Patent Publication Number: US-2009232982-A1

Title: Porous film production method and apparatus

Description:
FIELD OF THE INVENTION 
     The present invention relates to a production method of a porous film having a plurality of fine pores and a production apparatus of the same. 
     BACKGROUND OF THE INVENTION 
     In recent years, increase in integration degree, higher information density, and higher image definition have been desired more and more in fields of optical materials and electronic materials. Therefore, the film used in these fields is strongly required to have a more fine structure. As a film having the fine structure, there is a film having a honeycomb structure in which plural fine pores are formed on a surface of the film. 
     A method for producing a film having the honeycomb is disclosed in Japanese Patent Laid-Open Publications No. 2006-070254 and No. 2007-291367, for example. Each of these methods is a method for producing a film having a μm-scale honeycomb structure. In these methods, a polymer solution containing a predetermined polymer and a hydrophobic solvent is applied to a support to form a coating film. Air with high humidity is blown toward the coating film to cause condensation on the coating film. Water drops generated due to the condensation are evaporated from the coating film. 
     In the production methods disclosed in Japanese Patent Laid-Open Publications No. 2006-070254 and No. 2007-291367, the water drops generated due to the condensation make pores on the coating film. For the purpose of preventing evaporation of the water drops from the surface of the coating film, the hydrophobic solvent is evaporated from the coating film under a predetermined condition. In accordance the evaporation of the hydrophobic solvent, the coating film is hardened. Thereby, the form and size of each of the pores are not changed, and the pores are not irregularly arranged. The predetermined condition is as follows. In the method disclosed in Japanese Patent Laid-Open Publication No. 2006-070254, the surface temperature of the coating film is regulated to be equal to or less than the dew point of the gas around the coating film. In the method disclosed in Japanese Patent Laid-Open Publication No. 2007-291367, the surface temperature of the coating film is regulated to be less than the dew point of the gas around the coating film by a predetermined range. Next, under the condition that the surface temperature of the hardened coating film is higher than the dew point of the gas around the coating film by a predetermined range, the water drops are evaporated from the hardened coating film to form a μm-scale honeycomb structure on the coating film. 
     In the production methods disclosed in the above patent documents, it is necessary to adjust the surface temperature of the coating film and the dew point of the gas around the coating film within a predetermined range, respectively. However, the adjustment of the surface temperature of the coating film and the dew point of the gas is difficult. In particular, it is extremely difficult to adjust the dew point of the gas. 
     Additionally, it takes too much time for the hydrophobic solvent to evaporate from the coating film, and therefore it is impossible to shorten the time required for the production of the porous film as desired. In order to shorten the time required for the production of the porous film, it is effective to evaporate the hydrophobic solvent from the coating film rapidly by increasing the surface temperature of the coating film. However, when the surface temperature of the coating film is increased, there arises a problem in which the water droplets generated on the coating film are evaporated from the coating film prior to the evaporation of the hydrophobic solvent, and thereby resulting in a film having pores irregularly arranged with a nonuniform form and size. 
     SUMMARY OF THE INVENTION 
     In view of the above, an object of the present invention is to provide a porous film production method and apparatus capable of evaporating a hydrophobic solvent from a coating film rapidly and producing a porous film having a desired honeycomb structure provided with pores regularly arranged with a uniform form and size. 
     In order to achieve the above and other objects, a porous film production method of the present invention includes the following steps. A coating liquid containing a polymer and a hydrophobic solvent is applied to a support to form a coating film. Water vapor is condensed from ambient air on a surface of the coating film to form water drops. The hydrophobic solvent is evaporated until a content rate of the hydrophobic solvent in the coating film reaches 50 wt %. The coating film on the support is caused to contact with a liquid water after the first evaporating step defined as evaporating the hydrophobic solvent until the content rate of the hydrophobic solvent in the coating film reaches 50 wt %. The water, the hydrophobic solvent, and the water drops are evaporated from the coating film to form a porous film after the water contacting step defined as causing the coating film on the support to contact with the liquid water. 
     According to the porous film production method of the present invention, the following condition is preferably satisfied: 
         Tw−Tb&lt; 20° C. 
     where a boiling point of the hydrophobic solvent is denoted by Tb, and a temperature of the water to be caused to contact with the coating film in the water contacting step is denoted by Tw. Preferably, the coating film on the support is caused to contact with an alcohol having a boiling point lower than that of the water between the water contacting step and the second evaporating step defined as evaporating the water, the hydrophobic solvent, and the water drops from the coating film. 
     A porous film production apparatus of the present invention includes a coating device for applying a coating liquid containing a polymer and a hydrophobic solvent to a support to form a coating film, a condensation device for condensing water vapor from ambient air on a surface of the coating film to form water drops, a water contacting device for causing the coating film to contact with a liquid water, a first evaporating device for evaporating the hydrophobic solvent from the coating film before being caused to contact with the water by the water contacting device, and a second evaporating device for evaporating the water droplets generated due to the condensation, the water contacting with the coating film by the water contacting device, and the hydrophobic solvent from the coating film. 
     According to the porous film production method of the present invention, it is possible to shorten the time require for evaporation of the hydrophobic solvent from the coating film in the process for producing a porous film having a honeycomb structure. Therefore, it is possible to shorten the time required for the production of the porous film in comparison with conventional production methods. Accordingly, it is possible to produce a porous film having desired pores regularly arranged with a uniform form and size. 
    
    
     
       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. 1A  is a plane view of a porous film of the present invention,  FIG. 1B  is a cross sectional view taken along lines b-b of  FIG. 1A ,  FIG. 1C  is a cross sectional view taken along lines c-c of  FIG. 1A ; 
         FIG. 2  is an explanation view illustrating formation of the porous film of the present invention; 
         FIG. 3  is a schematic view illustrating a porous film production apparatus according to a first embodiment of the present invention; 
         FIG. 4  is a schematic view illustrating a tank according to a second embodiment of the present invention; 
         FIG. 5  is a table showing conditions and evaluation results in Example 1; and 
         FIG. 6  is a table showing conditions and evaluation results in Example 2. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention are described in detail. However, the present invention is not limited thereto. 
     As shown in  FIG. 1 , a porous film  29  has a surface on which a plurality of pores  30  are densely formed. Since the plurality of pores  30  are regularly arranged with an approximately uniform form and size, the porous film  29  has a honeycomb structure. 
     The pores  30  adjacent to each other are aligned in the porous film  29  so as to form a communicating path along the surface of the porous film  29 . In some cases, each of the pores  30  penetrates through both surfaces of the porous film  29 , that is, penetrates through the porous film  29  in its thickness direction, and in other cases, the pores  30  are formed as grooves on one of the surfaces of the porous film  29  without penetrating the porous film  29 . 
     According to this embodiment, a polymer is dissolved into a hydrophobic solvent to prepare a coating liquid  24 . The coating liquid  24  is applied to a support to form a coating film. The coating film becomes the porous film  29 . Accordingly, in a case where the pores  30  are formed so as to penetrate the porous film  29  in its thickness direction, when the porous film  29  on the support is viewed from a normal direction of the surface of the porous film  29 , the support appears to be exposed through the pores  30 . 
     In order to produce a long porous film, the support may be a film formed of a well-known polymer, such as a polymer film formed of polyethylene terephthalate (PET), for example. Further, in order to produce a sheet of porous film or a strip of porous film, the support may be not only a film formed of a polymer but also glass. 
     The coating liquid  24  preferably contains a polymer and an amphiphilic compound. The amphiphilic compound means a compound having both of a hydrophilic property and a lipophilic (hydrophobic) property, and concretely a compound having a hydrophilic group and a hydrophobic group. If the polymer has the amphiphilic property, it is not necessary to use other amphiphilic compounds with the polymer. However, if the polymer to be used for producing the porous film is regarded to have no amphiphilic property, it is preferable to use the amphiphilic compound with the polymer. 
     The sort of the polymer as a main component for the coating film may be chosen in accordance with intended use of the coating film. However, the number average molecular weight (Mn) of the polymer is preferably in the range of 10,000 to 10,000,000, and more preferably in the range of 50,000 to 1,000,000. 
     The polymer to be used with the amphiliphic compound is preferably dissolved into a nonaqueous solvent, namely, a hydrophobic solvent. For example, there are poly-ε-caprolactone, poly-3-hydroxybutyrate, agarose, poly-2-hydroxyethyl acrylate, polysulfone, and the like. In view of necessity of biodegradative properties, the cost, and the availability, poly-ε-caprolactone is particularly preferable. 
     Here, the hydrophobic property means low affinity for water. Concretely, when the degree of solubility of hydrophobic solvent to water is at most 5 wt %, the affinity for water is regarded as low. 
     As other examples of the polymer to be used with the amphiphilic compound, there are vinyl-type polymer (for example, polyethylene, polypropylene, polystyrene, polyacrylate, polymethacrylate, polyacrylamide, polymethacrylamide, polyvinyl chloride, polyvinylidene chloride, polyvinylidene fluoride, polyhexafluoropropene, polyvinyl ether, polyvinyl carbazol, polyvinyl acetate, polytetrafluoroethylene, and the like), polyesters (for example, polyethylene terephthalate, polyethylene naphthalate, polyethylene succinate, polybutylene succinate, polylactic acid, and the like), polylactone (for example, polycaprolactone and the like), polyamide or polyimide (for example, nylon, polyamic acid, and the like), polyurethane, polyurea, polycarbonate, polyaromatics, polysulfone, polyethersulfone, polysiloxane derivative, and the like. These may be used in the form of homo polymer, and otherwise used as copolymer, polymer blend, or polymer alloy, in view of solubility, optical physical properties, electric physical properties, film strength, elasticity, and the like. 
     As the polymer having the amphiphilic property, there is polyacrylamide, for example. As the other polymer having the amphiphilic property, there are a compound which has a main chain of polyacrylamide, a lipophilic side chain of dodecyl group, and hydrophilic side chain of carboxyl group, block copolymer of polyethylene glycol/polypropylene glycol, and the like. The lipophilic side chain is group which has nonpolar normal (linear) chain such as alkylene group, phenylene group, and the like, and preferably has a structure in which hydrophilic group such as polar group or ionic dissociative group is not divided until the end of the chain, except linking group such as ester group and amide group. The lipophilic side chain preferably has at least five methylene units if it is composed of alkylene group. The hydrophilic side chain preferably has a structure having hydrophilic part such as polar group, ionic dissociative group, or oxyethylene group on the end through a linking part such as alkylene group. 
     The amphiphilic compound to be mixed with the polymer is not only a monomer such as many sorts of commercially available surfactants but also an oligomer such as a dimmer and a trimer, and a polymer. When the amphiphilic compound and the polymer are mixed, water droplets are more easily formed on an exposed surface of the coating film. Further, when the dispersion condition of the polymer is controlled, the positions for forming the water droplets are more easily adjusted. When the polymer and the amphiphilic compound are mixed to be used, a weight ratio of the amphiphilic compound to the polymer is preferably in the range of 0.1% to 20%. Thus, the sizes of the formed water droplets tend to be uniform, and it becomes easy to obtain a porous film having the uniform pores. If the weight ratio of the amphiphilic compound to the polymer is less than 0.1%, the effect of adding the amphiphilic compounds is too low, and therefore the water droplets are unstably formed. Thus, the water droplets become nonuniform in size in some cases. In contrast, if the weight ratio of the amphiphilic compound to the polymer is more than 20%, the strength of the porous film becomes lower in some cases since the amphiphilic compound has low-molecular weight. 
     In the amphiphilic compound to be mixed with the polymer, a ratio of the number of hydrophilic group to the number of hydrophobic group is preferably in the range of 0.1/9.9 to 4.5/5.5. Thus, the more fine water droplets are formed on the coating film more densely. If the ratio of the number of hydrophilic group to the number of hydrophobic group doesn&#39;t satisfy the above range, the size of the pores becomes various, and concretely, a variation coefficient (unit: %) of the porous diameter that is determined as {(standard deviation of porous diameter)/(average of porous diameter)}×100 becomes 10% or more in some cases. Further, otherwise, it sometimes becomes hard to arrange the pores regularly. 
     Preferably, at least two sorts of the amphiphilic compounds different form each other are used. Thus, it becomes possible to control the position and the size of the water droplets. In addition, preferably, at least two sorts of the polymers are used, such that the same effects as described above can be achieved. 
     The sort of the solvent to be used in the coating liquid  24  is not restricted, so far as it is hydrophobic and the polymer can be dissolved into it. For example, there are chloroform, dichloromethane, tetrachloromethane, cyclohexane, methyl acetate, and the like. The solvent may contain at least two different sorts of the solvent components, and the mixture ratio thereof may be changed adequately. 
     Preferably, in the coating liquid  24 , the content of the polymer is in the range of 0.02 pts.wt. to 20 pts.wt., if the content of the solvent is determined as 100 pts.wt. 
     A porous film production process includes a coating process, a condensation process, and a drying process. In the coating process, the coating liquid  24  is applied to the support  22  to form the coating film  25 . In the condensation process, water vapor is condensed from ambient air on a surface of the coating film  25  to generate water droplets thereon. In the drying process, the coating film  25  is dried to form a porous film  29  having the plurality of pores  30 . As shown in  FIGS. 2A and 2B , in the condensation process after the coating process, moisture  19  is condensed from ambient air on a surface of the coating film  25  on the support  22  to form water drops  20 . Further, as shown in  FIG. 2C , in the drying process, solvents  27  are evaporated from the coating film  25 . In accordance with the evaporation of the solvents  27 , as shown in  FIG. 2D , the water droplets  20  enter the coating film  25 . The drying process includes a first evaporation process, a water contacting process, and a second evaporation process. The first evaporation process is performed for the purpose of evaporating the solvents  27  from the coating film  25  until the content rate of the solvents  27  in the coating film  25  reaches a predetermined level. The water contacting process is performed for the purpose of causing the coating film  25 , which has been subjected to the first evaporation process, to contact with water. The second evaporation process is performed for the purpose of evaporating the water and the solvents  27  from the coating film  25  after the water contacting process. The first evaporation process is performed such that the water droplets  20  enter the coating film  25  so as to have a honeycomb structure. Both of the water droplets  20  and the water adhered to the coating film  25  during the water contacting process are evaporated in the second evaporation process. Upon evaporation of the water and the solvents  27 , areas occupied by the water droplets  20  become the pores  30 . Thereby, the honeycomb structure can be obtained. 
     As shown in  FIG. 3 , according to a first embodiment, a porous film production apparatus  12  includes a coating chamber  13 . The coating chamber  13  is partitioned into a first area  14  for performing the coating process and the condensation process, a second area  15  for performing the first evaporation process, a third area  16  for performing the water contacting process, and a fourth area  21  for performing the second evaporation process. That is, the drying process is performed from the second area  15  to the fourth area  21 . Although the coating chamber  13  is an integrated chamber partitioned into the above-described areas in this embodiment, each of the areas may be formed of an independent chamber so as to constitute the porous film production apparatus by plural chambers. Note that an arrow A in  FIG. 3  shows a direction in which the support  22  moves. 
     A plurality of rollers  23  are disposed in the first area  14 , the second area  15 , the third area  16 , and the fourth area  21 . A peripheral surface of each of the rollers  23  supports the support  22 . The rollers  23  include drive rollers. The drive rollers rotate to convey the support  22 . The first area  14  includes a discharge die  26  for discharging the coating liquid  24  onto the support  22 , and an air feeding/sucking unit  31  disposed above a conveying path for the support  22 . The second area  15  includes air feeding/sucking units  32  and  33 . The third area  16  includes a water discharge die  35  for discharging water  34  onto the coating film  25 , and an alcohol discharge die  36  for discharging alcohol  37  onto the coating film  25 . The coating film  25  starts to contact with the water  34  at a contact start position P 1 . The fourth area  21  includes air feeding/sucking units  41  to  43 . Each of the air feeding/sucking units  41  to  43  may be two or more and arranged in line in a moving direction of the support  22 . The solvent vapor  27  in the coating chamber  13  is recovered by a not-shown recovery device, and then refined by a not-shown refining device provided outside the coating chamber  13  to be reused. Note that the structures of air feeding/sucking units  31  to  33  and  41  to  43  are approximately identical to each other. 
     In the first area  14 , the coating liquid  24  is discharged through the discharge die  26  onto the moving long support  22  to be the coating film  25 . The coating liquid  24  is preferably applied to the support  22  such that the thickness of the coating film  25  before being dried is constant within the range of 0.01 mm to 1 mm. Even when the thickness of the coating film  25  is within the range of 0.01 mm to 1 mm, variation in thickness of the coating film  25  makes it impossible to form the water droplets having a uniform diameter in some cases. When the thickness of the coating film  25  is less than 0.01 mm, the coating film  25  may have a problem such as thickness unevenness and pores irregularly arranged with nonuniform size and form in some cases. In contrast, when the thickness of the coating film  25  exceeds 1 mm, it takes too much time to dry the coating film  25 , and therefore the production efficiency may be decreased in some cases. 
     The air feeding/sucking unit  31  has an outlet  31   a  for feeding wet air at the vicinity of the coating film  25  and an inlet  31   b  for sucking gas around the coating film  25 . Further, the air feeding/sucking unit  31  is provided with a blowing controller (not shown) for independently controlling a temperature, a dew point, a humidity, and a wind speed of the air to be fed, and a suction force for sucking the air. The outlet  31   a  has a filter for keeping a dust level, namely a cleaning level of the wet air. A plurality of the air feeding/sucking unit  31  may be arranged in the moving direction of the support  22 . 
     The dew point of the air fed from the outlet  31   a  is denoted by TD, and a surface temperature of the coating film  25  is denoted by TS. A value obtained by subtracting TS from TD is denoted by AT. In this case, at least one of the values TD and TS is preferably controlled, such that a following condition is satisfied: 3° C.≦ΔT≦30° C. The surface temperature TS of the coating film  25  can be measured by, for example, a non-contact thermometer (such as an infrared thermometer commercially available) that is disposed near a conveying path of the coating film  25 . When the value AT is less than 3° C., the water droplets due to the condensation are hardly generated. In contrast, when the value ΔT is more than 30° C., the water droplets are generated suddenly. In this case, the water droplets become nonuniform in size, and otherwise, the water droplets, which should be arranged in two dimensional arrangement (in a matrix manner), are arranged in three dimensional arrangement in which one of the water droplets overlaps on the other one. 
     In the first area  14 , the surface temperature TS of the coating film  25  is controlled by the support  22  contacting with the rollers  23  and a temperature controlling plate (not shown) disposed so as to face the support  22 . However, the surface temperature TS may be controlled by one of the support  22  and the temperature controlling plate. Furthermore, the dew point TD is controlled by adjusting the humidification conditions of the wet air fed from the outlet  31   a  of the air feeding/sucking unit  31 . 
     In the second area  15 , the air feeding/sucking units  32  and  33  are arranged in this order from the upstream side in the conveying path of the coating film  25 . The air feeding/sucking unit  32  is disposed closely next to and downstream from the air feeding/sucking unit  31  of the first area  14 . 
     In the second area  15 , at least one of the values TS and TD is preferably controlled, such that a following condition is satisfied: 0° C.&lt;ΔT≦10° C. The surface temperature TS of the coating film  25  is controlled mainly by a temperature controlling plate (not shown) disposed at the vicinity of the coating film  25 . The temperature controlling plate has a structure basically equivalent to that disposed in the first area  14 , and can change the surface temperature TS along the moving direction of the support  22 . Furthermore, the dew point TD is controlled by adjusting the humidification conditions of the wet air fed from the outlet of the air feeding/sucking unit. 
     It is preferable that the coating film  25  is caused to contact with the water  34  in a liquid state in the third area  16  after the water droplets  20  enter the coating film  25  so as to have a honeycomb structure as shown in  FIG. 2D . Namely, the timing for causing the coating film  25  to contact with the water  34  is preferably determined with reference to the depth of the water droplets  20  entering the coating film  25 . However, it may be difficult to measure the depth of the water droplets  20  entering the coating film  25  promptly in some cases. In this case, the timing for causing the coating film  25  to contact with the water  34  in the third area  16  is preferably determined based on the content rate of the solvent in the coating film  25 . This is because there is a certain relation between the content rate of the solvent in the coating film  25  and the depth of the water droplets  20  entering the coating film  25  in a case where the above process is performed in the first area  14  and the second area  15 . 
     The solvents  27  are evaporated in the second area  15  such that the content rate of the solvent in the coating film  25  reaches 50 wt % at the time when the support  22  reaches the contact start position P 1 . When the content rate of the solvent in the coating film  25  reaches 50 wt %, the depth of thewater droplets  20  entering the coating film  25  is enough to have the honeycomb structure. When the content rate of the solvent in the coating film  25  reaches 50 wt % at the contact start position P 1 , the content rate of the solvent in the coating film  25  is at most 50 wt % at the contact start position P 1 . Although the content rate of the solvent in the coating film  25  at the contact start position P 1  may be less than 50 wt %, for the purpose of increasing the production efficiency, the water contacting process is preferably performed immediately after the content rate of the solvent in the coating film  25  reaches 50 wt %. Further, if the water contacting process is performed at the timing when the content rate of the solvent in the coating film  25  at the contact start position P 1  is more than 50 wt %, namely, at the timing when the content rate of the solvent in the coating film  25  is more than 50 wt %, the porous film  29  can be produced in some cases, however, in other cases, the water droplets  20  enter the coating film  25  insufficiently such that the pores  30  become shallow grooves on the coating film  25  in accordance with the sorts of the polymer and the solvent. Accordingly, it is preferable that the content rate of the solvent in the coating film  25  at the contact start position P 1  is at most 50 wt % for the purpose of achieving the effect surely. For example, it is preferable that the coating film  25  is dried in the second area  15  such that the content rate of the solvent in the coating film  25  reaches 50 wt % at the downstream end of the second area  15 . Note that the content rate of the solvent is obtained by 100×X/(X+Y) wherein the weight of the solvent  27  contained in the coating film  25  is denoted by X and the weight of the polymer contained in the coating film  25  is denoted by Y. 
     Whether the content rate of the solvent in the coating film  25  reaches 50 wt % or not can be judged by the thickness of the coating film  25 . For example, a non-contact thickness measuring device (not-shown) is disposed at the vicinity of the conveying path of the support  22 . Thus, whether the content rate of the solvent in the coating film  25  reaches 50 wt % or not can be judged by correlation between the target thickness of the porous film  29  and the thickness of the coating film  25  measured by the non-contact thickness measuring device. 
     Whether the depth of the water droplets  20  entering the coating film  25  is enough to have the honeycomb structure also can be judged by eyes. In this case, refraction of light on the water droplets  20  generated due to the condensation is utilized. While observing the coating film  25  in the second area  15  over time, it can be observed that the coating film  25  seems rainbow at first and then the rainbow disappears. The timing when the rainbow disappears approximately corresponds to the timing when the water droplets  20  enter the coating film  25  enough to have the honeycomb structure. Accordingly, a mode in which the solvents  27  are evaporated in the second area  15  until the content rate of the solvent reaches 50 wt % may be substituted with a mode in which the solvents  27  are evaporated in the second area  15  until the rainbow disappears from the coating film  25 . The method utilizing the refraction of light as described above can be easily performed by applying light toward the coating film  25 . 
     In the third area  16 , the liquid water  34  is discharged through the water discharge die  35  onto the coating film  25  fed from the second area  15  to cause the coating film  25  to contact with the water  34 . In the conventional manner, in a first evaporation process performed in the second area  15 , the hydrophobic solvent in the coating film  25  does not sufficiently evaporate in accordance with the sort of the polymer and the hydrophobic solvent. Thus, a porous film having a desired honeycomb structure can not be obtained. However, according to this embodiment, the coating film  25  is caused to contact with the water  34  such that the evaporation of the water droplets  20  filling the pores  30  in the coating film  25  can be prevented by the water  34 . Further, since the hydrophobic solvent moves to the water  34  and the water droplets  20  in the coating film  25 , it is possible to efficiently remove the hydrophobic solvent from the coating film  25  in conjunction with the process performed in the fourth area  21 . 
     It is preferable that the temperature of the water  34  is set so as not to cause bumping of the hydrophobic solvent in the coating film  25  contacting with the water  34 . In order to prevent the bumping of the hydrophobic solvent, the temperature of the water  34  should not be 20° C. higher than that of the hydrophobic solvent. Namely, when the boiling point of the hydrophobic solvent is denoted by Tb, and the temperature of the water  34  to contact with the coating film  25  is denoted by Tw, the value obtained by subtracting Tb from Tw is preferably less than 20° C. Accordingly, the value obtained by subtracting Tb from Tw preferably satisfies the following condition: Tw−Tb&lt;20° C. Note that, when the Tw is not larger than Tb, it is possible to prevent the bumping of the hydrophobic solvent approximately completely. In contrast, in order to increase the drying efficiency of the coating film  25 , Tw should not be 60° C. lower than Tb. Namely, the value obtained by subtracting Tb from Tw is preferably larger than −60° C., and more preferably satisfies the following condition: −60° C.&lt;Tw−Tb. Accordingly, it is preferable that the selection of the hydrophobic solvent and the setting of the temperature of the water  34  are performed such that the following condition is satisfied: −60° C.&lt;Tw−Tb&lt;20° C. However, −60° C. as the lower limit may change in accordance with the sort of the solvent, the target level of the production efficiency, and the like. In a case where the value obtained by subtracting Tb from Tw exceeds 20° C., the hydrophobic solvent contained in the coating film  25  is heated rapidly by the water  34  contacting with the coating film  25 , and the bumping of the hydrophobic solvent may occur in some cases. In contrast, in a case where the value obtained by subtracting Tb from Tw is −60° C. or less, the efficiency of drying the coating film  25  by evaporating the water  34  and the hydrophobic solvent from the coating film  25  is decreased. 
     A plurality of the water discharge dies  35  may be arranged along the conveying path of the support  22  such that the coating film  25  is caused to contact with the water  34  many times. Thereby, it becomes possible to remove hydrophobic solvents from the coating film  25  more efficiently. In this case, the temperature of the water discharged from the water discharge die in the downstream side is more preferably higher than that of the water discharged from the water discharge die in the upstream side. Higher the temperature of the water is, the faster the hydrophobic solvent moves to the water. 
     Although the coating film  25  is caused to contact with the water  34  in this embodiment, alcohol or hydrophilic solvent which does not dissolve the polymer may be used instead of the water  34 . 
     In the third area  16 , after the coating film  25  is caused to contact with the water  34 , the alcohol  37  is preferably discharged through the alcohol discharge die  36  onto the coating film  25  to cause the coating film  25  to contact with the alcohol  37  in an alcohol contacting process. The alcohol  37  preferably has the boiling point lower than that of the water  34 . Since the alcohol having the boiling point lower than that of the water has strong affinity for the water, the evaporation of the water is accelerated. Additionally, the alcohol also has affinity for the hydrophobic solvent, and therefore upon the evaporation of the water, the alcohol exerts the effect of evaporating the hydrophobic solvent. Accordingly, due to the contact with the alcohol, the efficiency of drying the coating film  25  in the fourth area  21  can be further increased. Further, when the temperature of the alcohol is denoted by Ta and the temperature of the water is denoted by Tw, it is preferable that the following condition is satisfied: Ta&gt;Tw. This is because the alcohol  37  easily evaporates, and in accordance with the evaporation of the alcohol  37 , the water also easily evaporates. 
     The alcohol contacting process may be performed between the water contacting process and a second evaporation process, or/and during the second evaporation process. The alcohol  37  is preferably ethanol, isopropyl alcohol, or the like to dry the coating film  25  efficiently and rapidly. 
     Instead of the alcohol discharge die  36 , a liquid bath containing alcohol maybe disposed. The coating film  25  is guided to the alcohol in the liquid bath, and caused to contact with the alcohol. In this case, there are disposed rollers  23  for contacting and supporting the support  22  in the liquid bath. In order to soak the coating film  25  in the alcohol without fail, it is preferable that the level of the lower end of at least one of the rollers  23  is below the liquid surface of the alcohol. 
     In the fourth area  21 , dry air is fed through the air feeding/sucking units  41  to  43  toward the coating film  25 , and thereby the water droplets  20  contained in the coating film  25 , the water adhered to the surface of the coating film  25  by the water contacting process, and the hydrophobic solvent are evaporated from the coating film  25  to dry the coating film  25 . Note that, in a case where the alcohol contacting process is performed in the third area  16 , the alcohol  37  is also evaporated from the coating film  25  in the fourth area  21 . 
     The porous film production apparatus  12  further includes a feeding means (not shown) for feeding the roll of long support to the first area  14 , and a winding means (not shown) for winding the porous film  29 . 
     According to the above production method, it is possible to shorten the time required for the evaporation of the hydrophobic solvents in comparison with the conventional method. 
     For the purpose of applying the coating liquid  24 , there are two methods. In the first method, the coating liquid is discharged onto a support disposed stationary and spread over the support. In the second method, the coating liquid is discharged through the discharge die onto a moving support. Both of them are applicable to the present invention. The first method is generally suitable for high-mix low-volume production. The second method is generally suitable for mass production. Note that, in the second method, if the coating liquid is continuously discharged, a long porous film can be produced, and if the coating liquid is intermittently discharged at a predetermined interval, a plurality of the porous films having a predetermined length can be produced one by one continuously. 
     Next, by referring to  FIG. 4 , a second embodiment is explained. The components identical to those in  FIG. 3  have the same reference numerals in  FIG. 4 . Instead of the water discharge die  35  (see  FIG. 3 ) used in the first embodiment, a tank  47  is used in the second embodiment. In other words, as the water contacting process, instead of applying the water  34  to the coating film  25 , the coating film  25  is soaked in the water  34  contained in the tank  47 .  FIG. 4  schematically shows only the conveying path of the coating film  25  contacting with the water  34  in the third area  16  (see  FIG. 3 ). The process performed in the upstream side from the third area  16  and the process performed from the alcohol discharge die  36  to the downstream side in the third area  16  are the same as those in the first embodiment, and therefore the explanation and drawing thereof will be omitted. 
     A tank  47  is disposed in the third area  16 . The rollers  23  for contacting and supporting the support  22  are disposed in the tank  47 . The level of the lower end of at least one of the rollers  23  is below the liquid surface of the water  34  such that the coating film  25  is soaked together with the support  22  in the water  34 . After passing through the second area  15  (see  FIG. 3 ), the coating film  25  is guided to the water  34  in the tank  47  in accordance with the movement of the support  22 , and then caused to contact with water  34 . Instead of using the water discharge die  35  in the first embodiment, the tank  47  is used in the second embodiment, and therefore an amount of the water  34  contacting with coating film  25  is increased and the time for causing the coating film  25  to contact with the water  34  is also increased in comparison with the first embodiment. Accordingly, even if the hydrophobic solvents are diffused toward the water  34 , the concentration of the hydrophobic solvents in the water  34  does not easily increased, and the difference between the concentration of the hydrophobic solvents at the vicinity of the coating film  25  and the concentration of the hydrophobic solvents in the water  34  can be easily maintained at a constant level. Thereby, the hydrophobic solvents more easily move to the water  34  due to the difference in concentration, and therefore the efficiency of drying the coating film  25  can be increased. 
     A plurality of the tanks  47  may be arranged in series along the moving direction of the support  22  to perform the water contacting process many times. Thus, the hydrophobic solvents can be more surely removed from the coating film  25 . Also in this case, the temperature of the water contained in the tank in the downstream side is preferably higher than that of the water contained in the tank in the upstream side. Thereby, it is possible to achieve more efficient drying of the coating film  25  while preventing bumping of the hydrophobic solvent. 
     Additionally, the coating film  25  is preferably caused to contact with alcohol  37  (see  FIG. 3 ) after passing through the tank  47  as in the case of the first embodiment. 
     According the film production method of the present invention, the coating film is caused to contact with the water in the process for producing the porous film having the honeycomb structure, and therefore it is possible to shorten the time required for evaporation of the hydrophobic solvents in comparison with the conventional method. Accordingly, it is possible to produce the porous film having pores regularly arranged with a uniform form and size in a short period of time. 
     EXAMPLE 1  
     [Experiments 1 to 11] 
     The porous film  29  was produced by the porous film production apparatus  12  under a condition different from each other in each of Experiments 1 to 11. Each of the conditions is shown in a table in  FIG. 5 . The support  22  was a film formed of PET. The solvent contained in the coating liquid  24  was dichloromethane having the boiling point Tb of 40° C., and the content rate of the solvent in the coating liquid  24  was 98.9 wt %. The composition of the coating liquid  24  is as follows. 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Polybutadiene 
                   1 pts. wt. 
               
               
                   
                 Dichloromethane (DCM) 
                 98.9 pts. wt. 
               
               
                   
                 Amphiphilic polyacrylamide 
                  0.1 pts. wt. 
               
               
                   
                   
               
            
           
         
       
     
     The coating film  25  was dried until the content rate of the solvent reaches a predetermined level in the second area  15 . Then, in the third area  16 , the water contacting process was performed such that the coating film  25  was caused to contact with the water  34  discharged through the water discharge die  35 . According to each Experiment, the content rate of solvent (unit: wt %) at the contact start position P 1  is shown in a column “Solvent content rate at P 1 ” in the table in  FIG. 5 , and the temperature Tw of the water  34  (unit: ° C.) is shown in a column “Water temperature Tw” in the table in  FIG. 5 , respectively. 
     In some Experiments, the alcohol contacting process was performed after the water contacting process in the third area  16 . Whether the alcohol contacting process was performed or not is shown in a column “With/without alcohol contacting process” in the table in  FIG. 5 . The alcohol  37  was continuously applied to the coating film  25  through the alcohol discharge die  36  to cause the coating film  25  to contact with the alcohol  37 . The alcohol  37  used in this example was ethanol having the boiling point of 78° C., and the temperature of the alcohol  37  was 30° C. 
     Next, the coating film  25  was dried in the fourth area  21  to obtain the porous film  29 . The quality of the porous film  29  obtained in each Experiment was evaluated. The production efficiency of the porous film  29  in each Experiment was also evaluated. Further, total evaluation was performed based on the above evaluation results. The quality of the porous film  29  was evaluated in accordance with whether or not irregularity appeared and degree of the irregularity, the uniformity of the forms and sizes of the pores, and regularity of the arrangement of the pores. The production efficiency was evaluated in accordance with the time required for drying the applied coating liquid  24  enough to be wound, that is, the length of production time of the porous film  29 . Note that the production efficiency is denoted by PE in  FIGS. 5 and 6 . The procedures and criteria for each evaluation are concretely explained below. 
     With regard to the evaluation of quality, whether or not irregularity appeared and degree of the irregularity were checked by eyes. With regard to the uniformity of the forms and sizes of the pores, and regularity of the arrangement of the pores, the porous diameter was measured by a laser microscope, and the measured porous diameter was evaluated. The criteria for evaluation of the quality were as follows. Note that the variation coefficient (unit: %) of the porous diameter was determined as {(standard deviation of porous diameter)/(average of porous diameter)}×100.
     A: There was no irregularity, uniformity was kept, and the variation coefficient was at most 5%.   B: There was no irregularity, uniformity was kept, and the variation coefficient was more than 5% and at most 10%.   C: There was little irregularity, uniformity was almost kept, and the variation coefficient was more than 10% and at most 15%.   D: There was irregularity, and the variation coefficient was more than 15%.   

     The criteria for evaluation of the production efficiency were as follows.
     A: less than 20 minutes.   B: not less than 20 minutes and not more than 40 minutes.   C: not less than 40 minutes and not more than 60 minutes.   D: more than 60 minutes.   

     Total evaluation was based on the following criteria. The total evaluation results are shown in a column “Total” in the table in  FIG. 5 .
     A: Both quality and production efficiency were evaluated as A.   B: Both quality and production efficiency were evaluated as A or B (except the case in which both of them were evaluated as A).   C: Both quality and production efficiency were evaluated as A, B, or C (except the case in which both of them were evaluated as A or B).   D: Any one of quality and production efficiency was evaluated as D.   

     COMPARATIVE EXAMPLE 1  
     The porous film was produced without performing the water contacting process and the alcohol contacting process. Concretely, the water discharge die  35  and the alcohol discharge die  36  in the third area  16  were substituted with air feeding/sucking units similar to the air feeding/sucking units  41  to  43  in the fourth area  21 , and the coating film  25  guided from the second area  15  was dried. Other conditions were the same as those in Experiment 1. The obtained porous film was evaluated in the similar manner as those in Experiments 1 to 11. 
     COMPARATIVE EXAMPLE 2  
     The alcohol contacting process was not performed, and the water temperature Tw during the water contacting process was set to a value shown in the table in  FIG. 5  such that the content rate of the solvent at P 1  became a level shown in the table in  FIG. 5 . Other conditions were the same as those in Experiment 7. The obtained porous film was evaluated in the similar manner as those in Experiments 1 to 11. 
     EXAMPLE 2  
     [Experiments 1 to 3] 
     The coating liquid  24  in Example 1 was substituted with the coating liquid  24  having the following composition. The solvent contained in the coating liquid  24  was chloroform having the boiling point Tb of 61° C., and content rate of solvent in the coating liquid  24  was 98.9 wt %. 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Polybutadiene 
                   1 pts. wt. 
               
               
                   
                 Chloroform (CLF) 
                 98.9 pts. wt. 
               
               
                   
                 Amphiphilic polyacrylamide 
                  0.1 pts. wt. 
               
               
                   
                   
               
            
           
         
       
     
     The porous film  29  was produced using the coating liquid  24  described above under a condition different from each other in each of Experiments 1 to 3. The obtained porous film  29  was evaluated in the similar manner as those in Example 1. The conditions and the evaluation results are shown in a table in  FIG. 6 . 
     COMPARATIVE EXAMPLE 1 
     The porous film was produced without performing the water contacting process and the alcohol contacting process. Concretely, the water discharge die  35  and the alcohol discharge die  36  in the third area  16  were substituted with air feeding/sucking units similar to the air feeding/sucking units  41  to  43  in the fourth area  21 , and the coating film  25  guided from the second area  15  was dried. Other conditions were the same as those in Experiment 1. The obtained porous film was evaluated in the similar manner as those in Experiments 1 to 3. 
     COMPARATIVE EXAMPLE 2  
     The alcohol contacting process was not performed, and the water temperature Tw during the water contacting process was set to a value shown in the table in  FIG. 6  such that the content rate of the solvent at P 1  became a level shown in the table in  FIG. 6 . Other conditions were the same as those in Experiment 1. The obtained porous film was evaluated in the similar manner as those in Experiments 1 to 3. 
     Various changes and modifications are possible in the present invention and may be understood to be within the present invention.