Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2001-12-20
2004-11-09
Jackson, Monique R. (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S375000, C428S379000, C428S383000
Reexamination Certificate
active
06815062
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a cable, in particular for electric energy transportation or distribution, and an insulating composition used therein.
In particular, the invention describes an electric cable and an insulating composition substantially free of lead or its derivatives, enabling cables to be produced having considerable mechanical and electrical properties and, in particular, an improved water resistance.
Cables have to be protected to limit damage deriving from moisture, abrasion, atmospheric chemicals etc., and have therefore to offer specific characteristics dictated and governed by international regulations.
2. Description of the Related Art
In particular, for electric energy transportation or distribution cables an extremely long operating life is required both to limit cost and to avoid their frequent replacement.
Cables for medium or high voltage electric energy transportation or distribution usually comprise a core formed from a conductor surrounded by one or more coverings of cross-linked polymer material, in particular polyethylene or ethylene copolymers suitably cross-linked after extrusion onto the conductor. These cross-linked materials maintain a high level of flexibility and satisfactory mechanical properties even under heating in continuous use and/or under current overload conditions.
To improve the performance of the insulating material, in particular its mechanical strength and hot-deformability, reinforcing fillers of various types, such as calcined kaolin talc and the like, are usually added to the base polymer material.
Generally, to prevent decay of dielectric strength with time, in particular in the presence of moisture, leading to formation of insulation defects commonly known as water trees, lead compounds are also added to the insulating composition, of which those currently most used are oxides, preferably Pb
2
O
3
and Pb
3
O
4
, phthalates, phosphites and mono-and di-stearates.
The need of substantially lead-free materials to provide increasing environmental compatibility has been felt in this field for a long time.
For example U.S. Pat. No. 4,204,024 describes a composition based on an ethylene-propylene elastomer (EPR, i.e. ethylene-propylene rubber) or, preferably, a non-conjugated ethylene-propylene-diene terpolymer (EPDM, i.e. ethylene propylene diene monomer), with vinyl silane, zinc oxide, a cross-linking agent (peroxide) and a cross-linking inhibitor to prevent premature cross-linking of the composition, the facility being provided to replace Pb
2
O
3
or Pb
3
O
4
with 0.1-2 parts by weight of lead peroxide, calculated on the compound, to eliminate the reddish coloration typically resulting from the use of said oxides. The use of this composition as electrical insulation should enable a reduction in the dielectric loss factor of the resultant cross-linked covering to be obtained.
WO 96/37899 describes a composition based on ethylene elastomer copolymers (EPR or EPDM, preferably obtained with metallocene catalysis) as the insulating material for cables, in which the lead compounds do not exceed one part by weight, calculated as the element. The compositions also contain a mineral filler in a quantity of 40-120 phr, i.e. parts per hundred rubber (of which at least 50% is calcined clay), a filler surface treatment agent (for example a silane) and zinc oxide in a quantity of between 10 and 30 phr.
To prepare water-resistant cables, U.S. Pat. No. 5,274,013 describes substantially lead-free elastomer compositions comprising a chlorinated polyolefin, preferably chlorinated polyethylene with a chlorine content of 25-45% by weight, an epoxide, a filler, a plasticizer, a cross-linking agent and possibly an EPDM polymer.
Patent application W097/00523 describes polymer compositions including EPDM polymers, in which the diene is vinyl norbornene, which enable a uniform surface of reduced roughness to be obtained, at a higher extrusion rate than conventional elastomer compositions based on EPDM polymers in which the diene is selected from the group consisting of 5-ethylidene-2-norbornene (ENB), 1,4-hexadiene (HEX), 1,6-octadiene, 5-methyl-1,4-hexadiene, 3,7-dimethyl-1,6-octadiene and the like. In particular the compositions described therein enable a good cross-linking rate and a high level of cross-linking to be obtained, with relatively low dielectric losses.
Patent application WO98/56012 describes polymer compositions for cable insulation in which the base polymer material is a mixture of an EPDM terpolymer, substantially equivalent to the EPDMs described in WO97/00523, with a minority component in the form of an ethylene/&agr;-olefin copolymer prepared by metallocene catalysis, able to further improve the physical properties of the resultant mixture over that described in WO97/00523.
However, the elastomer compositions described in both WO97/00523 and in WO98/56012 contemplate the presence of lead compounds, in the usual quantities (5 phr) for that field.
The substantial absence of lead in U.S. Pat. No. 4,204,024 or in WO96/37899 is compensated by using an extremely high percentage, of about 10-40 and 10-30 parts by weight respectively, of zinc oxide noting, explicitly in WO96/37899, that a quantity of 5% by weight, as is currently used in that field, is insufficient to obtain satisfactory electrical stability in the presence of moisture.
The use of a large quantity of zinc oxide evidently cannot constitute a desirable solution for a cable with low environmental impact, because of the pollution potential of zinc and its derivatives which, even though less than that of lead, is still not negligible.
On the other hand, the use of chlorinated polymers, as suggested in U.S. Pat. No. 5,274,013, involves evident environmental problems both during cable production and when the cable, at the end of its life, has to be disposed of in some manner. In this respect it is known that chlorinated polymers develop HCl on heating, with outstanding corrosion problems for the cable production plant, and environmental pollution.
SUMMARY OF THE INVENTION
On the basis of the experience of Applicant and of the aforedescribed known art, there still exists the technical problem of producing an electric cable with a covering consisting of a polymer material possessing mechanical and electrical properties adequate for the usual utilization conditions, while maintaining high electrical strength even in the presence of moisture, without using potentially polluting products which can present a problem when the cable has to be disposed of at the end of its life. The present applicant has unexpectedly found that the aforesaid technical problem can be solved by using as the base material for the insulating covering an elastomer terpolymer of ethylene, an &agr;-olefin and 5-vinyl-2-norbornene, without the addition of lead-containing products.
In particular the present applicant has found that the electrical properties of the insulating covering, with reference in particular to dielectric strength and dielectric losses, remain substantially unaltered with time even in the presence of moisture, if the base material used for the insulating covering is an elastomer terpolymer of the following composition:
a) 50-90 moles percent of ethylene;
b) 10-50 moles percent of an &agr;-olefin;
c) 0.16-5 moles percent of 5-vinyl-2-norbornene; the sum of the moles percent of a), b), c) being 100, said terpolymer having a branching index of less than or equal to 0.5 and a molecular weight distribution index M
w
/M
n
of greater than or equal to 6.
According to a first aspect, the invention therefore provides an electric cable (1) comprising at least one conductor (2) and at least one insulating layer (4), wherein said insulating layer comprises a substantially lead-free polymer composition comprising as base polymer material an elastomer terpolymer having the following composition:
a) 50-90 moles percent of ethylene;
b) 10-50 moles percent of an &agr;-olefin;
c) 0.16-5 moles percent of 5-vinyl-2-norbornene; the sum of the moles percent o
Albizzati Enrico
Caimi Luigi
Zaopo Antonio
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Jackson Monique R.
Pirelli Cavi E Sistemi S.p.A.
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