WO1998050597A1

WO1998050597A1 - Porous medium and method of preparing same

Info

Publication number: WO1998050597A1
Application number: PCT/US1998/009521
Authority: WO
Inventors: Charles Gelman; Timothy J. Gilson
Original assignee: Pall Corporation
Priority date: 1997-05-09
Filing date: 1998-05-08
Publication date: 1998-11-12
Also published as: AU7377198A

Abstract

The present invention provides a multilaminar porous medium comprising a porous substrate and an inorganic material sputtered coating thereon, as well as a method for preparing a multilaminar porous medium comprising sputtering an inorganic material onto a porous substrate. The present invention further provides a fluid treatment method and a filter element utilizing the present inventive multilaminar porous medium.

Description

POROUS MEDIUM AND METHOD OF PREPARING SAME

TECHNICAL FIELD OF THE INVENTION The present invention relates to a porous medium, specifically a multilaminar porous medium comprising a porous substrate and an inorganic coating thereon. The present invention further relates to a method of preparing such a multilaminar porous medium.

BACKGROUND OF THE INVENTION Porous media find wide-ranging uses in disparate applications. For example, porous media are employed in a variety of industrial, academic, and clinical settings such as in filtration, sterilization, dialysis, osmosis, etc. To this end, filters are manufactured with a variety of pore ratings and from a variety of natural (e.g., silk, cotton, glass, ceramic, metallic, etc.) and synthetic (e.g., polymeric, etc.) materials. The characteristics of the pores affect the filtration characteristics of the porous medium in terms of the porous medium's pore rating (e.g., pore size in terms of the size of particulates removed by the porous medium) , as well as the permeation properties through the porous medium (e.g., fluid flow rate through the porous medium or pressure drop across the porous medium) . The material from which the porous medium is constructed affects the physical and chemical properties of the porous medium (e.g., durability, hydrophobicity or hydrophilicity, oleophobicity, electroactivity, magnetic properties, thermal stability, conductivity, chemical stability or reactivity, bioactivity, etc.).

In many applications, a porous medium is prepared from a multiplicity of materials in an effort to take advantage of the desirable properties of each of the materials. Thus, for example, a porous substrate formed from one material can be coated with a different material to take advantage of certain desirable properties of both materials. One type of porous media for use in many applications is a metallized porous medium, which is a porous substrate, usually of polymeric material or glass, coated with an electrically conductive metal (see, e.g., U.S. Patents 3,351,487, 4,804,475, and 5,035,924). The metal coating enhances the electrical conductivity, thermal stability, electromagnetic wave shielding, and antistatic characteristics of the porous medium. Several processes are available for metallizing porous substrates. Metallizing can be achieved by arc- spray metal coating, vacuum metallizing, and the like. Additionally, U.S. Patent 3,351,487 describes a process for metallizing a porous substrate using an ion-exchange membrane. Another process, described in U.S. Patent

4,804,475, involves treating a porous polymeric membrane with an organometallic compound, exposing it to a reducing agent, and then metallizing it in a wet chemical bath. U.S. Patent 5,035,924 describes yet another approach to coating a polymeric porous medium with a metal coating by treating the medium with a polymeric film conducive to wet chemical metallizing. While such chemical metallizing processes have proven generally superior to the arc-spray and vacuum methods, such chemical plating methods are not suitable for many types of porous substrates. Additionally, the chemical plating of metal, particularly on polymeric materials, frequently produces heterogeneous metal layers with poor adhesion strength. Pretreating a polymer substrate to enhance adhesion strength does not entirely eliminate these problems, and many of these pretreatment steps are disadvantageous in that they require the porous substrate surfaces to be treated with highly toxic and caustic reagents . Another approach to metallizing porous media is by vapor deposition (see, e.g., ^". Membrane Sci . , 24 , 297- 307 (1985) ) . Vapor deposition, however, is accomplished through the use of vaporized material, e.g., metal, which necessitates careful control of conditions (e.g., temperature variables) to avoid adversely affecting the physical characteristics (e.g., pore rating and permeation characteristics) of the porous substrate. Moreover, this method only allows for coverage of the entire surface of the porous membrane, many times in a heterogeneous or striped fashion, and only with a relatively thin coating. Thus, there exists a need for an improved multilaminar porous medium, specifically a porous substrate having an inorganic material coating of closely controlled thickness and uniformity on a desired portion of the surface thereof . There is a corresponding need for an improved method of preparing such a multilaminar porous medium which, preferably, does not appreciably compromise the physical characteristics (e.g., pore rating and permeation properties) of the porous substrate, yet is effective in ensuring that a desired portion of the surface of the porous substrate is covered by the inorganic material coating. The present invention provides such a multilaminar porous medium and method of preparing same. These and other 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 multilaminar porous medium comprising a porous substrate and an inorganic material sputtered coating thereon, as well as a method for preparing a multilaminar porous medium comprising sputtering inorganic material onto a porous substrate . The present invention further provides a fluid treatment method and a filter element utilizing the present inventive multilaminar porous medium. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides a multilaminar porous medium comprising a porous substrate and a coating of inorganic material thereon, as well as a method of preparing such a multilaminar porous medium. In addition, the present invention provides a fluid treatment method and a filter element utilizing the present inventive multilaminar porous medium.

Specifically, the present inventive multilaminar porous medium comprises a porous substrate and an inorganic material sputtered coating thereon. The inorganic material sputtered coating can cover substantially (or actually) the entire surface of the porous substrate or a portion thereof. In particular, when the porous substrate is a sheet with two sides and a central portion therebetween, the inorganic material sputtered coating can cover one side (leaving the other side uncovered and the central portion covered or uncovered) or both sides (leaving the central portion covered or uncovered) . While the present inventive multilaminar porous medium can be prepared by any suitable technique, the present inventive multilaminar porous medium desirably is prepared by the present inventive method comprising sputtering inorganic material onto a porous substrate. Preferably, the sputtering is conducted under conditions which do not detrimentally affect the physical properties (e.g., pore rating and permeation characteristics) of the porous substrate.

Any suitable porous substrate can be utilized. The porous substrate can have any suitable physical dimensions and typically will be in sheet form having two opposing sides (e.g., an upstream side and an opposing downstream side, in relation to a fluid to be treated by being passed through the porous medium) with a central portion therebetween. The pores in the porous substrate generally will enable fluid communication between the two opposing sides (e.g., between the upstream and downstream sides) of the porous substrate. The pores can be of any suitable shape, which shape typically will be dictated by the nature of the porous substrate. In that respect, the porous substrate can be of any suitable nature, e.g., a fibrous nonwoven web, a fibrous woven web, a skinless membrane, a skinned membrane, combinations thereof, and the like. The porous substrate can have any suitable pore rating (e.g., ability to remove particles of a given size to a specified degree, as evidenced, for example, by bubble point) . Similarly, the porous substrate can have any suitable permeation properties (as indicated, for example, by water or air flow rate or pressure drop) . In addition, the porous substrate can be of any suitable thickness, e.g., generally about 1-100 μm, although thicker porous substrates of about 1-50 mm can be utilized. If the porous substrate is intended to be substantially (or entirely) covered with the inorganic material sputtered coating, then the porous substrate typically will be less than about 20 μm, preferably less than about 15 μm (e.g., about 1-10 μm) , in thickness. The porous substrate also can be supported by another material (e.g., a screen support) or unsupported.

The porous substrate can be prepared from any suitable material and can be homogeneous or comprised of a combination of materials. Preferably the porous substrate comprises (and more preferably consists essentially, or even consists entirely, of) a polymeric material. Suitable polymeric materials include nylon, polyethersulfone (PES) , fluorinated polymers (e.g., polytetrafluoroethylene [PTFE] ) , polyolefins (e.g, polyvinylchloride [PVC] and polyethyleneterephthalate [PET]), and polyester (e.g., melt-blown polyester).

Alternatively, the porous substrate comprises (and more preferably consists essentially, or even consists entirely, of) glass (e.g., glass fibers), a ceramic, or metal. The porous substrate also can comprise (or consist essentially or entirely of) paper, cotton, or any other material capable of being fashioned into a suitable porous substrate.

The inorganic material sputtered coating on the porous substrate can be of any suitable inorganic substance, e.g., an elemental or molecular species or a combination thereof. The inorganic material desirably adheres to the surface of the porous substrate (and, indeed, preferably, penetrates that surface to some extent to provide excellent adhesion characteristics) . A preferred inorganic material coating comprises (and more preferably consists essentially of, or even consists entirely of) a metal. Most preferably, the metal is thermoconductive, electroconductive, and/or resistant to fungal and microbial growth. Examples of such metals are chromium, copper, gold, nickel, stainless steel, palladium, platinum, silver, silicon and titanium. The inorganic material coating also can comprise an alloy (e.g., a combination of metals). Most preferably, the inorganic material coating comprises palladium or silver. Other preferred inorganic coating materials are ceramic materials, such as indium tin oxide, titanium oxide, silicon dioxide, silicon nitride, and the like.

The inorganic coating can adhere to any portion of the surface of the porous substrate. Thus, the inorganic coating can adhere to only a portion, such as to the edges or to a single side of the substrate, or in spots (e.g., dots) located across the surface of the substrate, or all or substantially all of the surface of the porous substrate. The sputtering of inorganic material onto the porous substrate generally is line-of-sight deposition.

Accordingly, one side or both sides of a porous substrate in sheet form can be coated with the inorganic material sputtered onto the porous substrate, or a portion, such as the edges or desired spots (e.g., dots) of the substrate, can be coated by any suitable technique, e.g., by suitably masking the substrate before sputtering. It is sometimes desirable to form a composite structure by attaching (e.g., laminating) the multilaminar porous medium of the present invention to another entity, which can be porous or nonporous (e.g., a support material, porous membrane, or other filter material) . Although any suitable method of attachment can be used, the sputtered coating on the inventive medium can directly or indirectly provide the means of attachment. For example, the inorganic coating material on the surface can be softened or melted and placed in contact with the other entity, so as to cause the inorganic material, and thus the inventive medium, to become attached to the other entity. Desirably, sputtered dots of inorganic material on the surface of the inventive medium are employed to attach the inventive medium to the other entity (e.g., by inductively heating the dots) .

The composite structure formed by attaching the multilaminar porous medium of the present invention to another entity thus comprises three identifiable subunits: (a) the coating of inorganic material, (b) the porous substrate, and (c) the other entity. If desired, the porous substrate can be wholly or partially eroded (e.g., vaporized or dissolved) from the composite structure. Preferably, the coating of inorganic material is not eroded during this process, but rather remains attached to the other entity after the porous substrate is eroded. For example, a composite structure comprising the multilaminar porous substrate of the present invention, such as a titanium-sputtered polyethersulfone

(PES) membrane, and a porous entity, such as a metal mesh (e.g., a powdered metal membrane), can be heated to vaporize (and thus erode) the PES membrane, and thereby provide a new multilaminar porous medium comprising the titanium sputtered coating attached to the metal mesh.

This technique can be used to decrease the pore rating of the porous entity (e.g., metal mesh) if the pore rating of the inventive porous medium (e.g., titanium-sputtered PES membrane) is lower than the pore rating of the porous entity. Of course, an inorganic material sputtered porous entity, such as an inorganic material sputtered metal mesh or an inorganic material sputtered powdered metal membrane, also can be prepared by sputtering an inorganic material (e.g., titanium) directly onto a suitable porous entity (e.g., a metal mesh) . The pore rating of the porous entity (e.g., metal mesh) is not significantly changed when a thin coating of inorganic material is applied, but can be significantly reduced, if desired, by sputtering a thick coating of inorganic material onto the porous entity.

The inner surface of pores of the porous substrate will be coated with the inorganic material depending on the nature of the pores. Tortuous pores in many polymeric porous substrates, for example, typically will be coated to a depth of up to about 10 μm below the outer surface of the porous substrate. If the porous substrate is sufficiently thin and/or the pores have an appropriate geometry, the inorganic material sputtered on one side of the porous substrate can coat the inner surface of the pores of the porous substrate throughout, i.e., from one side to the other side of the porous substrate. Similarly, if the porous substrate is sufficiently thin and/or the pores have an appropriate geometry, the entire surface (or at least substantially the entire surface) of the porous substrate (including the inner surface of the pores thereof) can be coated with inorganic material. Thus, one embodiment of the present invention involves a porous substrate with an inorganic coating on only one side thereof and with the inner surface of the pores of the porous substrate coated with the inorganic material either throughout (i.e., from one side to the opposing side of the porous substrate) or to a certain depth (e.g., about 1-10 μm) from the coated side of the porous substrate but not along the entire length of the pores to the opposing uncoated side of the porous substrate. Another embodiment of the present invention involves a porous substrate with an inorganic coating on both sides thereof and with the inner surface of the pores of the porous substrate coated with the inorganic material either throughout (i.e., from one side to the opposing side of the porous substrate) or to a certain depth (e.g., about 1-10 μm) from both sides of the porous substrate but not along the entire length of the pores between the sides of the porous substrate.

The inorganic coating can be of any suitable thickness (of a single or multiple layers) . For example, the inorganic coating can be a layer essentially one atom or molecule thick. The inorganic coating also can be considerably thicker, such as 5000 A or more.

Preferably, the inorganic coating is between about 100 A to about 5000 A thick, such as between about 200 A to about 4000 A thick (e.g., between about 400 A to about 2000 A thick) . More preferably, the inorganic coating is between about 100 A to about 1000 A thick, such as between about 200 A to about 800 A (e.g., between about 400 A to about 600 A thick) . Moreover, the inorganic coating need not be of a uniform thickness on the surface of the porous substrate. Preferably, however, the inorganic coating is substantially uniform in thickness on the surface of the porous substrate in order to promote uniformity in the surface qualities of the multilaminar porous medium. The excellent uniformity characteristics of an inorganic material sputtered coating as compared to, for example, a thermal vapor phase deposited coating is a benefit of the present invention (and can be reflected in, e.g., superior electrical properties such as significantly lower resistance values) . Desirably, the inorganic coating is such that it does not detrimentally affect the physical characteristics of the porous substrate. In particular, the inorganic coating preferably does not significantly occlude the pores of the porous substrate, nor does it adversely affect the ability of the porous substrate to be side-sealed, corrugated, or end-capped when used in, for example, a filtration device (e.g., a filter cartridge) . Of course, to the extent the inorganic coating covers the surface of the porous substrate, including the surface of the pores within the porous substrate, there will be some change in size (e.g., narrowing) of the pores of the porous substrate resulting from the inorganic coating. The effect of the inorganic coating on the porosity of the porous substrate typically will depend on the thickness of the inorganic coating relative to the size of the pores of the porous substrate.

Preferably, the sputtered inorganic material has a beneficial effect on the physical characteristics of the porous substrate. The sputtered coating can increase the mechanical stability, wettability and rigidity of the porous substrate, which is desirable in many applications. For example, whether applied over the whole surface or only the edges of the substrate, the sputtered coating can improve the ability of the substrate to be side-sealed and end-capped when used in a filtration device (e.g., a cartridge-type filtration device) . The end capping process, in particular, can be further improved by heating the coating (e.g., via induction heating) .

While the sputtered inorganic material can be used in a manner which does not significantly alter the pore rating of a porous substrate, the sputtered inorganic material also can be used to decrease the pore rating of a porous substrate, if desired. For example, substrates composed of powdered metal often have large pores, and as mentioned above, the pore size of a substrate, such as a metal, polymeric, glass, or ceramic substrate, can be reduced by either directly sputtering a thick coating of inorganic material onto the substrate, or by transferring a sputtered coating to the other entity (e.g., a metal, polymeric, glass, or ceramic substrate) from a porous substrate of the inventive medium having a pore rating lower than that of the other entity.

The porous medium of the present invention is multilaminar because the inorganic coating adheres to the surface of the porous substrate. The adherence of the inorganic coating to the surface of the porous substrate generally will be quite strong, characterized by the inorganic coating actually penetrating into the surface (i.e., below the surface) of the porous substrate. The depth of the penetration of the inorganic coating depends upon the nature of the materials involved. The excellent adhesion characteristics of an inorganic material sputtered coating as compared to, for example, a thermal vapor phase deposited coating is a benefit of the present invention.

The multilaminar porous medium of the present invention desirably is prepared by sputtering an inorganic material onto the desired surface of the porous substrate. The sputtering process can be repeated any suitable number of times to increase the thickness of the inorganic material coating. The sputtering process occurs in a relative vacuum (e.g., pressure of only a few millitorr) . The material to be sputtered (i.e., the inorganic material coating) is placed on a cathode containing magnets beneath its surface. Subsequently, a large direct current voltage is applied to the cathode so as to promote charged particles to attach to the surface.

Concurrently, an inert gas (typically argon) fed in close proximity to the sputtering cathode is ionized by the colliding charged particles. As a progressively larger number of gas atoms become ionized, an ionizing chain reaction occurs, promoting a large number of gas molecules to move toward the cathode, thereby gaining momentum. As the momentum is related to the charge on the ionized gas, it can be controlled by varying the potential and magnetic strength of the cathode. At some point, the momentum of the ionized gas particles exceeds the binding energy of the target atoms or molecules (i.e., the inorganic material coating on the cathode), and collision events cause the inorganic material to escape the surface of the cathode. Argon is a typical inert gas for sputtering, as it can acquire higher momentum than lighter gasses . Thus, when argon is employed as the gas, typically more than one inorganic material atom or molecule (particularly metal atom) escapes the cathode per collision event.

Through the sputtering process, an inorganic material can adhere to the surface of the porous substrate, including the surface of the pores within the porous substrate. Thus, the sputtering process of the present invention results in a multilaminar porous medium with an inorganic material sputtered coating on a desired portion of a porous substrate. The sputtering process permits the inorganic material to be adhered directly to the porous substrate, without the need to pretreat the porous substrate. Moreover, even without pretreatment, the inorganic material adheres very strongly to the porous substrate, and typically (and desirably) penetrates the porous substrate to a significant depth. Additionally, the sputtering process permits fine control over the thickness of the inorganic coating adhered to the porous substrate. Thus, through the sputtering process, a multilaminar porous medium can be manufactured with any desired inorganic material coating thickness, such as previously described herein. By controlling the thickness of the inorganic material adhered to the surface of the porous substrate, the multilaminar porous medium can be constructed without significantly occluding the pores of the porous substrate. Alternatively, the multilaminar porous medium can be constructed so as to purposefully narrow the pores of the porous substrate to provide a multilaminar porous medium with a selected pore rating, differing from the pore rating of the porous substrate. Also, the sputtering process is conducted under conditions (e.g., at temperatures) which do not significantly adversely affect the physical characteristics of the porous substrate (e.g., cause partial melting of the porous substrate when it is prepared from a relatively low melting temperature polymer) . As such, the sputtering process provides a preferred method of preparing multilaminar porous media without significantly compromising the pore rating and permeation properties of the porous substrate.

The present invention will find utility in any suitable application for a porous medium, such as in filtration (e.g., microfiltration, ultrafiltration, nanofiltration, reverse osmosis, etc.), sterilization, dialysis, osmosis, catalysis (e.g., in catalytic converters), diagnostics, medical applications (e.g., as wound or surgical dressings), etc. For example, silicon sputtered multilaminar porous media can be used in applications wherein abrasion resistance is desired (e.g., cross flow and dynamic filtration) . Sterilization applications include those wherein a silver sputtered multilaminar porous medium made from, e.g., polyvinylchloride, is used to fabricate a container, filter, or other device that kills bacteria on contact. Such devices include, for example, bags, tubes, and filters used for the sterile storage, transport and filtration (e.g., an i.v. filter) of sensitive substances (e.g., platelets and other blood products). Catalysis applications include those wherein a platinum sputtered multilaminar porous medium made from, e.g., PTFE, glass fiber, or PES, is used to catalyze a chemical reaction (e.g., reduction of olefins) . Also included are applications wherein a multilaminar porous medium is used as an in si tu filter in a catalyzed reaction. The multilaminar porous medium can be used directly as prepared, or modified in any suitable manner to optimize its properties for specific applications. For instance, the inorganic material sputtered coating can be modified after application to the porous substrate. Thus, a titanium sputtered coating can be oxidized to Ti0₂, and the resulting medium used in applications such as water purification. In addition, the multilaminar porous medium of the present invention can be side- sealed, corrugated, aήd/or end capped for use in filtration devices such as filter cartridges.

As previously discussed, an important criterion in selecting a suitable porous medium is the chemical properties of the porous medium. The present invention provides for porous media of a wide variety of chemical properties (by appropriately selecting the nature of the porous substrate and inorganic material coated thereon) . The multilaminar porous medium of the present invention will prove particularly useful in a variety of applications for which unilaminar porous media, or even currently available metallized membranes, are unsuitable. For example, porous media are often employed in intravenous infusion or in water filters (e.g., for waste or municipal water) ; however, currently available porous media are susceptible to biofouling (e.g., they act as substrates for microbial or fungal growth) . The multilaminar porous medium of the present invention can provide superior service in such end-uses because the inorganic material sputtered coating increases the resistance of the medium to biofouling. An electrical potential can be added to the multilaminar porous medium of the present invention to further improve its resistance to biofouling and/or to clean the medium (e.g., via electrolysis). An electrically conductive sputtered coating can be useful in other applications as well. For example, the multilaminar porous medium of the present invention can be grounded in order to prevent charge build-up on the medium (e.g., when the medium is used in a fluid having a high dielectric constant) . Alternatively, the medium can be used to transfer charge to selected components of a fluid, which can be useful, for example, for the separation of the charged species (e.g., by precipitation) . Indeed, the multilaminar porous medium of the present invention can improve the efficiency of a variety of separation protocols (e.g., osmotic, ultrafiltration, etc.) performed with currently available porous media by virtue of the magnetic and/or electrostatic properties of the inorganic material sputtered coating (e.g,. magnetic and/or charged species can be efficiently separated) . By measuring the change in conductivity that occurs across the multilaminar porous medium of the present invention during its use in filtration applications, the inorganic material sputtered coating can function as a contamination indicator, alerting the user when cleaning or replacement of the medium is required. Other uses for the present inventive multilaminar porous medium will be readily apparent to those of ordinary skill in the art.

Accordingly, the present invention further provides a fluid treatment method and a filter element utilizing the present inventive multilaminar porous medium. The fluid treatment method of the present invention comprises contacting a fluid to be treated with the present inventive multilaminar porous medium, e.g., such that the fluid to be treated passes through the porous medium, while the filter element of the present invention comprises a housing and the present inventive multilaminar porous medium.

The following examples further illustrate the present invention but, of course, should not be construed as in any way limiting its scope. EXAMPLE 1 This example illustrates the performance characteristics of multilaminar porous media comprising a porous substrate of a PES membrane coated on one side with palladium.

Three porous media were constructed, each with a different inorganic material sputtered coating thickness. To construct the multilaminar porous media, representative samples of Pall/Gelman Sciences SUPOR-20θ"^> polyethersulfone membrane having a nominal pore rating of 0.2 μm were sputtered with a coating of palladium to a thickness of 400 A, 800 A, and 3200 A. The palladium coating covered the surface of the pores of the porous substrates to a depth of about 8 μm. Each of these three porous media was characterized in terms of water flow, water bubble point, thickness, wetting properties, and membrane damage. An uncoated porous substrate, consisting of an uncoated Pall/Gelman Sciences SUPOR-200^Φ polyethersulfone membrane from the same lot as that used as the source of the porous substrates for the multilaminar porous media, served as a control . The data are set forth in Table 1.

The following procedure was used to calculate the water flow of the samples: A circular sample of the material being tested was wetted with de-ionized water (100%) and inserted into a filtration apparatus. Water was filtered through the sample, and the back pressure and time required to pass 100 ml of water through the sample were noted. The water flow was then calculated according to the following formula: Water flow = water flow rate (ml/min) /area of sample tested (cm²) /back pressure (kPa) .

The water bubble point was determined as described in ASTM F316-86, corresponding to the applied pressure resulting in the formation of the first bubble through the medium which is water wet. Table 1 : PES Membrane with Palladium Coating

As indicated by the data in Table 1, the inorganic material sputtered coating resulted in only a minor change in the pore rating (e.g., largest effective pore size) of the porous substrate. Specifically, the multilaminar porous media of the present invention had bubble points of about 6-9% higher than the control (uncoated) porous substrate. The relatively thinner inorganic material sputtered coating resulted in only minor occlusion of the pores of the porous substrate. In particular, the water flow of the multilaminar porous medium with a 400 A coating was not significantly different than the control (uncoated) porous substrate, although the thicker coatings resulted in decreased water flows (about 30% decreased flow with the 800 A coating and about 80% decreased flow with the 3200 A coating as compared to the control porous substrate) . As expected, the palladium coating imparted hydrophobicity to the porous substrate. No damage to the porous substrate, with reference to the non-coated side of the porous medium, was observed with the porous medium comprising the 400 A and 800 A thick palladium coatings. Some damage was observed on the non-coated side of the porous medium comprising the 3200 A thick palladium coating.

These results indicate that the inorganic material sputtered coating did not result in significant alteration of the physical characteristics (e.g., pore rating and permeation characteristics) of the porous substrate, particularly with respect to the coatings less than 1000 A thick, especially less than 500 A thick.

EXAMPLE 2 This example illustrates the performance characteristics of multilaminar porous media comprising a porous substrate of a PTFE membrane coated on one side with palladium.

Three porous media were constructed, each with a different inorganic material sputtered coating thickness. To construct the multilaminar porous media, representative samples of polytetrafluoroethylene membrane, supported on one side with a polypropylene screen support, and having a nominal pore rating of 0.2 μm, were sputtered with a coating of palladium to a thickness of 400 A, 800 A, and 3200 A.

Each of these three porous media was characterized in terms of water break-through, isopropyl alcohol bubble point, air flow rate, thickness, and membrane damage. An uncoated porous substrate, consisting of an uncoated polytetrafluoroethylene membrane from the same lot as that used as the source of the porous substrate for the multilaminar porous media, served as a control. The data from these evaluations are set forth in Table 2.

The isopropyl alcohol bubble point was determined as described in ASTM F316-86, corresponding to the applied pressure resulting in the formation of the first bubble through the medium which is isopropyl alcohol wet. Table 2: PTFE Membrane with Palladium Coating

As indicated by the data in Table 2, the inorganic material sputtered coating did not significantly affect the physical characteristics (e.g., pore rating and permeation characteristics) of the porous substrate. In particular, the isopropyl alcohol bubble point and water break-through point remained substantially the same, irrespective of the thickness of the palladium coating. The air flow rate of the multilaminar porous media decreased from about 24% to about 34% over the control porous substrate depending upon the thickness of the coating. These data are consistent with the increased thickness of the porous media having thicker layers of palladium. The decrease in air flow rate does not appear to represent significant pore occlusion. Additionally, while some damage of the porous substrate was observed with respect to the porous substrates coated with 800 A and 3200 A of palladium, these porous substrates were coated on the polypropylene side (i.e., the screen support side) of the porous substrate. No such damage of the porous substrate was observed with respect to the porous substrate coated with 400 A of palladium, which porous substrate was coated on the non-support side (i.e., the side opposing the screen support side) of the porous substrate . These results indicate that the inorganic material sputtered coating did not result in significant alteration of the physical characteristics (e.g., pore rating and permeation characteristics) of the porous substrate, particularly with respect to the coatings less than 500 A thick.

EXAMPLE 3 This example illustrates the performance characteristics of multilaminar porous media comprising a porous substrate of a glass fiber medium coated on one side with palladium.

Three porous media were constructed, each with a different inorganic material sputtered coating thickness. To construct the multilaminar porous media, representative samples of Pall/Gelman Sciences A/E^'* glass fiber media were sputtered with a coating of palladium to a thickness of 400 A, 800 A, and 3200 A.

Each of these three porous media was characterized in terms of water flow rate (measured as set forth in Example 1) and damage. An uncoated porous substrate, consisting of an uncoated Pall/Gelman Sciences A/E^® glass fiber medium from the same lot as that used as the source for the multilaminar porous media, served as a control. The data from these evaluations are set forth in Table 3

Table 3 : Glass Fiber Medium with Palladium Coating

As indicated by the data in Table 3, the metallic coating did not significantly affect the porous structure of the porous substrate. In particular, the water flow of the porous substrate was not significantly altered irrespective of the thickness of the palladium coating. Moreover, no damage to the glass fiber medium was detected with any of the amounts of palladium sputtered onto the porous substrates. Thus, these results indicate that the coating did not result in significant alteration of the physical characteristics of the porous substrate.

EXAMPLE 4 This example illustrates the performance characteristics of a multilaminar porous medium comprising a porous substrate of a PES membrane coated on one side with silver.

To construct the multilaminar porous medium, a representative sample of Pall/Gelman Sciences SUPOR-200^* polyethersulfone membrane having a nominal pore rating of 0.2 μm was sputtered with a coating of silver to a thickness of approximately 200-500 A.

The porous medium was characterized in terms of water bubble point (measured as set forth in Example 1) , water flow rate (measured as set forth in Example 1) , and membrane damage. An uncoated porous substrate, consisting of an uncoated Pall/Gelman Sciences SUPOR-200^® polyethersulfone membrane, from the same lot as that used as the source of the porous substrate for the multilaminar porous medium, served as a control. The data from these evaluations are set forth in Table 4.

Table 4 : Polyethersulfone Membrane with Silver Coating

As indicated by the data in Table 4, the inorganic material sputtered coating did not significantly change the pore rating (e.g., largest effective pore size) of the porous substrate. Specifically, the bubble point of the multilaminar porous medium of the present invention was not substantially different than the control

(uncoated) porous substrate. Similarly, the inorganic material sputtered coating did not result in significant occlusion of the pores of the porous substrate. In particular, the water flow rate of the multilaminar porous medium with a 200-500 A coating was not substantially different than the control (uncoated) porous substrate. No damage to the porous substrate, with reference to either the coated or the non-coated side of the porous medium, was observed. These results indicate that the inorganic material sputtered coating did not result in significant alteration of the physical characteristics (e.g., pore rating and permeation characteristics) of the porous substrate .

EXAMPLE 5

This example illustrates the performance characteristics of a multilaminar porous medium comprising a porous substrate of a PES membrane coated on one side with platinum. To construct the multilaminar porous medium, a representative sample of Pall/Gelman Sciences SUPOR-200^Φ polyethersulfone (PES) membrane was sputtered with a coating of platinum (Pt) to a thickness of 600 A.

The multilaminar PES-based medium was characterized in terms of water bubble point (measured as set forth in Example 1) , water flow rate (measured as set forth in Example 1), and thickness. The uncoated porous substrate of Example 4 served as a control . The data from these evaluations are set forth in Table 5.

Table 5: PES and Glass Fiber Media with Platinum Coating

As indicated by the data in Table 5, the metallic coating did not substantially affect the porous structure of the porous substrate. In particular, the water bubble point and water flow rate increased modestly. Moreover, no damage to the polyethersulfone membrane was detected after sputtering platinum onto the porous substrate. Thus, these results indicate that the coating did not result in substantial alteration of the physical characteristics of the porous substrate.

The examples herein indicate that multilaminar porous media according to the present invention can be prepared by sputtering an inorganic material, especially a metal, onto any suitable porous substrate to any suitable coating thickness. Moreover, through the sputtering method, the coating can be applied to the porous substrate in such a manner to strongly adhere to the porous substrate while, if desired, not significantly affecting the pore structure of the porous substrate (e.g., so as not to appreciably occlude the pores of the porous substrate) .

All of the references cited herein, including patents, patent applications, and publications, are hereby incorporated in their entireties by reference While this invention has been described with an emphasis upon preferred embodiments, it will be obvious to those of ordinary skill in the art that variations of the preferred embodiments may be used and that it is intended that the invention may be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications encompassed within the spirit and scope of the invention as defined by the following claims.

Claims

WHAT IS CLAIMED IS:

1. A multilaminar porous medium comprising a porous substrate and an inorganic material sputtered coating thereon.

2. The multilaminar porous medium of claim 1, wherein said inorganic material sputtered coating covers substantially the entire surface of said porous substrate.

3. The multilaminar porous medium of claim 1, wherein said porous substrate is a sheet with two sides, and said inorganic material sputtered coating covers one side of said porous substrate.

4. The multilaminar porous medium of claim 1, wherein said porous substrate is a sheet with two sides and a central portion therebetween, and said inorganic material sputtered coating covers both sides of said porous substrate while leaving said central portion uncovered with said inorganic material.

5. The multilaminar porous medium of any of claims 1-4, wherein said porous substrate comprises a polymeric material .

6. The multilaminar porous medium of claim 5, wherein said polymeric material is selected from the group consisting of polyethersulfone, polytetrafluoroethylene, and polyethyleneterephthalate.

7. The multilaminar porous medium of claim 6, wherein said polymeric material is polyethersulfone.

8. The multilaminar porous medium of any of claims 1-4, wherein said porous substrate comprises a material selected from the group consisting of glass, ceramics, and metal .

9. The multilaminar porous medium of any of claims 1-8, wherein said inorganic material comprises a metal.

10. The multilaminar porous medium of claim 9, wherein said metal is selected from the group consisting of copper, gold, nickel, palladium, platinum, titanium, silicon, and silver.

11. The multilaminar porous medium of claim 10, wherein said metal is palladium.

12. A method for preparing a multilaminar porous medium comprising sputtering inorganic material onto a porous substrate .

13. The method of claim 12, wherein said inorganic material sputtered coating covers substantially the entire surface of said porous substrate.

14. The method of claim 12, wherein said porous substrate is a sheet with two sides, and said inorganic material sputtered coating covers one side of said porous substrate .

15. The method of claim 12, wherein said porous substrate is a sheet with two sides and a central portion therebetween, and said inorganic material sputtered coating covers both sides of said porous substrate while leaving said central portion uncovered with said inorganic material .

16. The method of any of claims 12-15, wherein said porous substrate comprises a polymeric material .

17. The method of claim 16, wherein said polymeric material is selected from the group consisting of polyethersulfone, polytetrafluoroethylene, and polyethyleneterephthalate .

18. The method of claim 17, wherein said polymeric material is polyethersulfone.

19. The method of any of claims 12-15, wherein said porous substrate comprises a material selected from the group consisting of glass, ceramics, and metal.

20. The method of any of claims 12-19, wherein said inorganic material comprises a metal .

21. The method of claim 20, wherein said metal is selected from the group consisting of copper, gold, nickel, palladium, platinum, titanium, silicon, and silver.

22. The method of claim 21, wherein said metal is palladium.

23. A method of treating a fluid comprising contacting a fluid to be treated with the multilaminar porous medium of any of claims 1-11.

24. The method of claim 23, wherein said fluid to be treated is passed through said multilaminar porous medium.

25. A filter element comprising a housing and a multilaminar porous medium of any of claims 1-11.