FERROELECTRIC FIBERS AND APPLICATIONS THEREFOR

Abstract: A fiber which includes a thermoplastic polymer and particles of a ferroelectric material dispersed therein. The thermoplastic polymer may be, for example, a polyolefin, such as polypropylene or polyethylene, and the ferroelectric material may be barium titanate. The ferroelectric material may be present at a level of from about 0.01 to about 50 percent by weight (from about 0.001 to about 13 percent by volume), and will have a longest dimension in a range of from about 10 nanometers to about 10 micrometers. The fiber may be exposed to an electric field. A plurality of the fibers may be employed to form a knitted or woven fabric or a nonwoven web. Also provided is a method of preparing fibers containing particles of a ferroelectric material. The method includes destructuring the ferroelectric material in the presence of a liquid and a surfactant to give destructured particles; the liquid is a solvent for the surfactant and the surfactant is adapted to stabilize the destructured particles against agglomeration. A blend of the stabilized, destructured ferroelectric material particles and a thermoplastic polymer is then formed and extruded to form fibers. The extruded fibers may be collected on a moving foraminous support to form a nonwoven web and, if desired, may be exposed to an electric field. The fiber of the present invention, especially when in the form of a nonwoven web, is especially suited as a filtration medium. For example, the nonwoven web may be adapted to remove particulate matter from a gaseous stream.

Description: FERROELECTRIC FIBERS AND APPLICATIONS THEREFOR This application is a coπtinuation-in-part of Application Seπal No 08/653,562 which was filed on May 24, 1996 Background of the Invention The present invention relates to fibers, such as melt-extruded fibers, and to nonwoven webs prepared therefrom Air filter materials may be improved by treating the nonwovens in the presence of a high-intensity external electπc field, thereby endowing the web with local electπc fields which persist even after the high intensity electric field is removed (electret treatment) The electπc fields associated with the fibers of the web can be used to attract foreign particles from a fluid stream which typically is air, i e , the treatment imparts to the web an additional mechanism - attraction via electπc field - beyond physical entrapment, to filter out foreign particles The use of electπcaily charged fibrous mateπals as filtration media has been known for some time The advantage of materials of this type is that the charge on the fibers considerably augments the filtration efficiency without making any contπbution to the airflow resistance Among various dust filters, those made of electret fibers have high dust removing performances and are therefore suitable for attaining a high degree of cleanliness It is known that certain dielectric mateπals can be permanently electrostatically polarized, such as by heating the material, applying a high-voltage electπc field, and cooling the material while under the influence of the electπc field Upon the removal of the electric field, an appropriate dielectπc matenal becomes the electπcal equivalent of a permanent magnet A dielectπc becomes an electret when the rate of decay of the field-induced polarization can be slowed down so much that a significant fraction of the polaπzation is preserved long after the polaπzing field has been removed Such electrets can be created by vaπous methods, e g corona charging, tπboelectπc charging (fπction), or any other charging technique (e g by liquid contact) It has been established that air filters made of electret fibers are very effective in removing submicron aerosols The electrostatic collection mechanism increases the efficiency of these electrostatically charged fibrous nonwoven materials relative to conventional, uncharged fibers. The filters have an increased ability for the capture of particles with no corresponding increase in pressure drop. Dust filters have been made from films prepared from nonpolar polymeric materials in which the films are drawn, corona-charged, and treated with needle rolls to make fibrous materials which are then formed into the filters. Alternatively, a nonwoven fabric made of polypropylene fibers and rayon fibers may be subjected to resin processing, followed by bending or shearing, whereby the surface layer of the fabric is charged with static electricity. Electret formation may involve disposing a thread or filaments in an electro¬ static field established between parallel closely spaced electrodes. Alternatively, a monofilament fiber, such as a polypropylene fiber, is closely wound on a hollow winding roller which has been previously surfaced with a polyamide-faced aluminum foil. This process, however, is discontinuous and requires charging times in excess of three hours for the wrapped roll. Other processes for forming electrets involve softening the fibers in thermo- plastic polymer webs with heat and, while the fibers are soft, subjecting them to a suitable electrostatic field to produce a charged fibrous web. This technique may be carried out with a film which then is fibriilated to form fibers which are collected and formed into a filter. An electrostatic spinning process is known in which a fibrous material is sprayed electrostatically from a liquid state and deposited on a conductive support. Meltblown fibers may be charged after being formed and before being deposited to form a web. Several cold charging processes for the preparation of charged webs are known. Examples include the corona charging of combined webs made from layers of materials with differing conductivities. Charging is accompanied by utilizing a contact web, which is more conductive than the dielectric fibers of the filtration medium, and applying the charge through the more conductive medium. Another process involves placing a nonconductive web between the surface of a grounded metal electrode and a series of discharge electrodes. A suitable web (or film) may be conveniently cold charged by sequentially subjecting the web (or film) to a series of electric fields such that adjacent electric fields have substantially opposite polarities with respect to each other. In another method, a polymer film initially is passed across a corona discharge which imparts positive and negative charges on opposite sides of the film. The film then is mechanically split into small filaments, which are subsequently formed into a filter mat. In yet another process, a charge is released between fine wires and a surface electrode. The wires are biased with an electrostatic potential of several kilovolts. The structure to be charged, be it fiber or fabric, is positioned between the electrodes. Stable ions have been implanted, in the presence of a strong electric field, into the fibers of a polymeric filter structure which is at a temperature above the glass transition temperature but below the melt temperature of the polymer. Triboelectric charging involves bringing two or more polymers into close contact and, due to their different dielectric properties, charge is transferred from one to the other. After taking both polymers apart, they are left in a charged state. In a variation, the fibers of a filter mat are coated with particles of zinc colophony resin. The fibrous structure is mechanically needled to fracture the zinc resin crystals. The frictional effect of particle-to-particle attrition and/or crystal fracture along internal planes is sufficient to cause the particles to acquire a positive or negative charge. Many types of polymers have been investigated for use as air filters made of electret fibers. Suitable polymers for electrets are polyolefins (e.g., polypropylene and polyethylene), polycondensates (e.g., polyamides, polyesters, polycarbonates, and polyarylates), polyacrylates, polyacetals, polyimides, cellulose esters, polystyrenes, fluoropolymers, and polyphenylenesulfide. Also suitable are combinations of polymers (e.g., copolymers and polymer blends). It is known that certain additives improve the efficiency of electret performance, but with sometimes variable results. Examples of additives or additive/polymer combinations include titanium dioxide in polyacrylate, a fatty acid metal salt (such as magnesium stearate and aluminum palmitate) in an insulating polymer material (e.g., polypropylene, polyethylene, polyesters, and polyamides). Other additives include charge control agents, such as those employed in toners for electrophotographic processes. These agents have been blended with polyolefins and other polymers. Organic or organometallic charge control agents have been used in aromatic polyamid¬ es, polyolefins, and polyesters. Such materials as organic acids that are solids at room temperature, inorganic materials (e.g., ceramics, metal nitrides, and carbon black), and metallic materials (e.g., silver, copper, aluminum, and tin), have been attached to the surfaces of structures to be electrified. In a variation, the surfaces of fibrous webs have been subjected to a blast of a particle-containing aerosol or to metallic vapor deposition so as to provide solid discontinuous particles at the surfaces. The webs then are electrified. Most of the known poiymeπc electrets are composed solely of a nonpolar or polar poiymeπc material or binary electrets compπsing a nonpolar polymer and a polar polymer Binary electrets, compπsing both types of polymers, have been developed and produced so as to utilize the meπts of both the polar and nonpolar polymers and provide electrets retaining the excellent characteπstics of both the polymers It is known that a blend system, in which a nonpolar polymer is a matπx and a polar polymer is a domain, is supeπor as an electret over a blend system of a reverse structure, in which a polar polymer is a matrix and a nonpolar polymer is a domain Summary of the Invention The present invention teaches a new way to impart locally large eiectnc fields to fibers If ferroelectπc colloids, which possess permanent electπc dipole moments, are introduced into a fiber, the fiber wiil acquire locally large electπc fields The filtration efficiency of a web made from such fibers is thus enhanced The filtration efficiency of such a web may be further enhanced by treating the web in the presence of a high- mtensity electπc field (electret or corona treating) Thus, the present invention addresses some of the difficulties and problems discussed above by providing a fiber which includes a thermoplastic polymer and particles of a ferroelectric matenal dispersed therein The thermoplastic polymer may be, by way of example only, a polyolefin, such as polypropylene or polyethylene Examples of ferroelectπc mateπals include, by way of illustration only, perovskites, tungsten bronzes, bismuth oxide layered materials, pyrochlores, alums, Rochelle salts, dihydrogen phosphates dihydrogen arsenates, and colemamtes For example, the ferroelectπc matenal may be a perovskite, tungsten bronze, bismuth oxide layered matenal, or pyrochlore As another example, the ferroelectπc matenal may be baπum titanate In general, the ferroelectric matenal may be present at a levei of from about 0 01 to about 50 percent by weight, based on the weight of the fiber On a percent by volume basis, the ferroelectπc matenal may be present in the fiber at a level of from about 0 001 to about 13 Desirably, the ferroelectπc matenal will have a longest dimension in a range of from about 10 nanometers to about 10 micrometers Additionally, the fiber may be exposed to an electπc field The fiber generally may have any desired shape Thus, the fiber may be circular in cross section, bilobal, tπlobal, or any other desired configuration Additionally, the fiber may be a multicomponent fiber compπsed of two or more components, each of which includes a thermoplastic polymer, with at least one component also including particles of a ferroelectπc matenal dispersed in the thermoplastic polymer For example, the multicomponent fiber may be a bicomponent fiber in which the two components are arranged in a side-by-side configuration. Alternatively, the components may be arranged in a sheath-core configuration The fiber may be continuous or discontinuous The fiber also may be a mono¬ filament or a plurality of monofilaments If desired, the plurality of monofilaments may be braided, twisted, or false twisted If desired, a plurality of the fibers or mono¬ filaments may be employed to form a knitted or woven fabπc or a nonwoven web The present invention also provides a method of prepaπng fibers containing particles of a ferroelectπc matenal which includes destructuπng the ferroelectric matenal in the presence of a liquid and a surfactant to give destructured particles, wherein the liquid is a solvent for the surfactant and the surfactant is adapted to stabilize the destructured particles against agglomeration, forming a blend of the stabilized, destructured ferroelectπc matenal particles and a thermoplastic polymer, and extruding the blend to form fibers If desired, the method may include exposing the fibers to an electπc field. Additionally or alternatively, the extruded fibers may be collected on a moving foraminous support to form a nonwoven web The nonwoven web thus obtained may be exposed to an electric field Extrusion of the blend may be accomplished by any known means, including, but not limited to, melt extrusion, solution spinning, and gel spinning For example, the blend of the stabilized, destructured ferroelectric matenai particles and a thermoplastic polymer may be melted and the resulting molten blend melt extruded to form fibers. As another example, a solvent for the thermoplastic polymer may be added to the blend to form a solution of the thermoplastic polymer having dispersed therein the stabilized, destructured ferroelectπc material particles The resulting solution then may be solution spun in accordance with known procedures to form fibers The method further may include, after destructuπng the ferroelectπc matenal, adding the mixture of liquid and stabilized, destructured ferroelectπc matenal particles to a molten organic wax at a temperature sufficient to evaporate the liquid Alterna¬ tively, after destructuπng the ferroelectπc matenal, the liquid may be removed from the T 7/0 482 stabilized, destructured particles and the stabilized, destructured particles then may be added to a molten organic wax. If desired, after destructuπng the ferroelectric matenal, the liquid may be removed from the stabilized, destructured particles and the particles then may be redis- persed in water. The resulting aqueous dispersion then may be added to a molten organic wax at a temperature sufficient to evaporate the water. In certain desired embodiments of the method of the present invention: the thermoplastic polymer is polypropylene; the liquid is an aliphatic alcohol having no more than about 6 carbon atoms; the surfactant is an alcohol-soluble, tetraalkylammomum halide; an ethoxylated alkylamine; or a pπmary, secondary, or tertiary alkyl- or arylamine; destructuring is accomplished by means of a ball mill, attπter mill, or pin mill; removal of the liquid is achieved by evaporation under reduced pressure; and the organic wax is a polyethylene wax. By way of example, the ferroelectric material may be baπum titanate and the aliphatic alcohol may be 2-propanol or 1-butanol. The fiber of the present invention, especially when in the form of a nonwoven web, is especially suited as a filtration medium By way of illustration, the nonwoven web may be adapted to enhance the removal of particulate matter from a gaseous stream. For example, the nonwoven web may be a component of an air filter, such as a heating, ventilating, and air conditioning filter; an air conditioning or heating filter; a high efficiency particle abstraction (HEPA) filter; and an automotive air filter, such as an automobile engine air filter and an automobile cabin air filter As another example, the nonwoven web may be a component of a respirator and a face mask, such as a medical mask, examples of which are a surgical mask and a mask to protect an individual against air-borne allergens. An air filter including a nonwoven web of the present invention may be used to purify the air contained within a clean room, air which is to be supplied to buildings (particularly to hospitals, electronics plants or precision factoπes, where dust concentrations in the air must be kept tow), or gas discharged by factoπes. The air filter also may be a component of an air cieaner or a vacuum cleaner, for example, a component of a vacuum bag. The filtration efficiency of the nonwoven web generally is enhanced by exposure to an electric field The electret mateπals thus obtained by the present invention additionally may be suitably used as wiping mateπals, e.g., a dust wipe, absorbing mateπals, or mateπals for dust-protecting clothes Brief Description of the Drawings FIGS. 1 and 2 are plots of percent penetration versus pressure drop in mm water for the various nonwoven webs of Example 1. FIGS. 3 and 4 are plots of percent penetration versus the formulations from which the vaπous nonwoven webs of Example 2 were prepared. FIGS. 5 and 6 are plots of pressure drop and percent penetration, respectively, versus web cross deckle position for the vaπous nonwoven webs of Example 3. Detailed Description of the Invention As used herein, the term "thermoplastic polymer" refers to a polymer that softens when exposed to heat and returns to its oπgmal condition when cooled to room temperature. The thermoplastic polymer may be natural or synthetic. Examples of ther¬ moplastic polymers include, by way of illustration only and without limitation, end- capped polyacetals, such as poly(oxymethylene) or polyformaldehyde, poly(tπchloro- acetaldehyde), poly(π-valeraldehyde), poly(acetaldehyde), and poly(propionaldehyde); acrylic polymers, such as polyacrylamide, poly(acrylic acid), poly(methacryiic acid), poly(ethyl acrylate), and poly(methy! methacrylate), fluorocarbon polymers, such as poly(tetrafluoroethylene), perfluoπnated ethylene-propylene copolymers, ethylene- tetrafluoroethylene copolymers, poly(chlorotπfluoroethylene), ethylene-chlorotπfluoro- ethylene copolymers, poly(vιπylidene fluoride), and poly(vιnyl fluoπde), polyamides, such as poly(6-amιnocaproιc acid) or poly(ε-caprolactam), poly(hexamethylene adip- amide), poly(hexamethylene sebacamide), and poly(11-amιnoundecanoιc acid); polyar- amides, such as poly(imino-1 ,3-phenyleneiminoιsophthaloyl) or poly(m-phenylene isophthalamide); parylenes, such as poly-p_-xylylene and poly(chtoro-β-xylylene); polyaryl ethers, such as poly(oxy-2,6-dimethyl-1 ,4-phenylene) or poly(p_-phenylene oxide); polyaryl sulfones, such as poly(oxy-1 ,4-phenylenesulfonyl-1 ,4-phenyleneoxy- 1 ,4-phenylene-isopropylidene-1 ,4-phenylene) and poly(sulfonyl-1 ,4-phenyleneoxy-1 ,4- phenylenesulfonyl-4,4'-bιphenylene); polycarbonates, such as poly(bisphenol A) carbonate or poly(carbonyldioxy-1 ,4-pheπyleneιsopropylιdene-1 ,4-phenylene); poly- esters, such as poiy(ethylene terephthalate), poly(tetramethyleπe terephthalate), and poly(cyclohexylene-1 ,4-dιmethylene terephthalate) or poly(oxymethyiene-1 ,4-cyclo- hexylenemethyleneoxyterephthaloyl), polyaryl sulfides, such as poly(p.-phenylene sulfide) or poly(thιo-1 ,4-phenylene), polyimides, such as poly(pyromellιtιmιdo-1 ,4- phenylene), polyolefins, such as polyethylene, polypropylene, poly(l-butene), poly(2- butene), poly(l-pentene), poly(2-pentene), poly(3-methyl-1-pentene), and poly(4- methyl-1-pentene), vinyl polymers, such as poly(vιny) acetate), poly(vιnylιdene chlor¬ ide), and poly(vιnyl chloπde), diene polymers, such as 1 ,2-poly-1 ,3-butadιene, 1 ,4-poly- 1,3-butadιene, polyisoprene, and polychloroprene, polystyrenes, and copolymers of the foregoing, such as acrylonitnle-butadiene-styrene (ABS) copolymers ethylene- propylene copolymers, and ethylene-vinyl acetate copolymers The term "polyolefin" is used herein to mean addition polymers prepared from one or more unsaturated monomers which contain only carbon and hydrogen, examples of which are the polyolefins listed above In addition such term is meant to include blends of two or more polyolefins and random, block and graft copolymers prepared from two or more different unsaturated monomers Because of their commercial importance, the most desired polyolefins are polyethylene and poly¬ propylene The term "ferroelectric material" is used herein to mean a crystalline matenal which possesses a spontaneous polaπzation which may be reoπented by the application of an electπc field The term includes any phase or combination of phases exhibiting a spontaneous polaπzation, the magnitude and onentation of which can be altered as a function of temperature and externally applied electπc fields The term also is meant to include a single ferroelectπc matenal and mixtures of two or more ferroelectπc materials of the same class or of different classes The term further m- eludes a "doped" ferroelectπc material, i e , a ferroelectric matenal which contains minor amounts of elemental substituents, as well as solid solutions of such substituents in the host ferroelectπc matenal The structure of crystalline mateπals typically is descπbed in terms of 32 distinct symmetry groups Of these, 21 are noncentrosymmetπc That is they do not possess a center of symmetry Of the noncentrosymmetπc groups, 20 are piezoelectπc, and of these 20, only 10 are referred to as being pyroelectπc Pyroelectπc mateπals are unique in that they possess a spontaneous electπcal polaπzation which is directly attπbutable to permanent dipoles which exist on the unit cell level within individual crystals The alignment of dipoles along a crystallographic axis of the matenal yields a net spontaneous polaπzation in the material Pyroelectπc mateπals also are referred to as polar solids As the name implies, "pyroelectπc" refers to changes in the magnitude and direction of the spontaneous polaπzation with changes in temperature Ferroelectπc matenals are a subgroup of the spontaneously polaπzed pyroelectπc matenals The magnitude and direction of the spontaneous polaπzation in ferroelectπc mateπals respond to both temperature and the presence of externally applied electπc fields All ferroelectπc matenals exhibit a "Cuπe point" or "Cuπe temperature," which refers to a cπtical temperature above which the spontaneous polanzation vanishes. The Cuπe temperature often is indicated herein as "T c " Examples of ferroelectric mateπals include, without limitation, perovskites, tungsten bronzes, bismuth oxide layered mateπals, pyrochlores, alums, Rochelle salts, dihydrogen phosphates, dihydrogen arsenates, guanidine aluminum sulfate hexahydrate, tπglycme sulfate, colemanite, and thiourea. Thus, ferroelectπc mateπals may be inorganic or organic in nature Inorganic ferroelectπc mateπals are desired because of their generally supeπor thermal stabilities Several of the more useful of these classes are reviewed in detail below Perovskites Perovskites are mixed metal oxides of AB0 3 stoichiometry Perovskites have a very simple cubic structure made up of comer-shanng oxygen octahedra with small, highly-charged cations like titanium (Ti) tin (Sn), zirconium (Zr), niobium (Nb), tantalum (Ta), and tungsten (W) occupying the central octahedral B site, and lower charged, large cations like sodium (Na), potassium (K), rubidium (Rb), calcium (Ca), strontium (Sr), Baπum (Ba), and lead (Pb), etc , filling the interstices between the oxygen octahedra in the larger 12-coordιnated A sites The ferroelectπcty associated with these mateπals aπses from lattice distortions, occumng below the Cuπe temperature, which result in the development of very large dipoles within the crystals Perovskites are unique in their ability to form a wide vaπety of solid solutions, from simple binary and ternary solutions to very complex multicomponent solutions Some examples include, but are not limited to, BaSrTι0 3 , KBaTι0 3 , Pb(Cθo 2 sMn 025 vVo 5 )θ 3 , and numerous forms of baπum titanate and lead titanate doped with niobium oxide, antimony oxide, and lanthanum oxide, to name a few by way of illustration only The ability to form extensive solid solutions of perovskite-type compounds allows one skilled in the art to systematically alter the electπcal properties of the material by formation of a solid solution or addition of a dopant phase. For example, the Curie temperature of Barium titanate (BaTi0 3 ) can be systematically increased from 130°C to 490°C by substituting lead ions for baπum ions, the upper limit of T c being reached at 100 percent lead ion substitution. Likewise, it generally is known that the T c of barium titanate can be gradually decreased by substituting strontium ions for barium ions. Perovskite-Related Octahedral Structures These mateπals have a structure similar to that of perovskites, except that the oxygen octahedra are edge sharing rather than corner sharing. Only two mateπals in this class are of note, namely, lithium niobate (LiNb0 3 ) and lithium tantalate (LiTa0 3 ). For convenience, these mateπals are included in the term "perovskites." Tungsten Bronzes The tungsten bronzes are non-stoichiometπc substances having the general formula M ; Λ W0 3 , where 0<n<l and M is a monovalent metal cation, most typically sodium (Na). The ferroelectric tungsten bronzes typically have values of n<0.3. Within this family of mateπals are such compounds as lead metaniobate (PbNb 2 0 6 ) and lead metatantalate (PbTa 2 0 6 ) Bismuth Oxide Layered Mateπals These are complex layered structures of perovskite layers interleaved with bismuth oxide layers. A typical bismuth oxide layered compound is lead bismuth niobate (PbBiNb 2 0 9 ) Pyrochlores Pyrochlores are corner sharing oxygen octahedra similar to the perovskites. However, this family of compounds is more limited in the cation substitutions which can be made. Typical pyrochlores are cadmium niobate and tantalate and lead niobate and tantalate. These materials have Curie temperatures below 200°K (-73°C), which may limit their usefulness for some applications. The term "destructured" and variations thereof means a reduction in size of the ferroelectric particles. The terms "particles" and "agglomerated particles" are intended to mean particles of a ferroelectric material which have not been processed to reduce particle sizes. The term "destructured particles" refers to "particles" or "agglomerated particles" which have been processed, or "destructured," to reduce particle sizes. As used herein, the term "electric field" means an electπc field generated by any method known to those having ordinary skill in the art for charging nonconductive webs Such methods include, for example, thermal, liquid contact, electron beam, and corona discharge methods For example, corona discharge charging of nonconductive webs is described in U S Patent No 4,588,537 to Klaase et al , the contents of which regarding the charging of webs is incoφorated herein by reference As another example, charging of nonconductive webs between the surface of a grounded metal electrode and a seπes of discharge electrodes is descπbed in U S Patent No 4,592,815 to Makao, the contents of which regarding the charging of webs is incorporated herein by reference One technique of interest for the charging of webs involves applying high voltage electπc fields via direct current to form an electret or electπcal charge This "cold-charging" technique is descπbed in U S Patent No 5,401,446 to Tsai et al., which patent is incoφorated herein by reference In general, the technique involves subjecting a matenal to a pair of electπcal fields wherein the electπcat fields have opposite polarities For example the permeable matenal may be charged by sequentially subjecting the material to a series of electπcal fields such that adjacent electπcal fields have substantially opposite polaπties with respect to each other Thus, one side of the matenal is initially subjected to a positive charge while the other side of the matenal is initially subjected to a negative charge Then, the first side of the matenal is subjected to a negative charge and the other side of the matenal is subjected to a positive charge It is important to note that the terms "positive" and "negative" are meant to be relative terms For example, a pair of electrodes will have a positive electrode and a negative electrode any time there is a difference in potential between the two electrodes In general, the positive electrode will be the electrode with the more positive (or less negative) potential, while the negative electrode will be the electrode with the more negative (or less positive) potential The strength of the electπcal field used to charge the material may vary and can be appropπately determined by those of ordinary skill in the art As a practical matter, the strength of the electπcal field may vary from about 1 kV/cm to about 30 kV/cm For example the strength of the electπcal field may vary from about 4 kV/cm to about 12 kV/cm The term "melt extrusion" and vaπations thereof is meant to include any process by which a thermoplastic polymer composition is heated to a molten state and forced through a die to form a shaped article, such as, but not limited to a fiber As used herein, the term "solution spinning" means the formation of fibers by extruding a solution of a polymer composition from a die to form fine streams of fluid and includes both dry spinning and wet spinning. With dry spinning, the amount of polymer solvent is relatively low, so that the solvent evaporates quickly, thereby forming a fiber from each fluid stream. Wet spinning is similar to dry spinning, except that the solvent level is higher and the fluid streams are extruded into water (or other liquid) which extracts the solvent. See, by way of illustration only, H. F. Mark et al., Editors, "Encyclopedia or Polymer Science and Engineeπng," Vol. 6, John Wiley & Sons, New York, 1986, pp. 802- 821. As used herein, the term "nonwoven web" means a web of fibers in which the fibers are laid down in a random manner. Thus, a nonwoven web may be formed by such processes as wet laying, dry laying, meltblowing, coforming, spunbondiπg, and carding and bonding. A nonwoven web desirably may be prepared by a melt-extrusion process in which melt-extrusion to form fibers is followed concurrently by web formation on a foraminous support Such processes inctude, among others, meltblowing, coforming, and spunbondmg. By way of illustration only, such processes are exemplified by the following references- (a) meltblowing references include, by way of example, U S. Patent Nos. 3,016,599 to R W Perry, Jr , 3,704,198 to J S Prentice, 3,755,527 to J P Keller et al., 3,849,241 to R R. Butin et al., 3,978,185 to R. R Butin et al , and 4,663,220 to T. J. Wisneski et al. See, also, V. A. Wente, "Superfine Thermoplastic Fibers", Industrial and Enαmeeπnq Chemistry. Vol. 48, No 8, pp. 1342-1346 (1956), V A. Wente et al., "Manufacture of Superfine Organic Fibers", Navy Research Laboratory, Washington, D.C., NRL Report 4364 (111437), dated May 25, 1954, United States Department of Commerce, Office of Technical Services; and Robert R. Butin and Dwight T. Lohkamp, "Melt Blowing - A One-Step Web Process for New Nonwoven Products", Journal of the Technical Association of the Pulp and Paper Industry, Vol 56, No 4, pp 74-77 (1973), (b) coforming references include U.S Patent Nos 4,100,324 to R. A. Anderson et al and 4,118,531 to E. R. Hauser; and (c) spunbonding references include, among others, U S Patent Nos. 3,341 ,394 to Kmney, 3,655,862 to Dorschner et al., 3,692,618 to Dorschner et al., 3,705,068 to Dobo et al , 3,802,817 to Matsuki et al., 3,853,651 to Porte, 4,064,605 to Akiyama et al , 4,091 ,140 to Harmon, 4,100,319 to Schwartz, 4,340,563 to Appel and Moπman, 4,405,297 to Appel and Morman, 4,434,204 to Hartman et al , 4,627,811 to Greiser and Wagner, and 4,644,045 to Fowells The term "organic wax" is used herein to mean a matenal which may be a liquid, semisolid, or solid at ambient temperature, i e , at a temperature of 20-25°C Typical liquids include, by way of example only, low weight-average molecular weight (M w ) oligomeπc forms of polyethylene, polypropylene, and polyisobutylene Typical semisolids include, again by way of example only, polyisobutylene (M w =100,000) and atactic polypropylene Typical solids included, further by way of example only, polyethylene (M w = 1 ,000-4,000), polypropylene (M w = 1 ,000-4, 000), and vaπous carboxylate-, amide-, and alcohol-based waxes The choice of organic wax generally will be dictated by the thermoplastic polymer in which the destructured ferroelectπc matenal is to be dispersed As stated earlier, the present invention provides a fiber which includes a thermoplastic polymer and particles of a ferroelectπc matenal dispersed therein. In general, the thermoplastic polymer may be any thermoplastic polymer which is capable of being formed into fibers Desirably, the thermoplastic polymer will be a polyolefin, i e , an addition polymer prepared from one or more unsaturated monomers which contain only carbon and hydrogen Examples of polyolefins include polyethylene, polypropylene, poly(l-butene), poly(2-butene), poly(l-pentene), poly(2-pentene), poly(3-methy!-1-pentene) and poly(4-methyl-1-pentene) In addition, the term "polyolefin" includes blends of two or more polyolefins and random, block, and graft copolymers prepared from two or more different unsaturated monomers Because of their commercial importance the most desired polyolefins are polyethylene and poly¬ propylene Dispersed in the thermoplastic polymer are particles of a ferroelectπc matenal That is, the particles of the ferroelectπc medium are distπbuted throughout the fiber volume The distπbution of particles is substantially uniform in the sense that agglomerates of particles are not present adjacent to large regions of the fiber volume which are devoid of particles The particles are distπbuted in a random fashion, meaning that no effort is made to regularly space particles within the bulk of the fiber along the fiber axis Regions may exist where the particles are regularly spaced, but these regions occur by chance rather than by design The particle loading is expressed as either a weight fraction or volume fraction which is representative of the bulk loading of the ferroelectric matenal in the polymer from which the fibers are formed The amount of the particles of a ferroelectπc medium contained in the fiber in general will be in a range of from about 0 01 to about 50 percent by weight, based on the weight of the fiber For example, the amount of the particles of a ferroeiectπc matenal may be in a range of from about 0 05 to about 30 percent by weight As another example, the amount of the particles of a ferroelectric matenal may be in a range of from about 0 1 to about 20 percent by weight As a further example, the amount of such particles may be in a range of from about 0 5 to about 5 percent by weight On a percent by volume basis, the amount of the particles of a ferroelectπc matenal present in the fiber generally will be in a range of from about 0 001 to about 13 percent by voiume For example, the amount of the particles of a ferroelectπc matenal may be in a range of from about 0 01 to about 8 percent by volume As another example, the amount of the particles of a ferroelectπc matenal may be in a range of from about 0 1 to about 5 percent by volume As a further example, the amount of such particles may be in a range of from about 0 1 to about 2 percent by volume It will be appreciated by those having ordinary skill in the art, as demonstrated by the examples, that amounts of the materials necessary to prepare fibers coming within the scope of the present invention, e g , the thermoplastic polymer and ferroeiectπc matenal, are conveniently measured on a weight basis However, the percent by volume of the particles of the ferroelectπc matenal present in the fiber is the more significant parameter In general, there needs to be a sufficiently high level of thermoplastic polymer in order to provide a continuous matπx which will result in a fiber having the desired tensile strength characteristics That is, the strength of a fiber in large measure is a function of the strength of the continuous matrix of which the fiber is composed Thus, the percent-by-volume ranges for the particles of the ferroelectric matenal given herein provide sufficient guidance to one having ordinary skill in the art so that a sufficiently strong fiber may be obtained without undue expeπmentation being required In general, any size particles of the ferroelectπc matenal may be employed in the present invention, provided the particles are of a size which will not significantly adversely affect fiber formation For example, the longest dimension of the particles typically should be no greater than about 50 percent of the diameter of the oπfice through which the blend is to extruded Desirably, the ferroelectπc matenal will have a longest dimension in a range of from about 10 nanometers to about 10 micrometers Many ferroelectric materials are available as agglomerations of what are referred to herein as primary particles. These agglomerated particles may have longest dimensions which are greater than about 10 microns. When fibers having relatively large diameters are being prepared, such as those obtained from a spunbonding process, the dimensions of the agglomerated particles in general do not significantly adversely affect fiber formation. However, when fibers having smaller diameters are to be prepared, such as those which may be obtained from a meltblowing process in which fiber diameters may be in a range of from about 0.1 to about 10 micrometers, the agglomerated particles should be destructured. Of course, the particles may be destructured if desired, regardless of the diameters of the fibers to be prepared. The particles of ferroelectric material may be destructured by any means known to those having ordinary skill in the art. For example, destructuring may be accomplished by subjecting the ferroelectric material to processing in a ball mill, attriter mill, or pin miil. Although processing conditions will vary, depending upon the design and operation of the mill employed, suitable conditions may be readily determined by those having ordinary skill in the art. As already noted, destructuring is carried out in the presence of a liquid and a surfactant, wherein the liquid is a solvent for the surfactant and the surfactant is adapted to stabilize the destructured particles against agglomeration. The fiber generally may have any desired shape. Thus, the fiber may be circular in cross section, bilobal, trilobal, or any other desired configuration. Additionally, the fiber may be a multicomponent fiber comprised of two or more components, each of which include a thermoplastic polymer, with at least one component including a thermoplastic polymer and particles of a ferroelectric material dispersed therein. For example, the multicomponent fiber may be a bicomponent fiber in which the two components are arranged in a side-by-side configuration. Alternatively, the components may be arranged in a sheath-core configuration. The fiber may be continuous or discontinuous. The fiber also may be a mono¬ filament or a plurality of monofilaments. If desired, the plurality of monofilaments may be braided, twisted, or false twisted. If desired, a plurality of the fibers or mono¬ filaments may be employed to form a knitted or woven fabric or a nonwoven web. The present invention also provides a method of preparing fibers containing particles of a ferroelectric material, which method includes: destructuπng the ferroelectπc matenal in the presence of a liquid and a surfactant to give destructured particles, wherein the liquid is a solvent for the surfactant and the surfactant is adapted to stabilize the destructured particles against agglomeration, forming a blend of the stabilized, destructured ferroelectπc matenal particles and a thermoplastic polymer; and extruding the blend to form fibers. If desired, the method may include exposing the fibers to an electπc field. Additionally or alternatively, the extruded fibers may be collected on a moving foraminous support to form a nonwoven web. The nonwoven web thus obtained may be exposed to an electπc field In general, any liquid may be employed which is a solvent for the surfactant. The surfactant, in turn, is adapted to stabilize the destructured particles against agglomeration. Suitable liquids include, by way of example oniy, aliphatic hydro- carbons, such as hexane, heptane, octane, and decane, aromatic hydrocarbons, such as xylene, toluene, and cumene, aliphatic alcohols, such as 2-propanol 1-butanol, 1- hexanol, and benzyl alcohol, aliphatic ketones, such as methyl ethyl ketone; halogenated hydrocarbons, such as dichloromethane, chloroform, carbon tetrachloπde, and chlorobenzene, and polar solvents, such as water, tetrahydrofuran, and N,N- dimethylpyrolidmone Desirably, the liquid will be an aliphatic alcohol having no more than about ten carbon atoms. Examples of such alcohols include, by way of illustration only, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 1-pentanol, 2-pentanol, 3- pentanol, 2-methylbutanol, 3-methylbutanol, 1-hexaπol, 2-hexanol, 3-hexanol. 3,3- dimethylbutanol, 1-heptanol, 1-octanol, 1-nonanol, and 1-decanol The types of surfactants which may be employed in the method of the present invention include cationic, anionic, nonionic, and zwitterionic surfactants. In some cases, it may be desirable to use a mixture of two or more surfactants to stabilize the destructured ferroelectπc particles. Examples of cationic surfactants include, by way of illustration oniy, aliphatic and aromatic pnmary, secondary, and tertiary amines, amine oxides; amide-linked amines; and quaternary ammonium salts. Examples of anionic surfactants include, again by way of illustration only, carboxylic acids and salts; sulfonic acids and salts; lignosulfoπates; alkylbenzenesulfonates, alkylarylsulfonates; petroleum sulfonates, sulfonates with ester, ether, or amide linkages, sulfuπc acid esters and salts; sulfated alcohols; sulfated ethoxylated alcohols; sulfated ethoxylated alkylphenols; sulfated acids; sulfated amides; sulfated esters; sulfated natural fats and oils; phosphoπc acid and polyphosphoπc acid esters and salts; phosphated alcohols; phosphated phenols; phosphated alkoxylated alcohols; phosphated alkoxylated phenols; and salts of each class of phosphated anionic surfactant. Examples of nonionic surfactants include, also by way of illustration only, ethoxylated alcohols; ethoxylated alkylphenols; ethoxylated carboxylic acid esters; glycerol esters; polyethylene glycol esters; sorbitol esters; ethoxylated natural fats and oils; ethylene and diethylene glycol esters, propanediol esters; and ethoxylated carboxylic acid amides. The surfactant generally is employed in an amount sufficient to stabilize the destructured ferroelectnc material against agglomeration For example, the surfactant may be present in a range of from about 0.01 to about 10 percent by weight, based on the total amount of ferroelectπc matenal being destructured and stabilized against agglomeration Desirably, the surfactant will be present in a range of from about 0.01 to about 1 percent by weight A blend of the stabilized, destructured ferroelectπc matenal particles and a thermoplastic polymer may be prepared by a vaπety of methods For example, after destructuπng the ferroelectπc matenal, the mixture of liquid and stabilized, destructured ferroelectπc material particles may be added to a molten organic wax at a temperature sufficient to evaporate the liquid Alternatively, after destructuπng the ferroelectric matenal, the liquid may be removed from the stabilized, destructured particles and the stabilized, destructured particles then may be added to a molten organic wax If desired, the liquid may be removed from the stabilized, destructured particles after destructuπng and the particles then may be redispersed in water The resulting aqueous dispersion then may be added to a molten organic wax at a temperature suffi¬ cient to evaporate the water The above alternative procedures all result in the dispersion of the stabilized, destructured ferroelectric particles in an organic wax. Such wax dispersion then may be added to the thermoplastic polymer For example, the wax dispersion may be physically blended with thermoplastic polymer pellets at a temperature sufficient to maintain the wax in a molten condition The resulting blend may be further blended in, for example, a twin-screw extruder to give pellets composed of the thermoplastic polymer and the stabilized, destructured particles of ferroelectπc matenal Extrusion of the blend may be accomplished by any known means For example, the blend of the stabilized, destructured ferroelectnc matenal particles and a thermoplastic polymer may be melted pπor to extruding the blend to form fibers As another example, a solvent for the thermoplastic polymer may be added to the blend to foπn a solution of the thermoplastic polymer having dispersed therein the stabilized, destructured ferroelectπc matenal particles pπor to extruding the blend to form fibers. Thus, the fibers may be formed by melt extrusion or solution spinning In certain desired embodiments of the method of the present invention the thermoplastic polymer is polypropylene, the liquid is an aliphatic alcohol having no more than about ten carbon atoms; the surfactant is an alcohol-soluble, tetraalkylammonium halide, an ethoxylated alkylamine, or a pnmary, secondary, or tertiary alkyl- or arylamine, destructuring is accomplished by means of a ball mill, attπter mill, or pin mill, and the organic wax is a polyethylene wax By way of example, the ferroelectric material may be baπum titanate and the aliphatic alcohol may be 2-propanol or 1-butanol The fiber of the present invention, especially when in the form of a nonwoven web, is especially suited as a filtration medium For example, the nonwoven web may be adapted to remove particulate matter from a gaseous stream The present invention is further descπbed by the examples which follow Such examples, however, are not to be construed as limiting in any way either the spiπt or the scope of the present invention Example 1 Matenal Preparation A nonwoven web, made up of fibers composed of a thermoplastic polymer and having dispersed throughout the bulk of the fibers particles of a ferroelectnc matenal, was prepared in a four-step process The steps were (1) destructured particle preparation, (2) dispersion in low molecular weight polyethylene wax, (3) blending or compounding of the wax dispersion in polypropylene, and (4) nonwoven fabnc formation Destructured Particle Preparation Dispersions were prepared using two grades of baπum titanate (BaTi0 3 ) supplied by Tarn Ceramics, Inc. (Niagara Falls, New York) under the product names Ticon 15 5016 and Ticon ® HPB. Each dispersion was prepared by ball milling the barium titanate with 2-propanol and a stabilizing surfactant. The surfactant was an ethoxylated tallow amine (Rhodameen ® PN-430, Rhone-Poulenc, Cranberry, New Jersey). In a typical batch, approximately 1 kg of barium titanate was vigorously stirred with 2.6 L of 2-propanol and 4-5 mL of the surfactant. The resulting slurry was poured into a 6.2-L Roalox ceramic mill jar (U. S. Stoneware, East Palestine, Ohio) which had been charged with 12 lbs (about 5 4 kg) of Borundum ® (87 percent alumina) grinding media (U. S. Stoneware) The jar was rolled at 70 rpm for a penod of 48 hours on a U. S. Stoneware Unitized Jar Mill, Model 764AVM. At the end of the milling period, the resulting dispersion of stabilized, destructured barium titanate was removed from the jar The 2-propanol was removed by evaporation under reduced pressure and the resulting semi-dry powder was further dried at 90°C for 4 hours under reduced pressure In total, 50 lbs (about 22.7 kg) of each type of barium titanate were destructured and stabilized as descπbed above. Two 100-lb dispersions in deionized water containing about 50 percent by weight solids were prepared from the vacuum-dried baπum titanates. Wax Dispersion Preparation In general, a low molecular weight polyethylene wax was melted in a vat equipped with hydraulically driven interleaving blades The blades functioned to shear- mix the molten wax with the aqueous suspension The shear-mixing accelerated the rate of water evaporation and blended the dispersed particles with the wax. The vat was heated with steam at 38 psig to 48 psig, corresponding to a temperature range of 140°C to 147°C. Specifically, approximately 100 lbs (about 45 kg) of 50 weight percent aqueous baπum titanate was dispersed into 12.5 lbs (about 5.7 kg) of polyethylene wax (AC 16, Allied Signal, Ine , Morπstown, New Jersey). The baπum titanate/surfactant/wax disper- sions contained less than 0 1 weight percent residual water as determined by Karl Fischer titration. The actual compositions of the two wax dispersions are given in Table 1 , below The wax dispersions were cooled to dry ice temperature and ground to a coarse powder for dry blending with polypropylene Table 1 Compositions of Wax Dispersions Polyprooylene Compounding The two wax dispersions were separately dry blended with Hirπont Profax ® PF- 015 polypropylene (Montell Polymers, Wilmington, Delaware). The mixture was melt blended using a single screw compounding extruder. The compositions of the two blends are given in Table 2. Table 2 Polypropylene/Barium Titanate Formulations Nonwoven Fabric Formation Nonwoven fabrics were manufactured on a 100-inch (about 2.5-meter) melt- blown line essentially as described in U.S. Patent No. 3,849,241 to Buntin et al., which is incoφorated herein by reference. The 100-inch wide web was slit into five 20-inch is incoφorated herein by reference The 100-ιnch wide web was slit into five 20-ιnch (about 51 -cm) sections Of these, the outer 20 inches (about 51 cm) on either side of the web were discarded The remaining three slits represent cross deckle positions from 20 inches (about 51 cm) through 80 inches (about 203 cm) Meltblowing conditions were the same for all mateπals The line speed was vaπed to alter the basis weight Basis weights of 0 5 ounces per square yard or osy (about 17 grams per square meter or gsm), 0 6 osy (about 20 gsm), 0 75 osy (about 25 gsm), and 1 0 osy (about 34 gsm) were spun from the Profax ® PF-015 polypropylene alone (the control, coded PP-A) and from each formulation containing baπum titanate (see Table 2). All basis weights of meltblown webs were electret treated on-line under identical conditions The electret treatment was earned out in accordance with the teachings of U S Patent No 5,401 ,446, descπbed earlier RESULTS Air Filtration Measurements The air filtration efficiencies of the meltblown nonwoven webs prepared above were evaluated using a TSI Ine (St Paul, Minnesota) Model 8110 Automated Filter Tester (AFT) The Model 8110 AFT measures pressure drop and particle filtration charactenstics for air filtration media The AFT utilizes a compressed air nebulizer to generate a submicron aerosol of sodium chloride particles which serves as the challenge aerosol for measuπng filter performance The characteπstic size of the particles used in these measurements was 0 1 micrometer Typical air flow rates were between 31 liters per minute and 33 liters per minute The AFT test was performed on a sample area of about 140 cm 2 The performance or efficiency of a filter medium is expressed as the percentage of sodium chloπde particles which penetrate the filter Penetration is defined as transmission of a particle through the filter medium The transmitted particles were detected downstream from the filter The percent penetration (% P) reflects the ratio of the downstream particle count to the upstream particle count Light scattering was used for the detection and counting of the sodium chloπde par- tides Samples of meltblown material were taken from six cross-deckle (CD) positions (i e , 2 per 20-ιnch wide slit) of the nonwoven webs descπbed in the preceding section A sample of material is defined as a flat nonwoven web of a characteπstic basis weight cut to approximately 8 inches (about 20 cm) square Typically, 15-20 samples from penetration (% P). Tables 3-5 summarize the pressure drop and particle penetration data for the control and each barium titanate/polypropylene formulation. Table 3 Air Filtration Results for Control Webs Table 3, Continued Table 4 Air Filtration Results for PF-A Webs Table 4, Continued Table 5 Air Filtration Results for PF-B Webs Table 5, Continued The pressure drop and percent particle penetration data tabulated in Tables 3-5 clearly contrast the filtration properties of meltblown materials made from the base polypropylene alone and the barium titanate/poiypropylene formulations. The data illustrate improved filtration efficiencies at all basis weights. The low basis weight (0.5 osy and 0.6 osy) nonwovens exhibited no change in the pressure drop for filter media prepared using either polypropylene alone or the barium titanate/poiypropylene formulations. The higher basis weight (0.75 osy and 1.0 osy) nonwoven filter media exhibited a small increase in pressure drop. Irrespective of this change in pressure drop at the two higher basis weights, media made using the baπum titanate-containing formulations always had higher filtration efficiencies when compared to media made from the base polymer alone. The improved filtration performance of the barium titanate formulations compared to polypropylene alone is best illustrated by the graphs shown in FIGS. 1 and 2. Note that the data points representing the percent penetration versus pressure drop for the barium titanate/poiypropylene nonwoven media define a curve which always falls below the curve defined by the data points for the nonwoven media prepared from polypropylene alone. Therefore, for any given pressure drop, nonwoven media made from either of the two barium titanate formulations remove more particles from the air stream compared to nonwoven media made using polypropylene alone. Example 2 Matenal Preparation The ferroelectπc colloidally enhanced nonwoven fabncs descπbed in this example were prepared using the same procedure descnbed in Example 1 Therefore, only differences in the preparation and treatment of the nonwovens are descπbed below Destructured Particle Preparation Baπum titanate was obtained from the Transelco Division of Ferro Coφoration (Dresden, New York, Product Code 219-9) In this example, didodecyldimethyi- ammonium bromide (DDAB) was the stabilizing surfactant Typically, 200 g of baπum titanate was added to 1 L of 2-propanol and vigorously stirred Approximately 0 8 g of DDAB was added to the mixture The slurry was stirred and sonicated (Fisher Scientific Sonifier, Fischer Scientific Company, Philadelphia, Pennsylvania) for approximately 5 minutes The resulting slurry was poured into a 2-L Roalox ceramic mill jar which was charged with 4 5 lbs (about 2 kg) of Borundum gπnding media (see Example 1) The mill jar was then roiled at 70 φm for a peπod of 48 hours on a U S Stoneware Unitized Jar Mill, Model 764AVM After the milling was complete, the baπum titanate/stabihzing surfactant/2- propanol dispersion was removed from the mill jar The dispersion was poured into a large round-bottomed flask, and the 2-propanol was removed by vacuum evaporation The semi-dry baπum titanate/surfactant solid was further dπed at 90°C for 4 hours under reduced pressure A sufficient number of 200-g batches were prepared in accordance with this procedure to provide approximately 2 lbs (about 908 g) of surfactant-stabilized baπum titanate for dispersion into polypropylene The stabilized barium titanate was mixed with deionized water to give a 50 percent by weight aqueous dispersion for the next step Wax Dispersion Preparation In this example, 4 lbs (about 1 8 kg) of the 50 percent by weight aqueous stabilized baπum titanate dispersion was added to 0 5 lb (about 227 g) of the poly¬ ethylene (PE) wax employed in Example 1 The baπum titanate/DDAB/PE wax dispersion contained less than 0 1 percent by weight residual water (as determined by Karl Fischer titration) The composition of the wax dispersion was 80 percent by weight surfactant-stabilized baπum titanate and 20 percent by weight PE wax The wax P T/ S97/0 482 dispersion was cooled to dry ice temperature and ground to a coarse powder. The amount of coarse powder obtained was 2.3 lbs (about 1 kg). Polypropylene Compounding The 2.3 lbs (about 1 kg) of surfactant-stabilized barium titanate/PE wax powder was dry blended with 97.7 lbs (about 44 kg) of the same type of polypropylene employed in Example 1. The mixture was melt blended using a single screw com¬ pounding extruder operating at 330°F (about 166°C) and between 80 and 100 φm screw speed. The resultant blend was pelletized, dry blended, extruded, and pelletized a second time in an effort to ensure compositional homogeneity through-out the entire 100 lbs (about 45 kg) of material. The blend had a nominal baπum titanate con¬ centration of 2 percent by weight. This material then was used as a stock concentrate to produce three additional dilutions having nominal barium titanate concentrations ranging from 1 percent by weight to 0.1 percent by weight barium titanate as summarized in Table 6. Each dilution was dry blended, extruded, and pelletized twice to ensure compositional homogeneity. Table 6 Polypropylene/Barium Titanate Formulations Nonwoven Fabric Formation Meltblown nonwoven fabrics were prepared on a research meltblowing line essentially as descπbed in U.S. Patent No. 3,849,241 to Buntin et al., identified earlier. Meltblown fabrics were made from the Profax ® PF-015 polypropylene alone (PP-A) as a control and the polypropylene/stabilized barium titanate formulations containing 1.0 percent by weight and 0.1 percent by weight barium titanate (PF-C and PF-E, respec¬ tively). The formulation containing 0.5 percent by weight barium titanate (PF-D) was not melt spun. In each case, webs having basis weights ranging from 0.5 osy (about 17 gsm) to 2.0 osy (about 67 gsm) were prepared. The melt-spinning conditions used for the control and barium titanate formulations were identical. Two rolls of material were made at each basis weight for the control and the two barium titanate formulations. Each pair of rolls were identified as "A" and "B" rolls, in which the "A" and "B" designated the electret treatment conditions used during manufacturing. All "A" rolls were electret treated on-line with the spinning process. The electret treatment was earned out as described in Example 1. On-line electret treatment neces¬ sitated changing the rate at which the nonwoven web passed through the treatment zones to accommodate the line speed needed to produce fabric having various basis weights. In general, line speeds varied from 30 ft/min (about 15 cm/sec) to 120 ft/min (about 61 cm/sec), corresponding to nonwoven materials ranging in basis weight from 2.0 osy (about 68 gsm) to 0.5 osy (about 17 gsm). The electret treater utilized two treatment zones or stations. The upstream station had an air gap of 1 inch (about 2.5 cm) between the anode (charge bar) and the cathode (bias roll). The upstream bias roll and charge bar potentials were -5 kV and 0 kV, respectively. The down-stream station also had an air gap of 1 inch (about 2.5 cm) between the anode (charge bar) and the cathode (bias roll). The down-stream bias roll and charge bar potentials were -5 kV and 10 to 13 kV, respectively. The electret treater was purged with air at 20 standard cubic feet per minute (SCFM). The "B" rolls were electret treated off-line from the spinning process, also as described in Example 1. Off-line electret treatment was performed at a line speed of 30 ft/min (about 15 cm/sec) for all basis weights and all materials. The air gap in both upstream and down-stream treatment stations was 1 inch (about 2.5 cm). The bias roll potential was -5 kV and the charge bar potential was 10 to 13 kV for both treatment stations. The electret treater was purged with air at 23 SCFM. 7 482 RESULTS Air Filtration Measurements The air filtration efficiencies of the meltblown πonwovens prepared above were evaluated as descπbed in Example 1. Tables 7 through 9 summaπze the pressure drops and percent particle penetrations for mateπals electret treated on-line. Tables 10 through 12 summarize the pressure drops and percent particle penetrations for materials electret treated off-line. Table 7 Air Filtration Results for PF-A Webs Electret Treated On-Line ("A" Rolls) Table 8 Air Filtration Results for PF-C Webs Electret Treated On-Line ("A" Rolls) Table 9 Air Filtration Results for PF-E Webs Electret Treated On-Line ("A" Rolls) Table 9, Continued "Basis weight, osy (gsm). In mm water. e Standard deviation of pressure drop measurements. ""Standard deviation of percent penetration measurements. Table 10 Air Filtration Results for PF-A Webs Electret Treated Off-Line ("B" Rolls) Table 11 Air Filtration Results for PF-C Webs Electret Treated Off-Line ("B" Rolls) Pressure BW* Drop" < y {Apf % P <w*pf 2.0 (68) 4.52 0.13 0.72 0.10 1.5 (51) 3.29 0.10 0.95 0.10 1.0 (34) 2.57 0.08 2.59 0.39 0.75 (25) 2.07 0.05 5.25 0.56 0.5 (17) 1.31 0.05 12.31 0.97 "Basis weight, osy (gsm). b ln mm water. c Standard deviation of pressure drop measurements. d Standard deviation of percent penetration measurements. Table 12 Air Filtration Results for PF-E Webs Electret Treated Off-Line ("B" Rolls) Table 12, Continued "Basis weight, osy (gsm) In mm water ^Standard deviation of pressure drop measurements Standard deviation of percent penetration measurements Pressure Drop No significant change in pressure drop was observed at any given basis weight for meltblown matenal prepared from polypropylene alone as compared to the stabilized baπum titanate/polypropylene formulations independent of electret treatment conditions Notably, a small increase in pressure drop was measured for the mateπals electret treated off-line compared to those treated on-line This was attributed to compaction of the nonwoven web during unwinding and rewinding of the fabnc rolls dunng treatment Nonwoven Filter Media and Particle Penetration The filtration efficiency of the nonwoven media is given as the percent penetration The percent particle penetrations were lower for off-line electret treatment compared to on-line This difference was attributed to the differences in electret conditions cited above Filter media treated on-line and off-line both exhibited the same trend toward improved filtration efficiency with the addition of baπum titanate particles to the fibers, (see FIGS 3 and 4, descπbed below) The largest improvement was observed for matenal containing 1 0 percent by weight baπum titanate The efficiency systematically increased with the concentration of baπum titanate for all but the two highest basis weights (1 5 osy and 2 0 osy) At basis weights above 1 5 osy the effect of adding the baπum titanate was difficult to observe The improved filtration performance of the baπum titanate formulations compared to polypropylene alone are best illustrated by the graphs shown in FIGS. 3 and 4 In these graphs, the data points represent the percent penetration for a given type of nonwoven web The consistent behavior illustrated by FIGS 3 and 4 suggests that the baπum titanate particles were responsiDle for the improved filtration efficiencies of the nonwoven mateπa's described in this example Example 3 Material Preparation The ferroelectric colloidally enhanced nonwoven fabrics described in this example were prepared as described in Example 1 , except for the differences in preparation and treatment which are described below. 'Colloid Preparation The barium titanate was obtained from Tarn Ceramics, Inc. (Ticon* 5016). The colloidal dispersion was prepared using a large scale stainless steel mixing tank (about 130 gal or 492 L) equipped with a pneumatically driven stirrer. The dispersion was processed through a high speed stainless steel pin/attriter mill powered by a 50 hp electric motor operating at 1750 φm. The mixing tank and pin/attriter mill were custom built by Standridge Color Corporation (Social Circle, Georgia). The bottom half of the mixing tank was funnel shaped. The mixing tank was connected to a pneumatic pump and in turn the pump was connected to the pin/attriter mill using a 2.5-inch (about 10- cm) diameter flexible hose. The effluent from the mill was recycled into the top of the mixing tank. The pneumatic pump had a displacement of 0.25 gal (about 0.95 L) per stroke and was operated at a rate which provided a flow of 8-10 gal/min (about 0.5-0.6 L/sec). The mixing tank was filled with 190 lb. (86.4 kg) of technical grade 1-butanol. Then, 7.2 lb (3.27 kg) of Rhodameen PN430 (Rhone-Poulenc) was added with vigorous stirring. The barium titanate was added in 55-lb (about 25-kg) portions until a total of 770 lb (about 350 kg) had been added to the mixing tank. The slurry was pumped to the high speed pin/attriter mill and recycled to the mixing tank for approximately 30 min. The resultant 1-butanol dispersion was uniform in composition and contained 80 weight-percent barium titanate. Polyethylene Wax Dispersion Formation The 80 weight-percent barium titanate/1-butanol dispersion was added directly to a molten low molecular weight polyethylene (PE) wax (Allied Signal A-C 16). Note that in the previous examples a 50 weight-percent aqueous dispersion of barium titanate was added to the molten PE wax in a process commonly known in the art as flushing. In the present example, the stabilized colloidal particles of barium titanate were partitioned from a 1-butanol rich phase into a PE wax rich phase and the 1- butanoi was removed by vaporization. This process differs from water/wax flushing in that the 1-butaπol boils above the melting point of the A-C 16 PE wax. The wax was CT/US97/08482 melted in a 150-gal (about 568-L) steam-heated vat equipped with rotating blades which slowly blend the mixture Steam was supplied to the vat at 50 psig, corresponding to a temperature of about 297°F (about 147°C) In this example, 969.20 lb. (440.55 kg) of 1-butanol/banum titan- ate/Rhodameen ® PN-430 dispersion was combined with 190.8 lb. (86 73 kg) of A-C 16 PE wax. The molten wax and 1-butanol dispersion were blended continuously until no alcohol vapor was detected over the mixture At this point, the BaTiOs/Rhodameen ® PN-430/A-C 16 PE wax dispersion was poured into a tray to cool to room temperature. The solidified wax composite was further cooled to dry ice temperature and ground to a coarse powder for dry blending with polypropylene Polypropylene Compounding The BaTKtyRhodameen ® PN-430/A-C 16 PE wax composite, 832 lb (about 378 kg) was dry blended with 2,496 lb (about 1339 kg) of Montel Profax ® PF-015 polypropylene (PP) The dry mixture was melt blended using a single screw compounding extruder to give a mixture containing 20 weight-percent of baπum titanate A 600-lb (273-kg) portion of the 20 weight-percent concentrate prepared above was then blended with 1800 lb (about 818 kg) of Montel Profax ® PF-015 polypropylene This dry blend was melt blended using a single screw compounding extruder to yield a 5 weight-percent baπum titanate/polypropylene composite Nonwoven Fabnc Formation Nonwoven fabncs were manufactured on a 100-ιnch (about 2 5-meter) meltblown line essentially as described in U S Patent No 3,849,241 to Buntin et al The 100-ιπch wide web was slit into five 20-ιnch (about 51 -cm) sections Meltblowing conditions were held constant for all materials All fabncs had a nominal basis weight of 0 6 osy (about 20 gsm) The 20 weight-percent baπum titanate/polypropylene composite was dry blended at a rate of 1 part to 19 parts of virgin Montel Profax ® PF- 015 polypropylene to yield a meltblown fabric containing about 1 weight-percent baπum titanate In addition, the 5 weight-percent baπum titanate/polypropylene composite was processed without further dilution Finally, virgin Montel Profax ® PF- 015 polypropylene was meltspun to produce a control All meltblown nonwoven webs were electret treated on-line under identical conditions The electret treatment was earned out in accordance with the teachings of U S Patent 5,401 ,446 to Tsai et al., descnbed earlier RESULTS Air Filtration Measurements The air filtration efficiencies of the meltblown nonwoven webs were evaluated using a TSI, Inc. Model 8110 Automated Filter Tester as descπbed in the previous examples. Data is presented in tabular and figure form as percent penetration and pressure drop measured for a 32 L/miπ air stream containing 0.1 micron NaCI particles as the challenge aerosol. Samples of meltblown material were taken from ten cross deckle positions (i.e. 2 per 20-inch slit) of the nonwoven webs descπbed above. Samples were cut as flat sheets approximately 8-inches (about 20-cm) square. A minimum of 20 samples were evaluated for pressure drop (Δp in mm H 2 0)) and percent particle penetration (% P). Tables 13 through 15 summarize the pressure drop and particle penetration data for the control (Montel Profax " * PF-015) and the baπum titanate containing formulations. Table 13 Air Filtration Results for Control Webs Table 13, Continued Pressure CD" Drop* σ(Δpf % P* σi% pγ 95 (241) 2.32 0.06 14.09 1.06 "Cross-deckle position, inches (cm). In mm water. c Standard deviation of pressure drop measurements. Percent penetration "Standard deviation of percent penetration measurements. Table 14 Air Filtration Results for 1 Weight-Percent BaTi0 3 Table 14, Continued s ln mm water °Standard deviation of pressure drop measurements. Percent penetration "Standard deviation of percent penetration measurements. Table 15 Air Filtration Results for 5 Weight-Percent BaTiθ 3 The pressure drop (Δp) and percent particle penetration (% P) data presented in Tables 13 through 15 clearly demonstrate the superior filtration performance of the meltblown webs prepared from the baπum titanate/PP composite mateπals All the webs examined were charactenzed by a cross-deckle profile in the pressure drop and penetration data The shape of the profile was independent of the matenal The filtration data are best visualized in FIGS 5 and 6 The pressure drop measured across the web (FIG 5) was identical for each of the three mateπals depicted This suggests that fiber and web formation are independent of the matenal being spun (i e, polypropylene versus baπum titanate/PP composite) By contrast, the percent particle penetration (FIG 6) was significantly lower for both the 1 weight-percent and 5 weight- percent BaTι0 3 formulations compared to the control polypropylene Thus, for a given pressure drop through the web, the baπum titanate/PP composite evinces supeπor filter performance (i e , lower particle penetration) compared to the control polypropylene Example 4 A promising potential air filtration application of electret-treated meltblown is its use in medical and surgical face masks In order to veπfy its utility in such a product, the matenal must satisfy certain stπngent air filtration tests (bacteπal filtration efficiency greater than 98 5 %, when challenged by a 3 0-mιcron aqueous aerosol containing bacteπa, e g , Staphylococcus aureus, and an air filtration efficiency greater than 99 5 percent when challenged by 0 1-mιcron latex particles) The matenal also must be subject to only moderate pressure drops through the web (typically, less than 2 5 mm H 2 0 at a flow rate of 8 L/min) The meltblown nonwoven webs of Example 3 were tested for bacteπal filtration efficiency with differential pressure (in accordance with U S Department of Defense Index of Specifications and Standards #MIL-M-36954C) For compaπson with Example 3 all webs were sampled from the same CD position (50 inches) Five replicate samples of each code were tested The mean and standard deviation of these measurements are reported in Table 16 Pressure drops, Δp, are reported in mm H 2 0, efficiencies, ε, are reported in percent Where two values are reported, the first represents measurements made on matenal spun at the beginning of the production run and the second represents measurements made on matenal spun at the end of the production run The ferroeiectπc colloidally enhanced meltblown nonwoven webs of the present invention exceeded the targets for both filtration efficiency and pressure drop Table 16 Bacterial Filtration Efficiency Results The meltblown nonwoven webs of Example 3 also were tested for filtration efficiency when challenged by 0.1 micron latex particles (in accordance with ASTM Test Method F1215-89) Again, all webs were sampled from the same CD position (50 inches). Five replicate samples of each code were tested. The mean and standard deviation of these measurements are reported in Table 17. Again, efficiencies, ε, are reported in percent. Where two values are reported, the first represents measurements made on matenal spun at the beginning of the production run and the second represents measurements made on material spun at the end of the production run. The ferroelectπc colloidally enhanced meltblown nonwoven webs of the present invention met or exceeded the filtration efficiency targets. Table 17 Latex Particle Filtration Efficiency Results While the specification has been described in detail with respect to specific embodiments thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments.

Claims: WHAT IS CLAIMED IS 1 A fiber which compnses a thermoplastic polymer, and particles of a ferroelectnc matenal dispersed therein 2 The fiber of Claim 1 , in which the particles of a ferroelectπc matenal are present at a level of from about 0 01 to about 50 percent by weight, based on the weight of the fiber 3 The fiber of Claim 1 , in which the fiber has been exposed to an electπc field 4 The fiber of Claim 1 , in which the thermoplastic polymer is a polyolefin 5 The fiber of Claim 4, in which the polyolefin is polypropylene or polyethylene 6 The fiber of Claim 1 in which the particles of the ferroelectπc matenal have a longest dimension in a range of from about 10 nanometers to about 10 micrometers 7 The fiber of Claim 1 in which the ferroelectπc matenal is selected from the group consisting of perovskites tungsten bronzes, bismuth oxide layered mateπals, and pyrochlores 8 The fiber of Claim 7, in which the ferroelectπc matenal is baπum titanate 9 A multicomponent fiber comprised of two or more components, each of which is compnsed of a thermoplastic polymer, wherein at least one component is compπsed of a thermoplastic polymer and particles of a ferroelectπc matenal dispersed therein 10. The multicomponent fiber of Claim 9, in which the particles of a ferroelectric matenal are present at a level of from about 0.01 to about 50 percent by weight, based on the weight of the fiber. 11. The multicomponent fiber of Claim 9, in which the multicomponent fiber has been exposed to an electric field. 12. The multicomponent fiber of Claim 9, in which the particles of the ferroelectπc material have a longest dimension in a range of from about 10 nanometers to about 10 micrometers. 13. The multicomponent fiber of Claim 9, in which the ferroelectric material is selected from the group consisting of perovskites, tungsten bronzes, bismuth oxide layered matenals, and pyrochlores. 14. The multicomponent fiber of Claim 13, in which the ferroelectric material is baπum titanate 15. The multicomponent fiber of Claim 9, in which the multicomponent fiber is a bicomponent fiber in which the two components are arranged in a side-by-side configuration. 16. The multicomponent fiber of Claim 9, in which the multicomponent fiber is a bicomponent fiber in which the two components are arranged in a sheath-core configuration. 17. The multicomponent fiber of Claim 9, in which the thermoplastic polymer is a polyolefin. 18. The multicomponent fiber of Claim 17, in which the polyolefin is polypropylene or polyethylene. 19. A nonwoven web compπsed of the fiber of Claim 1 20. The nonwoven web of Claim 19, in which the nonwoven web has been exposed to an electric field. 21. A nonwoven web comprised of the multicomponent fiber of Claim 9. 22. The nonwoven web of Claim 21 , in which the nonwoven web has been exposed to an electric field. 23. A method of preparing fibers containing particles of a ferroelectric material, the method comprising: destructuring the ferroelectric material in the presence of a liquid and a surfactant to give destructured particles, wherein the liquid is a solvent for the surfactant and the surfactant is adapted to stabilize the destructured particles against agglomeration; forming a biend of the stabilized, destructured ferroelectric material particles and a thermoplastic polymer; and extruding the blend to form fibers. 24. The method of Claim 23, in which the thermoplastic polymer is a polyolefin. 25. The method of Claim 23, in which the particles of a ferroelectric material are present at a level of from about 0.01 to about 50 percent by weight, based on the weight of the fiber. 26. The method of Claim 23 which further comprises exposing the fibers to an electric field. 27. The method of Claim 23, in which the destructured particles of the ferroelectric material have a longest dimension in a range of from about 10 nanometers to about 10 micrometers. 28. The method of Claim 23, in which the ferroelectric material is selected from the group consisting of perovskites, tungsten bronzes, bismuth oxide layered materials, and pyrochlores. 29. The method of Claim 28, in which the ferroelectric material is barium titanate. 30. The method of Claim 23, which further comprises collecting the extruded fibers on a moving foraminous support to form a nonwoven web. 31. The method of Claim 30 which further comprises exposing the nonwoven web to an electric field. 32. The method of claim 23 which further comprises melting the blend of the stabilized, destructured ferroelectric material particles and a thermoplastic polymer and melt extruding the molten blend to form fibers. 33. The method of claim 23 which further comprises adding a solvent for the thermoplastic polymer to the blend to form a solution of the thermoplastic polymer having dispersed therein the stabilized, destructured ferroelectπc material particles and solution spinning the resulting solution to form fibers. 34. The method of Claim 23 which further compnses, after destructuπng the ferroelectric material: adding the mixture of liquid and stabilized, destructured ferroelectric material particles to a molten organic wax at a temperature sufficient to evaporate the liquid 35. The method of Claim 23 which further comprises, after destructuring the ferroelectric material: removing the liquid from the stabilized, destructured particles; and adding the stabilized, destructured particles to a molten organic wax. 36 The method of Claim 23 which further compnses, after destructuπng the ferroelectπc matenal removing the liquid from the stabilized, destructured particles, redispersmg the stabilized, destructured particles in water, and adding the resulting dispersion to a molten organic wax at a temperature sufficient to evaporate the water 37. The method of Claim 34, in which the liquid is an aliphatic alcohol having no more than about ten carbon atoms; the surfactant is an alcohol-soluble, tetraalkylammonium halide, ethoxylated alkylamtne, or pnmary, secondary, or tertiary alkyl- or arylamine, and the organic wax is a polyethylene wax 38 The method of Claim 36, in which the liquid is an aliphatic alcohol having no more than about ten carbon atoms, the surfactant is an alcohol-soluble, tetraalkylammonium halide, ethoxylated alkylamme, or pnmary, secondary, or tertiary alkyl- or arylamine, and the organic wax is a polyethylene wax 39 The method of Claim 23, in which destructunng is accomplished by means of a ball mill, attπter mill, or pin mill 40 The method of Claim 29, in which the aliphatic alcohol is 2-propanol or 1- butanol 41 A filtration medium compπsing the nonwoven web of Claim 20 42 The filtration medium of Claim 41 , in which the nonwoven web is adapted to remove particulate matter from a gaseous stream 43 The filtration medium of Claim 41 , in which the nonwoven web is a component of a heating, ventilating, and air conditioning filter 44. The filtration medium of Claim 41, in which the nonwoven web is a component of an air conditioning or heating filter. 45. . The filtration medium of Claim 41 , in which the nonwoven web is a component of a high efficiency particle abstraction filter. 46. The filtration medium of Claim 41, in which the nonwoven web is a component of an automotive air filter. 47. The filtration medium of Claim 46, in which the nonwoven web is a component of an automobile engine air filter. 48. The filtration medium of Claim 46, in which the nonwoven web is a component of an automobile cabin air filter. 49. A vacuum cleaner bag comprising the nonwoven web of Claim 20. 50. A face mask comprising the nonwoven web of Claim 20. 51. The face mask of Claim 50, in which the mask is a medical face mask. 52. A respirator comprising the nonwoven web of Claim 20. 53. A dust wipe comprising the nonwoven web of Claim 20.

Inventors: MYERS DAVID LEWIS, TURKEVICH LEONID ANTHONY, MYERS, DAVID, LEWIS, TURKEVICH, LEONID, ANTHONY
Applicants: KIMBERLY CLARK CO, KIMBERLY-CLARK WORLDWIDE, INC.
Application year: 1997
Main classification: D01F1
IPC codes: B01D39, C08K3, D01F1, D01F6, D01F8, D04H1