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 reactive fluorine-atom-containing species comprising fluorine atoms at a first location; and   contacting the nonwoven web with the reactive fluorine-atom-containing species at a second location remote from the first location so as to allow the fluorine atoms to be transferred to surfaces of the polymeric fibers, wherein the reactive fluorine-atom-containing species are distributed to the nonwoven web in a chamber remote from the first location through a manifold via a conduit.   
     
     
         2 . The method of  claim 1 , wherein the manifold is spaced at a distance of 2 to 30 centimeters from the web. 
     
     
         3 . The method of  claim 1 , wherein the manifold is positioned to traverse the web width and has an array of openings having up to about one opening every 8 centimeters. 
     
     
         4 . The method of  claim 3 , wherein the manifold has a ratio of about 0.1 to 1.5 units of inlet area to total area of outlet(s). 
     
     
         5 . (canceled) 
     
     
         6 . The method of  claim 1 , wherein the manifold traverses the width of the web, and wherein the web travels from a first roll to a second roll in a first direction, the manifold 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 reactive fluorine-atom-containing species. 
     
     
         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  surface area exposed to the reactive fluorine-atom-containing species between the rolls. 
     
     
         10 . A method of making a nonwoven fibrous electret, which method comprises:
 fluorinating a nonwoven web, which contains polymeric fibers that have a volume resistivity of 10 14  ohm·cm or greater at room temperature, with reactive fluorine-atom-containing species 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 10 , 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 electrically charged nonwoven fibrous electret exhibits a Q9 value of 1.5 or greater when tested in accordance with the Q9 Aging Test. 
     
     
         15 - 20 . (canceled) 
     
     
         21 . A method of making a fluorinated fibrous web, which method comprises:
 providing a nonwoven web that contains polymeric fibers;   creating a plasma that contains reactive fluorine-atom-containing species comprising fluorine atoms at a first location, the plasma being formed from fluorine containing species that include NF 3 ; and   contacting the nonwoven web with the reactive fluorine-atom-containing species at a second location remote from the first location so as to allow the fluorine atoms to be transferred to surfaces of the polymeric fibers.   
     
     
         22 . The method of  claim 21 , wherein the nonwoven web is contacted with the reactive fluorine-atom-containing species in a chamber and is delivered into the chamber through a conduit and a manifold, wherein the manifold distributes the reactive fluorine-atom-containing species to the nonwoven web in the chamber. 
     
     
         23 . The method of  claim 22 , wherein the manifold is spaced at a distance of 2 to 30 centimeters from the web. 
     
     
         24 . The method of  claim 23 , wherein the manifold traverses a width of the web, and wherein the web has a width greater than 1 meter. 
     
     
         25 . The method of  claim 21 , wherein the polymeric fibers comprise polypropylene and have a volume resistivity of 10 16  ohm·cm or greater. 
     
     
         26 . The method of  claim 25 , further comprising electrically charging the fluorinated nonwoven fibrous web.