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 . A method of making a fluorinated fibrous web, which method comprises:
 providing a nonwoven web that contains polymeric fibers;   creating a plasma that contains fluorine atoms at a first location; and   contacting the nonwoven web with products from the plasma at a second location remote from the first location so as to allow fluorine atoms to be transferred to surfaces of the polymeric fibers.   
     
     
         2 . The method of  claim 1 , wherein the plasma products are contacted with the nonwoven web in a chamber and are delivered into the chamber through a manifold that is spaced at a distance of 2 to 30 centimeters from the web. 
     
     
         3 . The method of  claim 2 , wherein the manifold is positioned to traverse the web width and has a continuous array of openings or up to about one opening every 8 centimeters. 
     
     
         4 . The method of  claim 3 , wherein the manifold has about 0.1 to 1.5 units of inlet area to total area of outlet(s). 
     
     
         5 . The method of  claim 2 , wherein the manifold comprises aluminum, stainless steel, nickel, fluoropolymers and combinations thereof. 
     
     
         6 . The method of  claim 2 , wherein the manifold that traverses the width web, and wherein the web travels from a first roll to a second roll in a first direction, the distribution means extending normal to the first direction. 
     
     
         7 . The method of  claim 1 , wherein the web is 0.25 to 5 mm thick and has a width greater than 1 meter. 
     
     
         8 . The method of  claim 1 , wherein the web moves at a speed of 1 to 100 meters per minute when being contacted with the plasma products. 
     
     
         9 . The method of  claim 8 , wherein the web travels from a first roll to a second roll such that the web has about 0.1 to 0.5 m 2  exposed to the plasma products. 
     
     
         10 . A method of making a nonwoven fibrous electret, which method comprises:
 fluorinating a nonwoven web, which contains polymeric fibers, with plasma products from a plasma that had been created at a location remote from where the fluorination occurs; and   electrically charging the fluorinated nonwoven fibrous web.   
     
     
         11 . The method of  claim 10 , wherein the polymeric fibers comprise polypropylene and have a volume resistivity of 10 16  ohm-cm or greater. 
     
     
         12 . The method of  claim 11 , wherein the polymeric fibers have a volume resistivity of 10 16  ohm-cm or greater, and wherein the nonwoven web has a basis weight of 2 to 500 grams per square meter. 
     
     
         13 . The method of  claim 11 , wherein the electrical charging step comprises hydrocharging. 
     
     
         14 . The method of  claim 10 , wherein the resulting nonwoven fibrous electret exhibits a Q9 value of 1.5 or greater when tested in accordance with the Q9 Aging Test. 
     
     
         15 . 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 RFT1 line for the atomic % fluorine.   
     
     
         16 . The electret article of  claim 15 , 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. 
     
     
         17 . The electret article of  claim 16 , wherein the ToF-SIMS C 3 F 4 H +  to C 2 F 5   +  ratio that is above an RFT2 line. 
     
     
         18 . The electret article of  claim 16 , wherein the ToF-SIMS C 3 F 4 H +  to C 2 F 5   +  ratio that is above an RFT3 line. 
     
     
         19 . A filter that comprises the electret article of  claim 15 . 
     
     
         20 . A respirator that comprises the filter of  claim 19 .