Disclosed herein are textile fabrics of metal-coated fibers having a far field shielding effectiveness for electromagnetic radiation and methods of designing, fabricating and using such fabrics. BACKGROUND OF THE INVENTION

Shielding of electromagnetic radiation is typically effected by absorbing or reflecting the radiation using a metallic material, e.g. metal foil, metal-filled coatings or metallized textile substrates such as mesh, knits, woven fabrics and non-woven fabrics. As the frequency of radiation increases, the wavelength decreases. The selection of metallic material for a shielding application depends on the nature of the radiation to be abated; for instance an open mesh can be effective in shielding radiation of wavelength larger than the mesh opening. Continuous copper foils are often selected for shielding high frequency radiation of small wavelength.

As disclosed in U.S. Patents 4, 037,009; 4,234,648 and 4,532,099 metallized fabrics have a desirable combination of properties that make them useful for a variety of electromagnetic radiation shielding applications. The substrate fibers of such fabrics provide the fabrics with flexibility, strength and air permeability. The metal coating provides a light weight conductive surface that can reflect and absorb electromagnetic radiation. A variety of textile configurations, e.g. woven and non-woven fabrics, coated with a conductive metal such as copper can provide shielding equivalent to copper foil for selected ranges of electromagnetic radiation. See for instance, U.S. Patents 4,631,214; 4,678,699 and 4,749,625 which disclose the use of metallic screens, ribbons and textiles in composite shielding materials which are effective in shielding at radiation up to about 1 gigahertz (GHz) .

Silver an and Henn disclose in "New Metallized Materials For EMI/RFI Shielding", EMC Technology. Vol. 9, No. 6, pages 37-42, (1990), that the shielding effectiveness of nylon non-woven fabric coated with metals such as copper, nickel over copper and tin over copper, varies with frequency of the radiation, e.g. in the range of 100 to 10,000 megahertz (0.1-10 Ghz) . At 10 GHz the shielding effectiveness for the various metallized fabrics ranged from about 60 to 80 decibels (db) . For example, nylon non-woven fabric (basis weight 1 ounce/square yard) coated with a single layer of copper (basis weight 0.45 ounces/square yard) and having a surface resistivity of 0.04 ohms per square exhibited about 82 db of shielding effectiveness at 10 gigahertz. The same fabric coated with a single layer of nickel (basis weight 0.16 ounces/square yard) over a single layer of copper (basis weight 0.29 ounces/square yard) and having a surface resistivity of 0.23 ohms per square exhibited about 58 db of shielding effectiveness at 10 GHz. The same fabric coated with a single layer of tin (basis weight 0.04 ounces/square yard) over a single layer of copper (basis weight 0.39 ounces/square yard) and having a surface resistivity of 0.1 ohms per square exhibited about 68 db of shielding effectiveness at 10 gigahertz. What is not disclosed is why the shielding effectiveness is so variable and how to design a metallized non-woven fabric to perform at a desired level of shielding effectiveness against known high frequency radiation.

SUMMARY OF THE INVENTION

This invention provides an algorithm based on characteristics of metallized fabrics that allows the design of electromagnetic shielding materials that provide a desired shielding effectiveness at selected frequencies. This invention also provides textile fabrics of metal-coated fibers having a far field shielding effectiveness for electromagnetic radiation at 10 GHz of at least 50 db. BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are shielding effectiveness curves for metallized fabrics. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As used herein the term "shielding effectiveness" applies to far field shielding, i.e. shielding at a distance greater than l/2τr wavelengths of the radiation from the source of the radiation.

Any metallized, fabric can be used to prepare the shielding materials of this invention provided the fabric can be characterized in terms of a few key properties, namely the thickness of the fabric, the size of apertures in the fabric, volume resistivity of the metal coating and surface resistivity of the metallized fabric. Useful textile types include woven, non-woven, knitted and fleeced fabrics prepared from a variety of substrate fibers including synthetic polymeric fibers such as nylon, polyester or acrylic fibers and inorganic fibers such as glass, carbon or graphite fibers. In many cases, especially in the case of non-woven fabrics, it is desirable that the fibers be bonded at cross over points to maintain structural integrity and avoidance of large apertures during handling or use; such bonding can be effected using adhesives or by fusion of fiber material. The fabric can be metallized by a variety of methods known in the art. A preferred

metallizing method is electroless deposition using catalyst materials disclosed in U.S. Patent 5,082,734. The thickness D of the metallized fabric is determined by well known techniques using flat surfaced calipers, commonly used for measuring the thickness of paper or textiles. In the algorithms of this invention D is in units of millimeters.

The size of the apertures in the fabric is determined by observing microscopic views of the fabric. The dimensions of the largest visible aperture in the plane of the surface of the fabric will comprise a maximum dimension L and a minimum dimension S. In the case of a circular aperture L should equal S. In the algorithms of this invention L and S are in units of millimeters.

The metallized fabric will exhibit a surface resistivity, p _s , which can be determined in accordance with procedures essentially as set forth in ASTM Standard Procedure F 390-78, entitled "Standard Test Method for Sheet Resistance of Thin Metallic Films

With a Collinear Four-Probe Array". In determining p _s of non-woven fabrics it is useful to employ probes having circular flat face about 3 mm in diameter, spaced about 25 mm apart, applied to the fabric with about 70 kiloPascals force. In the algorithms of this invention p _s is in units of ohms per square (often simplified to ohms) .

The metallized coating on the fabric will have an inherent volume resistivity, p _v , depending on the composition of the metal. When the coating comprises more than one layer of metal, e.g. a tin layer on a copper layer, the volume resistivity is determined by a weighted average of the volume resistivity of the separate layers. Regardless of whether the coating comprises one or a plurality of metal layers, volume resistivity, p _v , of the coating can be determined as ∑X _j P _vl -, where x. is the volume fraction of layer i in the metal coating and p _v] . is the

inherent volume resistivity of the metal species in layer i, where the fractions are summed for i=l,n, where n is the total number of metal layers in the coating. In the algorithms of this invention p _v is in units of ohms-millimeter (ohm-mm) .

For any metallized fabric characterized in terms of D, L, S, p _s and p _v , the shielding effectiveness SE at 10 GHz can be determined from the following algorithm: SE=e ^"12 - ^L [68.15 + 20(log ₁₀ (1-0.99961 e ^"3 - ^303a ) + 13.160 a] + [(l-e ^{' 2} - ^9L )(20 + 20 (log ₁₀ ((l+ln(L/S))/L) + 30 D/L) ] , where a=p _v /p _s .

Thus one aspect of this invention is a method for designing electromagnetic radiation shielding materials comprising metallized non-woven fabrics, where a desired shielding effectiveness at 10 GHz can be obtained by selecting non-woven fabric substrates characterized in terms of thickness D and aperture dimensions L and S and selecting the metal coatings characterized in terms of metal volume resistivity p _v to provide a metallized fabric characterized in terms of surface resistivity p _s . For instance, shielding materials comprising metallized non-woven fabrics can be selected and/or designed to provide shielding effectiveness at 10 GHz of at least 50 db provided that the metal coating is not a single layer of copper, a single layer of nickel or a bilayer of tin, silver or nickel over copper. Preferably the shielding effectiveness will be at least 60 db or higher, say at least 70 db, and even more preferably at least 80 db. For high performance shielding applications the metallized fabric can be designed to provide even higher shielding effectiveness of at least 85 db or higher, say about 90 or 95 db, even more preferably at least 100 or 110 db.

In the special case where the metal coatings comprise a layer of copper, a layer of nickel or a bilayer of copper covered with tin, nickel or silver,

SE at 10 GHz is at least 95 db, preferably at least 100 db, more preferably at least 105 db, even more preferably at least 110 db.

EXAMPLE 1 A nylon woven fabric of ripstop weave was coated by electroless deposition with copper providing a metal-coated fabric having apertures with a maximum dimension L=0.159 mm and a minimum dimension of 0.103 mm. The fabric had a caliper thickness of 0.089 mm and a surface resistivity of 0.046 ohms/square. The volume resistivity of copper is 17.182 x 10" ⁹ ohm- meter. Shielding effectiveness against radiation of frequencies from 0.01 to 10 GHz was calculated from the above-described algorithm. Shielding effectiveness was measured using a TEM cell for radiation of frequencies from 0.1 to 1 GHz and according to MIL-STD-285 at 0.01 GHz and for radiation of frequencies above 1 GHz. TEM cells are defined by J.E. Catrysse in a paper entitled "A New Test Cell For The Characterization Of Shielding Materials In The Far-Field" presented at the 7th International Conference on Electromagnetic Compatability, published by IEE, London, UK in 1990 (page 62). See also Catrysse ^, s paper entitled "Measuring Techniques For SE-Values Of Samples and Enclosures", IEE Colloquium on "Screening of Connectors, Cables and Enclosures", Digest No. 012 (1992), IEE, London, U.K. The results plotted in Figure 1 illustrate the value of using the algorithm of this invention in designing metal-coated fabrics for desired EMI shielding.

EXAMPLE 2 A polyethylene terephthalate (PET) non-woven fabric having a basis weight of 60 grams/square meter (g/m ² ) was coated by electroless deposition with copper providing a metal-coated fabric having apertures with a maximum dimension L=0.232 mm and a minimum dimension of 0.106 mm. The fabric had a caliper thickness of 0.404 mm and a surface resistivity of 0.025

ohms/square. Shielding effectiveness, calculated and measured as in Example 1, is plotted in Figure 2.

EXAMPLE 3 A PET woven fabric of taffeta weave was coated by electroless deposition with copper providing a metal-coated fabric having apertures with a maximum dimension L=0.122 mm and a minimum dimension of 0.095 mm. The fabric had a caliper thickness of 0.089 mm and a surface resistivity of 0.16 ohms/square. Shielding effectiveness, calculated and measured as in Example 1, is plotted in Figure 3.

While specific embodiments have been described herein, it should be apparent to those skilled in the art that various modifications thereof can be made without departing from the true spirit and scope of the invention. Accordingly, it is intended that the following claims cover all such modifications within the full inventive concept.