CA1122546A

CA1122546A - Melt-blown fibrous electrets

Info

Publication number: CA1122546A
Application number: CA319,225A
Authority: CA
Inventors: Donald A. Kubik; Charles I. Davis
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1978-02-06
Filing date: 1979-01-08
Publication date: 1982-04-27
Also published as: DK157286C; BR7900546A; JPS54113900A; ES477436A1; AU507773B2; ATA82779A; CH642277A5; NO790347L; DK157286B; NO151092C; NO151092B; SE444893B; US4215682A; FR2416535B1; DK45179A; GB2015253A; GB2015253B; SE7900929L; AT380126B; NL7900855A

Abstract

ABSTRACT
A persistent electric change is introduced into melt-blown fibers during the melt-blowing process. When these charged fibers are incorporated into fibrous webs, they provide unique properties, including improved filtering properties.

Description

~22546 F.N. 91~,293 ~

MEhT-BLOWN FIBROUS ELECTRETS
The present invention provides a new kind of fibrous electret, which may be conveniently and economically manufactured in an essentially one-step process, and which can comprise fibers of microscopic diameters, thereby pro-viding a unique combination of electret and microfiber pro-perties.
An early method for ~orming fibrous electrets taught in Thomas, U.S. Pat. ~,740,184, comprises disposing thermoplastic threads, filaments, fabrics, or sheets in an electrostatic field established between parallel, closely spaced electrodes. The fibrous material is heated to soften it and then cooled in the presence of the field, whereupon "more or less" permanent charges are introduced into the fibers.
Van Turnhout, U.S. Pat. 3,571,679, notes a drawback to this method in that it is difficult to introduce a reasonably high permanent charge into a fibrous web being treated, because application of high voltages to the charging electrodes causes arcing through the open pores of the fibrous web. Van Turnhout suggests covering the charging alectrodes with a poorly conductive sheet so as to distrib-ute a high applied voltage and dampen possible dielectric breakdown through the fibrous web.
The covered-electrode method is criticized in a later Van Turnhout patent, U.S. Pat. 3,998,916, on the ground that too long a time period is needed to charge the fibrous material to a desirably charged stateO To avoid this deficiency the '916 Van Turnhou~ patent proposes a :.

~ LZZ5~

somewhat circuitous or two-step procedure. In this proce-dure a film is first prepared and electrically charged, and the film is then fibrillated by passing it over needled rollers, and assembled in layers to form a fibrous web.
This reliance on film-formation to form fibers is part of a historical sequence in which the art has progressed from preparation of rather thick wax electrets to thinner films by use of polymeric materials and techniques which include several controls over the process, including for example, controls over the temperature of the film during charging; controls over the distance between the charging apparatus and the film; and controls over the time period of chargin~. In the '916 Van Turnhout patent (see also "The Use of Polymers for Electrets," J. Van Turnhout, Journal of Electrostatics, Volume 1 (1975), pages 147-163) electric charging of the film is achieved by heating the film to near its melting point, stretching it over a curved plate, and spraying it with positive or negative charges from a number of thin wires disposed above the curved plate.
In Sessler et al, U.S. Pat. 3,644,605, a thin polymeric film is supported over a coe~tensive dielectric plate and bom-barded with an electron beam. And In N~SA Technical Report R-457 (December, 1975), a spray or mist of li~uid dielectric is passed through the corona discharge from a brush elec-trode or from a grid of narrow wires and then collected ona dielectric sheet where the droplets harden as a film.
While formation of a fibrous web by the inter-mediate formation of a film ~enefits from the knowledge of film-charging techniques, it is a time-co~suming and expen-'~ ' , .. ~ ~,.

.

~225~
-- 3 --sive process. Further, the technique can only achie~e limited fiber sizes.
Such disadvantages are overcome by the new fibrous electrets of the present invention, which are based on melt-blown fibers. Melt-blown fibers are ~ibers prepared by extruding molten fiber-forming material through a plurality of orifices into a high-velocity gaseous stream where the extruded material is attenuated to form a stream of fibers.
According to the invention, melt-blown fibers are bombarded as they issue from the orifices with electrically charged particles such as electrons or ions. The fibers are col-lected at a point remote from the orifices where they have cooled to a solid fibrous shape-retaining form and where they are found to carry a persistent electric charge. The collected web or mat can be used directly, except, typically, for trimming or cutting to size.
The conditions under which a method of the inven-tion must be performed are in sharp contrast to the controlled conditions which have been possible in past methods for form-ing film electrets: the fibers are moving at extremely highspeeds; they are blown turbulently by the high-velocity gaseous stream; and they are enveloped and dispersed in large volumes of diluting, high-velocity air. Still, the electrically charged particles enter the fiber stream and are retained in useful amounts in the melt-blown fibers~
Injection of the particles into the fiber stream necessarily occurs during the small fraction of a second ~less than one millisecond) when the fibers are both near the source of the electrically charged particles and are in a molten or near-~Z25 ~

molten stage. After such injection the fibers solidify extremely rapidly, and thereby freeze the electrical charges into the fibers, where they provide the collected mass of fibers with a persistent electric charge.
In brief summary a fibrous ~eb electret of the invention comprises melt-blown fibers which hold elec-trically charged particles and thereby carry a persistent electric charge that is useful to enhance filtering pro-perties of the web, said charge having a half-life of at least one week in a room-temperature, 100-percent-relative-humidity environment.
A method for forming a fibrous web electret of the invention comprises, in brief summary, 1) extruding molten fiber-forming material lS that exhibits a volume resistivity of at least 1014 ohm-centimeters through a plurality of orifices into a high-velocity gaseous stream where the extruded molten fiber-forming material is attenuated to form a stream of fibers;

2) bombarding electrically charged par-ticles at said stream of fibers as the fibers issue from the orifices; and

3) collecting said fibers at a point suf-ficiently remote from the orifices for the fibers to have cooled to a solid fibrous shape-retaining form.
The persistent charge in fibrous webs of the invention is distinctive from the temporary charge that has been applied to other fibrous products in the past~ often , - 4a -as an aid to manuacture of the product. For example, such charges have been applied to assist coating of the fibers by an oppositely charged liquid (see Bennett et al, U.S. Pat. 2,491,889); or to improve the distribution and separation of the fibers and to draw them toward a collector, thereby providing a more uniform fibrous mat (see Miller, U.S. Pat. 2,466,906; Till et al, U.S. Pat.
2,810,426; Fowler, U.S. Pat. 3,824,052; Rasmussen, U.S.
Pat. 3,003,304; and such fibrillated strand patents as Owens et al, U.S. Pat. 3,490,115; and Kilby et al, U.S.
Pat. 3,456,156).
The charges applied in these manufacturing pro-cedures are only temporary in nature. For example, the fiber-forming material may not have sufficient volume-resis-tivity, or too much conductive solvent may be present in theformed fibers, to allow a permanent charge to be maintained~
Or the charge may be applied ater the fibers have been formed so that only a surface charge is applied. Or the charging conditions, such as the applied voltage, may be insufficient to develop a permanent charge. Or the charge may be neutralized after collection of the fibers. If any residue of such a temporary charge remains after manufacture o fibrous mats according to the listed references, it is .: : ;.
,:: ~ ,: ,, . , : :
- :, -- :, - ~ :: :

~Z2S ~

rapidly dissipated during storage or usage.
By contrast, the fibrous webs of the present in-vention carry a persistent or "permanent" charge. When stored under typical conditions, fibrous webs of the inven-tion can retain a useful charge for many years. ~nder ac-celerated testing, such as storage in a room-temperature 100-percent-relative-humidity environment, the charge on fibrous webs of the invention generally has a half-life of at least one week, and preferably of six months or a year.
With such a persistence of charge, fibers and fibrous webs of the invention can properly be termed electrets, and the terms "fiber electrets," "fibrous web electrets," or the more general "fibrous electret" will be used herein to describe them.
For many fibrous web electrets of the invention a good indication of the magnitude of charge can be made by measuring surface voltage in the web with an isoprobe elec-trostatic voltmeter. However, such a measurement is less accurate if a web comprises a mixture of oppositely charged fibers. A mixed-charge web is still useful, for example, for enhanci~g filtering properties, but the net charge measured on the web will not represent the full magnitude of the charge. For fibrous web electrets of the invention that carry a persistent charge of only one sign, the charge is generally measured as at least 10 8 coulombs per gram of melt-blown fibers. For fibrous web electrets that include both postiveIy and negatively charged fibers, the net charge will usually be at least lQ 9 coulom~s per gram of melt-blown fibers. An indica~ion o~ electric charge can also be l~Z2S~6 obtained with other tests, such as application of toner powder to ~he web, but not necessarily in numerically quan-tified measurements.
The melt-~lown charged fibers prepared according to the invention can be tailored to have a desired fiber diameter. For many purposes, the fibers are in microfiber sizes (i.e., of a size best viewed under a microscope), and for some applications, the smaller in diameter the better.
For example, the microfibers can average less than 25, 10, or even one micrometer in diameter.
Microfiber sizes are known to achieve several use-ful properties, including improvement in certain aspects of filtering; and the combination of microfiber sizes with a permanent electric charge gives fibrous web electrets of the invention unique filtering properties. One particularly significant use for fibrous wQb electrets of the invention is in respirators, especially in cup-like shaped face masks as shown in Figure 3. Use of fi~rous web electrets of the invention to replace the webs of melt-blown microfibers used in previous masks of the type shown can improve filter effi-ciencies by a factor of two or more. Masks o~ the invention of the type shown in Figure 3 can be inexpensively manu-factured, and their low cost and high efficiency offers a widespread utility not available with any other known faca masks.
Figure 1 is a schematic view of representative apparatus for forming a fibrous web electret of the present invention;
Figure 2 is an elevational view along the lines ' , - -t ' ; : ~ :
: , , ': ' ' ' ~

1~2Z546 2-2 of Figure l, and includes a schematic wiring diagram for a source of electrically charged particles included in the apparatus of Figure l;
Figures 3 and ~ show a representative face mask that incorporates a fibrous web electret of the present in-vention, Figure 3 being a perspective view showing use of the mask, and Figure 4 being a sectional view along the lines

4-4 of Figure 3;
Figure 5 is a schematic diagram of an apparatus for testing the filter properties of a fibrous web electret of the present invention; and ~ igure 6 is a plot of particle penetration (ordinate) versus particle size (abscissa) for fibrous web electrets of the present invention and comparative uncharged lS webs.
Figures l and 2 show a representative apparatus 10 for preparing fibrous web electrets of the present invention.
A portion of this apparatus can be conventional melt-blowing apparatus of the type described in Report No. 4364 of the U.S. Naval Research Laboratories, published May 25, 1954, entitled "Manufacture o Super Fine Organic Fibers" by Wente, V. A.; Boone, C. D., and Fluharty, E. L. Such a fiber-blowing apparatus includes a die ll which is formed with a row of narrow side-by-side orifices 12 for extruding molten material, and with slots 13 on each side of the row of orifices through which a gas, usually air, is blown at high velocity. The stream of gas draws out the extruded material into fibers; cools the fibers to a solidified form;
and carries the fibers to a collector 14 as a fiber stream .- .
'` .: - ' ~

15. The collector 14 shown in Figure 1 comprises a finely perforated screen arranged as a drum or cylinder, but the collector can also take other forms such as a flat screen or a closed-loop belt traveling around rollers. Gas-withdrawal apparatus may be positioned behind the screento assist in deposition of fibers and removal of gas. The stream 15 of blown fibers is deposited on the collector as a randomly intertangled coherent mass which is handleable as a ~at 16 that may be unwound from the collector and wound into a storage roll 17.
To bombard eIectrically charged particles at the melt-blown fibers, one or more sources of such particles is placed adjacent the die orifices 12. In the apparatus of Figures 1 and 2, two sources 18 and 19 are used, one on each side of the fiber stream 15. Each source comprises an elec-trical conductor 20 or 21 connected to a high-voltage source 22, and disposed within a metal shell 23 or 24 which is con-nected through a resistor 25 to ground. As shown in Figure 2, the conductors can be mounted in insulators 26 and 27.
Upon energization of the conductor at a voltage high enough (usually 15 kilovolts or more), a corona orms around the conductor, and the air or other gas around the conductor ionizes. The electrically charged ions or particles are propelled into the fiber stream by a combination of aero-dynamic and electro~tatic forces acting on the charged par-ticles. The flow of charged particles may be assisted by a fan or by use of a voltage on the shells 23 or 24 which propels the particles away. Instead of a cylindrical shell or tube, flat metal plates positioned on each side of the , ,. ; . ~ .
: - ;.

;;~12254~

g .
conductor can be used, or any other arrangement which estab-lishes a desired voltage gradient between the electrode and the surrounding shield. Alternative sources of electrically charged particles are electron beams and radiation sources, such as x-ray guns.
The sources 18 and 19 of electrically charged par-ticles are placed close to the lip of the die 11, where the fibers are in a molten or near-molten stage. Under such con-ditions, the mobil-ity of the free charge carriers within the fibers is high, and introduction of a charge into the fibers is facilitated. The closer the source of electrically charged particles to the lip of the die, the more molten the fibers are, and the easier it is to introduce the charge.
As the fibers solidify and cool, the bombarded charges become frozen into the fibers and the fibers become persistently charged (heating the fibers will remove the charge). In accordance with the common terminology of elec-trets, this charge is called a homo-charge, and it has the same sign as the volta~ applied to tha conductors. Either a positive or negative voltage may be applied to the source of electrically charged particles, and sources of oppositely charged particles may be used simultaneously, as on opposite sides of the fiber stream.
A static charge on the surface of the fibers (which may be opposite in sign to that bombarded~ may also develop during production of a web of the invention. However, such a charge will quickly decay, in the same manner as the decay of a static charge applied to a completed fibrous web.
The temperature of the gas around the fibers tends ~lZZ546 to decline rapidly with increasing distance from the die orifice. For example, for conditions as described in Example 1 where the temperature of the air at the die orifice is about 550F (290C), the temperature will ~e about 370F
(190C) a half inch (1.25 centimeter) from the die, about 300F (150C) an inch (2.5 centimeters) from the die, about 240F (120C) one and one-half inches (3.75 centimeters) from the die, and about 200F (95C) two inches (5 centi-meters) from the die. Thus, charges bom~arded at the molten or near-molten fibers near the die lip rapidly become frozen into the fibers.
A variety of polymeric materials having dielectric properties that permit electrically charged particles to remain in the fiber without draining away of the charge may be used to prepare blown fibers in webs of the invention.
Polypropylene, which has a volume-resistivity of approxi-mately 1016 ohm-centimeters, is especially useful. Other polymers such as polycarbonates and polyhalocarbons that may be melt-blown and have appropriate volume-resistivities under expected environmental conditions may also be used.
In general, the useful polymeric materials have a volume-resistivity of at least 1014 ohm-centimeters, and avoid absorption of moisture in amounts that prevent the desired half-life for the charge. Pigments, dyes, fillers, and other additives may be mixed into the polymeric material, if they do not remo~e needed properties of, for e~ample, resistivity.
The diameter of the blown fibexs prepared varies with such parameters as the size of the die orifice, the :` :

~2:25 ~6 viscosity o the polymeric material, and the velocity of the air stream. Blown microfibers are generally regarded as discontinuous, though their aspect ratio (the ratio of length to diameter) should approach infinity to allow preparation of useful webs. Some workers estimate the fiber lengths to be up to several inches (i.e., 10 centimeters or more).
The fiber-forming procedure may be modified to introduce other fibers or particles into the web. For example, Braun, U.S. Pat. 3,971,373, describes apparatus and procedures for introducing solid particles into a blown fiber web. A wide variety of particles are useful, par-ticularly for filtering or purifying purposes; illustrations include activated carbon, alumina, sodium bicarbonate, and silver, which remove a component from a fluid by adsorption, chemical reaction, or amalgamation; and such particulate catalytic agents as hopcalite, which catalyzes the conver-sion of a hazardous gas to a harmless form. The particles may vary in size, at least from 5 micromet0rs to 5 milli-meters in average diameter. For respirators, the particles generally average less than 1 millimeter in diameter.
Preformed fibers may also be introduced into a blown fiber web during formation of the web; see, for example, Perry, U.S. Pat. 3,016,599, and Hauser, U.S. Pat. 4,11~,531.
For example, staple fibers, including crimped staple fibers, can be added to a stream of melt-blown fibers ~in the case of crimped staple fibers by picking the crimped fibers from a web by means of a lickerin roll) to form a more open or porous web, having reduced pressure drops but good filtering properties.

: '-~ .':: ~ , "':, : :

25~6 Many other additions or variations in the basic melt-blowing process are possible. For example, melt-blown fibers may be collected in a pattern of compacted and low-density regions, see Krueger, U.S. Pat. 4,042,740. Also, collected webs of melt-blown fibers may be further processed, e.g., by chopping to form fibers useful for inclusion in other products; by compacting in a pattern (see Francis, U.S.
Pat. 2,464,301); by spraying or otherwise adding ingredients to the web; by laminating the web to other webs or sheet products; or by shaping or cutting the web.
~ igures 3 and 4 illustrate a convenient configura-tion and construction for face masks in which fibrous web electrets of the invention may be used. The mask 2B includes a generally cup-like shaped member 29 which is adapted to fit over the mouth and nose of a person, and a strap 30 for sup-porting the mask. The edge of the mask tends to fit rather closely to the contours of the face and thus defines the air inlet to the wearer of the mask; i.e., most of the air breathed by a wearer of the mask must pass through the mask.
The cup-shaped member may comprise an inner non-woven web of air-laid fibers 31, two layars 32 and 33 of fibrous web electrets of the present invention, and an outer non-woven web 34 of air-laid fibers.
The invention will be further illustrated by tha following examples. Two different tests used in the examples for testing filtration capability of the prepared ~ebs -- one using dioctylphthalate droplets (DOP test~, and the other using silica dust in a test established by the National Institut~ for Occupational Safety and Health (NIOSH

:

~s~

silica dust test) -- are described in detail in the U.S.
Federal Register, Title 30, part 11.
Examples 1-8 Blown microfibers were prepared from polypropylene B 5 resin (Hercules "Profax~6330") on apparatus as illustrated in Figure 1. Conditions for Examples 1, 2, 4-6 and 8 were as follows: The die was 20 inches (50 centimeters) wide;
the temperatures of, respectively, the melt in the die, the die itself, and the air expelled from the die, were 346C, 370C and 400C. The air pressure at the die was 0.43 kilo-gram per square centimeter and the polypropylene was ex-truded at a rate of 15 pounds (6.8 kilograms) pe~ hour. The die lip was 60 centimeters from the collector; the distance 35 in Figure 1 from the die lip to the conductors was 3 centimeters; and the distance 36 between the center-line 37 of the fiber stream and conductors 20 or 21 was 2.5 centi-meters. A voltage of 15 kilovolts was applied to each of the conductors 20 and 21 and a voltage of 3 kilovolts was applied to the shells 23 and 24. For Examples 3 and 7 all the conditions were the same except that the melt tempera-ture was 360C, the air temperature was 370C, and the air pressure was 0.5 kilogram per sguare centimeter. Webs were prepared in varying thicknesses and varying weights as listed in Table I~ Most of the examples included a positively charged web (indicated by a + in the tablè below and made by applying a positive voltage to both electrodes 20 and ~1 in Figure 1), a negatively charged web (-), and an uncharged or comparative web (C~. Pressure drop (~P) and particle penetration ~%P~ as measured by the DOP test are given in ~ rr~

..

., -, , ~ .
- : - ~ , . ..

1~;2Z54bi Ta~le I.
TABLE I
Example No.Basis Weight ~P %P
(grams/sq. meter) tmillimeters of water) (percent)

5 1 + 0.29 0.6 61 1 - 0.30 0.6 60 1 C 0.32 . 0.8 83 2 + 0.25 0.9 51 2 - 0.25 0.7 65 2 C 0.25 0.7 80 3 +- 0.26 0.9 54 3 - 0.26 0.9 58 103 C 0.28 . 1.0 78 4 + 0.33 1.1 44 4 - 0.33 1.2 53 4.C 0.32 1.1 7~
5 - 0.45 0.8 61 5 C 0.45 0.7 81

6 ~ 0.52 1.1 46 156 - 0.52 1.3 52 6 C 0.52 1.3 73

7 + 0.52 1.1 44 7 - 0.52 1.2 53 7 C 0.52 1.1 70

8 - 0.65 2.1 32 8 C 0.65 2.1 55 Examples 9-12 Masks as shown in Figures 3 and 4 were prepared from webs of Examples 1-, 1+, 2+ and 3~. Results in the NIOSH silica dust test are given in Table II.
TABLE II
Example Initial Final Final Particle No. Inhalation Inhalation Exhalation Penetration (millimeters (millimeters (millimeters (milligrams) of water) of water~ of watsr)

9 7.9 13.9 9.3 1.39 8.1 14.7 10.0 .66 11 11.6 16.6 16.4 .19 12 12.0 17.8 13.8 .23 ~1~5`~

Charge Decay Tests The decay of the charge on the fibrous web electret of Example 6~ over a period of time was tested by storing samples of the web in polyethylene containers at normal room conditions. The charge decay was determined by measuring the surface voltage with a Monroe isoprobe electrostatic volt-meter and using the relationship between charge and surface voltage (Q=CV, where Q is charge, C is capacitance, and V is surface voltage) to calculate the effective surface charge density. Table III shows the proportion between the initial surface charge and the surface charge measured at various time intervals.
TABLE III
Proportion of Surface Charge Retained After Days of Storage Example 100 200 325 No. Days ~y~ ~y~
6 + 0.96 0.94 0.94 In addition, measurements were made of the decay in charge for samples of the Example 6+ and 6 C webs after storage in a desiccator at 20C and 100 percent relative humidity. The samples were placed in the desiccator 120 days after their manufacture. The proportion of surface charge retained after different periods of exposure is shown in Ta~le IV.

TABLE IV
Proportion of Surface Charge Retained After Days of Storage Example 5 10 25 100 180 No. Days Days Days Days Days 6 + 0.99 Q.98 O.g7 6 C 0.35 0.15 0.1 1~2:;~

In addition to tests on the decay of surface charge, the change in particle penetration through an Example 6+ web after various periods of storage in a 100-percent-relative-humity environment was measured, and the results are shown in Table V. The measurements were made on an apparatus 39 as shown in Figure 5. Air entering a 3-inch-diameter aerosol transport tube 40 is passed through an absolute filter 41 to insure that background particle con-centration is held to a minimum. The challenge aerosol is injected downstream of the a~solute filter at an inlet 42, and passed through a section 43 where, if necessary, the aerosol can be neutralized using a krypton-85 radiation source. The challenge aerosol was a fumed silica dust as described in the NIOSH silica dust test.
The output of the aerosol source is monitored with an aerosol photometer 44 which is mounted on the transport tube. The aerosol photometer employs a photodiode 45 to measure the forward scattered light from particles which pass through the beam from a helium neon laser 46. The amount of scattered light is related to the aerosol concen-tration if the si~e distri~ution of the aerosol population is constant ~ith time. A sample of the aerosol is drawn from the ma~n aerosol stream through conduit 47 and passed through the test filtration media 48. With appropriate valving, the size and concentration of the challenge par-ticles ranging from 0.15 to 3 micrometers are monitored upstream and downstream o~ the filtration med;a using a Particle Measuring System ASAS-200 Asrosol Spactrometer connected to condu;t 4g. Continuous measur~ments are made ~ .

~Li2:~S~

of the pressure drop across the filter tby a pressure gauge 50), the dewpoint temperature as measured in the conduit 51, and the air temperature. The data obtained from this test instrument enables description of filter penetration as a function of particle size rather than on a mass basis.
Typical penetration results on the apparatus of Figure 5 for the webs of Examples 3+ (squares), 6+ (circles) and 6 C (solid dots) are shown in Figure 6. A peak in par-ticle-penetration occurs in the particle size range of 0.3 to 0.6 micrometers, where neither diffusion on inertial depo-sition are very effective. Ho~ever, as seen, fibrous web electrets of the invention provide an improvement for all of the particle sizes.
As noted above, Table V shows penetration results in the apparatus of Figure 5 after the test webs had ex-perienced different lengths of exposure in a lO0-percent-relative-humidity environment. The results reported in Table V are cumulative particle penetrations measured for particles less than a given diameter ~0.3 micrometer, l micrometer and 3 micrometers~; i.e. the result reported in the column headed "3 micrometers" is the percentage of particles up to 3 micrometers in size that penetrated through the test web; the result reported in the column headed "l micrometer" is the percentage of particles up to 1 micrometer in size that penetrated, etc.

- , ;;4~i TABLB V
~ays of Exposure Cumulative Mass Penetration in Percent in 100-%-R.H. at Different Particle Sizes - 0.3 micrometer 1 micrometer ~ m crometers 0 0.012 0.24 2.4 1 0.019 0.30 3.3 7 0.008 . 0.34 3.0 0.009 0.24 1.7 180 0.008 0.29 2.6 - ~ . .

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A fibrous web electret comprising melt-blown fibers which hold electrically charged particles and there-by carry a persistent electric charge that is useful to enhance filtering properties of the web, said charge having a half-life of at least one week in a room-tempera-ture, 100-percent-relative-humidity environment.

2. A fibrous web electret of claim 1 in which said melt-blown fibers comprise polypropylene.

3. A fibrous web electret of claim 1 in which said melt-blown fibers average less than about 25 micro-meters in diameter.

4. A fibrous web electret of claims 1, 2 or 3 which further includes staple fibers interspersed with said melt-blown fibers.

5. A fibrous web electret of claims 1, 2 or 3 which further includes solid particles dispersed in the web.

6. Filter apparatus comprising in combination a fibrous web electret of claims 1, 2 or 3 and a support structure supporting the fibrous web electret across a stream of fluid that is to be filtered.

7. A respirator comprising support structure for mounting the respirator on a person, and filter means through which air is drawn from the ambient environment to the mouth and nose of a person wearing the respirator, said filter means including a fibrous web electret of claims 1, 2 or 3 disposed so as to filter air drawn into the respirator.

8. A respirator which comprises a cup-like member adapted to fit over the mouth and nose of a person wearing the respirator, and in which at least a layer of said cup-like member comprises a fibrous web electret of claims 1, 2 or 3.

9. A fibrous web electret comprising a coherent mass of randomly intertangled melt-blown microfibers which average less than 10 micrometers in diameter and which hold electrically charged particles in an amount sufficient to provide a persistent electric charge of at least 10 8 coulombs per gram of said melt-blown microfibers, with a half-life of at least six months in a room-tamperature, 100-percent-relative-humidity environment.

10. A method for forming a fibrous web electret comprising 1) extruding molten fiber-forming material that exhibits a volume resistivity of at least 1014 ohm-centimeters through a plurality of orifices into a high-velocity gaseous stream where the extruded molten fiber-forming material is attenuated to form a stream of fibers;
2) bombarding electrically charged particles at said stream of fibers as the fibers issue from the orifices; and 3) collecting said fibers at a point sufficiently remote from the orifices for the fibers to have cooled to a solid fibrous shape-retaining form.

11. A method of claim 10 in which staple fibers are intro-duced into said stream of fibers after the stream has been bombard-ed with charged particles.

12. A method of claims 10 or 11 in which solid particles are introduced into said stream of fibers after the stream has been bombarded with charged particles.