ELECTROMAGNETIC INTERFERENCE SHIELDING FILTER

Abstract: A porous composite material made of porous electrically conductive materials coated with or laminated to porous hydrophobic or oleophobic materials. The porous composite material has excellent electromagnetic radiation shielding properties and resists passage of particles through it while permitting air or other gases to flow through it.

Description: TITLE Electromagnetic Interference Shielding Filter FIELD OF THE INVENTION This invention relates to electromagnetic radiation shielding materials and, more particularly, to porous , oleophobic, hydrophobic, chemical- and gas-resistant materials having high electromagnetic radiation shielding characteristics and high particle filtration performance for use in electronic equipment enclosures. BACKGROUND OF THE INVENTION Many shielded electronic equipment enclosures and housings provide internal ventilation, temperature control and ambient pressure equilibration by air circulation through them. Air passage into the enclosures is through electromagnetic radiation shielding devices and air filters located in the enclosure vents. It is expected of these shielding devices and filters that they prevent, or reduce to acceptable levels, passage of electromagnetic radiation or interference (EMI); and that they also prevent passage of particulate materials, to specified size levels; while minimizing impediments to flow of air through the enclosure. For example, metal honeycomb assemblies are very effective in shielding against EMI, however, they are quite bulky, ranging in thickness from about 1/2" to several inches depending on the electromagnetic radiation frequency range over which they are providing shielding. Furthermore, the honeycomb assemblies are usually arrays of open tubes that provide no barrier properties to passage of dust, dirt, or other particles through them and, consequently, require that separate filtration units be included in the equipment if prevention of passage of particles into the enclosure is important. In general, for installations in which there is great flexibility in space and weight considerations TIT E HEET effective EMI-shielding filter assemblies can be constructed from methods and materials well known in the art. On the other hand, for install tions in small, portable electronic devices such as hand-held nuclear radiation detectors, hip-mounted air samplers, and other electronic sensors and instruments or, in mobile electronic systems such as radio/telephone communication systems, missile guidance and control systems, and the like, in which there are severe constraints on the size and weight of components, there are few suitable materials from which compact, lightweight, effective EMI-shielding air filters capable of operating in a broad range of environments can be constructed. SUMMARY OF THE INVENTION This invention provides a lightweight porous material that has high electromagnetic radiation shielding properties; excellent barrier properties to fine liquid and solid particulates; high resistance to wetting and penetration by water and oils; high resistance to attack or damage by chemicals; and excellent air flow characteristics. Combination of the preceding stated properties in a single material has been heretofore unknown. The material of the invention can be readily fabricated by conventional means to form compact, space- and weight-saving air vent filters for electronic equipment that protect the equipment from adverse influence or damage by electromagnetic radiation or intrusion by airborne particles, and which can be used in almost any indoor or outdoor environment without loss of effectiveness. Broadly described, the material of the invention is a porous composite material comprising a porous electrically-conductive material to which is adhered a porous hydrophobic material or a porous oleophobic material. More specifically, referring to the drawings, Figure 1 illustrates embodiments having a layer of porous electrically- conductive material laminated to an outer protective layer of porous hydrophobic material (Figure la) or of porous oleophobic material (Figure lb). Figure 2 illustrates embodiments having a porous protective layer laminated to each side of a porous electrically-conductive layer. The porous outer protective layer of one embodiment is a porous hydrophobic material (Figure 2a) and in the other embodiment the porous outer protective layer is a porous oleophobic material (Figure lb). Figure 3 illustrates another embodiment of the invention, having a single layer, of porous electrically-conductive material which has been coated with an oleophobic material. Porous is used herein to describe materials that allow passage of gases, especially air. These are materials that comprise interconnected pores and voids that form passages extending through the thickness of the material. The passageways open on both sides of the material. BRIEF DESCRIPTION OF THE DRAWINGS Figures la and lb are exploded cross-section views of two- layer embodiments of the invention. Figures 2a and 2b are exploded cross-section views of three- layer embodiments of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Referring to Figure 1, two-layer embodiments of the invention comprise a porous layer of electrically conductive material 2 adhered to a porous outer protective layer of porous hydrophobic material 5 or porous oleophobic coated material 15 by a porous layer of adhesive 3. The electrically conductive material 2 may be any porous material having sufficient electrical conductivity and having a structure such that incident electromagnetic radiation presented to it will be substantially reflected, intercepted, or otherwise attenuated. Such electrically conductive materials include woven wire mesh, sintered porous metals, metal sponges, etc. Another electrically-conductive material is film or sheet of porous expanded polytetrafluoroethylene containing electrically-conductive T TE SHEET particles. Such a particle-filled sheet or film can be prepared by coagulation of a mixture of electrically-conductive particles and polytetrafluoroethylene particles suspended in an aqueous dispersion and processing the mixture into a porous expanded form as described in U.S.Patents Nos. 3,953,566 and 4,187,390. Suitable electrically-conductive filler particles include metal particles such as silver, gold, nickel, stainless steels, and the like or, preferably, high conductivity carbon particles. The materials most preferred for the electrically-conductive layer 2 are porous non-woven sheet or film of synthetic polymers. Such porous non-woven sheet or film may be prepared from the classes of polymers including, but not limited to, polyamides, polyesters, polyolefins, polyurethanes, fluoropolymers, and the li e, by fiber processes known in the art such as spin bonding, felting, paper-making, and the like, or by other pore-forming processes known in the art, for example, foaming, stretching, or expansion processes. The non-woven sheet or film are made electrically conductive by application of metals to their surfaces by metallizing processes such as vapor deposition, sputter coating, and the like, or preferably, by electroless chemical deposition processes. U.S. Patent Nos. 4,557,957 and 4,720,400 describe electroless chemical plating methods suitable for metallizing internal pore surfaces as well as external surfaces of porous expanded polytetrafluoroethylene. The methods described can also be used to plate other porous synthetic polymer film and sheet which, although not as inherently hydrophobic as polytetrafluoroethylene, are substantially resistant to penetration of water and aqueous chemical solutions. The porous outer protective layer of porous hydrophobic material 5 or porous oleophobic coated material 15 forms the outermost layer of the layered material of the invention, serving as a barrier to passage of liquid and solid particulates in the local environment while allowing passage of gases through it, for example, into or out of an equipment enclosure for cooling purposes or pressure equilibration. The porous hydrophobic outer protective layer 5 can be any material having suitable air flow and filtration characteristics and which is inherently hydrophobic or oleophobic or, which can be treated to develop hydrophobic and/or oleophobic properties in it. The material can be a fabric, such as a non-woven, woven, or knit fabric. The material can be made as a paper, for example, a synthetic or natural filter paper or cloth. The material can also be a porous film or membrane of synthetic polymers including polya ides, polyesters, polyolefins such as polyethylene or polypropylene, polyacrylics, polyvinylidene fluoride, and the like. Preferred as a gas permeable material for the porous hydrophobic outer protective layer 5 is porous polytetrafluoroethylene, most preferably, porous expanded polytetrafluoroethylene membrane or film made as described in U.S. Patent No. 3,953.566, which has a porous microstructure of nodes interconnected by fibrils. The resulting pores or voids allow for good gas or air flow, provide excellent particle filtration properties, and, due to the high inherent hydrophobicity of polytetrafluoroethylene, resistance to penetration of liquid water or aqueous solutions. Additionally, the porous expanded polytetrafluoroethylene membrane or film imparts excellent chemical resistance and broad temperature range of use to the material of the invention due to the chemical inertness and thermal properties of polytetrafluoroethylene. The porous expanded polytetrafluoroethylene and other porous gas permeable materials described above can be coated with an amorphous copoiymer of tetrafluoroethylene and perfluoro-2,2- dimethyl-l,3-dioxole to significantly enhance their hydrophobic properties and to develop oleophobic properties in them. The copoiymer is available from the DuPont Company as Teflon (R) AF 1600 in which the dioxole concentration is about 65 mole percent, and as Teflon (R) AF 2400 in which the dioxole concentration is about 85 mole percent. The oleophobic copoiymer coating is applied from a dilute solution, for example, a solution of perfluoro-(2- butyltetrahydrofuran) solvent (also known as Fluoroinert-75, or FC- 75, from Dow Chemical Co.) containing copoiymer solids in the concentration range 0.01 to 5.0 wt. percent. The solution can be applied to the porous permeable materials by conventional means such as spraying, transfer roll coating, dip coating, brushing, and the like, after which the solvent is dried by any convenient means. SUBSTITUTE SHEET The coating coats all the outside surfaces and at least a portion of the interior surfaces of the pores without blocking the pores or excessively reducing the void volume of the porous material to which it is applied. The adhesive of the porous layer of adhesive 3 may be selected from many well known in the art. The adhesives can be thermopl stic, thermosetting, or reaction curing types, in liquid or solid form, selected from the classes including, but not limited to, polyamides, polyacrylamides, polyesters, polyolefins, polyurethanes, and the like. The adhesive must be applied so that it forms a porous gas permeable layer which minimizes resistance to air flow while adhering the porous outer protective layer 5 to the porous electrically-conductive layer 2. Suitable application means include gravure printing, spray coating, powder coating, and the like. A preferred adhesive is a thermoplastic polymer in the form of a non-woven web or mesh. The non-woven thermoplastic web or mesh is placed between the porous electrically-conductive layer 2 and the porous hydrophobic or oleophobic outer protective layer 5 and, by applying heat and pressure, for example, by a hot-roll and a pinch- roll or by a heated press, the thermoplastic adhesive web is melted and the porous electrically-conductive material layer 2 is adhered to the porous hydrophobic or oleophobic material of the outer protective layer 5, thus forming the porous layered material of the invention. For process and cost considerations it may be desirable to alter the order in which the electroless chemical plating, oleophobic coating, and/or lamination take place. For example, electroless chemical plating of a porous material laminated to an oleophobic outer protective layer can be done after lamination without difficulty and without loss of function. Likewise, oleophobic coatings can be applied to single layers or laminated constructions equally well. By the same token, for applications where an outer protective layer is deemed unnecessary or undesirable, a second embodiment, shown in Figure 3, of the porous composite material of the invention which comprises a single layer 12 of porous electrically- conductive material coated with the amorphous copoiymer of tetrafluoroethylene and perfluoro-2,2-dimethyl-l,3-dioxole can be used. The porous electrically-conductive material, selected from the classes and prepared as described above, provides the same level of EMI shielding as in the laminated form and, when coated with the amorphous copoiymer of tetrafluoroethylene and perfluoro - 2,2-dimethyl-l,3-dioxole as described above, exhibits good hydrophobic and oleophobic properties while having good gas or air flow through the material. Three-layer embodiments of the invention are shown in Figure 2. These embodiments comprise a porous layer of electrically- conductive material 2 adhered to a porous outer protective layer of porous hydrophobic material 5 or porous oleophobic coated material 15 by a porous layer of adhesive 3, all as specified above in the description of the broadest embodiment of the invention. It further comprises a porous inner protective layer 6 adhered to the inside surface of the electrically-conductive layer 2 by a porous layer of adhesive 4. The porous inner protective layer 6 prevents contact with the porous electrically-conductive material layer 2, thus reducing the likelihood of damage to the porous electrically- conductive layer during assembly, handling, and use. The porous inner protective material layer 6 provides further protection by preventing migration of any particles that may be shed by the porous electrically-conductive material layer 2. The porous inner protective material layer 6 can be of any material having suitable airflow and filtration characteristics and also having excellent non-1inting or non-shedding characteristics. The material can be a fabric or paper as described above with respect to the material of the porous outer protective layer, however, more preferable materials are porous films, membranes, non-woven webs and meshes of synthetic polymers also described above. Porous polytetrafluoroethylene, preferably porous expanded polytetrafluoroethylene membrane or film made as described in U.S. Patent No. 3,953,566 is most preferred as the material for the porous inner protective layer 6. In addition to the properties described earlier, the porous expanded polytetrafluoroethylene membrane or film has especially good non-linting and non-shedding characteristics. BSTITUTE SHEET Selection, application, and processing of the material of the porous layer of adhesive 4 is as described above with respect to the porous layer of adhesive 3, however, referring to Figure 2, the materials forming the porous layers of adhesive 3, 4 may be, but are not necessarily, the same. It may be desirable, for process considerations, to use a different material in each porous layer of adhesive, for example, a different adhesive polymer in each layer or an adhesive polymer type having a different melt point in each layer * TEST PROCEDURES Air Permeability The resistance of samples to air flow was measured by a Gurley densometer (manufactured by W. & L.E. Gurley & Sons) in accordance with Test Standard ASTM D726-58. The results are reported in terms of Gurley-number (also Gurley-seconds) which is the time in seconds for 100 cubic centimeters of air to pass through 1 square inch of a test sample at a pressure drop of 4.88 inches of water. Oil Repel!ency The resistance of samples to wetting by oily liquids was tested by challenging them with drops of test liquids as specified in AATCC Test Method 118-1989. The test liquids are a series of eight liquid hydrocarbons of different surface tensions, small drops of which are placed on the sample surface and observed for wetting. The Oil Repellency Rating is the highest numbered test liquid which does not wet the test surface. Testing was done according to the test method except that samples smaller than 8 in. x 8 in. were used and the samples were not necessarily conditioned as specified prior to testing. Electromaqnetic Interference Shielding Effectiveness EMI Shielding Effectiveness of test samples was measured according to ASTM Standard Test Method D4935-89 for measuring the Electromagnetic Shielding Effectiveness of Planar Materials; modified for use in measuring in the frequency range 1 to 18 Gigaherz (GHz). Shielding Effectiveness (SE) is reported in decibels (db) of attenuation. EXAMPLES Example 1 A layered porous material of the invention having a hydrophobic outer protective layer was prepared as follows: A first porous expanded polytetrafluoroethylene membrane, approximately 0.020 inches thick, was made electrically-conductive by electroless chemical plating the membrane with a first layer of copper and a second layer of nickel by the methods set forth in U.S.Patent Nos. 4,557,957 and 4,720,400. Properties of the membrane before and after plating are tabulated below. Before After Plating Plating Density, g/cc 0.24 0.5 Pore Volume, % 89 77 Volume Resistivity, ohm-cm >1 megohm 0.004 A second porous expanded polytetrafluoroethylene membrane, approximately 0.003 inches thick, and the electrically-conductive membrane were laminated together by interposing a layer of Sharnet 4200 (non-woven polyester adhesive web, manufactured by Applied Extrusion Technologies, Inc.) between them and applying heat and pressure to the assembly by passing the assembly through the nip of a hot-roll and a pinch-roll. The finished layered material of the invention was tested and SUBSTITUTE SHEET found to have Air Permeability of 5.6 Gurley-seconds and EMI Shielding Effectiveness of -70 decibels. Example Z A layered porous material of the invention having a hydrophobic outer protective layer was prepared as follows: A spun-bonded polyester fiber web (Reemay #2033, manufactured by Reemay, Inc.), approximately 0.020 inches thick, was made electrically-conductive by electroless chemical plating the web with a first layer of copper and a second layer of nickel as described in Example 1 above. Properties of the Reemay 2033 web before and after plating are tabulated below. Before After Plating Plating Density, g/cc 0.23 0.4 Pore Volume, % 85 71 Volume Resistivity, ohm-cm >1 megohm 0.003 The electrically-conductive Reemay 2033 polyester fiber web was laminated to a section of the 0.003 inch thick porous expanded polytetrafluoroethylene membrane described above using the materials and methods also described in Example 1 above. The finished layered material of the invention was tested and found to have Air Permeability of 1.9 Gurley-seconds and EMI Shielding Effectiveness of -85 decibels. Example 3 A layered porous material of the invention having an oleophobic outer protective layer was prepared as follows: A 0.003 inch thick porous expanded polytetrafluoroethylene membrane was coated with Teflon AF 1600 Amorphous Fluoropolymer to make it oleophobic. The membrane was immersed in a bath containing a solution of 0.5 % Teflon AF 1600 in FC-75 solvent after which it was oven-dried to remove the solvent. The membrane had an Oil Repellency Rating of 0 before coating and 6 after coating. The oleophobic porous expanded polytetrafluoroethylene was laminated to a section of the electrically-conductive Reemay 2033 polyester fiber web described in Example 2 with Sharnet 4200 adhesive as described in Example 1. The finished layered material of the invention was tested and was found to have Air Permeability of 36 Gurley-seconds, Oil Repellency Rating of 4, and EMI Shielding Effectiveness of -85 decibels. Example 4 A layered porous material of the invention having an oleophobic outer protective layer was prepared as follows: Porous spun-bonded polyester fiber web (Reemay #2033) was laminated to the oleophobic porous expanded polytetrafluoroethylene membrane described in Example 3 above. Lamination of the layers was effected using Sharnet 4200 adhesive with heat and pressure applied by passing them through the nip of a hot-roll and a pinch- roll. The spun-bonded polyester web layer was then selectively made electrically-conductive by electroless chemical plating it with a first layer of copper and a second layer of nickel. The finished layered material of the invention was tested and found to have Air Permeability of 23 Gurley-seconds, Oil Repellency Rating of 3, and EMI Shielding Effectiveness of -75 decibels. BSTITUTE SHEET

Claims: CLAIMS: 1. A porous composite material having electromagnetic radiation shielding properties comprising a layer of porous electrically-conductive material adhered to a layer of porous hydrophobic material. 2. The porous composite material of claim 1, wherein the porous electrically-conductive material is a porous metal mesh. 3. The porous composite material of claim 2, wherein the porous electrically-conductive material is a metallized porous synthetic polymer fabric. 4. The porous composite material of claim 3, wherein the porous electrically-conductive material is a metallized porous non- woven polyester web. 5. The porous composite material of claim 4, wherein the porous electrically-conductive material is a metallized porous expanded polytetrafluoroethylene film. 6. The porous composite material of claims 1,2,3,4 or 5, wherein the porous hydrophobic material is porous expanded polytetrafluoroethylene film. 7. A porous composite material having electromagnetic radiation shielding properties comprising a layer of porous electrically-conductive material adhered to a layer of porous material coated with an amorphous copoiymer of tetrafluoroethylene and perfluoro-2,2-dimethyll,3-dioxole. 8. The porous composite material of claim 7, wherein the porous electrically-conductive material is a porous metal mesh. 9. The porous composite material of claim 8, wherein the porous electrically-conductive material is a metallized porous synthetic polymer fabric. 10. The porous composite material of claim 9, wherein the porous electrically-conductive material is a metallized porous non- woven polyester web. 11. The porous composite material of claim 10, wherein the porous electrically-conductive material is a metallized porous expanded polytetrafl oroethylene film. 12. The porous composite material of claims 7,8,9,10 or 11, wherein the porous material coated with an amorphous copoiymer of tetrafluoroethylene and perfluoro-2,2-dimethyl-l,3-dioxole is porous expanded polytetrafluoroethylene film. 13. A porous composite material having electromagnetic radiation shielding properties comprising a layer of porous electrically-conductive material to each side of which is adhered a porous expanded polytetrafluoroethylene film. 14. The porous composite material of claim 13, wherein the porous electrically-conductive material is a porous metal mesh. 15. The porous composite material of claim 14, wherein the porous electrically-conductive material is a porous metallized synthetic polymer fabric. 16. The porous composite material of claim 15, wherein the porous electrically-conductive material is a porous metallized non- woven polyester web. 17. The porous composite material of claim 16, wherein the porous electrically-conductive material is a metallized porous expanded polytetrafluoroethylene film. 18.. A porous composite material having electromagnetic radiation shielding properties comprising a layer of porous electrically-conductive material to one side of which is adhered a porous expanded polytetrafluoroethylene film coated with an amorphous copoiymer of tetrafluoroethylene and perfluoro-2,2- dimethyl-l,3-dioxole, and to the opposite side of which is adhered an uncoated porous expanded polytetrafluoroethylene film. 19. The porous composite material of claim 18, wherein the porous electrically-conductive material is a porous metal mesh. 20. The porous composite material of claim 19, wherein the porous electrically-conductive material is a metallized porous synthetic polymer fabric. 21. The porous composite material of claim 20, wherein the porous electrically-conductive material is a metallized porous non- woven polyester web. 22. The porous composite material of claim 21, wherein the porous electrically-conductive material is a metallized porous expanded polytetrafluoroethylene film. 23. A porous composite material having electromagnetic radiation shielding properties comprising a porous electrically- conductive material coated with an amorphous copoiymer of BSTITUTE SHEET tetrafluoroethylene and perfluoro-2,2-dimethyl-1,3-dioxole. 24. The porous composite material of claim 23, wherein the porous electrically-conductive material is a porous metal mesh. 25. The porous composite material of claim 24, wherein the porous electrically-conductive material is a metallized porous synthetic polymer fabric. 26. The porous composite material of claim 25, wherein the porous electrically-conductive material is a metallized porous non- woven polyester web. 27. The porous composite material of claim 26, wherein the porous electrically-conductive material is a metallized porous expanded polytetrafluoroethylene film. 28. A process for shielding electronic equipment having a vent against electromagnetic radiation and particle intrusion which comprises placing a porous composite material of claim 1,7,13,18 or 23 in the vent.

Inventors: HEDRICK FRED G, LACK CRAIG D, HEDRICK, FRED, G., LACK, CRAIG, D.
Applicants: GORE & ASS, W.L. GORE & ASSOCIATES, INC.
Application year: 1992
Main classification: H05K9
IPC codes: B32B5, C08J9, C23C18, H01B1, H05K9