Electret filter media and process for producing the same

Abstract

It is intended to provide an electret filter which has a large surface charge density, sustains stable electret properties over a long time even in a high-temperature atmosphere and yet degrades to reduce its volume when buried for disposal; and a process for producing the same. The molar ratio of an L-lactic acid monomer to a D-lactic acid monomer ranges from 100 to 85:0 to 15 or 0 to 15:85 to 100. An electret filter having a surface charge density of 1.2×10 −9 C/cm 2 or more can be obtained by crystallizing by heating at a temperature from the glass transition temperature to the melting point and then, while heating to 60 to 140° C., applying a direct current corona electrical field to thereby cool to 40° C. or lower under.

1 . An electret filter medium, comprising a lactic acid polymer having a molar ratio of an L-lactic acid monomer to a D-lactic acid monomer in the range from 100 to 85:0 to 15.  
   
   
       2 . The electret filter medium according to  claim 1 , wherein it is mainly composed of a lactic acid polymer that produces an endotherm of at least 0.5 J/g accompanied with crystal fusion.  
   
   
       3 . The electret filter medium according to  claim 1 , wherein the content of lactide is at most 15% based on the weight of the medium.  
   
   
       4 . The electret filter medium according to  claim 1 , wherein it has a surface charge density of at least 1.2×10 −9 /cm 2 .  
   
   
       5 . The electret filter medium according to  claim 1 , further comprising 0.01 to 0.3 parts by weight of a nucleating agent based on 100 parts by weight of the lactic acid polymer.  
   
   
       6 . A process for producing the electret filter medium according to  claim 1 , comprising: 
 applying a direct current corona electric field to a nonwoven fabric while heating it to a temperature of 60° C. to 140° C., wherein the nonwoven fabric comprises fibers mainly composed of a lactic acid polymer; and    then cooling the nonwoven fabric to a temperature of 40° C. or lower while applying the electric field to the nonwoven fabric.    
   
   
       7 . An electret filter medium, comprising a lactic acid polymer having a molar ratio of an L-lactic acid monomer to a D-lactic acid monomer in the range from 0 to 15:85 to 100.  
   
   
       8 . The electret filter medium according to  claim 7 , wherein it is mainly composed of a lactic acid polymer that produces an endotherm of at least 0.5 J/g accompanied with crystal fusion.  
   
   
       9 . The electret filter medium according to  claim 7 , wherein the content of lactide is at most 15% based on the weight of the medium.  
   
   
       10 . The electret filter medium according to  claim 7 , wherein it has a surface charge density of at least 1.2×10 −9 /cm 2 .  
   
   
       11 . The electret filter medium according to  claim 7 , further comprising 0.01 to 0.3 parts by weight of a nucleating agent based on 100 parts by weight of the lactic acid polymer.  
   
   
       12 . A process for producing the electret filter medium according to  claim 7 , comprising: 
 applying a direct current corona electric field to a nonwoven fabric while heating the nonwoven fabric to a temperature of 60° C. to 140° C., wherein the nonwoven fabric comprises fibers mainly composed of a lactic acid polymer; and    then cooling the nonwoven fabric to a temperature of 40° C. or lower while applying the electric field to the nonwoven fabric.