Remote fluorination of fibrous filter webs

Abstract

A method of making a fluorinated fibrous web, which method includes providing a nonwoven web 22 that contains polymeric fibers, creating a plasma that contains fluorine atoms at a first location 14 , and contacting the nonwoven web with products from the plasma at a second location 26 remote from the first location 14 . The method avoids exposure of the web to the plasma and hence expands the manufacturing processing window. Webs so fluorinated have a different C 3 F 4 H + to C 2 F 5 + ratio when compared to locally fluorinated webs having similar levels of surface fluorination. The remote fluorinated webs can be subsequently charged electrically to provide a good performing electret filter 40 suitable for use in an air purifying respirator 30 . Webs fluorinated in accordance with this invention also may exhibit good performance even after being “aged” at high temperatures.

1 . An electret article that comprises:
 a nonwoven web that comprises melt-blown fibers that contain polypropylene and that have fluorine atoms on the surfaces of the melt-blown fibers in the web such that (a) an atomic % fluorine is greater than 40%, and (b) a ToF-SIMS C 3 F 4 H +  to C 2 F 5   +  ratio that is greater than 0.3 and that is above a Remote Fluorination Threshold RFT 1  line for the atomic % fluorine.   
     
     
         2 . The electret article of  claim 1 , wherein the polypropylene melt-blown fibers are microfibers that have an effective fiber diameter of 1 to 10 μm and that have a volume resistivity of 10 14  ohm-cm or greater, and wherein the article exhibits a Q 0  of at least about 2 and a Q9 of at least about 1.5 mm H 2 O. 
     
     
         3 . The electret article of  claim 2 , wherein the ToF-SIMS C 3 F 4 H +  to C 2 F 5   +  ratio is above an RFT 2  line. 
     
     
         4 . The electret article of  claim 2 , wherein the ToF-SIMS C 3 F 4 H +  to C 2 F 5   +  ratio is above an RFT 3  line. 
     
     
         5 . A filter that comprises the electret article of  claim 1 . 
     
     
         6 . A respirator that comprises the filter of  claim 5 . 
     
     
         7 . The electret article of  claim 1 , wherein the article exhibits a Q9 of at least 1.6/mm H 2 O. 
     
     
         8 . The electret article of  claim 1 , wherein the article exhibits a Q9 of at least 1.8/mm H 2 O. 
     
     
         9 . The electret article of  claim 1 , wherein the article has an atomic % fluorine of greater than 40%. 
     
     
         10 . The electret article of  claim 1 , wherein the article has an atomic % fluorine of greater than 45%. 
     
     
         11 . The electret article of  claim 1  having a ToF-SIMS C 3 F 4 H +  to C 2 F 5   +  ratio of at least about 0.4. 
     
     
         12 . The electret article of  claim 1  having a basis weight of 20 to 150 g/m 2 , a solidity of 3 to 10%, and a thickness of 0.5 to 2 mm.