The invention relates to semiconductive shields for use in electrical conductors such as power cables, and particularly to a vulcanizable semiconductive conductor or bonded insulation shield composition that exhibits improved physical properties and processability compared to known semiconductive conductor and bonded insulation shields.
Power cables typically include a core electrical conductor, an overlaying semiconductive shield (also called a "conductor screen"), an insulation layer formed over the semiconductive shield, and an outermost insulation shield (or screen). The semiconductive shield used to screen the electrical conductor is conventionally formed by dispersing various furnace-type carbon blacks, such as ASTM N-472 or P-type grades, in an ethylene copolymer resin base. These furnace blacks often have poor dispersion characteristics in polymers and contribute high levels of ionic contaminants. Consequently, protrusions and contaminants occur at the cable's shield/dielectric interface, causing increased stress gradients in an electrical field. This electrical field enhancement, combined with the migration of water and ions into the insulation, may lead to the formation of water trees and subsequent dielectric breakdown and premature cable failure.
Other commercially available high performance semiconductive shield compositions contain other types of carbon blacks, such as acetylene blacks, and an ethylene/ethylacrylate copolymer, ethylene/vinylacetate copolymer, ethylene/butylacrylate copolymer or blends of these materials with polyethylene. These materials typically contain reduced levels of ionic contamination and exhibit good dispersion and very smooth extrusion surfaces. Such shield compositions have a high viscosity due to the high carbon black loadings needed to achieve adequate conductivities and, therefore, abrade and/or corrode cable extrusion equipment. This wear results in poor extrusion cable surfaces and interfaces, thus reducing the shield's electrical performance properties.
Semiconductive shield compositions containing acetylene black and an ethylene/ethylacrylate copolymer often demonstrate "shrinkback" on the cable. Shrinkback occurs when the semiconductive shield anneals and shrinks following cable manufacture. Shrinkback causes the semiconductive shield to lose its adhesion to the conductor. As a result, the conductor protrudes out of the cable core, thus diminishing the integrity of the cable system, particularly at splices.
Efforts have been made to improve semiconductive shield compositions. High performance semiconductive conductor shield compositions that include an ethylene/vinyl acetate copolymer, acetylene carbon black, and an organic peroxide crosslinking agent are often used for these applications. Vinyl acetate resins, however, may only be used with aluminum conductors because they are corrosive to copper conductors. Furthermore, high loadings of acetylene black combined with ethylene/vinyl acetate resin lead to the formation of acids in the extruder which then corrode and abrade extrusion die tooling, resulting in cable dimension variations over time.