FIBROUS NONWOVEN WEB

Abstract: The present invention provides a polymeric, particularly aramid, fibrous nonwoven web having a relatively small pore size. The present inventive fibrous nonwoven web preferably has a high titer reduction with respect to bacteria such as Pseudomonas diminuta and viruses as exemplified by PR772 coliphage. The present invention further provides a method of filtering a fluid by passing a fluid through the present inventive fibrous nonwoven web, as well as a filter element incorporating the present inventive fibrous nonwoven web. The web may be coated to render it hydrophobic and oleophobic, and thus repellent to liquid chemicals. Such chemically repellent webs may be adapted for human apparel, and fashioned into garments suitable for use in adverse environments such as biological and toxic environments.

Description: FIBROUS NONWOVEN WEB TECHNICAL FIELD OF THE INVENTION The present invention pertains to a fibrous nonwoven web. In particular, the present invention relates to a polymeric fiber sheet, especially an aramid fiber sheet, as well as a method of preparing and using such a sheet, particularly as a filtration medium. BACKGROUND OF THE INVENTION Numerous fibrous filtration sheets have been prepared from a variety of fibers, such as glass, polyethylene, polypropylene, polyester, and aramid fibers. While these sheets are suitable for many applications, particularly many filtration applications, they are generally incapable of reliably removing small micron and submicron particulate matter and bacteria and viruses. While the thickness of such a fibrous sheet can be increased to compensate for the relatively large pore size of the sheet, the flow resistance of the sheet then becomes impracticably high. This inability to prepare fibrous filtration sheets of sufficiently small pore size is generally the result of the unavailability of fibers of sufficiently fine diameter, inasmuch as the pore size of a fibrous filtration sheet is related to some extent to the fiber diameter used to prepare such a sheet, i.e., uniformly smaller diameter fibers will more easily allow for the preparation of a fibrous filtration sheet of uniformly smaller pore size. While membranes, such as microporous and ultrafiltration membranes, are capable of providing the desirable smaller pore size as compared to fibrous nonwoven webs, membranes suffer from many performance disadvantages as compared to fibrous nonwoven webs. For example, membranes are relatively brittle and require considerable care in handling. As a result, membranes are difficult to convert into corrugated structures for filter elements without introducing cracks in the membranes, particularly in preparing small radii filter elements, with the consequent loss of filtration efficiency. The relative fragility of membranes also render membranes susceptible to failure upon pulsing of the fluid flow through the membranes. Accordingly, there remains a need for a fibrous nonwoven web with a relatively small pore size, preferably sufficiently small to remove submicron particles, especially bacteria and viruses, at a reasonable thickness and/or flow resistance through the fibrous nonwoven web. Such a fibrous nonwoven web preferably also would have a high modulus, as well as sufficient elongation, so as to withstand filtration conditions, such as pulsing, which can be encountered in commercial filtration processes. The handling characteristics of such a fibrous nonwoven web also should be such that the fibrous nonwoven web can be converted into a corrugated structure for a filter element without damaging the web in any way so as to affect filtration efficiency. A high temperature resistance and good chemical stability are other useful and desirable characteristics of such a fibrous nonwoven web. The present invention provides such a fibrous nonwoven web, as well as a method of preparing and of using such a fibrous nonwoven web, particularly in filtration applications. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein. BRIEF SUMMARY OF THE INVENTION The present invention provides a fibrous nonwoven web useful as a filtration medium. The present inventive fibrous nonwoven web is characterized by having a titer reduction of at least about 10 β , preferably at least about IO 8 , and most preferably at least about 10 10 , with respect to Pseudomonas diminuta, and a titer reduction of at least about 10 1 , preferably at least about IO 3 , more preferably at least about IO 5 , and most preferably at least about IO 8 , or even at least about 10 10 , with respect to PR772 coliphage, and an air flow resistance of about 30 psi (about 200 kPa) or less at a thickness of about 0.02 inch (about 500 μm) or less. Accordingly, the present inventive fibrous nonwoven web can be prepared so as to be capable of removing all bacteria and a significant portion of, and preferably all, viruses from a fluid. The present invention further provides a method of filtering a fluid by passing a fluid through the present inventive fibrous nonwoven web. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph depicting a curve showing the relationship between the air flow resistance (psi and kPa) of fibrous nonwoven webs prepared in accordance with Example 1 and the 1500 cc/min bubble points (inch water column and kPa) of the fibrous nonwoven webs. Figure 2 is a graph depicting a curve showing the relationship between the air flow resistance (psi and kPa) of fibrous nonwoven webs prepared in accordance with Example 6 and the 1500 cc/min bubble points (inch water column and kPa) of the fibrous nonwoven webs DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides a fibrous nonwoven web with the pore size, e.g., microporosity, advantages of a membrane and the handling advantages of a fibrous nonwoven web such as ruggedness, extended life, filter depth, and strength, with few of the disadvantages of either of those filtration media. While membranes, e.g., microporous and ultrafiltration membranes, can be prepared which are capable of having submicron pore ratings which could be used to exclude, for example, the smallest bacteria and viruses, such membranes suffer from the significant disadvantage of being quite fragile. In many situations requiring more rugged filtration media, fibrous nonwoven webs are utilized; however, such fibrous nonwoven webs are generally limited to the filtration of relatively larger particles from fluids and are not typically suitable for removing submicron particles such as bacteria and viruses from fluids. The present inventive fibrous nonwoven web provides a combination of the advantages of conventional membrane and fibrous nonwoven web filtration media. In particular, the present invention provides a fibrous nonwoven web characterized by a titer reduction of at least about IO 6 , preferably at least about IO 8 , and most preferably at least about 10 10 , with respect to Pseudomonas diminuta. Since Pseudomonas diminuta is generally accepted as the smallest and highest penetrating test bacteria, its retention by a material is evidence of the material's ability to provide absolute removal of bacteria, particularly liquid-borne bacteria, in most applications. Thus, the present invention is generally capable of the absolute removal of bacteria from fluids under most conditions. Similarly, the present inventive fibrous nonwoven web can be further characterized by a titer reduction of at least about 10 1 , preferably at least about IO 3 , more preferably at least about IO 5 , and most preferably at least about IO 8 , or even at least about 10 10 , with respect to PR772 coliphage. While viruses are not all of the same size, viruses are typically characterized as either "large" viruses or "small" viruses. The large viruses include viruses of about 0.08 μm in effective diameter and larger, e.g., adenoviruses, rheoviruses, and herpes viruses. The small viruses include viruses which are about 0.025-0.028 μm in effective diameter, e.g., hepatitis viruses, polio viruses, and parvoviruses. Inasmuch as the size of the PR772 coliphage is about 0.053 μm, and the size of the PP7 bacteriophage is about 0.027 μm, these phages provide excellent models for assessing the removal efficiency of a filtration medium with regard to various sized viruses. A filtration medium is generally considered to have an absolute removal ability with respect to a particular particle, e.g., the PR772 coliphage as representative of intermediate-sized viruses, when it has at least a IO 8 , and preferably at least a 10 10 , titer reduction with respect to that particle. Of course, an absolute removal ability of a filtration medium with respect to the PR772 or PP7 phages would confirm the absolute removal ability of that filtration medium with respect to larger viruses. The titer reduction of a filtration medium with respect to a particular particle, e.g., PR772 coliphage, is the ratio of that particle contained in the influent to that contained in the effluent. Surprisingly, the aforesaid titer reduction is capable of being achieved by the present inventive fibrous nonwoven web at a reasonable thickness and flow resistance. In particular, the aforesaid titer reductions preferably can be achieved with the present inventive fibrous nonwoven web having a thickness of about 0.02 inch (about 500 μm) or less, more preferably a thickness of about 0.01 inch (about 250 μm) or less, and most preferably a thickness of about 0.008 inch (about 200 μm) or less, e.g., about 0.001-0.006 inch (about 25-150 μm) . The present inventive fibrous nonwoven web desirably has an air flow resistance of about 30 psi (about 200 kPa) or less, preferably about 20 psi (about 140 kPa) or less, more preferably about 15 psi (about 100 kPa) or less, and most preferably about 7 psi (about 50 kPa) or less, e.g., about 1.5-6 psi (about 10-40 kPa) . All of the air flow resistance (i.e., pressure drop or ΔP) values set forth herein to describe and illustrate the present invention reflect values determined using an air flow rate of 28 ft/min (about 8.5 m/min) in accordance with the general procedure described in U.S. Patent 4,340,479. The present inventive fibrous nonwoven web can have any suitable sheet (or basis) weight. The desirable sheet weight of the present inventive fibrous nonwoven web will vary depending on the particular application for the fibrous nonwoven web. In many applications, the present inventive fibrous nonwoven web generally will have a sheet weight no more than about 80 g/m 2 and desirably will have a sheet weight of no more than about 60 g/m 2 , preferably no more than about 40 g/m 2 . While the present inventive fibrous nonwoven web can have a sheet weight of less than about 10 g/m 2 , such a filtration fibrous nonwoven web will typically have a sheet weight of at least about 20 g/m 2 , e.g., 20-40 g/m 2 . Generally, the present inventive fibrous nonwoven web will have as low a sheet weight as possible as consistent with application objectives. A lower sheet weight typically will result in a lower flow resistance and a thinner fibrous nonwoven web, which in turn reduces material costs and enables the easier manipulation of the fibrous nonwoven web, particularly the easier corrugation of the fibrous nonwoven web without damage for use in a filter cartridge. Further, this will provide higher filter surface area with respect to a particular volume and longer service life. Surprisingly, the desirable properties of the present inventive fibrous nonwoven web, including consistent and predictable removal efficiencies, are attainable at very low sheet weights, e.g., to as low as 20 g/m 2 and below. The present invention also provides a method of preparing the fibers which are useful in the construction of the present inventive fibrous nonwoven web. Specifically, the present invention includes a method of reducing the diameter, and preferably length, of fibers comprising preparing a dispersion of fibers in a fluid and subjecting the dispersion to fibrillation, particularly mechanical fibrillation, under conditions sufficient to reduce the average diameter, and preferably length, of the fibers. The present invention also includes a method of preparing a fibrous nonwoven web comprising reducing the diameter, and preferably length, of fibers as described herein and preparing a fibrous nonwoven web from the thus processed fibers. Such fibers can include any suitable fiberε, preferably polymeric fibers. Suitable fibers include aramid, titinate, polyester, asbestos, and cellulosic fibers (particularly esparto fibers) . While the present inventive fibrous nonwoven web can comprise any of such fibers or combinations thereof, with or without other fibers, e.g., glass fibers, cotton fibers, or other materials, the present inventive fibrous nonwoven web preferably consists essentially of such fibers alone. Such fibers most preferably are aramid fibers, which are fibers of poly(paraphenylene terephthalamide) and related compounds wherein the majority of the amide groupε are directly attached to two aromatic rings. Aramid fibers are commercially available as Kevlar® fibers (duPont, Wilmington, Delaware) , Twaron® fibers (Akzo, Arnhem, Netherlands), Apyeil® fibers (Unitika, Osaka, Japan), and Conex® fibers (Teijin, Osaka, Japan) . While any suitable aramid fibers may be used in conjunction with the present invention, Kevlar® 361 and Twaron® 1094 aramid fibers are preferably utilized for purposes of the present invention. Although it is preferred to use aramid fibers alone, a composite web may be prepared with aramid fibers and other fibers, such as, for example, polyester fibers, cotton fibers, glass fibers, cellulose fiberε, such as esparto, or other polymeric fibers, or combinationε among the foregoing fibers. Polyester fibers may be added, for example, to increase the strength of the web or to vary the pore rating of the web. Examples of suitable polyester fibers include Reemay® fibers (20 μm diameter fibers, Reemay, Old Hickory, Tennessee) , polyethylene terephthalate (PET) fibers, polybutylene terephthalate (PBT) fibers, poly (1, 4-cyclohexylene dimethylene terephthalate), and so forth. The choice of a particular polyester will depend on the application to which the web will be put. The fibers can be incorporated into the aramid web in any suitable amount, typically in an amount from about 5% to about 50% by weight. A composite polyester-aramid fiber web thus prepared can be used in a number of applications, for example, for removing residue from rollers in a photocopy machine. Cotton fibers, such as those sold by Barnhardt Co., or glass fibers, such as thoεe εold under the trade name Evanite by Evans Co., can be added to increase the dirt capacity of the aramid fiber web when the web is used as a filter. Such cotton or glass fibers can be incorporated into the aramid web in any suitable amount, typically in an amount from about 5% to about 60% by weight. Other fibers can be incorporated into the web, for example, carbon, steel, aluminum, and so forth. In accordance with one embodiment of the invention, the fibrous web includeε cellulose fibers. The cellulose fibers can form a composite web with aramid fibers, or, optionally, the web may be prepared with cellulose fibers alone. When a composite web is prepared with aramid fibers and cellulose fibers, the aramid fibers preferably are present in an amount ranging from about 40% to about 95% by weight. The preferred cellulose fibers for use in the present invention are esparto fibers, such as those sold by Morgan Co. A celluloεe fiber web in accordance with the invention iε particularly suitable for use aε a filter press pad, for example, for filtering comestible liquids such as beverage syrupε. The cellulose fibers can form composite webε with other fibers, such as, for example, polyester fibers, glasε fiberε, cotton fibers, and so forth. Such composite webs can be prepared in accordance with the teachings set forth above. For example, a disperεion containing the fiberε can be subjected to fibrillation, such as mechanical fibrillation, and a nonwoven web subεequently prepared therefrom. When a composite fibrous web is being prepared, the dispersion of fibers preferably corresponds to the percentage of fibers desired in the composite web. For example, if a composite web comprising 25% aramid fibers and 75% cellulose fibers is desired, the initial dispersion subject to fibrillation desirably contains this ratio of fibers. The fibers useful in the context of the present invention can be of any suitable length and typically will be cut fiberε of about 4 mm or leεε in length. In general, shorter length fibers, such as fibers having lengths of about 2 mm or less, desirably are used in connection with the present invention. The use of shorter length fibers can reεult in a decreaεe or avoidance of undesirable entanglements during the preparation of the present inventive fibrous nonwoven web and can lead to the preparation of a thinner and more uniform fibrous nonwoven web with reduced flow resistance while retaining the desirable properties of the preεent inventive fibrouε nonwoven web. The reduction of the diameter of the fibers can be carried out by subjecting any suitable dispersion of the fibers, e.g., aqueous disperεions of about 50 g/1 or lesε, to any suitable fibrillation means under conditions εufficient to reduce the average diameter of the fiberε. It haε been diεcovered that more concentrated fiber dispersions can be beneficial in achieving greater fibrillation, i.e., greater diameter reduction, of the fibers. Thus, the fibrillation in the context of the present invention is preferably carried out with fiber dispersions of at least about 20 g/1, e.g., about 25-40 g/1. Such fibrillation can be carried out by any suitable device, such as a mill mixer, particularly a Kady Mill® mixer (Kinetic Diεperεion Corp., Scarborough, Maine), for a sufficient period of time, a homogenizer, particularly a Union® HTD28 Homogenizer (Union Pump Co., North Andover, Massachusetts) , for a sufficient number of pasεeε, or a microfluidizer, particularly a Microfluidizer Model MllOY or M110EH (Microfluidicε International Corp., Newton, Massachusettε) , for a sufficient number of pasεes. It has been found that the processing time in conjunction with the fibers solids and fibrillation energy, e.g., mechanical energy, is directly related to the filtration efficiency of the resulting fibrouε nonwoven web prepared from the processed fibers. Thus, for example, it is possible to provide a fibrous nonwoven web with a consistent and predictable titer reduction by controlling the processing time of the fibers in a given piece of equipment under conεtant operating conditions. It has been observed that extending the time of processing will better separate the fibers, and will shorten the length to diameter ratioε of the fibers. The pore rating of the web produced with such fibers will decrease, and the preεsure drop across the web will increase when the web is used as a filter. The uεe of a mill mixer to treat fiberε in the context of the preεent invention is preferred inaεmuch aε εuch equipment provides for fibers with smaller diameters and, thus, fibrous nonwoven webs with εmaller pores. Such equipment preferably is operated at maximum recommended operating conditions to achieve the smallest diameter fibers and εmalleεt pore rated fibrouε nonwoven webε in the least amount of processing time. The mechanical fibrillation in the context of the present invention is quite different than the mixing which typically takes place in conventional Fourdrinier paper making processes which utilize relatively low-energy beaters such as the Jordan®, Cowles®, and Claflin® beaters. Such relatively low-energy mixers, however, may be uεed to prepare an initial diεperεion of the raw fiberε prior to εubjecting the diεperεion to the mechanical fibrillation in accordance with the preεent invention. For example, a raw fiber dispersion may be prepared by such a relatively low- energy beater until a satisfactory dispersion is reached, e.g., no lumps are readily apparent in the diεpersion, which point typically will be reached within an hour or two. This disperεion then may be diluted aε desired before being subjected to the fibrillation process in accordance with the present invention as described above. The present invention accordingly provides fiber which is capable of being formed into the present inventive fibrous nonwoven web. Surprisingly, the present invention is capable of reducing the diameter of fibers without substantially adversely affecting other, deεirable characteristics of the fibers, including the modulus, elongation, temperature resistance, and chemical stability of such fibers. Thus, fibrous nonwoven webs prepared from the thus processed fibers retain the deεirable propertieε of conventional fibrous nonwoven webs while also exhibiting improved pore size and pore size distribution characteristics akin to those of membranes. The present inventive fibrous nonwoven web can be prepared from εuch proceεεed fibers by fibrouε εheet techniqueε which are well-known in the art, εuch aε conventionally modified Fourdrinier paper making processeε. While a binder resin is generally not necessary to provide structural integrity to the fibrous nonwoven web (surpriεingly, it iε believed that the fibers' natural surface charge holds the fibers together) , a binder resin nevertheless can be used in preparing the present inventive fibrous nonwoven web so as to improve the mechanical properties, particularly the tensile properties, of the fibrous nonwoven web. Any εuitable binder resin can be utilized in the context of the present invention. Suitable binder resinε include epoxy binderε, for example, EPON 826 or EPI-REZ (Shell Corporation) , binderε which would impart a positive zeta potential to the fibrous nonwoven web, acrylic binders, tetrafluoroethylene binders, fluorinated ethylene propylene (FEP) binders, ethylene vinyl acetate (EVA) , phenolic resin binders, and fluoroethylene polymer binders, or combinations among the foregoing, such as, for example, phenolic and epoxy binders. The binder reεin can be utilized with the preεent inventive fibrous nonwoven web by use of a variety of techniques, which are well-known in the art. For example, the fibrous nonwoven web can be passed through an appropriate resin εolution, dried, and then cured. Alternatively, an appropriate binder resin can be exhausted onto the fibers in slurry form preparatory to a Fourdrinier operation to convert the fibrous slurry into a fibrous nonwoven web. Additional methods of binder application include spraying, pattern printing, and powder resin coating. Although a binder is not necesεary, a binder preferably iε used when additional strength is desired. Thus, for example, a binder preferably is used when the fibrous web is used to process fluids at relatively high pressures, for example, presεureε exceeding about 200 psig (about 1380 kPa) . A binder further preferably is used when the filter is designed to be backwashed. The choice of binders for the present inventive fibrous' nonwoven web may be based upon the particular application to which the web will be put, and may be based upon factorε including the temperature and chemical composition of the filtered fluid. For example, FEP is a preferred binder when the web iε uεed at temperatureε of about 225 °C to about 270 °C. At higher temperatures, such as temperatures ranging from about 550 °C to about 600 °C, a polytetrafluoroethylene (PTFE) binder is preferred. PTFE binders are advantageous in that they render the fibrouε web chemically inert to many chemically aggreεεive environmentε. The PTFE binderε alεo render the εurface of the fibrouε web hydrophobic, and thuε particularly suitable for filtering certain media. For lower temperature applications, epoxy binders, such as EPON 826 (Shell Corporation) , may be used. A binder that changes the zeta potential of the fibrous web can be used to ionically enhance the filtration properties of the nonwoven web. For example, the surface of the nonwoven web can be modified with a surface- modifying binder to impart a charge to the surface of the nonwoven web. Suitable surface modifying binders include, for example, cationic polymers. One such cationic polymer is a polyamine resin εold under the trademark KYMENE by Herculeε Co. Such a positively-charged, surface-modified fibrous web is especially well suited for the filtration of negatively charged species. The present inventive fibrous nonwoven web also can be surface modified to, for example, modify the wetting properties, e.g., critical wetting surface tension (CWST) , of the fibrous nonwoven web (e.g., to render it hydrophobic, hydrophilic, oleophobic, or oleophilic) . The measurement of CWST is diεcloεed in U.S. Patent 4,880,548. Such εurface modification of the preεent inventive fibrous nonwoven web can be carried out in any suitable manner and preferably is accompliεhed by graft polymerizing a suitable monomer onto the surface of the fibrous nonwoven web. Preferred examples of εuch monomers include acrylic or methacrylic monomers having alcohol functional groups, such as, for example, hydroxyethylacrylate, hydroxyethylmethacrylate, hydroxypropylacrylate, hydroxypropylmethacrylate, and combinations thereof, particularly hydroxypropylacrylate and/or hydroxyethylmethacrylate. These monomers can be combined with small amounts of acrylic monomers having no alcohol functional groups, such as methyl methacrylate, as described in U.S. Patent 5,019,260. Any suitable meanε may be used to polymerize the suitable monomers onto the fibrous nonwoven web of the present invention. Radiation grafting is the preferred technique to achieve such a result. The source of radiation can be from radioactive isotopes like Cobalt 60, Strontium 90, and Cesium 137, or from machines like x-ray machines, electron accelerators, and ultraviolet equipment. Preferably, however, the radiation is in the form of electron beam radiation. It has been found that, by using this form of radiation, a very uniform distribution of radiation can be provided. This in turn results in a final product which is grafted more uniformly as compared to those fibrous nonwoven webs which are grafted using other radiation sources, e.g., Cobalt 60. Grafting typically will be achieved by either irradiating the fibrous nonwoven web and then expoεing it to a εuitable solution of the monomer or irradiating the fibrous nonwoven web while it is expoεed to a εuitable solution of the monomer. Regardless of which procedure is used, the grafting should be conducted in the absence of oxygen, e.g., under a nitrogen atmosphere, since oxygen will react with the reactive siteε created by radiation exposure, thereby lowering the number of siteε available for the deεired polymer bonding. If the fibrouε nonwoven web iε irradiated prior to immerεion in the monomer solution, the fibrous nonwoven web εhould contact the monomer solution as quickly aε poεεible to avoid undeεirable reactionε resulting in the losε of reactive εites for bonding the polymer to the surface of the fibrous nonwoven web. The monomer solution can comprise any suitable concentration of the monomer to be graft polymerized, typically 1-10 vol. % monomer in a solvent syεtem, generally water by itself or with a suitable alcohol εuch aε t-butyl alcohol. The graft polymerization can be carried out in the preεence or absence of crosεlinking agentε, although crosslinking agents preferably are uεed when the aforementioned acrylate monomers are graft polymerized onto the surface of the fibrous nonwoven web. Any suitable crosslinking agent can be used in the context of the present invention. Suitable crosεlinking agentε include di- or poly-acrylates and methacrylates of diols and polyols, particularly linear or branched aliphatic diols such as ethylene glycol, 1, 2-propylene glycol, diethylene glycol, dipropylene glycol, dipentylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene-oxide glycol, and poly(ethylene oxide-copropylene-oxide) glycol, as well as triol acrylates such as trimethylolpropane triacrylate. The crosslinking agent can be utilized in any suitable amount. Typically, the crosεlinking agent will be added to the grafting solution in an amount of about 0.025 vol.% to about 5 vol.%, more typically in an amount of about 0.05 vol.% to about 2 vol.%. The details and parameters of polymer grafting are well known in the art. While the preεent inventive fibrous nonwoven web is inherently self-supporting at appropriate basis weight, the present inventive fibrous nonwoven web preferably iε mated to a suitable support material, e.g., conventional low preεsure drop, high strength support layer, particularly when the present inventive fibrouε nonwoven web iε prepared without added binder resin. The support layer can comprise any suitable porous structure that provideε the desired support characteristicε while not adversely affecting the desirable properties of the present inventive fibrous nonwoven web, e.g., does not significantly adversely affect flow resistance (ΔP) . Suitable materials for the εupport layer include polymerε εuch aε aramid, polyeεter, polyethylene, polypropylene, and polyamide, as well as glasε and cotton. Ideally, the εupport layer εhould be compatible with the particular environment, e.g., fluid and operating conditionε, within which the preεent inventive fibrouε nonwoven web is intended to operate. Thus, the εupport layer can be of the εame compoεition as the fibrous nonwoven web, although polyester is generally a desirable support material irrespective of the composition of the fibrous nonwoven web. In accordance with another embodiment of the invention, a composite filter is formed by laminating a porous membrane, such aε a PTFE membrane, to the nonwoven fibrouε web of the preεent invention. Preferably, the nonwoven web of the compoεite filter compriεes aramid fibers, and is prepared without a binder resin. The compoεite filter iε formed by laminating a porouε membrane to at least one side, and preferably two oppoεing εides, of the aramid fiber web. When the composite filter is made with a PTFE membrane, the compoεite filter will have the surface properties of polytetrafluoroethylene, and, as such, will withstand aggreεεive chemical and temperature environmentε. The compoεite filter will retain the advantage of providing the titer reduction afforded by the aramid fiber web. Any suitable porous membrane can be used in conjunction with the fibrous nonwoven web of the present invention. Such membranes . include those prepared in accordance with U.S. Patent 4,340,479. Many commercially available membranes are suitable for use with the present invention. For example, a PTFE membrane sold under the tradename EMFLON by Pall Corporation may be used. Other suitable membranes include, for example, polyamide and polyethylene membranes. Preferred microporous membranes that may be uεed in accordance with the preεent invention include those sold by Pall Corporation under the trademarks Ultipor N 6 e®/ Fluorodyne®, and Posidyne®; those available from Cuno Corporation under the trademark Zetapor®, and those sold by Millipore under the trademark Durapore®. Preferably, the membrane has a pore size in the range of about 0.1 μm to about 10 μm, more preferably about 1 μm to about 3 μm. The compoεite filter can be prepared by laminating the membrane to the fibrouε web by any εuitable meanε, εuch aε, for example, by heating and melting the fiberε on the surface of the web and affixing the membrane thereto. The present invention further provides a method of treating, e.g., filtering, a fluid by passing a fluid through the present inventive fibrous nonwoven web. In particular, the filtration of a fluid by passing it through the present inventive fibrous nonwoven web can reduce the quantity of bacteria and preferably viruses in the fluid. Thus, the present inventive filtration method includes pasεing a fluid through the preεent inventive fibrouε nonwoven web, which fluid containε bacteria, e.g., more than about 10 b /ml, and even more than about lOVml, and possibly as much as 10 10 /ml or more, before being passed through the fibrous nonwoven web and contains no bacteria after being passed through the fibrous nonwoven web. More preferably, the present inventive method involves passing a fluid through the present inventive fibrous nonwoven web, which fluid contains at least about lOVml, possibly at least about lOVml, 10 8 /ml, or even 10 10 /ml, viruseε before being passed through said fibrouε nonwoven web and contains lesε than about 10 2 /ml viruεeε after being passed through the fibrous nonwoven web, most preferably wherein the fluid contains no viruses after being passed through the fibrouε nonwoven web. Thuε, the preεent inventive fibrouε nonwoven web is particularly useful in the filtration of fluids to remove bacteria and reduce, if not totally remove, many viruses therefrom, depending upon the particular application and the configuration of the present inventive fibrous nonwoven web. The present inventive fibrous nonwoven web can be used to filter any such fluidε, εuch aε pharmacological and biotechnology process fluids. The excellent removal and handling characteristicε of the preεent inventive fibrouε nonwoven web, as compared to many other fibrous nonwoven web and membrane filtration media, also render the preεent inventive fibrous nonwoven web suitable for a variety of other filtration applications, including most applications in which lesε effective fibrouε nonwoven webε currently are being used, as well as in many applications in which membranes are being uεed and have been considered for uεe. Exemplary uεes of the present inventive fibrous nonwoven web include such applications as a prefilter coalescer for fuel (particularly aircraft fuel, e.g., JP100 fuel), a filter to remove colloidal level iron oxide (which would rapidly plug an equivalent pore rated membrane) , and a filter pad for filter preεεeε. The present inventive fibrouε nonwoven web alεo iε uεeful in a wide variety of other applicationε. For example, the preεent inventive fibrouε nonwoven web can be utilized in "breathable" protective garments, especially when treated with a fluoropolymer, e.g., polytetrafluoroethylene, repellent or other suitable coating. Examples of other suitable coatingε include silicones, waxes, fluorinated ethylene propylene, hydrocarbonε, and combinationε of the foregoing. In a preferred embodiment, the coating is a combination of fluoropolymers selected to render the web oleophobic and hydrophobic. Preferably, the coating comprises a hydrophobic polyolefin in combination with an oleophobic fluoropolymer. Examples of εuitable hydrophobic polyolefins include compounds such as PTFE, FEP, copolymerized ethylene and chlorotrifluoroethylene (ECTFE) , polyvinylidene fluoride (PVDF) , copolymerized ethylene and tetrafluoroethylene (ETFE) , perfluoroalkoxylated polytetra¬ fluoroethylene (PFA PTFE) , and polychlorotrifluoroethylene (PCTFE) . Examples of suitable oleophobic fluoropolymers include fluoroaliphatic esterε, εuch aε FC824 (available from 3M) , fluoroalkyl polymerε εuch aε FX824 (available from 3M) , and releaεe agentε εuch aε Mileaεe F53 and F31X (available from ICI) . A garment in accordance with the preεent invention comprises a fibrous nonwoven web adapted for human apparel. For example, the garment may be a liner for a laboratory smock. Garmentε prepared in accordance with the present invention are suitable for use in adverεe environments such as biological and toxic atmoεphereε. Moreover, εince aramidε are flame/heat-resistant and radiation tolerant, with some chemical resistance, the present inventive fibrouε nonwoven web, at least when prepared from aramid fibers, can be uεed in high temperature and/or radiation environmentε. At very high temperatureε, such as temperatures typically employed in conventional incinerators, the webε of the invention may be fully or nearly fully incinerable; that iε, they will leave no or very little aεh upon incineration, thereby allowing for the efficient diεpoεal of εuch a garment. Preferably, εuch a fully incinerable garment in accordance with the present invention does not contain fibers in addition to the aramid fibers which are not fully or nearly fully incinerable, e.g., glass fibers. In one embodiment, the garment of the invention comprises a composite material. The composite material includes a membrane, such as a PTFE membrane, laminated to the preεent inventive fibrouε web. In a preferred embodiment, the membrane is a fluid-impermeable, gas- permeable PTFE membrane that includes a white pigment, for example, Ti0 2 . Preferably, the pigment has been incorporated into the membrane, for example, during caεting of the membrane. The PTFE membrane provideε protection in harεh chemical environments, and the white color assists in providing maximum thermal protection. The fibrous nonwoven web of the present invention can be formed into any suitable configuration. The present inventive polymeric fibrous nonwoven web can be used in suitable filters, filtration cartridgeε, and the like. The present inventive fibrous nonwoven web can be used in dead¬ end filtration applications, as well as in tangential, cross-flow, and dynamic filtration applications. The preεent inventive fibrouε nonwoven web iε expected to be eεpecially useful in filter elements, such aε filter cartridgeε, which are generally deεcribed in U.S. Patent 4,340,479. Preferred filter elements utilizing the present inventive fibrous nonwoven web comprise the present inventive fibrouε nonwoven web, wherein the εideε of the fibrous nonwoven web have been overlapped and sealed to form a tubular configuration having an exterior surface, an interior, and two ends, and end caps sealed to the ends of the tube, wherein at least one of the end caps has a central aperture providing access to the interior of the tube, and all of the sealε are fluid-tight. The preεent inventive fibrouε nonwoven web preferably is corrugated or pleated in such a filter element so as to provide a large surface area for the volume of the filter element. At leaεt one of the εides of the fibrous nonwoven web can be mated to a porous support layer, and, in such a situation, both the fibrous nonwoven web and porous support layer generally will be corrugated. The filter element can comprise a single fibrous nonwoven web of the preεent invention or, more preferably, multiple such fibrous nonwoven webs mated together. When there are multiple fibrouε nonwoven webε in the filter element, the fibrouε nonwoven webε can be εeparated by a porouε support layer to which each fibrous nonwoven web iε mated, or, when there are two fibrouε nonwoven webε, the fibrouε nonwoven webε can be positioned such that there is no porous support layer therebetween. The other aspects of the filter element may be of any suitable construction and prepared from any suitable material. For example, the end caps can be prepared from a εuitable thermoplaεtic material, such as polyolefin, polyamide, and polyester, particularly, polybutylene glycol terephthalate or polyethylene glycol terephthalate. The filter element can be constructed using techniques which are well-known in the art. The present inventive fibrouε nonwoven web iε expected to be also useful in helically wrapped filter elementε, εuch as filter septa. Such filter elements are generally deεcribed in U.S. Patent 5,290,446. Helically wrapped filter elementε in accordance with the present invention will typically comprise the preεent inventive fibrouε nonwoven web which is helically wrapped around a pervious, hollow tube to form an overlap of 0% to about 95% of the width of the fibrous nonwoven web. There may be one or more such helically wrapped fibrouε nonwoven webε, and there is preferably at least one diffusion layer which allows for lateral flow of fluid within the filter element positioned between the fibrous nonwoven web and the pervious, hollow tube and/or between succeεεive fibrouε nonwoven webε. In accordance with one embodiment of the preεent invention, the preεent inventive fibrouε nonwoven web can be calendared to reduce the thickness and pore rating of the web and to impart the web with a smoother and more uniform surface. Calendaring is useful to reduce the thickness of the web and to improve the thicknesε uniformity of the web. Calendaring, however, can adverεely affect the εtrength characteristics of the web, and, thus, care should be exercised in calendaring the web so as not to adversely affect the strength of the web to such an extent that the web will not have sufficient strength to withstand its intended filtering operation. In addition, the material should not be calendared so as to render the filter nonporous, or so as to adversely affect the pressure drop across the web to a significant extent, as meaεured by, for example, the Gurley air flow across the web. Preferably, the pressure drop across the material will be no more than 10 times aε great after calendaring aε before calendaring. More preferably, the increaεe in preεsure drop will be no more than about 5 to about 7 times as great after calendaring as before calendaring. The calendaring of the support layer can be carried out using any suitable procesε. The calendaring proceεε preferably iε performed in accordance with the method diεclosed in U.S. Patent 5,435,957. In this method, the nonwoven material is calendared between a reεilient roll and a nondeformable roll. The nondeformable roll compriεeε or iε manufactured from any material that will not deform upon contact with the nonwoven material, for example, stainless steel. The reεilient roll compriεeε or is covered with a coating of plastic, rubber, or fibrous material, such as silicone rubber, urethane, nylon, or cotton. Preferably, the roll has a combined fiber and thermoεetting reεin covering, εuch as one of the Beloit Supertex™ materialε. A thorough explanation of theεe proceεε variableε and their effect on the calendaring process may be found in the aforementioned U.S. Patent 5,435,957. The following exampleε further illustrate the preεent invention and, of course, should not be construed aε in any way limiting its scope. Example 1 Thiε example illustrates the fine pore structure in combination with a relatively low air flow resistance which can be a characteristic of the fibrous nonwoven web of the present invention. A series of fibrous nonwoven webε was prepared by conventional fibrous sheet techniques from an aqueous slurry of 25 g/1 aramid fiber which was subjected to mechanical fibrillation for various times up to about 145 hr using a mill mixer in an open tank system with a 6 inch (about 15 cm) diameter rotor having angular slotε against a similarly slotted stator with 0.005-0.008 inch (about 127- 203 μm) clearance. While not intending to bound to any particular theory, it iε believed that the aramid fiber slurry concentration was εuch that the rotor-stator clearance effectively simulated zero or a negative clearance. The resulting fibrous nonwoven webs had thicknesses of about 0.01 inch (about 254 μm) . The air flow resistance at 28 ft/min (about 8.5 m/min) and 1500 cc/min bubble point (ASTM F316-86) was determined for each of the fibrous nonwoven webs. The 1500 cc/min bubble point is the applied presεure resulting in the passage of 1500 cc/min of air through the sheet which is alcohol wet (specifically with isopropyl alcohol), i.e., general air flow through the εheet indicating the expulεion of εubεtantially all the alcohol wetting liquid from the pores of the sheet. The observed air flow resistance and 1500 cc/min bubble point values are set forth in Figure 1, which is a graph of air flow resistance versus 1500 cc/min bubble point for the fibrous nonwoven webs. The 1500 cc/min bubble point is an indication of the pore size of the fibrouε nonwoven sheet, with the higher bubb e points indicative of the smaller pore sizes achievable with increasing processing time of the aramid fiber in the mill mixer. As is apparent from the graph of Figure 1, the present inventive fibrous nonwoven web can be characterized by a relatively high 1500 cc/min bubble point, e.g., about 800 inches water column (about 200 kPa) or more, with a reaεonable air flow reεistance, e.g., about 10-15 psi (about 69-103 kPa) . Example 2 This example illustrates the excellent titer reduction with respect to viruseε of a fibrouε nonwoven web of the preεent invention. A fibrous nonwoven web was prepared in the manner described in Example 1. The fibrous nonwoven web had a thicknesε of about 0.01 inch (about 254 μm) and possessed an air flow resistance at 28 ft/min (about 8.5 m/min) of about 12 psi (about 83 kPa) and a 1500 cc/min bubble point (ASTM F316-86) of about 900 inches water column (about 225 kPa) . Two 142 mm diameter disc sampleε of the fibrous nonwoven web were subjected to a challenge fluid comprising 0.2 μm filtered Tryptin soy broth containing 1.6 x lOVml PR772 coliphage. The challenge was run via a pressure vessel at 100-200 ml/min, with a total of 500 ml effluent being collected for each test disc. The effluent then was tested to determine the titer reduction achieved by each disc. The two disc εampleε were found to have titer reductionε with respect to PR772 coliphage of 4.6 x IO 3 and 5.0 x 10 3 , respectively. Example 3 This example further illustrates the excellent titer reduction with respect to viruses of a fibrous nonwoven web of the present invention. The fibrous nonwoven web of Example 2 was subjected to the same viral titer reduction testing set forth in Example 2, except that the challenge fluid contained 1.5 x 10 8 /ml PR772 coliphage and two layers of the fibrous nonwoven web were used in each of two teεtε. Thuε, the thickness of the tested disc sampleε was about 0.020 inch (about 508 μm) , with a total air flow resistance at 28 ft/min (about 8.5 m/min) of about 24 psi (about 166 kPa) . Each of the two disc samples were found to have titer reductions with reεpect to PR772 coliphage of > 1.5 x IO 8 . Thuε, the preεent inventive fibrous nonwoven web removed all of the PR772 coliphage from the challenge fluid. Example 4 This example illustrates the effect of incorporating a binder into the present inventive fibrous nonwoven web. A series of aramid fiber webs was prepared in accordance with the method set forth in Example 1. The fiberε were εubjected to mechanical fibrillation for approximately 16 hours using a mill mixer in an open tank system with a 6 inch (about 15 cm) diameter rotor having angular slots against a similarly εlotted stator with 0.005-0.008 inch (about 127-203 μm) clearance. The first web waε prepared without a binder. The εecond web waε prepared by passing the aramid fiber web through a solution of an epoxy binder, then drying and curing the web. Further webε were similarly prepared using other binders. The tensile strength of each web was evaluated by gripping a 1-inch (2.54 cm) sample between two jaws, then separating the jaws at a rate of 15 inches (38.1 cm) per minute until the web ruptured. The force at rupture was then evaluated. The tensile strength of each fibrouε web was as follows: Tensile Strength Example Binder lb/in 2 (kPa) 4A None 1.6 (11.0) 4B Epoxy 8.2 (56.5) 4C Phenolic 5.3 (36.5) 4D EVA 7.2 (49.6) 4E KYMENE 10.9 (75.2) 4F FEP 4 (27.6) It is thus seen that the binders increaεed the strength of the fibrous nonwoven web of the present invention. Example 5 Thiε example illustrates the properties of cellulose- aramid fiber composite webs of the present invention. Two nonwoven composite fibrous webs were prepared in accordance with the present invention. The webs were prepared from an aqueous slurry of aramid and cellulose fibrous webs, in a manner similar to that set forth in Example 1. The webε had the following compositions: Example Aramid fibers Cellulose fiberε 5A 50% 50% 5B 25% 75% The web of Example 5A had a thickness of about 0.012 inch (about 304.8 μm) , and that of Example 5B had a thickness of about 0.027 inch (685.8 μm) . The air flow resistance at 28 ft/min (about 8.5 m/min) and 1500 cc/min bubble point (ASTM F316-86) for each of the fibrous nonwoven webs were aε followε: Bubble point Preεεure drop Example inch H 2 0 (kPa) psi (kPa) 5A 450 (112) 9 (65) 5B 100 (25) 5 (34) The webs of thiε example are useful as filter presε pads for filtering yeast from beverage syrups, for example, to obtain a beverage syrup having a reduced yeast level. Example 6 This example further illustrateε the preparation of aramid fiber webε having fine fiber diameterε in accordance with the present invention. Nonwoven aramid fibrous webs were prepared in accordance with the method of Example 1. The webs had a nominal thickness ranging from about 0.004 inch to 0.006 inch (102 μm to 152 μm) , and a basis weight of about 32-43 g/m 2 . The air flow resiεtance at 28 ft/min (about 8.5 m/min) and 1500 cc/min bubble point (ASTM F316-86) were determined for each of the fibrous nonwoven webs. The obεerved air flow reεiεtance and 1500 cc/min bubble point valueε are εet forth in Figure 2, which iε a graph of air flow reεiεtance verεuε bubble point valueε, aε graphed for variouε webs. When the bubble point value exceeds about 900 inch water column (about 225 kPa) , the pore rating of the web is lesε than about 0.05 μm, thus indicating that the web provides a barrier to pesticides, viruses, and bacteria. All of the references cited herein, including publications, patentε, and patent applications, are hereby incorporated in their entireties by reference. While this invention has been deεcribed with an emphaεis upon preferred embodiments, it will be obvious to those of ordinary skill in the art that variationε of the preferred embodimentε may be used and that it is intended that the invention may be practiced otherwise than as specifically deεcribed herein. Accordingly, thiε invention includeε all modificationε encompassed within the spirit and scope of the invention as defined by the following claims.

Claims: WHAT IS CLAIMED IS: 1. A fibrouε nonwoven web characterized by having a titer reduction of at leaεt about 10 1 with reεpect to PR772 coliphage and an air flow reεiεtance of about 30 psi (207 kPa) or lesε at a thickneεε of about 0.02 inch (0.05 cm) or less. 2. The fibrous nonwoven web of claim 1, wherein said fibrous nonwoven web comprises aramid fibers. 3. The fibrous nonwoven web of claim 1 or 2, wherein the web has a titer reduction of at least about IO 8 with respect to PR772 coliphage. 4. The fibrous nonwoven web of any of claims 1-3, wherein said web compriseε cellulose fibers and optionally other fibers selected from the group consisting of polyester fiberε, polytetrafluoroethylene fiberε, cotton fiberε, and glaεε fibers. 5. The fibrous nonwoven web of any of claims 1-4, wherein said web comprises aramid fibers and non-aramid fibers, and said non-aramid fibers are selected from the group consisting of polyester fibers, polytetrafluoroethylene fibers, cotton fibers, glasε fiberε, and celluloεic fiberε. 6. The fibrous nonwoven web of any of claims 1-5, wherein said fibrous nonwoven web has a surface coating of a polymer thereon. 7. The fibrous nonwoven web of claim 6, wherein said polymer has a positive zeta potential. 8. The fibrous nonwoven web of any of claims 1-7, wherein said fibrous nonwoven web has a sheet weight of about 80 g/m 2 or lesε. 9. The fibrous nonwoven web of any of claims 1-8, wherein said fibrous nonwoven web haε an air flow resistance of about 15 psi (103.5 kPa) or lesε. 10. The fibrouε nonwoven web of any of claimε 1-9, wherein said fibrous nonwoven web includes a binder resin. 11. The fibrous nonwoven web of claim 10, wherein said binder resin is selected from the group consisting of polytetrafluoroethylene, fluoroethylene polymer, and epoxy binders. 12. A composite structure comprising the fibrous nonwoven web of any of claims 1-11 laminated to a polymeric membrane on at least one surface of said fibrous nonwoven web. 13. A filter element compriεing a houεing and the fibrouε nonwoven web of any of claims 1-11 or the composite εtructure of claim 12. 14. A method of filtering a fluid comprising passing a fluid through the fibrous nonwoven web of any of claims 1-11 or the composite structure of claim 12. 15. A garment comprising the fibrous nonwoven web of any of claims 1-11 or the composite structure of claim 12 adapted for human apparel. 16. A garment according to claim 15, wherein said fibrouε nonwoven web iε laminated to a nonporouε polytetrafluoroethylene membrane, wherein said nonporous polytetrafluoroethylene membrane includes a white pigment.

Inventors: ADILETTA JOSEPH G, ADILETTA, JOSEPH, G.
Applicants: ADILETTA JOSEPH G, PALL CORP, ADILETTA, JOSEPH, G., PALL CORPORATION
Application year: 1996
Main classification: D04M1
IPC codes: A41D31, B01D39, D04H1, D06M14, D06M15