Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1999-05-07
2003-01-14
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S379000, C428S383000, C252S511000, C174S1100PM, C174S1200SR, C524S495000
Reexamination Certificate
active
06506492
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to cables, and more particularly to compositions suitable for semiconductive jackets especially for medium and high voltage power cables and cables jacketed therewith.
BACKGROUND OF THE INVENTION
Electric power cables for medium and high voltages typically include a core electrical conductor, an overlaying semiconductive shield, an insulation layer formed over the semiconductive shield, an outermost insulation shield, and some type of metallic component. The metallic component may include, for example, a lead sheath, a longitudinally applied corrugated copper tape with overlapped seam, or helically applied wires, tapes, or flat strips. U.S. Pat. No. 5,281,757 assigned to the current assignee, and U.S. Pat. No. 5,246,783, the contents of both of which are herein incorporated by reference, disclose examples of electric power cables and methods of making the same.
Electric power cables for medium and high voltage applications also typically include an overall extruded plastic jacket which physically protects the cable thereby extending the useful life of the cable. The afore-described overall jacket may be insulating or semiconducting. If the overall jacket is insulating, it may overlay or encapsulate the metallic component of the cable as discussed in the September/October 1995 Vol. 11, No. 5, IEEE Electrical Insulation Magazine article, entitled,
Insulating and Semiconductive Jackets for Medium and High Voltage Underground Power Cable Applications,
the contents of which are herein incorporated by reference.
According to the National Electrical Safety Code, power cables employing insulating jackets must be grounded every 0.125 to 0.25 mile depending on the application, or at every splice for cable in duct (at every manhole). Such grounding reduces or eliminates the losses in a cable system. Furthermore, as the neutral to ground voltage may be very high, such grounding is also required for safety purposes.
In contrast to insulating jackets, semiconductive jackets are advantageously grounded throughout the length of the cable and therefore do not need periodic grounding previously described. Accordingly, semiconductive jackets are only grounded at the transformer and at the termination.
Although semiconductive jackets are advantageous for the foregoing reasons, they are not widely employed in the power cable industry. Prior art semiconductive jacket materials were usually not developed for jacketing applications, and as such, often do not meet performance criteria for long-life cable protection.
The Insulated Cable Engineers Association (ICEA) specifies in ICEA S-94-649-1997, “Semiconducting Jacket Type 1”, mechanical properties for semiconductive electrical cable jackets and references American Society for Testing and Materials (ASTM) test methods to test materials suitable for these applications.
Prior art semiconductive jackets, even if they do meet performance criteria for long-life cable protection, are often cost prohibitive for widespread industry employment. This high cost is primarily due to the high weight percentage of conductive additive necessary in the jacket material to make the jacket semiconductive. Typically this weight percentage is greater than 15 to 30 weight percent to achieve the required conductivity or volume resistivity for the jacket. See, for example, U.S. Pat. No. 3,735,025, the contents of which are herein incorporated by reference, which discloses an electric cable jacketed with a thermoplastic semiconducting composition comprising chlorinated polyethylene, ethylene ethyl acrylate, and 30 to 75 or 40 to 60 parts by weight of semiconducting carbon black.
Prior art polymer compounds used in the role of a semiconductive jackets are normally thermoplastic and get their conductivity by use of a large weight percentage of a conductive filler material, usually conductive grades of carbon black, to incur a high level of conductivity (or low level of resistivity), to the compound. The National Electrical Safety Code (Section 354D2-c) requires a radial resistivity of the semiconducting jacket to be not more than 100&OHgr;·m and shall remain essentially stable in service. Prior art compositions required loadings of conductive filler material of at least about 15% to 60% by weight to achieve this criteria. These high levels of conductive filler material inherently add significantly to the cost of such compositions, inhibit the ease of extrusion of the jacketing composition, and decrease the mechanical flexibility of the resultant cable.
Percolation theory is relatively successful in modeling the general conductivity characteristics of conducting polymer composite (CPC) materials by predicting the convergence of conducting particles to distances at which the transfer of charge carriers between them becomes probable. The percolation threshold (p
c
), which is the level at which a minor phase material is just sufficiently incorporated volumetrically into a major phase material resulting in both phases being co-continuous, that is, the lowest concentration of conducting particles needed to form continuous conducting chains when incorporated into another material, can be determined from the experimentally determined dependence of conductivity of the CPC material on the filler concentration. For a general discussion on percolation theory, see the October 1973 Vol. 45, No. 4, Review of Moderm Physics article, entitled,
Percolation and Conduction,
the contents of which are herein incorporated by reference. Much work has been done on determining the parameters influencing the percolation threshold with regard to conductive filler material. See for example,
Models Proposed to Explain the Electrical Conductivity of Mixtures Made of Conductive and Insulating Materials,
1993 Journal of Materials Science, Vol. 28;
Resistivity of Filled Electrically Conductive Crosslinked Polyethylene,
1984 Journal of Applied Polymer Science, Vol. 29; and
Electron Transport Processes in Conductor-Filled Polymers,
1983 Polymer Engineering and Science Vol. 23, No. 1; the contents of each of which are herein incorporated by reference. See also,
Multiple Percolation in Conducting Polymer Blends,
1993 Macromolecules Vol. 26, which discusses “double percolation”, the contents of which are also herein incorporated by reference.
Attempts for the reduction of conductive filler content in CPC materials have been reported for polyethylene/polystyrene and for polypropylene/polyamide, both employing carbon black as the conductive filler. See
Design of Electrical Conductive Composites: Key role of the Morphology on the Electrical Properties of Carbon Black Filled Polymer Blends,
1995 Macromolecules, Vol. 28 No. 5 and
Conductive Polymer Blends with Low Carbon Black Loading: Polypropylene/Polyamide,
1996 Polymer Engineering and Science, Vol. 36, No. 10, the contents of both of which are herein incorporated by reference.
However, none of the prior art concerned with minimizing the conductive filler content has addressed materials suitable for use as a semiconductive jacket material for cables which must meet not only the electrical requirements, but also stringent mechanical requirements as discussed heretofore.
What is needed, and apparently lacking in the art is a semiconductive jacket material which has a significant reduction of conductive filler material, thereby decreasing the cost of the material and the processing by increasing the ease of extrusion and mechanical flexibility of the jacketed cable, while maintaining industry performance criteria for resistivity and mechanical properties.
SUMMARY OF THE INVENTION
The present invention provides a conductive polymer composite (CPC) material for semiconductive jackets for cables which has a significant reduction in conductive filler content while maintaining the required conductivity and mechanical properties specified by industry by selecting materials and processing approaches to reduce the percolation threshold of the conductive filler in the composite, while balancing the mater
Gray J. M.
Kelly Cynthia H.
Norris & McLaughlin & Marcus
Pirelli Cables & Systems, LLC
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