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 C3F4H+ to C2F5+ 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.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application is a divisional of U.S. Ser. No. 12 / 418,290, filed Apr. 3, 2009, the disclosure of which is incorporated by reference in its entirety herein.[0002]The present invention pertains to a new method of fluorinating a nonwoven fibrous web. The method uses fluorine-containing plasma products created from a plasma formed at a location remote from a point where the fluorine atoms are transferred to the fibrous web. Webs fluorinated in accordance with the present invention also may be subjected to electrical charging so that they can subsequently be used as electret filters.BACKGROUND[0003]Electret articles—that is, dielectric articles that exhibit at least quasi-permanent electric charge—are known to exhibit good filtration properties. The articles have been fashioned in a variety of constructions, but for air filtration purposes, the articles commonly take the form of a nonwoven polymeric fibrous web. An example of such a product...

AI Technical Summary

Benefits of technology

[0017]Another advantage of the present invention is that higher power levels can be used in the plasma reaction, allowing more complete disassociation of the fluorine-containing feed gases. In known methods of plasma fluorinating fibrous webs, the higher power levels needed for full disassociation of the reactant gas are more difficult to achieve over a large electrode area. There is no concern for risking damage to the web when it is not present at the location where the plasma is made.

[0018]The present invention also is beneficial in that it allows for NF3 use rather than fluorine (F2) gas or fluorocarbons like C3F8. Surprisingly applicants discovered that nitrogen is not incorporated in the nonwoven web substantially when NF3 is used as the starting material for producing the plasma. The avoidance of fluorinated carbons is particularly beneficial in that their deposits do not need to be cleaned off the plasma fluorination equipment or the fluorinated web.

[0019]Articles of the present invention exhibit a different C3F4H+ to C2F5+ ratio when compared to similar fibrous webs that are locally fluorinated. The C3F4H+ to C2F5+ ratio is greater in nonwoven webs of the present invention for a given atomic % fluorine. This ratio may be distinguished from known articles using a Remote Fluorination Threshold line described below. Nonwoven webs produced in accordance with the present invention will have a C3F4H+:C2F5+ ratio that is above the local fluorination line. Articles of the invention may have a ratio so much greater that it is above a remote fluorination threshold line RFT1, and even may be above a further remote fluorination line RFT2, and even above a still further remote fluorination threshold line RFT3. Thus, the C3F4H+:C2F5+ ratio that is manifested under a ToF-SIMS analysis is remarkably distinct from the ratio provided in local fluorination, particularly above atomic % fluorine levels of 40% or greater, more particularly above about 42% atomic fluorine. The inventive nonwoven webs that exhibit such fluorine content have been able to demonstrate good performance after being aged under accelerated conditions.GLOSSARY

[0020]“comprises (or comprising)” means its definition as is standard in patent terminology, being an open-ended term that is generally synonymous with “includes”, “having”, or “containing”. Although “comprises”, “includes”, “having”, and “containing” and variations thereof are commonly-used, open-ended terms, this invention also may be suitably described using narrower terms such as “consists essentially of”, which is semi open-ended term in that it excludes only those things or elements that would have a deleterious effect on the fluorinated article, the fluorinated electret, or its method of being produced;

[0021]“electret” means a dielectric article that exhibits at least quasi-permanent electric charge;

[0023]“fluorine atoms” means atomic fluorine and / or any molecular fragment or molecule that contains fluorine;

Problems solved by technology

Although there are a number of documents that describe the use of remote plasma discharge techniques to fluorinate various articles, there is no known technique for using a remote plasma to deliver fluorine atoms to the surface of a nonwoven web that contains polymeric fibers.

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