Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2000-03-24
2002-07-09
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S372000, C428S379000, C252S511000, C174S1050SC, C174S1100PM, C174S1200SC
Reexamination Certificate
active
06416860
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electric cables and particularly the invention relates to inner semiconducting layers of electric cables, preferably to cross-linked, inner semiconducting layers of electric cables, and to a method of producing same.
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 polymerization 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 ethylene-vinyl acetate copolymer (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 a strippable composition, mention may be made of EP-B1-0 420 271 which discloses a semiconducting insulation shielding composition for electric cables which, based on the total weight of the composition, consists essentially of (A) 40-64% by weight of an ethylene-vinyl acetate copolymer with 27-45% of vinyl acetate, (B) 5-30% by weight of an acrylonitrile-butadiene copolymer with 25-55% of acrylonitrile, (C) 25-45% by weight of carbon black having a surface area of 30-60 m
2
/g, and (D) 0.2-5% by weight of an organic peroxide cross-linking agent. In addition the composition may include 0.05-3% by weight of conventional additives.
As a further example of prior art strippable semiconducting 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-ethyl-hexyl 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 acrylo-nitrile, (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 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.
Besides being semiconducting it is often desired that the outer semiconducting layer is strippable from the other layers (i.e. the insulating layer) to facilitate the joining of two cable ends. This strippability is achieved by making the outer semiconducting layer more polar (e.g. with the aid of a polar polymer, such as EVA) than He underlying insulating layer and cross-linking the outer semiconducting layer. The strippability of the outer semiconducting layer from the insulating layer is also influenced by other factors such as e.g. the choice of carbon black in the semiconducting layer.
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 o: 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.
A disadvantage of EEA and EBA as polymers for semiconducting layers of electric cables is that when heated to high temperatures they decompose and split off decomposition products. The main decomposition products of EEA are ethylene in gas form and carboxylic and anhydride groups on the EEA main chain. Similarly, the main decomposition products of EBA are butene in gas form and carboxylic and anhydride groups on the EBA main chain. Although the thermal stability of EEA and EBA is about 100° C. higher than that of EVA, thermal decomposition of EEA and EBA may occur at high temperature processing, such as the vulcanising of cables. The decomposition of EEA and EBA means that there remains less EEA and EBA in the semiconducting layer and consequently that the polarity of the layer is lowered. Further, the decomposition product that is s
Fagrell Ola
Gustafsson Bill
Borealis A/S
Gray J. M.
Kelly Cynthia H.
Merchant & Gould P.C.
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