CA1194448A

CA1194448A - Method of making fibrous electrets

Info

Publication number: CA1194448A
Application number: CA000398121A
Authority: CA
Inventors: Larry C. Wadsworth; Solomon P. Hersh
Original assignee: Surgikos Inc
Current assignee: Ethicon Inc
Priority date: 1981-03-12
Filing date: 1982-03-11
Publication date: 1985-10-01
Also published as: MX152157A; NO820790L; FI74213C; AU8129082A; FI820841L; AU550114B2; BR8201328A; IE52787B1; DE3276458D1; ES510351A0; EP0060687B1; FI74213B; ES8304841A1; EP0060687A3; IE820557L; ATE27510T1; DK107282A; ZA821640B; US4375718A; EP0060687A2

Abstract

METHOD OF MAKING FIBROUS ELECTRETS

Abstract of the Disclosure A process of manufacturing an electrostatically charged filtration medium is disclosed. A web made of noncon-ductive thermoplastic fibers is contacted on each side with a more conductive web to form a combined web. The combined web is charged with electrically charged particles from corona charging elements on opposite sides of the web. The charging elements are operated at a voltage of from 5 to 25 kV but with opposite polarity.

Description

;rSU-36 METE30D OF lMAKING FIBROUS ELECTRETS

This invention relates to a me~hod of forming f ibr~us electr@ts ~or use as a filtra~iGn medium.

Eleckrically charged f ibrou~ ma~erials to be us~d as a f iltra'ciorl medium have been known for some time . In UOS. Patent 2,740,184, Thomas discloses a process of charging ~hermoplas ic, fibrous w~s by sof~enirlg the 10 fibers in t:he webs wi~h heat and, whil~ such fibers are soft, subjecting them to a ~uitable electro~tatic field to produce a charged f ibrous web.

U.S. Patent 3,998,916 to Van Turnhout discloses a process 15 o:E manufacturing electrically charged fibro~ ilters usi~g a ibrou~ material produced from a f ibrillat~d f ilm.
The film i~ hea~ed to a temperature rlear i~s melting point and is subjec~ed to an elec~rostatic charge from a plurality o~ corona charging elemen~s. The charge~ web is 20 then f ibrillated, and the resulting f ibers are colleGted and processed in~o a fil~er. The Van Turnhout U.,S. Patent 4,178,157 disclose~ a similar process in which ~hP fib2rs are also crimped.

Simm et al UOS. Pa~ent 4t0~9,025 discloses an electrostatic spinning process in which fiber ma~eri 1 is sprayed electrostatically from a liquid s~a~e and deposited on a conductive ~uppor~.

UoK~ Patent ~pplication 2,015,253A to 3M publi~hed SeptO5~ 1979~
di~closes a proce~s of forming fibrous electrets by a melt blown process in which the melt blown fibers are charged with an electrostatic charge immediately after they are formed and then deposited on a web. The patent discloses that these fihrous webs are useful as ilters and spec~fically for face masks.

. JS7~-36

2~

U.S. Patent 3,571/679 discloses a process for forming a f ibrous electret using contacting electrodes and elevated temperatures in which the electrodes are covered with weakly conductive covers m~ from an asbestos cement mixture to prevent arcing.

All of these references indica~e that it is n~cessary to charge the fibrous material or fiber-forming material when the material is at a temperature near its melting point in lQ order ~o trap the electric charge within the fibrous material.

According to the present invention, there is disclosed a lS ~ethod of applying a permanent electric charge by means of corona charging to a f ibrous web when the weh is at room ~emperature, i.e., approximately 20C ~o 25C~ which is a temperature considerably below the melting temperature of the filaments of ~he web. The charging is accomplished by ~0 applying a contact web, which is more conductive than the dielectric fibers of the filtration medium, ~o the fil ration medium and applying the charge by corona charging through the more conductive medium.
~ process is thus pro~.ided of making a filtration web which comprises joining a fibrous conduc-tive web to each surface of a web of filtration medium, the web of filtration medium comprising thermoplastic fibers which have a diameter of from 0.3 to 80 microns, charging one surface of the joined web wlth electrically charged paxticles fxom a pair of corona charging ele-ments with a first corona cha.rging element connected to a voltage of from 5 to 25 kV, charging the opposlte 5ur~
face of the joined web with electrically charged part icles with a second corona charging element connected ~4~

to a voltage of from about 5 to 25 kV but with the oppo-site polarity as that ~f the first element.

The filtration medium is composed of fibers which have dielectric proper~ies, The contact web, which is in contact with such fibers, is composed of material which will conduc~ ~he electrical charge to the dielec~ric f il~ration mediumc The f ibers in ~he dielec~ric filtration medlum are thermoplastic and are made of a polyolefin such as polypropylene or polye~hylene or may be made f rom polycarbonates or polyhalocarbons . The contact web may be a woven or nonwoven web made of cellulosic fiber such as cotton, rayon, woodpulp or hemp or mixtures of these f ibers, or may be a nonwoven web ~ade f rom highly ~ ~ !

-3 dielectric fibers but bonded together with a conductive binder. The nonwoven webs contain an adhesive binder~
The contact web will conduct the electrostatic charge but has poorer dielectric properties than the filtration medium fibers and will not maintain any signif icant charge.

The room temperature or cold charging of fibrous electrets according to the present process offers certain advantages over the elevated ~emperature or hot charg ing Gf ibrous electrets according to the prior art paten~s mentioned above. With a cold charging system, a ~reater range of fibrous materials can be easily charged to form a filtration medium. The fibrous webs need not be melt-blown or fibrillated films but may be formed intowebs by any of the standard nonwoven fabric processing techniques such as air layingr carding~ or spun bonding, as well as webs formed from fibrillated films or melt-blown webs. The ability to charye web~ of different constructions allows a processor greater flexibility in making a filtration medium than would be available to the processor employing a hot~-charging technique where the charge is applied to the fiber of the web when the fiber is ~ade~ It is also possible, using the cold-charging technique, to process or reprocess filtration we~s which may have previously been charged and whose charge has been dissipated by inadvertence or upon aging of the webs. The contact dwell time, that is, the time during which the fibrous web is subject to the corona chargingJ can be varied in the present processO Gen rally, it is a much longer period ~han in the prior art process. For example, the contact or dwell time in the proce~s disclosed in U~S. Patent 4~215/682 is less han ~ne millisecondt and the dwell time in the present process is from about ~01 to 35 1 seconds. Because of the greater dwell time in the pres~nt process, there is a better opportunity for the ~ibers to acquire a charge.

Th electrostatic charye is applied to the fibrous web using corona discharge bars~ These bars have point emitters which produce a corona in the vicinity of the bars causing the air arou~d ~he b~r~ to ionize thereby forming charged par~icles. These charged particles will migra~e to the contact web and induce or convey a charge to the filtration medium. The charge bars have a voltage of from 5 to 25 kV. There is at least one charging bar on each side of the webJ and they are preferably maintained at the same potential level but with opposite polarity.

Contrary to the indications in ~he prior art refer~nces that the application of an electrostatic charge to a fiber after the fiber has been fully formed will not be permanent, applican~s have found that by applying the charge according to the process of the present invention the charge is permanent, and the filtration webs produced by the present process have mainta.ined their charge as long as filtration webs produced by the prior art processes.

~
Figure l is a cross-sectional view of a filtration medium made in accordance with the present invention~

Figure 2 is a schematic illu6tration of the process for carrying out the present invention.

Figure 3 is a schematic illustration of another embodiment for carrying out the process o the present invention.

Figures 4 and 5 are illustr~tions showing the charge that is applied to the filtration medium~

Electret fibrous filters are highly efficient in filtering air because of the combination of mechanical entrapment of particles in the air combined with the trapping of particles based on ~he electrical or electrostatic characteristics of the ibers. Both charged and uncharged particles in the air, of a si~e that would not be mechanically trapped by the filtration medium~ w.ill be trapped by the charged nature of the filtration medium.
The filtration medium is composed o fibers which have dielectric properties. The fibers are made from thermoplastic polymers which are hydrophobic and thermally stable over temperatures which will be encountered in conditions of use. These polymers are preferably polypropylene or polyethylene. The fibers themselves may be formed by any one of the commonly employed methods of forming f ibers . The fibers may be initially collected as individual ~ilaments and subsequently formed in~o a web, or they may initially be formed as a nonwoven f ibrous web.
If the fibers are initially collected as individual filaments, the filaments may be processed into webs by any of the common nonwoven fabric manufacturing processes.
These include air laying, carding, or other known nonwoven fabric manufacturing processes. The nonwoven webs may be bonded with an adhesive binder as long as the binder will not interfere with the ability of the web to mainl:ain its dielectric properties.

The fibers that are used in the f iltration medium may be of any size which is normally made by a particular fiber-making process. For example, melt-blown fibers are usually from about 0.3 to 5 microns in diameter and are usually made in a length of several inches~ Spun-bonded fibers are about 5 to 50 microns ln diameter and are usually made in continuous length~. If the fibers are o:E
the type that are formed lnto webs by carding~ th2y are ~6--~sually made in sizes of approximately 10 to 30 microns, and their length may be from approximately 1/4 to 1 9/16 inches~ I~ a wet-laid process is employed to manufacture ,he -~bs of the filtration medium, the fiber length may be from approximately 1/8 inch to 1 9/16 inches. In general, the fibers that are useful in this process may have a diameter from approximately 0.3 microns ~o about 50 microns. It should be understood, that for particular f ilter applications, a particular diameter f iber may be more advantageous to employ than a fiber of a different diameter.

The weight of the webs that may be employed as khe f iltration medium may vary over a wide rangeO If the filtration medium is to be used as a sur~ical face mask, the weight of ~he medium would b~ from about 0~4 ounce per square yard to about 1 ounce per square yardO If the filtration medium is to be used in an industrial or home f ilter~ the weight may vary from about 3 ounces to about 6 ounces per square yard.

The differerlce in the conductivity between the dielectric filtration medium and the contact web i~ expressed as a di~ference in the resistivity of the materials. The volume resistivity of the fil~ration medium is greater than abou'c 1016 ohm-cm, The resistivity of the contact webs is about 101 to about 1013 ohm~cm.
It has been empirically de~ermined that the resistivi'cy of the ~iltration web ;shou:Ld be at least 1~3 times the contact web's resistivity, The contact webs which are used in the present process may be woven or nonwoven webs made from cotton, rayon, or mixtures of cot'con and rayon with woodpulp or other f ibers ~uch as hemp and may contain conductive ibers corltaining dispersed carbonO These webs I if nonwoven, may be bonded , . . .

by any conventional, nonwoven bonding system which may employ a hydrophilic or hydrophobic binder. The contact web which is employed in the present process does not maintain the charge whic~ is maintained by the filtration web. The nonwoven contact webs may also be made from non-cellulosic fibers such as polyethylene, polypropylene, polyamide or polyester and bonded with a binder that is conductive so that the conductivity of the contact web is greater than the conductivity o the filtration medium lO web. The weight of the contact web may vary from 0.3 ounces p~r square yard to about 6 ounces per square yard.

The present process may be better understood by referense ts ~he drawings~
1~
In Figure 1 there is shown a partial cross-section 12 of filtra~ion medium 11 with a cont~c~ web 10 on each side of the filtration medium. These webs are in contact with the filtration medium during the time that the composite web is subjecked to the electrostatic charging elements.

Figure 2 shows one embodiment or the process for manufacturing he f ltration elemen~s of the present invention. A source of filtration medium web 13 is unwound and passed between rollers 23 where it is brought into contact with contact webs 17J There is a contact web 17 on each side of the filtration web 13~ The webs are then brought into proximity of two corona discharging units 18 and l9. There may be an additional set of corona discharging bars 18' and 19' a~ hereinafter de~cribed.
The corona discharging units are spaced from the webs 17 a distance of approximately l/2 ~o 2 l/2 inchesO The distance should be such that the voltage applied to the corona discharge elements will not jump the air gap between ~he corona discharg~ elements~ This dis~ance is dependent not only on the space but also on the voltage of 3L~ JSU 36 the elements. As the distance between opposing bars is increased, the voltage on the bars may be increased~ The discharge elemen~s are charged at a level of from approximately 5 to 25 kY. The discharge elements are normally in balance, that is both elem~nts 18 and 19 are at the same charge voltage, but the elements have opposite polarity. If element 18 has a positive ch~rge, then element 19 has a negative charge. It i~ possible to apply a charge to the filtration medium with ~he opposing charging units af different voltages~ There should be some voltage applied to ~oth charging uni~s and he voltage on opposite sides of the web must be of the oppos i te po 1 arity.

Multiple charge bars 18 7 18 ' and 19, 19 ' on each side of the web may also be employed. The bars on the same side of the web should be spaced apart a suf f icient distance so that there is no arcing between ad jacent bars O The adjacen.t bars may have the same charge or an opposite 20 charge and may be charge~ at dif ferent voltage levels The bars on the opposite side of the web must have opposing charges. An adequate spacing b~tween adjacent charge bars is from about 5 to about 10 inches. A:Eter the charge has been applied to the web, the rontact webs are separated from the filtration medium by passage over rollers 15 and the filtra~lon medium is wound on reel 1~
As shown in Figure 4, the charge that the present process applies to the surface of the filtration medium may be opposite the polarity of the charging bar nearest the surfaceO That is, if charging bar 18 (Figure 2) is negatively charged, the surface of the filtration medium close~t to the bar 18 will be positively charged. This is known as heterocharging as contrasted to homocharging where the charge on the medium would be of the same 35 polarity as the charging bar. E~eterocharging has not previou~ly been observed to occur while employing any ~ype ~ JSU 36 ._9 of corona or charge deposition or lnjection charging process. It has been observed only as a resul~ of dipole alignment or charge separation in thermal charging processes usin~ contact electrodesO
s Under some conditions of operation of ~he present process, the surface of the filtration medium nearest to the positive charging bar retains a positive charge f which is illustrated in Figure 5. This is homocharging.
Generally; if the contact web is thin and is made of fibers which are polar in nature~ e.g., cellulosic or polyamide~ heterocharging will occur. If the contact web is thick, for example a cotton prlnt cloth or is made of non-polar or hydrophobic fibers, polyester9 polyethylene, etc., homocharging will occur. The filtration efficiency of the fil~ration medium is substantially identical regardless of whether the charging process is homocharging or he te rochar~ ing .

An alternate method of forming the filtration webs o~ the present invention is shown in Figure 3. In the me hod shown in this figure, the contact webs are not separated from the iltra~ion web after charging but are retained on the web and become part of the final filtration unit.
25 This method is especially useful in the manufacture of surgical face masks. A source of filtration medium 20 is unwound and brought into contact with two facing layers which are unwound from rolls 21 and ~2. The three layers pass between rollers 23, which bring the facing layers 30 into contact with the f iltration medium. The combined webs are then passed in proxi.mity to the corona discharge elements 18 and 19.

The contact webs become the facing layers of the surgical face mask. In a typical surgical face maskl ~he con~act webs would be nonwoven fabrics made from rayon fibersO

~ U 36 lû--The rayon fibers are conductive~ The combined web is subsequently passed through rollers 24 and then w~und on reel 25. In the manufacture or face masks, the web would be unwound from the roll 25, cut to size, folded and a seam of binding a~d ties applied. If desired, the web can be fed directly from the charging station to a face mask manufacturing station.

In either of the above-mentioned processesj the line speed of the web passing in proximity to the corona charging elements can be varied over a relatively wide range. In general, the line speeds can be from about 5 to 60 feet per minute, and in some instance, if the web is very thin, khe line speeds can be in excess of 100 feet per minuteO
lS The contact webs will not retain any significant electro~tatic charge after they are remov~d from proximi~y of the corona charging bars.

In khe following Examples/ the ~iltration efficiency of the webs was kested by two different methods. The test reported as l'B.F.E.~ is a bacterial filtration efficiency test. This test is run in the following manner.
Staphylococcus aureus bacteria ~e nebulized into a spray mist and forced through an aperture in a closed condui~.
The bac~eria pas~ing through the aperture are ~rapped on a Millipore Filter and th n innoculated on agar plates. The ~ame procedure is repeated with the filtration medium to be tested blocking the aperkure o~ the conduit~ ~fter a period of 24r48 hours, the bacteria colonies are counted.
The efficiency of the filtratiGn medium is determined by eomparing ~he colony coun~ on ~he pla~es wi~h and wi~hou~
the filtration m~dium in the ap rture, The results are expressed as a percenkage which represents the reduction of ~h~ bacteria colsnies when the f ltration medium is in place~
* Tradem~rk , .

~ SV-36 The test reported as "F.E.T. " is a test employing uniform polymer latex microspheres dispersed in water in place of the bacteria of the B.F.E. test. An ~erosol of the polymer partiele~ dispersed in water is diluted in an air stream which is then passed through a filtration medium holding device and then to a particle counter where the polymer particles are counted. ~efore the air stream reaches the particle counter, the liquid water ln the system is evaporated and removed from the air stream. A
sample of the filtra~ion medium to be tested is then inserted in the holding device, and the air stream is again d.irec~ed to the particle counterO The difference in the particles counted with and without the filtration medium in place is an indicatlon of the efficiency of the filtration medium and i~ expressed as a percentage of the parkicles removed from the ~ir stream by the filtration medium. O the two tes~s, the F.E.T. test, because it does ~ot have the variables of a biological ~es~, gives more reproducible results.
The following Examples show the effec~ of eold charging on the filtration efficiency of a filtration medium prep~red according to the present invention compared to the same webs which were not charged and to webs which were charged without the con~act web in contact with ~he filtration medium. Examples 2~6 in Table 1 were all charged with a contact web on each surface of the fil~ration ~dium during char~ng. The media in Examples 2-6 was a mel~ ~lown polypropylene webO It should be noted that in Example 4 the polarity of the charge bars was iden~ical and tha~ the filtration efficiency of the resulting web wa~ inadequate, 'l', Table 1 Ao Laminatel Weight and Charge Conditions Space Spe~od 5Web We igh~ TopBottom From We~ of Web ~ ~ ~ ~ Bar kVBar lcV (Inches) Ft/Min.
___ 2 . 4 4 No Charge 2 2049 -18 +18 2~5 30 3 2. 46 -10 +10 ~. 5 30

4 2.46 +18 ~18 2.~ 30 2 . 59 -18 +18 2 . 5 60 6 2. 52 -18 ~1~ 2 . 5 15 B. Data Surface Potential2(Volts~ Latex Microsphere 15Example ~3 Bot~om4 Filtration Eff . (F.E.T.
-40 ~30 40 . 9 2 +785 -20 g5. 2 3 +3~0 ~140 950 9 4 -5 1~ S1 . 3 ~625 -~L5 g5 . 2 6 +7~0 -300 9~ . 8 1 Laminate weights in examples 1-6 include weight of filtration medium plus 1.23 oz~sq. yd. combined weight 25 of both facings.

2 Voltage potentials measured on the media after the facings were rem~ved with a Keithley i!lodel 2501 5tatic Detector Head and a Model 621 Electrometer. The media 30 samples were placed on a grounded steel plate, and the surface poten~:ial of the sur~ace opposi~ the steel plate is measured.

3 Side o~ filter medlurrl closest to top charge 35 bar~

~ SV ~6 4 Side of filter medium closest to bottom charge bar, In the following Table 2, the filtration media in Examples 8 and 9 were charged directly without a contact web. All other charged examples in Table 2 were charsed in contact with a cellulosic contact web. All filtration tests and air resistance tests were performed on laminates consisting of filter media plus two facings~ Examples 8 10 and g were laminated af ter charging O

Table 2 A. Laminatedl Weight and Charge Conditions Space Speed Web Weight Top Bottom From Web o:E Web ~@g~ Bar kV Bar kV ( Inches ) Ft/Min .
7 2 .15 No Cha rge 8 2, 11 ~12~12 1 . 5 15 9 2.15 -12 ~12 1.5 15 10 10 ~.14 No Charge 11 2. 20 -12 ~12 1~ 5 5 12 2. 15 12+12 1 . 5 10 13 2 . 1 3 -12 ~12 1 . 5 30 14 2.14 ~ 12 1.5 60 B. Data Surface Potential2 Filtration Eff (~) ( Volts ) Air Resistance ~ ~3 BO~tom4 F.E oT~ B._~E. ( in . water ) 2a 7 -47 -~05~ . 0 64 . 0 0 . 22 8 l-70 -377~.0 83.6 0.21 9 +197 -1678Q . ~ 82 . 2 0 . 21 -37 -2748 O 6 68 . 4 0 . 23 11 ~2~7 ~2395. 5 97 . 9 0 O 26 ~5 12 ~400 -1209~.9 97~1 0.25 13 ~227 ~7795,5 97O3 0.23 14 +167 +16395 O 5 98 . 0 0 O 24 1 Larninate weight~ in Pxamples 7-14 include we.i~h~ c:f filtration medium plus 1.15 oz./sq. yd~ combined weight of both facingsO

2 Vol~cage poten~ials were measured a~ set forth in Note2 of Table I on the media after the facings are JSU~36 removed with a Keithley Model 2501 Static Detector Head and a Model 621 Electrometer.

3 Side of filter medium closest to top charye bar.

4 Side of filter medium clcsest to bottom charge bar.

All surface potential measurements in the Examples shown in Tables I and II were performed on the media after the facings were removed as described by Weiss and Thibodeaux in "Cotton as an Electret~" Textile Res. ~., 47r 471-476 (1977)~ The uncharged examples 1, 7 and 10 all had less than 50 negative volts of surface potential on both sides of the filtration medium, and the filtration efficiencies were lik4wise rather low with F~EoT~ v~lues o 40.9, 58.0 and 48.6%, respec~ively. The slight negative charges on the uncharged medium are attrlbutable to tribo~l~ctric and ~0 separation charges resulting from contact potential dif--ference between different materials. On the other hand, the cold charged laminate samples 2-6 and 11 14 all had positive poten~ials on the top surface of ~he filtration medium nearest to the negative charge bar, ranging from ~167 to +785 volts. The filtration efficiencies of all cold charged laminates were considerably improved over that of the uncharged laminates with FoE oT~ values of the charged laminates ranging from 94.9 to 95.9%, and In Vltr~
Bacterial Filtra~ion Efficienci~s of (~.F.~.) ranging from 97.1 to 98,0%~ In contrast, the iltration medium which was cold charged without facings (Examples 8 and 9) generally had lower surface potentials than the filtration medium of the cold charged laminates, with the resultan~
charges being either heteropolar or homopolarr Examples 8 and 9 being heteropolar. The corresponding filtration effici2ncies of the examples and other unreported tests were only of intermediate levels betweerl unchargecl and JS~-36 cold charged laminate with F.E.T~ values ranging from 61.1-80.2%, with a single sample having a F.E.T. value in the 90~92% range.

Example 15 Samples of a 1 ply, 2 ply and 4 ply carded, unbonded polypropylene fiber web were placed in contact with a layer of a rayon nonwoven contact web on each side of the polypropylene web and tested for flltr~tion efflciency by the F.E.T. test. Identical samples were then charged under the process conditions set forth for Example 8 and 9 and tested for filtration efficiency. The total weight of the contact webs for each sample was 1.23 oz./sq. yd. Th~
weight of the laminate of polypropylene web and contact web and the results of the F.E.T. are shown in the followiny Table 3.

Table 3 Lamina te We i gh t F . E . T ~ %
_oæ./sq~ yd~ Uncharged Charged 1 ply polypropylene 1.74 0.0 42~4 2 ply polypropylene 1~ 95 35, 9 59 . 6 4 ply polypropylene 2.88 66.9 96.4 ~5 Example 16 A series of dif ferent woven and nonwoven webs were placed in contact wi.h a melt blown polypropylene filtration medium. The lamina~es were char~ed according to the process o this invention with the charging bars at 18 kV, a spacing of 2~5 inches and at a line speed of forty feet per minute. Samples of each laminate were tested for filtr~tion efficiency by the F.E~T. test~ In some samples, the contact web was removed prior to the F . E . T . tPst and replaced wi th ~ standard rayon nonwoven facing material for testing. In additionl the volume resistivity of each facing was determined in a Keithley Model 6105 Resistivity Chamber~ The results are reported in Table 40 Tab_e_4 Faclng Used for F~EoT~ Volume Resis Contact Web r~ ~ %

Rayon NonwovenSame 99.7 6~7 x 101 Spunbonded Nylon Rayon 0.3 oz./sq. yd.Nonwoven 99.0 2.2 ~ 1013 Spunbonded Nylon 0~3 oz./sq.yd.Same 93-9 ~.~ x 1013 Spunbonded Nylon Rayon 0.5 oæ./sq.yd.Nonwoven 99.4 104 X lol3 Spunbonded Nylon 0.5 oz~/sq.yd.Same 99.4 12~ X lol3 Cotton Print Rayon Cloth Nonwoven 99.0 9.5 x 101 Woven Polyester with Conductive Dispersed Rayon Carbon Filaments Nonwoven 96O8 1.4 x 1 ol3 Woven Polyes~er/Cotton/
Conductive Dispersed Rayon Carbon Fiber Blen~ Nonwov,on 38 . 8 1~ 9 x 10 _xample 17 Samples of a melt blown pol~propylene filtration medium were joined with a rayon nonw3ven con~act fabric on each side of the filtration medium. The combined weight of the contact fabric was 1.15 oz./ sq. yd. The combined web was passed be~ween two sets of corona charging bars and the polarity of the charging bars was varied. Under configuration No. 1, the upper bars were both negatively charged. Under configuration No . 2, the upper bars were both positively charged. Under configuratiGn No. 3, the first upper bar was positively charged, and the ~econd upper bar was negatively charged. The lower bars were always charged tG the opposite polarity of the bar on the opposite side of the w~b. The charge was 18 kV on all bars, and the spacing of opposing bars was 205 inches, and the spacing of adjacent bars was 5 and 9/16 inches. The F~To Of the laminates was determined and showed that the particular placement configuration o the charging bars 20 had no deleterious effect on the F.E,T~ of the laminates.
The results are reported in Table 5.
Table 5 ~ F.E.T. %

No charge 68.3 No. 1 98.6 No. 3 98.0 No. 2 990~

No. 3 9805 No. 3 98~8 No. 2 99.2 JSU~36 Example 18 To demonstrate the ability of filtration media made by the process of the present invention to maintaln their filtration efficiency on aging, filtration media made by the present process were aged for twelve weeks at a temperature of 120F and 90% relative humidity. The results are shown in Table 6.

Table 6 Filtration Efficiency Weeks Aging % B.F.E.
1 97.1 2 93.4 3 96.9 4 95.7 89.6 6 95.2 7 93.9 8 89.8 9 92.7 89.5 11 94.3 12 94.1

Claims

Claims:

1. A process of making a filtration web comprising:

joining a fibrous conductive web to each surface of a web of filtration medium, the web of filtration medium comprising thermoplastic fibers having a diameter of from 0.3 to 80 microns, charging one surface of the joined web with electrically charged particles from a pair of corona charging elements with a first corona charging element connected to a voltage of from 5 to 25 kV, charging the opposite surface of the joined web with electrically charged particles with a second corona charging element connected to a voltage of from 5 to 25 kV
but with the opposite polarity as that of the first element.

2. The process of Claim 1 in which the conductive web is removed from contact with the filtration medium after charging.

3. The process of claim 1 in which the second corona charging element is connected to the same voltage as the first element but with the opposite polarity.

4. The process of Claim 1 in which the filtration medium is composed of polyolefin fibers.

5. The process of Claim 1 in which the voltage applied to the corona charging elements is between 10 and 20 kV.

6. The process of Claim 4 in which the polyolefin fibers are polyproylene, the major portion of which are between 0.3 and 5 microns in diameter.

7, The process of Claim 6 in which the weight of the filtration medium is between 0.4 and 4 ounces per square meter.

8. The process of Claim 1 in which the joined web is charged by a second pair of corona charging elements which are connected to voltage of from 5 to 25 kV and with the elements having opposite polarity.

9. The process of Claim 1 in which the combined web is maintained in the corona of the corona charging elements for from 0.01 to 1 seconds to produce a per-manent charge on said filtration web.

10. The process of Claim 9 in which the volume resis-tivity of the conductive web is less than the volume resistivity of the dielectric fiber web by a factor of at least 103.

11. The process of Claim 9 in which the conductive web is a cellulosic fabric and is permanently attached to the filtration medium.

12. The process of Claim 9 in which the voltage applied to the corona charging elements is between 10 and 20 kV.

13. The process of Claim 9 in which the filtration medium is composed of polyolefin fibers.

14. The process of Claim 13 in which the polyolefin fibers are polypropylene, the major portion of which are between 0.3 and 5 microns in diameter.

15. The process of Claim 9 in which the temperature of the webs during charging is at room temperature.

16. The process of Claim 9 in which the combined web is moved between a second pair of corona charging elements which are connected to a voltage of from 5 to 25 kV and with the elements on opposite sides of the web having opposite polarity.