Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2001-04-20
2004-01-06
Yamnitzky, Marie (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S372000, C428S383000, C252S511000, C427S118000, C174S1100PM, C174S1020SC, C174S1200SC
Reexamination Certificate
active
06673448
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electric cables and particularly the invention relates to semiconducting layers of electric cables, preferably to cross-linked, semiconducting layers of electric cables, and more preferably to cross-linked, inner and non-strippable outer semiconducting layers of electric cables.
BACKGROUND OF THE INVENTION
Electric cables and particularly electric power cables for medium and high voltages are composed of a plurality of polymer layers extruded around the electric conductor. The electric conductor is usually coated first with an inner semiconducting layer followed by an insulating layer, then an outer semiconducting layer. To these layers further layers may be added, such as a water barrier layer and a sheath layer.
The insulating layer and the semiconducting layers normally consist of ethylene homo- and/or copolymers which preferably are cross-linked. LDPE (low density polyethylene, i.e. polyethylene prepared by radical polymerisation at a high pressure) cross-linked by adding peroxide, for instance dicumyl peroxide, in connection with the extrusion of the cable, is today the predominant cable insulating material. The inner semiconducting layer normally comprises an ethylene copolymer, such as an ethylene-vinyl acetate copolymer (EVA). The composition of the outer semiconducting layer differs depending on whether it has to be strippable or not. Normally a strippable semiconducting layer comprises an ethylene copolymer, such as an EVA together with an acrylonitrile-butadiene rubber (NBR) and sufficient carbon black to make the composition semiconducting. A non-strippable (bonded), outer semiconducting layer may comprise EVA, EEA or EBA together with an amount of carbon black sufficient to make the composition semiconducting.
As an example of non-strippable type semiconducting compositions, mention may be made of EP 0 057 604 which discloses, in its examples, three semiconductive compositions, where semiconductive composition A contained 59.25% by weight of an EVA containing 20% by weight of vinyl acetate (VA) comonomer, 40.00% by weight of acetylene black, 0.2% by weight of antioxidant and 0.55% by weight of dicumyl peroxide (DCP) cross-linking agent, composition B contained 83.8% by weight of an EVA containing 18% by weight of VA comonomer, 15% by weight of Ketchen black, 0.2% by weight of antioxidant and 1.0% by weight of DCP and composition C contained 61.2% by weight of EVA containing 20% by weight of VA comonomer, 38% by weight of furnace black, 0.2% by weight of antioxidant and 0.6% by weight of DCP.
As an example of prior art strippable semi-conducting compositions for electric cables, mention may be made of U.S. Pat. No. 4,286,023 which discloses a polymer composition for electric cables comprising (A) an ethylene copolymer selected from the group consisting of ethylene-alkyl acrylate copolymers containing about 15-45% by weight of alkyl acrylate, said alkyl acrylate being selected from the group consisting of C
1
-C
8
alkyl esters of (meth)-acrylic acid, such as methyl acrylate, ethyl acrylate, methyl methacrylate, butyl acrylate, 2-ethylhexyl acrylate and the like, and ethylene-vinyl acetate copolymers containing about 15-45% by weight of vinyl acetate, (B) a butadiene-acrylonitrile copolymer (nitrile rubber) containing about 10-50% by weight of acrylonitrile, (C) conductive carbon black, and (D) a peroxide cross-linking agent, wherein the weight ratio A:B=1:9 to 9:1; C:(A+B)=0.1 to 1.5, and D is present in an amount of 0.2-5% by weight of the total composition.
It should be noted that U.S. Pat. No. 4,286,023 relates to strippable outer semiconducting layers. Inner and non-strippable outer semiconducting layers are not disclosed.
It should also be noted that ethylene-vinyl acetate copolymer is the preferred component (A) according to U.S. Pat. No. 4,286,023. If component (A) is selected from C
1
-C
8
alkyl esters of acrylic acid and methacrylic acid, the preferred copolymer is ethylene-ethyl acrylate copolymer.
Although prior art compositions for semiconducting layers in electric cables are satisfactory for many applications, there is always a desire to improve their characteristics and eliminate or reduce any disadvantages they may have.
One disadvantage of EVA conventionally used in semiconducting layers is that at elevated temperatures, such as during compounding of the semiconducting composition, EVA starts to decompose and generate acetic acid at about 150° C. At the same time double-bonds are formed in the polymer chain. The acetic acid, which is very corrosive, especially at high temperatures, attacks the processing equipment and leads to an undesired corrosion thereof. To a certain extent this may be counteracted by making the equipment of special, corrosion-resistant materials which, however, are expensive and add to the investment cost for manufacturing the cable. The release of acetic acid is also a negative factor from an environmental point of view. Further, the formation of double-bonds in the polymer chain at the generation of acetic acid may lead to undesired cross-linking and gel formation.
Another disadvantage of EVA as a material for the semiconducting layers of electric cables manifests itself when cross-linking (vulcanising) cables. The cross-linking is usually conducted in an about 100-200 m long vulcanising tube, where cross-linking should take place as rapidly and completely as possible. For conventional cables having semiconducting EVA-containing layers, cross-linking is carried out at a temperature of about 260-300° C., preferably 270-285° C. A nitrogen-gas pressure of about 8-10 bar is applied in the vulcanising tube and contributes to the preventing of oxidation processes by keeping away the oxygen of the air and to reducing the formation of microcavities, so-called voids, in the polymer layers. As explained above in connection with compounding of EVA, the elevated temperature at the cross-linking of EVA also causes generation of acetic acid and gel formation. The more elevated temperature at the cross-linking step compared to the compounding step results in a correspondingly increased generation of acetic acid and formation of gel. Besides having an obnoxious smell, the acetic acid generated means a loss of VA from the EVA-containing layer and, probably connected therewith, a reduced strippability when making cables with a strippable outer semiconducting EVA-containing layer. Further, the acetic acid released condenses in the vulcanising tube together with other volatile substances and forms a viscous sticky liquid at the bottom of the vulcanising tube. This liquid must be removed from the vulcanising tube as otherwise it tends to adhere to and contaminate the surface of the cable. This implies production stops and lower productivity.
Yet another problem with electric cables is the so-called “shrink-back” phenomenon, which manifests itself in particular when standard type EEA or EBA based semiconducting compositions are used as inner semiconductive layers on cables with solid conductors. This problem is related to the fact that the metal conductor of the cable and the polymer coating layers of the cable shrink differently when cooled. After making the cable by extrusion and cross-linking of the polymer layers around the metal conductor as described earlier, the cable is cut into lengths of a suitable dimension and cooled to ambient temperature. Upon cooling the polymer layers of the cable shrink more than the metallic conductor. This shrinking decreases the diameter of the cable coating and also decreases its length along the cable. The last mentioned lengthwise shrinking makes the metallic conductor protrude beyond the cable coating at both ends of the cable (the coating shrinks back from the metallic conductor). The shrink-back of the cable coating also depends on adhesion between the coating, more particularly the inner semiconducting layer, and the metal conductor. The better the adhesion between the inner semiconducting layer and the metal conductor, the smal
Dammert Ruth
Eklind Hans
Gustafsson Bill
Borealis Technology Oy
Gray J. M.
Yamnitzky Marie
LandOfFree
Electric cable and a method for the production thereof does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electric cable and a method for the production thereof, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electric cable and a method for the production thereof will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3233139