Method for manufacturing a filter of electrically charged electret fiber material and electret filters obtained according to said method

Abstract

A method for forming a filter of electrically charged electret fiber material, consisting of a high molecular weight, nonpolar polymeric substance, comprising drawing, charging, and fibrillating a foil of said high molecular weight substance. The fibrillated foil is wound and then crimping is effected.

We claim: 
     
       1. A method for manufacturing a filter of electrically charged electret fiber material, consisting of a high-molecular weight, non-polar polymeric substance, comprising the steps of: continuously supplying a foil of said substance, drawing, charging and fibrillating of the foil and the processing of the thus obtained filter material into a filter, selecting the foil and/or the type of drawing to manufacture spontaneously crimpable fiber material, winding said material in uncrimped condition to a fibrous package and subsequently effecting crimping to interlock the fibers of said fibrous package. 
     
     
       2. A method according to claim 1, further comprising the step of heat treating the filter. 
     
     
       3. A method according to claim 1, wherein a foil is used in which at least two substances with different drawing or crimping characteristics are incorporated in a non-homogeneous mixture. 
     
     
       4. A method according to claim 3, wherein a foil is used being a laminate of said at least two substances. 
     
     
       5. A method according to claim 1, further comprising the step of deforming the foil during or after the step of drawing. 
     
     
       6. A filter produced by the method according to claim 1. .Iadd. 7. An electret fibrous web comprising fibers fibrillated from a film of a high-molecular-weight nonpolar substance and which contain electric charges embedded therein so as to enhance filtration properties of the web, said fibers being crimped after fibrillation and intermeshed with one another, said fibrous web being capable of being loaded with high amounts of dust while still maintaining high filtration efficiency. .Iaddend..Iadd. 8. An electret fibrous web of claim 7 in which said fibers comprise polypropylene. .Iaddend..Iadd. 9. An electret fibrous web of claim 7 in which said fibers are fibrillated from a film that comprises a laminate of at least two different high-molecular-weight nonpolar substances. .Iaddend..Iadd. 10. An electret fibrous web of claim 9 in which both of said different substances comprise polypropylene. .Iaddend.