Importance

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Abstract

The application is directed toward a process for forming an electret filter medium. The process includes the steps of: (1) providing a thin film of a dielectric material; (2) placing the thin film on a pattern anvil having a pattern of raised areas wherein the height of the raised areas is greater than the thickness of the film; (3) conveying the film, while placed on the pattern anvil, through an area where a fluid selected from the group including one or more of water, mineral oil, a chlorinated hydrocarbon, ethylene glycol and a solution of 50 volume percent water and 50 volume percent 1 propanol is applied to the film; (4) subjecting the film to a sufficient amount of ultrasonic vibrations in the area where the fluid is applied to the film to microaperture the film with at least about 1,000 microapertures per square inch in a pattern generally the same as the pattern of raised areas on the pattern anvil and with each of the microapertures having an area ranging from about 10 to about 100,000 square micrometers; and (5) corona discharge treating the film to instill a positive charge of at least about 100 volts on one surface of the film and a negative charge of at least about 100 volts on the other surface of the film.

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First Claim