Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1999-01-05
2001-08-14
Sellers, Robert E. L. (Department: 1712)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C174S1050SC, C174S1100PM, C174S1210SR, C525S208000
Reexamination Certificate
active
06274239
ABSTRACT:
DESCRIPTION
In a general aspect, the present invention relates to electric cables comprising at least one conducting core and at least one insulation coating comprising a polyolefin polymeric base.
More particularly, the present invention relates to an electric cable comprising an insulation coating having an improved resistance against the formation of the so-called water trees.
The present invention also relates to a polymeric composition which has a preferred, although not exclusive, use in the manufacture of an insulation coating in electric cables for power transmission at medium or high voltage.
In the following description, the terms: medium and high voltage, are used to indicate voltages of from 1 to 35 kV and, respectively, over 35 kV.
As is known, one of the more difficult problems to solve in the manufacture of electric cables for power transmission or energy cable, is that of ensuring that the insulation coating of the conducting core achieves an effective control of the electric field generated during the energy transmission, dissipates as little power as possible and preserves its dielectric and structural characteristics along time.
It is also known that to obtain such characteristics the best material for manufacturing the insulation coating is constituted by olefin polymers and in particular by polyethylene and copolymers or terpolymers thereof.
More particularly, among the latter, cross-linked polyethylene—commonly indicated by the acronym XLPE—has either good dieletric characteristics or a low loss factor (minimum dissipated power).
But together with these good characteristics, olefin polymers generally possess a low resistance against a particular degradation phenomenon, known in the art by the term: “water treeing”, which may cause in time a degradation of the electric characteristic of the insulation material.
Such phenomenon essentially consists in the formation of microfractures having a branched shape (trees), progressively growing in time and responsible, in some cases, for an electric weakening of the insulation coating.
Even though the mechanism leading to the formation of these microfractures or weak zones has not yet been fully clarified, the formation of such zones or “trees” is anyhow attributed to the combined action of the electric field generated by the current flow in the conducting core of the cable and of the moisture existing in the inside of the insulation coating.
The problem represented by the formation of the above water trees is particularly felt in the cables for power transmission at medium or high voltage which are not provided with external protection elements, wherein the insulation coating may be directly in touch with water or anyhow with humid environments.
In order to reduce somehow the formation of water trees, different solutions have been proposed in the art, which are essentially based either on the selection of suitable polymeric materials for the manufacture of the insulation coatings, or on the use of suitable retarding additives, so-called tree-retardants.
So, for instance, it is known from U.S. Pat. No. 5,246,783 the use of an ethylene copolymer and of an alpha-olefin having 3 to 20 carbon atoms, having a molecular weight distribution of from 1.5 to 30 and a distribution index of the alpha-olefin comonomer greater than 45%.
European Patent EP 0 179 845 discloses, on the other hand, the combined use of an ethylene polymer or of an ethylene copolymer with an alpha-olefin, with an ethylene-alkylacrylate or an ethylene-alkylmethacrylate copolymer, in a cross-linkable coating composition resistant to the formation of water trees, for medium/high voltage energy cables.
As to the use of the so-called tree-retardant additives, it is known from the U.S. Pat. Nos. 4,212,756 and 4,144,202 the use of particular organo silanes comprising an acrylic/methacrylic group and, respectively, an epoxy group.
According to the present invention, it has now been found that the simultaneous presence of ester groups and epoxy groups in a polyolefin polymeric composition may impart to said composition a particular resistance to the phenomenon of water trees in the working condition of an electric cable.
According to the invention, in fact, a surprising synergistic effect—in terms of increased resistance to the water treeing phenomenon—has been observed, due to the simultaneous presence of ester groups and epoxy groups within a selected concentration range.
When the aforementioned groups are simultaneously present in the polymeric base forming the insulation coating of the cable, it has been noticed in particular that the retarding effect against water tree formation is evidently greater than the sum of the effects of the same groups when the latter are present alone.
According to a first aspect thereof, the present invention therefore provides an electric cable comprising at least a conductor and at least an insulation coating comprising a polyolefin polymeric base, which is characterized in that said polyolefin polymeric base comprises, in parts by weight to the total weight of the same:
from 0.5 to 15 parts of ester groups; and
from 0.01 to 5 parts of epoxy groups.
According to a further aspect of the invention, the problem of an adequate resistance to the phenomenon of water trees is therefore solved by a cable which is characterized in that said polyolefin polymeric base comprises a first predetermined amount of ester groups and a second predetermined amount of epoxy groups, said first and second predetermined amounts being such as to reduce the water tree formation in the insulation coating material after electric ageing in water.
In the following description and the subsequent claims, the term: electric water ageing, is used to indicate an ageing treatment of the insulation coating carried out in water and in the presence of an electric field such as—for instance—the treatment proposed by EFI (Norwegian Electric Power Research Institute), illustrated below, or analogous treatments well known in the art.
According to a further aspect thereof, the present invention also provides a polyolefin polymeric composition resistant to water treeing, in particular for the manufacture of an insulation coating for electric cables, which is characterized in that it comprises—in parts by weight to the total weight thereof—from 0.05 to 15 parts of ester groups and from 0.01 to 5 parts of epoxy groups.
In the following description and the subsequent claims, the term: polyolefin polymeric base, is used to indicate a polymer selected from the group comprising high-, medium- and low-density polyethylene homopolymers, ethylene copolymers and ethylene terpolymers with an alpha-olefin having 3 to 20 carbon atoms, ethylene-alpha-olefin-diene terpolymers and mixtures thereof.
The term: polyolefin polymeric composition, on the other hand, is used to indicate a polymeric composition comprising a polyolefin polymeric base of the above defined type.
Preferably, the polyolefin polymeric base of the invention is an ethylene polymer selected from the group comprising: polyethylene, copolymers obtainable by polymerizing ethylene with at least one alpha-olefin, linear or branched, having 3 to 14 carbon atoms, terpolymers obtainable by polymerizing ethylene, an alpha-olefin, linear or branched, having 3 to 14 carbon atoms and a diene having 4 to 25 carbon atoms having a density (measured according to ASTM D-792) of from 0.860 g/cm
3
to 0.940 g/cm
3
and a Melt Index (measured according to ASTM D-1238) of from 0.1 g/10′ to 40 g/10′.
In the terpolymers of the invention, the above diene is preferably selected from the group comprising: 1,4 pentadiene, 1,4 hexadiene, 1,5 hexadiene, dicyclopentadiene, 4-vinyl-cyclohexene, 1-vinyl-1-cyclopentene, ethyl norbornene (LNB), alkylbicyclononadiene, indene, norbornene and mixtures thereof.
According to the invention, it has been observed that to achieve an adequate resistance to the water tree formation, the polyolefin matrix forming the insulation coating of the cable conducting core should preferably comprise at least
Garcia Valeria
Peruzzotti Franco
Norris & McLaughlin & Marcus
Pirelli Cavi S.p.A.
Sellers Robert E. L.
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