Core-sheath fibers, nonwoven fibrous webs, and filtration articles comprising same

Abstract

Described herein are thermoplastic core-sheath fibers. The thermoplastic core-sheath fibers include a core having a coextensive sheath layer disposed thereon, the core including a first polymeric resin and a charge-enhancing additive, and the sheath layer including a second polymeric resin, with the proviso that when the second polymeric resin includes poly(4-methyl-1-pentene), the second polymeric resin does not include 100% by weight of poly(4-methyl-1-pentene). These thermoplastic core-sheath fibers can be used in filtration applications.

Description

[Technical Field]

[0001] FIELD OF THE DISCLOSURE The present disclosure relates generally to nonwoven fibrous webs containing charge-enhancing additives and articles comprising them. Summary of the Invention

[0002] Electrets are dielectric materials that have a permanent or semi-permanent electric charge or dipole polarization. Electrets are useful in a variety of devices, including, for example, food packaging wrap, air filters, filtering facepieces, and respirators, as well as in electroacoustic devices such as microphones and headphones, and as electrostatic elements in electrostatic recording devices.

[0003] Typically, electrets are made by incorporating a charging additive into a polymer material and then inducing a charge on the polymer material using corona treatment, tribocharging treatment, hydrocharging treatment, or a combination thereof. Both corona treatment, tribocharging, and hydrocharging are considered surface treatment techniques. Therefore, when making fibrous electrets, the charge-enhancing additive used to create a quasi-permanent charge is placed in the surface layer (see U.S. Pat. No. 4,375,718 (Wadsworth et al.) and JP 2008150753 (Hane et al.)). In the present application, it has been unexpectedly discovered that a charging additive added to the core of a core-sheath fiber can produce an electret.

[0004] In one aspect, a thermoplastic core-sheath fiber is disclosed. The thermoplastic core-sheath fiber includes a core having a coextensive sheath layer disposed thereon, the core including a first polymer resin and an electrostatic charge enhancing additive, and the sheath including a second polymer resin, with the proviso that when the second polymer resin includes poly(4-methyl-1-pentene), the second polymer resin does not include 100% by weight of poly(4-methyl-1-pentene).

[0005] In one embodiment, the thermoplastic core-sheath fibers disclosed herein can be used in filtration articles such as respirators.

[0006] In another aspect, a method for making an electret is described. The method includes providing a thermoplastic core-sheath fiber including a core having a coextensive sheath layer disposed thereon, the core including a first polymer resin and a charge-enhancing additive, and the sheath including a second polymer resin, with the proviso that when the second polymer resin includes poly(4-methyl-1-pentene), the second polymer resin does not include 100% by weight of poly(4-methyl-1-pentene), and charging the thermoplastic core-sheath fiber via corona treatment, hydrocharging, tribocharging, or a combination thereof to form an electret.

[0007] The above summary is not intended to describe every embodiment. The details of one or more embodiments of the invention are also set forth in the following detailed description. Other features, objects, and advantages will be apparent from the detailed description and claims. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a schematic cross-sectional view of an exemplary core-sheath fiber according to the present disclosure.

[0009] [Figure 2] 1 is a schematic perspective view of a nonwoven fibrous web according to the present disclosure.

[0010] [Figure 3] FIG. 1 is a schematic front view of an exemplary respirator 40 according to one embodiment of the present disclosure.

[0011] [Figure 4] FIG. 4 is a schematic cross-sectional view of the mask body 42 of FIG. 3.

[0012] Repeat use of reference characters in the specification and drawings is intended to represent the same or analogous features or elements of the present disclosure. It is to be understood that those skilled in the art may devise numerous other modifications and embodiments that fall within the scope and spirit of the principles of the present disclosure. The figures may not be drawn to scale. DETAILED DESCRIPTION OF THE INVENTION

[0013] As used herein, The terms "a," "an," and "the" are used interchangeably and mean one or more; "And / or" is used to indicate that either or both of the stated cases may occur; for example, A and / or B includes (A and B) as well as (A or B).

[0014] Further herein, the recitations of ranges by endpoints include all numbers subsumed within that range (eg, 1 to 10 includes 1.4, 1.9, 2.33, 5.75, 9.98, etc.).

[0015] Further, as used herein, the term "at least 1" includes all numbers greater than or equal to 1 (e.g., at least 2, at least 4, at least 6, at least 8, at least 10, at least 25, at least 50, at least 100, etc.).

[0016] As used herein, "comprising at least one of" A, B, and C refers to a single element A, a single element B, a single element C, A and B, A and C, B and C, and combinations of all three.

[0017] Referring now to Figure 1, a core-sheath fiber 100 includes a core 110 having a sheath layer 120 disposed thereon. Although not shown, the sheath layer 120 is coextensive along the fiber length (excluding the fiber ends). The core-sheath fiber and core shown in Figure 1 have circular cross-sections, although other cross-sections may be used, such as triangular, square, rectangular, pentagonal, hexagonal, heptagonal, octagonal, star-shaped, elliptical, trilobal, and tetralobal cross-sections. Similarly, although Figure 1 shows a centrally located core, the core may also be off-center.

[0018] In one embodiment, the core-sheath fibers of the present disclosure are not so-called "islands in the sea" extrudates, but rather have multiple fiber cores (i.e., one, two, four, or even more than six cores) distributed within a polymer matrix that also forms the sheath.

[0019] Thermoplastic resins useful in the core of the present disclosure include any thermoplastic non-conductive polymer that is capable of retaining a significant amount of trapped charge when formed into a web and charged. Typically, such polymeric resins have a thermal conductivity of 10 to 30% at the temperature of the intended application. 14 It has a DC (direct current) resistivity of more than Ω-cm. Polymers capable of acquiring trapped charges include polypropylene; polyethylene (e.g., HDPE, LDPE, LLDPE, VLDPE; ULDPE, UHMW-PE grades); polyolefins such as poly(1-butene); poly(3-methylbutene); poly(4-methyl-1-pentene); polyvinyl chloride; polystyrene; polycarbonate; polyesters including polylactide; and perfluorinated polymers and copolymers. Preferably, the thermoplastic resin comprises polypropylene.

[0020] Examples of suitable thermoplastic resins include, for example, polypropylene resins: ESCORENE PP 3746G available from Exxon-Mobil Corporation (Irving, TX); TOTAL PP3960, TOTAL PP3860, and TOTAL PP3868 available from Total Petrochemicals USA Inc. (Houston, TX); and METOCENE MF 650W available from LyondellBasell Industries, Inc. (Rotterdam, Netherlands); and poly-4-methyl-1-pentene resins TPX-DX820, TPX-DX470, and TPX-MX002 available from Mitsui Chemicals (Tokyo).

[0021] In the present disclosure, the core of the fiber contains a charge-enhancing additive. Many charge-enhancing additives for producing electret-containing fibrous webs are known in the art. The charge-enhancing additive is a material that increases the initial quality factor (Q0) and / or enhances the charge stability (Q3 / Q0 ratio) of webs produced using core-sheath fibers, as discussed below. Exemplary charge-enhancing additives include pigments, light stabilizers, primary and secondary antioxidants, metal deactivators, hindered amines, hindered phenols, metal salts, phosphite triesters, phosphates, fluorine-containing compounds, and combinations thereof. Preferably, the charge-enhancing additive is solid at ambient conditions to prevent migration within the resin and does not decompose at moderate temperatures. In one embodiment, the charge-enhancing additive is solid at temperatures of at least 25, 30, 40, 50, 60, 80, or even 100°C. In one embodiment, the charge enhancing additive does not decompose, e.g., does not lose significant weight (i.e., less than 5, 1, or even 0.1 wt %), when measured under nitrogen by thermogravimetric analysis using a temperature ramp rate of 10°C / min to heat to 235°C.

[0022] Particularly preferred charge enhancing additives include hindered amine additives, triazine additives, and hindered phenol additives.

[0023] Specific examples of hindered amine or triazine additives include poly[[6-(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl][[(2,2,6,6-tetramethyl-4-piperidyl)imino]hexamethylene[(2,2,6,6-tetramethyl-4-piperidyl)imino]] available from BASF, Ludwigshafen, Germany under the trade name "CHIMASSORB 944"; dimethyl succinate-1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidine polycondensate available from BASF under the trade name "TINUVIN 622"; and dimethyl succinate-1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidine polycondensate available from BASF under the trade name "TINUVIN 622" (poly[[6-(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl][[(2,2,6,6-tetramethyl-4-piperidyl)imino]] available from BASF, Ludwigshafen, Germany under the trade name "CHIMASSORB 944" (poly[[6-(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl][[(2,2,6,6-tetramethyl-4-piperidyl)imino]]hexamethylene ... di-tert-butyl-4-hydroxybenzyl)-2-n-butylmalonate bis(1,2,2,6,6-pentamethyl-4-piperidyl) available under the trade name "144" from BASF; polycondensate of dibutylamine-1,3,5-triazine-N,N'-bis(2,2,6,6-tetramethyl-4-piperidyl-1,6-hexamethylenediamine-N-(2,2,6,6-tetramethyl-4-piperidyl)butylamine available under the trade name "CHIMASSORB 2020" from BASF; 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-((hexyl)oxy)-phenol available under the trade name "TINUVIN 1577" from BASF; N-substituted amino aromatic compounds, particularly triamino-substituted compounds, such as 2,4,6-trianilino-p-(carbo-2'-ethylhexyl-l'-oxy)-l,3,5-triazine, available under the tradename "T-150"; and 2,4,6-tris-(octadecylamino)triazine, also known as tristearylmelamine ("TSM").

[0024] The hindered phenolic additive has a hydroxyl group as a terminal functional group. The hindered phenol additive is not particularly limited, and specific examples include pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (Irganox 1010, manufactured by BASF), octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (Irganox 1076, manufactured by BASF), tris-(3,5-di-tert-butyl-4-hydroxybenzyl)-isocyanurate (Irganox 3114, manufactured by BASF), and 3,9-bis-{2-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)-propionyloxy]-1,1-dimethylethyl}-2,4,8,10-tetraoxaspiro-[5,5]undecane (Sumilizer-GA-80, manufactured by Sumitomo Chemical Co., Ltd.).

[0025] Further thermally stable organic triazine compounds or oligomers, in which the compound or oligomer contains at least one nitrogen atom in addition to the nitrogen atom in the triazine ring, are disclosed in U.S. Pat. Nos. 6,268,495, 5,976,208, 5,968,635, 5,919,847, and 5,908,598 to Rousseau et al.

[0026] Further examples of charge-enhancing additives are provided in U.S. Patent Application Publication No. 2011 / 0137082 (Li et al.); U.S. Patent No. 8,613,795 (Li et al.), U.S. Patent No. 7,390,351 (Leir et al.), U.S. Patent No. 5,057,710 (Nishiura et al.), and U.S. Patent Nos. 4,652,282 and 4,789,504 (both to Susumu et al.), and U.S. Patent No. 8,790,449(B2) (Li et al.).

[0027] The charge-enhancing additive can be added in any suitable amount. The charge-enhancing additives of the present disclosure can be effective even in relatively small amounts. Typically, the charge-enhancing additive is present in the blend of thermoplastic resin and charge-enhancing additive in an amount up to about 10 wt %, more typically in the range of 0.02 to 5 wt %, based on the total weight of the blend. In some embodiments, the charge-enhancing additive is present in an amount of 0.1 to 3 wt %, 0.1 to 2 wt %, 0.2 to 1.0 wt %, or 0.25 to 0.5 wt %.

[0028] Blends of thermoplastic resins and charge-enhancing additives can be prepared by well-known methods. The charge-enhancing additive can be added directly to the thermoplastic resin to form the core, or the charge-enhancing additive can be concentrated in the thermoplastic resin in a so-called masterbatch, which can then be added to the thermoplastic resin to form the core. In the case of a masterbatch, the thermoplastic resin of the masterbatch can be different from the thermoplastic resin forming the core. In one embodiment, the charge-enhancing additive is present in the masterbatch in an amount of 10 to 30 wt. %. Typically, blends of charge-enhancing additives and thermoplastic resins are processed using melt extrusion techniques, so the blend can be preblended to form pellets in a batch process, or the thermoplastic resin and charge-enhancing additive can be mixed in an extruder in a continuous process. When using a continuous process, the thermoplastic resin and charge-enhancing additive can be preblended as solids or added separately to the extruder and mixed in the molten state.

[0029] Examples of melt mixers that can be used to form pre-blended pellets include those that provide dispersive mixing, distributive mixing, or a combination of dispersive and distributive mixing. Examples of batch processes include those using a BRABENDER (e.g., the BRABENDER PREP CENTER, available from CW Brabender Instruments, Inc., South Hackensack, New Jersey) or a BANBURY internal mixing and roll milling device (e.g., available from Farrel Co., Ansonia, Connecticut). After batch mixing, the resulting mixture may be immediately quenched and stored below the mixture's melting temperature for later processing.

[0030] Examples of continuous processes include single screw extrusion, twin screw extrusion, disk extrusion, reciprocating single screw extrusion, and pin barrel single screw extrusion. Continuous processes can include the use of both cavity transfer mixers (e.g., CTM available from RAPRA Technology, Ltd., Shrewsbury, England) and distributive or dispersive mixing elements such as pin mixing elements, static mixing elements (e.g., available from MADDOCK mixing elements or SAXTON mixing elements).

[0031] Examples of extruders that can be used to extrude pre-blended pellets prepared by a batch process include the same types of equipment described above for continuous processing. Useful extrusion conditions are generally those suitable for extruding resins without additives.

[0032] The core can have any average diameter, but preferably ranges from 1 to 100 microns, more preferably from 5 to 50 microns, and even more preferably from 10 to 25 microns.

[0033] The core is encapsulated by a sheath layer. The sheath layer forms a layer coextensive with the outer surface of the fiber core, except for the ends of the fiber core, which may or may not be covered by the sheath layer. While not a requirement, it is preferred that the sheath layer be substantially uniform and complete. In one embodiment, the sheath layer can be a thin layer, e.g., at least 0.05, 0.1, 0.2, 0.4, 0.5, or even 0.6 microns thick, and having an average thickness of up to 0.8, 1.0, 1.5, 2.0, 2.5, 2.8, or even 3.0 microns. In one embodiment, the core-to-sheath volume ratio is at least 60:40, 70:30, or even 75:25. In one embodiment, the core-to-sheath volume ratio is up to 80:20, 85:15, 90:10, or even 95:5. In one embodiment, the weight percent of the sheath layer in the core-sheath fiber is at least 3, 5, 8, 10, 15, or even 25 weight percent. In one embodiment, the weight percent of the sheath layer in the core-sheath fiber is at most 30, 40, 50, 60, or even 70 weight percent.

[0034] The sheath layer comprises a thermoplastic polymer. Exemplary thermoplastic polymers include styrene block copolymers (e.g., SIS, SEBS, SBS), thermoplastic polyolefins, elastomeric alloys (e.g., elastomeric thermoplastic acrylate block copolymers such as polymethyl methacrylate-block-poly(butyl acrylate)-block-polymethyl methacrylate, commercially available as KuraRity from Kuraray (Okayama), Ltd.), thermoplastic polyurethanes (TPUs), thermoplastic polyesters and copolyesters; polyvinyl chloride; polystyrene; polycarbonate; thermoplastic polyesters (e.g., polylactide and polyethylene terephthalate); perfluorinated polymers and copolymers, thermoplastic polyamides, and blends of any of the foregoing.

[0035] Thermoplastic copolyesters can be useful as the thermoplastic polymer. Thermoplastic aliphatic polyesters, which may further comprise polylactic acid, polycaprolactone, and other biodegradable polymers, are particularly useful. Melt-processible (filament-forming) polylactic acid polymer materials (e.g., LD copolymers) are commercially available, for example, from NatureWorks LLC (Minnetonka, Minnesota) under the trade names INGEO 6100D, 6202D, and 6260D. Melt-processible polylactic acid polymer materials (e.g., D-lactic acid homopolymers) are available, for example, from Synbra Technologies, The Netherlands, under the trade name "SYNTERRA PDLA 1010." Many other potentially suitable polylactic acid materials are also available.

[0036] Exemplary thermoplastic polyurethanes (TPUs) include polyester-based TPUs and polyether-based TPUs. One exemplary polyester-based thermoplastic polyurethane is available from The Huntsman Corporation (The Woodlands, Texas) as IROGRAN (model PS 440-200). Exemplary polyether TPU resins include those commercially available from BFGoodrich Company (Cleveland, Ohio) as Estane.

[0037] Exemplary thermoplastic polyolefins include homopolymers and copolymers of propylene, ethylene, 1-butene, 1-hexene, 1-octene, 1-decene, and 1-octadecene. Of these, homopolymers and copolymers of ethylene and / or propylene are preferred, with propylene generally being preferred. Representative examples include polyethylene (e.g., HDPE, LDPE, LLDPE, VLDPE; ULDPE, UHMW-PE grades), polypropylene, poly(1-butene), poly(3-methylbutene), and copolymers of the olefin monomers discussed herein.

[0038] In one embodiment, the first polymer resin of the core is the same as the second polymer resin used in the sheath, while in another embodiment, the first polymer resin of the core is different from the second polymer resin used in the sheath.

[0039] In one embodiment, the polymeric resin of the sheath (or second polymeric resin) comprises poly(4-methyl-1-pentene). In one embodiment, the polymeric resin of the sheath comprises less than 100, 99, 98, 97, 95, 90, 85, 80, or even 75 weight percent poly(4-methyl-1-pentene). In one embodiment, the polymeric resin of the sheath is substantially free of 4-methyl-1-pentene or polymers thereof (e.g., poly(4-methyl-1-pentene)), meaning containing less than 10, 8, 6, 5, 4, 3, 2, 1, 0.5, or even 0.1 weight percent.

[0040] In one embodiment, the core-sheath fiber is substantially free of polyarylene sulfide, in other words, the core-sheath fiber contains less than 10, 8, 6, 5, 4, 3, 2, 1, 0.5, 0.1, or even 0.01 weight percent polyarylene sulfide in the sheath portion, the core portion, or the core-sheath fiber.

[0041] In one embodiment, the core-sheath fiber is substantially free of polytetrafluoroethylene, in other words, the core-sheath fiber contains less than 10, 8, 6, 5, 4, 3, 2, 1, 0.5, 0.1, or even 0.01 weight percent polytetrafluoroethylene in the sheath portion, the core portion, or the core-sheath fiber.

[0042] The sheath should be substantially free (containing less than 0.1, 0.05, or even 0.01% by weight) of materials such as antistatic agents that may enhance electrical conductivity or otherwise interfere with the ability of the fiber core to accept and retain an electrostatic charge.

[0043] In one embodiment, the sheath is substantially free of charge-enhancing additives (containing less than 0.1% by weight, or even less than 0.01% by weight).

[0044] In one embodiment, the sheath also includes a charge-enhancing additive.

[0045] The sheath and core may have different compositions, and the core may include a different thermoplastic resin and / or a different charge-enhancing additive than the sheath layer.

[0046] In one embodiment, the core and / or sheath may include one or more conventional adjuvants, such as antioxidants, light stabilizers, plasticizers, acid neutralizers, fillers, antimicrobial agents, surfactants, antiblocking agents, pigments, primers, dispersants, and other adhesion promoters. The incorporation of antimicrobial agents and toughening agents discussed in U.S. Patent No. 7,879,746 (Klun et al.) (incorporated herein by reference) may be particularly beneficial for medical applications. In certain applications, the incorporation of surfactants discussed in U.S. Patent Application Publication No. 2012 / 0077886 (Scholz et al.) (incorporated herein by reference) may be particularly beneficial for certain applications.

[0047] In one embodiment, the core-sheath fibers of the present disclosure can advantageously have better performance (such as longevity and / or filtration capacity) due to the charge-enhancing additive located in the core of the fiber.

[0048] In one embodiment, an additive compound can be added to the sheath to modify the surface of the core-sheath fiber. For example, the core-sheath fiber can be fluorinated. In one embodiment, a fluorinated compound (e.g., a fluorinated compound available as Repellent Polymer Melt Additive PM-870 from 3M Co., Maplewood, MN) can be added to the polymer resin of the sheath layer. In another embodiment, the core-sheath fiber can be placed in an atmosphere containing a fluorine-containing species and an inert gas, and then subjected to an electrical discharge to modify the surface chemistry of the sheath layer. The electrical discharge can be in the form of a plasma, such as an AC corona discharge. This plasma fluorination process results in the presence of fluorine atoms on the surface of the polymer article. Plasma fluorination processes are described in many U.S. Patents to Jones / Lyons et al., including U.S. Patent Nos. 6,397,458, 6,398,847, 6,409,806, 6,432,175, 6,562,112, 6,660,210, and 6,808,551. Electret articles with high fluorine saturation ratios are described in U.S. Patent No. 7,244,291 (Spartz et al.), and electret articles with low fluorine saturation ratios due to heteroatoms are described in U.S. Patent No. 7,244,292 (Kirk et al.). Other publications disclosing fluorination technology include: U.S. Pat. Nos. 6,419,871, 6,238,466, 6,214,094, 6,213,122, 5,908,598, 4,557,945, 4,508,781, and 4,264,750; U.S. Patent Application Publication Nos. 2003 / 0134515(A1) and 2002 / 0174869(A1); and WO 01 / 07144.

[0049] The core-sheath fibers used in the practice of the present disclosure may have any average fiber diameter and may be continuous, random, and / or staple fibers. For example, in some embodiments, the fibers (i.e., individual fibers) may have an average fiber diameter of 5 microns or more (e.g., 6 microns or more, 8 microns or more, 10 microns or more), up to 15 microns, up to 18 microns, up to 20 microns, up to 22 microns, or even up to 25 microns).

[0050] In one embodiment, the diameter of a core-sheath fiber can be determined by microscopy (e.g., optical microscopy or scanning electron microscopy), where the fiber is cross-sectioned and viewed under magnification to determine the fiber diameter, core diameter, and / or sheath thickness.

[0051] In one embodiment, the diameter of the core-sheath fiber can be calculated by measuring the pressure drop across the fiber web. The effective fiber diameter (EFD) can be calculated as described in CNDavies, The Separation of Airborne Dust and Particulates, Institution of Mechanical Engineers, London Proceedings, IB (1952). In practice, sheath thickness may exhibit some experimental variation as a result of routine experimental variation and the averaging nature of EFD.

[0052] Methods for making core-sheath fibers are well known and need not be described in detail herein. In one embodiment, core-sheath fibers are made by coextrusion. For example, at least two polymers are extruded separately and fed into a polymer distribution system, where the polymers are introduced into a segmented spinneret plate. The polymers follow separate paths and combine within the spinneret holes, thereby providing a core-sheath fiber. See, for example, U.S. Pat. Nos. 4,789,592 (Taniguchi et al.) and 5,336,552 (Strack et al.), both of which are incorporated herein by reference in their entireties. In another embodiment, a sheath layer is deposited onto the core fiber using deposition and coating techniques known in the art. For example, vapor deposition can be used to encase the fiber core with a sheath material above the melting temperature of the resin. Such techniques may be more useful with a neat polymer resin sheath. See, for example, U.S. Pat. No. 10,213,716 (Kitagawa et al.), which is incorporated herein by reference in its entirety. The fiber core can be coated with the sheath composition using a coating technique, such as spray coating, dip coating, etc. See, for example, WO201688692 (Kitagawa).

[0053] The fibers (filaments) described herein can generally be made using techniques known in the art for making filaments, including wet spinning, dry spinning, melt spinning, melt blowing, or gel spinning.

[0054] Particularly advantageous for forming core-sheath filaments is melt spinning. In melt spinning, a polymer is heated and passed through a spinneret, causing the fiber to solidify upon cooling. For example, a melt spinning process can be performed to collect multicomponent filaments. As used herein, the term "melt spinning" refers to filaments formed by extruding molten filaments through a set of orifices, allowing the filaments to cool and (at least partially) solidify to form filaments, where the filaments pass through an air space (which may include a moving air stream) to aid in the cooling and solidification of the filaments, and then passing the thus-formed fibers through an attenuation (i.e., drawing) unit to draw the fibers.

[0055] Melt spinning can be distinguished from melt blowing in that melt blowing involves the extrusion of molten filaments into a converging, high-velocity air stream introduced by an air-blowing orifice positioned very close to the extrusion orifice. Melt spinning can also be distinguished from electrospinning in that electrospinning can be described as extrusion from a desired solvent solution. Spinneret modifications result in multicomponent (e.g., core-sheath) fibers (see, e.g., U.S. Pat. Nos. 4,406,850 (Hills), 5,458,972 (Hagen), 5,411,693 (Wust), 5,618,479 (Lijten), and 5,989,004 (Cook)). Filaments according to the present disclosure can also be made by fibrillating films, which can provide filaments with rectangular cross-sections.

[0056] 2, an exemplary nonwoven fibrous web 200 includes core-sheath fibers 210 and optional secondary fibers 220. The core-sheath fibers 210 have an average fiber diameter of 2 to 100 microns and include core-sheath fibers according to the present disclosure. The optional secondary fibers may be any fiber type and / or have any average fiber diameter.

[0057] Nonwoven fibrous webs can be made, for example, by conventional airlaid, carded, stitchbonded, spunbonded, wetlaid, and / or meltblown procedures.

[0058] Spunbond nonwoven fibrous webs can be formed according to well-known conventional methods in which melt-spun fibers are deposited on a moving belt, forming a nonwoven continuous fibrous web with interfiber bonds. Meltblown nonwoven fibrous webs are made by a similar process, except that high-velocity gas impinges on the extruded fibers, stretching and thinning them before they are collected on a rotating drum. Meltblown fibrous webs also have interfiber bonds, but generally do not have the cohesive strength of the corresponding spunbond fibrous webs.

[0059] In some embodiments, nonwoven webs can be made by air-laying fibers (e.g., core-sheath fibers and optional secondary fibers). Air-laid nonwoven fibrous webs can be made using equipment such as that available from Rando Machine Company (Macedon, New York) under the name RANDO WEBBER. In some embodiments, a type of air-laying process known as gravity laying can be used, as disclosed in U.S. Patent Application Publication No. 2011 / 0247839 (Lalouch), the disclosure of which is incorporated herein by reference. Nonwoven fibrous webs can be densified and strengthened by techniques such as cross-lapping, stitch-bonding, needle-tucking, hydroentangling, chemical bonding, and / or thermal bonding.

[0060] Nonwoven fibrous webs according to the present disclosure can have any basis weight, thickness, porosity, and / or density unless otherwise specified. In some embodiments, the nonwoven fibrous web is a high-loft open nonwoven fibrous web. In some embodiments, the fibers of the nonwoven fibrous web have an effective fiber diameter of at least 3, 4, 5, 10, 15, 20, or 25 micrometers and up to 125, 100, 90, 80, 75, 50, 40, or even 30 micrometers.

[0061] The core-sheath fibers and / or nonwoven fibrous webs comprising the core-sheath fibers may be charged as they are formed or after they are formed. In the case of electret filter media (e.g., nonwoven fibrous webs), the media is generally charged after the fibrous web is formed.

[0062] Generally, any standard charging method known in the art can be used. For example, charging can be performed by various methods, including tribocharging, hydrocharging, and corona discharge. A combination of methods can also be used. As mentioned above, in some embodiments, the electret web of the present disclosure has the desirable feature of being able to be charged by corona discharge alone, particularly DC corona discharge, without requiring an additional charging method. Examples of suitable corona discharge processes are described in U.S. Patent Re. 30,782 (van Turnhout), U.S. Patent Re. 31,285 (van Turnhout), U.S. Patent Re. 32,171 (van Turnhout), U.S. Patent No. 4,215,682 (Davis et al.), U.S. Patent No. 4,375,718 (Wadsworth et al.), U.S. Patent No. 5,401,446 (Wadsworth et al.), U.S. Patent No. 4,588,537 (Klaase et al.), U.S. Patent No. 4,592,815 (Nakao), U.S. Patent No. 6,365,088 (Knight et al.), British Patent No. 384,052 (Hansen), U.S. Patent No. 5,643,525 (McGinty et al.), and Japanese Patent No. 4,141,679 (B2) (Kawabe et al.). Further methods are discussed by M. Paajanen et al. in Journal of Physics D: Applied Physics (2001), vol. 34, pp. 2482-2488, and by GM Essler and JE West in Journal of Electrostatics (1975), 1, pp. 111-123.

[0063] Another technique that can be used to charge an electret web is hydrocharging. Web hydrocharging is accomplished by contacting the fibers with water in a manner sufficient to impart an electric charge to the fibers, followed by drying the web. One example of hydrocharging involves directing a water jet or stream of water droplets at the web at a pressure sufficient to impart a filtration-enhancing electret charge to the web, followed by drying the web. The pressure required to achieve optimal results will vary depending on the type of sprayer used, the type of polymer from which the web is formed, the type and concentration of additives added to the polymer, the thickness and density of the web, and whether or not a pretreatment, such as a corona surface treatment, is performed prior to hydrocharging. Generally, water pressures in the range of about 10 to 500 psi (69 to 3450 kPa) are suitable. The water jet or stream of water droplets can be provided by any suitable spraying device. An example of a useful spraying device is one used for hydroentangling fibers. An example of a suitable method of hydrocharging is described in U.S. Patent No. 5,496,507 (Angadjivand et al.). Other methods are described in U.S. Patent No. 6,824,718 (Eitzman et al.), U.S. Patent No. 6,743,464 (Insley et al.), U.S. Patent No. 6,454,986 (Eitzman et al.), U.S. Patent No. 6,406,657 (Eitzman et al.), and U.S. Patent No. 6,375,886 (Angadjivand et al.). Hydrocharging of the web may also be accomplished using the method disclosed in U.S. Patent No. 7,765,698 (Sebastian et al.).

[0064] Surprisingly, it has been discovered that core-sheath fibers containing a charge-enhancing additive in the core possess an electret charge. Electret charge refers to the presence of at least a quasi-permanent charge, where "quasi-permanent" means that the charge exists for a period long enough to be significantly measurable under standard atmospheric conditions (22°C, atmospheric pressure of 101,300 Pascals, and 50% relative humidity). The charge can be characterized by an X-ray discharge test, such as that described in U.S. Pat. No. 9,815,067 (Schultz et al.), column 18, lines 12-42, which is incorporated herein by reference. Unlike static charge that dissipates immediately (such as may be generated as a result of friction), the electret charge of a (e.g., nonwoven) web article is a quasi-permanent charge that is substantially maintained over the intended product life of the article. Thus, sufficient charge is evident during use and at least six or twelve months after manufacture.

[0065] To confirm that a particular filter material is, in fact, electrically charged, its performance can be examined after exposure to ionizing X-ray radiation. As described in "Air Filtration" by R.C. Brown (Pergamon Press, 1993) and "Application of Cavity Theory to the Discharge of Electrostatic Dust Filters by X-Rays," by A.J. Waker and R.C. Brown, Applied Radiation and Isotopes, Vol. 39, No. 7, pp. 677-684, 1988, if a charged filter is exposed to X-rays, more aerosol will penetrate the filter after exposure than before, because the ions generated by the X-rays in the pores between the fibers neutralize some of the charge. Thus, a plot of transmission versus cumulative X-ray exposure can be obtained, which steadily increases up to a certain level and then remains constant with further exposure. At this point, all charge has been removed from the filter.

[0066] In some embodiments, the electret charge of a (eg, single) core-sheath fiber web can be characterized by exhibiting a % transmittance of at least 50% when tested according to the X-ray discharge test.

[0067] In other embodiments, the electret charge of a (e.g., single) core-sheath fiber web may be characterized by exhibiting an initial quality factor of at least 0.2, the quality factor being at least 50% less than the initial quality factor after 40 minutes when tested according to the X-ray discharge test (as described in the Examples).

[0068] Core-sheath fibers according to the present disclosure are useful, for example, in the manufacture of nonwoven filtration media, particularly nonwoven electret filtration media.

[0069] In one embodiment, the core-sheath fiber can be included as an air filter element in a respirator, such as a filtering mask, or for filtration articles such as home and commercial air conditioners, air purifiers, vacuum cleaners, medical air line filters, and air conditioning systems for vehicles and general appliances such as computers, computer disk drives, and electronic devices. In some embodiments, the filtration article is combined with a respirator assembly to form a respiratory device designed for use by a person. In respirator applications, the filtration article may be in the form of a molded, pleated, or folded half-face respirator, a replaceable cartridge or canister, or a prefilter. As used herein, the term "respirator" refers to a system or device worn over a person's respiratory tract to prevent contaminants from entering the wearer's respiratory tract and / or to protect other persons or objects from exposure to pathogens or other contaminants exhaled by the wearer during breathing, including, but not limited to, a filtering face mask.

[0070] 3 and 4 show an example of a respirator. Respirator 40 includes a mask body 42, which may be curved, hemispherical, or may assume any other desired shape (see, for example, U.S. Pat. Nos. 5,307,796 (Kronzer et al.) and 4,827,924 (Japuntich)). In mask 40, an electret nonwoven fibrous web (i.e., filtration material) 200 according to the present disclosure is sandwiched between a cover web 43 and an inner molding layer 45. Molding layer 45 provides structure to mask body 42 and supports filtration material 200.

[0071] The shaping layer 45 can be disposed on either side of the filtration medium 200 and can be made, for example, from a nonwoven web of thermally bonded fibers formed into a cup-shaped configuration. The shaping layer can be formed according to known procedures (see, for example, U.S. Pat. No. 5,307,796 (Kronzer et al.), the disclosure of which is incorporated herein by reference). The shaping layer(s) are typically made from bicomponent fibers having a core of a high-melting material, such as polyethylene terephthalate, surrounded by a sheath of a lower-melting material, such that when heated in a mold, the shaping layer conforms to the shape of the mold and retains this shape when cooled to room temperature. When pressed together with another layer, such as a filter layer, the low-melting sheath material can also serve to bond the layers together.

[0072] To hold the mask 40 snugly against the wearer's face, the mask body 42 may have straps 52, ties, a mask harness, or the like attached to it. Flexible bands 54 made of metal, such as aluminum, may be provided on the mask body 42 so that they can be shaped to hold the mask 40 in a desired fit over the wearer's nose (see, e.g., U.S. Pat. No. 5,558,089 (Castiglione et al.)). Respirators according to the present disclosure may also include additional layers, valves (see, e.g., U.S. Pat. No. 5,509,436 (Japuntich et al.)), shaped masks, and the like. Examples of respirators that can incorporate electret filtration media according to the present disclosure include those described in U.S. Pat. Nos. 4,536,440 (Berg), 4,827,924 (Japuntich), 5,325,892 (Japuntich et al.), 4,807,619 (Dyrud et al.), 4,886,058 (Brostrom et al.), and U.S. Reissue Pat. No. 35,062 (Brostrom et al.).

[0073] Various filtration test protocols have been developed to evaluate filtration performance. These tests involve measuring the aerosol penetration of a filter web (usually expressed as the percent of aerosol that penetrates the filter web (%Pen)) using a standard challenge aerosol such as dioctyl phthalate (DOP), and measuring the pressure drop (ΔP) across the filter web. From these two measurements, a quantity known as the Quality Factor (QF) can be calculated by the following formula: QF=-ln(%Pen / 100) / ΔPΔ (where ln represents the natural logarithm). A higher QF value indicates better filtration performance, and a decrease in QF value correlates perfectly with a decrease in filtration performance. Details on measuring these values ​​are provided in the Examples section. Typically, the filtration media of the present disclosure have a face velocity of 0.3 (mmH2O) at a face velocity of 13.8 cm / sec. -1 It has a measured QF value of 100 or more.

[0074] The initial quality factor (Q0) is typically at least 0.2, preferably at least 0.3, 0.4, or even 0.5, for a face velocity of 13.8 cm / sec, when tested according to the Filtration Performance Test Method, as described in the Examples below. More preferably, the initial quality factor is at least 0.6 or 0.7. In some embodiments, the initial quality factor is at least 0.8, at least 0.90, at least 1.0, or even greater than 1.0. To test the performance of a filter web, the filter web is exposed to X-rays at room temperature (e.g., 23°C) for a specified time, and the quality factor is measured again. In one embodiment, the quality factor after 40 minutes of X-ray exposure is typically at least 50% less than the initial quality factor.

[0075] In one embodiment, the ratio of the quality factor (Q3) of the exposed filter web to the quality factor (Q0) of the initial web is at least 0.75, 0.80, 0.85, 0.90, or even 0.95, with 1.00 representing no change in charge retention after exposure.

[0076] For a web to have sufficient charge for use as a filter, the % transmittance is typically at least 50%. As the % transmittance increases, the filtration performance of the web also increases. In some embodiments, the % transmittance is at least 55%, 60%, or 70%. In preferred embodiments, the % transmittance is at least 75% or 80%. In some embodiments, a single web exhibits a % transmittance of at least 85%, at least, or at least 95%.

[0077] In one embodiment, a filter web made from the core-sheath fibers of the present disclosure has an oil repellency test of at least 3, 4, or even 5, when measured by the oil repellency test disclosed herein. [Example]

[0078] Unless otherwise noted, all parts, percentages, ratios, etc. in the examples and the rest of the specification are by weight, and all reagents used in the examples were obtained or are available from general chemical suppliers, such as Sigma-Aldrich Company (Saint Louis, Missouri), or can be synthesized by conventional methods. [Table 1]

[0079] Filtration performance test method for nonwoven webs

[0080] Initial filtration performance

[0081] Samples were tested for % aerosol penetration (%Pen) and pressure drop (ΔP), and the quality factor (QF) was calculated from these two values. The filtration performance (%Pen and QF) of nonwoven microfiber webs was evaluated using an AFT Model 8130 Automatic Filter Tester (available from TSI, Inc., St. Paul, MN) using dioctyl phthalate (DOP) as the challenge aerosol and a pressure transducer measuring the pressure drop across the filter (ΔP (mmH2O)). The DOP aerosol was nominally 50-200 mg / m2. 3 Upstream concentrations of 100 mg / m 3 The aerosol was monodisperse, 0.33 micrometer mass median diameter (MMD), with a target value of 0.33 micrometers. The aerosol was passed through the media sample at a calibrated flow rate of 85 liters / minute (13.8 cm / sec face velocity). The aerosol ionizer was turned off for these tests. The total test time was 23 seconds (15-second rise time, 4-second sample time, and 4-second purge time). A calibrated photometer was used to measure the DOP aerosol concentration both upstream and downstream of the media by light scattering. DOP %Pen is defined as follows: %Pen = 100 x (DOP concentration downstream / DOP concentration upstream). For each material, six separate measurements were taken at different locations on the web, and the results were averaged.

[0082] An initial quality factor (Q0) was determined. For Q3 values, six samples were thermally aged at 72°C for 3 days (ambient humidity), then the quality factor was determined, and the average of these six QFs was reported as Q3.

[0083] Oil repellency test

[0084] All webs are tested for oil repellency using the 3M Oil Repellency Test I (Drop Test) (April 2020). In this test, samples are subjected to penetration by oils or oil mixtures of various surface tensions. The oils and oil mixtures are given a rating corresponding to the table below. [Table 2]

[0085] Preparation of fibrous and nonwoven samples

[0086] Step A - Fiber and Web Formation:

[0087] For each example, the filtration media was formed by first dry blending the electrostatic additive (if applicable) with the resin (as listed in the table below) and then extruding the fibers into a spunbond web using a core-sheath die. The nominal web specifications used are listed in Table 3 below and are referred to as Spec 1, Spec 2, and Spec 3. [Table 3]

[0088] Step B - Preparation of electrets:

[0089] Each of the spunbond webs in step A was charged by one of the following electret charging methods: corona charging, hydrocharging, or corona pretreatment followed by hydrocharging. These methods are designated charging methods C, H, and CH, respectively.

[0090] Charging method C - Corona charging:

[0091] Corona charging was achieved by passing the web on a grounded surface under a corona brush source at a speed of about 3 centimeters per second with a corona current of about 0.01 milliamps per centimeter of source length. The corona source was positioned about 3.5 centimeters above the grounded surface on which the web rested. The corona source was driven by a positive DC voltage.

[0092] Charging method H-Hydrocharging:

[0093] A fine spray of high-purity water having a conductivity of less than 5 microSiemens / cm was continuously generated from a nozzle operating at a pressure of 896 kilopascals (130 psig) and a flow rate of approximately 1.4 liters / minute. Selected webs prepared in step A were conveyed by a porous belt and passed through the water spray at a velocity of approximately 10 centimeters / second while a vacuum was drawn through the web from below. Each web was passed through the hydrocharger twice (once on each side in turn) and then allowed to dry thoroughly overnight before filter testing.

[0094] Charging Method CH - Corona Pretreatment and Hydrocharging:

[0095] Selected webs prepared in step A above were pretreated by DC corona discharge as described in Charging Method C and then charged by hydrocharging as described in Charging Method H.

[0096] Effective Fiber Diameter (EFD)

[0097] The EFD is calculated from the pressure drop, target thicknesses of approximately 0.028 inches (for Spec 1) and 0.047 inches (for Spec 2), and a face velocity of 13.8 cm / sec at 1 atmosphere pressure. The pressure drop is determined as follows: A high-speed automatic filter tester (obtained from TSI Inc., Shoreview, MN, under the trade designation "8130") was operated with particle generation and measurement turned off. The flow rate was adjusted to 85 liters per minute (LPM), and a sample diameter of 5.25 inches (13.34 cm) was used. The sample was placed in the lower circular plenum opening, and the tester was engaged. A pressure transducer (obtained from MKS Instruments, Inc., Andover, MA) within the device measured the pressure drop in mm H2O. Based on the measured pressure drop, the effective fiber diameter is calculated as described in CNDavies, The Separation of Airborne Dust and Particulates, Institution of Mechanical Engineers, London Proceedings, IB (1952).

[0098] Sheath Thickness

[0099] The effective sheath thickness of selected samples was calculated using the following formula: This formula is derived using the volumetric S / C ratio assuming a cylindrical cross section. r シース =(d / 2)-[C 体積% / 100×(d / 2) 2 ] (1 / 2) In the formula, r シース is the radius of the sheath, d is the diameter of the core-sheath fiber determined by EFD, and C 体積% is the volume percent of the core (based on the S / C ratio). Table 5 below shows the EFD (effective fiber diameter) and calculated sheath thickness for the samples. [Table 4] [Table 5] [Table 6] [Table 7] [Table 8] [Table 9] [Table 10] [Table 11]

[0100] In Table 12, the web of Example 8-2 was annealed at 110° C. for 10 minutes before charging. As shown in Table 12, there is virtually no effect on Q0 or Q3. [Table 12]

[0101] In Table 13, the webs were annealed for 10 minutes before charging as specified. As shown in Table 13, oil resistance is imparted upon annealing. [Table 13]

[0102] Foreseeable modifications and variations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. The present invention is not limited to the embodiments set forth in this application for illustrative purposes. In the event of any inconsistency or discrepancy between the description in this specification and the disclosure in any document set forth in this specification or incorporated by reference, the description in this specification shall control. The present invention includes the following aspects. (1) Thermoplastic core-sheath fibers, 1. A thermoplastic core-sheath fiber comprising a core having a coextensive sheath layer disposed thereon, said core comprising a first polymeric resin and a charge-enhancing additive, and said sheath comprising a second polymeric resin, with the proviso that when said second polymeric resin comprises poly(4-methyl-1-pentene), said second polymeric resin does not comprise 100% by weight of poly(4-methyl-1-pentene). (2) The thermoplastic core-sheath fiber according to item 1, wherein the sheath has a thickness of at least 0.1 micrometers and at most 3 micrometers. (3) The thermoplastic core-sheath fiber according to item 1 or 2, wherein the volume ratio of the sheath to the core is at least 60:40. (4) The thermoplastic core-sheath fiber according to any one of items 1 to 3, wherein the sheath is substantially free of electrostatically enhancing additives. (5) The thermoplastic core-sheath fiber according to any one of items 1 to 3, wherein the sheath comprises a charge-enhancing additive. (6) The thermoplastic core-sheath fiber according to any one of items 1 to 5, wherein the core comprises at least 0.1 wt. % of the charge-enhancing additive. (7) The thermoplastic core-sheath fiber according to any one of items 1 to 6, wherein the charge-enhancing additive is selected from the group consisting of pigments, light stabilizers, primary and secondary antioxidants, metal deactivators, hindered amines, hindered phenols, metal salts, phosphite triesters, phosphates, fluorine-containing compounds, and combinations thereof. (8) The thermoplastic core-sheath fiber according to any one of items 1 to 7, wherein the core-sheath fiber has a diameter of at least 4 micrometers. (9) The thermoplastic core-sheath fiber according to any one of items 1 to 8, wherein the core comprises polypropylene. (10) The thermoplastic core-sheath fiber according to any one of items 1 to 9, wherein the second polymer resin comprises at least one of polypropylene, polyethylene, polylactic acid, polyester, or polystyrene. (11) The thermoplastic core-sheath fiber according to any one of items 1 to 10, wherein the core-sheath fiber has an electret charge. (12) The thermoplastic core-sheath fiber according to any one of items 1 to 11, wherein the sheath layer comprises a fluorinated compound. (13) The thermoplastic core-sheath fiber according to any one of items 1 to 12, wherein the first polymer resin and the second polymer resin comprise the same polymer. (14) A nonwoven fiber web comprising the thermoplastic core-sheath fiber according to any one of items 1 to 13. (15) A medical article comprising the nonwoven fibrous web according to item 14. (16) A filtration article comprising the nonwoven fibrous web according to item 14. (17) The filtration article of item 16, wherein the filtration article is a respirator. (18) The filtration article of item 16 or 17, wherein the nonwoven fibrous web is pleated. (19) A method for producing an electret, comprising: (i) providing a thermoplastic core-sheath fiber comprising a core having a coextensive sheath layer disposed thereon, said core comprising a first polymeric resin and a charge-enhancing additive, with the proviso that when said second polymeric resin comprises poly(4-methyl-1-pentene), said second polymeric resin does not comprise 100% by weight of poly(4-methyl-1-pentene); (ii) charging the thermoplastic core-sheath fibers via corona treatment, hydrocharging, tribocharging, or a combination thereof to form the electret.

Claims

1. A thermoplastic core-sheath fiber having an electret charge, a core having a coextensive sheath layer disposed thereon; the core comprises a first polymeric resin and a charge-enhancing additive; the sheath comprises a second polymer resin; A thermoplastic core-sheath fiber, wherein when said second polymer resin comprises poly(4-methyl-1-pentene), said second polymer resin does not comprise 100% by weight of poly(4-methyl-1-pentene).

2. 10. The thermoplastic core-sheath fiber of claim 1, wherein the sheath has a thickness of at least 0.1 micrometers and at most 3 micrometers.

3. The thermoplastic core-sheath fiber of claim 1, wherein the sheath comprises a static enhancing additive.

4. 10. The thermoplastic core-sheath fiber of claim 1, wherein said core comprises at least 0.1% by weight of said charge-enhancing additive.

5. 2. The thermoplastic core-sheath fiber of claim 1, wherein the charge-enhancing additive is selected from the group consisting of pigments, light stabilizers, primary and secondary antioxidants, metal deactivators, hindered amines, hindered phenols, metal salts, phosphite triesters, phosphates, fluorine-containing compounds, and combinations thereof.

6. 10. The thermoplastic core-sheath fiber of claim 1, wherein said core-sheath fiber has a diameter of at least 4 micrometers.

7. The thermoplastic core-sheath fiber of claim 1 , wherein the core comprises polypropylene.

8. 10. The thermoplastic core-sheath fiber of claim 1, wherein the second polymer resin comprises at least one of polypropylene, polyethylene, polylactic acid, polyester, or polystyrene.

9. The thermoplastic core-sheath fiber of claim 1, wherein the sheath is substantially free of the electrostatic charge-enhancing additive.

10. The thermoplastic core-sheath fiber of claim 1 , wherein the first polymer resin and the second polymer resin comprise the same polymer.

11. A nonwoven fibrous web comprising the thermoplastic core-sheath fibers of claim 1.

12. A medical article comprising the nonwoven fibrous web of claim 11.

13. A filtration article comprising the nonwoven fibrous web of claim 11.

14. 14. The filtration article of claim 13, wherein the filtration article is a respirator.

15. 14. The filtration article of claim 13, wherein the nonwoven fibrous web is pleated.

16. 1. A method of making an electret, comprising: (i) providing a thermoplastic core-sheath fiber comprising a core having a coextensive sheath layer disposed thereon; the core comprising a first polymeric resin and a charge-enhancing additive; the sheath comprises a second polymer resin; If the second polymer resin comprises poly(4-methyl-1-pentene), the second polymer resin does not comprise 100% by weight of poly(4-methyl-1-pentene); (ii) charging the thermoplastic core-sheath fibers via corona treatment, hydrocharging, tribocharging, or a combination thereof to form the electret.

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