Abstract:
A production method of cellulose film wherein cellulose film is produced by preparing a polymer solution through dissolving cellulose ester in a solvent containing a prescribed organic solvent as the main component, forming a filmy object from the prepared polymer solution, and evaporating the solvent in the filmy object; the residual amount of the organic solvent is reduced while the film quality is not degraded, and the production efficiency is degraded to a least possible extent; a poor solvent, highest in boiling point among the materials contained in the solvent, is added in the content ranging from 0.1 wt % to 1.0 wt %, taking the total amount of the solvent in the prepared polymer solution to be 100 wt %; and the solubility of cellulose ester in the poor solvent is inferior to the solubility of the cellulose ester in the organic solvent which is the main component of the solvent.

Description:
This application is a division of co-pending application Ser. No. 10/385,857, filed on Mar. 12, 2003, now U.S. Pat. No. 6,887,415 which claims the benefit of Japanese application no. 2002-067086, filed on Mar. 12, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a production method of cellulose film in which cellulose film is produced by preparing a polymer solution through dissolving cellulose ester in a solvent containing a prescribed organic solvent as the main component, forming a filmy object from the prepared polymer solution, and evaporating the solvent in the filmy object, and a cellulose film thus produced; and protective film for polarizing plate, optical functional film, polarizing plate, and liquid crystal displays produced by the above described production method of cellulose film. 
     2. Description of the Related Art 
     Conventionally, cellulose film is used as optical materials for optical functional films for use in widening the viewing angle and preventing glare; protective films for the polarizing plates in liquid crystal displays; and the like. The cellulose films used for such optical materials are produced by means of the solution method for forming film. In the solution method for forming film, a filmy object is formed from a polymer solution in which cellulose ester or the like is dissolved in an organic solvent, the filmy object formed is heated to evaporate the organic solvent in the filmy object, and a polymer film is thereby obtained. In this connection, when the organic solvent remains in the produced cellulose film, there occur adverse effects on the dimensional stability of the film, or the coloring of the film is degraded. Accordingly, the control of the residual amount of the organic solvent in the produced cellulose film has heretofore been performed from the viewpoint of the quality. The produced cellulose film is subjected to saponification and the like in post-processes, and subsequently is commercialized as optical functional film, or protective film for polarizing plate. 
     Now, when the cellulose film produced by the solution method for forming film is considered from the standpoint of the environment conservation, nowadays considered to be important, there is concern that, with the level of the residual amount of the organic solvent as controlled from the viewpoint of the quality, a slight amount of the organic solvent is evaporated from the produced film in the post-processes subsequent to the film production. 
     However, as for the residual amount of the organic solvent, investigation has hitherto been performed from the viewpoint of the quality, but no investigation has been performed from the viewpoint of the environment conservation, and hence it is not clear how far the level of the residual amount of the organic solvent should be lowered so that the effects on the environment substantially vanish. 
     Additionally, in order to reduce the residual amount of the organic solvent, the following treatments are suggested: the heating period of time is extended in the evaporation process, the heating temperature is raised, and the amount of the organic solvent is reduced in relation to the amount of cellulose ester. However, in the current solution method for forming film, the production efficiency is improved while the quality of the film being maintained at a high level, and hence a variety of measures are adopted for shortening the time required for the evaporation process as much as possible; these measures include the following measures in which the amount of the organic solvent is decreased to a level as low as possible in relation to the amount of cellulose ester, and the heating temperature is raised to a level below which cellulose ester is not thermally decomposed. Consequently, it is anticipated that not only the quality of the film is degraded, but also the production efficiency is remarkably degraded, owing to extending of the heating time in the evaporation process, raising the heating temperature, and reducing the amount of the organic solvent in relation to the amount of cellulose ester, for the purpose of reducing the residual amount of the organic solvent. 
     SUMMARY OF THE INVENTION 
     The present invention, in view of the above circumstances, takes as its object the provision of a production method of cellulose film which can reduce the residual amount of the organic solvent, without degrading the film quality, and with degrading the production efficiency to the least possible extent; a cellulose film which substantially has little effects on the environment due to the residual organic solvent; and protective film for polarizing plate, optical functional film, polarizing plate, and a liquid crystal display produced by the above described production method of cellulose film. 
     The production method of cellulose film of the present invention, which achieves the above described object, is a production method of cellulose film which method produces cellulose film by preparing a polymer solution through dissolving cellulose ester in a solvent having a prescribed organic solvent as the main component, forming a filmy object from the prepared polymer solution, and evaporating the solvent in the filmy object; and wherein: 
     the polymer solution is prepared by adding a poor solvent, having the highest boiling point among the materials contained in the solvent, so as to have the content of 0.1 to 1.0 wt % where the total amount of the solvent in the prepared polymer solution is taken as 100 wt %; and 
     the solubility of cellulose ester in the poor solvent is inferior to the solubility of cellulose ester in the organic solvent which is the main component of the solvent. 
     The addition amount of the poor solvent is very small, so that the addition of the poor solvent scarcely degrades the production efficiency of the cellulose film. Additionally, since the poor solvent is highest in boiling point among the materials contained in the solvent, it is most difficult to be evaporated and tends to remain. Furthermore, since the solubility of cellulose ester in the poor solvent is inferior to the solubility of cellulose ester in the organic solvent which is the main component of the solvent, the intermolecular bond between the poor solvent and the cellulose ester is difficult to be formed as compared to the intermolecular bond between the main-component organic solvent and the cellulose ester. In the solvent during the evaporation process, the action of the remaining poor solvent prevents the formation of the intermolecular bond between the main-component organic solvent and cellulose ester, and the evaporation of the main-component organic solvent is thereby promoted. Additionally, the remaining poor solvent hardly forms the intermolecular bond with cellulose ester so that cellulose ester is scarcely restrained by cellulose ester, and the addition amount of the poor solvent is very small; hence the poor solvent is evaporated at the end of the evaporation process, and the added poor solvent does not affect the film characteristics. 
     Additionally, in the production method of cellulose film of the present invention, the main-component organic solvent is dichloromethane, and it is preferable that the polymer solution is prepared by adding an alcohol having one to two carbon atoms in addition to the poor solvent. 
     The compatibility of dichloromethane with cellulose ester is satisfactory, and hence adoption of dichloromethane as the main component of the solvent leads to reduction of the total amount of the solvent in relation to the amount of cellulose ester. Additionally, addition of alcohols having 1 to 2 carbon atoms improves the dimensional stability (self-supporting property) of the filmy object, making the transportation of the film-like material be convenient. 
     Furthermore, in the production method of cellulose film of the present invention, taking the total amount of the solvent in the prepared polymer solution to be 100 wt %, it is preferable to prepare the polymer solution in such a way that dichloromethane is added in a content of 70 to 99 wt %, and simultaneously an alcohol having 1 to 2 carbon atoms is added in a content of 0.9 to 29.0 wt %. 
     Additionally, in the production method of cellulose film of the present invention, it is preferable that the poor solvent is an alcohol having the boiling point in the range from 80 to 170° C. 
     The boiling point of dichloromethane, the main solvent component of the solvent, is about 40° C.; accordingly, when the boiling point of the added alcohol is 80° C. or above, the alcohol remains in the solvent during the evaporation process, preventing without fail the intermolecular bonding formation of dichloromethane with cellulose ester. On the other hand, when the boiling point of the added alcohol is chosen to be 170° C. or below, the alcohol can be evaporated in the final stage of the evaporation process without causing the thermal decomposition of cellulose ester. 
     In this connection, in the production method of cellulose film of the present invention, when the mixing of the poor solvent is performed in an in-line mode, a static mixer may be used in the piping for addition and mixing; or 
     at least two or more kinds of polymer solutions maybe subjected to simultaneous flow casting or successive flow casting. 
     Additionally, in the production method of cellulose film of the present invention, it is also preferable that the polymer solution of cellulose ester film has the solid content ranging from 15 to 30 wt %. 
     Additionally, in the production method of cellulose film of the present invention, it is also preferable that the material containing the cellulose acetate synthesized from wood pulp as the main component is used as cellulose ester. 
     As cellulose ester, the cellulose acetate synthesized from cotton linter is known, in addition to the cellulose acetate synthesized from wood pulp; however, adoption of the cellulose acetate synthesized from wood pulp as the main component makes it possible to reduce the costs for cellulose film. 
     Additionally, in the production method of cellulose film of the present invention, it is preferable that the film is made to be swollen and then dried on the way of the drying process thereof, or after drying, during the film formation process by flow casting of the polymer solution of cellulose ester. 
     As above, through swelling once the filmy object, while the solvent being evaporated from the filmy object, or after the solvent has been evaporated, even when the molecules composing the solvent form the intermolecular bond with cellulose ester, the intermolecular bond can be broken; namely, the evaporation of the solvent can be further promoted by swelling once the filmy object and then evaporating the solvent therein again. 
     In this connection, it is preferable that for the purpose of swelling once the film (filmy object), the film may be swollen with water, a solvent may be applied onto the film, or exposure to a solvent gas may be performed. Incidentally, it is preferable that an alcohol-based substance (for example, an alcohol having 1 to 2 carbon atoms, etc.) is used as the solvent to be applied and the solvent gas. 
     The cellulose film of the present invention, which achieves the object of the present invention, is characterized in that, in the form of the finished film product, the residual amount of dichloromethane is 0.1 wt % or less, and additionally the total residual amount of the solvent is 0.5 wt % or less. 
     By controlling the residual amount of dichloromethane, and the total residual amount of the solvent to the values as specified above, the effects on the environment of the residual solvent in the cellulose film having been produced can be substantially prevented. 
     The protective film for polarizing plate, optical functional film, polarizing plate, and liquid crystal displays, which achieve the object of the present invention, are characterized in that each thereof is produced by use of the production method of cellulose film of the present invention, or by use of the cellulose film of the present invention. 
     As above, the present invention can provide the production method of cellulose film in which the residual amount of the organic solvent in the film can be reduced, without degrading the film quality and with degrading the production efficiency to a least possible extent; the cellulose film which gives the substantially vanishing effects of the residual solvent on the environment; and the protective film for polarizing plate, optical functional film, polarizing plate, and a liquid crystal display, all produced by the aforementioned production method of cellulose film. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified schematic view of the production line while the cellulose film is being produced by flow casting of the polymer solution onto a round cylindrical drum; and 
         FIG. 2  is a simplified schematic view of the production line while the cellulose film is being produced by flow casting of the polymer solution onto an endless belt. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Description will be made below on the embodiments of the present invention. 
     In the embodiments of the production method of cellulose film of the present invention, the cellulose film is produced by flow casting of a polymer solution onto a supporting body. The supporting bodies onto which the flow casting of the polymer solution is performed include the following two bodies, a round cylindrical drum and an endless belt. Now, with reference to  FIG. 1 , description is made on the production line wherein a cellulose film is produced by the flow casting of the polymer solution onto the round cylindrical drum. 
       FIG. 1  is a simplified schematic view of the production line while the cellulose film is being produced by flow casting of the polymer solution onto a round cylindrical drum. 
     The production line  1  shown in  FIG. 1  is one embodiment of the production method of cellulose film of the present invention, namely, a production line of TAC (triacetyl cellulose) film; from the upstream side of the production line  1  in order, there are arranged a polymer solution preparation apparatus  100 , a flow casting die  200 , a flow casting drum  300 , and a peeling roll  400 . 
     In the production line  1 , there are arranged three polymer solution preparation apparatuses  100 , each preparing a different type of polymer solution. Here, as an example, description will be made on the polymer solution preparation apparatus  100  shown on the right in  FIG. 1 . The polymer solution preparation apparatus  100  prepares the polymer solution of triacetyl cellulose. In preparation of the polymer solution, triacetyl cellulose is dissolved in a solvent containing dichloromethane as the main component, and a small amount of n-butanol is added to the solution in which triacetyl cellulose is dissolved. n-Butanol is higher in boiling point than dichloromethane. Additionally, the solubility of triacetyl cellulose in n-butanol is inferior to the solubility of triacetyl cellulose in dichloromethane. The polymer solution preparation apparatus  100  comprises a storage tank  110 , a liquid delivery pump  120 , a poor solvent supply device  130 , a static mixer  140 , and a filter  150 . In the storage tank  110 , a mixed solution CM of dichloromethane and methanol wherein triacetyl cellulose is dissolved is stored while being stirred by stirring blades  111 . The solution CM stored in the storage tank  110  is delivered to a flow casting die  200  by the liquid delivery pump  120 ; the static mixer  140  and the filter  150  are arranged in some midway points along the liquid delivery path. Additionally, the poor solvent supply device  130  supplies n-butanol B to the static mixer  140  under favor of a liquid delivery pump  131 . In the static mixer  140 , n-butanol B is added to and mixed with the mixed solution CM. The addition amount of n-butanol is so small that the adding and mixing of n-butanol little degrades the production efficiency of the TAC film. Incidentally, instead of using the poor solvent supply device  130 , n-butanol may be added to and mixed in the storage tank  110 . The filter  150  removes, from the solution delivered from the static mixer  140 , foreign objects, undissolved raw materials, etc., and then the solution is delivered to a flow casting die  200 . 
     In the flow casting die  200 , the polymer solutions prepared respectively in the three polymer solution preparation apparatuses  100  are supplied. More specifically, from the polymer solution preparation apparatus  100  depicted at the right of  FIG. 1 , of the three polymer solution preparation apparatuses, the polymer solution constituting the film surface layer is delivered; from the polymer solution preparation apparatus  100  depicted at the center, the polymer solution constituting the film central part is delivered; and from the polymer solution preparation apparatus  100  depicted on the left, the polymer solution constituting the film back surface layer is delivered. The respective delivered polymer solutions are discharged from the outlet of the flow casting die  200 . Incidentally, the number of the polymer solution preparation apparatuses  100  is not necessarily limited to 3, but it may be one, two, or more than three depending on the specification of the produced cellulose film. 
     The flow casting drum  300  is revolved along the direction of the arrow A. The flow casting die  200  is arranged above the flow casting drum  300  in such a way that the outlet faces onto the circumferential surface of the flow casting drum  300 . 
     The respective polymer solutions discharged from the outlet of the flow casting die  200  are subjected to simultaneous flow casting onto the circumferential surface of the flow casting drum  300  rotating along the direction of the arrow A. The polymer solutions discharged onto the circumferential surface of the flow casting drum  300 , during about three-quarter revolution along the direction of the arrow A, is water-cooled from the inside of the flow casting drum  300  and simultaneously air-cooled from the outside by blasting cooling air, and accordingly the gelation is promoted to form a filmy object having self-supporting property. 
     Subsequently, the filmy object reaches the position, where a peeling roll  400  is installed, to be peeled off. 
     A winding device  500  is arranged at the downstream end of the production line  1  shown in  FIG. 1 . A soft film drying zone  11  and a late stage drying zone  12  are arranged between the peeling roll  400  and the winding roll  500 , both zones being the zones where the solvent in the filmy object is evaporated. Furthermore, a swelling device  600  is arranged between the soft film drying zone  11  and the late stage drying zone  12 . The filmy object peeled off by the peeling roll  400  is delivered by two driving rolls along the direction of the arrow B, via the soft film drying zone  11 →the swelling device  600 →the late stage drying zone  12 , and wound by the winding roll  500 . 
     A tenter  700  is arranged in the soft film drying zone  11 . The filmy object peeled off by the peeling roll  400  is delivered to the soft film drying zone  11 , and passes through the interior of the tenter  700 . Inside the tenter  700 , the filmy object is heated, the solvent contained in the filmy object is further evaporated. The filmy object discharged from the tenter  700  is delivered to the swelling device  600 . A swelling device is a device in which the filmy object delivered thereto is once made to be swollen; the swelling device  600  in  FIG. 1  is a device where the solvent gas composed of ethanol is sprayed onto the filmy object being delivered. Incidentally, instead of ethanol, alcohols such as methanol and water vapor may be sprayed. The filmy object discharged from the swelling device  600  is delivered to the late stage drying zone  12 . Plural rolls  800  are arranged in the late stage drying zone  12 , and the filmy object is delivered while being wrapped around the plural rolls  800 . The filmy object is heated in the upstream section  12   a  of the late stage drying device  12 , and the solvent remaining in the filmy object is evaporated. 
     As for the n-butanol added to and mixed in the polymer solution preparation apparatus  100 , it is most resistant to evaporation and tends to remain in the solvent, since n-butanol is highest in boiling point among the materials contained in the solvent. Additionally, since the solubility of triacetyl cellulose in n-butanol is inferior to the solubility of triacetyl cellulose in dichloromethane, it is more difficult to form the intermolecular bond of n-butanol with triacetyl cellulose than the intermolecular bond of dichloromethane with triacetyl cellulose. In the solvent having been added and mixed with n-butanol, the action of n-butanol breaks the intermolecular bond between dichloromethane and triacetyl cellulose, promoting the evaporation of dichloromethane. Additionally, since the remaining n-butanol does not tend to form intermolecular bond with triacetyl cellulose, it does not tend to be engaged to triacetyl cellulose, and its addition amount is small, it is evaporated by the time when the filmy object has passed the upstream section  12   a  of the late stage drying zone  12 , so that the added n-butanol does not affect adversely the characteristics of the film. Furthermore, since in the production line  1 , the solvent in the filmy object is once evaporated in the soft film drying zone  11  and is subsequently swollen by the swelling device  600 , even the intermolecular bond between dichloromethane and triacetyl cellulose which remains unbroken by n-butanol can be broken. Then, in the upstream section  12   a  of the late stage drying zone  12 , the solvents remaining in the filmy object, such as the dichloromethane broken out of triacetyl cellulose by the swelling action, is evaporated. Consequently, in the production line  1 , the evaporation of the solvent in the filmy object is promoted, and the remaining amount of dichloromethane in the filmy object having passed the delivery zone  12  can be reduced to be 0.1 wt % or less, and simultaneously the total residual amount of the solvent is also reduced to be 0.5 wt % or less. 
     In the downstream section  12   b  of the late stage drying zone  12 , the filmy object is cooled down to room temperature, and the filmy object (TAC film) takes the form of the finished TAC film product. The filmy object (TAC film) discharged from the late stage drying zone  12  is wound by the winding device  500 . The TAC film thus produced is subsequently delivered to the subsequent processes, unshown in the figure, and is commercialized as the optical functional films such as protective film for polarizing plate and anti-glare film. Additionally, polarizing plate is formed by attaching the protective film for polarizing plate onto both sides of a polarization element made of polyvinyl alcohol etc.; and a part of a liquid crystal display is made by using the polarizing plate. 
     Now, with reference to  FIG. 2 , description will be made below on the production line wherein cellulose film is produced by flow casting of the polymer solution onto an endless belt. 
       FIG. 2  is a simplified schematic view of the production line while the cellulose film is being produced by flow casting of the polymer solution onto an endless belt. 
     The production line  2  shown in  FIG. 2  is the TAC (triacetyl cellulose) film production line which is an embodiment of the production method of cellulose film of the present invention, as the production line  1  shown in  FIG. 1 , wherein a flow casting band  900  is arranged instead of the flow casting drum  300  arranged in the production line  1  shown in  FIG. 1 . With the same reference numerals for the same constituent elements as those of the production line  1  in  FIG. 1 , description is made below with a focus on the points different from those in the production line  1  shown in  FIG. 1 . 
     In the production line  2  shown in  FIG. 2 , the three same polymer solution preparation apparatuses  100  as the three polymer solution preparation apparatuses shown in  FIG. 1 , and three flow casting dies  200  are arranged. The three polymer solution preparation apparatuses  100  are respectively connected to the three flow casting dies  200  in a one-to-one relation. Additionally, the three flow casting dies  200 , flow casting band  900 , and peeling roll  400  are arranged in a drying chamber  10 . 
     The flow casting band  900  is formed by wrapping an endless belt  930  around a driving drum  910  and a driven drum  920 . The belt  930  displaces circularly along the direction of the arrow C in the drying chamber  10 . The three flow casting dies  200  are arranged along the running direction of the belt  930  and above the belt  930 , with the die outlets facing onto the surface of the belt  930 . 
     The polymer solutions delivered to the respective flow casting dies  200  are successively subjected to flow casting onto the surface of the belt  930  circularly running along the direction of the arrow C, the solvent is gradually evaporated while the belt  930  is circularly running in the drying chamber  10 , and becomes a film to yield the self-supporting property. Namely, the evaporation of the solvent leads to a filmy object having shape stability. After the belt  930  has finished about one round, the filmy object is peeled off by the peeling roll  400 , and delivered to the soft film drying zone  11 . 
     In the soft film delivery zone  11  of the production line  2  shown in  FIG. 2 , plural rolls are arranged; the filmy object going into the soft film drying zone  11  is delivered along the direction of the arrow D, by being guided by the plural rolls. The swelling device  600  is arranged in a midway position in the soft film delivery zone  11 . Incidentally, the swelling device  600  may be arranged in a midway position in the upstream section  12   a  of the late stage drying zone  12 . The swelling device  600  shown in  FIG. 2  is different from the swelling device shown in  FIG. 1  in that the filmy object being delivered is water soaked and rinsed with water. Incidentally, the application of an alcohol such as ethanol may replace the watersoaking. Both in the upstream section and in the downstream section of the swelling device  600  of the soft film drying zone  11 , dry air is blasted onto the filmy object being delivered, resulting in evaporation of the solvent in the filmy object. In the upstream section  12   a  of the late stage drying zone  12 , next to the soft film delivery zone  11 , the filmy object is heated, and the solvent remaining in the filmy object is evaporated. Additionally, in the downstream section  12   b  of the late stage drying zone  12 , the filmy object is cooled down nearly to room temperature, to take a form of the finished TAC film product. The filmy object (TAC film) discharged from the late stage drying zone  12  is wound by the winding device  500 . 
     Now, detailed description is made below on the preparation of the polymer solution. In the polymer solution preparation performed in the polymer solution preparation apparatuses  100  as shown in  FIGS. 1 and 2 , at the beginning, triacetyl cellulose grains are dissolved in an organic solvent having dichloromethane as the main component, in the storage tank  110 . The triacetyl cellulose is a mixture of those synthesized from wood pulp and cotton linter, wherein the content of that synthesized from wood pulp is 60 wt % and the rest of 40 wt % is allotted to that synthesized from cotton linter. As above, making that synthesized from wood pulp be the main component can reduce the cost for the TAC film. Incidentally, that synthesized from cotton linter may be completely excluded to make the whole comprise only that synthesized from wood pulp. 
     The compatibility between the dichloromethane and triacetyl cellulose is satisfactory, and hence adopting dichloromethane as the main component of the organic solvent leads to the reduction of the total amount of the solvent in relation to the amount of triacetyl cellulose. Additionally, the organic solvent in the storage tank  110  contains methanol as a component of the mixed solvent. The addition of methanol leads to the improvement of the shape stability (self-supporting property) of the filmy object peeled off by the peeling roll  400 , and the easiness in transporting the filmy object. The composition ratio between the dichloromethane and methanol is so adjusted in the storage tank  110  that dichloromethane is contained in the content of from 70 wt % to 99 wt %, and methanol is contained in the content of from 0.9 wt % to 29.0 wt %, taking the total amount of the solvent in the polymer solution prepared in the polymer solution preparation apparatus 100 to be 100 wt %. Incidentally, ethanol may replace methanol, or water may be added with modified composition ratio of methanol. Furthermore, in the organic solvent in the storage tank  110 , a plasticizer, an ultra violet light absorber, an anti-deterioration agent, etc. are dissolved as additives. In the storage tank  110 , the solid content such as triacetyl cellulose and the additives is adjusted so as to be from 15 to 30 wt %, taking the amount of the polymer solution prepared in the polymer solution preparation apparatus 100 to be 100 wt %. n-Butanol, a poor solvent, is so added that the content thereof falls in the range from 0.1 wt % to 1.0 wt %, taking the amount of the polymer solution prepared in the polymer solution preparation apparatus 100 to be 100 wt %. Incidentally, as a poor solvent, any alcohol having the boiling point in the range from 80 to 1700, other than n-butanol, may be used. The boiling point of dichloromethane is about 40° C.; accordingly, when the boiling point of the poor solvent is 80° C. or higher, the poor solvent remains in the solvent during evaporation of the solvent, and the intermolecular bonding of dichloromethane to triacetyly cellulose is prevented without fail. On the other hand, when the boiling point of the poor solvent is 170° C. or lower, the poor solvent can be evaporated without thermally decomposing triacetyl cellulose. 
     As a result of the preparation described above, the solvent of the polymer solution delivered to the flow casting die  200  is composed of dichloromethane and n-butanol. Additionally, the composition ratios thereof are such that the content of dichloromethane ranges from 70 wt % to 99 wt %, the content of methanol ranges from 0.9 wt % to 29.0 wt %, and the content of n-butanol ranges from 0.1 wt % to 1.0 wt %, talking the total amount of the solvent to be 100 wt %. 
     Incidentally, until this point, description has been made on the production method of TAC film using the polymer solution in which triacetyl cellulose is dissolved in the solvent containing dichloromethane as the main component; however, in the production method of cellulose film of the present invention, the main solvent component may be an organic solvent such as lower fatty alcohols, and a chloride of a lower fatty hydrocarbon other than dichloromethane. Additionally, the solute may be a cellulose ester other than triacetyl cellulose. Furthermore, the added poor solvent is not limited to n-butanol, but it may be any solvent which is highest in boiling point among the materials contained in the solvent of the prepared polymer solution, and is inferior in the solubility of cellulose ester to the organic solvent which is the main component of the solvent. 
     EXAMPLES 
     Description will be made below on the TAC film production by applying the production method of cellulose film of the present invention, and the performed measurement of the residual amounts of the organic solvents, together with the comparative examples. 
     At the beginning, example 1 produced the TAC film by using the production line  1  shown in  FIG. 1 . In the preparation of the polymer solution, the triacetyl cellulose synthesized from cotton linter was not mixed; the triacetyl cellulose synthesized from wood pulp (20 parts by weight), a plasticizer (2.2 parts by weight), and an ultraviolet light absorber (0.02 parts by weight) were used; and the solvent was prepared so as to give the composition ratios specified below, for which the polymer solution prepared by the polymer solution preparation apparatus  100  was taken to be 100 wt %. Additionally, in the swelling device  600 , a solvent gas composed of nitrogen gas and added methanol (methanol:nitrogen=2:8) was sprayed onto the filmy object discharged from the soft film drying zone  11 , thereby swelling once the filmy object. 
     Example 1 
     
         
         Dichloromethane: 79.6 wt % 
         Methanol: 19.9 wt % 
         n-Butanol: 0.5 wt % 
       
    
     Additionally, in examples 2 to 4, the TAC films were produced under the same conditions as those in example 1, except that the conditions under which the filmy object discharged from the soft film drying zone  11  was once swollen, was changed to each condition specified below. In other words, the composition ratios of the polymer solutions were the same as those in example 1. 
     Example 2 
     Application of a Solvent (Methanol:Water=1:1) in 0.5 cc/m 2 . 
     Example 3 
     Spray of Water Vapor at 120° C. 
     Example 4 
     Rinsing with Water by Watersoaking. 
     Furthermore, in example 5, the TAC film was produced under the same conditions (the composition ratios of the polymer solution, etc.) as those in example 1, except that the filmy object discharged from the soft film drying zone  11  was not once swollen. 
     Example 5 
     Between the soft film drying zone  11  and the late stage drying zone  12  shown in  FIG. 1 , the filmy object was not once swollen, and the filmy object discharged from the soft film drying zone  11  as delivered to the late stage drying zone  12 , thereby performing the continuous drying. 
     Additionally, furthermore, in respective examples 6 and 7 and comparative examples 1 and 2, the TAC films were produced under the same conditions as those in example 5, except that the solvent composition ratios of the polymer solution were changed as the respective conditions specified below. In other words, in the same manner as that in Example 5, the filmy object discharged from the soft film drying zone  11  was not once swollen. 
     Example 6 
     
         
         Dichloromethane: 99.0 wt % 
         Methanol: 0.9 wt % 
         n-Butanol: 0.1 wt % 
       
    
     Example 7 
     
         
         Dichloromethane: 70.0 wt % 
         Methanol: 29.0 wt % 
         n-Butanol: 1.0 wt % 
       
    
     Comparative Example 1 
     
         
         Dichloromethane: 79.6 wt % 
         Methanol: 20.31 wt % 
         n-Butanol: 0.09 wt % 
       
    
     Comparative Example 2 
     
         
         Dichloromethane: 79.6 wt % 
         Methanol: 19.29 wt % 
         n-Butanol: 1.01 wt % 
       
    
     On the TAC films produced in respective examples 1 to 7 and comparative examples 1 and 2, described above, the total residual amount of the organic solvent, the residual amount of dichloromethane, and the residual amount of n-butanol were respectively measured by gas chromatography, and the results as shown in Table 1 were obtained. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Total 
                 Residual amount 
                 Residual 
               
               
                   
                 residual 
                 of 
                 amount 
               
               
                   
                 amount 
                 dichloromethane 
                 of n-butanol 
               
               
                   
                 (wt %) 
                 (wt %) 
                 (wt %) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 1 
                 0.34 
                 0.03 
                 0.31 
               
               
                 [0.5 Wt %, Solvent gas] 
               
               
                 Example 2 
                 0.34 
                 0.03 
                 0.31 
               
               
                 [0.5 Wt %, Solvent 
               
               
                 application] 
               
               
                 Example 3 
                 0.35 
                 0.04 
                 0.31 
               
               
                 [0.5 Wt %, Water vapor] 
               
               
                 Example 4 
                 0.35 
                 0.04 
                 0.31 
               
               
                 [0.5 Wt %, Watersoaking] 
               
               
                 Example 5 
                 0.37 
                 0.05 
                 0.32 
               
               
                 [0.5 Wt %, No swelling] 
               
               
                 Example 6 
                 0.18 
                 0.09 
                 0.09 
               
               
                 [0.1 Wt %, No swelling] 
               
               
                 Example 7 
                 0.48 
                 0.04 
                 0.44 
               
               
                 [1.0 Wt %, No swelling] 
               
               
                 Comparative Example 1 
                 0.45 
                 0.11 
                 0.34 
               
               
                 [0.09 Wt %, No swelling] 
               
               
                 Comparative Example 2 
                 0.51 
                 0.03 
                 0.48 
               
               
                 [1.01 Wt %, No swelling] 
               
               
                   
               
             
          
         
       
     
     Table 1 shows the total residual amount (wt %) of the organic solvent, the residual amount of dichloromethane (wt %), and the residual amount of n-butanol (wt %), in a single horizontal row, for each example or each comparative example. These three residual amounts are the residual amounts in the TAC film immediately after having been discharged from the late stage drying zone  12 . 
     The present inventors discovered, as a result of diligent research, that in order to substantially reduce the effects on the environment ascribable to the solvent remaining in the TAC film after production, in the form of the finished TAC film product, the residual amount of dichloromethane is required to be 0.1 wt % or less, and additionally the total residual amount of the organic solvent is required to be 0.5 wt % or less. From the results shown in Table 1, for the TAC film produced in any of examples 1 to 7, the residual amount of dichloromethane is 0.1 wt % or less, and additionally the total residual amount of the organic solvent is 0.5 wt % or less. Accordingly, in the TAC film produced in any of examples, the effects of the residual solvent on the environment can substantially be suppressed. However, in the TAC film produced in comparative example 1, wherein the content of n-butanol is 0.09 wt %, the total residual amount of the organic solvent is 0.5 wt % or less, but the residual amount of dichloromethane takes a slightly higher value of 0.11 wt %. On the contrary to comparative example 1, in the TAC film produced in comparative example 2, wherein the content of n-butanol is 1.01 wt %, the residual amount of dichloromethane is 0.1 wt % or less, but the total residual amount of the organic solvent takes a slightly higher value of 0.51 wt %. As can be seen from these results, in order to produce the cellulose film which substantially vanishes the effects of the residual solvent on the environment, n-butanol has only to be added in the content range from 0.10 wt % to 1.00 wt % in the preparation process of the polymer solution, taking the total amount of the solvent in the prepared polymer solution to be 100 wt %. Turning to a comparison of example 1 with example 5, both examples being the same in the addition amount of n-butanol, the residual amount of dichloromethane remaining in the TAC film is larger in example 5 than in example 1. Such a matter is also the case in comparison of any example of examples 2 to 4 with example 5. As can be seen from these results, by swelling once the filmy object between the soft film drying zone  11  and the late stage drying zone  12 , the evaporation of dichloromethane in the late stage drying zone  12  is promoted. A comparison of examples 1 and 2 with examples 3 and 4 indicates that the evaporation of dichloromethane is more promoted by swelling the filmy object with a solvent than with water. Incidentally, the peeling off operation with the peeling roll was able to be more rapidly performed in the production of the TAC film in any example than in the production of the TAC film in any comparative example.