WO2016081937A1

WO2016081937A1 - In-situ charging fiber spinning method for producing a nonwoven electret

Info

Publication number: WO2016081937A1
Application number: PCT/US2015/062168
Authority: WO
Inventors: Tao Huang
Original assignee: E. I. Du Pont De Nemours And Company
Priority date: 2014-11-21
Filing date: 2015-11-23
Publication date: 2016-05-26

Abstract

A method and apparatus for producing charged polymer fibers is provided, wherein charging occurs by passing the fibers through a mist zone containing vapor and/or droplets of a polar liquid prior to incorporating them into a fibrous web.

Description

TITLE

IN-SITU CHARGING FIBER SPINNING METHOD FOR

PRODUCING A NONWOVEN ELECTRET

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of US Provisional Patent Application Serial No. 62/ 082673, filed November 21 , 2014, and entitled "In-Situ

Charging Fiber Spinning Method For Producing A Nonwoven Electret;" and US Provisional Patent Application Serial No. 62/082,664, filed November 21 , 2014, and entitled "Melt Spun Filtration Media For Respiratory Devices And Face Masks," which applications are both incorporated herein in their entirety for all purposes by reference thereto.

FIELD OF THE INVENTION

This invention relates to a method and apparatus that incorporate in- situ charging for producing a nonwoven filtration web. The web is useful as a filtration media that has improved filtration efficiency

BACKGROUND

The aerosol filtration efficiency of nonwoven fibrous webs can be improved by imparting an electrostatic charge to the fibers, resulting in the formation of an electret fabric. Electrostatically charged fibrous materials to be used as a filtration medium have been known for some time. U.S. Pat. No. 2,740,184, discloses a process of charging thermoplastic, fibrous webs by softening the fibers in the webs with heat and, while the fibers are soft, subjecting them to a suitable electrostatic field to produce a charged web. A number of methods are known for forming such electret materials. For example, U.S. Pat. No. 4,215,682 teaches a "hot electrically charging" method by bombarding meltblown fibers as they issue from the die orifices with electrically charged particles such as electrons or ions; US 5,401 ,446 teaches a "cold corona charging" by charging fibers by means of a corona discharge after fiber formation; US 4,798,850 describes a mechanical approach referred to as "tribocharging" by mixing two different crimped synthetic polymer fibers that have been carded into a fleece and then needled to form a felt so that they become electrically charged during the carding. A more recent development termed "hydro-charging" uses water to impart electric charge to a nonwoven fibrous web. US 5,496,507 discloses the use of jets of water impinging on a nonwoven web at a sufficient pressure to provide a filtration enhancing property to the web. Other hydrocharging methods that use water to produce electret articles are known, including soaking the web (US 6,406,657), condensing water on the web (US 6,743,464), and sucking water through the web (US 6,783,574 and US 2004/0023577 A1 ).

Nonaqueous polar liquids also have been used in making fibrous electret articles (US 6,454,986).

An apparatus or equipment suitable for hydraulically entangling fibers is generally useful in the method of hydrocharging the nonwoven web, although the operation is carried out at lower pressures in hydrocharging than generally used in hydroentangling. All forms of hydrocharging are, by their nature, carried out on already-formed webs, and so may be termed post- processing. Thus, hydrocharging must be follwed by a drying process to remove residual water. Typically the amount of charging that may be imparted by hydrocharging is limited, so in most cases other processes like prior corona charging must also be carried out, entailing additional capital investment and operating cost. Precautions necessitated by use of high voltage must also be considered.

Hydrochaging and other post processing techniques may also involve use of hydraulic and lubricating oils and other liquids that may contaminate filter media used for breathing air. Such contamination often diminishes either the initial or long term performance of electrets.

Multiple passes of hydrotreatment and use of wetting solvents may be needed to be able to completely wet substantially all the surfaces in pits, voids, pores and internal spaces of a web. In hydrocharging, the pressure necessary to achieve optimum results will vary depending on the type of jet used, the type of polymer, the type and concentration of additives, the thickness and density of the web, whether pre-treatment such as corona surface treatment, is carried out prior to hydrocharging. The pressure required to attain sufficient charging is often enough to deform or damage the web, often resulting in a lower porosity and reduced dust loading capacity. More robust and effective processes are thus sought.

What is needed is a method and apparatus to produce a nonwoven filtration media, especially a nanofibrous electret filtration media, of which the media has improved filtration efficiency, with lower resistance and higher dust loading capacity. SUMMARY

In one aspect, the present invention is directed toward a process for producing a nonwoven web comprising electret fibers. The process comprises the steps of:

(i) supplying a molten spinning melt to a spinning surface of a rotating member having a departure edge,

(ii) rotating the rotating member at a rotational speed sufficient to form a film of the melt on the member,

(iii) ejecting from the departure edge a plurality of discrete, continuous filaments derived from the film melt into a stretching zone, each filament having a filament surface;

(iv) passing the filaments through a mist zone comprising vapor or liquid droplets of a polar liquid, whereby ions are deposited onto the filament surfaces to electrostatically charge the filament;

(v) attenuating the ejected discrete filaments by centrifugal force to form continuous electret fibers having a fiber surface, the attenuating being carried out in the stretching zone;

(vi) solidifying the attenuated fibers in a shaping zone external to the stretching zone;

(vii) subsequently collecting the continuous electret fibers on a collection surface to form the nonwoven web.

There is further provided a spinning apparatus for making a

nanofibrous web comprising electret fibers. The apparatus comprises:

(i) a molten polymer supply tube configured to deliver a polymeric spinning melt from a polymer source;

(ii) a rotating member configured for high speed rotation about a spin axis and having a spinning surface and a departure edge at a periphery of the spinning surface, the rotating member being configured to receive the polymeric spinning melt onto the spinning surface and to eject molten discrete continuous filaments from the departure edge into a stretching zone at a speed sufficient to cause attenuation by centrifugal force of the filaments into continuous fibers having a smaller diameter;

(iii) a mist generator adapted to deliver a vapor or droplets of a polar liquid into a mist zone; and

(iv) a collection surface configured to collect the attenuated fibers into the fibrous web.

Upon passage through the mist zone, the fibers are electrostatically charged to become electret fibers.

There is further provided a nonwoven fibrous electret that may be produced by the foregoing process and a nonwoven fibrous electret comprising the properties of:

(i) a basis weight of between about 10 g/m2 to about 40 g/m2,

(ii) a porosity greater than 95%,

(iii) a web strength in a machine direction greater than 2.0 N/cm, and a tensile elongation of greater than about 30%. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic illustration of the in-situ charging method in the present invention.

FIG. 2 is a schematic illustration the ion-transfer mechanism of the contact electrification for polymer fibers in the present invention.

FIG. 3 is a schematic illustration of an apparatus of making an eiectret nonwoven web media when applied the in-situ corona charging according to the present invention in the present invention.

FIG. 4 is a schematic illustration of an apparatus of making an eiectret nonwoven web media when applied the in-situ hydrocharging using a atomization ring according to the present invention in the present invention.

FIG. 5 is a schematic illustration of an apparatus of making an eiectret nonwoven web media when applied the in-situ hydrocharging using a atomization disk according to the present invention in the present invention.

FIG. 6 is a schematic illustration of an apparatus of making the electrofied fibers when applied the combination of the corona charging and the in-situ hydrocharging according to the present invention in the present invention.

FIG. 7 is a schematic illustration of an apparatus of making the electrofied fibers in the present invention.

FIG. 8 is a schematic illustration of an apparatus of making an eiectret nonwoven web media and web laydown, drying and wind-up facility in the present invention.

FIG. 9 is a schematic illustration of an apparatus of making an eiectret nonwoven web media and web laydown, drying and wind-up facility in the present invention.

FIG. 10 is a schematic illustration of an eiectret nonwoven fibrous media in the present invention.

FIG. 1 1 is a schematic depiction of a temperature profile used in an implementation of the present fiber spinning method.

DETAILED DESCRIPTION Definitions

The term "web" as used herein refers to a network of fibers in a layer, commonly as a nonwoven web.

The term "nonwoven" as used herein refers to a web that includes a multitude of essentially randomly oriented fibers, such that the naked eye cannot discern any overall repeating structure in the arrangement of the fibers. The fibers can be bonded to each other, or can be unbounded but entangled to impart strength and integrity to the web. The fibers can be staple fibers or continuous fibers, and can comprise a single material or a multitude of materials, either as a combination of different fibers or as a combination of similar fibers each comprised of different materials.

The term "nanofibrous web" as used herein refers to a web

constructed predominantly of nanofibers. "Predominantly" means that greater than 50% of the fibers by number percentage in the web are nanofibers. The term "nanofibers" as used herein refers to fibers having a diameter less than 1000 nm. In the case of fibers that are not round, the term

"diameter" as used herein refers to the greatest dimension of a cross-section taken perpendicular to the fiber's long axis.

The term "melt-spun nanofibers" as used herein refers to nanofibers made from a centrifugal melt spinning process. Melt-spun nanofibers include, without limitation, nanofibers made with a process disclosed by U.S. Pat. No. 8,277,71 1 .

The term "melt-spun nanofibrous web" as used herein refers to a nanofibrous web comprising melt-spun nanofibers.

The term "melt-blown nanofibers" as used herein refers to nanofibers made using a melt blowing process, e.g., the process disclosed by US 2008/0023888.

The term "melt-blown nanofibrous web" as used herein refers to a nanofibrous web made of melt-blown nanofibers.

The term "electroblown nanofibers" as used herein refers to nanofibers made from an electroblowing process, e.g., the process disclosed by WO 2003/080905.

The term "electroblown nanofibrous web" as used herein refers to a nanofibrous web made of electroblown nanofibers.

The term "microfibers" as used herein refers to fibers having a diameter from about 1 .0 μιτι to about 3.0 μιτι.

By "scrim" is meant a woven or unwoven substrate on which a fibrous web may be attached and/or supported. Melt-blown and spun-bond webs are commonly used as scrim materials.

By "electret" is meant an electrically-charged dielectric article, i.e., one having a quasi-permanent electric charge or dipole polarization. As known to a skilled person, an electret generates internal and external electric fields, and is the electrostatic analog of a permanent magnet.

The term "stand-alone" used herein in reference to a fibrous web indicates that the web is self-contained and itself has sufficient mechanical integrity for its configuration to be maintained without any supporting substrate such as a scrim.

The term "single source" used herein in reference to a fibrous web indicates that the fibers of the web are all produced in a single spinning process, such that the web formation does not entail the blending of separately-sourced fibers.

By "centrifugal spinning process" is meant any process in which fibers are formed by ejection from a rotating member.

By "spin disk" is meant a rotating member that has a disk shape with a concave, frustoconical or flat open inner surface. By "spin bowl" is meant that a rotating member has a bowl shape with a surface that may be, without limitation, concave, convex, or frustoconical.

By "filament" is meant an elongated structure that may be formed as a precursor to a fine fiber resulting from attenuation of the filament. Filaments are formed in spinning processes at a departure point of a rotating member, which may be an edge, serrations or an orifice at the periphery of the member, through which fluid passes.

The term "nozzle-free" is used herein with reference to a process in which the production of filaments, fibrils, or fibers does not entail passage of a spinning fluid or melt through a nozzle or other similar constriction that defines the shape of the exiting fluid, and that no spinning apparatus is used that includes any rotating member having nozzles or other similar

constrictions through which a spinning fluid or melt is appointed to pass.

By "air flow field" is meant a vector field that describes the speed and direction of air flow at any point or physical location in the process of the invention. The term "air" is used herein to mean ordinary, ambient air itself or any other inert gas or gaseous fluid, or mixtures of such.

By "low-velocity air flow field" is meant an air flow field wherein the speed of air flow imposed does not exceed 5 m/s at any point.

The term "charged" is used herein to indicate that an object has a net electric charge, which can be of positive or negative polarity, relative to uncharged objects or those objects with no net electric charge.

By "mist zone" is meant a volume defined by the presence therein of a vapor or droplets of a polar liquid.

The term "apparent density" used herein with respect to a fibrous web refers to the mass density, or mass per unit volume, with the volume being that of the free-standing web measured without imposing any external force that would compress the web thickness. For example, a thickness

determination may be accomplished by an optical imaging technique that measures the free-standing height of the web. Apparent density may then be calculated by dividing a measured basis weight (i.e., the weight per unit area) of a web by the measured free-standing height of the web.

By "essentially" is meant that if a parameter is held "essentially" at a certain value, then changes in the numerical value that describes the parameter away from that value that do not affect the functioning of the invention are to be considered within the scope of the description of the parameter.

Quality Factor and Effective Quality factor: Quality Factor (QF) has frequently been used to compare the performance of different media types, as defined as:

QF = -Ιη(Ρ/100)/ΔΡ,

wherein P is penetration and ΔΡ is pressure drop. Both can be measured by the TSI instrument described above. Quality factor QF can be specified in units of inverse pressure, e.g. (Pa)^"1 , wherein 1 pascal (Pa) = 1 N/m². The thickness, porosity, and fiber diameter of the nanofibrous nonwoven media enter the quality factor (QF) indirectly by their effect on P and ΔΡ. However, an ideal filtration media would also exhibit low basis weight and low apparent density, to account for the desirability of accomplishing filtration with the smallest possible amount of media. Accordingly, filtration media can further be characterized by an effective quality factor (eQF), which is defined herein as the quality factor divided by apparent density (p_apparent), or:

eQF = QF / Papparent = (-ln(P/100)/ΔΡ) / Papparent Effective quality factor can be specified in units of (Pa g/cm³)^"1.

Description

One aspect of the present disclosure is directed toward a manufacturing method by for fiber spinning a nonwoven polymeric media. The method includes in-situ charging to provide a high quality and high performance electret product. Also provided is an apparatus for

accomplishing such a spinning operation.

The process and apparatus of the present invention can be used to melt spin any of a wide variety of melt spinnable polymers. Suitable polymers include thermoplastic materials comprising polyolefins, such as polyethylene polymers and copolymers, polypropylene polymers and copolymers;

polyesters and co-polyesters, such as poly(ethylene terephthalate), biopolyesters, thermotropic liquid crystal polymers and PET coployesters; polyamides (nylons); polyaramids; polycarbonates; acrylics and meth- acrylics, such as poly(meth)acrylates; polystyrene-based polymers and copolymers; cellulose esters; thermoplastic cellulose; cellulosics; acrylonitrile- butadiene-styrene (ABS) resins; acetals; chlorinated polyethers;

fluoropolymers, such as polychlorotrifluoroethylenes (CTFE), fluorinated- ethylene-propylene (FEP); and polyvinylidene fluoride (PVDF); vinyls;

biodegradable polymers, bio-based polymers, bi-composite engineering polymers and blends; embedded nanocomposites; natural polymers; and combinations thereof.

The polymer used in the present process and fibrous web can further comprise functional additives, either incorporated directly in the fibers or as a coating thereof. The term "functional additives" refers generically to any additive formulated in the polymeric material, which materially affects the properties or processing of the present fibers or fibrous web produced therewith. Such additives may include, without limitation, one or more of: a charging promoting agent that enhances the fibers' ability to accept and retain electrostatic charge; an antioxidant; an antimicrobial agent; activated carbon; or other polymer processing enhancement agent. Efficacious charging promoting agents include, without limitation, fatty acid amides and oligomeric hindered amine light stabilizers, such as octadecanamide (CAS No. 124-26- 5) and Chimassorb 944 (CAS No. 71878-19-8), respectively.

The present disclosure is further directed toward a nanofibrous web and filtration media constructed therewith. The web comprises polymeric fibers that are intimately comingled and entangled in a single layer, stand- alone network. In an implementation, the comingling and entanglement is attained by producing the fibers in a single spinning operation that provides fibers having diameters ranging from below 1 μιτι (nanofibers) up to 3 μιτι (microfibers). Preferably coarse fibers having diameter greater than 3 μιτι are also produced and comingled in the network. The web has an open, fluffy structure indicated by a low apparent density and is electrostatically charged. As a result of the structure and the charging, the web provides a high effective quality factor indicative of its ability to function as a good filtration element.

FIG. 1 schematically illustrates an exemplary implementation of the present process. A rotating member 100 is rotated at high speed. Filament 101 is ejected from member 100 and is spirally stretched during its flight by centrifugal force. Arrow 102 shows the direction of motion of the filament; arrows 103 indicate the direction of the friction force applied to filament's surface as it moves at high speed through a mist space. This results in the contact wetting of the filament surface by water molecules derived from the mist. These molecules are separated from the fiber surface by evaporation as the fiber moves to a collection point. The wrapping instability 104 of the fiber helps the evaporation of the hydro molecules, and the electrostatic charging to the fiber surface is generated by the ion-transfer before the fiber is laid down to form the nonwoven web. For clarity of illustration, FIG. 1 shows only a single filaments, but it will be understood that ordinarily a large plurality of filaments are simultaneously being ejected from the rotating member and being attenuated to fibers that are laid down on the nonwoven web. In an embodiment, the vapor, steam, or liquid droplets within the mist zone have a size of at most 50 μιτι, or even at most 20 μιτι or 10 μιτι. In an embodiment, the mist zone and the stretching zone are approximately coextensive. Alternatively, the mist zone may be located entirely within the stretching zone or the shaping zone, or it may extend within all or part of both zones. The droplets in the mist zone may be produced by ultrasonic or other known atomization techniques, or they may be produced as gas evolved from a heated or boiling liquid. Although water is preferable, other polar liquids may also possibly be used, but may present environmental concerns and ordinarily have to be recovered and recycled to be cost-effective.

FIG. 1 1 is a schematic illustration of a spatial temperature profile used in an embodiment of the present process. T1 is the temperature of a melt spinning zone immediately surrounding the rotating member, T2 is the temperature in an intermediate stretching zone through which the filaments 1 1 pass, and T3 is the temperature in the outer rapid quenching and fiber solidifying zone. The temperature profile is arranged such that T1 >T2> Tm (the melting point of polymer) and T3<< Tm, i.e. well below the melting point of the polymer. Thus, the region around the rotating member is ordinarily heated so that the fiber is above its melting point when it leaves the member and remains hot enough to be attenuated as it passes through the stretching zone. Thereafter, it has cooled below its melting point as it is collected on the web collector. Of course, a skilled person will recognize that FIG. 1 1 presents an idealized representation, since the actual ambient temperature will vary smoothly within the volume of the spinning apparatus.

The ion-transfer mechanism by which the polymer fibers are contact electrified is elucidated by FIG. 2, which shows covalently bound ions and mobile counterions, which could be counterions from the polymer, or other aqueous ions, such as H⁺ or OH^". FIG. 2A illustrates the covalently bound ions and mobile counterions in the wetting of the fiber surface by water molecules; FIG. 2B illustrates charging of the web by the ion separation at the interface of the polymer fibers, resulting from the evaporation of water molecules applied in the melt zone as the fiber spirals down from its ejection to collection to form the nonwoven web.

Apparatus for melt spinning that incorporates in-situ charging is illustrated in FIG. 3. It comprises: (1 ) a mounting frame 300 for a rotating member; (2) a corona charging ring 301 ; (3) a rotating member 302

connected by a rotating shaft to a high speed motor (not shown); (4) a stretching zone of the molten-state fibril or thread, 304; (5) a shaping zone of the solid-state fiber, 305. The rotating member 302 is a spin bowl in this embodiment. Alternatively the rotating member can have a flat or concave spinning surface. For example, the rotating member can have the cross- section of an ellipse, a hyperbola, a parabola, or other like curved surfaces, or the spinning surface can be frustoconical or have other more intricate shapes having one or more flat surfaces.

The spin bowl is optionally heated by an indirect, non-contact heating device 10, such as an infrared source, an induction heating device, or other such radiational heating source, to a temperature at or above the melting point of the polymer. During operation, the spinning melt spreads on the rotating spin bowl, preferably fully wetting the spin bowl's inner surface.

Typically, a film a few μιτι thick forms and flows along the bowl's inner surface until it reaches the departure edge. The rotational speed of spin bowl 302 is controlled to between about 1 ,000 rpm and about 100,000 rpm, even between about 5,000 rpm and about 100,000 rpm, or even between about 10,000 rpm and about 50,000 rpm. At the departure edge, the thin film splits into multiple discrete filaments 303 that are ejected from the bowl into the surrounding space. The throughput rate of the melt can be between about 0.1 cc/min to about 200 cc/min, even between about 0.1 cc/min to about 500 cc/min.

The molten-state filaments are further stretched by centrifugal force into fiber in the stretching zone 304. A corona charging ring energized by a high voltage supply (not shown) is located above the rotating spin bowl to provide the corona charging to the filament, especially while it is being stretching into fiber. The solidification of the fiber typically occurs in the shaping zone while the fiber is spirally flying downward. The attenuated and charged fibers ultimately impinge on a web collector, whereby they are formed into a web.

After exiting the stretching zone, the attenuated fibers pass through a volume termed the "shaping zone" and are collected or "laid down" to form a non-woven, nanofibrous web network. No action is taken during the spinning and laydown process to cut, chop, break, or otherwise define the length of the attenuated fibers, and so they may be of an indefinite length. It is found by direct imaging of the spinning process that at least a large preponderance of fibers remain intact from the point of ejection at least to the point of collection, so that that the fibers are ordinarily at least 30-50 cm, and more commonly at least 1 m long before any spontaneous breakage occurs. In some instances individual fibers may have unbroken lengths well over 1 m or more.

Accordingly, the fibers are herein termed "continuous."

In addition, it is found that in situ charging, as described above, is more effective than post charging processes, wherein the fibers are charged only after they have already been incorporated in a fibrous web

It is further found that electrostatic charging is very effectively accomplished while the fibers are relatively close to the melting point, as demonstrated by the method of thermally stimulated currents (TSCs). For polypropylene, the temperature regime for polymer melt and fibril threads to take charging most effectively is around 165°C to 195°C, so that corona charging is beneficially applied with the fibers in the stretching zone at a temperature in this range, with about 180°C being preferred. Incorporating a suitable charging agent in melts of non-polar polymers (e.g., polyolefins) further enhances charging near the melting point.

Without being bound by any theory, it is believed that charging the fibers in situ during their production offers benefits not attainable with post charging processes. For example, US Patent 6,375,886 to Angadjivand et al. describes a hydrocharging process wherein high pressure water impinges on a finished web. A relatively high flow rate that can disrupt the web structure is required, and charging, especially of polypropylene, is generally less effective at temperatures below 100°C than it would be at temperatures at or near the polymer melting point.

The laydown, or collection of fibers into a fibrous web, can be enhanced by use of electrostatic forces. For example, an electrostatic charge voltage potential can be applied and maintained in the spinning space between the distribution disc and the collector to improve the uniformity of the fibrous web laydown. The electrostatic charge can be applied by any high voltage charging device known in the art. For example, the rotating member and the collector can be made of electrically conductive material, so they can function as electrodes to which the charging device is connected.

Alternatively, the charging device can be connected to an electrode disposed within the spinning space and either the spinneret or the collector. The voltage potential applied to the spinning unit can be in the range between about 1 kV and about 150 kV.

Another implementation of the present apparatus and process is illustrated in FIG. 4, It comprises the same basic elements as shown in FIG. 3, including a mounting frame; a rotating member on a high speed rotating shaft; a stretching zone for the molten-state filaments; and a shaping zone for the solidified fiber; it does not employ a corona charging ring. An atomization ring 400 on the mounting frame provides a mist zone 401 containing water droplets around the high speed rotating spin bowl in the fiber shaping zone. The fiber is stretched by centrifugal force in the mist zone, wherein contact wetting of water molecules on the fiber surface results from friction between the mist droplets and fiber, which is moving at high speed. As a result, the fiber is electrostatically charged by ion transfer due to the separation of the water molecules from the fiber surface when the fiber is spirally laid down.

Still another implementation is illustrated in FIG. 5. It comprises the basic elements shown in FIG. 3, including a mounting frame; a rotating member on a high speed rotating shaft; a stretching zone for molten-state filaments; a shaping zone for solidified fibers, and again lacks a corona charging ring. A co-axial rotating atomization disk 502 under the high speed rotating spin bowl introduces water droplets into a mist zone 501 .

Downwardly directed shaping air from a shaping air ring on the mounting frame assists with the fiber laydown.

A further implementation is illustrated in FIG. 6. It comprises the basic elements shown in FIG. 4, including a mounting frame; a rotating member on a high speed rotating shaft; a stretching zone for the molten-state filaments; an atomization ring, and a shaping zone for the solidified fiber. A corona charging ring 600 is located above the rotating spin bowl to provide the corona charging to the molten-state filament in the stretching zone 601 where the filament is stretched into fiber. There is a mist zone 602 around the high speed rotating spin bowl in the fiber shaping zone. The filament is stretched by centrifugal force in the mist zone, wherein the fiber surface is contact- wetted by water molecules to because of friction as the fiber moves through the mist zone. Electrostatic charging of the fiber surface results, as described above.

An apparatus combining elements shown in FIGS. 5 and 6 is illustrated in FIG. 7. It includes a mounting frame; a rotating member on a high speed rotating shaft; a stretching zone for molten-state filaments; an atomization ring, a shaping zone for solidified fiber, a corona charging ring 700 is located above the rotating spin bowl to provide the corona charging for molten-state filaments in the stretching zone 704 that are being stretched into fibers. A co-axial rotating atomization disk 703 under the high speed rotating spin bowl provides vapor droplets within mist zone 705 around the high speed rotating spin bowl. The fiber is stretched by centrifugal force in the mist zone, wherein the fiber surface is contact-wetted by water molecules to because of friction as the fiber moves through the mist zone. Electrostatic charging of the fiber surface results, as described above.

Yet another apparatus and process for fiber spinning with in-situ charging and making a nonwoven media in accordance with the present disclosure is illustrated in FIG. 8. It comprises a spin pack 801 including elements shown in FIG. 6. A vacuum box 802 and a loop, continuously advancing converyer belt 803 are situated under spin pack 801 . Charged fibers are laid down in a non-woven web 804, which moves along belt 803. Nonwoven web 804 advances as it is being formed and is removed onto a guiding roll 805 that rotates in the direction indicated by arrow. The web then passes into a drying box 806. Thereafter, the finished web is wound onto a take-up roll 807, so that it may conveniently be further processed,

transported, or stored.

Yet another manufacturing apparatus implementing the present method is illustrated in FIG. 9. It comprises a spin pack 900 including the same elements as FIG. 7. Under the spin pack 900 is an atomization disk 901 .

Vacuum box 902 and a porous converyer belt 903 are provided for laying down the fibers to form nonwoven web 904. Applying a slight vacuum through vacuum box 902 (e.g. with a blower) beneficially removes any hot and/or cooling gases away from the fibers and helps pin the fibers to the collector, thereby promoting the collection of the spun fibers into a web with an even and uniform distribution. Optionally, the web collection system is cooled, e.g. with a cooling liquid or gas, such as flowing cold water or dry ice. Cooling the collected web is found to improve web strength, especially when the system is used with polymers that are higher melting than polypropylene, such as polyesters. The nonwoven web passes from conveyor belt 903 across a guiding roll 905 into a drying box 906, then the web is wound on a take-up roll 907. Alternatively, the web can be formed on a scrim material or the like, which is placed on the collector to collect the fiber directly onto the scrim thereby making a composite material. For example, a nonwoven web or other porous scrim material, such as a spunbond web, a melt blown web, a carded web or the like, can be placed on the collector and the fiber deposited onto the nonwoven web or scrim. In this way composite fabrics can be produced.

FIG. 10 depicts a 3D image of a nonwoven nanofibrous electret made using the present method and apparatus, specifically a stand-alone

nanofibrous web comprising electret fibers that are intimately comingled and entangled in a single layer, stand-alone network comprising nanofibers and microfibers with the average diameter of all fibers that is less than 1000 nm.

The present in-situ charging method combining the corona charging and the hydrocharging deposits both positive and negative charge onto the surface of fibers such that the positive and negative charge is randomly dispersed throughout the web. The corona charging produces a polarized web surface. The hydrocharging produces random unpolarized trapped charge throughout the volume of the web. Thus, a nonwoven nanofibrous electret web produced in accordance with the present invention may be substantially unpolarized in a plane normal to the plane of the web.

The web laydown of fibers into the present non-woven web is facilitated in some embodiments by at least one of a beneficially configured air flow field and the electrostatic charging arrangement. Judicious use of an air flow field helps to direct the fiber in its flight path from ejection to

incorporation in the fibrous web. The operational parameters characterizing the air flow field include the air temperatures and air flow speed and direction, e.g., within of the stretching zone and the shaping zone. The air flow field may further comprise air flow in a center zone, which may be delivered through an anti-swirling shield located on the bottom of the rotating

apparatus. This center air acts in some embodiments to inhibit an undesirable vortex-like action, wherein the ejected fibers falling downward from the spinning apparatus become entwined and entangled in a narrow zone below, and generally aligned with, an extension of the rotational axis of the spinning structure. Formation of such a vortex tends to inhibit a smooth and uniform laydown. Use of central air in conjunction with a centrifugal melt spinning process is discussed in more detail in International Patent

Publication WO 2013/096672 (of which US Patent Application Serial No. 14/364,708 claims benefit). Certain aspects of centrifugal melt spinning applicable in the manufacture of the present fibers are also discussed in US Patent Application Publication US2015/01 1 1455A1 to Huang et al. Both these references are incorporated herein in their entirety for all purposes by reference thereto.

In an embodiment, the air flow field in the present process is used solely to direct the flight path of the ejected fibers, ultimately to their collection point, so that the air velocity throughout the field can be maintained at relatively modest values. In contrast, melt blowing processes rely on high velocity air to attenuate the fiber while it is still molten. Thus, far higher air speed, such as 100 - 200 m/s, is typically required, whereas the fiber direction herein can be accomplished by imposing a gentle flow having a maximum speed of 5 m/s, or even 2 m/s, or even 1 m/s or less. The high speeds employed in melt blowing processes are likely to impede electrostatic charging, e.g., by blowing ions created by the electrostatic field away before they can attach to the fibers being created.

The air flow may include flow supplied from a nozzle that has an opening that is located on a radius of the rotating member, and the air flow may be directed at an angle to the radius of between 0 and 60 degrees and in a direction opposite to the direction of rotation of the rotating member.

The air flow also may include flow in the space proximate to the collector but that is distal from the periphery of the rotating member. In this region, the air flow is essentially perpendicular to the collector surface. The air therefore directs the fibers onto the surface of the collector where they are held in position by the electrostatic charge on the fibers and the electric field between the collector and the rotating member. Air in this region may be supplied by nozzles located on the underside of the rotating member, on the surface thereof facing the collector. The nozzles may be directed towards the collector.

The air flow field may further include a flow of air into the collector that is essentially perpendicular to the collector from a region between the body of the rotating member and the collector surface.

In some embodiments, an additional electrostatic charging may be carried out on the individual fibers at one or more stages during the spinning and web collection and assembly process, and beyond the charging accomplished by passage through the mist zone. The charge may be imposed while the polymer is molten on the spinning surface before ejection or in the ejected fiber as it is being attenuated. The additional charging may also occur after the fiber is cooled and already attenuated but before it is assembled into the fibrous web. In other embodiments, the additional charging occurs at any one or more of these stages. The present fibers retain electrostatic charge after they are incorporated into the non-woven web of the present disclosure.

For example, the additional charging may be accomplished by imposition of an electric field. Any high voltage direct current (d.c.) or even alternating current (a.c.) source may be used to supply the electric field. The spinning melt, filaments, or fibers may even be charged by induction from a charge held on or near the collector.

In an implementation, the charging arrangement may comprise electrifying both a corona ring located near the rotating member and the collector belt. Ordinarily, voltages of different sign and magnitude are applied to these locations, with the voltages all referenced to earth ground. The presence of these voltages beneficially results in a finished web that retains an electrostatic charge.

The current drawn in the charging process is expected to be small (preferably less than 10 mA). The source should have variable voltage settings, e.g., 0 kV to 80 kV, preferably -5 kV to -15kV for corona ring and +50 to +70 kV for collection plate, and preferably (-) and (+) polarity settings to permit adjustments in establishing the electrostatic field

In an embodiment, the present apparatus is used to produce a non- woven web comprising polymeric fibers that are intimately comingled and entangled in a single layer, stand-alone network. In an implementation, the comingling and entanglement is attained by producing the fibers in a single spinning operation. The fibers may nanofibers having a relatively narrow range of diameters, but under other processing conditions, the apparatus can produce a broad distribution of fiber diameters. In particular, the operation of the present spinning process is dependent on operating parameters that include temperatures, melt feeding rate, and speed of the rotating member. It has been found that adjustment of these parameters affects the distribution of fiber diameters produced. As further detailed below, it has been found that certain combinations of these operational parameters, together with the geometry of the rotating member and the selection of the polymeric material, result in a surprising and unexpected instability in the spinning melt on the rotating member. Under particular conditions, there is formed a wavy, non-uniform film thickness pattern that is characterized by a dynamic pattern of alternating regions or bands of undulating thickness that extend generally radially outward from the center of the rotating member to its edge. This variation of the thickness of the film results in turn in the ejection of discrete, continuous filaments of varying diameter derived from the film melt, with larger diameter filaments typically produced from regions of higher thickness and smaller diameter filaments from regions of lower thickness. The variation in diameter may persist after the filaments are attenuated into fibers.

In some embodiments, fibers produced using the present method and apparatus have diameters ranging from below 1 μιτι (nanofibers) up to 3 μιτι (microfibers). Preferably coarse fibers having diameter greater than 3 μιτι are also produced and comingled in the network. Construction of the web with a wide range of diameters promotes an open, fluffy structure indicated by a low apparent density and is electrostatically charged, rendering useful as a filtration element.

In an embodiment, the fibers comprise (by number): (a) at least about

70% nanofibers, about 5%-25% microfibers, and 0 to about 5% coarse fibers. The number average diameter of all the fibers is less than about 1000 nm and a median diameter of all the fibers is less than about 500 nm.

In another embodiment, the web fibers comprise (by number): (a) at least about 70% nanofibers having in combination a number average fiber diameter ranging from a lower limit of 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, or 700 nm to an upper limit of 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, or 1000 nm and a median fiber diameter ranging from a lower limit of 200, 250, 300, 350, or 400 nm to an upper limit of 350, 400, 450, 500, 550, or 600 nm; (b) about 5%-25% microfibers; and (c) 0%-5% coarse fibers, with the proviso that the number average diameter of the nanofibers is greater than the median diameter of the nanofibers. Such embodiments further include ones wherein the ranges of number average fiber diameter and the median fiber diameter are non-overlapping.

In still another embodiment, the web fibers comprise nanofibers, microfibers, and optionally coarse fibers, wherein: (a) the number average diameter of all the fibers ranges from a lower limit of 550, 600, 650, 700, or 750 nm to an upper limit of 800, 850, 900, 950, or 1000 nm and (b) the median diameter of all the fibers ranges from a lower limit of 150, 200, 250, 300, or 350 nm to an upper limit of 400, 450, or 500 nm.

In yet another embodiment, the web fibers comprise nanofibers, microfibers, and coarse fibers, with a mass percentage of the microfibers in a range from about 15% to about 30% and a mass percentage of the coarse fibers in a range from about 50% to about 70%. The fiber surface area within the fibrous web is dominated by area on the nanofibers, so that the

percentage of the specific surface area of the nanofibers may be greater than 90% of the specific surface area of the entire fibrous web. Thus, the relative surface charge density on the nanofibers may be about 10 times or more greater than the relative surface charge density of the microfibers.

Embodiments of the present nanofibrous web exhibit an apparent density ranging from a lower limit of 0.01 , 0.015, 0.02, 0.025, or 0.03 g/cm³ to an upper limit of 0.035, 0.04, 0.045, or 0.05 g/cm³.

A nonwoven fibrous electret web made using the present method and apparatus also affords a desirable combination of mechanical properties. In an embodiment, the of the nonwoven web provides a basis weight of between about 10 g/m² to about 40 g/m², a porosity greater than 95%, and a web strength in a machine direction greater than 2.0 N/cm, and the tensile elongation of greater than about 30%.

The presence of larger diameter fibers intimately comingled and entangled with smaller diameter nanofibers is found to be beneficial. In particular, it is believed that the presence of a modest number of microfibers and even coarse fibers creates a structure wherein the larger fibers impart a good web strength and other desirable mechanical properties, while supporting and maintaining a relatively open network in which the nanofibers are disposed. Pore size is thereby increased, beneficially reducing pressure drop in a filtration element. The open structure accommodates a relatively large number of nanofibers that in turn provide the web with a high area of surface that can be electrostatically charged, which is believed to enhance the web structure's ability to capture incident particulates.

In comparison, previous filtration structures have sometimes employed both nanofibers and the larger microfibers and/or coarse fibers, but have segregated them into a filtration layer and a support layer scrim, respectively. Such configurations ordinarily do not afford a network in which the nanofibers are sustained by the larger fibers in a relatively open and fluffy configuration. Instead, the nanofiber filtration layer tends to be more compact, deleteriously decreasing average pore size and increasing air flow resistance.

The open structure of the present nanofibrous web is further believed to permit the web to accept and retain a substantial electrostatic charge. With a large proportion of nanofibers, there is a large surface area that is able to be charged. The open structure is also beneficial, since there is minimal loss of surface area and charging due to the relative paucity of points of fiber tangency. The inclusion of the present continuous melt-spun nanofibers is also believed beneficial over shorter chopped fibers, such melt-spun or solution-spun fibers that may be as short as 1 cm or less. In addition, it is found that in situ charging, as described above, is more effective than post charging processes, wherein the fibers are charged only after they have already been incorporated in a fibrous web.

The present nanofibrous web exhibits desirable values of quality factor (QF) and effective quality factor (eQF), which are described below in further detail. Embodiments of the present nanofibrous web may have an effective quality factor (eQF) of at least 2.6, 2.8, or 3.0 (Pa ^χ g/cm³)^"1, up to 4.5, 4.75, 5, 5.25, 5.5, 5.75, or 6 (Pa g/cm³)^"1.

TEST METHODS

In the non-limiting Examples that follow, the following test methods are employed to determine various reported characteristics and properties.

Some of these are determined in accordance with published ASTM Standard Test Methods, which are promulgated by ASTM International, West

Conshohocken, PA. Each such ASTM Standard referenced herein is incorporated in its entirety for all purposes by reference thereto.

Thermally-Stimulated Discharge Current Analysis: The charge in a fibrous web is characterized by the detectable discharge current using thermally-simulated discharge current (TSDC). A thermal analysis instrument of Solomat TSC/RMA model 91000 with a pivot electrode is used in measurement. The proceedure of thermally-stimulated discharge

measurement is to heat an electret web so that the frozen or trapped charge regains mobility and moves to some lower energy configuration to generate a detectable external discharge current. An electric charge polarization can be induced in a web that has been charged according to the present invention by elevating the temperature to some level above the glass transition

temperature (Tg) of the polymer. After raising the polymer above its Tg, the sample is cooled in the presence of an electric field to freeze-in the polarization of the trapped charge. Thermally-stimulated discharge currents can then be measured by reheating the electret material at a constant heating rate and measuring the current generated in an external circuit. The measurement result of discharge current can be ploted is plotted against the temperature. The peak maximum and shape of discharge current are related to the configuration of the charge trapped in the electret material. The integration of the discharge peak(s) determines the amount of charge produced in the outside circuit due to movement of the charge inside the electret web to a lower energy state upon heating.

3D Web Imaging: Scanning electron microscopy (SEM) and other 2D imaging techniques typically give projected images that do not faithfully show how fibers are oriented within a nonwoven article in the depth (thickness) direction or the geometrical and topological features of a nanoweb's pore structure. Hence, 3D volume rendering of nanowebs is vital for understanding the pore structure and the fiber orientation represented in actual nanowebs.

Optical microscopy has not been widely used heretofore for imaging nanowebs, due to diffraction-limitations, as well as noise due to scattering. The characterization of sub-wavelength structures using a microscope is difficult because of the Abbe diffraction limit. Light with wavelength λ, traveling in a medium with refractive index n and converging to a spot with angle Θ will make a spot with radius d=A/(2nsin0). The denominator (nsinO ) is called the numerical aperture {NA), which can reach about 1 .4 in modern optical devices. Hence, the Abbe limit is roughly d-h/2. For green light with wavelength of 500 nm, the Abbe limit is 250 nm. A polymer nanoweb contains nanofibers, some of which may have diameters as small as 250 nm or less. An optical illumination system with a high-aperture cardioid annular condenser and a high numerical aperture makes it possible to get a useful image stack of the nanoweb with a high megapixel digital camera and precise control of vertical resolution (down to 10 nm). Stacks of images are taken using an automatic z range control with 10nm to 100 nm resolution. Individually, these images give little information as to how fibers are structurally related. But with a 3D volume image reconstruction algorithm, a stack of images can be transformed into a 3D volume rendering of a nanofibrous web, and the resulting 3D images can be rendered in different view directions. To improve accuracy, the data reported herein were obtained over an area that was expanded from that provided in a single image stack. Hence, stacks of images were taken over an effective image field formed by a three by three array of adjacent individual image fields and suitably combined, thereby increasing the sampling area by a factor of about seven, to roughly 271 m x 210 μιτι versus 101 m x 81 μιτι, while still maintaining a manageable file size. FIG. 1 shows a reconstructed 3D image of the nanoweb of Example 1 taken over such an effective image field after enhanced image processing and morphological operations.

Fiber Size Measurement Fiber Diameter is measured using scanning electron microscopy (SEM). In order to reveal the fiber morphology in different levels of detail, SEM images are taken at nominal magnifications of X25, X100, X250, X500, X1 .000, X2,500, X5,000 and X10,000. For fiber diameter counting, fibers are counted from at least 5 (up to 10) images at a magnification of 5000x or 2500x.

Basis Weight (BW) is determined by in accordance with ASTM

D3776/D3776M - 09a (2013), "Standard Test Methods for Mass Per Unit Area (Weight) of Fabric," and reported in g/m² or gms. Option C of the ASTM method is used to characterize a handsheet cut to about 10 cm for each web.

Web Thickness is measured using an optical microscopic method. In order to obtain a representative thickness measurement of the selected web examples, a non-contact measurement method is employed in order to preserve each example's web morphology. An Alicona Infinite Focus microscope, which utilizes an automated leveled stage to accurately obtain vertical and horizontal measurements, is used for the calculation of thickness. A 3D scan of each example is conducted to produce a 3D optical surface profile, from which the Alicona software produces a number average surface height (thickness) of the imaged area. This method produces a non-biased and non-destructive measurement of the thickness for each example.

Web Porosity is defined as a ratio of the volumes of the fluid space in a filter divided by the whole volume of the filter, and can be computed from the measured pore volume and bulk density of the material. The porosity of the sample is calculated from the basis weight and the thickness measurement for each sample. In practice, the basis weight (BW) of the sheet is calculated by the weight of a given sample size (W) divided by the sample area (A). The basis weight of the sample sheet is measured by punching out three samples of a fixed area across the transverse direction of the sheet and weighing them using a standard balance. The volume of this sample size is thus Α*δ where δ is the thickness of the sample. The weight of the sample is the weight of the fibers in the sample volume. If the solid fraction of the sheet is φ and the bulk polymer density is p is then

W = φ ρΑ*δ Since BW= W/A, Thus φ = BW/ρδ and polymer density p

Porosity = 1 -Solid Fraction

= 1 - BW/ρδ The thickness is measured using an optical microscopic method and is averaged over three measurements at different points of the sample across the transverse direction. In order to obtain a representative thickness measurement of the selected examples, a non-contact measurement method is used in order to preserve each example's web morphology. Each example is placed between layers of DuPont FEP film to prevent any damage during the punching of a 35mm diameter circle. Each example is then weighed for the calculations of basis weight and porosity. An Alicona Infinite Focus microscope, which utilizes an automated leveled stage to accurately obtain vertical and horizontal measurements, is employed. Before each example, the stage would be centered and zeroed while focused on the surface of the stage in order to have precise control on the vertical height measurement of the example. Following the zeroing of the stage, each example would be placed onto the stage so that the center would be directly beneath the microscopes lens. In order to ensure each example is resting flat on the stage surface, a 6.4g metal washer, with an outer diameter of 31 mm and an inner diameter of 7mm, is carefully placed onto the center of each example so that imaging could be conducted through the center of the washer. The stage would then be lowered until the example passed through the field of focus until the sample could no longer be observed. Using the stage's height position above the example as the top height parameter and the stage's zeroed surface as the bottom height parameter, a 3D scan is commenced starting at the stage's surface with the microscope collecting a 1 ,025.10μηη by 820.09 m image every 1 μηη until the stage reaches the set top height parameter. Following the collection of images, the Alicona imaging software analyzes each image to construct a 3D optical surface profile of the selected example. Using the Alicona imaging software, an area analysis of the example's surface texture is conducted to provide a series of software calculated surface parameters including the average height of the imaged area. By using this technique and the Alicona imaging software, a non-bias representative measurement of each example's thickness can be collected.

Frazier Air Permeability is a measure of the amount of time required for a certain volume of air to pass through a test specimen. The air pressure is generated by a gravity loaded cylinder that captures an air volume within a chamber using a liquid seal. This pressurized volume of air is directed to the clamping gasket ring, which holds the test specimen. Air that passes through the specimen escapes to atmosphere through holes in the downstream clamping plate. Frazier air permeability measurements are carried out using either a FAP-5390F3 or an FX3300 instrument, both manufactured by Frazier Precision Instrument Co Inc. (Hagerstown, MD).

In using the FAP-5390F3 instrument, the test specimen is mounted at the sample stand. The pump is so adjusted that the inclined type air pressure gauge shows the pressure of 12.7 cm at the water column by use of the resistor for pressure adjustment use. From the scale indication observed then of the vertical type air pressure gauge and the kind of used orifice, the air amount passes the test specimen, is obtained. The size of the nozzle is varied depending upon the porosity of the material.

In using the FX3300 instrument, a powerful, muffled vacuum pump draws air through an interchangeable test head with a circular opening. For measurement the test head appropriate for the selected test standard is mounted to the instrument. The specimen is clamped over the test head opening by pressing down the clamping arm which automatically starts the vacuum pump. The preselected test pressure is automatically maintained, and after a few seconds the air permeability of the test specimen is digitally displayed in the pre-selected unit of measure. By pressing down the clamping arm a second time the test specimen is released and the vacuum pump is shut-off. Since the vacuum pump is automatically started when the test specimen is clamped in place over the test head opening, the test pressure builds up only after the test specimen has been clamped. The test pressure is digitally pre-selected in accordance with the test standard and is automatically controlled and maintained by the instrument. By using a true differential measurement the test pressure is measured accurately, even at high air flow rates. The air flow through the test specimen is measured with a variable orifice. The air permeability of the test specimen is determined from the pressure drop across this orifice, and is digitally displayed in the selected unit of measure for direct reading. High stability, precision pressure sensors provide for an excellent measuring accuracy and reproducibility of the test results. In this measurement, a pressure difference of 124.5 N/m² is applied to a suitably clamped media sample and the resultant air flow rate is measured as Frazier air permeability and is reported in units of cm³/min/cm². Frazier air permeability is normalized to 34 g/m² basis weight by multiplying the Frazier air permeability by the basis weight and divided by 34 and is reported in cm³/min/cm². High Frazier air permeability corresponds to high air flow permeability and low Frazier air permeability corresponds to low air flow permeability.

Electrostatic Charge (E.S.) is measured using SIMCO FMX-003 Electrostatic Fieldmeter. The FMX-003 measures static voltages within +1- 22kV (22,000V) at a distance of 25 mm.

Mean Flow Pore Size is measured according to ASTM E 1294-89 (1999, now withdrawn), "Standard Test Method for Pore Size Characteristics of Membrane Filters Using Automated Liquid Porosimeter." Individual samples of different size (8 mm diameter) are wetted with the low surface tension fluid as described above and placed in a holder, and a differential pressure of air was applied and the fluid is removed from the sample. The differential pressure at which wet flow is equal to one-half the dry flow (flow without wetting solvent) is used to calculate the mean flow pore size using supplied software. Mean flow pore size is reported in μιτι.

Bubble Point is measured according to ASTM F316-03 (201 1 ),

"Standard Test Methods for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test." Individual samples (8 mm diameter) are wetted with the low surface tension fluid as described above. After placing the sample in the holder, differential pressure (air) is applied and the fluid is removed from the sample. The bubble point is the first open pore after the compressed air pressure is applied to the sample sheet and is calculated using vendor supplied software.

Filtration Efficiency (FE) and Pressure Drop (ΔΡ): Media and mask performance are characterized by two main factors, Percent Penetration (P) and Pressure Drop (ΔΡ). The Percent Penetration represents the fraction of incident particles which penetrate the filter without being removed under a specified testing condition. The same penetration concept can likewise be quantified by the Percent Filtration Efficiency (FE) can be determined from the more easily measured value of P using the relationship

FE = 100 - P

Pressure Drop (ΔΡ) characterizes a filter's resistance to air flow, which is conveniently measured using the test method described in DIN Standard EN 1822 (1998), ΔΡ is reported in units of pressure, such as mm H₂O/cm² or Pa.

It is known that both P and ΔΡ vary as a function of the challenge aerosol and its face velocity, so that comparison of data for different samples is proper only if the measurements are done under the same conditions. The challenge aerosol is typically described by variables that include particle (or aerosol) peak size and distribution, the chemistry and form of the aerosol (solid vs. liquid), and the aerosol's charge distribution (neutralized, neutral, or charged). The face velocity is a function of the challenge flow rate used and the surface area of media in the test fixture.

A TSI 8130 Filtration Tester (The Model 8130 Automated Filter Tester) is used to measure filter efficiency and penetration versus particle size.

Challenging filters and/or filter media with a known particle size is achieved by using atomizers and the Electrostatic Classifier to generate particles. Upstream and downstream particle detection is accomplished using tow Condensation Particle Counters. Fine particle dust-loading tests are conducted on flat-sheet media with a circular opening of 1 1 .3 cm diameter (area = 100 cm²). A 2 wt% sodium chloride aqueous solution is used to generate fine aerosol with a mass mean diameter of 0.26 μιτι, which is used in the loading test. The air flow rate is 32 liter/min which corresponds to a face velocity of 5.3 cm/s. According to the equipment manufacturer, the aerosol concentration is about 16 img/m³. Filtration efficiency and initial pressure drop are measured at the beginning of the test and the final pressure drop is measured at the end of the test. Pressure drop increase is calculated by subtracting the initial pressure drop from the final pressure drop.

Quality factor and Effective Quality factor: Quality Factor (QF) has frequently been used to compare the performance of different media types, as defined as: QF = -Ιη(Ρ/100)/ΔΡ wherein P is penetration and ΔΡ is pressure drop. Quality factor QF can be specified in units of inverse pressure, e.g. (Pa)^"1, wherein 1 pascal (Pa) = 1 N/m². The thickness, porosity, and fiber diameter of the nanofibrous nonwoven media enter the quality factor (QF) indirectly by their effect on P and ΔΡ. However, an ideal filtration media would also exhibit low basis weight and low apparent density, to account for the desirability of

accomplishing filtration with the smallest possible amount of media.

Accordingly, filtration media can further be characterized by an effective quality factor (eQF), which is defined herein as the quality factor divided by apparent density (p_apparent), or:

eQF = QF / Papparent = (-ln(P/100)/ΔΡ) / Papparent

Effective quality factor can be specified in units of (Pa g/cm³)^"1.

Dust Loading Capacity Test: The dust holding capacity is a measure of the life of the filter. It is usually defined as the weight of dust per square foot which a filter will hold before the pressure drop across the filter reaches a given level at a given air face velocity. A loading test is performed to determine the effect particle accumulation has on filter resistance and penetration. Fine particle dust-loading tests are conducted here on flat-sheet media using automated filter test (TSI Model No. 8130) with a circular opening of 1 1 .3 cm diameter (area=100 cm²). A 2 wt.% sodium chloride aqueous solution is used to generate fine aerosol with a mass mean diameter of 0.28 μπτι, which was used in the loading test. The air flow rate is 32 liter/min which corresponded to a face velocity of 5.3 cm/s. According to equipment manufacturer, the aerosol concentration was about 18 mg/nr³.

Filtration efficiency and initial pressure drop are measured at the beginning of the test and the final pressure drop is measured at the end of the test. Pressure drop increase is calculated by subtracting the initial pressure drop from the final pressure drop. The amount (weight) of material collected on media during a loading test is determined by the following formula:

W = {C Qf f)/1000 wherein W = weight of particle accumulation in mg; C = aerosol concentration in mg/m³. The time of how long a test required based on the weight of the aerosol to be collected is simply rearrange by the equation shown above:

EXAMPLE

A nanofibrous web media consisting of continuous polypropylene fibers is made using centrifugal melt spin process with a rotating spin bowl. The fibers are charged using a combination of corona and vapor charging in accordance with the present disclosure. Parameters of the casting process, including temperature, melt feeding rate, and spin bowl rotational speed are chosen to provide an instability in the film near the departure edge of the spinning disk. Under these conditions, a relatively thicker film moves outward, with radial banding from the center to the edge, with an undulating thickness. Nanofibers are formed from the thinner region of the film; thicker regions result in microfibers and/or coarse fibers.

The charged and attenuated fibers are laid on a belt collector to form web media. The web laydown of fibers is controlled by a combination of the designed air flow field and a charging arrangement. The operation

parameters of air flow field are the air temperatures and air flow rates of the stretching zone air, shaping air and a center air applied through the hollow rotating shaft and an anti-swirling hub. There is dual high voltage charging on the collector belt and on the corona ring around the spinning bowl. The web is laid down at a distance under the spin bowl.

The Example of this invention is made under the following conditions. A PRISM extruder with a gear pump is used to deliver the polymer melt to the rotating spin bowl through the melt transfer line. The extrusion temperature is set at 200°C. The temperature of the spinning melt from the melt transfer line is set to 200°C and the melt feeding rate was 10 gram/m in/bowl. The rotating spin bowl has a diameter of 152.4 mm. The rotation speed of the spin bowl is set to a constant 10,000 rpm. Induction heating is used to heat up the rotating spin bowl. The stretching zone air flow is set at 150°C and 8.0 SCFM. The shaping air flow is set at 80°C and 7.0 SCFM. The center air flow through the hollow rotating shaft and anti-swirling hub is set at 50°C and 2.5 SCFM. The nanofibrous web is laid down on a belt collector with a laydown distance of 127 mm.

The polymer used in the Example is polypropylene. However, polypropylene-polypropylene blends wherein the polypropylenes in the blend are different can also be used. The polypropylene can also contain 1 .0 wt.% octadecanamide as a charging additive. Octadecanamide with a CAS number of 124-26-5, is fatty acid amide with a chemical name of N-(1 ,3-Benzodioxol- 5-ylmethyl)octadecanamide; or C18H37NO; or Stearamide; or Amide C18, can be obtained from Sigma-Aldrich. It has a melting point of 98 - 102°C and Flash Point of 297.34°C. Additional processing conditions for making this example include the spin bowl temperature, the spin enclosure temperature and humidity, the corona charging voltage and current, and the temperature and concentration of the steam cloud, which is made by atomizing deionized water, the collector belt charging voltage and current, and the collector belt moving speed as the web wind-up speed. Under the given rotating speed and melt feeding rate, the spin bowl temperature is the key parameter determining the fiber diameter and its distribution. For the given fiber spinning and the web laydown conditions, the basis weight of the resulting web can be determined by the collector belt moving speed. The Example has a basis weight range between about 10 g/m² to about 40 g/m², a porosity greater than 95%, a web strength in a machine direction greater than 2.0 N/cm, and a tensile elongation of greater than about 30%. Having thus described the invention in rather full detail, it will be understood that this detail need not be strictly adhered to but that further changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the invention as defined by the subjoined claims.

Embodiments of the present nanofibrous web are contemplated that comprise a combination of any two or more of the aforementioned

dimensional, physical, or functional characteristics, as are embodiments of the present process including any two or more of the aforementioned process steps or features and embodiments of the present apparatus including any two or more of the structural features recited above.

Where a range of numerical values is recited or established herein, the range includes the endpoints thereof and all the individual integers and fractions within the range, and also includes each of the narrower ranges therein formed by all the various possible combinations of those endpoints and internal integers and fractions to form subgroups of the larger group of values within the stated range to the same extent as if each of those narrower ranges was explicitly recited. Where a range of numerical values is stated herein as being greater than a stated value, the range is nevertheless finite and is bounded on its upper end by a value that is operable within the context of the invention as described herein. Where a range of numerical values is stated herein as being less than a stated value, the range is nevertheless bounded on its lower end by a non-zero value.

In this specification, unless explicitly stated otherwise or indicated to the contrary by the context of usage, where an embodiment of the subject matter hereof is stated or described as comprising, including, containing, having, being composed of, or being constituted by or of certain features or elements, one or more features or elements in addition to those explicitly stated or described may be present in the embodiment. An alternative embodiment of the subject matter hereof, however, may be stated or described as consisting essentially of certain features or elements, in which embodiment features or elements that would materially alter the principle of operation or the distinguishing characteristics of the embodiment are not present therein. A further alternative embodiment of the subject matter hereof may be stated or described as consisting of certain features or elements, in which embodiment, or in insubstantial variations thereof, only the features or elements specifically stated or described are present. Additionally, the term "comprising" is intended to include examples encompassed by the terms "consisting essentially of and "consisting of." Similarly, the term "consisting essentially of is intended to include examples encompassed by the term "consisting of."

When an amount, concentration, or other value or parameter is given as either a range, preferred range, or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range.

In this specification, unless explicitly stated otherwise or indicated to the contrary by the context of usage, amounts, sizes, ranges, formulations, parameters, and other quantities and characteristics recited herein, particularly when modified by the term "about," may but need not be exact, and may also be approximate and/or larger or smaller (as desired) than stated, reflecting tolerances, conversion factors, rounding off, measurement error, and the like, as well as the inclusion within a stated value of those values outside it that have, within the context of this invention, functional and/or operable equivalence to the stated value.

Claims

What is claimed is: 1 . A process for producing a nonwoven web comprising electret fibers, comprising the steps of:

(iii) ejecting from the departure edge a plurality of discrete,

continuous filaments derived from the film melt into a stretching zone, each filament having a filament surface;

2. The process of claim 1 , wherein the spinning melt comprises a polyolefin.

3. The process of claim 2, wherein the spinning melt comprises a polypropylene or a blend of multiple different polypropylenes.

4. The process of claim 1 , wherein the electret fibers in the nonwoven web are intimately comingled and entangled in a single layer, stand-alone network.

5. The process of claim 1 , wherein the fibers are not subjected to any imposed air flow having an air speed exceeding 5 m/s.

6. The process of claim 1 , wherein the electret fibers have a number average diameter less than 1000 nm.

7. The process of claim 1 , wherein the electret fibers comprise nanofibers, microfibers, and optionally coarse fibers.

8. The process of claim 1 , further comprising the step of:

(viii) imposing an electrostatic field on the filaments for at least a portion of a duration occurring between steps (iii) and (vii).

9. The process of claim 8, wherein the electrostatic field is provided by a voltage potential that is between about 1 kV and about 150 kV

10. The process of claim 1 , further comprising the step of:

(ix) imposing an electrostatic field on the filaments during at least one of steps (iv) or (v).

1 1 . The process of claim 9, wherein the electrostatic field is provided by a voltage potential maintained between the rotating member and the collection surface.

12. The process of claim 9, wherein the electrostatic field is provided by a voltage potential maintained between the rotating member and an electrode positioned between the rotating member and the collection surface.

13. The process of claim 9, wherein the electrostatic field is provided by a voltage potential maintained between the collection surface and an electrode positioned between the rotating member and the collection surface.

14. The process of claim 1 , further comprising the step of:

(x) delivering air to form a low-velocity air flow field that directs the attenuated fibers to the collection surface, the air flow field being such that the fibers are not subjected to any imposed air flow having an air speed exceeding 5 m/s.

15. The process of claim 14, wherein the air flow field includes within the stretching zone a flow that is proximate the departure edge and directed radially outward therefrom.

16. The process of claim 14, wherein the air flow field includes within the shaping zone a flow that urges the fibers toward the collection surface.

17. The process of claim 1 , wherein the polar liquid is water.

18. The process of claim 1 , wherein the mist zone is present within the stretching zone.

19. The process of claim 1 , wherein the mist zone is present within the shaping zone.

20. The process of claim 1 , wherein the spinning melt comprises a charging promoting agent.

21 . A spinning apparatus for making a nanofibrous web comprising electret fibers, comprising:

(iv) a collection surface configured to collect the attenuated fibers into the fibrous web, and

wherein, upon passage through the mist zone, the fibers are electrostatically charged to become electret fibers.

22. The spinning apparatus of claim 21 , further comprising at least one electrode configured to be connected to a high voltage source.

23. The spinning apparatus of claim 22, wherein the at least one electrode comprises a corona charging ring.

24. The spinning apparatus of claim 21 , further comprising at least one air flow outlet for delivering air from an air source to maintain a low- velocity air flow field that directs the fibers to the collection surface.

25. A nonwoven fibrous electret comprising the properties of:

(i) a basis weight of between about 10 g/m2 to about 40 g/m2,

(ii) a porosity greater than 95%,

(iii) a web strength in a machine direction greater than 2.0 N/cm, and a tensile elongation of greater than about 30%.