Patent Publication Number: US-2013234360-A1

Title: Method and apparatus for producing cellulose acylate film

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and an apparatus for producing a cellulose acylate film. 
     2. Description of the Related Art 
     A solution casting method, as is well known, is a method for producing a film by casting on a support a dope obtained by dissolving a polymer in a solvent to form a casting film, peeling off the casting film as a wet film from the support, and drying the wet film. Cellulose acylate films that are frequently used for optical applications have been produced by such a solution casting method. 
     Cellulose acylate films have been used as, for example, viewing-angle widening films, polarizing plate protective films, and the like that constitute liquid crystal displays. In recent years, along with rapid extension of a market of the liquid crystal displays, demand for the cellulose acylate films has also rapidly increased. Thus, it is necessary to increase production volume of the cellulose acylate films in existing apparatuses to a large extent. 
     In order to increase the production volume of the films in existing solution casting apparatuses, a belt or drum as a casting support is caused to move at a higher speed. On the casting support, casting of the dope and peeling off of the casting film are repeatedly performed, and the number of repetition per unit time increases as the speed of film production is made faster. As the speed of film production is made faster, the casting support becomes more rapidly contaminated, though the speed of the casting support being contaminated varies depending on the composition of the dope, the casting conditions, the peeling conditions, and the like. Furthermore, if the production of films is continued while the casting support is contaminated, problems may occur. For example, smoothness of the film surface may be impaired, or contaminants of the casting support may adhere to the film. Therefore, the casting support needs to be washed. However, washing must be carried out while production of the films is halted. Consequently, as the time required for the washing becomes longer, or as the frequency of the washing becomes higher, the film productivity is deteriorated. 
     Regarding the contaminants of the casting support, there are occasions in which materials that are contained in a casting film but are not visually recognized slowly increase to the extent that clouding is eventually recognized. In the following explanations, the clouding phenomenon that is recognized as such is referred to as plate-out. Various suggestions have been hitherto made regarding the method for preventing plate-out. For example, in the method described in United States Patent Application Publication No. 2008/054215 (corresponding to JP-A-2008-063403), the proportion of the mass of cellulose acylate to the mass of magnesium contained in cellulose acylate is adjusted to a predetermined range, and thereby plate-out is prevented. Furthermore, JP-A-2011-006603 describes a method of adjusting the amount of calcium, the amount of magnesium, and the amount of sulfuric acid in the dope to predetermined ranges, such that a compound having a predetermined skeletal structure is contained in the dope. Thereby, yellowing of the film is prevented while plate-out is suppressed. 
     The methods described in United States Patent Application Publication No. 2008/054215 (corresponding to JP-A-2008-063403) and JP-A-2011-006603 have certain effects as the method for preventing plate-out. However, even if these methods are used, plate-out may still occur, and therefore, it cannot be said that these methods are perfect methods for suppressing plate-out. The contaminant recognized in the plate-out is so-called wax contained in cellulose acylate. The wax is mainly composed of fatty acid, fatty acid salt, and fatty acid ester. Among them, fatty acid ester is prone to impair the smoothness of the film surface. Furthermore, since cold casting can increase the production speed of the films as compared with dry casting, the cold casting exhibits excellent productivity. However, according to the cold casting, plate-out is more likely to occur as compared with the dry casting, and in particular, fatty acid ester is likely to precipitate out on the casting support. Since fatty acid ester has high molecular weight, it does not easily dissolve even in liquids that are commonly used as solvents. Therefore, fatty acid ester is not easily removed even if it is washed, and cannot be completely removed even if it is wiped with a cloth soaked with a solvent or the like, for example. Even if fatty acid ester can be removed, it takes an enormous amount of time. Meanwhile, the cold casting is a method of hardening a casting film by cooling it on a casting support, and the dry casting is a method of hardening a casting film by drying it on a casting support. 
     SUMMARY OF THE INVENTION 
     In view of the above, an object of the present invention is to provide a method and an apparatus for producing a cellulose acylate film, capable of easily removing contaminants from a casting support by washing the casting support, even if plate-out occurs, by using cellulose acylate containing fatty acid ester. 
     The method for producing a cellulose acylate film of the present invention includes an adding step (step A), a casting step (step B), a peeling step (step C), and a drying step (step D). In the step A, fatty acid having a carbon number in the range of not less than 12 to not more than 22 is added to a cellulose acylate solution. The cellulose acylate solution is a liquid in which cellulose acylate and a plasticizer are dissolved in a solvent. The fatty acid is added to the cellulose acylate solution in a state that proportion of mass of the fatty acid with respect to sum of mass of the cellulose acylate and mass of the plasticizer is in the range of 1×10 −4  to 3×10 3 . In the step B, the cellulose acylate solution containing the fatty acid is cast on a surface of a support to form a casting film. The support circulates through a casting position and a peeling position. The casting position is a position at which the cellulose acylate solution is cast. The peeling position is a position at which the casting film formed by the casting is peeled off. In the step C, the casting film is peeled off from the surface of the support. In the step D, the casting film thus peeled off is dried. 
     The fatty acid is preferably linear fatty acid. 
     The fatty acid is preferably saturated fatty acid. 
     It is preferable that the fatty acid is dissolved in a solvent to prepare a fatty acid solution, and the step A is carried out by adding the fatty acid solution to the cellulose acylate solution. 
     It is preferable that the cellulose acylate solution is continuously supplied to a casting device for carrying out the step B, and the fatty acid solution is continuously added to the cellulose acylate solution that flows toward the casting device. 
     The apparatus for producing a cellulose acylate film of the present invention includes an adding section, a casting device, a casting die, a support, a peeling device, and a drying device. The adding section adds fatty acid having a carbon number in the range of not less than 12 to not more than 22 to a cellulose acylate solution. The cellulose acylate solution is a liquid in which cellulose acylate and a plasticizer are dissolved in a solvent. The fatty acid is added to the cellulose acylate solution in a state that proportion of mass of the fatty acid with respect to sum of mass of the cellulose acylate and mass of the plasticizer is in the range of 1×10 −4  to 3×10 −3 . The casting device forms a casting film from the cellulose acylate solution containing the fatty acid. The casting die discharges the cellulose acylate solution containing the fatty acid. The casting die is installed in the casting device. The support circulates through a casting position and a peeling position. The support is installed in the casting device. The casting position is a position at which the cellulose acylate solution is cast. The peeling position is a position at which the casting film formed by the casting is peeled off. The peeling device peels the casting film from the support. The drying device dries the casting film thus peeled off. 
     According to the present invention, it is possible to easily remove contaminants from the casting support by washing the casting support, even if plate-out occurs, by using cellulose acylate containing fatty acid ester. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and advantages of the present invention will be more apparent from the following detailed description of the preferred embodiments when read in connection with the accompanied drawings, wherein like reference numerals designate like or corresponding parts throughout the several views, and wherein: 
         FIG. 1  is a schematic diagram of a solution casting apparatus for carrying out a solution casting method. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Solution Casting Apparatus 
     Cellulose acylate films are produced by, for example, using a solution casting apparatus  10  shown in  FIG. 1 . The solution casting apparatus  10  includes a dope producing unit  11  and a film producing unit  12 . 
     The dope producing unit  11  includes a first dissolution device  15 , a second dissolution device  16 , and a mixing device  17 . The first dissolution device  15  is connected to a casting die  18  of the film producing unit  12  through the mixing device  17 . In the first dissolution device  15 , cellulose acylate  20  supplied thereto is mixed with a plasticizer  19  and a solvent  21  for cellulose acylate  20  (hereinafter, referred to as a first solvent  21 ), and the mixture is stirred or heated such that the cellulose acylate  20  is dissolved in the first solvent  21 . Thereby, a cellulose acylate solution  22  (hereinafter, referred to as a dope  22 ) is obtained. In the first dissolution device  15 , other materials that are different from the cellulose acylate  20  and the first solvent  21  may be supplied and mixed with the cellulose acylate  20  and the first solvent  21 . Examples of the other materials include a matting agent and an ultraviolet absorber. 
     In the second dissolution device  16 , a predetermined fatty acid  25  that has been supplied thereto is mixed with a solvent  26  for the fatty acid  25  (hereinafter, referred to as a second solvent  25 ), and the mixture is stirred or heated such that the fatty acid  25  is dissolved in the second solvent  26 . Thereby, a fatty acid solution  27  is obtained. The predetermined fatty acid  25  will be described in detail later. 
     The second dissolution device  16  is connected to a first piping L 1  for connecting the first dissolution device  15  and the mixing device  17 . Thereby, the fatty acid solution  27  is added to the dope  22 . When the fatty acid solution  27  is added to the dope  22 , a casting dope  30  containing the fatty acid  25  is obtained. A process of adding the fatty acid  25  as described above is hereinafter referred to as an adding process. The first piping L 1  extends from the mixing device  17  to the casting die  18  so as to connect them. In a second piping L 2  for connecting the second dissolution device  16  and the first piping L 1 , a valve  31  is provided, and by regulating the opening degree of the valve  31 , the amount of the fatty acid solution  27  to be supplied to the first piping L 1  is controlled. As the supply amount of the fatty acid solution  27  is controlled, the mass of the fatty acid  25  to be added to the dope  22  is controlled. The amount of the fatty acid  25  to be added to the dope  22  will be described in detail later. 
     In this embodiment, the dope  22  obtained in the first dissolution device  15  is continuously supplied to the casting die  18 , and the fatty acid solution  27  is continuously added to the dope  22  flowing from the first dissolution device  15  to the casting die  18 . When the viscosity of the dope  22  is greatly different from that of the fatty acid solution  27 , it may be difficult for the two materials to be uniformly mixed. In such a case, as in this embodiment, the mixing device  17  may be disposed between the casting die  18  and an adding position PA at which the fatty acid solution  27  is added to the dope  22 , such that a mixture of the dope  22  and the fatty acid solution  27  that has been guided to the mixing device  17  is prepared as a uniform solution by the mixing device  17 . The mixing device  17  is, for example, preferably a static type mixing device. Examples of the static type mixing device include a static mixer and a Sulzer mixer. 
     The adding process may be such that the fatty acid  25  is directly supplied to the dope  22  without being dissolved in the second solvent  26 . In this case, the second dissolution device  16  is not used. However, from the viewpoint of mixing the fatty acid  25  with the dope  22  such that the mixture of them becomes uniform more rapidly and reliably, it is preferable that the fatty acid solution  27  is prepared by dissolving the fatty acid  25  in the second solvent  26 , and the fatty acid solution  27  is supplied to the dope  22 , as in the case of this embodiment. 
     The film producing unit  12  includes a casting device  34 , a tenter  35 , a roller drying device  36 , and a winding device  37 , in this order from the upstream side. 
     Meanwhile, in the present specification, the solvent content (unit; %) is a value based on dry weight. Specifically, when the mass of the solvent (sum of the mass of the first solvent  21  and the mass of the second solvent  26 ) is designated as x, and the mass of a film  40  is designated as y, the solvent content (unit; %) is the percentage determined by the formula: {x/(y−x)}×100. 
     The casting device  34  includes a drum  41  as a casting support and the casting die  18  for discharging the casting dope  30  supplied thereto. The casing die  18  is disposed above the drum  41 . In a casting process, the casting dope  30  is continuously discharged from the casting die  18  onto the drum  41  that is rotating in the circumferential direction. Thereby, the casting dope  30  is cast on the drum  41 , and thus a casting film  42  is formed. In  FIG. 1 , a position at which the casting dope  30  is brought into contact with the drum  41  and the formation of the casting film  42  is initiated (hereinafter, referred to as a casting position) is assigned with symbol PC. 
     The drum  41  includes a temperature controller (not shown in the drawing) that controls a circumferential surface temperature of the drum  41 . The temperature of the casting film  42  is adjusted by the drum  41  whose circumferential surface temperature is controlled. For example, in the case of cold casting, if the circumferential surface temperature is adjusted within the range of −15° C. to 10° C., the casting film  42  is cooled and turns into a gel. Through this gelation, the casting film  42  is hardened to be conveyable. 
     An endless belt formed into a circular shape (not shown in the drawing) may be used as the casting support instead of the drum  41 . In the case of using the belt as the casting support, the belt is wound around the circumferential surfaces of a pair of rollers (not shown in the drawing) that rotate in the circumferential direction. At least any one of the pair of rollers may be a driving roller provided with a driving means. As the driving roller rotates in the circumferential direction, the belt that is in contact with the circumferential surface of the driving roller is conveyed. As a result of this conveyance, the belt continuously moves in the longitudinal direction by circulating. In the case of using the belt as the casting support, a temperature of the belt that is in contact with the circumferential surfaces of the rollers may be controlled by a temperature controller (not shown in the drawing) provided to the pair of rollers, for controlling the circumferential surface temperatures of the respective rollers. 
     In regard to the casting dope  30  extending from the casting die  18  to the drum  41 , that is, a so-called bead, a decompression chamber (not shown in the drawing) is provided in an upstream side in the rotational direction of the drum  41 . The decompression chamber sucks the atmosphere in an upstream-side area of the casting dope  30  that has been discharged from the casting die  18 , and decompresses the area. 
     The casting film  42  is hardened to the extent that the casting film  42  can be conveyed to the tenter  35 , and then the casting film  42  in the state of containing the solvent is peeled off from the circumferential surface of the drum  41 . This peeling process is such that in the case of cold casting, for example, the casting film  42  having a solvent content within the range of 150 mass % to 280 mass % is peeled off. At the time of peeling off the casting film  42 , the casting film  42  is supported by a roller  45  for peeling (hereinafter, referred to as a peeling roller  45 ), and a peeling position PP at which the casting film  42  is peeled off from the drum  41  is maintained to be constant. 
     As discussed above, a film  40  is formed from the casting dope  30  by using the casting device  34 . As the circumferential surface of the rotating drum  41  circulates through the casting position PC and the peeling position PP, casting of the casting dope  30  and peeling off of the casting film  42  are repeatedly carried out on the drum  41 . 
     In a conveyance path between the casting device  34  and the tenter  35 , an air blowing device (not shown in the drawing) may be disposed. By blowing air from the air blowing device, drying of the film  40  is progressed. The casting film  42  thus peeled off, that is, the film  40 , is guided to the tenter  35 . 
     The tenter  35  includes plural pin plates  46  as holding members for holding side ends of the lengthy film  40 , a pair of rails (not shown in the drawing), and a pair of chains (not shown in the drawing). The pin plates  46  are disposed such that plural pins (not shown in the drawing) stand up on the top surface of a stand. The tenter  35  holds the side ends of the film  40  by penetrating pins of the pin plates  46  through the side ends of the film  40  guided thereto. 
     The rails are provided to lateral sides of the conveyance path of the film  40  on a one-to-one basis, and the paired rails are disposed so as to be spaced from each other. The chains are bridged over a driving sprocket and a driven sprocket (not shown in the drawing), and are attached movably along the rails. The plural pin plates  46  are mounted on the chains at a predetermined interval. The rotation of the driving sprocket causes the pin plates  46  to perform circulatory movement along the rails. The pin plates  46  initiate holding of the film  40  thus guided in the vicinity of an inlet port of the tenter  35 , and move toward an outlet port of the tenter  35 . The holding of the film  40  by the pin plates  46  is released in the vicinity of the outlet port. The pin plates  46  that have released the holding of the film  40  move again to the vicinity of the inlet port, and hold the film  40  that has been newly guided. The side ends of the film  40  are held by the pin plates  46 , and the film  40  is conveyed in the longitudinal direction. 
     The tenter  35  is provided with a duct  47  disposed above the conveyance path for the film  40 . At a lower surface of the duct  47 , a discharge port for discharging a dry gas (not shown in the drawing) is formed. By blowing a dry gas, drying of the film  40  is progressed while the film  40  is conveyed in the tenter  35 . A duct having the same structure may also be provided under the conveyance path of the film  40 . 
     The roller drying device  36  includes plural rollers  50  and an air conditioner (not shown in the drawing). The plural rollers  50  support the film  40  by the circumferential surfaces thereof. The film  40  is wound around the rollers  50 , and thereby the conveyance path for the film  40  is defined. The air conditioner regulates the temperature, humidity, and the like inside the roller drying device  36 . Thereby, drying of the film  40  is progressed even while the film  40  is conveyed in the roller drying device  36 . As such, drying of the film  40  is carried out in both the tenter  35  and the roller drying device  36 . 
     After the film  40  is sent to the winding device  37 , the film  40  is wound into a roll. In this manner, the film  40  is produced from the casting dope  30  in the film producing unit  12 . 
     A slitter (not shown in the drawing) may be disposed in the downstream side from the tenter  35  so as to cut off holding marks of the film  40  made by the pins of the pin plates  46 . In order to impart a desired optical property, for example, retardation, to the film  40 , an additional tenter (not shown in the drawing) may be further disposed between the tenter  35  and the roller drying device  36 , or in the downstream side from the roller drying device  36 , such that treatments such as stretching of the film  40  in the width direction thereof is carried out by using the tenter. 
     The film  40  thus obtained can be utilized as an optical film. Examples of the optical film include a protective film for a polarizing plate, or a phase difference film. 
     In the case of forming a casting film composed of plural layers by sequential casting or co-casting, among the plural casting dopes to be used, as a casting dope for forming the layer to be in contact with the drum  41 , the above-mentioned casting dope  30  is preferably used. 
     The adding process will be described in detail. In the adding process, a predetermined fatty acid  25  is added to the dope  22  in a state that the proportion of the mass of the fatty acid  25  relative to the mass of the solid content of the dope  22  is in the range of 1×10 −4  to 3×10 −3 . That is, when the mass of the solid content of the dope  22  is designated as M 1 , and the mass of the fatty acid  25  is designated as M 2 , the mass of the fatty acid  25  which brings a value of the ratio M 2 /M 1  into the range of 1×10 −4  to 3×10 −3  is determined, and the determined mass of the fatty acid  25  is added to the dope  22 . Thereby, the washing property is enhanced. The washing property being satisfactory means that removal of contaminants by washing is easy, that is, contaminants are removed more reliably in a shorter time. The solid content of the dope  22  is composed of the cellulose acylate  20  and the plasticizer  19 . If solid components other than the cellulose acylate  20  and the plasticizer  19  are included in the dope  22 , the mass of the other solid components may be ignored. 
     When the predetermined fatty acid  25  is added to the dope  22 , the added fatty acid  25  is precipitated out on the circumferential surface of the drum  41  together with fatty acid ester. As compared with the fatty acid ester, the fatty acid  25  has higher solubility relative to the solvent, and therefore the fatty acid  25  is more easily removed by washing. The fatty acid ester is removed together with the fatty acid  25 . 
     However, if the proportion of the mass of the fatty acid  25  with respect to the mass of the solid content of the dope  22  is below 1×10 −4 , it cannot be said that the effect of the washing property is reliably obtained. Furthermore, if the proportion of the mass of the fatty acid  25  with respect to the mass of the solid content of the dope  22  is larger than 3×10 −3 , the effect of the washing property may be obtained, but whitening is observed in the film  40  thus obtained, which is not preferable. 
     As in the case of this embodiment, when the fatty acid  25  in the state of being dissolved in the second solvent  26  is added to the dope  22 , the fatty acid solution  27  is supplied to the dope  22 , such that the fatty acid  25  is added to the dope  22  in a state that the proportion of the mass of the fatty acid  25  relative to the mass of the solid content of the dope  22  is in the range of 1×10 −4  to 3×10 −3 . For the purpose of supplying the fatty acid solution  27  to the dope  22 , there are two methods of adjusting the ratio M 2 /M 1  to the range of 1×10 −4  to 3×10 −3 . According to one of the methods, the mass of the fatty acid solution  27  to be supplied per unit mass of the dope  22  is made to be constant and the concentration of the fatty acid  25  in the fatty acid solution  27  is adjusted before adding the fatty acid solution  27  to the dope  22 . According to the other method, the concentration of the fatty acid  25  in the fatty acid solution  27  is made to be constant and the mass of the fatty acid solution  27  to be supplied per unit mass of the dope  22  is adjusted before adding the fatty acid solution  27  to the dope  22 . 
     In view of the solubility of the fatty acid  25  in the dope  22 , it is preferable that the concentration of the fatty acid  25  in the fatty acid solution  27  is made to be constant within the range of 1% to 5% and the mass of the fatty acid solution  27  to be supplied per unit mass of the dope  22  is adjusted before adding the fatty acid solution  27  to the dope  22 . The concentration of the fatty acid  25  to be added is adjusted by controlling the flow rate of the fatty acid solution  27  relative to the flow rate of the dope  22 . 
     When the fatty acid  25  is added to the dope  22 , plate-out is suppressed. 
     &lt;Fatty Acid&gt; 
     The fatty acid  25  used in the present invention is fatty acid having a carbon number in the range of 12 to 22. In particular, linear fatty acid is preferably used, saturated fatty acid is more preferably used, and the fatty acid listed in Table 1 is particularly preferably used. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Carbon 
                 Name 
                   
               
               
                 number 
                 (Trivial name inside parentheses) 
                 Molecular formula 
               
               
                   
               
             
            
               
                 10 
                 Decanoic acid 
                 CH 3 —(CH 2 ) 8 —COOH 
               
               
                 12 
                 Dodecanoic acid (Lauric acid) 
                 CH 3 —(CH 2 ) 10 —COOH 
               
               
                 16 
                 Hexadecanoic acid (Palmitic acid) 
                 CH 3 —(CH 2 ) 14 —COOH 
               
               
                 18 
                 Octadecanoic acid (Stearic acid) 
                 CH 3 —(CH 2 ) 16 —COOH 
               
               
                 22 
                 Docosanoic acid (Behenic acid) 
                 CH 3 —(CH 2 ) 20 —COOH 
               
               
                 20 
                 Eicosanoic acid 
                 CH 3 —(CH 2 ) 18 —COOH 
               
               
                 24 
                 Tetracosanoic acid 
                 CH 3 —(CH 2 ) 22 —COOH 
               
               
                 26 
                 Hexacosanoic acid 
                 CH 3 —(CH 2 ) 24 —COOH 
               
               
                   
               
            
           
         
       
     
     &lt;Cellulose Acylate&gt; 
     As for cellulose acylate  20 , it is particularly preferable that the degree of hydrogen groups of cellulose esterified for carboxylic acid, that is, the degree of acylation satisfies all of the following formulae (1) to (3). In these formulae (1) to (3), “A” is the degree of substitution of acetyl groups for the hydrogen atoms on the hydroxyl groups of cellulose, and “B” is the degree of substitution of acyl groups for the hydrogen atoms on the hydroxyl groups of cellulose while each acyl group has carbon atoms whose number is from 3 to 22. 
       2.4 ≦A+B≦ 3.0  (1)
 
       0 ≦A≦ 3.0  (2)
 
       0 ≦B≦ 2.9  (3)
 
     A glucose unit constructing cellulose with β-1,4 bond has free hydroxyl groups at 2nd, 3rd, and 6th positions. The cellulose acylate  20  is a polymer in which, by esterification, the hydrogen atoms on part or all of the hydroxyl groups of cellulose are substituted by the acyl groups having 2 or more carbon atoms. When the esterification of one hydroxyl group in the glucose unit is made at 100%, the degree of substitution is 1. As for cellulose acylate, when the esterification in each hydroxyl group at the 2nd, 3rd, and 6th position is made at 100%, the degree of substitution is 3. 
     Here, the degree of acylation at the 2nd position in the glucose unit is described as DS2, the degree of acylation at the 3rd position in the glucose unit is described as DS3, and the degree of acylation at the 6th position in the glucose unit is described as DS6. The sum of the degree of acylation, “DS2+DS3+DS6”, is preferably in the range of 2.00 to 3.00, more preferably in the range of 2.22 to 2.90, and further more preferably in the range of 2.40 to 2.88. Moreover, “DS6/(DS2+DS3+DS6)” is preferably at least 0.32, more preferably at least 0.322, and further more preferably in the range of 0.324 to 0.340. 
     The cellulose acylate may be composed of either one kind of the acyl group, or two or more kinds thereof. It is preferable that, when two or more kinds of the acyl groups are used, one of them is the acetyl group. When the sum of the degree of substitution of the acetyl groups for the hydroxyl groups at the 2nd, 3rd, and 6th positions is represented by DSA, and the sum of the degree of substitution of the acyl groups other than the acetyl groups for the hydroxyl groups at the 2nd, 3rd, and 6th positions is represented by DSB, the value of “DSA+DSB” is preferably in the range of 2.2 to 2.86, and particularly preferably in the range of 2.40 to 2.80. DSB is preferably at least 1.50, and particularly preferably at least 1.7. Additionally, it is preferable that the hydroxyl groups at the 6th position account for at least 28% of DSB, and more preferably at least 30%, and further more preferably at least 31%, and particularly preferably at least 32%. The value of “DSA+DSB” at the 6th position of cellulose acylate is preferably at least 0.75, and more preferably at least 0.80, and particularly preferably at least 0.85. Cellulose acylate with such a composition provides excellent solubility for preparing a polymer solution to be used in the solution casting. 
     The acyl group with 2 or more carbon atoms in cellulose acylate is not limited particularly, and may be either an aliphatic group or an aryl group. Such acyl group may be, for example, alkylcarbonyl ester of cellulose, alkenylcarbonyl ester of cellulose, aromatic carbonyl ester of cellulose, and aromatic alkylcarbonyl ester of cellulose, and each of them may have further substitutents. Exemplary substitutents are a propionyl group, a butanoyl group, a pentanoyl group, a hexanoyl group, an octanoyl group, a decanoyl group, a dodecanoyl group, a tridecanoyl group, a tetradecanoyl group, a hexadecanoyl group, an octadecanoyl group, an iso-butanoyl group, a t-butanoyl group, a cyclohexane carbonyl group, an oleoyl group, a benzoyl group, a naphthyl carbonyl group, and a cinnamoyl group. Among them, preferably used are the propionyl group, the butanoyl group, the dodecanoyl group, the octadecanoyl group, the t-butanoyl group, the oleoyl group, the benzoyl group, the naphthyl carbonyl group, and the cinnamoyl group, and more particularly used are the propionyl group and the butanoyl group. 
     &lt;First Solvent&gt; 
     As the first solvent  21 , solvents which are known as solvents for the dope in the case of producing a cellulose acylate film by solution casting can be used. For example, dichloromethane, various kinds of alcohol, and various kinds of ketone may be used. Furthermore, a mixture of plural solvent components may be used as the first solvent  21 . In this case, a mixture obtained by adding a poor solvent of cellulose acylate to a good solvent thereof may also be used. 
     &lt;Second Solvent&gt; 
     The second solvent  26  may be a single component or a mixture of plural components, as long as the second solvent  26  includes a good solvent of the fatty acid  25 . From the viewpoint of being more reliably and more rapidly homogenized after being added to and mixed with the dope  22 , the second solvent  26  more preferably contains compounds common to those used as the components of the first solvent  21 . 
     Hereinafter, Examples of the present invention and Comparative Examples compared with the present invention will be described. The details are described in Examples, and only the conditions that are different from those of Examples will be described in Comparative Examples. 
     Examples 
     The dope  22  having the following composition was prepared by using the first dissolution device  15 . The first solvent  21  is a mixture composed of three components, and the first component is dichloromethane, the second component is methanol, and the third component is n-butanol. 
     
       
         
           
               
               
               
             
               
                   
               
             
            
               
                 Cellulose acetate 
                 100 
                 parts by mass 
               
               
                 (based on pulp as a raw material, 
               
               
                 substitution degree of acetyl group: 2.85) 
               
               
                 Triphenyl phosphate 
                 13 
                 parts by mass 
               
               
                 Dichloromethane 
                 384 
                 parts by mass 
               
               
                 Methanol 
                 54 
                 parts by mass 
               
               
                 n-Butanol 
                 15 
                 parts by mass 
               
               
                 Matting agent 
                 0.03 
                 parts by mass 
               
               
                 (AS972, average primary particle 
               
               
                 diameter: 10 nm) 
               
               
                   
               
            
           
         
       
     
     In the second dissolution device  16 , the fatty acid  25  and the second solvent  26  were supplied to prepare the fatty acid solution  27 . Example 1 to Example 9 were prepared by varying the kind of the fatty acid  25  to be used and the proportion of the mass of the fatty acid  25  relative to the mass of the solid content of the dope  22  (M 2 /M 1 ). The kind of the fatty acid  25  and M 2 /M 1  for the respective Examples are listed in Table 2. The second solvent  26  had the same composition as that of the first solvent  21 . 
     The fatty acid solution  27  was added to the dope  22  flowing through the first piping L 1  from the first dissolution device  15  toward the casting die  18 , and thus the casting dope  30  was prepared. The mixing device  17  was provided in the downstream side from the adding position PA of the first piping L 1 , and the casting dope  30  was mixed and homogenized therein. 
     The casting dope  30  was continuously cast on the drum  41  of the film producing unit  12  for 3 hours. The width of the circumferential surface of the drum.  41  was 10 cm, and the diameter of the drum  41  was 5 cm. The following evaluation was carried out with regard to the contaminants adhering on the drum  41 . 
     In the respective Examples, the washing property, suppression of plate-out, and transparency of the film  40  were respectively evaluated by the following methods and criteria. The evaluation results for the respective Examples are shown in Table 2. In Table 2, “Ex” denotes Example, and “Com” denotes Comparative Example. 
     1. Washing Property 
     While the drum  41  was rotated in a single direction at the speed of 1 m per minute, contaminants adhering to the circumferential surface of the drum  41  were wiped out with a cloth soaked with a solvent. The solvent soaked in the cloth was a mixture obtained by mixing acetone and methylene chloride at a mass ratio of 1/1. The washing property was evaluated according to the following criteria, on the basis of the number of rotations of the drum  41  required to eliminate the contaminants on the drum  41 . 
     A: Contaminants on the drum  41  were eliminated when the number of rotations of the drum  41  was in the range of note less than 1 to not more than 3. 
     B: Contaminants on the drum  41  were eliminated when the number of rotations of the drum  41  was in the range of note less than 4 to not more than 5. 
     C: Contaminants on the drum  41  were eliminated when the number of rotations of the drum  41  was in the range of not less than 6 to not more than 10. 
     D: Contaminants remained on the drum  41  even when the number of rotations of the drum  41  was 11 or more. 
     Here, A and B are acceptable levels for the washing property, and C and D are unacceptable levels. 
     2. Suppression of Plate-Out 
     When plate-out occurs, gloss of the circumferential surface of the drum  41  disappears depending on the amount of contaminants, and the circumferential surface of the drum  41  whitens. Thus, the glossiness of the circumferential surface of the drum  41  was measured, and the effect of suppressing plate-out was evaluated on the basis of the glossiness. The evaluation method was specifically as follows. 
     (1) Before the film  40  was produced, the drum  41  was disposed in a dark room in advance. In this dark room, the circumferential surface of the drum  41  was irradiated with white light emitted in a predetermined direction with a predetermined angle relative to the circumferential surface of the drum  41 . A photoreceiver was disposed at a position different from that of the light source for the white light, and the amount of the white light reflected at the drum was measured by using the photoreceiver. The amount of light thus measured was designated as Q 1 . 
     (2) Continuous production of the film  40  was initiated, and at the point of time when 40 hours had passed, production was momentarily stopped. 
     (3) The drum  41  was detached from the casting device, and the drum  41  was disposed in the dark room again. The amount of reflected light was measured under the same conditions as those in item (1). The amount of light thus measured was designated as Q 2 . 
     (4) The value determined by the formula: (Q 2 /Q 1 )×100 was designated as the glossiness (unit: %). 
     A: The glossiness is in the range of not less than 80% to not more than 100%. 
     B: The glossiness is in the range of not less than 65% to less than 80%. 
     C: The glossiness is in the range of not less than 50% to less than 65%. 
     D: The glossiness is in the range of not less than 0% to less than 50%. 
     Here, A and B are acceptable levels for the effect of suppressing plate-out, and C and D are unacceptable levels. 
     3. Transparency of Film 
     If the amount of the fatty acid  25  added to the dope  22  is too large, the fatty acid  25  is precipitated out on the surface of the film  40  as time passes, and the film  40  whitens. The degree of whitening of the film  40  can be measured based on haze. As the degree of whitening increases, the value of haze also increases, so that transparency of the film becomes poor. Furthermore, if the film is subjected to heating, the possibility of precipitation of the fatty acid  25  increases. Thus, transparency of the film  40  was evaluated by the following method. 
     (1) The film  40  thus obtained was cut into a size of 40 mm×80 mm, and the haze was measured in an environment at 25° C. and 60% RH. The haze was measured according to JIS-6714 by using a haze meter (trade name: HGM-2DP, manufactured by Suga Test Instruments Co., Ltd.). The value thus measured was designated as H 1 . 
     (2) The film  40  that had been subjected to the measurement of item (1) was left to stand in an environment at 150° C. for 2 hours. 
     (3) After the heating treatment of the item (2), the haze of the film  40  was measured with the same haze meter as that used in item (1). The value thus measured was designated as H 2 . 
     (4) The change in the haze caused by the heating treatment was determined as the value of H 2 /H 1 . 
     A: The value of H 2 /H 1  is less than 1.2. 
     B: The value of H 2 /H 1  is in the range of not less than 1.2 to less than 1.5%. 
     C: the value of H 2 /H 1  is 1.5 or more. 
     Here, A and B are acceptable levels for the transparency of the film  40 , and C is an unacceptable level. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Fatty acid 
                   
                 Evaluation results 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Carbon 
                 Molecular 
                 M2/M1 
                 Washing 
                   
                   
               
               
                   
                 Name 
                 Number 
                 Weight 
                 (ppm) 
                 property 
                 Plate-out 
                 Transparency of film 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Ex 1 
                 Octadecanoic acid 
                 18 
                 284 
                 100 
                 A 
                 B 
                 A 
               
               
                 Ex 2 
                 Octadecanoic acid 
                 18 
                 284 
                 500 
                 A 
                 A 
                 A 
               
               
                 Ex 3 
                 Octadecanoic acid 
                 18 
                 284 
                 1000 
                 A 
                 A 
                 A 
               
               
                 Ex 4 
                 Octadecanoic acid 
                 18 
                 284 
                 2000 
                 A 
                 A 
                 A 
               
               
                 Ex 5 
                 Octadecanoic acid 
                 18 
                 284 
                 3000 
                 A 
                 A 
                 B 
               
               
                 Ex 6 
                 Dodecanoic acid 
                 12 
                 200 
                 500 
                 A 
                 A 
                 A 
               
               
                 Ex 7 
                 Hexadecanoic acid 
                 16 
                 256 
                 500 
                 A 
                 A 
                 B 
               
               
                 Ex 8 
                 Eicosanoic acid 
                 20 
                 313 
                 500 
                 A 
                 A 
                 B 
               
               
                 Ex 9 
                 Docosanoic acid 
                 22 
                 340 
                 500 
                 A 
                 A 
                 B 
               
               
                 Com 1 
                 Not added 
                 — 
                 — 
                 0 
                 D 
                 D 
                 A 
               
               
                 Com 2 
                 Octadecanoic acid 
                 18 
                 284 
                 50 
                 C 
                 C 
                 A 
               
               
                 Com 3 
                 Octadecanoic acid 
                 18 
                 284 
                 4000 
                 A 
                 C 
                 D 
               
               
                 Com 4 
                 Octadecanoic acid 
                 18 
                 284 
                 80 
                 C 
                 C 
                 A 
               
               
                 Com 5 
                 Decanoic acid 
                 10 
                 172 
                 500 
                 C 
                 C 
                 A 
               
               
                 Com 6 
                 Tetracosanoic acid 
                 24 
                 369 
                 500 
                 B 
                 C 
                 D 
               
               
                 Com 7 
                 Hexacosanoic acid 
                 26 
                 396 
                 500 
                 B 
                 D 
                 D 
               
               
                   
               
            
           
         
       
     
     Comparative Examples 
     In Comparative Example 1, the fatty acid  25  was not added to the dope  22 , and the dope  22  was directly cast. In Comparative Examples 2 to 7, the fatty acid  25  and the second solvent  26  were supplied to the second dissolution device  16 , and a fatty acid solution was prepared. The kind of the fatty acid  25  to be used, and the proportion of the mass of the fatty acid  25  with respect to the mass of the solid content of the dope  22  (M 2 /M 1 ) were varied. The kind of the fatty acid  25  and M 2 /M 1  for the respective Comparative Examples are listed in Table 2. The other conditions were the same as the conditions used in the Examples. 
     The respective Comparative Examples were subjected to the same evaluations as those carried out for the Examples. The evaluation results are shown in Table 2. 
     Various changes and modifications are possible in the present invention and may be understood to be within the present invention.