Crystalline polymer microporous membrane, method for producing same, and filter for filtration

Abstract

To provide a membrane capable of efficiently collecting fine particles for a long period of time, disclosed is a polytetrafluoroethylene microporous membrane wherein the mean pore size in the surface of the membrane is larger than the mean pore size in the back thereof and the mean pore size continuously changes from the surface toward the back.

1 . A crystalline polymer microporous membrane wherein the mean pore size in the surface of the membrane is larger than the mean pore size in the back thereof and the mean pore size continuously changes from the surface toward the back. 
   
   
       2 . The crystalline polymer microporous membrane according to  claim 1 , wherein dD/dt in the surface of the membrane is negative in which D indicates the mean pore size, and t indicates the distance from the surface in the thickness direction. 
   
   
       3 . The crystalline polymer microporous membrane according to  claim 2 , wherein dD/dt in the back of the membrane is larger than dD/dt in the surface of the membrane. 
   
   
       4 . A single-layered crystalline polymer microporous membrane wherein the mean pore size in the surface of the membrane is larger than the mean pore size in the back thereof. 
   
   
       5 . The single-layered crystalline polymer microporous membrane according to  claim 4 , which has a face having a mean pore size smaller than the mean pore size in the surface of the membrane and larger than the mean pore size in the back thereof. 
   
   
       6 . The crystalline polymer microporous membrane according to  claim 4 , wherein the mean pore size continuously reduces from the surface of the membrane toward the back thereof. 
   
   
       7 . The crystalline polymer microporous membrane according to  claim 1 , wherein the crystalline polymer is a polyalkylene. 
   
   
       8 . The crystalline polymer microporous membrane according to  claim 1 , wherein the crystalline polymer is a polytetrafluoroethylene. 
   
   
       9 . A method for producing a crystalline polymer microporous membrane, comprising semi-baking an unbaked film according to a process of imparting thermal energy to the surface of the film to thereby form a temperature gradient in the thickness direction of the film. 
   
   
       10 . A method for producing a crystalline polymer microporous membrane, comprising semi-baking an unbaked film under a condition under which thermal energy is applied more to the surface of the film than to the back thereof. 
   
   
       11 . The method for producing a crystalline polymer microporous membrane according to  claim 9 , wherein the semi-baking is conducted continuously. 
   
   
       12 . The method for producing a crystalline polymer microporous membrane according to  claim 11 , wherein the semi-baking is conducted continuously by heating the surface of the unbaked film and cooling the back thereof. 
   
   
       13 . The method for producing a crystalline polymer microporous membrane according to  claim 9 , wherein the semi-baking is conducted intermittently. 
   
   
       14 . The method for producing a crystalline polymer microporous membrane according to  claim 13 , wherein the semi-baking is conducted intermittently by intermittently heating and cooling the surface of the unbaked film and preventing the temperature elevation in the back thereof. 
   
   
       15 . The method for producing a crystalline polymer microporous membrane according to  claim 9 , wherein a heating roll is contacted with the surface of the unbaked film to semi-bake it. 
   
   
       16 . The method for producing a crystalline polymer microporous membrane according to  claim 15 , wherein the heating roll has a temperature under which the crystal condition varies. 
   
   
       17 . The method for producing a crystalline polymer microporous membrane according to  claim 16 , wherein the crystalline polymer is a polytetrafluoroethylene and the temperature of the heating roll is from 327 to 345° C. 
   
   
       18 . The method for producing a crystalline polymer microporous membrane according to  claim 9 , wherein the crystalline polymer is a polyalkylene. 
   
   
       19 . The method for producing a crystalline polymer microporous membrane according to  claim 9 , wherein the crystalline polymer is a polytetrafluoroethylene. 
   
   
       20 . The method for producing a crystalline polymer microporous membrane according to  claim 9 , further comprising stretching the semi-baked film in at least one direction. 
   
   
       21 . The method for producing a crystalline polymer microporous membrane according to  claim 9 , further comprising stretching the semi-baked film biaxially. 
   
   
       22 . A crystalline polymer microporous membrane produced by the method for producing a crystalline polymer microporous membrane according to  claim 9 . 
   
   
       23 . A filter for filtration, comprising the crystalline polymer microporous membrane according to  claim 1 . 
   
   
       24 . A filter for filtration produced by working the crystalline polymer microporous membrane according to  claim 1  into a pleated one. 
   
   
       25 . The filter for filtration according to  claim 23 , which is used with its surface side having a larger pore size kept as the filtration face of the filter.