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/cm2 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.