U.S. Pat. No. 6,656,986 teaches various polyethylene, peroxide-crosslinkable compositions useful in the manufacture of power cable insulation. Some of these compositions have achieved commercial success in the medium voltage power cable market, and an interest exists in extending these commercially successful compositions into the high and extra high voltage power cable markets.
The manufacture of power cable insulation is a multistep process that can be separated into two broad parts, i.e., first making a composition from which the cable insulation is made, and second, extruding the composition over single or stranded conductor as an insulation.
In one embodiment of the first part of the process, i.e., the part in which the composition is made, a base polymer, e.g., polyethylene, is mixed with one or more additives and then formed into pellets which are soaked with peroxide and subsequently stored and/or shipped to a fabricator who performs the second part of the process, i.e., converting the pellets to a wire or cable coating. To avoid acid catalyzed decomposition of peroxide during storage and shipping, U.S. Pat. No. 6,656,986 teaches inclusion of oligomeric and/or high molecular weight hindered amine stabilizers (HAS).
In the making of the pellets, care is taken not to introduce or create impurities that can adversely affect the utility of the composition once formed into a wire or cable sheath. However, some impurities are inevitably introduced into the composition either as, for example, contaminants associated with feed materials to the process, or are made during the process as, for example, gels that result from degradation of the base polymer. Efforts are made, of course, to minimize and remove these impurities before the composition is extruded as a power cable sheath. Some of the impurities are in the form of fine, e.g., less than 100 microns (μm), particulates and are susceptible to removal from the composition by filtering. In those embodiments in which the composition is compounded within an extruder, a fine-mesh screen is typically located at or near the die head of the extruder such that the melt within the extruder must pass through the screen before it leaves the extruder. As the filter becomes plugged with particulates, pressure builds within the extruder and the operational efficiency of the extruder drops until the filter is cleansed or replaced. In those embodiments in which an oligomeric or high molecular weight base, e.g., a oligomeric or high molecular weight HAS, is present in the composition prior to melt filtration, it tends to contribute to the plugging of the screen and diminishing the operational efficiency of the extruder and overall run efficiency of the process.
Insulation for use in medium voltage power cable applications can typically tolerate more impurities than those for use in high or extra high voltage power cable applications. As such, the screen used to filter the composition before extrusion into pellets can be more coarse, i.e., have a larger openings, than that used for filtering compositions for use in high or extra high voltage power cable applications. As a consequence and all else being equal, the finer (smaller) the screen mesh through which a melt must pass, the more particulate it will trap, the faster it will plug, and the shorter the time interval will be between filter cleaning and/or replacement. This, in turn, affects the operational efficiency of the compounding process.
Of particular interest to the extension of compositions currently designed for use in medium voltage power cable applications to high and extra high voltage power cable applications is the reduction and/or elimination of particulate contaminants and gels during the compounding of the base polymer with additives and/or fillers and to the extent that such gels are made, their removal by filtering before the composition is fabricated into pellets. Further to this interest is maintaining the relative stability of the pellet against loss of crosslinking efficiency during shipping and/or storage, and the minimizing of water generation during cure.