, and cinnamoyl. Further, specifically preferable groups are propionyl and butanoyl. 
     [Solvent] 
     Solvent as raw material of polymer dope is preferably an organic compound in which polymer is soluble. Examples of solvents for preparing the polymer dope include: 
     aromatic hydrocarbons, such as benzene and toluene; 
     halogenated hydrocarbons, such as dichloromethane and chlorobenzene; 
     alcohols, such as methanol, ethanol, n-propanol, n-butanol, and diethylene glycol; 
     ketones, such as acetone and methyl ethyl ketone; 
     esters, such as methyl acetate, ethyl acetate, and propyl acetate; 
     ethers, such as tetrahydrofuran and methyl cellosolve. 
     The term of polymer dope in the invention is used as mixture obtained by dissolution or dispersion of polymer in a solvent. 
     Halogenated hydrocarbons containing 1-7 carbon atoms are preferably used, for example, dichloromethane. Specifically, it is preferable in a mixed solvent to mix one or more alcohols containing 1-5 carbon atoms with the dichloromethane, for the purpose of high solubility, easy separability from a support for casting, mechanical strength of film material, and various optical characteristics of cellulose triacetate (TAC). Such alcohols are contained in the mixed solvent preferably in a range of 2-25 wt. %, and desirably in a range of 5-20 wt. %. Preferable examples of alcohols are methanol, ethanol, n-propanol, isopropanol, n-butanol and the like. Among those, specifically preferable alcohols are methanol, ethanol, n-butanol, and mixture of two or more of them. 
     Solvents not containing dichloromethane are effectively used in the publicly suggested requirement, for the purpose of minimizing influence to environment. Examples of compounds useful to this end are ethers having 4-12 carbon atoms, ketones having 3-12 carbon atoms, esters having 3-12 carbon atoms, and alcohols having 1-12 carbon atoms. Those can be mixed for use. For example, methyl acetate, acetone, ethanol, n-butanol and the like can be used in mixed solvent. Ethers, ketones, esters and alcohols of the examples may have a cyclic structure. Compounds having two or more functional groups of —O—, —CO—, —COO— and —OH, namely groups of ethers, ketones, esters and alcohols can be used as a solvent. 
     Details of cellulose acylates are according to various relevant techniques suggested in JP-A 2005-104148. Those examples and their various features can be used in the present invention. 
     I. Specific Examples of Cellulose Acylates 
     Suggested in JP A 57-182737 (corresponding to U.S. Pat. No. 4,499,043), JP A 10-45803 (corresponding to U.S. Pat. No. 5,856,468), JP A 11-269304 (corresponding to U.S. Pat. No. 6,139,785), JP A 8-231761, JP A10-60170, JPA9-40792, JPA11-5851, JPA9-90101, JPA4-277530, JP A 11-292989, JP A 2000-131524, and JP A 2000-137115. 
     II. Specific Examples of Solvents for Esters and Their Dissolution 
     Suggested in JP A 10-324774, JP A 8-152514, JP A 10-330538, JP A 9-95538 (corresponding to U.S. Pat. No. 5,663,310), JP A 9-95557 (corresponding to U.S. Pat. No. 5,705,632), JPA10-235664 (corresponding to U.S. Pat. No. 6,036,913), JP A 2000-63534, JP A 11-21379, JP A 10-182853, JP A 10-278056, JP A 10-279702, JP A 10-323853 (corresponding to U.S. Pat. No. 6,036,913), JP A 10-237186, JP A 11-60807, JP A 11-152342, JPA11-292988, JPA11-60752, JPA2000-95876, and JPA2000-95877. 
     Uses of various materials in relation to the polymer have been suggested in JP-A 2005-104148, including solvents, plasticizers, deterioration inhibitors, ultraviolet (UV) absorbers, lubricants, stripping accelerators, optical anisotropy control agents, retardation control agents, dyes, mat agents, release agents, and other additives. 
     I. Plasticizers 
     Suggested in JP A 4-227941, JP A 5-194788, JP A 60-250053, JP A 6-16869, JP A 5-271471, JP A 7-286068, JP A 5-5047 (corresponding to U.S. Pat. No. 5,279,659)7 JP A 11-80381, JP A 7-20317, JP A 8-57879, JP A 10-152568, and JP A 10-120824. 
     II. Deterioration Inhibitors and UV Absorbers 
     Suggested in JP A 60-235852, JP A 3-199201, JP A 5-190707, JP A 5-194789, JP A 5-197073, JP A 5-271471, JP A 6-107854, JP A6-118233, JPA 6-148430, JPA7-11055, JPA7-11056, JPA8-29619, JPA8-239509 (corresponding to U.S. Pat. No. 5,806,834), JPA2000-204173, and JP A 2000-193821. 
     Density of cellulose triacetate (TAC) in the polymer dope  21  is equal to or more than 5 wt. % and equal to or less than 40 wt. %, preferably equal to or more than 15 wt. % and equal to or less than 30 wt. %, and desirably equal to or more than 17 wt. % and equal to or less than 25 wt. %. Viscosity of the polymer dope  21  is equal to or more than 20 Pa·s and equal to or less than 200 Pa·s, and preferably equal to or more than 30 Pa·s and equal to or less than 100 Pa·s. 
     In the polymer dope production from cellulose triacetate, various techniques suggested in JP-A 2005-104148 for dissolution of materials and additives, filtration, elimination of bubbles, mixing of additives can be used. 
     No. 1. Dissolution Related to Casting 
     Suggested in JPA 9-95544 (corresponding to U.S. Pat. No. 5,663,310), JP A 10-45950, JP A 10-95854 (corresponding to U.S. Pat. No. 5,783,121), and JP A 2000-53784. 
     No. 2. Specific Preparing Methods of Solutions 
     Suggested in JP A 11-310640 (corresponding to U.S. Pat. No. 6,211,358), JP A 11-323017, JP A 11-302388, and JP A 2000-273184. 
     No. 3. Condensation of Solutions 
     Suggested in JPA 4-259511; U.S. Pat. No. 2,541,012, U.S. Pat. No. 2,858,229, U.S. Pat. No. 4,414,341, and U.S. Pat. No. 4,504,355. 
     [Additives] 
     The liquid additive  27 , as solution, dispersion or the like, contains a plurality of additive components and a solvent for various uses. Examples of the additive components include stripping accelerators, plasticizers, ultraviolet (UV) absorbers, deterioration inhibitors, fine particles, optical performance control agents, and the like. The solvent is preferably the same composition as that contained in the polymer dope  21 . Viscosity of the liquid additive  27  is equal to or more than 8×10 −4  Pa·s and equal to or less than 0.1 Pa·s, and preferably equal to or more than 1×10 −3  Pa·s and equal to or less than 0.05 Pa·s. 
     [Casting Dope] 
     The casting dope  29  is prepared by use of the polymer dope  21  and the liquid additive  27  described heretofore. Density of the liquid additive  27  in the casting dope  29  is in a preferable range equal to or more than 1 wt. % and equal to or less than 20 wt. % in 100 wt. % of the solid content of the casting dope  29 . 
     In the invention, polymer film cast by a solution casting apparatus and process may have a multi-layer structure. To this end, casting may be a co-casting type, a successive co-casting type, or the like. Also, both of those can be combined. Examples of a die for the co-casting type are a casting die with a feed block, a multi-manifold die, and the like. In the polymer film with plural layers, at least one of a skin layer (air side) and a core layer (support side) can have a preferable thickness being 0.5-30% as great as the thickness of the entirety of the polymer film. In the co-casting, edge portions of a bead of the high viscosity dope are preferably enveloped by edge portions of a bead of the low viscosity dope in the course of a flow from the casting die slot to a support for casting. Also, in the co-casting, a ratio of the alcohol in the polymer dope for the skin layer (air side) is preferably higher than a ratio of the alcohol in the polymer dope for the core layer (support side) in a bead from the casting die slot to a support for casting. 
     Also, polymers for use in the invention may be various examples other than cellulose acylates or cellulose acetates, for example, cellulose alkylates, cellulose acetate propionate (CAP), cellulose acetate butylate (CAB), polyethylene terephthalate (PET), polyethylene, and the like. For use of such examples, the temperature of the polymer film  20  described in the above embodiments can be determined according to various factors including glass transition temperature Tg of the polymer, molecular interaction, and the like. 
     Various methods suggested in JP-A 2005-104148 are usable in combination with the casting of the invention, the methods including construction of the casting die, decompression chamber, support and other mechanical elements, co-casting, stripping, stretching, conditioning for drying in respective steps, polymer film handling, winding after eliminating a curl for flatness, solvent collection and polymer film collection. Those can be used in the present invention. 
     A. Support of Metal for Solution Casting 
     Suggested in JP A 2000-84960, U.S. Pat. No. 2,336,310, U.S. Pat. No. 2,367,503, U.S. Pat. No. 2,492,078, U.S. Pat. No. 2,492,977, U.S. Pat. No. 2,492,978, U.S. Pat. No. 2,607,704, U.S. Pat. No. 2,739,069, U.S. Pat. No. 2,739,070, GB A 640731 (corresponding to U.S. Pat. No. 2,492,977), GB A 735892, JP B 45-4554, JP B 49-5614, JP A 60-176834, JP A 60-203430, and JP A 62-115035. 
     B. Co-Casting 
     Suggested in JP B 62-43846; JP A 61-158414, JP A 1-122419, JP B 60-27562, JP A 61-94724, JP A 61-947245, JP A 61-104813, JP A 61-158413, JP A 6-134933; JP A 56-162617; JP A 61-94724, JP A 61-94725, and LP A 11-198285. 
     C. Specific Methods of Casting of Cellulose Esters 
     Suggested in JP A 61-94724, JP A 61-148013, JP A 4-85011 (corresponding to U.S. Pat. No. 5,188,788), JPA 4-286611, JPA 5-185443, JP A 5-185445, JP A 6-278149, and JP A 8-207210. 
     D. Stretching 
     Suggested in JP A 62-115035, JP A 4-152125, JP A 4-284211, JP A 4-298310, and JP A 11-48271. 
     E. Specific Methods of Drying 
     Suggested in JPA8-134336, JPA8-259706, and JPA8-325388. 
     F. Drying of Specific Controls of Heat 
     Suggested in JP A 04-001009 (corresponding to U.S. Pat. No. 5,152,947), JP A 62-046626, JP A 04-286611, and JP A 2000-002809. 
     G. Drying in Preventing Wrinkles 
     Suggested in JP A 11-123732, JP A 11-138568, and JP A 2000-176950. 
     Examples of the invention are hereinafter described. Among those, Examples 1-4 are according to the feature of the invention. Comparative examples 1-3 have been made for comparison with Examples 1-4. Example 1 will be described in detail. Portions of Examples 2-4 and Comparative examples 1-3 the same as those of Example 1 are not described because of repetition. 
     Example 1 
     Preparation of Polymer Dope 
     The following were solute (solid content) to prepare the polymer dope  21 . 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Cellulose triacetate 
                 89.3 parts by weight  
               
               
                   
                 Plasticizer A 
                 7.1 parts by weight 
               
               
                   
                 Plasticizer B 
                 3.6 parts by weight 
               
               
                   
                   
               
            
           
         
       
     
     The cellulose triacetate had substitution degree of 2.8. The plasticizer A was triphenylphosphate. The plasticizer B was biphenyl diphenylphosphate. The following were components of mixed solvent for the polymer dope  21 . 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Dichloromethane 
                  80 parts by weight 
               
               
                   
                 Methanol 
                 13.5 parts by weight  
               
               
                   
                 n-butanol 
                 6.5 parts by weight 
               
               
                   
                   
               
            
           
         
       
     
     The solute was added to the solvent, which was stirred and mixed to prepare the polymer dope  21 . The polymer dope  21  was adjusted to have density of cellulose triacetate of approximately 23 wt. %. The polymer dope  21  was passed through and filtered by a filter paper #63LB (trade name, manufactured by Toyo Roshi Kaisha, Ltd.), then through a sintered metal filter 06N (manufactured by Nippon Seisen Co., Ltd.) having a nominal pore diameter of 10 microns, and furthermore through a mesh filter. The polymer dope  21  was stored in the storage tank  11 . The polymer dope  21  of the composition according to the above list is herein referred to as polymer dope A. The polymer dope A had viscosity of 100 Pa·s. 
     [Cellulose Triacetate] 
     In the cellulose triacetate (TAC), an amount of the residual acetic acid was 0.1 wt. % or less. The TAC contained 58 ppm of Ca, 42 ppm of Mg, 0.5 ppm of Fe, 40 ppm of the free acid content of acetic acid, and 15 ppm of sulfur ion. In the TAC, a degree of acetyl substitution of 6-position was 0.91. A ratio of the acetyl group of the substitution of 6-position relative to all of the acyl groups was 32.5%. In the TAC, an extracted amount of acetone was 8 wt. %. A ratio of the weight average molecular weight to the number average molecular weight was 2.5. In the TAC, the yellow index was 1.7. The haze was 0.08. A factor of transparency was 93.5%. Raw material of cellulose for the TAC was fibrous material collected from cotton. The TAC herein will be referred to as cotton-derived TAC. 
     [Preparation of Additives] 
     The following were components of mixed solvent for the additives. 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Dichloromethane 
                  80 parts by weight 
               
               
                   
                 Methanol 
                 13.5 parts by weight  
               
               
                   
                 n-butanol 
                 6.5 parts by weight 
               
               
                   
                   
               
            
           
         
       
     
     The additives were added to the solvent, and stirred and mixed to obtain the liquid additive  27 . Viscosity of the liquid additive  27  was 0.001 Pa·s. 
     The casting dope  29  was produced in the solution casting apparatus or system  10 . The polymer dope A in the storage tank  11  was supplied by the gear pump  23  to the flow line  22  at a volumetric flow rate Q 1 . The additive feeder  28  fed the solution casting system  10  with the liquid additive  27  at a volumetric flow rate Q 2 . The supply conduit  77  supplied the liquid additive  27  to the flow line  22 . The first solution  100  containing the polymer dope A and the liquid additive  27  was supplied to the in-line mixer  25 . The flow rate Q 1  was 45 liters per minute. The flow rate Q 2  was 0.3 liter per minute. The in-line mixer  25  rotated the rotor hub  73  with the traveling speed V 3  of the peripheral surface  73   d  to set the Reynolds value Re in the above-described equation equal to 0.02. So the first solution  100  passing the flow space  85  was caused to flow finely, to obtain the casting dope  29 . 
     Example 2 
     Preparation of Polymer Dope 
     The following were solute (solid content) to prepare the polymer dope  21 . 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Cellulose triacetate 
                 89.3 parts by weight  
               
               
                   
                 Plasticizer A 
                 7.1 parts by weight 
               
               
                   
                 Plasticizer B 
                 3.6 parts by weight 
               
               
                   
                   
               
            
           
         
       
     
     The cellulose triacetate had substitution degree of 2.8. The following were components of mixed solvent for the polymer dope  21 . 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Dichloromethane 
                  80 parts by weight 
               
               
                   
                 Methanol 
                 13.5 parts by weight  
               
               
                   
                 n-butanol 
                 6.5 parts by weight 
               
               
                   
                   
               
            
           
         
       
     
     The solute was added to the solvent, which was stirred and mixed to prepare the polymer dope  21 . The polymer dope  21  was passed through and filtered by a filter paper #63LB (trade name, manufactured by Toyo Roshi Kaisha, Ltd.), then through a sintered metal filter 06N (manufactured by Nippon Seisen Co., Ltd.) having a nominal pore diameter of 10 microns, and furthermore through a mesh filter. The polymer dope  21  was stored in the storage tank  11 . The polymer dope  21  of the composition according to the above list is herein referred to as polymer dope B. The polymer dope B had viscosity of 50 Pa·s. 
     For the in-line mixer  25 , Example 1 was repeated. The casting dope  29  was obtained by mixing the polymer dope B and the liquid additive  27 . 
     Example 3 
     Example 1 was repeated with a difference in that three in-line mixers  25  were arranged in series and resided in the flow line  22 . The casting dope  29  was prepared from the polymer dope A and the liquid additive  27 . 
     Example 4 
     Example 1 was repeated with a difference in that the in-line mixer  225  was provided in the flow line  22  and that two of the in-line mixers  25  were serially connected on a downstream side of the in-line mixer  225 . The casting dope  29  was formed from the polymer dope A and the liquid additive  27 . 
     Comparative Examples 1-3 
     Examples 1-3 were repeated respectively with a difference in which one or more well-known static mixers 6-N16-22 (6) -1 (trade name, manufactured by Noritake Co., Ltd.) were used in place of the in-line mixer  25  or  225 . 
     [Evaluation of the Casting Dope] 
     The prepared casting dope was observed by human eyes and evaluated. The following were grades of the evaluation. 
     A: The liquid additive  27  was found uniformly mixed in the polymer dope. 
     B: The liquid additive  27  was found mixed in the polymer dope. 
     F: The liquid additive  27  was not found mixed in the polymer dope. 
     In Table 1, results of experiments conducted for the Examples and Comparative examples are indicated. The static mixer was constituted by the in-line mixer  225  of the embodiment. The at least one dynamic mixer was constituted by the in-line mixer  25 . The results include the type and number of the static mixer, the type and number of the at least one dynamic mixer, the flow rate Q 1  of the polymer dope, the viscosity ηd of the polymer dope, the flow rate Q 2  of the liquid additive, the viscosity ηt of the liquid additive, and evaluation of the casting dope. The inner widths of the flow line  22  and the liquid additive supply line  26  were so determined that a ratio V 1 /V 2  between the flow speeds V 1  and V 2  of the polymer dope and liquid additive was equal to 3. Among the signs in the table, A designates any one of the static mixer and at least one dynamic mixer of the invention. B designates the well-known static mixer, 6-N16-22 (6) -1 (trade name, manufactured by Noritake Co., Ltd.). 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                   
                   
                   
                 Comparative 
                 Comparative 
               
               
                   
                 Example 1 
                 Example 2 
                 example 1 
                 example 2 
               
               
                   
               
               
                 Type of static 
                 — 
                 — 
                 — 
                 — 
               
               
                 mixers 
               
               
                 No. of static 
                 0 
                 0 
                 0 
                 0 
               
               
                 mixers 
               
               
                 Type of dynamic 
                 A 
                 A 
                 B 
                 B 
               
               
                 mixers 
               
               
                 No. of dynamic 
                 1 
                 1 
                 1 
                 1 
               
               
                 mixers 
               
               
                 Q1 (liters per 
                 45 
                 45 
                 45 
                 45 
               
               
                 min.) 
               
               
                 ηd (Pa · s) 
                 100 
                 50 
                 100 
                 50 
               
               
                 Q2 (liters per 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
               
               
                 min.) 
               
               
                 ηt (Pa · s) 
                 0.001 
                 0.001 
                 0.001 
                 0.001 
               
               
                 Evaluation 
                 B 
                 B 
                 F 
                 F 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Comparative 
                   
               
               
                   
                 Example 3 
                 example 3 
                 Example 4 
               
               
                   
               
               
                 Type of static mixers 
                 — 
                 — 
                 A 
               
               
                 No. of static mixers 
                 0 
                 0 
                 1 
               
               
                 Type of dynamic mixers 
                 A 
                 B 
                 A 
               
               
                 No. of dynamic mixers 
                 3 
                 3 
                 2 
               
               
                 Q1 (liters per min.) 
                 45 
                 45 
                 45 
               
               
                 ηd (Pa · s) 
                 100 
                 100 
                 100 
               
               
                 Q2 (liters per min.) 
                 0.3 
                 0.3 
                 0.3 
               
               
                 ηt (Pa · s) 
                 0.001 
                 0.001 
                 0.001 
               
               
                 Evaluation 
                 A 
                 F 
                 B 
               
               
                   
               
            
           
         
       
     
     As a result, the casting dope  29  of a uniform quality was obtained according to Examples 1-4. In contrast, no casting dope  29  of a uniform quality was obtained according to Comparative examples 1-3. It is concluded that the casting dope  29  with high uniformity can be obtained easily by mixing the liquid additive  27  with the polymer dope  21  with high efficiency according to the invention. 
     Although the present invention has been fully described by way of the preferred embodiments thereof with reference to the accompanying drawings, various changes and modifications will be apparent to those having skill in this field. Therefore, unless otherwise these changes and modifications depart from the scope of the present invention, they should be construed as included therein.