Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
2001-01-22
2002-12-10
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C174S1020SC, C174S1050SC
Reexamination Certificate
active
06491849
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
This invention relates to compositions useful in the preparation of semiconductive conductor shields in power cables and to semiconductive conductor shields and power cables utilizing the composition.
2. Description of the Related Art
A typical insulated electric power cable generally comprises a conductor in a cable core that is surrounded by several layers of polymeric materials including an inner semiconducting shield layer (conductor or strand shield), an insulating layer, an outer semiconducting shield layer (insulation shield), a metallic wire or tape shield used as the ground phase, and a protective jacket. Additional layers within this construction such as moisture impervious materials, are often incorporated. The invention pertains to the inner semiconducting shield layer, i.e., the conductor shield.
Semiconductive shields have been used in power cables as shields for the cable conductor and insulation for many years. The conductor shield is typically extruded over the cable conductor to provide a layer of intermediate conductivity between the conductor and cable insulation in the power cable. Conventional compositions for these conductor shields include a base polymer as the predominant component of the composition compounded with, carbon black to provide conductivity for the composition and various additives.
The primary purpose of the semiconducting conductor shield between the conductor and insulation in an electrical power cable is to ensure the long term viability of the primary insulation. There is always a need for improved semiconductive conductor shield compositions that balance cost and performance.
Examples of polymer compositions used as shields in power cables are found in the disclosures of U.S. Pat. Nos. 4,612,139 and 4,305,846 to Kawasaki et al., U.S. Pat. No. 4,857,232 to Burns, Jr., U.S. Pat. No. 3,849,333 to Lloyd et al., U.S. Pat. No. 5,889,117 to Flenniken, and U.S. Pat. No. 6,086,792 to Reid et al., the disclosures of which are hereby incorporated by reference.
It would be desirable to have a conductor shield material with improved performance that does not require the use of expensive additives, complex polymer formulations, or specially prepared carbon black, as performance must always be balanced with cost in the manufacture of electric cable.
SUMMARY OF THE INVENTION
The invention provides a conductor shield material with improved performance without the need for expensive additives, complex polymer formulations, or specially prepared carbon black.
In particular, the composition of the invention, conductor shields and cables made with conductor shields in accordance with the invention exhibit superior performance over time as demonstrated by accelerated cable life testing (ACLT) as compared to conventional high performance conductor shield compositions.
In particular, the invention provides a conductor shield comprising a base polymer selected from the group consisting of copolymers of ethylene and a mono-unsaturated ester, copolymers of ethylene and one or more alpha olefins having three to six carbon atoms, EPR and EDPM rubbers, low density polyethylene and linear low density polyethylene; conductive carbon black; and a waxy additive selected from the group consisting of at least one amide wax, at least one ethylene vinyl acetate wax and mixtures of at least one amide wax and at least one ethylene vinyl acetate wax.
The invention also provides a conductor shield consisting essentially of a base polymer selected from the group consisting of copolymers of ethylene and a mono-unsaturated ester, copolymers of ethylene and one or more alpha olefins having three to six carbon atoms, EPR and EDPM rubbers, low density polyethylene and linear low density polyethylene; conductive carbon black; and a waxy additive selected from the group consisting of at least one amide wax, at least one ethylene vinyl acetate wax and mixtures of at least one amide wax and at least one ethylene vinyl acetate wax.
In addition to the composition matter, the invention includes a semiconductive shield for the conductor or insulation in a power cable formed by extruding the composition over the conductor or insulation of the power cable and the resulting power cable that employs the composition as a conductor shield.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The base polymer of the composition of the invention can be selected from a variety of polymers including various homopolymers, copolymers and terpolymers known in the art, the selection being based upon the ultimate desired use of the polymer composition. For example, the polymers used in the polymeric compositions of the present invention may include, but are not limited to, homopolymers, copolymers and graft polymers of ethylene where the co-monomers are selected from butene, hexene, vinyl acetate, acrylic acid, methacrylic acid, esters of acrylic acid, esters of methacrylic acid, maleic anhydride, half esters of maleic anhydride, carbon monoxide and the like; elastomers selected from natural rubber, polybutadiene, polyisoprene, random styrene butadiene rubber, polychloroprene, nitrile rubbers, ethylene propylene copolymers and terpolymers and the like; homopolymers and copolymers of styrene, including styrene-butadiene, styrene-butadiene-styrene linear and radial polymers, acrylonitrile-butadiene-styrene, styrene acrylonitrile and the like; linear and branched polyether or polyester polyols; crystalline and amorphous polyesters and polyamides; alkyd resins, rosin acids or rosin esters; hydrocarbon resins produced from thermal or Friedal Crafts polymerization of cyclic diene monomers such as dicyclopentadiene, indene, cumene and the like; ethylene/silane copolymers; ethylene/.alpha.-olefin/diene terpolymers such as ethylene/propylene/1,4-hexadiene, ethylene/1-butene/1,4-hexadiene and the like; mixtures thereof and the like. Additionally, the polymer used in compositions of the present invention may include copolymers and terpolymers containing the above-identified polymers as major components of the copolymer or terpolymer.
Preferably, the base polymer of the composition of the invention is selected from a variety of polymers including copolymers of ethylene and a mono-unsaturated ester such as ethylene-ethyl acrylate, ethylene-methyl acrylate, ethylene-methyl methacrylate and ethylene-vinyl acetate, copolymers of ethylene and one or more alpha olefins having three to six carbon atoms, as well as EPR and EDPM rubbers, low density polyethylene (LDPE) and linear low density polyethylene (LLDPE). Of these copolymers, ethylene-vinyl acetate (EVA) is more preferred. More particularly, EVA having a vinyl acetate content between 18 and 20% is most preferred for use as the base polymer of the invention. The base polymer of the composition of the invention is present in amounts from about 30% to about 99.4% by weight, based upon the weight of the of the total composition.
In the present invention, commercially available, conventional carbon black is added to the polymer compositions to impart semi-conductive properties to the composition. The ability to use such commercially available, conventional carbon blacks to achieve improved ACLT results is an advantage of the invention. The carbon black added to the polymer may be one of the various available conventional carbon blacks, including finely divided carbon such as lamp black, furnace black, or acetylene black, i.e. carbon black made by pyrolyzing acetylene. Ketjin black may be used in the compositions of the invention as well as many of the commercial carbon black grades described in ASTM D 1765 98b, for example, N351, N293 and N550. Preferably, to avoid problems associated with carbon black dust, the carbon black is pelletized, although non-pelletized carbon black, such as in its fluffy form, may also be used with equal success. The carbon black is generally present in the composition in the amount of from about 0.1% to about 65% by weight of the polymer composition. Preferably the carbon black is p
Blank Rome Comiskey & McCauley
General Cable Technologies Corp.
Kopec Mark
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