Plastic and nonmetallic article shaping or treating: processes – Carbonizing to form article – Agglomeration or accretion
Patent
1984-02-02
1986-05-13
Czaja, Donald
Plastic and nonmetallic article shaping or treating: processes
Carbonizing to form article
Agglomeration or accretion
29592E, 55155, 55DIG5, 264248, 264DIG48, B29C 7104
Patent
active
045885370
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a continuous method for manufacturing an electret filter medium from dielectric material having an open or porous structure, said method comprising the steps of continuously feeding a web of dielectric material with a substantially closed dielectric foil adjacent to at least one major face thereof into a corona discharge device, reducing the thickness of the web of dielectric material and charging the web of reduced thickness dielectric material by means of a corona discharge and to apparatus for carrying out said method.
A somewhat analagous but discontinous, high-temperature method for manufacturing an electret filter medium is disclosed in U.S. Pat. No. 4,308,223. In said method, a filter web of polypropylene fibers of a relatively high basis weight, viz., 410 g/m.sup.2 and a thickness of 2 mm is charged by two effects, viz., orientation of dipoles and implantation of charges into the fibers. For optimum orientation of the dipoles, the charging is conducted at a high temperature (120.degree. C.) and consequently, requires quite a long time, viz., 15 minutes in total. The charging is carried out by inserting the mat between two electrodes, one of which is provided with a large number of corona points connected to a high DC-voltage. The other electrode is earthed and covered with a dielectric foil for the purpose of preventing the ions produced by the corona from flowing to earth. In order to inject negative CO.sub.3 -ions, a negative corona voltage is used. For the purpose of improving the uniformity of the charging of the filter medium, the corona points are positioned in close proximity to one another on the upper electrode. The coronas originating from the points oppose one another and are operative over a limited distance (a few millimeters). The points are positioned a short distance above or in the filter material to be charged. High corona voltages cannot be used because they cause sparking from the points to the grounded electrode. The risk of sparking is notably greater when the points are pressed into the filter material. The sparks can cause a short circuit which may produce holes in both the foil and the filter material. The holes in the foil may even become so wide that the foil is rendered useless. The holes in the filter material allow the passage of dust particles at an unacceptable rate.
Because the points as well as the upper electrode are at a high voltage the arrangement can be unsafe. A further problem is that the filter material is attached with screws onto the lower electrode so that part of the filter material remains uncharged and has to be discarded.
Furthermore, the charging is somewhat variable as appears from the penetration data in Table II of U.S. Pat. No. 4,308,223. This variation from 1.3 to 7 mg. penetration is likely due to the non-uniform charging of the filter material in which some parts of the filter material are not charged optimally.
U.S. Pat. No. 4,375,718 discloses a continuous, room-temperature process for manufacturing an electrostatically charged filter medium. In this method, a web of non-conductive, thermoplastic fibers is contacted on each side with a more conductive web to form a combined composite web. The composite web is charged by corona charging elements of opposite polarity on opposite sides of the composite web.
The present invention is directed to a continuous method of manufacturing an electret or electrostatic filter medium having superior filtration performance compared to media produced by prior art methods. The use of a substantially closed supporting dielectric foil has the advantage of protecting against electrical breakdown or sparks during the charging process.
The method according to the present invention is very well suited for a continuous process. Since the charge injection is so strong, it is not necessary to aim at orientation of permanent dipoles and one may start with dielectric materials without polar groups. These have a much higher insulation resistance than do polar dielectric materials thereby
REFERENCES:
patent: Re30782 (1981-10-01), van Turnhout
patent: 4375718 (1983-03-01), Wadsworth et al.
Klaase Petrus T. A.
van Turnhout Jan
Czaja Donald
Fertig Mary Lynn
Minnesota Mining and Manufacturing Company
Okubo Edward T.
Sell Donald M.
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