Gas separation: apparatus – Electric field separation apparatus
Reexamination Certificate
1997-10-01
2001-05-29
Chiesa, Richard L. (Department: 1724)
Gas separation: apparatus
Electric field separation apparatus
C055S524000, C055S528000, C055SDIG005, C055SDIG003, C095S057000, C096S069000, C442S082000, C442S092000
Reexamination Certificate
active
06238466
ABSTRACT:
TECHNICAL FIELD
This invention pertains to electret articles, electret filters, respirators that employ electret filters, and the use of electret filters in removing particles from a gas, especially removing aerosols from air. This invention is especially concerned with electret filters that have improved properties such as electret stability in the presence of oily mists (i.e., liquid aerosols).
BACKGROUND
Scientists and engineers have long sought to improve filtration performance of air filters. Some of the most effective air filters use electret articles. Electret articles exhibit a persistent or quasi-permanent electrical charge. See G. M. Sessler, Electrets, Springer Verlag, New York, 1987. Researchers have expended considerable efforts to improve the properties of electret articles for use in filters. Despite extensive research directed toward producing improved electret articles, the effects of processing variables are not well understood, and, in general, the effects of varying processing conditions are difficult if not impossible to predict.
Electret articles have special property requirements such as charge stability, loading performance, resistance to moisture and oil exposure, et cetera, that can be significantly affected by processing steps that would be generally innocuous or beneficial for nonwoven fabrics and fabric-like materials. Thus, in the absence of extensive empirical data, it is often very difficult to understand the effects that a particular processing step (for example quenching) might or might not have on the resulting product's electret properties.
One method that has been reported to improve electret filter performance is blending a performance-enhancing additive into a polymer that is used to form electret fibers. For example, Jones et al. in U.S. Pat. Nos. 5,411,576 and 5,472,481 disclose electret filters that are made by extruding a blend of polymer and a melt-processable fluorochemical to form a microfibrous web that is subsequently annealed and corona treated. Lifshutz et al. in WO 96/26783 (corresponding to U.S. Pat. No. 5,645,627) report electret filters that are made by extruding a blend of polymer and a fatty acid amide or a fluorochemical oxazolidinone fluorochemical to form a microfibrous web that is subsequently annealed and corona treated.
Other techniques have been reported which improve an electret article's charge properties. For example, Klaase et al. in U.S. Pat. No. 4,588,537 report using corona treatment to inject charge into an electret filter. Angadjivand et al. in U.S. Pat. No. 5,496,507 found that impinging water droplets onto a nonwoven microfiber web imparted a charge to the web, and Rousseau et al. in WO 97/07272 disclose electret filters that are made by extruding blends of a polymer and a fluorochemical or organic triazine compound to form a microfibrous web that is subsequently impinged with water droplets to impart charge and thereby improve the hydrocharged web's filtration performance.
Matsuura et al. in U.S. Pat. No. 5,256,176 disclose a process of making stable electrets by exposing an electret to alternating cycles of applying electric charges and subsequently heating the article. Matsuura et al. do not disclose electrets having additives that enhance oily mist loading performance.
SUMMARY OF THE INVENTION
This invention provides an electret article containing a polymer and a performance-enhancing additive (other ingredients may also be added as described below). The electret article can, for example, be in the form of a fiber or film, or it may be in the form of a nonwoven web, especially when used as a filter. The inventors discovered that low crystallinity compositions containing a polymer and a performance-enhancing additive are particularly valuable because they can be converted to electret filters that have superior properties. As described below, the low crystallinity compositions can be made by introducing a quenching step during processing.
Quenching reduces a material's order (e.g. crystallinity) as compared to the material's order without the quenching process. The quenching step occurs concurrently with or shortly after converting a molten material into a desired shape. Usually the material is shaped by being extruded through a die orifice and quenched (typically by applying a cooling fluid to the extrudate) immediately after it exits the orifice.
The invention also provides a unique electret article containing a polymer and a performance-enhancing additive that may be characterized by certain features in a Thermally Stimulated Discharge Current (TSDC) spectrum. Electret filters incorporating the electret articles exhibiting these unique TSDC spectral features can exhibit surprisingly superior filtration performance.
The invention includes articles that incorporate the electret articles, and also includes methods of removing particulate solid or liquid aerosol from a gas using the inventive electret articles.
The invention further provides electret filters that exhibit superior properties not achieved in similarly constructed filters that do not use the inventive electret articles. These filters contain fibers made from a blend of polymer and performance-enhancing additive and they exhibit superior dioctylphthalate (DOP) liquid aerosol loading performance. DOP liquid aerosol loading performance is defined in relation to particular tests in the Examples section. Preferred filters exhibit enhanced oily mist loading performance and decreased penetration of aerosols or particulates while at the same time exhibiting a small pressure drop across the filter.
The present invention can provide numerous advantages over known electret filters including enhanced oily mist aerosol loading performance, charge stability in the presence of liquid aerosol, and decreased penetration of aerosols or particulates with a small pressure drop across the filter.
Electret articles of the present invention may find use in numerous filtration applications, including respirators such as face masks, home and industrial air-conditioners, furnaces, air cleaners, vacuum cleaners, medical and air line filters, and air cleaning systems in vehicles and in electronic equipment such as computers and disk drives.
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Lavergne, C. and Lacabanne, C., “A Review of Thermo-Stimulated Current,”IEEE Electrical Insulation Magazine, Mar./Apr. 1993, vol. 9, No. 2, pp. 5-21.
Touilhet, Yves, Dr., “Index 81 Congress Papers,” May 5-7, 1981, Section 3—Advances in Web Forming, pp. 2-18.
Charge Density vs. Poling Time, Undated.
TSDC Data For Corona Charged Web With Fluorochemical Additive That Has Been Aged, Undated.
Jones Marvin E.
Mei Betty Z.
Rousseau Alan D.
3M Innovative Properties Company
Chiesa Richard L.
Hanson Karl G.
Rosenberg Frank S.
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