ELECTRET FILTER MEDIUM

PURPOSE:To obtain a filter medium which is little in pressure drop and excellent in removal efficiency by fixing the intervals between the ridge parts of adjacent folds of a folded electret filter medium by bridging the ridges with a slender molded material made of resin having m.p. lower than the m.p. of material constituting the filter medium. CONSTITUTION:Only the ridge parts of the folds of an electret filter medium are bridged and fixed with a slender molded material made of resin having m.p. lower than that of material constituting the medium. As a result, since leak of static electricity permanently electrified in the electret filter medium is not almost caused, removal efficiency of fine particles such as dust is not almost lowered. Further the effective filtration area of the filter medium is not reduced and increase of pressure drop due to clogging can be prevented because no molded material sticks to the slanted parts of the flutes of the electret filter medium. Furthermore the intervals between the sufficiently folded ridge parts are effectively held and therefore bonding and overlapping of the slanted faces of the adjacent folds can be prevented.