U.S. patent application number 09/791226 was filed with the patent office on 2001-11-29 for polymeric composition for coating electric cables having an improved resistance to water treeing and electric cable comprising said composition.
Invention is credited to Garcia, Valeria, Peruzzotti, Franco.
Application Number | 20010046601 09/791226 |
Document ID | / |
Family ID | 11371847 |
Filed Date | 2001-11-29 |
United States Patent
Application |
20010046601 |
Kind Code |
A1 |
Peruzzotti, Franco ; et
al. |
November 29, 2001 |
Polymeric composition for coating electric cables having an
improved resistance to water treeing and electric cable comprising
said composition
Abstract
A polymeric composition for coating electric cables for power
transmission at medium/high voltage, having an improved resistance
against the formation of the so-called water trees, comprises a
polymeric polyolefin base including, in parts by weight to the
total weight thereof, from 0.5 to 15 parts of ester groups and from
0.01 to 5 parts of epoxy groups. Electric cables according to the
invention comprise at least one electrically conducting core (2)
and an insulation coating (5) comprising the aforementioned
polymeric composition.
Inventors: |
Peruzzotti, Franco;
(Legnano, IT) ; Garcia, Valeria; (Brugherio,
IT) |
Correspondence
Address: |
NORRIS MCLAUGHLIN & MARCUS, P.A.
P O BOX 1018
SOMERVILLE
NJ
08876
|
Family ID: |
11371847 |
Appl. No.: |
09/791226 |
Filed: |
February 22, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09791226 |
Feb 22, 2001 |
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09226059 |
Jan 5, 1999 |
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6274239 |
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09226059 |
Jan 5, 1999 |
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08654280 |
May 28, 1996 |
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Current U.S.
Class: |
428/375 |
Current CPC
Class: |
C08L 23/0884 20130101;
C08K 5/14 20130101; C08L 2203/202 20130101; Y10T 428/2933 20150115;
C08L 23/06 20130101; C08L 2666/04 20130101; C08L 2666/06 20130101;
H01B 7/2813 20130101; C08L 2666/22 20130101; H01B 7/2825 20130101;
C08L 23/02 20130101; C08L 23/02 20130101; H01B 7/282 20130101; H01B
3/441 20130101; C08L 23/06 20130101; C08L 23/02 20130101 |
Class at
Publication: |
428/375 |
International
Class: |
D02G 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 21, 1995 |
IT |
MI 95 A 001330 |
Claims
1. An electric cable comprising at least one conducting core (2)
and at least one insulation coating (5) including a polyolefin
polymeric base, characterized in that said polyolefin polymeric
base comprises, in parts by weight to the total weight thereof:
from 0.5 to 15 parts of ester groups; and from 0.01 to 5 parts of
epoxy groups.
2. An electric cable comprising at least one conducting core (2)
and at least one insulation coating (5) including a polyolefin
polymeric base, 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 said
second predetermined amounts being such as to reduce the formation
of water trees within the material of the insulation coating (5)
after electric ageing in water.
3. An electric cable according to claims 1 or 2, characterized in
that said polyolefin polymeric base comprises: a) at least one
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.sup.3 to 0.940 g/cm.sup.3 and a Melt
Index (measured according to ASTM D-1238) of from 0.1 g/10' to 40
g/10'; b) at least one acrylic or vinyl polymer selected from the
group comprising: i) copolymers obtainable by polymerizing ethylene
with at least one acrylic ester of the formula: 11wherein R.sub.1
is H or CH.sub.3, R.sub.2 is an alkyl or aryl hydrocarbon group,
linear or branched, preferably a phenyl, having 1 to 10 carbon
atoms, and R.sub.3 is hydrogen or an alkyl or aryl hydrocarbon
group, linear or branched, preferably a phenyl, having 1 to 10
carbon atoms; ii) copolymers obtainable by polymerizing ethylene
with at least one vinyl ester of a carboxylic acid of the formula:
12wherein R.sub.2 is an alkyl or aryl hydrocarbon group, linear or
branched, preferably a phenyl, having 1 to 10 carbon atoms; c) at
least one glycidyl ether of the formula: 13wherein R.sub.4, R.sub.6
and R.sub.7 are independently a hydrogen atom, an alkyl or aryl
hydrocarbon group, linear or branched, preferably an optionally
substituted phenyl, having I to 25 carbon atoms or a glycidyl ether
group of the formula: 14or polyfunctional derivatives thereof.
4. An electric cable according to claim 3, characterized in that
said polyolefin polymeric base comprises from 5 to 40 parts by
weight of said at least one acrylic or vinyl polymer (b) for each
100 parts by weight of said ethylene polymer (a).
5. An electric cable according to claims 1 or 2, characterized in
that said polyolefin polymeric base comprises: a) at least one
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.sup.3 to 0.940 g/cm.sup.3 and a Melt
Index (measured according to ASTM D-1238) of from 0.1 g/10' to 40
g/10'; b) at least one glycidyl ester of the formula: 15wherein
R.sub.3 and R.sub.5 are independently H, an alkyl or aryl
hydrocarbon group, linear or branched, preferably a phenyl, having
1 to 10 carbon atoms; R.sub.1 is H or CH.sub.3.
6. An electric cable according to claim 5, characterized in that
said at least one glycidyl ester is glycidyl methacrilate.
7. An electric cable according to anyone of the preceding claims,
characterized in that said polyolefin polymeric base further
comprises an epoxy resin.
8. An electric cable according to claims 1 or 2, characterized in
that said polyolefin polymeric base comprises: a) at least one
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.sup.3 to 0.940 g/cm.sup.3 and a Melt
Index (measured according to ASTM D-1238) of from 0.1 g/10' to 40
g/10'; b) at least one acrylic or vinyl polymer selected from the
group comprising: i) copolymers obtainable by polymerizing ethylene
with at least one acrylic ester of the formula: 16wherein R.sub.1
is H or CH.sub.3, R.sub.2 is an alkyl or aryl hydrocarbon group,
linear or branched, preferably a phenyl, having 1 to 10 carbon
atoms, and R.sub.3 is hydrogen or an alkyl or aryl hydrocarbon
group, linear or branched, preferably a phenyl, having 1 to 10
carbon atoms; ii) copolymers obtainable by polymerizing ethylene
with at least one vinyl ester of a carboxylic acid of the formula:
17wherein R.sub.2 is an alkyl or aryl hydrocarbon group, linear or
branched, preferably a phenyl, having 1 to 10 carbon atoms; c) at
least one epoxy resin.
9. An electric cable according to claims 7 or 8, characterized in
that said epoxy resin is aliphatic, cycloaliphatic or aromatic.
10. An electric cable according to claim 9, characterized in that
said epoxy resin has a dynamic viscosity at 25.degree. C. of from
60 to 55,000 mPa.times.s and an epoxy content of from 0.1 to 0.7
gram eq/100 g of resin.
11. An electric cable according to claims 1 or 2, characterized in
that said polyolefin polymeric base further comprises a terpolymer
having a Melt Index of from 0.1 g/10' to 40 g/10' obtainable by
polymerizing ethylene with i) at least one acrylic ester of the
formula: 18wherein R.sub.1 is H or CH.sub.3, R.sub.2 is an alkyl or
aryl hydrocarbon group, linear or branched, preferably a phenyl,
having 1 to 10 carbon atoms, and R.sub.3 is hydrogen or an alkyl or
aryl hydrocarbon group, linear or branched, preferably a phenyl,
having 1 to 10 carbon atoms; ii) at least one glycidyl ester of the
formula: 19wherein R.sub.3 and R.sub.5 are independently H, an
alkyl or aryl hydrocarbon group, linear or branched, preferably a
phenyl, having 1 to 10 carbon atoms; R.sub.1 is H or CH.sub.3.
12. An electric cable according to claims 1 or 2, characterized in
that said polyolefin polymeric base further comprises a terpolymer
having a Melt Index of from 0.1 g/10' to 40 g/10' obtainable by
polymerizing ethylene with i) at least one vinyl ester of a
carboxylic acid of the formula: 20wherein R.sub.2 is an alkyl or
aryl hydrocarbon group, linear or branched, preferably a phenyl,
having 1 to 10 carbon atoms; ii) at least one glycidyl ester of the
formula: 21wherein R.sub.3 and R.sub.5 are independently H, an
alkyl or aryl hydrocarbon group, linear or branched, preferably a
phenyl, having 1 to 10 carbon atoms; R.sub.1 is H or CH.sub.3.
13. An electric cable according to claims 11 or 12, characterized
in that said at least one glycidyl ester is glycidyl
methacrilate.
14. An electric cable according to claims 1 or 2, characterized in
that said polyolefin polymeric base further comprises a copolymer
obtainable by polymerizing ethylene with at least one glycidyl
ester of the formula 22wherein R.sub.3 and R.sub.5 are
independently H, an alkyl or aryl hydrocarbon group, linear or
branched, preferably a phenyl, having 1 to 10 carbon atoms; R.sub.1
is H or CH.sub.3.
15. An electric cable according to claim 14, characterized in that
said at least one glycidyl ester is glycidyl methacrylate.
16. An electric cable according to anyone of the preceding claims,
characterized in that said polyolefin polymeric base further
comprises an effective amount of at least one cross-linking
agent.
17. An electric cable according to claim 16, characterized in: ,
that said at least one cross-linking agent is
ter-butyl-cumylperoxide.
18. An electric cable according to anyone of the preceding claims,
characterized in that said polyolefin polymeric base further
comprises an effective amount of at least one antioxidant
agent.
19. A polyolefin polymeric composition resistant to water treeing,
in particular for the manufacture of an insulating coating for
electric cables, characterized in that it comprises in parts by
weights to the total weight thereof: from 0.5 to 15 parts of ester
groups; and from 0.01 to 5 parts of epoxy groups.
20. A polymeric composition according to claim 19, characterized in
that it comprises: a) at least one 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.sup.3 to 0.940 g/cm.sup.3 and a Melt Index (measured according
to ASTM D-1238) of from 0.1 g/10' to 40 g/10'; b) at least one
acrylic or vinyl polymer selected from the group comprising: i)
copolymers obtainable by polymerizing ethylene with at least one
acrylic ester of the formula: 23wherein R.sub.1 is H or CH.sub.3,
R.sub.2 is an alkyl or aryl hydrocarbon group, linear or branched,
preferably a phenyl, having 1 to 10 carbon atoms, and R.sub.3 is
hydrogen or an alkyl or aryl hydrocarbon group, linear or branched,
preferably a phenyl, having 1 to 10 carbon atoms; ii) copolymers
obtainable by polymerizing ethylene with at least one vinyl ester
of a carboxylic acid of the formula: 24wherein R.sub.2 is an alkyl
or ary hydrocarbon group, linear or branched, preferably a phenyl,
having 1 to 10 carbon atoms; c) at least one glycidyl ether of the
formula: 25wherein R.sub.4, R.sub.6 and R.sub.7 are independently a
hydrogen atom, an alkyl or aryl hydrocarbon group, linear or
branched, preferably an optionally substituted phenyl, having 1 to
25 carbon atoms or a glycidyl ether group of the formula: 26or
polyfunctional derivatives thereof.
21. A polymeric composition according to claim 20, characterized in
that it comprises from 5 to 40 parts by weight of said at least one
acrylic or vinyl polymer (b) for each 100 parts by eight of said
ethylene polymer (a).
22. A polymeric composition according to claim 19, characterized in
that it comprises: a) at least one 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.sup.3 to 0.940 g/cm.sup.3 and a Melt Index (measured according
to ASTM D-1238) of from 0.1 g/10' to 40 g/10'; b) at least one
glycidyl ester of the formula: 27wherein R.sub.3 and R.sub.5 are
independently H, an alkyl or aryl hydrocarbon group, linear or
branched, preferably a phenyl, having 1 to 10 carbon atoms; R.sub.1
is H or CH.sub.3.
23. A polymeric composition according to claim 22, characterized in
that said at least one glycidyl ester is glycidyl methacrylate.
24. A polymeric composition according to anyone of claims 19-23,
characterized in that it further comprises an epoxy resin.
25. A polymeric composition according to claim 19, characterized in
that said polyolefin polymer base comprises: a) at least one
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.sup.3 to 0.940 g/cm.sup.3 and a Melt
Index (measured according to ASTM D-1238) of from 0.1 g/10' to 40
g/10'; b) at least one acrylic or vinyl polymer selected from the
group comprising: i) copolymers obtainable by polymerizing ethylene
with at least one acrylic ester of the formula: 28wherein R.sub.1
is H or CH.sub.3, R.sub.2 is an alkyl or aryl hydrocarbon group,
linear or branched, preferably a phenyl, having 1 to 10 carbon
atoms, and R.sub.3 is hydrogen or an alkyl or aryl hydrocarbon
group, linear or branched, preferably a phenyl, having 1 to 10
carbon atoms; ii) copolymers obtainable by polymerizing ethylene
with at least one vinyl ester of a carboxylic acid of the formula:
29wherein R.sub.2 is an alkyl or aryl hydrocarbon group, linear or
branched, preferably a phenyl, having 1 to 10 carbon atoms; c) at
least one epoxy resin.
26. A polymeric composition according to claims 24 or 25,
characterized in that said epoxy resin is aliphatic, cycloaliphatic
or aromatic.
27. A polymeric composition according to claim 26, characterized in
that said epoxy resin has a dynamic viscosity at 25.degree. C. of
from 60 to 55,000 mPa.times.s and an epoxy content of from 0.1 to
0.7 gram eq/100 g of resin.
28. A polymeric composition according to claim 19, characterized in
that it further comprises a terpolymer having a Melt Index of from
0.1 g/10' to 40 g/10' obtainable by polymerizing ethylene with i)
at least one acrylic ester of the formula: 30wherein R.sub.1 is H
or CH.sub.3, R.sub.2 is an alkyl or aryl hydrocarbon group, linear
or branched, preferably a phenyl, having 1 to 10 carbon atoms, and
R.sub.3 is hydrogen or an alkyl or aryl hydrocarbon group, linear
or branched, preferably a phenyl, having 1 to 10 carbon atoms; ii)
at least one glycidyl ester of the formula: 31wherein R.sub.3 and
R.sub.5 are independently H, an alkyl or aryl hydrocarbon group,
linear or branched, preferably a phenyl, having 1 to 10 carbon
atoms; R.sub.1 is H or CH.sub.3.
29. A polymeric composition according to claim 19, characterized in
that it further comprises a terpolymer having a a Melt Index of
from 0.1 g/10' to 40 g/10' obtainable by polymerizing ethylene with
i) at least one vinyl ester of a carboxylic acid of the formula:
32wherein R.sub.2 is an alkyl or aryl hydrocarbon group, linear or
branched, preferably a phenyl, having 1 to 10 carbon atoms; ii) at
least one glycidyl ester of the formula: 33wherein R.sub.3 and
R.sub.5 are independently H, an alkyl or aryl hydrocarbon group,
linear or branched, preferably a phenyl, having 1 to 10 carbon
atoms; R.sub.1 is H or CH.sub.3.
30. A polymeric composition according to claims 28 or 29,
characterized in that said at least one glycidyl ester is glycidyl
methacrylate.
31. A polymeric composition according to claim 19, characterized in
that it further comprises a copolymer obtainable by polymerizing
ethylene with at least one glycidyl ester of the formula: 34wherein
R.sub.3 and R.sub.5 are independently H, an alkyl or aryl
hydrocarbon group, linear or branched, preferably a phenyl, having
1 to 10 carbon atoms; R.sub.1 is H or CH.sub.3.
32. A polymeric composition according to claim 31, characterized in
that said at least one glycidyl ester is glycidyl methacrylate.
33. A polymeric composition according to anyone of the preceding
claims, characterized in that it further comprises an effective
amount of at least one cross-linking agent.
34. A polymeric composition according to claim 33, characterized in
that said at least one cross-linking agent is
ter-butyl-cumylperoxide.
35. A polymeric composition according to anyone of the preceding
claims, characterized in that it further comprises an effective
amount of at least one antioxidant agent.
36. Use of a polymeric composition according to anyone of claims
19-35 for the insulation of electric cables.
37. Use of a glycidyl ester of the formula: 35wherein R.sub.3 and
R.sub.5 are independently H, an alkyl or aryl hydrocarbon group,
linear or branched, preferably a phenyl, having 1 to 10 carbon
atoms; R.sub.1 is H or CH.sub.3, as tree retardant additive in a
polymeric composition for coating an electric cable.
38. Use according to claim 37, characterized in that said at least
one glycidyl ester is glycidyl methacrylate.
39. Use of a terpolymer obtainable by polymerizing ethylene with i)
at least one acrylic ester of the formula: 36wherein R.sub.1 is H
or CH.sub.3, R.sub.2 is an alkyl or aryl hydrocarbon group, linear
or branched, preferably a phenyl, having 1 to10 carbon atoms, and
R.sub.3 is hydrogen or an alkyl or aryl hydrocarbon group, linear
or branched, preferably a phenyl, having 1 to 10 carbon atoms; ii)
at least one glycidyl ester of the formula: 37wherein R.sub.3 and
R.sub.5 are independently H, an alkyl or aryl hydrocarbon group,
linear or branched, preferably a phenyl, having 1 to 10 carbon
atoms; R.sub.1 is H or CH.sub.3, as tree retardant additive in a
polymeric composition for coating an electric cable.
40. Use of a terpolymer obtainable by polymerizing ethylene with i)
at least one vinyl ester of a carboxylic acid of the formula:
38wherein R.sub.2 is an alkyl or aryl hydrocarbon group, linear or
branched, preferably a phenyl, having 1 to 10 carbon atoms; ii) at
least one glycidyl ester of the formula: 39wherein R.sub.3 and
R.sub.5 are independently H, an alkyl or aryl hydrocarbon group,
linear or branched, preferably a phenyl, having 1 to 10 carbon
atoms; R.sub.1 is H or CH.sub.3, as tree retardant additive in a
polymeric composition for coating an electric cable.
41. Use according to claims 39 or 40, characterized in that said
terpolymer has a Melt Index of from 0.1 g/10' to 40 g/10'.
42. Use according to claims 39 or 40, characterized in that said at
least one glycidyl ester is glycidyl methacrylate.
43. Use of a terpolymer obtainable by polymerizing ethylene with at
least one glycidyl ester of the formula: 40wherein R.sub.3 and
R.sub.5 are independently H, an alkyl or aryl hydrocarbon group,
linear or branched, preferably a phenyl, having 1 to 10 carbon
atoms; R.sub.1 is H or CH.sub.3, as tree retardant additive in a
polymeric composition for coating an electric cable.
44. Use according to claim 43, characterized in that said at least
one glycidyl ester is glycidyl methacrylate.
45. Use according to claim 43, characterized in that said copolymer
has a Melt Index of from 0.1 g/10' to 40 g/10'.
Description
[0001] 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.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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 terpolimers thereof.
[0007] 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).
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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%.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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:
[0020] from 0.5 to 15 parts of ester groups; and
[0021] from 0.01 to 5 parts of epoxy groups.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.sup.3 to 0.940
g/cm.sup.3 and a Melt Index (measured according to ASTM D-1238) of
from 0.1 g/10' to 40 g/10'.
[0028] 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.
[0029] 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 0.5% by weight
of ester groups and at least 0.01% by weight of epoxy groups.
[0030] On the other hand, it has been observed that amounts
exceeding 15% by weight of ester groups and, respectively, 5% by
weight of epoxy groups do not produce a substantial additional
benefit in terms of resistance to the phenomenon of water trees,
against a marked increase in the power dissipated by the insulation
coating (increase of the loss factor or tg delta), with ensuing
increase in the energy transmission costs.
[0031] According to the invention, is has also been observed that
the aforementioned improved resistance to the water trees formation
is not substantially affected by the way in which the ester groups
and the epoxy groups are incorporated into the polymeric
composition, provided that these groups are present in the
aforementioned amounts, as indicated above.
[0032] So, for instance, in a first embodiment of the invention, a
polymeric composition comprising the above minimum amount of ester
groups and epoxy groups may be prepared by adding to a polyolefin
polymeric base a first compound, either polymeric or not,
incorporating an ester group and, respectively, a second compound,
either polymeric or not, incorporating an epoxy group.
[0033] Preferably, the above compound incorporating an ester group
is an acrylic or vinyl polymer selected from the group
comprising:
[0034] i) copolymers obtainable by polymerizing ethylene with at
least an acrylic ester of the formula: 1
[0035] wherein R.sub.1 is H or CH.sub.3, R.sub.2 is an alkyl or
aryl hydrocarbon group, linear or branched, preferably a phenyl,
having 1 to 10 carbon atoms, and R.sub.3 is hydrogen or an alkyl or
aryl hydrocarbon group, preferably a phenyl, linear or branched,
having 1 to 10 carbon atoms;
[0036] ii) copolymers obtainable by polymerizing ethylene with at
least a vinyl ester of a carboxylic acid of the formula: 2
[0037] wherein R.sub.2 is an alkyl or aryl hydrocarbon group,
linear or branched, preferably a phenyl, having 1 to 10 carbon
atoms.
[0038] For the purposes of the invention, the acrylic copolymers
having the formula (I) of preferred use comprise ethylene
copolymers with a comonomer selected from the group comprising the
following acrylic esters: methyl acrylate, ethyl acrylate, propyl
acrylate, n-butyl acrylate, iso-butyl acrylate, methyl
methacrylate, ethyl methacrylate, and mixtures thereof.
[0039] Among them, particularly preferred are the acrylic
ester-ethylene copolymers and, more particularly, those having an
amount of acrylic comonomer of from 2% to 40% by weight, such as
for instance those commercially available under the trade names
ENATHENE EA.TM. (Quantum Chemical Corporation, Cincinnati, Ohio,
U.S.A.), OPTEMATM (Exxon Chemical) and LOTRYL.TM. (Elf
Atochem).
[0040] For the purposes of the invention, the vinyl copolymers of
preferred use comprise ethylene copolymers with a comonomer
selected from the group comprising vinyl acetate and vinyl
propionate, such as for instance those commercially available under
the trade names LEVAPREN.TM. (Bayer), ESCORENE.TM. (Exxon
Chemical), ELVAX.TM. (Du Pont de Nemours International S.A.) and
EVATANE.TM. (Elf Atochem).
[0041] According to an advantageous aspect of the invention,
optimal homogeneity characteristics of the polymeric composition
may be achieved when the acrylic or vinyl copolymers having the
formula (I) and (II) have a Melt Index value near that of the
polyolefin polymeric base in which they may be easily
incorporated.
[0042] For the purposes of the invention, such Melt Index value
(measured according to ASTM D-1238) preferably ranges from 0.1
g/10' to 40 g/10'.
[0043] According to a preferred embodiment, the above amount of
ester groups may be reached when the polymeric composition
comprises at least 5% by weight of the above acrylic or vinyl
polymers having the formula (I) and (II), or greater values
depending upon the ester groups content within said polymers.
[0044] Furthermore, the polymeric composition of the invention
preferably comprises from 5% to 40% by weight of the aforementioned
acrylic or vinyl polmers having the formula (I) and (II).
[0045] According to the invention, the above-identified compound
incorporating an epoxy group may be:
[0046] a) a glycidyl ether of the formula: 3
[0047] wherein R.sub.4, R.sub.6 and R.sub.7 are independently a
hydrogen atom, an alkyl or aryl hydrocarbon group, linear or
branched, preferably an optionally substituted phenyl, having 1 to
25 carbon atoms or a glycidyl ether group of the formula: 4
[0048] b) a polyfunctional derivative of the glycidyl ether of
formula (III), or
[0049] c) an epoxy resin.
[0050] For the purposes of the invention, the glycidyl ethers
having the formula (III) may be mono-, bi-, tri- or
tetrafunctional; those of preferred use are mono- and bifunctional
glycidyl ethers selected from the group comprising:
p-t-butyl-phenyl-glycidyl ether, 2-ethyl-hexyl-glycidyl ether,
dodecyl-glycidyl ether, tetradecyl-glycidyl ether,
glycidyl-isopropyl-etherbutyl-glycidyl ether,
1,4-butanediol-diglycidyl ether, 1,6-hexen-diol-diglycidyl ether
and mixtures thereof.
[0051] Epoxy resins of preferred use comprise instead resins of the
aliphatic, cycloaliphatic or aromatic type, preferably having a
dynamic viscosity at 25.degree. C. of from 60 to 55,000
mPa.times.s, preferably between 7,000 and 10,000 mPa.times.s, and
an epoxy content (measured according to ASTM D-1652) of from 0.1 to
0.7 gram eq/100 g of resin, preferably between 0.53 and 0.55 gram
eq/100 g of resin.
[0052] Examples of such resins are those commercially available
under the trade name EUREPOX.TM. (SCHERING) and preferably those
named EUREPOX.TM. 730.
[0053] Preferably, the polymeric composition comprises from 0.2 to
10% by weight of at least one glicydyl ether having the formula
(III) or polyfunctional derivatives thereof and/or from 0.2 to 10%
by weight of said epoxy resin, so as to reach the aforementioned
amount of epoxy groups indicated hereinabove.
[0054] According to a further embodiment of the invention, the
above-identified minimum amount of ester groups and epoxy groups
may be reached by adding to a polyolefin polymeric base a compound,
either polymeric or not, incorporating either an ester group or an
epoxy group.
[0055] Bifunctional compounds of preferred and advantageous use
are, more particularly, those selected from the group comprising
the glycidyl esters of the formula: 5
[0056] wherein R.sub.3 and R.sub.5 are independently H, an alkyl or
aryl hydrocarbon group, linear or branched, preferably a phenyl,
having 1 to 10 carbon atoms; R.sub.1 is H or CH.sub.3.
[0057] Among them, the glycidyl esters of the acrylic or
methacrylic acid are preferred.
[0058] For the purposes of the invention, particularly preferred is
glycidyl methacrylate (GMA), commercially available under the trade
name BLEMMER G.TM. (Blemmer Chemical Corp.).
[0059] In this case, the aforementioned amount of ester and epoxy
groups may be reached when the polymeric composition preferably
comprises from 0.03 to 15% by weight of at least a glycidyl ester
having the formula (IV).
[0060] Bifunctional compounds of the polymeric type of preferred
and advantageous use are those selected from the group
comprising:
[0061] a) terpolymers obtainable by polymerizing ethylene with:
[0062] i) at least one acrylic ester of the formula: 6
[0063] wherein R.sub.1 is H or CH.sub.3, R.sub.2 is an alkyl or
aryl hydrocarbon group, linear or branched, preferably a phenyl,
having 1 to 10 carbon atoms and R.sub.3 is hydrogen or an alkyl or
aryl hydrocarbon group, preferably a phenyl, linear or branched,
having 1 to 10 carbon atoms;
[0064] ii) at least one glycidyl ester of the formula: 7
[0065] wherein the meaning of R.sub.1, R.sub.3 and R.sub.5 are
those indicated hereinabove;
[0066] b) terpolymers obtainable by polymerizing ethylene with:
[0067] i) at least one vinyl ester of a carboxylic acid of the
formula: 8
[0068] wherein R.sub.2 is an alkyl or aryl hydrocarbon group,
linear or branched, preferably a phenyl, having 1 to 10 carbon
atoms;
[0069] ii) at least one glycidyl ester of the formula: 9
[0070] wherein the meaning of R.sub.1, R.sub.3 and R.sub.5 are
those indicated hereinabove;
[0071] c) copolymers obtainable by polymerizing ethylene with at
least one glycidyl ester of the formula: 10
[0072] wherein R.sub.3 and R.sub.5 are independently H, an alkyl or
aryl hydrocarbon group, linear or branched, preferably a phenyl,
having 1 to 10 carbon atoms; R.sub.1 is H or CH.sub.3.
[0073] Also in these cases, glycidyl esters having the formula (IV)
of preferred use are the glycidyl esters of the acrylic or
methacrylic acids and, particularly, glycidyl methacrylate.
[0074] Ethylene/acrylic ester/glycidyl methacrylate terpolymers and
ethylene/glycidyl methacrylate copolymers of preferred and
advantageous use are commercially available under the trade names
LOTADER.TM. GMA AX8900 and LOTADER.TM. GMA AX8840 (Elf Atochem),
respectively.
[0075] As previously explained, in this case too optimum
homogeneity characteristics of the polymeric composition may be
obtained when the above bifunctional terpolymers or copolymers have
a Melt Index of from 0.1 g/10' to 40 g/10'.
[0076] In this case, the aforementioned amount of ester and epoxy
groups may be reached when the polymeric composition comprises from
3 to 30% by weight of at least an ethylene/acrylic ester/glycidyl
methacrylate terpolymer or ethylene/vinyl ester/glycidyl
methacrylate terpolymer and from 1 to 40% by weight of at least an
ethylene/glycidyl methacrylate copolymer.
[0077] Obviously, both the bifunctional compounds, either polymeric
or not, may be used in the polymeric composition of the invention,
either alone or combined with the aforementioned monofunctional
compounds (acrylic or vinyl polymers, glycidyl ether, epoxy resin)
, so as to reach the desired amount of ester and epoxy groups.
[0078] According to a further aspect thereof, the present invention
relates to a new use of one of the above bifunctional
compounds--either polymeric or not--incorporating either an ester
group or an epoxy group, as a tree retardant additive in a
polymeric composition for coating an electric cable.
[0079] In fact, it is advantageously possible to confer to a
polyolefin polymeric base the desired characteristics of resistance
to the water treeing phenomenon by simply adding said bifunctional
compounds to said base in the aforementioned amounts.
[0080] Preferably, the amount of the ester and epoxy groups falls
within said range of 0.5-15% parts by weight and, respectively, of
0.01-5% parts by weight to the total weight of the composition so
obtained.
[0081] In a preferred embodiment, the polymeric composition of the
invention is cross-linked by means of one of the methods known in
the art to this end.
[0082] Preferably, the polymeric composition is chemically
cross-linked; for this purpose, it incorporates an effective amount
of at least one cross-linking agent, such as for instance
ter-butyl-cumyl peroxide.
[0083] In order to achieve an improved stability, furthermore, the
polymeric composition of the invention advantageously incorporates
an effective amount of at least one antioxidant agent, such as for
instance 4,4'-thio-bis(3-methyl-6-ter-butyl)phenol.
[0084] Depending upon the particular use of the cable, moreover,
the polymeric composition of the invention may incorporate other
additives and fillers conventional in themselves, such as for
instance pigments, dyes, stabilizers, lubricants, etc.
[0085] Further advantages and characteristics of the invention will
be better apparent from the following description of some preferred
embodiments thereof, which are reported in the following by way of
non-limitative illustration, with reference to the attached
drawing, whose only figure shows, in perspective view and partial
cross-section, a cable according to the invention.
[0086] In such figure, reference 1 indicates a cable comprising an
electric conducting core 2 including a plurality of wires, i.e. of
copper, all indicated by 3.
[0087] The conducting core 2 is enclosed within several coaxial
coating layers, including an inner semiconducting layer 4, an
insulation layer 5, an outer semiconducting layer 6, a metal screen
7 and an outer polymeric sheath 8.
[0088] The above described cable 1 may be produced starting from
the conducting core 2 according to known methods, for instance by
subsequently extruding layers 4, 5 and 6, by applying the metal
screen 7 and by finally extruding the external sheath 8.
[0089] With reference to the description hereinabove, some merely
illustrative and not limitative examples of polymeric compositions
according to the invention, particularly suitable for the
manufacture of the insulation layer of a cable, such as for
instance the layer 5 of the cable described above, will be provided
in the following.
EXAMPLE 1
[0090] A polymeric composition according to the invention was
prepared by mixing in an extruder the following ingredients, in
parts by
1 polymeric base 90 phr ethylene/acrylic ester/glycidyl
methacrilate 10 phr terpolymer peroxide 2 phr antioxidant 0.34
phr
[0091] As polymeric base low-density polyethylene (LDPE) was used,
having a density of 0.923 g/cm.sup.3 and a Melt Flow Index of 2
g/10' (Enichem).
[0092] As ethylene/acrylic ester/glydidyl methacrylate terpolymer,
LOTADER.TM. GMA AX.sub.8900.TM. (Elf Atochem) was used.
[0093] The peroxide and the antioxidant used were ter-butyl-cumyl
peroxide (TRIGONOX.TM. T produced by AKZO) and
4-4'-thio-bis(3-methyl-6-ter-butyl)- phenol (SANTONOX.TM. R
produced by MONSANTO).
EXAMPLE 2
[0094] According to the same preparation methods and using the same
ingredients as the previous Example 1, a polymeric composition was
prepared having the following composition in parts by weight for
each 100 parts of polymeric base (phr):
2 polymeric base 85 phr ethylene/acrylic ester/glycidyl
methacrilate 15 phr terpolymer peroxide 2 phr antioxidant 0.34
phr
[0095] According to the same preparation methods and using the same
ingredients as the previous Example 1, a polymeric composition was
prepared having the following composition in parts by weight for
each 100 parts of polymeric base (phr):
3 polymeric base 80 phr ethylene/acrylic ester/glycidyl
methacrilate 20 phr terpolymer peroxide 2 phr antioxidant 0.34
phr
EXAMPLE 4
Comparison
[0096] According to conventional preparation methods known in the
art and using the same ingredients of the previous Example 1, a
polymeric composition including only the polyolefin polymer, the
cross-linking agent and the antioxidant agent was prepared, having
the following composition in parts by weight for each 100 parts of
polymeric base (phr):
4 polymeric base 100 phr peroxide 2 phr antioxidant 0.34 phr
EXAMPLE 5
Comparison
[0097] According to conventional preparation methods known in the
art and using the same ingredients of the previous Example 1, a
polymeric composition including acrylic ester groups was prepared,
having the following composition in parts by weight for each 100
parts of polymeric base (phr):
5 polymeric base 82.5 phr ethylene/acrylic-ester copolymer 17.5 phr
peroxide 2 phr antioxidant 0.34 phr
[0098] As ethylene/acrylic-ester copolymer the ethylene/butyl
acrylate copolymer commercially available under the trade name
ENATHENE.TM. EA 720 (USI QUANTUM) was used.
EXAMPLE 6
Comparison
[0099] According to conventional preparation methods known in the
art and using the same ingredients of the previous Example 1, a
polymeric composition according to the prior art and including
epoxy groups was prepared, having the following composition in
parts by weight for each 100 parts of polymeric base (phr):
6 polymeric base 100 phr epoxy resin 1.5 phr peroxide 2 phr
antioxidant 0.34 phr
[0100] As epoxy resin the resin commercially available under the
trade name EUREPOX.TM. (SCHERING) was used.
EXAMPLE 7
Evaluation of the Water Tree Resistance
[0101] The resistance properties to the formation of water trees of
the polymeric compositions according to Examples 1-6 hereinabove,
were evaluated according to the methodology proposed by EFI
(Norwegian Electric Power Research Institute) in the publication
"The EFI Test Method for Accelerated Growth of Water Trees",
presented at the "1990 IEEE International Symposium on Electrical
Insulation", held in Toronto, Canada, on Jun. 3-6 1990.
[0102] According to such a methodology, the cable is simulated by
preparing cup-shaped multilayered test-samples, wherein the
material constituting the insulation coating is sandwiched between
two layers of semiconducting material.
[0103] More particularly, the layer of insulation material is
heat-molded in the shape of a cup at the temperature of 120.degree.
C., starting from a tape having a thickness of 5-7 mm, in an
electric press capable of developing a pressure of about 90 t, so
as to obtain a thickness of about 0.50 mm.
[0104] The layers of semiconducting material, extruded and
preshaped in an analogous way until a thickness of about 0.5 mm is
obtained, are then pressed and heat-welded on opposite sides of the
insulation layer at a temperature of about 180.degree. C. for 15
minutes in an electric press similar to that used to form the same
layers.
[0105] The test-samples so obtained, once cooled at
room-temperature, are then submitted to an accelerated electric
ageing test, filling with water the cavity defined inside the
cup-shaped test-sample, immersing in the water a high voltage
electrode and laying the resulting equipment on a metal plate
(earth electrode).
[0106] The accelerated trees growth is then induced in the
insulation layer by applying a voltage generally of from 2 to 15 kV
between the electrodes.
[0107] In order to further accelerate the phenomenon, the test is
heat-performed, for instance in a suitable oven.
[0108] In the tests carried out, the polymeric compositions of
Examples 1-6 were coupled to a semiconducting screen constituted by
a XLPE mix, commercially available under the trade name NCPE
0592.TM. (Borealis N. V., Bruxelles, Belgium).
[0109] According to the above described EFI methodology, 5
test-samples were produced for each polymeric composition, which
were submitted to accelerated ageing in the following test
conditions:
7 electric gradient: 5 kV/mm temperature: 70.degree. C.
[0110] At the end of a 30-day period, 20 100 .mu.m-thick sections
were taken off from each test-piece, dyed with methylene blue
according to the CIGRE standards and then examined with an optical
microscope at a magnification of from 100 to 200.times..
[0111] From such observation the density of water trees, expressed
in number of trees per cm.sup.3, was then calculated for each
test-sample. The mean values are shown in the following Table
1.
8 TABLE 1 nr. Example 1 2 3 4 5 6 Trees density (nr./cm.sup.3) 190
140 95 980 470 780
[0112] From the data shown in the table, it may be observed that
the resistance to the water tree formation. of the polymeric
compositions of the invention (Examples 1-3), comprising ester
groups and epoxy groups, is markedly greater that that offered by
the control polymeric compositions, incorporating only acrylic
groups (Example 5) or epoxy groups (Example 6).
[0113] In this connection, it has to be observed that, quite
surprisingly, the combined effects of the above ester groups and
epoxy groups is of a synergistic type, i.e., much greater than the
sum of the individual effects ascribable to each of them considered
in isolation.
[0114] This synergistic effect is so marked that, in the case of
the composition of Example 3, a density of water trees has been
observed whose order of magnitude was even lower than that of the
cross-linked polyethylene taken as control.
EXAMPLE 8
Evaluation of Dielectric Strength
[0115] The dielectric strength properties of the polymeric
compositions according to the previous Examples 1-6 were evaluated
on test-samples obtained by the ageing methodology proposed by EFI,
described in the preceding example.
[0116] In this case, 20 test-samples were produced which were
submitted to accelerated water ageing in the following test
conditions:
9 electric gradient: 5 kV/mm temperature: 70.degree. C.
[0117] On a batch of 5 non-aged test-samples (control) and on three
batches of 5 test-samples taken after 7, 15 and 30 days
respectively from the beginning of the accelerated electric ageing,
the value of the dielectric strength was then measured according to
the ASTM D-149 standard.
[0118] The dielectric strength tests were carried out with silicone
oil in the inside and on the outside of the test-samples, using a
circular electrode and applying a voltage gradient of 2 kV/s.
[0119] The results of the tests carried out (mean values of 5
tests) are shown in the following Table 2.
10TABLE 2 Example 1 2 3 4 5 6 Strength as is 120 130 135 105 120
110 (kV/mm) 7 days 60 75 85 55 60 58 15 days 55 70 80 45 50 47 30
days 55 70 75 45 50 47
[0120] From the data shown in the table, it may be observed
that--after ageing--the dielectric strength of the polymeric
compositions of the invention (Examples 1-3), is as a whole greater
than that of the control compositions, independently from the
original starting values, with an advantageous improvement of the
insulating properties of the composition.
EXAMPLE 9
Evaluation of the Loss Factor
[0121] The evaluation of the so-called loss factor (tan delta) of
the polymeric compositions according to the previous Examples 1-6
was carried out according to the ASTM D-150 standard (AC Loss
Characteristics and Dielectric Constant (Permettivity) of Solid
Electrical Insulating Material).
[0122] More particularly, the loss factor was measured by using as
test-samples molded flat plates of 20.times.20 cm of side, 1.0 mm
thick, and using circular electrodes with guard ring.
[0123] Before taking the measures, the test-samples had been
submitted to heat treatment at 90.degree. C. in order to remove the
cross-linking by-products in each plate.
[0124] The results of the tests performed (mean values out of 5
tests) are shown in the following Table 3.
11 TABLE 3 nr. Example 1 2 3 4 5 6 tg delta 0.0002 0.0003 0.0004
0.0001 0.0001 0.0010 (room T) tg delta 0.0008 0.0024 0.0110 0.0003
0.0004 0.0110 (90.degree. C.)
[0125] As it may be observed from the above table, the presence of
ester groups and epoxy groups in the polymeric composition of the
invention may bring about an increase in the loss factor; hence,
the content of said groups may be selected having regard to the
desired resistance characteristics to the formation of water trees
and to the other performances required to the cable.
[0126] To this end, the optimum amount of ester groups and epoxy
groups in the polymeric composition may be selected by a man
skilled in the art in relation to the specific application
requirements of the cable.
[0127] From what has been described and illustrated above it is
immediately evident that the cable of the invention possesses a
combination of features that render the same useable for all those
applications--and in particular for power transmission at
medium/high voltage--where a particular resistance to water treeing
is required.
[0128] In particular, according to the invention, the use of ester
groups and epoxy groups in predetermined amount in a given
polymeric composition allows to increase the resistance to the
phenomenon of water treeing compared to the case when--in the same
polymeric composition--such groups are not present and, as a
consequence, to increase, after electric water ageing, the
dielectric strength of the insulation coating of a cable
manufactured with the same polymeric base.
[0129] Obviously, those skilled in the art may introduce variants
and modifications to the above described invention, in order to
satisfy specific and contingent requirements, variants and
modifications which fall anyhow within the scope of protection as
is defined in the following claims.
* * * * *