CA2087998A1

CA2087998A1 - Process for forming electret filter media and electret filter media

Info

Publication number: CA2087998A1
Application number: CA002087998A
Authority: CA
Inventors: Lee K. Jameson; Bernard Cohen
Original assignee: Kimberly Clark Corp
Current assignee: Kimberly Clark Worldwide Inc
Priority date: 1992-09-23
Filing date: 1993-01-25
Publication date: 1994-03-24
Also published as: MX9305691A; US5370830A

Abstract

ABSTRACT OF THE DISCLOSURE

The invention is directed toward a process for forming an electret filter medium. The invention is also directed to electret filter media.

Description

2~,8rl,998 .. RELATED APPLICATIONS

Other patent applications dealin~ wi.th applications o~
- hydrosonics by the present inventors include U.S. patent application serial number 07/769,050 entitled "Hydrosonically Microapertured Thin Thermoset Sheet Materials"; U.s. paterlt application serial number 07/769,0~7 entitled "Hydrosonically Microapertured Thin 'I'hermoplastic Sheet Materials"; UOS.
patent application serial number 07/768,7,32 entitled "Pressure Sensitive Valve System and Process For Forming Said System";
U.S. patent application serial number 07~768,494 entitled Hydrosonically Embedded Soft Thin Film Materials and Process For Fo:rming Said Materials'7; U.S. patent application number 07/768,788 entitletl "Hydrosonically MicroaF,ertured Thin Na~urally Occurring Polymeric Sheet Materials and Me~hod of Making the Samel'; U.S. patent application serial number 07/769,048 entitled "Hydrosonically Microapertured Thin Metallic Sheet Materials"; U.S. patent application serial number 07/769,045 entitled "Process For Hydrosunically Microaperturing Thin Sheet ~aterials"; and U. S . patent application serial number 07/767,727 entitled i'Rrocess For Hydrosonically Area Thinning Thin Sheet Mat~rials". This ~il7il25 first group of applications was filed on September 30, 1991.
, All of these applications are hereby incorporated by reference.
Additional applications, by the present inventors, .' directed to applications of hydrosonic energy which are being 3 30 filed contemporaneously with the present application include ., 1 .
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U.S. patent application serlal number entitled "Hydrosonically Bonded Nonwoven/Paper Material and Process for Forming the Same"; U.S. patent applica~ion serial number __ entitled l'Method for Forming a Net-Like Material From a Thermoplastic Film"; U.S. patent application serial number entitled "Process for Hyclrosonically Area Embossing ~ Thin Th~rmoplastic Film Materials and Products Formed ; Thereby", and U.S. patent application serial number _entitled "Method for Forming Ultra-Microapertures in Thin Thermoplastic Film Materials and Products Formed Thereby".
All of these applications are hereby incorporated by ` reference.
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The field of the present invention encompasses electrets and processes for their formation.

¦ BACKGROUND OF THE INVRNTION

, 1 U.S. patent number 4,944,854 to Felton et al. states that it is known that certain dielectric materials can be permanently electrostatically polarized and tha~ the~e materials are polarized by, (1) exciting the material by heating, (2) applying a high-voltage electric field, and (3j cooling the material while under the influence of the electric field. Upon removal of the electric field, appropriate dielectric materials will be found to have become the , .. , . .. . _ . .

electrical analog of a permanent magnet. A dlelectric becomes an electret when the rate of decay of the field-induced polarization can be slowed dow~ so much that a significant fraction of the polari~ation is preserved long after the polarizing field has been removed.
The ,Felton et al. '854 patent continues by stating that early methods for the formation of fibrous electrets from thermoplastic fllms or filaments involved disposing the thread or filaments in an electrostatic field established between parallel closely spaced electrodes. This process, which is disclosed in U.S. Pat. No. 2,740,184, heats the thread or filamentary material to soften it and then cools the mat~rial in the presence of the electrostatiG fleld whereupon charges are introduced in~o the fibers. The voltage employed in ' 15 charging the material is ~000 vol~s or more o~ direct current.
-` The filamentary material itself is a hollow filamentary :,;
;~, material having a wax core such as carnauba wax, the resulkant product being described as having a "more or less permanent l charge". In order to produce that degree of permanence of ,~, 20 charge in a non-wax electret, such as for instance a ,l polyolefin electret, it has been found that substantially , higher voltages must be employed, that is to say direct current voltages in the range of 8000 volts or more. When ' such higher voltages are employed, the electret field will 'i~ 25 break down and arcing will occur in the ~ree air space ~,' , surroundiny the single fiber or filamentary material employed according to the teachings ~f U.S, Pat. No. 2,740,184.

~3 ~t~o~8 The Felton et al. ~854 patent further states t.ha-~ arcing produced fro~ high voltages, that is to say voltages of 8000 volts direct currenk or higher, may be circumvented by covering the electrod~s with a poorly conductive mate~ial so as to evenly distribute the applied voltage and dampen possible dielectric breakdown. For instance, U.S. Pat. No.
3,571,673 discloses a process ~or forming electre~s by closely ~: winding a monofilament fiber such as a polypropyle~P fiber on a hollow winding roller which has been previously surfaced with a polyamide faced aluminum foil. Subsequent to winding the layer of fibers, a second polyamide faced aluminum foil is wound about the yarn layer. The fiber and foil wound roll is then disposed between two electrodes where it is polarized for three hours at a temperature of about 1200 C. wikh a vol~age of 200 volts. This method, however, is disconkinuous and extremely slow in that charging times of a~out three ~ours ~` for the wrapped roll are required.
The Felton et al. '854 patent also states that as a result of such deficiencies, electrets were then commonly producPd by either a spray spinning technique such as that sek fGrth ¦ in u.s. Pat. No. 4,215,682 wherein an eleckric charge is introduced into meltblown fihers during the melt-blowing process, or alternatively, the electrets are prepared from a ~` film which is homopolarly charged and subsequently fibrillated (see U.S~ Pat. No. 3,998,91~
Other background materials dealing with electreks include (1) the Introduction of Topics in Applied Physics, G. M.
Sessler, Vol. 33, 2nd ed., 1987, pp 1-12.; (2) Recent Proqress ' 4 ; .:

in_Electr2t Research, Topics in Appliecl Physics, R. Gerhard-Multhaupt et al., Vol. 33, 2nd ed., 1987, pp 383-431 and (3) Electrets and Rclated Electrostatic Charqe Storaqe Phenome~a, L. M. saxt et. al., The Electro~hemlcal Society~ Inc. 1968 (LC
no. 68 23768).
One ~f the dlfficulties with which those in the art have b~en faced with is the ability to form an electret which may be used in filtration devices and which is formed from a thin film material without having to subsequently fibrillate khe film into fibers or filaments.
As can be seen in the paragraph citing related applications, we have filed several patant applications ; dealinq with applications of hydrosonic energy. A starting poinE in the understanding of the princlples of hydrosonic energy is a fundamental ~nowledge of ultrasonicsO Ultrasonics is basically the science of the ef~ects of sound vibrations beyond the limit of audible ~requencies. Ultrasonics has been used in a wide variety of applications. For example, ultrasonics has been used ~or (1~ dust, smokP and mist ., 20 precipitation; (2) preparation of colloidal dispersions; (3~
cleaning of metal parts and fabrics; (~) fri.ction welding;
(5) the ~ormation of catalysts; (6) the degassing and . solidification of molten metals; (7) the extraction of flavor oils in brewing; (8) electroplating; (9) drilling hard ma~erials; (10) fluxless soldering and (lQ) nondestructive testing ~uch as in diagnostic medicine.
Tha object of high power ultrasonic applications is to bring about some permanent physical change in the material ~ '' ~ i 5 2 ~

treated. This process requires the flow of vlbratory power per unit of area or volume. Depending on the application, the power density may range from less than a watt to thousands of watts per square centimeter. Although -the original ultrasonic pow~r devlces operated at radio frequencies, today most operate at 20-69 kHz.
The piezoelectric sandwich-type transducer driven by an - ~ electronic power supply has emerged as the most common sourceof ultrasonic power; the overall efficiency of such equipment (net acoustic power per electric-line power~ ypically greater than 70%. The maximum power from a conventional transducer is inversely proportional to the square of the frequency. Some applications, such as cleaning, may have many transducers working into a common load.
lS Other, more particular areas where ultrasonic vibratory ,J force has been utilized are in the areas of thin nonwov~n websand thin films. For example, ultrasonic force has been use t~a bond or weld nonwoven webs. See, for example, U.S. patent ~, numbers 3,575,752 to Carpenter/ 3,660,186 to _a~3~_~_~l~, ~i 20 3,966,51g to Mitchell et al. and 4,695,454 to Sayo_itz et al.
which disclose the use of ultrasonics to bond or weld nonwoven webs. U.S. patent numbers 3,4~38,240 to Roberts, describes the ! use o~ ultrasonics to bond or weld thin films such a5 orientedi `1 ~
polyesters.
Ultrasonic force has also been utilized to aperture nonwoven webs. See, for example, U.~. patent numbers 3,949,127 to Ostermeier et al. and 3,966,519 to Mitchell et al..

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Lastly, ultrasonic force has been used to aperture thin film material. See, for example, U. S. patent number 3,756,880 to Graczyk.
Other methods for the aperturing of thin film have been developed. For example, U.S. patent number 4,815,714 to Douqlas discusses the aperturing of a thin film by first abrading the film, which is in filled and unoriented form, and th~n subjecting the film to corona discharge treatment.
As has previously been stated, those in the art had recognized that ultrasonics could be utilized to form apertures in nonwoven webs. See, U.S~ patent to Mltchell._et al.. Additionally, the Mitc~lell et al. patent discloses that the amount of ultrasonic ~nergy being subjected to a nonwoven :; web could be controlled by applying enough of a fluid to the 1 15 area at which the ultrasonic energy was being applied to the ;~ nonwoven web so that the fluid was present in uncombined form.
Importantly, the _ patent states that the fluid is moved by the action of the ultrasonic force within the nonwoven web to cause aperture formation in the web by fiber ~'20 rearrangement and entanglement. The Mitchell et al. patent ..1 also states that, in its broadest aspects, since these ef~ects are obtained primarily through physical movement of fibers, the method of their invPntion may be utili~ed to bond or increase the strength of a wide variety of fibrous webs.

DEFII!~ITIONS

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As used herein, the term '~dielectric" refers to an alternative n~me for an electrically insulating material. A
dielectric is a substance that has very low electrical conductivity, i.e., an insulator. S~ch substances have electrical conductivities of l~ss than 1 millionth mho per cm. Most polymers, excepting those that are specially produced as conductive or semiconducting polymers, are thus dielectrics.
As used herein, the term "electret" refers to a permanently or semi-permanently polarized piece of dielectric ~'~material; the analog of a magnet.
~:~As used herein the term "film" refers to a generally nonporous item that can be arranged in generally planar ,,configuration~ For example, films may be formed from ' 15 thermoplastic, thermoset materials.
~As used herein the term "thin ilm" refers to a film imaterial having an average thickness generally of less than about ten (lO) mils. Average thickness is determined by randomly selecting five (5) locations on a given film, measuring the thickness of thP film at each location to the nearest 0~1 mil, and averaging the five values (sum of the ~: ~fi~e values divided by five) .
As used herein the term "mesh count" refers to the number which is the product of the number of wires in a wire mesh ~, 25 screen in both the machine (MD) and cross-machine (CD) .,directions in a given unit area. For example, a wire mesh .screen having 100 wires per inch in the machine direction and :100 wires per inch in the cross machine direction would have ,, g ~Y,~$

a mesh count of 10,000 per square inch. As a result of the interweaving of these wires, raised areas are present on both sides of the mesh screen. The number of ralsed areas on one side of such a wire mesh screen is generally one~half of the mesh count.
As used herein the term "aperture" refers to a generally linear hole or passageway. Aperture is to be distinguished from and does not include holes or passageways havin~ the greatly tortuous path or passageways found in membranes. This term is also meant to include an aperture which i5 present only upon an application of a phy~ical stimulus. For example, ~; a puncture point in an elastomeric film which opens to form an aperture upon the film being stretched or elongated.
As used herein the term "microaperture" refers to an aperture which has an area of less than about 100,000 square micrometers. The area of the microaperture is to be measured at the narrowest point in the linear passageway or hole.
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; ~ As used herein the term "ultrasonic vibra~ions" refers to vibrations having a frequency of at least about 20,000 cycle~ per second. The frequancy Of ~he ul~rasonic vibra~ions may range from about 20,000 to about 400,000 cycles per second ox more~
As used herein the term "hydrosonics" refers ~o the ~` application of ultrasonic vibrations to a material where the area of such application is has had a liquid applied thsreto to th~ extent that the liquid is present in sufficient ~ , ~ quantity to generally fill the gap between the tip of the ~, .
ultrasonic horn and the surf ace of ~he material.
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OBJECTS OF THE INVENTION

Accordingly, it is a general object of the present invention is to pro~ide a process for forming an electre~ from a hydrosonically apertured f llm.
Another object of the present invention is to provide an , electret formed from a hydrosonically apertured film.
Still further objects and the broad scope of applicability of the present invention will become apparent to thos~ of -~ skill in the art from the d~tails given hereinafter. However, it shou].d be understood that the detailed description o the pre~ently preferred embodiments of the present invention is given only by way o~ illustration because various changes and ! 15 modifications well within the spirit and scope of the '~'t invention will become apparent to those of skill. in ths art in view of this detailed description.

~, SUMMARY OF THE INVRNTION

l 20 i In response to the foregoing problems and dlfficulties encountered by those in the art, we have developed a method ~,i for forming an electret filter medium including the steps of:
'~ ~ (l) hydrosonically providing a thin film material with at :~l 25 least about 1,000 microapertures per square inch with each of the microapertures having an area ranging from about lO to ,,~
a~out 100,000 square micrometers; and (2) corona discharge ~1 `~l treating the thin film material to instill a positlve charge J
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of at lea~t about 100 volts on one surface of the film and a ; negative charge Gf at least about 100 volts on the other .
surface of the fi lm .
In some embodiments, the positively charyed surface is instilled with a positlve charge of at least about 150 volts.
For example, the positively charged surface may be instilled with a positive charge of at least about 250 volts. More particularly, the positively charged surface may be instilled with a positive charge of at leask about 500 volts.
In somQ embodiments, the negatlvely charged surface may be instilled with a negative charge of at least abo~lt 150 volts. For example, the negatively charged surface may be ~. instilled with a negatlve charge of at least about 250 volts.
;~ More particularly, the negatively charged surface may be :, 15 instilled with a negative charge of at least about 500 ~lolts.
~: The invention i5 also directed to an electret filter ~;~'J medium which includes a thin film material haviny least about -3 1,000 microapertures per square inch with each oP the ; ; microapertures having an area ranging from about 10 to about 100,000 square micrometers.
The f ilm material is a dielectric material selected from .~ the group including one or more polyolefins r polyesters, polycarbonates, polyhalocarbons or natuxal rubber. For .~.j example, the polyolefin may be a polyethylene or a polypropylene.
. In some embodiments, the film material may be provided with at least about 5,000 microapertures per square inch. For ~;~ example the film material may be provided with at least about ,~ 1 1 ' ~,~

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20,000 microapertu~es per squ~re inch. More particularly, the film materlal may be provided with ~t least about 90,000 microapertures per square inch. Even more particularly, the film material may be provided with at least about 160,000 ; 5 microapertures per square i~ch. In some of these embodiments the area of each of the formed microapertures may yenerally range from at least about 10 square micrometers to about 10,000 square micrometers. For example, th~ area of each of -; the formed microapertures may generally range fr~m ak least about 10 square micrometers to about 5,000 square micrometers.
More particularly, the area of each of the formed microapertures or may generally range from at least about 10 square ~icrometers to about 1,000 square micrometers. Even more particularly, the area of each of the formed microapertures may generally rangP from at least about 10 square micrometers to about 100 square micrometers.

" I

Figure I is a schematic representation of apparatus which may hydrosonically microaperture a thin film material and also provide one surface of the film material with a positive ~.
charqe and the other surface of the film material with a negative charge.
2S Figure II is a cross sectional view of the transport mechanism depicted in Figure I taken along lines A-A in Figure ~, 12 ' .

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Figure III is a schematic representation of the dotted linei~rea depicted in Figure I where the ultrasonic vibrations are applied to the f ilm which is to be transfor~ed into an electre t .
: 5 DETAILED DESCRIPTION OF THE INVENTION
.:
Turning now to the figures where like reference numerals : represent like structure and, in particular to Figure I which is a schematic representation of an apparatus which can carry out the method of the present invention, it can be seen that the apparatus is generally represen~ed by the reference numeral 10. In operation, a supply roll 12 of.a thin .. dielectric film 14 ls provided. As has been p~viously stated, the kerm thin film refers to films which have an average ~,~ thickness of about ten (10) mils or less. Additionally~
.~ generally speaking, the average thickness of the thin film 14 .~ .
~-:J will be at leaist about 0.25 mil. For example, the average thickness of the thin film l~ material may range from about 20 0.25 mil to about 5 mils. More particularly, the average thicknesR of the thin film 14 may range from about 0.2S mil ,...~
.~i to about 2 mils. Even more specifically, the average thickness of the thin film 14 may range from abou~ 0.5 mil to about l mil.
The thin dielectric film 14 may be formed from a wi~e variety of dielectric materials. For example, the thin film ~ ~ 14 may be ~ormed from a material selected from the group ; ~: including one or more polyolefins, polyesters, polycarbonates, ~ ~ 3 r~ 9 ~ ~

polyhalocarbons or natural rubber. For example, the polyolefin may be a polye~hylene or a polypropylene.
The thin film 14 is transported to a first nip 16 formed ~- by a first transport roll 18 and a first nip roller 20 by the 5 action of an endless transport mechanism 22 which moves in the direction indicated by the arrow 24. The transport mechanism 22 is dri~en by the rotation of the first transport roller 18 in conjunction with a second transport roller 26 which, in tuxn are driven by a conventional power source, not shown.

Figure II is a cross sectional view of the transport mechanism 22 taken along lines A-A in Figure I. Figure II
` d.iscloses th~t the transport rnechanism 22 includes a heavy : :' duty transport wire mesh screen 28 usually having a mesh count ~' of less than about 400 (i.e. less than a 20 wires per inch MD

by 20 wires per inch CD meish screen if machine dir~ction (MD) and cross machine direction (CD~ wire count is the same~.
Heavy duty mesh wire screens ~f this type may be made from a variety of materlals isuch as, for example, plastics, nylons or polyesters, and are readily available to those in the art.
~- 20 Located above and attached to the transport screen 28 is an endles~ flat shim plate 30. The shim plate 30 desirably is form~d from stainless steel. However, those of skill in the art w111 readily recognize that other materials may be utilized. Located above and attached to the shim plate 30 is ,~
-i,; 25 a ~ine mesh wire pattern screen 32 usually having a mesh count ;~ of at least about 2,000 (i.e. at least about 45 wires per inch MD by 45 wires per inch CD mesh screen if MD and CD wire , coUnt is the same). Fine mesh wire screens of this type are : .i ~ 14 ,~ .
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readily available to those in the art . The f ine mesh wi~e screen 32 has .raised areas or knuckles 3~ which perform the function of a pattern anvil as will he discussed later.
From the first nip 16 the thln film 1~ is transported by the transport mechanism 22 over a tension roll 36 to an area 38 (defined in Figure I by the dotted line~l circle) where the thin film 14 is subjected to ultrasonic vibrations.
: The assembly for subjecting the thin film 14 to the ultrasonic vibrations ii~i conventional and is generally designated at 40. The assembly 40 includes a power supply 42 which, through a power control 44, supplies power to a piezoi31ectric transducer 46. As is well known in the art, the piezoelectric transducer 46 transforms electrical energy into : mechanical m~ovement as a result of the transducerrs vibrating 15 in respons2 to an input of electrical energy. The vibrations created by the piezoelectric transducer 46 are transferred, in conventional manner, to a mechanical movement booster or .l ampli~ier 48. As is well known in the art, the mechanical ~: movement booster 48 may be desi~ned to increase the amplitude ~:j 20 of the vibrations ~mechanical movement) by a known factor ~l depending upon the configuration of the booster 48. In further :~.i con~entional manner, the mechanical movement (vibrational energy) is transferred from the mechanical movement booster 48 to a conventional knife edge ultrasonic horn 50. It should :~ 25 be reali~ed that other types of ultrasonic horns 50 could be ~:~i utilized. For example, a rotary type ultrasonic horn could b~
: I
~ used. The ultrasonic horn 50 may be designed to effect yet : .1 ~, another boost or increase in the amplitude of the mechanical movement (vibrations) which is to be applied to the thin film 14. Lastly, the assembly includes an actu~tor 52 which includes a pneumatic cylinder, no-t shown. The actuator 52 provides a mechanism for raising and lowering the assemb].y 40 so that the tip 54 of the ultrasonic horn 50 can apply tension to the transport mechanlsm 22 upon the assembly 40 being loweredO It has been found that it is necessary to haYe some degree of tension applied -to the transport mechanism 22 upon the lowering of the assembly for proper application of i. 10 vibrational energy to the thin fllm 14 to ~orm microapertures in the thi~ film 14. One deslrable aspect of this tensioned '~ arrangem~nt is that the need to design a finely toleranced gap between the tip 54 of the horn 50 and the raised ~reas or .~ knuckles 34 of the fine mesh wire screen 32 is not necessary.
`S 15 Figure III is a schematic representation of the area 38 where the ultrasonic vihrations are applied to the thin film 14. As can be seen in Figure III, the transport mechanism 22 ~ ,:
. forms an angle 56 with the tip 54 of the ultrasonic horn 50.
While some microaperturing will occur if the anyle 56 is as great as 45 degrees, it has been found that it is desirable for the angle 56 to range from about 5 degrees to about 15 degrees. For example, the angle. 56 may range from about 7 to about 13 deqrees. More particularly, the angle 56 may range ~; from about 9 to about 11 de~rees.
.~ 25 Figure III also illustrates that the transport mechanism i 22 is supported from below by the first tension roll 36 and ~, a second tension roll 58. Positioned somewhak prior to the tip 54 of the ultrasonic horn 50 is a spray noæzle 60 which is `~ 16 ~ 1 configured to apply a fluid 62 to the surface of th2 thin film 14 just prior to t^.~ sheet material's 14 being subjected to ul~rasonic vibratlons by the tip 54 of the ultrasonic horn 50. The fluid 62 de~irably may be selected :Erom the group including onP or more of water; mineral oil; a chlori~ated hydrocarbon; ethylene glycol; or a solution of 50 vol.ume percent water and 50 volume percent 2 propanol. For example, in some embodiments the chlorinated hydrocarbon may be select-ed from the group including 1,1,1 trichloroethane or carbon tetrachloride. It should be noted that the wedge-shaped area 64 formed by the tip 54 of the ultrasonic horn 50 and the transport mechanism 22 should be s~bjected to a suf:Eicient amount of the fluid 62 for the fl~lid 62 to act as both a heat sink and a coupling agent for the most desirable resultsO
Positioned below the transpoxt mechanism 22 in the area where the tip 54 of the ultrasonic horn 50 is located is a fluid ,, collection tank 66. (See Figure I.) ThP fluid collection tank ,66 serves to collect fluid 62 which has been applied to the sur~ace of the thin film 14 and which has either been driven .i 20through the film 14 and/or the transport mechanism 22 or over .;i' the edges of the transport mechanism 22 by the action of the .) . vibrations of the tip 54 of the ultrasonic horn 50. Fluid 62 which is collected iA the collection tank 66 is transported by tubing 68 to a fluid holding tank 70.
25Figur~ I illustrates that the fluid holding tank 70 con~ains a pump 72 which, by way of additional tubing 74, ~:~ .J
~ suppli~s the fluid 62 to the fluid spray nozzle 60. According~

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ly, the fluid 62 may be re-cycled for a consiclerable period of time.
While the mechanism of action may not be fully understood . ~nd the present application shoulcl not be bound to any particular theory or mechanism of action, it i5 believed that ~`. the presence of the fluid 62 in the wedge-shaped area 64 during operation of the ultrasonic horn 50 acco~plishes two separate and distinct functions. First, the presence of the fluid 62 allows the fluid 62 to act as a heat sink which `~ 10 allows the ultrasonic vihrations to be applied to the thin film 14 without the thin film 14 being altered or destroyed - as by melting. Secondly, the presence of the fluid 62 in the `~ wedge-shaped area 64 allows the fluid 62 to act as a ~ouplin~3 agent in the application of the vibrations from the ultrasonic ~:: 15 horn 50 to the thin film 14.
It has been discovered that the action of the ultrasonic horn 50 on the thin film 14 microapertures the khin film 14 `~ in spite of the fact that there are no fibers to re~arrange 1 to form microapertures as was the case in Mitchell et al . The microapertures are punched through the thin fil~ 14 in the pattern of the raised areas or knuckles 34 of the fine mesh wire pattern screen 32. Generally, the number of :, microapertures produced will be equal to the number of raised ,~ areas or knuckles 34 on the upper surface of the fine mesh wire screen 32. That is, the number of microapertures will generally be one-half the mesh count of a given area of .~, pattern screen 32~ For example, if the pattern screen 32 is 100 wires per inch MD by 100 wires per inch CD, the total :i .

number of knuckles or ralsed areas 34 on one slde of the pattern wire 32, per square inch, will be loo times loo divided by 2. This equals 5,000 microapertures per square inch. For a 200 wires per inch MD by 200 wires per inch CD
pattern screen 32 the calculation yields 20,000 microapertures per square inch. Depending somewhat on the thickness of the thin thermoplastic sheet material 14, at a mesh count of about 90,000 (300 wires per inch MD by 300 wires per inch CD) the wires are so thin as to allow the knuckles 34 on both sides to microaperture the thin thermoplastic sheet material 14 if suf~icient force is applied. Thus, a 300 wires per inch MD by 300 wires per inch CD mesh screen yields 90,000 microapertures per square inch; for a 400 wires per inch MD by 400 wir~s per inch CD mesh--160,000 microapertures per square inch. Of course the MD and CD wire count of the wire mesh scree~ does not have to be the same.
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It should also be noted that the number of microapertures ~ormed may also vary with the number of ultrasonic vibrations '~ to which the thin fllm 1~ is subjected per unit area for a given period of time. This factor may be varied in a number of ways. For example, the number and si~e of the microapertures will vary somewhat with the line speed of the thin film 14 as it passes underneath the tip 54 of the ~;, ultrasonic horn 50. Generally speaking, as line speed increases, first the size of the microapertures decreases and ;~ then the number of microapertures decreases. As the number of microapertures decreases the less the pattern of i microapertures resembles the pattern of raised areas 34 on the ''i ;. ;''~ 19 . 'i ' .
.~
~ i g pattern screen 32. The range of line speeds that usually yields microapertures varies with the type of material utilized to form the thin film 14 and the material used as the fluid 62. For thermoplastic polyethylene having a thickness of about 0.5 mll, typi~al line speeds which usually yield microapertures for a wide variety of fluids range from ~ibou-t 5 to about 25 feet per minute. For example, if water is used as the fluid with polyethylene typical line speeds which usually yield microapertures range from about 5 to about 23 .~
feet per minute. It is believed that, to some extent, the variations in the number of microapertures formed and the size of the microapertures occurs due to the minute variations in !' the height of the raised areas or knuckles 34 of the fine mesh ~1 pattern screen 32. It should be not~d that the fine mesh pattern screens uied to date have been obtained from conventional everyday sources such as a hardware storeO It is ,~
also believed that if a pattern screen 32 could be created where all of the raised areas 34 of the screen 32 were oE
exactly the same height these variatlons would only occur in uniform fashion with variakions of line speed.
l A~ was stated above, the area or size of each of the :.~ microapertures formed will vary with the parameters discussed . above. The area of the microapertures will also vary with the .,~ .
~ area of the raised areas of the pattern anvil such as the '""~J
knuckles 34 on the fine mesh wire screen 32. It is believPd that the type of material used in forming the thin film 14 ~-~jl will also vary the area of the microapertures formed if all other parameters are maintained the same. For example, the " ,J

' ~`1 ` .~, soft~r the thln film 14, the easier i-t is to push the thin film l~ through the raised areas of the fine mesh p~ttern screen 32. Because the raisPd areas (knuckles) on the fine mesh screen are generally pyramidal in shape, the deeper the raised area penetrates the thin f ilm 14, the larger the microaperture In such situations the shape of the microaperture wlll conform generally to the pyramidal shape of the raised area of the fine mesh screen and the microaperture will be generally pyramidally shaped in the z direction, and will have an area which is greater at onP end than at the other. As has been previously stat~d, the area of the microaperture should be measured at the narrowPst point of the aperture. Of course, the height of the raised 2reas i`;
must be greater tha~ the thickness of the thin sheet m~terial .~ 15 14 for microaperture~ to b~ formed and the degree of excess, if any, necessary may vary with the type of film 14 to be ; microapertured. In any ev~n~, the height of the raised areas must be isufficient to punch through the film 1~ including any i elasticity which might be encountered in the punching i 20 operation. That is, the more elastic the film 14, the greater the height of the raised areas has to exceed the thickness of the khin film 14.
In some embodimen~s it may be necessary to subject the ~ thin film 14 to multiple passes through the apparatus 10 in -i~ 25 order to microaperture the film 14. In such situations the i~ thin film 14 will initially only be thinned in the pattern of :~ the pattern anvil's raised areas. However, after two or more ! passes through the apparatus 10, with the thin film 14 being , 21 :, -.

aliyned in the same config~lration wi-th respect to the patte~n anvil, microapertures may be formed. Essentially what is ; happening in these situations is that the thin film 14 is repeatedly thinned by repeated application of ultrasonic vibrational force until such time as microapertures are formed. Alternatively, the fine mesh wire diameter size may be increased with the consequent decrease in mesh countO
Increasing the wire diameter size of the fine mesh screen 37 increases the likelihood that ~icroapertures will be formed.
Another feature of the present invention is the fact that the microapertures can be formed in a predesignated area or areas of the thin thermoplastic sheet materlal 14. This can be accomplished in a number of ways. For example, tlle thin . .
-~ film 14 may be su~jected to ultrasonic vibrations only at certain areas of the film 14, thus, microaperturing would occur only in those areas. Alternatively, the entirP thin film 14 could be subjected to ultrasonic vibrations with the ~3 pattern anvil having raised areas only at certain locations and otherwise being flat. Accordingly, the thin film 14 would . . ,j be microapertured only in those areas which correspond to ~;l area~ on the pattern anvil having raised areas.
, ~
It should also be noted that some limitation exists in the `e number of microapertures which can be formed in a given thin film 14 on a single application of vibrational energy, i.e.
~l 25 a single pass through the apparatus if a wire mesh screen is used as the pattern anvil. This follows from the fact that, as was stated above, the height of the raised areas must exceed the thickness of the thin film 14 in conjunction with , ,,~
` ~ 22 ; , ~ ,1 .. ,~ . . ,__ _ . _ , .. . .. . . . .

~37~

the fact that, generally, as the mesh count increases the height of the ra sed area~ or knuckles decreases. In such situations, if the number of microapertures desired per unit area is greater than the number which can be formed in one pass through the apparatus, multiple passes are necessary with ; the alignment of the thin film 14 with respect to the raised areas being altered or shifted slightly on each pas~.
Generally speaking the area of each of the microapertures is greater than about ten square micrometersO That is the area of each of th microapertures may range from at least about 10 square micrometers to about 100,000 square micrometers. For example, the area of each of the formed microapertures may generally range from at least about 10 square micrometers to ; about 10,000 square micrometers. More particularly, the area of each of the formed microapertures may generally rang2 fro~
at lea6t about 10 square micrometers ~o about 1,000 square ~ micrometers. Even more particularly, the area of each of the : ::. formed microapertures may generally range from at least about10 square micrometers to about 100 square micrometer~.
l 20 A number of important observations about the process may i , now be made. For example, it should be understood that the ! presence of the fluid 62 is highly important to the present ' 'j! inventive process which usas tha fluid 62 as a coupling agent.
~ecause a coupling agent is present, the microapertures are `~, 25 punched through the film 14 as opposed to being formed by ~' melting. Additionally, the presence of the shim plate 30 or ;, its equivalent is necessary in order to provide an ~anvil i~rl mechanism against which the thin film 14 may be worked, that ''~'`.`', :
~, I
~ 23 : I

,, ~ ~;

~ ~ ~ p~

; is apertured, by the action of the tlp 54 of the ultrasonic horn 5rf`f. Because the vlbrating tip 54 of the ultrasonlfc~ horn 50 is acting in a hammer and anvil manner when operated in conjunction with -the heavy duty mesh screen 28/shi~ plate ~; 5 30/fine wire mesh 32 ccfmbination, it should be readily recognized that ~ certain degree of tension must be placed upon the transport mechanism 22 by the downward displacement - of the ultrasonic horn _fO. If there is little or no tf2nsion placed upon the transport mechanism 22, the sh~m plate 30 cannot perform its function as an anvil and microaperturing generally does not occur. Because bcfth the shim plate 30 and the fine mesh pat~ern wlre 32 form the resistance that the :
;f ultrasonic horn 50 works ayainst, they may be collectively referred to as a pattern anvil combination. It shcf~.lld be lS easily recognized by those in-the art that the function of the pattern anvil can be accomplished by other arranfi~ements than the heavy duty mesh screen 28/shim plate 30/fine mesh screen ~' 32 combination. For example, the pattern anvil could be a flfat plate with raised portions acting to direct l-he ~j 20 microaperturing force of the ultrasonic horn 50.
`~i Alternatively, the pattern anvil could be a cylindrical roller having raised areas. If the pattern anvil is a cylindrical ~:, roller with raised areas, it i5 desirable for the pattern anvil to be wrapped or coated wi~h or made from a resilient materialt Where the pattern anvil is a mesh screen the j~ resiliency is provided by the fact that the screen is unsupported directly below the point of application of ~ ff ;~ ultrasonic vibrations to the mesh screen.

-;f ~7~
Figure I reveals that, after ~he thin film 14 has been microapertured, the fllm 14 is passed through a corona discharge treating apparatus 76. Generally speaklng, the apparatus 76 includes a positive electrode 78 positioned adjacent one surface of the film 14 and a negative electrode 80 positioned adjacent the o~her surface of the film 14. Also ~ present are transport rollers 82 which are arranged around ,~ each of the electrodes 78, 80. Two contact webs 84, 86 are arranged, respectively, about each of the electrodes 78, 80.
The film 14 is fed into the nip 88 formed by the two leading transport rollers 8i2 and is sandwiched between the two transport webs 84, 86 at the time that the film 14 is subjected to the applicaticn of corona discharge from the ~; electrodes 78, 80. It should br2 no~ed that, for the process to work properly, the transpor~ webs (continuous belts) should : ;
b~ formed from a material which will conduct the electrical .j ~` charge o~ the corona unit to the dielectric film 140 . .`"i ;, Desirable materials for forming the transport webs 84, 86 include, for example, cellulosic fiber such as cotton, wood pulp, or hemp or mixtures of the fibers. It may also be made from fibers of a dielectric material which are bonded together with a conductive binder. The transport wabs 84, 86 should 1, have poorer dielectric properties than the film 14 which is ;~ being subjected to corona discharge treatment. Accordingly, ~, 25 the transport webs 84, 86 should not retain any significant -~ electrical charge. The apparatus 76 also includes a ~. ,. ;~7 conventional power source (not shown) which energizes the electrodes 78 and 80. The transport rollers 82 are separately ? ;~

r . ~

2 ~

powered by a conventional drive motor so th~t the transport - webs 84, 86 move with the film 14 as it passes between the ; electrodes 78, 80. The electrodes are configured and desiyned to apply a positive charge of at least about 100 volts to one surface of the film 14 and/or a negative ~harge of at least about 100 volts to the other surface of the film 1~. For ~xample, the electrod~s may apply a positive charge of at ;~ least about 150 volts to one surface of the film 14 and/or a ne,gativ2 charge of at least about 150 volts to the other surface of the film 14. More particularly, the electrodes may apply a positive charge of at least about 250 volts to one surface of the film 14 and/or a negative charge of at leas~
about 250 volts to the other surface of the film 1~. Even ~ more particularly, the electrodes may apply a positive charge :, 15 of at least about 500 volts to one surface of the film 14 :~J and/or a negative charge of at least about 500 volts to the .. i other surface of the film 14. Of course, if desired, the sam~
~'J amount o~ charge does not have to be applied to each surface , ,, 1 of the film 14. To assist in the application of the charge 1, 20 to the film 14, it may be desirable to elevate the temperature of the film 14 while the film 14 is bein~ subjected to the '. corona discharge treatment~ The degree of elevation of the : temperature of the film 14 will vary with the type of film 14 which is being utiliæed. Generally speaking, as is known in ~$ 25 the art, the film 14 is elevated to a temperature in excess ~. of the material's glass transition temperature.
-:1 ~' ', ~1 .
.~.j 26 ~7~

After application of the corona discharge treatment, -the charged, microapertured film is wound up for storage on roll 9 0 .
The invention will llOW be discussed with regard to ~; 5 specific examples which will aid those of skill in the art in a full and complete understanding thereof.
' .

EXAMPLE I
:
A sheet of 0.5 mil thick polyethylene film obtained from Edison Plastics of Plainfleld, N~J. and having the trade designation of S/E 702 was cut into a l ngth of about 87 5 ` inches and a width of about ~.1 lnches. The sample was . . .
; processed in accordance with the present invention.
A model 1120 power supply obtained from the ~.3ranson l Company of Danbury, Connecticut, was utilized. This power .¦ supply, which has the capacity to deliver l,300 watts of electrical energy, was used to convert 115 volt, 60 cycle ~3 electrical energy to 20 kilohertz alternating currentO A
.l 20 Branson type J4 power level control, which has the ability to `, regulate the ultimate output of the model 1120 power supply ., from 0 to 100~, was connected to the model 1120 power supply.
.3 i~ In this example~ the power level control was set at 100%. The ~:!3l actual amount of power consumed was indicated by a Branson -model A410A Wattmeter~ This amount was about 660 watts.
.;~ The output of the power supply was fed to a model 402 piezoelectric ultrasonic transducer obtained from the Branson Company. The transducer converts th~ electrical energy to `-: 3 . . ~

,..

~: 3 .., mechanici~l movem~nt. At 100~ power the i-~mount of mechanical movement of the transducer is abou~ 0.8 micrometersO
The piezoelectrlc transducer was connected to a mechanical movement booster section obtained from the Branson Company.
The booster is a solid titanium metal shaft with a length equal to one-half of the wave length of the 20 ~silohertz resonant frequency. Boosters can be machined so that the amoun~ o~ mechanical movement at their output end is increased or decreased as compared to the amount of movemen-t of the transducer. In this example the booster increased the amount of movement and has a gain ratio of about 1:2.5. That is, the amount of mechanical movement at the output end of the booster is about 2.5 times the amount of m~vement of the transducer.
`~The output end of the booster was connected to an ultrasonic horn obtained from the Branson Company. The horn '~in this example is made of titanium with a working face of ~iabout 9 inches by about 1/~ inch. The leading and trailing edges of the working face o~ the horn are each curved on a ,.,~
Iradius of about 1/8 inch. The horn step area is exponential ii ~ 20 in shape and yields about a two-fold increase in the :, mechanical movement of the booster~ That is, the horn step area has about a 1:2 gain ratio. The combined increase, by the ~,booster and the horn step area, in the original mechanical .. ...
; mo~ement created by the transducer yields a mechanical movement of about 4.0 micrometers.

The forming table arrangement included a small forming table which wa~ utilized to transport and support the ~'polyethylene film. The forming table included two 2-inch , ! 28 ~7~

diameter idler rollers whlch were spaced about 12 inches apart on the surface of -the forming table. A transpo~t mesh belt encircles the two idler rollers so that a continuous conveying or transport surface ls created~ The transport mesh belt is a square weave 20 x 20 mesh web of 0.020 inch diameter plastic filaments. The belt is about 10 inches wide and ls raised above the surface of the forming table.
. .
The transducer/boosterlhorn assembly, hereinaftPr the assembly, is secured in a Branson series 400 actuator. When power is switched on to the transducer, the actuator, by means ;~ of a pneumatic cylinder with a piston area of about 4.4 square inches, lowers the assembly so that the output end of the horn : "
contacts the polyethylene film being processed. The actuator also raises the assembly so that the output end of the horn ., is removed from contact with the film when power is switched ~' ~f~.
.,;
The assembly is positioned so that the output end of th2 ~s horn is adapted so that it may be lowered to contact the -, tra~sport mesh belt between the two idler rollers. An 8 inch ~ 20 wide 0.005-inch thick stainless steel shim stock having a :.~
;i length of about 60 inches was placed on the plastic mesh transport belt to provide a firm support for a pattern screen which is placed on top of the stainless steel shim. In this example the pattern screen is a 200 by 200 mesh wire size .i,~, weave skainless steel screen. The film which was then fastaned onto the pattern wire using masking tape.
~ The forming table arrangement also included a fluid - 1 circulating system. The circulating system includes a fluid :1 ` .
~l 29 :';'~ ' , : ~:

~7~

reservoir tank, a fluid circulating pump which rnay convenient-ly be located within the tank, associated tllbing for transpor-ting the fluid from the tank to a slotted boom which is designed to direct a curtain of fluid into the junc~ure of the output end of the horn and the film. .-In operation, the assembly was positioned so that the ~ output end of the horn was at an angle of from about 10 to 15 .~ degrees to the film. Accordingly, a wedge shaped chamber was formed between the output end of the horn and the film. It is into this wedge shaped chamber that the fluid, in thisexample water, at room temperature, was directed by the slotted boom.
It should be noted that the actuator was positioned at a , height to insure that, when the assembly is lowered, the ; 15 downward movement of the output end of the horn is stopped by the tension of the transport mesh before the actuator reaches the ~imit of its stroke. In this example, actuatin~ pressura was adjusted to 15 pounds per square inch as read on a pressure gauge which is attached to the pneumatlc cylinder of th~ actuator. This adjustment results in a total downward force of 66.0 pounds. (15 psi times 4.4 square inches of piston area equals 66. 0 pounds of .~orce. ) -The sequence of operation was (1) the fluid pump was switched on and the area where the output end of the horn was :l 25 to contact the film was flooded with water; (2) -the transport mesh conveyor system was switched on and the film started moving at 7.7 feet per minute; and (3) power to the assembly ~ was supplied and the assembly was lowered so that the output ., ~j , ~

end of the horn c~ntacted the film while the sample continued to pass under the output end of the horn until the end of the sample was reached. The reading o~ the A410A wattmeter durlng the process is an indication of the energy required to maintain maximum mechanical movement at the output end of the horn while working against the combined mass of the water, the polyethylene film, the pattern wire, the shim stock, and the transport wireO
After the film had been microapertured, the film was : 10 passed through a corona discharge treatment assembly whi~h consisted of a positive and negative elec-trode spaced 1.5 inches abov~ and below the film wi-th each being powered by a ~5 kilovolt power supply. (R.C.-3 Chargemaster charging bar electrode # 400I165; power units P/N 25A Simco Co, Inc.
Hatfield, Pa.) The transport webs were obtained from the , Dexter Corp. of Windsor Locks, Ct. and were wetlaid latex ,, .'~' bonded contact webs (i~03768W9303). The film was passed through the corona discharge unit at a speed ~f ahout 5 to 10 feet per minute. The corona discharge treatment assembly l 20 applied 238 positive volts to one surface of the film and 157 ;~ neqative volts to the other surface of the film~
This example yielded an electret formed from a 0.5 mil ~:;
~ thick polyethylene film having a maximum microaperture density :: ' of about 20, 000 microapertures per square inch.

.; ~
EXAMPLE II

-~ 31 :~

" .~

.!,, ' ' ' ~ ; ' ~ ' ~

The process of Example I was repeated. The corona discharge treatment assembly applie~ 195 pcsitive volts to one side of the film and 182 nega~ive volts to the other side of the film.
This example yielded an electret havln~ a maximum ; microaperture density of about 20,000 microapertures per square inch.

EXAMPLE III
The process of Example I was repeated except the film was 0.5 mil Phanex polyester film obtained from the Pilcher Hamilton Corp. of Broadview, Ill. The line speed was about 2.6 feet per minute, a 120 by 120 fine wire mesh stainless i stéel screen was used, the actuating pressure was about 15 'I , pounds per square inch and the watts consumed were about 9350.
:.
Four passes were made in this example. The corona discharge ~ treatment assembly applied 563 positive vol~s ~o one side and ; ~i 452 negative volts to the other side of the film. This exampla yielded an electret formed from 0.5 mil thick polyester ~ilm having a maximum microaperture density of about 28,000 microapertures per square inch.
~,:
,~
EXAMPLE IV

The process of Example I was repeated excepted a sheet of :~9 4.O mil thick, lightly cross-linked natural rubber having the trade designation Softlastic Rubber TS 3000 (J. P. Stevens `~,'.i .

Co., Northamp~o~, Massachuset~s). The sheet of natural rubber was stretched loo percent and held in -the stretched position by a jig laid on the wire with the ji~ being fastened to the wire. The wire screen was 18 by 23 mesh stainless steel.
Actuating pressure was about 8 pounds, the line speed 8.5 feek per minute, about 880 wa~ts of power were consumed. The corona discharge ~reatmen~ assembly applied 209 positive vol~s to one side of the sheet and 150 volts to the other side of the sheet. This example yielded an electret iormed from a 4.0 o mil natural rubber shee~ having a m~ximum valved area of 82s ,, - valves per square inch. The valve area opening depends upon the applied force with a maximum functional aperture area in .,. .~
~ excess of 100,000 square micrometers.

; i i; 15 EXAMPLE V

; ~
;~ ' The process of Example IV was repeated excPpt -the passage through the corona discharge unit was at one foot per minute.

`i The corona discharge treatment assembly applied 532 positive volts to one side of the sheet and 563 volts to the other side ,~
of the sheet. This example yielded an electret formed from " a 5.0 mil natural rubbar sheet having a ma~imum valved area ~, of 828 valves per square inch. The valve area opening depends . ~ .
~,~l, upon the applied force with a maximum functional aperture area in excess of 103,000 square micrometers.

~i~ The uses to -~hich the electret of the present invention may be put are numerous. These include, for example, various 7~

air filtration applications; capacitive transducers to constructmicrominiaturecapacitivemicrophones;piezoelectric polymer devices such as audio frequency transducers; medical product applications for diagnostic measurements and prosthetic devices; as electrornechanical transducers in vibration devices and generators and as infrared sensors in a variety of devices (pyroelectric polymer electrets are employed in infrared videcon tubes).
It is to be understood that variations and modifications of the present invention may be made without departing from the scope of the invention. For example, in some embodiments the use of multlple ultrasonic horns aligned abreast or sequentially may be desirable. It is also to be unclerstood ^: that the scope of the present invention is not to be ,, 15 in~erpreted as limited to the specific embodiments disclosed :~ herein, but only in accordance with the appended claims when read in light of the foregoing disclosure.

., , ,~
. ,1. !
. ., !
.(' ,f~i ~ e ~,,, ~:~ 34 ''i

Claims

1. A process for forming an electret filter medium comprising the steps of:
hydrosonically providing a thin film of a dielectric material having first and second surfaces and least about 1,000 microapertures per square inch with each of the microapertures having an area ranging from about 10 to about 100,000 square micrometers; and corona discharge treating the thin film material to instill a positive charge of at least about 100 volts on said first surface and a negative charge of at least about 100 volts on said second surface.

2. The process according to claim 1, wherein the first surface is instilled with a positive charge of at least about 150 volts and the second surface is instilled with a negative charge of at least about 150 volts.

3. The process according to claim 1, wherein the first surface is instilled with a positive charge of at least about 250 volts and the second surface is instilled with a negative charge of at least about 250 volts.

4. The process according to claim 1, wherein the first surface is instilled with a positive charge of at least about 500 volts and the second surface is instilled with a negative charge of at least about 500 volts.

5. An electret filter medium comprising:
a microapertured thin film of dielectric material, having first and second surfaces, said film material having least about 1,000 microapertures per square inch with each of the microapertures having an area ranging from about 10 to about 100,000 square micrometers; and wherein a positive charge of at least about 100 volts exists on said first surface and a negative charge of at least about 100 volts exists on said second surface.

6. The electret filter medium of claim 5, wherein the film material is a thermoplastic material selected from one or more of the group consisting of one or more of polyolefins, polyesters, polycarbonates and polyhalocarbons.

7. The electret filter medium of claim 6, wherein the polyolefin is polyethylene.

8. The electret filter medium of claim 5, wherein the film material is a thermoset material.

9. The electret filter medium of claim 5, wherein the film material is a naturally occurring material.

10. The electret filter medium of claim 5, having at least about 5,000 microapertures per square inch.

11. The electret filter medium of claim 5, having at least about 20,000 microapertures per square inch.

12. The electret filter medium of claim 5, having at least about 90,000 microapertures per square inch.

13. The electret filter medium of claim 5, having at least about 160,000 microapertures per square inch.

14. The electret filter medium of claim 5, wherein the area of each of the formed microapertures generally ranges from at least about 10 square micrometers to about 10,000 square micrometers.

15. The electret filter medium of claim 5, wherein the area of each of the formed microapertures generally ranges from a least about 10 square micrometers to about 5,000 square micrometers.

16. The electret filter medium of claim 5, wherein the area of each of the formed microapertures generally ranges from at least about 10 square micrometers to about 1,000 square micrometers.

17. The electret filter medium of claim 5, wherein the first surface has a positive charge of at least about 150 volts and the second surface has a negative charge of at least about 150 volts.

18. The electret filter medium of claim 5, wherein the first surface has a positive charge of at least about 250 volts and the second surface has a negative charge of at least about 250 volts.

19. The electret filter medium of claim 5, wherein the first surface has a positive charge of at least about 500 volts and the second surface has a negative charge of at least about 500 volts.