Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2000-03-13
2001-10-09
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C252S511000, C428S383000, C428S372000, C174S1100PM, C174S1200SR, C174S1200SC, C524S524000, C427S118000
Reexamination Certificate
active
06299978
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to compositions suitable for use as a conductor shield in multi-layered extruded coatings on power cables that are intended to provide protection for the power cable in use, and especially to such coatings that extend the life of the cable when it is exposed to moisture. In particular, the present invention relates to such compositions that are based on polyolefins, conductive carbon blacks and a ter-polymer of ethylene, vinyl acetate and vinyl alcohol. Such ter-polymers may be referred to as EVA(OH) ter-polymers.
BACKGROUND OF THE INVENTION
High voltage power cables have multiple coatings that are extruded onto the conductor to provide protection and to extend the life of the power cable. In particular, high voltage power cables have an inner semiconducting layer (referred to as conductor shield) surrounding the conductor, an intermediate layer of cross-linked polyethylene insulation and an outer semiconducting layer surrounding the insulation layer (referred as insulation shield). The purpose of the inner semiconducting layer is to relax or relieve the heterogeneous electrical stress attributed to irregularities in the conductor and also to increase the adhesion between the conductor and insulation. The external semiconducting layer, which could be either a bonded layer or a strippable layer, homogenizes the electrical stress on the insulation surface.
The semiconducting layer, which is also known as the conductor shield, is typically formed from ethylene based polymers e.g. ethylene/vinyl acetate or ethylene/ethyl (or butyl) acrylate or ethylene/alkene copolymers. The ethylene based polymer is blended with conducting carbon black e.g. furnace black or acetylene black, appropriate antioxidants and an organic peroxide cross-linking agent.
The level or amount of the trace elements (impurities) in the carbon black, conductor shield asperities at the insulation interface and the additives in the composition all tend to influence the useful life of the insulation.
A common cause of reduction in the life of an extruded power cable is the formation of so-called water trees, which are believed to result from the presence of water and water-soluble ions, such as sulphur and metallic cations, within the conductor shield. Carbon blacks with low sulphur and cation impurity levels are advantageous for use in long-life underground cables. Acetylene black manufactured from pure acetylene is considered to be the industry standard for a clean carbon black.
The asperities at the conductor shield—insulation interface tend to increase localized electrical stress on the insulation and hence reduce the life of the cable. The actual size and number density of these asperities can be reduced so as to increase the cable life, by choosing appropriate manufacturing conditions or carbon black of appropriate morphology and cleanliness characteristics, or both. U.S. Pat. No. 5,352,289 describes a furnace carbon black having an ash level and sulphur content less than or equal to 50 ppm as being suitable for cable applications. However, it is also known that, under optimum processing conditions, furnace blacks of low ash and grit content exhibited a smoothness quality comparable to acetylene black.
Some of the low molecular additives conventionally added to highly-filled conductor shield compositions have also been found to be advantageous in increasing or extending the performance of extruded power cables. For instance, U.S. Pat. No. 4,909,960 describes one such composition containing a low molecular weight polyethylene with an average molecular weight of 1000-4000. U.S. Pat. No. 4,612,139 describes the use of polyethylene glycol having molecular weight of 1000-20,000 as imparting advantages in retarding water tree growth. U.S. Pat. No. 4,801,766 describes use of a conductor shield composition containing n-vinyl carbazole to increase the breakdown strength of the cable.
U.S. Pat. No. 5,719,218 discloses addition of an ethylene/vinyl acetate(vinyl alcohol) ter-polymer to the intermediate layer of insulation, to improve resistance to moisture induced degradation.
Other methods to impart to the insulation an intrinsic resistance to the growth of water trees are known. However, there remains a need for an improved conductor shield composition which, when laid (layered) with a conventional non-tree retardant insulation, will extend cable life.
SUMMARY OF THE INVENTION
It has now been found, unexpectedly, that the life of a cable having an insulation layer of cross-linked polyethylene can be extended by adding ethylene/vinyl acetate(vinyl alcohol) ter-polymer to the composition used to form the conductor shield.
Accordingly, an aspect of the present invention provides a semiconducting composition for use as conductor shield in extruded coatings on high voltage electrical cables, said composition consisting essentially of (a) a polymeric component of a blend of 0-99% by weight of polyolefin and 1-100% by weight of a ter-polymer of ethylene/ vinyl acetate (vinyl alcohol), (b) conducting carbon black, said carbon black containing less than 50 ppm of each of ash, ions and sulphur, and (c) an antioxidant.
A further aspect of the present invention provides a high voltage electrical cable coated with a conductor shield composition consisting essentially of (a) a polymeric component of a blend of 0-99% by weight of polyolefin and 1-100% by weight of a ter-polymer of ethylene/vinyl acetate (vinyl alcohol), (b) conducting carbon black, said carbon black containing less than 50 ppm of each of ash, ions and sulphur, and (c) an antioxidant, and over-coated with cross-linked polyolefin composition.
An additional aspect of the present invention provides a method of providing a coating on high voltage electrical cable comprising:
(a) extrusion coating the conductor of such cable with a conductor shield composition consisting essentially of (a) a polymeric component of a blend of 0-99% by weight of polyolefin and 1-100% by weight of a ter-polymer of ethylene/ vinyl acetate (vinyl alcohol), (b) conducting carbon black, said carbon black containing less than 50 ppm of each of ash, ions and sulphur, and (c) an antioxidant;
(b) extrusion coating a cross-linkable polyolefin composition over said conductor shield composition; and
(c) effecting cross-linking of said cross-linkable polyolefin composition.
In an embodiment of the method of the invention, a protective coating is extruded over said cross-linkable coating prior to effecting said cross-linking.
DETAILED DESCRIPTION OF THE INVENTION
An aspect of the present invention provides a semi-conducting composition for use as conductor shield in extruded coatings on high voltage electrical cables. The composition consists essentially of a polyolefin, conducting carbon black, an antioxidant and a ter-polymer of ethylene/ vinyl acetate (vinyl alcohol).
The polyolefin of the semi-conducting composition of the invention may be selected from a wide variety of polyolefins, but must be capable of being blended with the EVA(OH) polymer as described herein. Such polymers include polymers of ethylene and other monomers e.g. polymers generally referred to as polyethylene, including homopolymers of ethylene and copolymers of ethylene with other alpha-unsaturated hydrocarbon monomers, and other ethylene-a olefin copolymers e.g. ethylene/vinyl acetate, ethylene/ethyl acrylate and ethylene/vinyl silane copolymers. The polymers may be manufactured by a variety of techniques known to those skilled in the manufacture of such polyolefins. Examples of processes of manufacture include under high pressure using a tubular or autoclave reactor with any of the known free radical initiators or coordination catalysts, using slurry, solution or gas phase polymerization techniques with coordination catalysts, including the catalysts known as metallocene catalysts and the transition metal catalysts.
Carbon blacks for use in coatings for cables are well known in the art, and include carbon blacks known as acetylene black and furnace black. The carbon black has l
Equistar Chemicals LP
Gray J. M.
Kelly Cynthia H.
LandOfFree
Semiconductive polyolefin compositions and cables covered... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Semiconductive polyolefin compositions and cables covered..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Semiconductive polyolefin compositions and cables covered... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2567513