Porous film production method

Abstract

A coating liquid containing a polymer and a hydrophobic solvent is applied to a support to form a coating layer. In a humidifying process with high humidity, the coating layer is set under an atmosphere having a first dew point higher than a surface temperature of the coating layer. Then, in a humidifying process with low humidity, the coating layer is set under an atmosphere having a second dew point higher than the surface temperature of the coating layer and lower than the first dew point. Thereby, water vapor is condensed from ambient air on the coating layer to generate minuscule water drops. Thereafter, the coating layer is dried to form a porous film having plural pores made by the minuscule water drops as a template. Since the humidifying process with high humidity is performed for a short period of time and then the humidifying process with low humidity is performed, the minuscule water drops are prevented from growing up, to obtain a porous film having minute pores.

1 . A porous film production method comprising the steps of:
 (A) applying a coating liquid containing a polymer and a hydrophobic solvent to a support to form a coating layer;   (B) setting said coating layer under an atmosphere having a first dew point Td 1  higher than a surface temperature Ts of said coating layer (Td 1 >Ts);   (C) setting said coating layer under an atmosphere having a second dew point Td 2  higher than said surface temperature Ts of said coating layer and lower than said first dew point Td 1  (Td 1 >Td 2 >Ts) after said step B; and   (D) drying said coating layer subjected to said step B and said step C to form a porous film having a plurality of pores made by water drops as a template for said porous film, and said water drops being formed by said step B and said step C.   
   
   
       2 . A porous film production method as defined in  claim 1 , wherein a relation between said dew point Td 1  and said dew point Td 2  satisfies the following condition:
     Td 1− Td 2≧1° C.   
   
   
       3 . A porous film production method as defined in  claim 2 , wherein t 1  is a remaining time of said coating layer in said step B, t 2  is a remaining time of said coating layer in said step C, and said t 1  and said t 2  satisfy the following condition:
   t1<t2   
   
   
       4 . A porous film production method as defined in  claim 3 , wherein said remaining time t 1  of said coating layer in said step B is not less than 0.1 seconds and not more than 60 seconds. 
   
   
       5 . A porous film production method as defined in  claim 4 , wherein a thickness of said coating layer before being dried is at most 500 μm.