The present invention relates to an article, specifically a wire or cable, which has been shielded against electromagnetic interference (EMI) and radio frequency interference (RFI) by a coating or layer that includes metal plated conductive fibers.

The increased use and sensitivity of electronics has led to a need for materials that protect against EMI and RFI. Presently, various types of wires and cables, including coaxial cables, utilize a metal foil or woven, served, or braided wire filaments as an electrically conductive shielding layer to inhibit interfering signals from reaching the center conductor. In other applications, metal coated fibers in which the base fiber is carbon, graphite, fiberglass, aramid, stainless steel, or steel nano-fibers have been used as discontinuous fillers molded in an insulating matrix into rigid shielding structures.

The shielding effectiveness of such discontinuous fillers is known to be directly proportional to the amount of filler used. Therefore, while shielding properties increase with increasing amounts of filler, other non-desirable properties, such as stiffness and weight of the resulting product also increase. Accordingly, the material not only becomes difficult to fabricate, but the resulting shield structure loses its ability to bend or twist without creating shielding gaps, or"windows,"thus rendering objects enclosed within the shield structure susceptible to EMI and RFI.

Thus, while discontinuous fiber-based shielding material may find uses in molded structures, its use in wires and cables is limite. It would be desirable to provide a shielding structure incorporating discontinuous fibers which can be easily manipulated and incorporated in processes such as extrusion to enable lower-cost wires and cables shielded against EMI and RFI.

To solve the above-mentioned problems associated with the prior art, the present invention utilizes metal plated conductive fibers as an integral part of a shielding layer, rather than a separate layer. This new shielding layer can be used alone or in combination with a conventional metal shielding layer. The small diameter associated with such metal plated fibers not only enables a large amount of metal plated fiber to be incorporated into the shielding layer, which increases shielding effectiveness, but allows the product to be fabricated by high speed extrusion. Unlike the prior art, therefore, the present invention offers improved shielding effectiveness, enhanced rate of fabrication and decreased cost.

SUMMARY OF THE INVENTION Accordingly, a subject of the present invention is an article comprising an inner conductor formed of at least one electrically conductive wire; an insulating layer surrounding the inner conductor; and a first shielding layer surrounding the insulating layer, the first shielding layer comprising a plurality of metal plated fibers in an insulating polymer matrix, the metal plated fibers being present in an amount effective to shield against EMI and RFI.

In another aspect, the invention provides a method of producing an article comprising an inner conductor formed of at least one electrically conductive wire, an insulating layer surrounding the inner conductor; and a first shielding layer surrounding the insulating layer. The first shielding layer comprises a plurality of metal plated fibers in an insulating polymer matrix, wherein the metal plated fibers being present in an amount effective to shield against EMI and RFI. The method comprises extruding the insulating layer over the wire; and extruding the first shielding layer over the insulating layer and wire.

BRIEF DESCRIPTION OF THE DRAWINGS The following figures, which form a part of the disclosure of the present invention depict additional aspects of the invention. In the figures: Fig. 1 illustrates an article comprising an inner conductive core, a dielectric insulating layer, an extruded metal plated fiber shield, an optional drain wire, and an outer jacket; Fig. 2 illustrates an article comprising an inner conductive core, a dielectric insulating layer, a metal shield layer, an extruded metal plated fiber shield and an outer jacket; Fig. 3 illustrates the article of Fig. 2, wherein the metal shield layer is in the form of a helically wrapped metal plated fiber tape over an optional braided or served metal shield layer; Fig. 4 illustrates the article of Fig. 2, wherein the metal shield layer is in the form of a longitudinally wrapped metal plated fiber tape over an optional braided or served metal shield layer; and Fig 5. illustrates the article of Fig. 2, wherein the inner conductive core comprises a plurality of individually insulated wires with an optional extruded inner dielectric layer and an optional braided or served metal shield.

Fig. 1 shows a cable 10 constituting an embodiment consistent with the present invention. The cable 10 includes a center conductor 12, usually copper, covered by a primary dielectric insulating layer 14 such as PVDF, which is covered by an extruded metal plated fiber shield 16. Over the shielding layer 16 is an outer jacket 18, which can comprise ETFE, cross-linked ETFE, FEP, or other suitable polymers. The cable 10 is shown with an optional drain wire 20.

Fig. 2 shows a second embodiment of cable 10 including a braided or served metal shield 30.

Fig. 3 shows a third embodiment of cable 10, wherein the first shielding layer comprises a helically wrapped tape formed of metal plated fibers in a polymer matrix 40.

Fig. 4 shows a fourth embodiment of cable 10, wherein the first shielding layer comprises a longitudinally wrapped tape formed of metal plated fibers in a polymer matrix 50.

Fig. 5 shows a fifth embodiment of cable 10 consistent with the present invention. This embodiment includes a plurality of center conductors 60, individually covered by a primary insulating layer 62, which is optionally covered by an extruded inner dielectric layer 64. The inner dielectric layer 64 is shown covered by an optional braided or served metal shield 66, which is covered by an extruded metal plated fiber shield 68. Over the shielding layer 68 is an outer jacket 70.

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to an article comprising: an inner conductor formed of at least one electrically conductive wire; an insulating layer surrounding the inner conductor; and a first shielding layer surrounding the insulating layer. The first shielding layer comprises a plurality of metal plated fibers in an insulating polymer matrix. According to the present invention, the metal plated fibers are present in an amount effective to shield against EMI and RFI. The article may also comprise an outer jacket of insulating material surrounding the first shielding layer.

It has been found that an article comprising metal plated fiber in an amount up to about 50%, preferably about 35% by weight of the polymer matrix, enables the article to be fabricated using an extrusion process. The metal plated fiber of the present invention comprises a core material of carbon, glass, polymer or metal, which is covered with a fiber layer formed of material selected from the group consisting of electroplated metal, chemical vapor deposited metal, and electroless plated metal. The method of coating and the use of such metal coated fibers are described in U. S. Patent Nos. 4,680,093,4,609,449 and 4,661,403, the entire contents of which are incorporated by reference herein. Preferably, the core material comprises aramid, fiberglass, graphite, stainless steel.

By using the metal plated fibers described above, the inventive article does not require the use of an additional metal shielded layer. Therefore, an article fabricated with a layer comprising a metal plated fiber in an insulating polymer matrix is not only lighter in weight than traditional articles having a separate metal shield layer, but has improved flexibility and shielding properties over these traditional articles.

However, certain applications may require an additional woven, braided or served metal layer disposed over the dielectric insulating layer. If this is the case, the article comprises a second shielding layer of metal disposed over the insulating layer, the second shielding layer being formed by a process selected from the group consisting of weaving, braiding, and serving. If necessary, the article can further comprise a drain wire disposed in electrical contact with the first shielding layer. In addition, when shielding from magnetic interference, particularly in the H range, the metal plated fiber can comprise a core material covered with a fiber layer formed of material selected from the group consisting of magnetic material and ferro- magnetic material.

It has also been shown that when the metal plated fibers are coated with a sizing agent and cut to a length from about 20 microns to about 65 millimeter before incorporation into the polymer matrix, improvement is realized in both the process and the resulting properties. The wetting properties between the metal coated fibers and the polymer matrix are enhanced when a coating formed of a sizing agent is applied to the metal plated fibers before they are incorporated into the polymer matrix.

It has further been discovered that a length-to-diameter aspect ratio of at least 700, preferably about 780, for the metal coated fibers allows the material to be extruded at high speeds by cross-head or other well-known extrusion techniques.

In addition, the layer comprising a metal plated fiber in an insulating polymer matrix may be extruded in the form of a tape, which can then be helically or longitudinally wrapped around the dielectric or around the optional woven, braided, or served metal layer. Upon heat treatment, which is sometimes referred to as a sintering step, the extruded tape can be fused to form a seamless shielding layer.

Improvements in the previously described properties are realized by incorporating a conductive material, in addition to a metal plated fiber, into the insulating polymer matrix. This embodiment comprises a first shielding layer surrounding the insulating layer, wherein the first shielding layer comprises a plurality of metal plated fibers in a matrix, the matrix comprising an insulating polymer and a conductive filler material. Again, the metal plated fibers being present in an amount effective to shield against EMI and RFI. Preferably, the conductive material includes conductive powders, uncoated graphite fibers, and metal powders.

More preferably, the conductive material comprises carbon black. By adding such conductive materials to the insulating polymer matrix, it is possible to minimize the amount of the more expensive metal plated fibers, without sacrificing shielding properties. Alternatively, by adding conductive powders without decreasing the amount of metal plated fibers, a synergistic improvement in mechanical and shielding properties is realized.

As previously stated, the combination of components presently used enables an extrusion method to produce an article comprising an inner conductor formed of at least one electrically conductive wire; an insulating layer surrounding the inner conductor; and a first shielding layer surrounding the insulating layer. The first shielding layer comprising a plurality of metal plated fibers in an insulating polymer matrix, wherein the metal plated fibers being present in an amount effective to shield against EMI and RFI. The method comprises extruding the insulating layer over the wire, and extruding the first shielding layer over the insulating layer and wire. The process preferably utilizes a cross-head extruder in which the inner conductive wire is fed perpendicular to the extruder. If necessary, the method can further comprise forming a second shielding layer over the insulated wire prior to extruding the first shielding layer. The step of forming the second shielding layer comprises forming the second shielding layer of conductive wires by one of weaving, braiding, and serving.

It has been discovered that the step of extruding the first shielding layer can comprise feeding a continuous length of metal plated fiber or metal plated fiber in the form of chopped metal fibers into an extruder during extrusion.

Alternatively, the step of extruding the first shielding layer can comprises first compounding the metal plated fiber into a compound pellet and adding the compound pellet to an extruder during extrusion. Such a compounding step includes mixing the metal plated fibers with an polymeric material and subsequently forming a pellet.

Regardless of the method of feeding the metal plated fiber into the extruder, the preferred finished article is a wire, cable or coaxial cable. Both the application as a wire, cable or coaxial cable, and the extrusion technique used to produce such an article, preclude the use of metal filaments, flakes, needles or the like in the present application. The sharp edges associated with the morphology of filaments, flakes or needles increase the likelihood that, during the extrusion process, they would cut through the various extruded layers and contact the conductive core. This potential of shorting out the cable or wire prohibits the use of a filler having sharp edges in the polymer matrix layer.

The present invention has been disclosed generally and by reference to embodiments thereof. The scope of the invention is not limited to the disclosed embodiments but is defined by the appended claims and their equivalents.