CA1190486A - Filter media comprising dissimilar electrically charged layers - Google Patents

Abstract

FILTER MEDIA COMPRISING
DISSIMILAR ELECTRICALLY CHARGED LAYERS
Abstract Filter media comprising at least two electrically charged electrostatically dissimilar filter layers. The layers are electrostatically dissimilar in that they produce electrostatic fields that differ in kind. Representative different electrically charged filter layers are webs of electrically charged melt-blown fibers; webs of electri-cally charged fibrillated-film fibers; and rosin-wool filter webs. The combination of at least two of such differing webs removes surprisingly high percentages of particles from an aerosol, greater than predicted by what each layer normally removes by itself.

Description

176,487 CAN/RRT

_ 1 _ FILTER MEDIA COMPRISING
DISSIMIL~R ELECTRICALLY CHARGED LAYERS

Technical Field The present invention provides unique electrically charged filter media and filter devices, which remove unpredictably high percentages of particles from a gaseous stream.
Background Prior Art According to conventional filter theory and experience, if two or more filter layers are combined in one filter device, each layer will remove particles from an aerosol at the same rate as it would if functioning separately by itself and not in combination with other layers. The individual rates obtained by testing the filter layers by themselves can thus be used to predict the total number of particles that will be removed by the filter layer in combination. For example, the total number of particles (NT) removed by two or more filter layers in combination is conventionally given by the formula NT = PlNl + P2N2~
where Pl and P2 are the percentages of particles removed by the filter layers when they are tested by themselves, and Nl and W2 are the individual numbers of particles presented to each of the filter layers when the layers are present in the combination.l (Footnotes are at the end of the specification).
We now find that despite the apparent mathematic logic and necessity of conventional theory and experience, such theory underestimates the number of particles removed when certain electrically charged filter layers are used in combination. Electrostatic fields contained within electrically charged filter media have long been recognized to increase the proportion of particles removed from an aerosol. Such electrically charged filter media are exemplified by the well-known cornmercial "Hansen" or "rosln-wool" filters or the more recently developed electrically charged fibrillated-:Eilm fi.bers taught in van Turnhow-t U.S.
Patent 3,998,916, or -the electrically charged melt-blown fibers taught in Kubik et al, U.S. Pat. 4,215,682. However, in the past, the effect of electric charge has only been thought to improve the performance of the :Eilter layer that is electrically charged. There has been no suggestion that if two electr.:ically charged layers were combined together, the combined ef:Eect of the two layers would be greater -than the summation o:E what each layer provides by i-tself.
Summary of the Invention The present invention relates to a filter element useful as part o:E a filter device that exhibits a pressure drop of no more than 50 millimeters water in a gaseous stream flowing through the device at a rate of 85 litres per minu-te, said filter element comprising at least -two electrically charged fibrous layers, at least one of which includes a carrier for -the electric charge that is not included in the other layer.
The invention also relates to a filter device compris-ing a filter element as described above and means for supportingthe filter element across a gaseous stream to be filtered.
We have now found that when two or more electrically charged filter layers are used in combination i.n one filter device, and the ].ayers are electrostatically dissimilar as dis-cussed below, the combination will remove a significantly higher percentage of particles from an aerosol than conventional theory and experience would predict. In o-ther words, the -- 2 ~
~ ~, combination of layers removes more than a summation of what each layer would be predicted to remove. At least one layer apparent-ly removes more in the combination than it will remove when operatiny outside the combination.
sy "electrostatically dissimilar" it is meant that the filter layers establish elec-trostatic fields that differ in kind. For example, -the unpredic-ted high removal of particles can be ob-tained by combining any two or more of a rosin-wool filter layer, a layer of electrically charged melt-blown fibers taught in Kubik et al, U.S. Pat. ~,215,6B2, a layer of electrically charged fibrillated-film fibers as taught in van Turnhout, U.S. Pat. 3,998,916, or a layer of resin-impregnated glass fibers. The carriers of electric charge in these layers, i.e., the fibers or the generally particulate or globular resin, ingredients, are different as to composition, physical configura-tion and/or me-thod of preparation, and these differen-t carriers establish electrostatic fields which - 2a -- , ,;

differ as to the pattern or polarity of electrost~tic field established within the filter layer.
We have not confirmed any definite mechanism for the increased removal of particles; but one possible explanation is that particles penetrating through a first electrically charged layer are modified during their passage, i.e., by induction of a charge, so as to be more susceptible to removal by a second electrically charged layer. Whatever the exact mechanism, however, the increased percentage of removal is an important advantaye, allowing greater purification of an aerosol, reduction in thickness or density of filt2r layers without an increase in penetration, and lower pressure drops.
The present invention also overcomes a deficiency of certain filters such as rosin-wool filters.
As stated by F. J. Feltham in a review article "The Hansen Filter," Filtration and 5eparatlon, pages 370-372, July/August (1979):
It has been known since the early 1930s that the Hansen filter can be very rapidly discharyed by aerosols of certain fluids that soften or dissolve the resin. It is reasonable to suppose that these deposit on the resin, and by softening the surface allow the charge to migrate towards the wool resin interface, thus reducing the effective radius of the nonuniform electric field essential to the Hansen effect.
Oddly enough water itself has little effect, presumably because it is unable to wet the 3C surface of the resin, but paraffin oil and dibutyl phthalate both have a considerable effect, hence the difficulty in producing a Hansen type filter to pass the DLN 3181 para~fin oil mist test in Germany or the DO~
test in AmericaO
Rosin-wool filters are desirable because of their low cost, and the provision of a rosin-wool-based $~

filter medium capable of passing the parafEin oil mist test in German~ or the DOP (dioctylphthalate) test in the U.S. would be of significant benefit. The present inven-tion achieves rosin-wool-based filters capable of passing such tests, and makes this achievement by relying on another surprising attribute of certain fibrous filter materials, especially the melt-blown electrically charged oleophilic fibers taught in Kubik et all U.S. Pat. 4,215,682. Filter media comprising such ~melt-blown fibers have been found to exhibit a decreasing penetration of particles (i.e., an increasing removal of particles) such as dioctylphthalate droplets over a usefully long period of time. When such a layer is combined with a rosin-wool filter layer, the combined iltra-tion is substantially level over a useful period of time and the total combination has a desirable pressure drop and cost.
Other fibrous filter media besides rosin wool filter media benefit from this aspect of the invention, including -the fibrillated-film electrically charged fibers taught in van Turnhout, U.S. Pat. 3,998,916. Such filter media exhibit an increasing penetration of dioctylphthalate droplets over time, but a level response over a useful period of time can be obtained by combining them with a layer such as the melt blown fibrous layer.
Other fibrous layers which may be used instead of charged melt-blown fibrous layers to achieve an approximately level or steady penetration of dioctylphthalate droplets over time are uncharged melt-blown fibers and glass fiber layers.
Description of the Drawings Figure 1 is a perspective view showing a representative respirator or face mask of the invention Figure 2 is a sectional view through the ~ask shown in Figure 1 taken along the lines 2-2 in Figure l Figure 3 is a perspective view, partially in section, of a representative cartridge filter element of the invention; and Figures 4 to 11 are plots of particle penetration versus time for filtration tes-ts on filter media of -the inven-tion, on individual filter layers tha-t are components of Eilter media oE the inven-tion, and on other filter media for comparison to filter media oE the invention.
Detailed Description Figure 1 is a perspective view showing a representative respirator or face mask 10 in place on a person. The respirator 10 comprises two elec-trically charged fibrous layers 11 and 12 disposed within the mask wall, and pro-tective outer and inner molded shells 13 and 14 disposed on each side oE the fibrous layers. The shells 13 and 14 can be formed from polymer-impregnated air-laid fibrous webs, and as shown for the shell 13, may be spaced somewhat from the fibrous layers as a result of the stiffness of the shell. A metal strip 15 is attached over the nose portion of the mask, and may be pinched to assure a tight fit with the nose of the wearer. A soEt rubber strip 16 surrounds -the periphery of the mask, and straps 17 and 18 hold the mask tigh-tly against the face of the wearer. It is thereby required that air brea-thed by -the wearer pass through -the mask.
Figure 3 is a sec-tional view through a different representa-tive embodiment oE the inven-tion, namely, a threaded car-tridge 20 adapted to be removably received in a respirator. The cartridge 20 comprises a casing 21, which is generally made of metal and which is perforated with outer openings 22 and inner openings 23. Two or more fibrous layers, which in this illustra-tive embodiment comprise two electrically charged fibrous layers 24 and 25, are disposed within the casing 21.
The preferred electrically charged layers for use in the present in-vention comprise electrically charged melt-blown fibers as taught in Kubik et al, United States Patent 4,215,682, and elec-trically charged Eibrillated-film fibers as taught in van Turnhout, United States Patent 3,998,916. Bo-th kinds of Eibers have the advantage that they carry persis-tent electric charges. For example, under accelera-ted testing in a room--tel~lperature 100-percent-relative-humidity environment, the charge on webs of such fibers generally have a half-life of a-t least one week and preEerably of six months or a year. Also, filter media that include a-t least one layer of charged melt-blown fibers and one layer of charged fibrillated-Eilm Eibers exhibit especially advantageous com-bina-tions of low particle penetrations and low pressure drops.
Electrically charged melt-blown and fibrillated-Eilm Eibers may be made Erom a variety of polymeric materials of high resis-tivity, such as 10 or more ohm-centimeters. Polypropylene is an especially desired material.
AEter preparation, the Eibersmaybegathered into self-supporting webs that consist only of the fibers. Alternatively webs can be prepared that include other Eibers or particles in mixture with the fibers; or webs can include another layer such as a reinforcing fabric. The webs may be made in different thicknesses and at different bulk densities to adap-t them to differen-t Eilter-ing uses.
The dimensions of mel-t-blown or fibrillated-film fibers may vary depending upon -the particular processing conditions. The melt-blown fibers are generally circular in cross section and range in diameter from a-t least 0.1 micrometer to 50 micrometers, though they preferably average less -than 10 micrometers in diameter. The fibrillated-film fibers are generally somewha-t rectangular in cross section and generally have different charges on their -two opposite faces. This double-polarity charge arises during charging of the film that is fibrillated. Charges of opposite polarity may be injected into each side c>f the film; or, if charges of one polarity are injected into one side of the film, -the air on the other side of -the film may be ionized to produce charges of opposite polarity, which become drawn into that side of the film. The double-polarity charge exhibited by fibrillated-film fibers is desirable, since it establishes preferred electrostatic-field gradients within the filter.
Other electrically charged layers useful in the invention include the previously noted "Hansen" filter media, which generally comprise synthetic or naturally occurring Eibers and particles or globules of a natural rosin or synthetic resin adhered to the fibers as taught in the prior art. Both the Eibers and resinous particles serve as carriers for the electric charges, which are less-persistent triboelectric or surface charges de~eloped during manufacture of the layer. Layers of resin-bonded glass fibers also can acquire an electrostatic charge and may be useful in filters of the invention. Filter media that include at least one layer comprising glass fibers and at least one layer comprising charged melt-blown fibers or charged fibrillated-film fibers provide especially desirable combinations of filtering properties, and are well adapted to low-cos~ incorporation into respirators and other filter devices.
The synergistic decrease in penetration with dissimilar electrically charged filter layers occurs even if the layers are spaced apart in the f ilter rather than being disposed in side-by-side contact. However, the two layers are commonly assembled side-by-side, and sometimes held together in one sheet by friction, needle-tacking, enclosing within a porous sleeve, etc. Three or more dissimilar electrically charged filter layers may be used in combination, or two or more dissimilar electrically charged filter layers may be used in combination with non-charged filtering layers. The order in which the electr.cally charged layers are presented to the challenge aerosol may affect the magnitude of particle removal, though improved results generally occur irrespective of the order.

Pilter rnedia of the invention are especially useful in respirators such as face masks, but they may be used in other filtration devices, such as in hood-type respirators, or room or building air-filtering systems.
Filter media oE the invention offer low particle penetrations and low pressure drops for any of these uses, i.e., pressure drops less than 50, and preferably less than 25, millimeters water at a flow rate of 85 liters per minute. The filter media may be used in a variety of forms, e.g., flat sheets, pleated sheets, or bags and may be used against a variety of challenge aerosols. Filter media are usually tested for penetration by dioctylphthalate, sodium chloride, or parafEin oil, which are industry-accepted indications of the filteriny ability of the media for a variety of aerosols.
Filter layers may generally be determined to be electrically charged by measuring particle penetration through the layer before and after treatment with an antistatic liquid. We generally use a solution that comprises one part of "Staticide" antistatic treating 1iquid (manufactured by Analytical Chemical I.aboratories, Elk Grove Village, Illinois) and 100 parts of water. We lightly spray each side of the layer with a fine mist of the solution (so as not to saturate the layer) and then test the layer for DOP particle penetration. If the particle penetration through a filter layer increases about 5 percent or more after treatment with the described solution, the layer is regarded as electrically charged.
The solution apparently removes or masks the electric charge, whereupon the filtering properties depend more on the mechanical filtering capacity of the fibrous layer.
Preferred electrically charged filter layers exhibit an increase in penetration of 10 or 20 percent in this test, showing that they apparently have a larger electrostatic charge.
The invention will be further illustrated by reference to Figures 4-11, which include graphs of the ~Trc~de ~k _9_ results oE particle penetration tests on a variety of exemplary filter media of the invention and on a co~para-tive filter medium. The standard dioctylphthalate (DOP) test used in these e~amples and reEerred to elsewhere herein was conducted in accordance with principles set forth in Title 30, CFR 11, Sub Part ~, 11.140-11, as printed in the Federal Register, Volume 37, No. S9, ~arch 25, 1972. The challenge aerosol directed through the test media comprised 100 micrograms of dioctylphthalate p~r liter o~ air and flowed at a continuous rate of 32 li.ters per minute. The dioctylphthalate particle size was targeted at 0.3 micrometer. ~easurements were made with a ~odel Q 127 DOP Penetrometer, manufactured by ~ir Technique Inc., Baltimore, Maryland. Percent penetrations - 15 were recorded continuously.
The sodium chloride test reported ~or Figure ~
was similar to British Standard BS 4400 and used a sodium chloride aerosol from the chamber of a Sodium Chloride Tester, Model TD~ 6n, made by ~ir Technique Inc. The aerosol was drawn through the test media at a rate o~ 32 liters per minute. The nominal particle size of the sodium chloride particles was 0.5 micrometer.
The electrically charged melt-blown fiber layer used in all of the examples except the comparative example reported in Figure 7 comprised three separately collected sublayers of fibers made by the procedure taught in Kubik and Davis, ~.S. Pat. 4)215,682. The fibers averaged about 5 micrometers in diameter, and each layer weighed approxi-mately 45 grams per square meter. In the comparative testing reported in Eigure 7, one of the 45 grams-per-square-meter sublayers was used by itself and compared with a layer that was identical except that it was un-chargedO The uncharged layer was prepared in the same way as the charged layer except that the corona charging unit described in Kubik et al, U.S. Pat. 4,215,682, was not activated, i.e., was not supplied with electric power.

The fibrillated-film charged fibrous layers used in the examples comprised a web o fibers available from N. V. Verto, Rotterdam, Holland, under the ,~ identification "Filtrete" G 1150. The fibers averaged approximately 40 micrometers wide and 10 micrometers thick and were charged on each side with charges of opposite polarity. The web was 8 millimeters thick and weighed 150 grams per square meter. It was understood to have been formed generally by the procedures taught in van Turnhout, U.S. Pat. 3,998,916.
The Willson No. 1410 filter layer used in the examples is understood to be a rosin-wool type of filter medium. It was 3 millimeters thick and weighed 1000 grams per square meter.
Two glass fiber layers were used in the examples, an AF-3 and an AF-4 filter medium obtained from Johns Manville. Both were understood to comprise 0.5-diameter-micrometer glass fibers and a phenolic binder. The AF-3 medium had a thickness of 10 millimeters and a weight of 60 grams per square meter, and the AF-4 medium had a thickness of 15 millimeters, and a weight of 95 grams per square meter.
Figure 4 shows the results of DOP tests on the described layer of fibrillated-film fibers by itself (Curve A), on the described layer of melt-blown fibers by itself (Curve B), the theoretical results to be expected if the fibrillated-film layer and melt-blown fiber layer were used in combination (Curve C~, and the results actually obtained in a test of a fil~er madium comprising the described melt-blown fiber layer and fibrillated--film layer in combination, with the melt-blown fiber layer facing the challenge aerosol (Curve D).
Figure 5 shows the results in the DOP test on the Willson No. 1410 layer by itself (Curve A), the melt-blown fibrous layer by itself (Curve B), the theoretical result to be obtained by combining the ~elt-blown fibrous layer and the Willson No5 1410 layer (Curve C), and the actual results obtained in a test of a filter medium comprising the two layers, with the melt~blown fibrous layer directed to the challenge aerosol (Curve D) .
Figure 6 shows the results in the DOP test on the fibrillated-film fibrous layer hy itself (Curve A), the Willson NoO 1410 layer by itself (Curve B), the theoretical results to be expected by combining the fibrillated-film fibrous layer and the Willson No. 1410 layer (Curve C), and the actual results obtained from a filter medium comprising the two layers, with the fibrillated-film layer directed to the challenge aerosol (Curve D).
Figure 7 shows the results of comparative DOP
tests on the described layer of uncharged melt-blown fibers by itself (Curve A), on the described melt-blown charged fibrous layer by itself (Curve B), the theoretical result to be obtained by combining the uncharged and charged melt-blown fibrous layers (Curve C, which is the dotted curve), and the actual results obtained with a filter medium c~mposed of the two layers (Curve D). As may be seen, the theoretical and actual results are approximately the same.
Figure 8 shows the actual results obtained using a filter medium composed of the described fibrillated-~ilm layer and the described melt-blown charged layer in combination, Curve A showing the results when the fibrillated-film la~er is directed to the challenge fluid, and Curve B showing the results when the melt-blown layer is directed to the challenge fluid.
Figure 9 shows the results of the described standard sodium chloride test on the charged melt-blown layer by itself (Curve A), on the charged fibrillated-filrn layer by itself ~Curve B), the theoretical resul~ to be obtained by combining the charged fibrillated-film layer and the charged melt-blown layer (Curve C), and the actual results obtained on a filter medium composed of the two layers, with the melt~blown layer directed to the challenge aerosol (Curve D).
Figure 10 shows the results obtained in the DOP
test on the AF-3 glass fiber layer by itself (Curve A), the fibrilla~ed-film layer by itself (Curve ~), the theoretical result to be expected by combining the AF-3 glass fiber layer and fibrillated-film layer (Curve C), and the actual results obtained by combining the two layers, with the glass fiber layer directed to the challenge aerosol (Curve D).
Figure 11 shows the results of the DOP test on the AF-4 glass fiber layer by itself (Curve A), on the melt-blown charged layer by itself (Curve B), the theoretical result to be expected by combining the two layers (Curve C), and the actual results obtained on a filter medium composed of the two layers, with the melt-blown layer directed to the challenge aerosol (Curve D), and with the glass ~iber layer directed to the challenge aerosol (Curve E).

1 According to convention, the number of particles removed by an individual filter layer in a combination of layers can be calculated by multiplying the percentage of particles normally removed by that filter layer when the layer is tested by itself times the number of particles presented to the filter layer. The total number of particles (NT~ removed from an aerosol by two filter layers ~ill be the summation of 1) the multiplication product of a) the percentage of particles (Pl) ~ormally removed by the first filter layer to which ~he aerosol is presented when that filter layer is tested by itself and b) the number of particles INl) in the ae~osol, and 2) the multiplication product of a) a percentage of particles (P2) normally removed by the second filter layer when tested by itself and b) the number of particles (N2) that escape or penetrate through the first filter layer. In o~her words, NT = PlNl + P2N2

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A filler element useful as part of a filter device that exhibits a pressure drop of no more than 50 millimeters water in a gaseous stream flowing through the device at a rate of 85 litres per minute, said filter element comprising at least two electrically charged fibrous layers, at least one of which includes a carrier for the electric charge that is not included in the other layer.

2. The filter element of claim 1 in which at least one of the electrically charged layers comprises electrically charged melt-blown fibers.

3. The filter element of claim 1 in which at least one of the electrically charged layers comprises electrically charged fibrillated-film fibers.

4. The filter element of claim 2 in which at least one of the electrically charged layers comprises electrically charged fibrillated-film fibers.

5. The filter element of claim 2 or 3 in which at least one of the electrically charged layers is a Hansen filter layer.

6. The filter element of claim 2 or 3 in which at least one of the electrically charged layers comprises glass fibers.

7. The filter element of claim 1 in which the fibers of said at least two filter layers are comprised of organic fibers.

8. A filter device comprising a filter element of claim 1, 2 or 3 and means for supporting the filter element across a gaseous stream to be filtered.

9. The filter device of claim 8 in which the filter element is present in a cup-shaped mask and the means for support-ing the filter element across a gaseous stream to be filtered comprises means for holding the mask over a person's nose and mouth, with the edge of the mask against the wearer's face so as to define a path through the mask to the person's nose and mouth.

10. The filter device of claim 9 in which one of the electrically charged layers comprises charged melt-blown fibers and a different one of the electrically charged layers comprises charged fibrillated-film fibers.