CABLE WITH RECYCLABLE COVERING

CABLE A ENVELOPPE RECYCLABLE

English Abstract

The invention describes a cable with recyclable covering, particularly for
transporitng or distributing medium or high voltage energy, in which at least
one covering layer is based on thermoplastic polymer material comprising a
propylene homopolymer or a copolymer of propylene with ethylene or an .prop.-
olefin other than propylene in mixture with a dielectric liquid. The cable of
the invention possesses superior mechanical and electrical properties,
including high dielectric strength, in particular enabling it to be used at
high operating temperature.

French Abstract

L'invention concerne un câble doté d'une enveloppe recyclable destiné notamment à transporter ou à distribuer un milieu ou une énergie à haute tension, dans lequel au moins une couche de couverture est constituée d'une matière polymère thermoplastique comprenant un homopolymère propylénique ou un copolymère de propylène comportant de l'éthylène ou une .alpha.-oléfine autre que le propylène en un mélange avec un liquide diélectrique. Le câble de l'invention possède des propriétés mécaniques et électriques supérieures, notamment une haute rigidité diélectrique, lui permettant en particulier d'être utilisé à des températures de fonctionnement élevées.

Claims

17
CLAIMS
1. A cable comprising at least one electrical conductor and at least one
extruded
covering layer based on a thermoplastic polymer material in admixture with a
dielectric
liquid, wherein:
said thermoplastic material comprises a propylene homopolymer or a copolymer
of propylene with at least one olefin comonomer selected from ethylene and an
.alpha.-olefin
other than propylene, said homopolymer or copolymer having a melting point
greater
than or equal to 140°C. and a melting enthalpy of from 30 to 100 J/g;
and
said dielectric liquid comprises at least one diphenyl ether, non-substituted
or
substituted with at least one linear or branched, aliphatic, aromatic or mixed
aliphatic and
aromatic C1 - C30 hydrocarbon radical.
2. The cable as claimed in claim 1, wherein the propylene homopolymer or
copolymer has a melting point of from 145 to 170°C.
3. The cable as claimed in claim 1, wherein the propylene homopolymer or
copolymer has a melting enthalpy of from 30 to 85 J/g.
4. The cable as claimed in claim 1, wherein the propylene homopolymer
or copolymer has a flexural modulus, measured at room temperature, of from
30 to 1400 MPa.
5. The cable as claimed in claim 1, wherein the propylene homopolymer
or copolymer has a flexural modulus, measured at room temperature, of from
60 to 1000 MPa.
6. The cable as claimed in claim 1, wherein the propylene homopolymer or
copolymer has a melt flow index, measured at 230°C., of from 0.05 to
10.0 dg/min,
7. The cable as claimed in claim 1, wherein the propylene homopolymer or
copolymer has a melt flow index, measured at 230 °C., of from 0.5 to
5.0 dg/min.

18
8. The cable as claimed in claim 1, wherein the olefin comonomer is present in
a
quantity of less than or equal to 15 mol %.
9. The cable as claimed in claim 1, wherein the olefin comonomer is present in
a
quantity of less than or equal to 10 mol %.
10. The cable as claimed in claim 1, wherein the olefin comonomer is ethylene
or an
.alpha.-olefin of formula CH2 = CH-R, where R is a linear or branched C2 - C10
alkyl.
11. The cable as claimed in claim 1, wherein the .alpha.-olefin is selected
from 1-butene,
1-pentene, 4-methyl-l-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene and
combinations thereof.
12. The cable as claimed claim 1, wherein the thermoplastic material is
selected from:
(a) a propylene homopolymer or a copolymer of propylene with at least one
olefin comonomer selected from ethylene and an .alpha.-olefin other than
propylene, having a
flexural modulus of from 30 to 900 MPa; and
(b) a heterophase copolymer comprising a thermoplastic phase based on
propylene and an elastomeric phase based on ethylene copolymerized with an
.alpha.-olefin, in
which the elastomeric phase is present in a quantity of at least 45 wt % based
on the total
weight of the heterophase copolymer.
13. The cable as claimed in claim 12, wherein the propylene homopolymer or
copolymer under (a) has a flexural modulus of from 50 to 400 MPa.
14. The cable as claimed in claim 12, wherein the propylene homopolymer or
copolymer under (a) has:
a melting point of from 140 to 165°C.;
a melting enthalpy of from 30 to 80 J/g;
a fraction soluble in boiling diethyl ether in an amount of less than or equal
to
12 wt %, having a melting enthalpy of less than or equal to 4 J/g;

19
a fraction soluble in boiling n-heptane in an amount of from 15 to 60 wt %,
having
a melting enthalpy of from 10 to 40 J/g; and
a fraction insoluble in boiling n-heptane in an amount of from 40 to 85 wt %,
having a melting enthalpy of greater than or equal to 45 J/g.
15. The cable as claimed in claim 12, wherein the propylene homopolymer or
copolymer of (a) has:
a fraction soluble in boiling diethyl ether in an amount of from 1 to 10 wt %,
having a melting enthalpy of less than or equal to 2 J/g;
a fraction soluble in boiling n-heptane in an amount of from 20 to 50 wt %,
having
a melting enthalpy of from 15 to 30 J/g; and
a fraction insoluble in boiling n-heptane in an amount of from 50 to 80 wt %,
having a melting enthalpy of from 50 to 95 J/g.
16. The cable as claimed in claim 12, wherein the .alpha.-olefin included in
the elastomeric
phase of the heterophase copolymer under (b) is propylene.
17. The cable as claimed in claim 16, wherein the elastomeric phase consists
of an
elastomeric copolymer of ethylene and propylene comprising from 15 to 50 wt %
of
ethylene and from 50 to 85 wt % of propylene on the weight of the elastomeric
phase.
18. The cable as claimed in claim 1, wherein the thermoplastic polymer
material is the
propylene homopolymer or copolymer in mechanical mixture with a low
crystallinity
polymer having a melting enthalpy of less than or equal to 30 J/g, and a
quantity of less
than or equal to 70 wt % on the total weight of the thermoplastic material.
19. The cable as claimed in claim 18, wherein the low crystallinity polymer is
in a
quantity of from 20 to 60 wt % on the total weight of the thermoplastic
material.
20. The cable as claimed in claim 18, wherein the low crystallinity polymer is
a
copolymer of ethylene with a C3 - C12 .alpha.-olefin.

20
21. The cable as claimed in claim 18, wherein the low crystallinity polymer is
a
copolymer of ethylene with an .alpha.-olefin and a diene.
22. The cable as claimed in claim 20 or claim 21, wherein the copolymer of
ethylene
is selected from:
(i) a copolymer having the following monomer composition:
35-90 mol % of ethylene;
10-65 mol % of .alpha.-olefin; and
0-10 mol % of a diene; and
(ii) a copolymer having the following monomer composition:
75-97 mol % of ethylene; and
3-25 mol % of .alpha.-olefin;
0-5 mol % of a diene.
23. The cable as claimed in claim 22, wherein the ethylene copolymer is
selected from
a copolymer having the following monomer composition:
90-95 mol % of ethylene;
5-10 mol % of .alpha.-olefin; and
0-2 mol % of a diene.
24. The cable as claimed in claim 20 or claim 21, wherein the .alpha.-olefin
is selected
from propylene, 1-hexene and 1-octene.
25. The cable as claimed in claim 21, wherein the diene has from 4 to 20
carbon
atoms.
26. The cable as claimed in claim 21, wherein the diene is selected from a
conjugated
or non-conjugated linear diolefin, and a monocyclic or polycyclic diene.

21
27. The cable as claimed in claim 21, wherein the diene is selected from 1,3-
butadiene, 1,4-hexadiene, 1,6-octadiene, 1,4-cyclohexadiene, 5-ethylidene-2-
norbornene,
5-methylene-2-norbornene, 5-vinyl-2-norbornene, and their mixtures.
28. The cable as claimed in claim 1, wherein the hydrocarbon radical has from
1 to 24
carbon atoms.
29. The cable as claimed in claim 1, wherein the dielectric liquid comprises
at least
one diphenyl ether having the following structural formula:
<IMG>
where R1 and R2 are equal or different and represent hydrogen, a phenyl group
non-substituted or substituted by at least one alkyl group, or an alkyl group
non-
substituted or substituted by at least one phenyl.
30. The cable as claimed in claim 29, wherein the alkyl group has from 1 to 20
carbon
atoms.
31. The cable as claimed in claim 1, wherein the dielectric liquid is selected
from
phenyl toluyl ether, 2,3'-ditoluyl ether, 2,2'-ditoluyl ether, 2,4'-ditoluyl
ether,
3,3'-ditoluyl ether, 3,4'-ditoluyl ether, 4,4'-ditoluyl ether, octadecyl
diphenyl ether, either
as pure isomers or as combinations thereof.
32. The cable as claimed in claim 29, wherein the ratio of number of aryl
carbon
atoms to number of total carbon atoms of the dielectric liquid is greater than
or equal
to 0.4.
33. The cable as claimed in claim 29, wherein the ratio of number of aryl
carbon
atoms to number of total carbon atoms of the dielectric liquid is greater than
or equal
to 0.7.

22
34. The cable as claimed in claim 29, wherein the diphenyl ether has a
dielectric
constant, at 25°C., of less than or equal to 8.
35. The cable as claimed in claim 29, wherein the diphenyl ether has a
dielectric
constant, at 25°C., of less than or equal to 4.
36. The cable as claimed in claim 29, wherein the dielectric liquid has a
kinematic
viscosity at 20°C. of between 1 and 100 mm2 /s.
37. The cable as claimed in claim 29, wherein the dielectric liquid has a
kinematic
viscosity at 20°C. of between 3 and 50 mm 2 /s.
38. The cable as claimed in claim 29, wherein the diphenyl ether has a
hydrogen
absorption capacity of greater than or equal to 5 mm3 /min.
39. The cable as claimed in claim 38, wherein the hydrogen absorption capacity
is
greater than or equal to 50 mm3 /min.
40. The cable as claimed in claim 1, further comprising an epoxy resin added
to the
dielectric liquid in a quantity of less than or equal to 1 wt % based on the
weight of the
liquid.
41. The cable as claimed in claim 1, wherein the weight ratio of dielectric
liquid to
base polymer material is from 1:99 to 25:75.
42. The cable as claimed in claim 1, wherein the weight ratio of dielectric
liquid to
base polymer material is from 2:98 to 20:80.
43. The cable as claimed in claim 1, wherein the weight ratio of dielectric
liquid to
base polymer material is from 3:97 to 15:85.

23
44. The cable as claimed in claim 1, wherein the base polymer material is
selected
from propylene homopolymers or copolymers comprising at least 40 wt % of
amorphous
phase, based on the total polymer weight.
45. The cable as claimed in claim 1, wherein the extruded covering layer is a
layer
with electrical insulation properties.
46. The cable as claimed in claim 1, wherein the extruded covering layer is a
layer
with semiconductive properties.
47. The cable as claimed in claim 1, further comprising a conductive filler
with
semiconductive properties dispersed in the extruded covering layer.
48. The cable as claimed in claim 1, further comprising at least one layer
with
electrical insulation properties and at least one layer with semiconductive
properties.
49. A polymer composition comprising a thermoplastic polymer material in
admixture
with a dielectric liquid wherein:
said thermoplastic material comprises a propylene homopolymer or a copolymer
of propylene with at least one olefin comonomer selected from ethylene and an
.alpha.-olefin
other than propylene, said homopolymer or copolymer having a melting point
greater
than or equal to 140°C. and melting enthalpy of from 30 to 100 J/g; and
said dielectric liquid comprises at least one diphenyl ether, non-substituted
or
substituted with at least one linear or branched, aliphatic, aromatic or mixed
aliphatic and
aromatic C1 - C30 hydrocarbon radical.

Description

CA 02425382 2003-03-13
WO 02/27731 PCT/EP01/09700
1
CABLE WITH RECYCLABLE COVERING
*~*****
The present invention relates to a cable with recyclable covering. In
particular, the invention relates to a cable for transporting or distributing
medium or high voltage electrical energy, wherein an extruded covering layer
based on a thermoplastic polymer material in admixture with a dielectric
liquid
with high mechanical and electrical properties is present, enabling, in
particular, the use of high operating temperatures and the transportation of
high power energy.
The requirement for products of high environmental compatibility,
composed of materials which, in addition to not being harmful to the
environment both during production and utilization, can be easily recycled at
the end of their life, is now fully accepted in the field of electrical and
telecommunications cables.
However the use of materials compatible with the environment is
certainly conditioned by the need to limit costs and, for the more common
uses, guaranteeing a performance equivalent to or even better than that of
conventional materials anyway.
In the case of cables for transporting medium and high voltage energy,
the various coverings surrounding the conductor commonly consist of
polyolefin-based crosslinked polymer material, in particular crosslinked
polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or
ethylene/propylene/diene (EPDM) copolymers, also crosslinked. The
crosslinking, effected after the step of extrusion of the polymer material on
the
conductor, gives the material satisfactory mechanical performance even under
hot conditions during continuous use and with current overload.
It is well known however that crosslinked materials cannot be recycled,
so that manufacturing wastes and the covering material of cables which have
reached the end of their life can be disposed of only by incineration.
Electric cables are also known having their insulation consisting of a
multi-layer wrapping of a paper or paper/polypropylene laminate impregnated
with a large quantity of a dielectric liquid (commonly known as mass
impregnated cables or also oil-filled cables). By completely filling the
spaces
present in the multi-layer wrapping, the dielectric liquid prevents partial
discharges arising with consequent perforation of the electrical insulation.
As

CA 02425382 2003-03-13
WO 02/27731 PCT/EP01/09700
2
dielectric liquids, products are commonly used such as mineral oils,
polybutenes, alkylbenzenes and the like (see for example US-4,543,207, US-
4,621,302, EP-A-0987718, WO 98/32137).
It is however well known that mass impregnated cables have numerous
drawbacks compared with extruded insulation cables, so that their use is
currently restricted to specific fields of application, in particular to the
construction of high and very high voltage direct current transmission lines,
both for terrestrial and in particular for underwater installations. In this
respect, the production of mass impregnated cables is particularly complex and
costly, both for the high cost of the laminates and for the difficulties
encountered during the steps of wrapping the laminate and then of
impregnating it with the dielectric liquid. In particular, the dielectric
liquid
used must have low viscosity under cold conditions to allow rapid and uniform
impregnation, while at the same time it must have a low tendency to migrate
during installation and operation of the cable to prevent liquid loss from the
cable ends or following breakage. In addition, mass impregnated cables cannot
be recycled and their use is limited to an operating temperature less than
90 C.
Within non-crosslinked polymer materials, it is known to use high
density polyethylene (HDPE) for covering high voltage cables. HDPE has
however the drawback of a lower temperature resistance than XLPE, both to
current overload and during operation.
Thermoplastic low density polyethylene (LDPE) insulating coverings are
also used in medium and high voltage cables; again in this case, these
coverings are limited by too low an operating temperature (about 70 C).
WO 99/13477 describes an insulating material consisting of a
thermoplastic polymer forming a continuous phase which incorporates a liquid
or easily meltable dielectric forming a mobile interpenetrating phase within
the
solid polymer structure. The weight ratio of thermoplastic polymer to
dielectric
is between 95:5 and 25:75. The insulating material can be produced by mixing
the two components while hot either batchwise or continuously (for example by
means of an extruder). The resultant mixture is then granulated and used as
insulating material for producing a high voltage electric cable by extrusion
onto
a conductor. The material can be used either in thermoplastic or crosslinked
form. As thermoplastic polymers are indicated: polyolefins, polyacetates,

CA 02425382 2003-03-13
WO 02/27731 PCT/EP01/09700
3
cellulose polymers, polyesters, polyketones, polyacrylates, polyamides and
polyamines. The use of polymers of low crystallinity is particularly
suggested.
The dielectric is preferably a synthetic or mineral oil of low or high
viscosity, in
particular a polyisobutene, naphthene, polyaromatic, a-olefin or silicone oil.
US 4410869 describes dielectric compositions comprising a mixture of
ditoluyl ether isomers, optionally in the presence of hydroquinone or a
derivative thereof, used for impregnating electrical devices, including
capacitors and transformers.
US 4543207 describes dielectric compositions comprising dielectric oils
and aromatic mono-olefins and/or diolefins having condensed or non-
condensed aromatic nuclei. Said compositions comprise, in particular,
mixtures of organic acid esters, vegetable or animal oils and aromatic ethers
with 0.01-50% aromatic mono- and/or diolefins having two condensed or non-
condensed aromatic rings. The compositions are used to impregnate
capacitors, transformers and electric cables.
The Applicant considers as still unsolved the technical problem of
producing an electric cable with a covering made from a thermoplastic polymer
material having mechanical and electrical properties comparable to those of
cables with an insulating covering of crosslinked material. In particular, the
Applicant has considered the problem of producing a cable with a non-
crosslinked insulating covering having good flexibility and high mechanical
strength under both hot and cold conditions, while at the same time
possessing high dielectric strength.
In view of said problem, the Applicant considers that the addition of
dielectric liquids to polymer materials as proposed in the cited WO 99/13477
gives totally unsatisfactory results. In this respect, the Applicant maintains
that adding a dielectric liquid to an insulating material should on the one
hand
determine a significant increase in its electrical properties (in particular
its
dielectric strength), while on the other hand maintaining the material
characteristics (thermomechanical properties, manageability) unchanged, even
at high operating temperature (at least 90 C and beyond).
The Applicant has now found it possible to solve said technical problem
by using, as recyclable polymer base material, a thermoplastic propylene
homopolymer or copolymer mixed with a dielectric liquid as hereinafter
defined. The resultant composition possesses good flexibility even when cold,

CA 02425382 2003-03-13
WO 02/27731 PCT/EP01/09700
4
excellent thermomechanical strength and high electrical performance, such as
to make it particularly suitable for forming at least one covering layer, and
in
particular an electrical insulating layer, of a medium or high voltage cable
of
high operating temperature, of at least 90 C and beyond. The dielectric liquid
suitable for implementing the invention has high compatibility with the base
polymer and high efficiency in the sense of improving electrical performance,
consequently allowing the use of small quantities of additive such as not to
impair the thermomechanical characteristics of the insulating layer.
High compatibility between the dielectric liquid and the base polymer
ensures homogeneous dispersion of the liquid in the polymer matrix and
improves cold behaviour of the polymer.
According to a first aspect, the invention therefore relates to a cable (1)
comprising at least one electrical conductor (2) and at least one extruded
covering layer (3, 4, 5) based on a thermoplastic polymer material in
admixture
with a dielectric liquid, wherein:
- said thermoplastic material comprises a propylene homopolymer or a
copolymer of propylene with at least one olefin comonomer selected from
ethylene and an a-olefin other than propylene, said homopolymer or copolymer
having a melting point greater than or equal to 140 C and a melting enthalpy
of from 30 to 100 J/g;
- said dielectric liquid comprises at least one diphenyl ether, non-
substituted or substituted with at least one linear or branched, aliphatic,
aromatic or mixed aliphatic and aromatic C1-C30, preferably C1-C24a
hydrocarbon radical.
According to a first embodiment, said extruded covering layer based on
said thermoplastic polymer material in admixture with said dielectric liquid
is
an electrically insulating layer.
According to a further embodiment, said extruded covering layer based
on said thermoplastic polymer material in admixture with said dielectric
liquid
is a semiconductive layer.
Preferably, the propylene homopolymer or copolymer has a melting point
of from 145 to 170 C.
Preferably, the propylene homopolymer or copolymer has a melting
enthalpy of from 30 to 85 J/g.
Preferably, the propylene homopolymer or copolymer has a flexural

CA 02425382 2003-03-13
WO 02/27731 PCT/EP01/09700
modulus, measured in accordance with ASTM D790, at room temperature, of
from 30 to 1400 MPa, and more preferably from 60 to 1000 MPa.
Preferably, the propylene homopolymer or copolymer has a melt flow
index (MFI), measured at 230 C with a load of 21.6 N in accordance with ASTM
5 D1238/L, of from 0.05 to 10.0 dg/min, more preferably from 0.5 to 5.0
dg/min.
If a copolymer of propylene with an olefin comonomer is used, this latter
is preferably present in a quantity of less than or equal to 15 mol%, and more
preferably less than or equal to 10 mol%. The olefin comonomer is, in
particular, ethylene or an a-olefin of formula CH2=CH-R, where R is a linear
or
branched C2-Cio alkyl, selected for example from: 1-butene, 1-pentene, 4-
methyl-l-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene and the like, or
combinations thereof. Propylene/ethylene copolymers are particularly
preferred.
Preferably, said thermoplastic material is selected from:
(a) a propylene homopolymer or a copolymer of propylene with at least
one olefin comonomer selected from ethylene and an a-olefin other than
propylene, having a flexural modulus generally of from 30 to 900 MPa,
preferably of from 50 to 400 MPa;
(b) a heterophase copolymer comprising a thermoplastic phase based on
propylene and an elastomeric phase based on ethylene copolymerized with an
a-olefin, preferably with propylene, in which the elastomeric phase is present
in a quantity of at least 45 wt% on the total weight of the heterophase
copolymer.
The homopolymers or copolymers of class a) show a single-phase
microscopic structure, ie substantially devoid of heterogeneous phases
dispersed as molecular domains of size greater than one micron. These
materials do not show, in fact, the optical phenomena typical of heterophase
polymer materials, and in particular are characterised by better transparency
and reduced whitening due to local mechanical stresses (commonly known as
"stress whitening").
Particularly preferred of said class a) is a propylene homopolymer or a
copolymer of propylene with at least one olefin comonomer selected from
ethylene and an a-olefin other than propylene, said homopolymer or copolymer
having:
a melting point of from 140 to 165 C;

CA 02425382 2009-04-17
WO 02/27731 PCT/EPO1/09700
6
a melting enthalpy of from 30 to 80 J/g;
a fraction soluble in boiling diethyl ether in an amount less than or
equal to 12 wt%, preferably from 1 to 10 wt%, having a melting enthalpy of
less
than or equal to 4 J/g, preferably less than or equal to 2 J/g;
a fraction soluble in boiling n-heptane in an amount of from 15 to 60
wt%, preferably from 20 to 50 wt%, having a melting enthalpy of from 10 to 40
J/g, preferably from 15 to 30 J/g; and
a fraction insoluble in boiling n-heptane in an amount of from 40 to 85
wt%, preferably from 50 to 80 wt%, having a melting enthalpy of greater than
or equal to 45 J/g, preferably from 50 to 95 J/g.
Further details of these materials and their use in covering cables are
given in European patent application 99122840 filed on 17.11.1999 in the
name of the Applicant.
The heterophase copolymers of class b) are thermoplastic elastomers
obtained by sequential copolymerization of: i) propylene, possibly containing
minor quantities of at least one oleffm comonomer selected from ethylene and
an a-olefin other than propylene; and then of: ii) a mixture of ethylene with
an
a-olefm, in particular propylene, and possibly with minor portions of a diene.
This class of product is also commonly known b-y the term "thermoplastic
reactor eiastomers".
Particularly preferred of the said class b) is a heterophase copolymer in
which the elastomeric phase consists of an elastomeric copolymer of ethylene
and propylene comprising from 15 to 50 wt% of ethylene and from 50 to 85
wt% of propylene on the weight of the elastomeric phase. Further details of
these materials and their use in covering cables are given in patent
application
W000/41187 in the name of the Applicant.
Products of class a) are available commercially for example under the
trademark RexflexR of the Huntsman Polymer Corporation.
Products of class b) are available commercially for example under the
trademark HifaxR of Montell.
Alternatively, as thermoplastic base material, a propylene homopolymer
or copolymer as hereinbefore defined can be used in mechanical mixture with a
low crystallinity polymer, generally with a melting enthalpy of less than 30
J/g,
which mainly acts to increase flexibility of the material. The quantity of low
crystallinity polymer is generally less than 70 wt%, and preferably from 20 to

CA 02425382 2003-03-13
WO 02/27731 PCT/EP01/09700
7
60 wt%, on the total weight of the thermoplastic material.
Preferably, the low crystallinity polymer is a copolymer of ethylene with
a C3-C12 a-olefin, and possibly with a diene. The a-olefin is preferably
selected
from propylene, 1-hexene and 1-octene. If a diene comonomer is present, this
is generally C4-C2o, and is preferably selected from conjugated or non-
conjugated linear diolefins, such as 1,3-butadiene, 1,4-hexadiene, 1,6-
octadiene or their mixtures and the like; monocyclic or polycyclic dienes,
such
as 1,4-cyclohexadiene, 5-ethylidene-2-norbornene, 5-methylene-2-norbornene,
5-vinyl-2-norbornene or their mixtures and the like.
Particularly preferred ethylene copolymers are:
(i) copolymers having the following monomer composition: 35-90 mol%
of ethylene; 10-65 mol% of an a-olefin, preferably propylene; 0-10 mol% of a
diene, preferably 1,4-hexadiene or 5-ethylene-2-norbornene (EPR and EPDM
rubbers fall within this class);
(ii) copolymers having the following monomer composition: 75-97 mol%,
preferably 90-95 mol%, of ethylene; 3-25 mol%, preferably 5-10 mol%, of an a-
olefin; 0-5 mol%, preferably 0-2 mol%, of a diene (for example ethylene/ 1-
octene copolymers, such as the products EngageR of Dow-DuPont Elastomers).
The dielectric liquid according to the invention preferably comprises at
least one diphenyl ether having the following structural formula:
R1 - R2
~ ~ 0 ~ - ~
where Rl and R2 are equal or different and represent hydrogen, a phenyl
group non-substituted or substituted by at least one alkyl group, or an alkyl
group non-substituted or substituted by at least one phenyl.
By alkyl group it is meant a linear or branched Ci-C24, preferably C1-C20,
hydrocarbon radical.
Liquids advantageously usable in the present invention are for example
phenyl toluyl ether, 2,3'-ditoluyl ether, 2,2'-ditoluyl ether, 2,4'-ditoluyl
ether,
3,3'-ditoluyl ether, 3,4'-ditoluyl ether, 4,4'-ditoluyl ether, octadecyl
diphenyl
ether either as pure isomers or in mixture with each other. Said dielectric
liquid has a ratio of number of aryl carbon atoms to number of total carbon

CA 02425382 2003-03-13
WO 02/27731 PCT/EP01/09700
8
atoms greater than or equal to 0.4, preferably greater than or equal to 0.7.
The diphenyl ether of the invention preferably has a dielectric constant,
at 25 C, of less than or equal to 8, preferably less than 4 (measured in
accordance with IEC 247).
According to a further preferred aspect, the diphenyl ether of the
invention has a predetermined viscosity such as to prevent fast diffusion of
the
liquid within the insulating layer and hence its outward migration, while at
the
same time such as to enable it to be easily fed and mixed into the polymer.
Generally, the dielectric liquid of the invention has a kinematic viscosity,
at
20 C, of between 1 and 100 mm2/s, preferably between 3 and 50 mm2/s
(measured in accordance with ISO 3104).
According to a further preferred aspect, the diphenyl ether of the
invention has a hydrogen absorption capacity greater than or equal to 5
mm3/min, preferably greater than or equal to 50 mm3/min (measured in
accordance with IEC 628-A).
According to a preferred aspect, an epoxy resin can be added to the
dielectric liquid suitable for forming the cable of the invention, generally
in a
quantity of less than or equal to 1 wt% on the weight of the liquid, this
being
considered to mainly act to reduce the ion migration rate under an electrical
field, and hence the dielectric loss of the insulating material.
The dielectric liquid suitable for implementing the invention has good
heat resistance, considerable gas absorption capacity, in particular for
hydrogen, and hence high resistance to partial discharges, so that dielectric
loss is not high even at high temperature and high electrical gradient. The
weight ratio of dielectric liquid to base polymer material of the invention is
generally between 1:99 and 25:75, preferably between 2:98 and 20:80, and
more preferably between 3:97 and 15:85.
Dielectric liquids of the present invention can be prepared for example
by reacting a cresol, in the form of a salt of an alkaline metal, with halogen
toluene possibly in the presence of a copper or copper salt-based catalyst.
Further details regarding the preparation of the dielectric liquids of the
invention are reported for example in US 4410869.
According to a preferred aspect, the cable of the invention has at least
one extruded covering layer with electrical insulation properties formed from
the thermoplastic polymer material in admixture with the aforedescribed

CA 02425382 2003-03-13
WO 02/27731 PCT/EP01/09700
9
dielectric liquid.
According to a further preferred embodiment, the cable of the invention
has at least one extruded covering layer with semiconductive properties formed
from the thermoplastic polymer material in admixture with the aforedescribed
dielectric liquid. To form a semiconductive layer, a conductive filler is
generally
added to the polymer material. To ensure good dispersion of the conductive
filler within the base polymer material, this latter is preferably selected
from
propylene homopolymers or copolymers comprising at least 40 wt% of
amorphous phase, on the total polymer weight.
In a preferred embodiment, the cable of the invention has at least one
electrical insulation layer and at least one semiconductive layer formed from
a
thermoplastic polymer material in admixture with a dielectric liquid as
hereinabove described. This prevents the semiconductive layers from
absorbing, with time, part of the dielectric liquid present in the insulating
layer, so reducing its quantity just at the interface between the insulating
layer
and semiconductive layer, in particular the inner semiconductive layer where
the electrical field is higher.
According to a further aspect, the invention relates to a polymer
composition comprising a thermoplastic polymer material in admixture with a
dielectric liquid, in which:
- said thermoplastic material comprises a propylene homopolymer or a
copolymer of propylene with at least one olefin comonomer selected from
ethylene and an a-olefin other than propylene, said homopolymer or copolymer
having a melting point of greater than or equal to 140 C and a melting
enthalpy of from 30 to 100 J/g;
- said dielectric liquid comprises at least one diphenyl ether, non-
substituted or substituted with at least one linear or branched, aliphatic,
aromatic or mixed aliphatic and aromatic C1'C30a preferably Ci-C24a
hydrocarbon radical.
According to a further aspect, the invention relates to the use of a
polymer composition, as described hereinabove, as the base polymer material
for preparing a covering layer (4) with electrical insulation properties, or
for
preparing a covering layer (3, 5) with semiconductive properties.
In forming a covering layer for the cable of the invention, other
conventional components can be added to the aforedefined polymer

CA 02425382 2003-03-13
WO 02/27731 PCT/EP01/09700
composition, such as antioxidants, processing aids, water tree retardants, and
the like.
Conventional antioxidants suitable for the purpose are for example
distearylthio-propionate, pentaerithryl-tetrakis [3-(3,5-di-tertbutyl-4-
5 hydroxyphenyl)propionate] and 1,3,5-trimethyl-2,4,6-tris(3,5-di-tertbutyl-4-
hydroxy-benzyl)benzene and the like, or mixtures thereof.
Processing aids which can be added to the polymer base include, for
example, calcium stearate, zinc stearate, stearic acid, paraffin wax and the
like,
or their mixtures.
10 With particular reference to medium and high voltage cables, the
polymer materials as hereinabove defined can be advantageously used to form
an insulating layer. As stated above, these polymer materials show indeed good
mechanical characteristics both at ambient temperature and under hot
conditions, and also show improved electrical properties, in particular they
enable high operating temperature to be employed, comparable with or even
exceeding that of cables with coverings consisting of crosslinked polymer base
materials.
If a semiconductive layer is to be formed, a conductive filler, in
particular carbon black, is generally dispersed within the polymer material in
a
quantity such as to provide the material with semiconductive characteristics
(i.e. such as to obtain a resistivity of less than 5 Ohm.m at ambient
temperature). This quantity is generally between 5 and 80 wt%, and preferably
between 10 and 50 wt%, of the total weight of the mixture.
The possibility to use the same type of polymer composition for both the
insulating layer and the semiconductive layers is particularly advantageous in
producing cables for medium or high voltage, in that it ensures excellent
adhesion between adjacent layers and hence better electrical behaviour,
particularly at the interface between the insulating layer and the inner
semiconductive layer, where the electrical field and hence the risk of partial
discharges are higher.
The compositions of the present invention can be prepared by mixing
together the base polymer material, the dielectric liquid and any other
additives possibly present by methods known in the art. Mixing can be carried
out for example by an internal mixer of the type with tangential rotors
(Banbury) or with interpenetrating rotors, or, preferably, in a continuous
mixer

CA 02425382 2003-03-13
WO 02/27731 PCT/EP01/09700
11
of Ko-Kneader (Buss) type, or of co- or counter-rotating double-screw type.
Alternatively, the dielectric liquid of the invention can be added to the
polymer material during the extrusion step by direct injection into the
extruder
cylinder.
According to the present invention, the use of the aforedefined polymer
composition in covering cables for medium or high voltage enables recyclable,
flexible coverings to be obtained with excellent mechanical and electrical
properties.
Greater compatibility has also been found between the dielectric liquid
and thermoplastic base polymer of the invention than in the case of similar
mixtures of the same polymer material with other dielectric liquids known in
the art. This greater compatibility leads, inter alia, to less exudation of
the
dielectric liquid and hence a reduction of the already discussed migration
phenomena. Because of their high operating temperature and their low
dielectric loss, the cables of the invention can carry, for the same voltage,
a
power at least equal to or even greater than that transportable by a
traditional
cable with XLPE covering.
For the purposes of the invention the term "medium voltage" generally
means a voltage of between 1 and 35 kV, whereas "high voltage" means
voltages higher than 35 W.
Although this description is mainly focused on the production of cables
for transporting or distributing medium or high voltage electrical energy, the
polymer composition of the invention can be used for covering electrical
devices in general and in particular cables of different type, for example low
voltage cables, telecommunications cables or combined
energy/telecommunications cables, or accessories used in constructing
electrical lines, such as terminals or connectors.
Further characteristics will be apparent from the detailed description
given hereinafter with reference to the accompanying drawing, in which:
- Figure 1 is a perspective view of an electric cable, particularly suitable
for medium or high voltage, according to the invention.
In Figure 1, the cable 1 comprises a conductor 2, an inner layer with
semiconductive properties 3, an intermediate layer with insulating properties
4, an outer layer with semiconductive properties 5, a metal screen 6, and an
outer sheath 7.

CA 02425382 2003-03-13
WO 02/27731 PCT/EP01/09700
12
The conductor 2 generally consists of metal wires, preferably of copper
or aluminium, stranded together by conventional methods. At least one
covering layer selected from the insulating layer 4 and the semiconductive
layers 3 and 5 comprises the composition of the invention as hereinbefore
defined. Around the outer semiconductive layer 5 there is usually positioned a
screen 6, generally of electrically conducting wires or strips wound
helically.
This screen is then covered by a sheath 7 of a thermoplastic material, for
example non-crosslinked polyethylene (PE) or preferably a propylene
homopolymer or copolymer as hereinbefore defined.
The cable can also be provided with an outer protective structure (not
shown in Figure 1) the main purpose of which is to mechanically protect the
cable against impact and/or compression. This protective structure can be, for
example, a metal reinforcement or a layer of expanded polymer as described in
WO 98/52197.
Figure 1 shows only one possible embodiment of a cable of the present
invention. Suitable modifications known in the art can evidently be made to
this embodiment, but without departing from the scope of the invention.
The cable of the invention can be constructed in accordance with known
methods for depositing layers of thermoplastic material, for example by
extrusion. The extrusion is advantageously carried out in a single pass, for
example by the tandem method in which individual extruders are arranged in
series, or by co-extrusion with a multiple extrusion head.
The following examples illustrate the invention, but without limiting it.
EXAMPLES
The dielectric liquids according to the invention used in the following
examples were:
- BaylectrolR 4900: ditoluyl ether (Bayer AG), dielectric constant at 25 C
equal to 3.5, measured in accordance with IEC 247;
- NeovacR SY: octadecyl diphenyl ether (Matsumura Oil Research Corp.),
dielectric constant at 25 C equal to 2.7, measured in accordance with IEC 247.
The comparison dielectric liquids used in the following examples were:
- BaysiloneRPD5 (General Electric - Bayer), dielectric constant at 25 C
equal to 2.6, measured in accordance with IEC 247;
- polyphenylmethylsiloxane (PPMS), polyaromatic dielectric oil as
described in IEEE Transactions on Electrical Insulation Vol. 26, No.4, 1991),

CA 02425382 2008-12-02
WO 02/27731 PCT/EP01/09700
13
having a viscosity of 4 mm2/ sec at 25 C;
- FlexonR641 (commercial product of Esso): naphthene-based aromatic
oil having a viscosity of 22 mm2/sec at 40 C, consisting of 40 wt% aromatic
hydrocarbons, 57 wt% saturated hydrocarbons and 3 wt% polar compounds.
As polymer materials were used:
- a flexible propylene homopolymer with melting point 160 C, melting
enthalpy 56.7 J/g, MFI 1.8 dg/min and flexural modulus 290 MPa
(RexflexRWL105 - commercial product of Huntsman Polymer Corp.) (Table 1,
Examples 1-6)
- a propylene heterophase copolymer with an ethylene/propylene
elastomeric phase content of about 65 wt% (propylene 72 wt% in the
elastomeric phase), melting enthalpy 32 J/g, melting point 163 C, MFI 0.8
dg/min and flexural modulus of about 70 MPa (HifaxRKSO81 - commercial
product of Montell) (Table 1, Examples 7-8).
Composition preparation -
The polymer in granular form was preheated to 80 C in a turbomixer.
The dielectric liquid was added, in the quantities specified for the
formulations
given in Table 1, to the polymer preheated in the turbomixer under agitation
at
80 C over 15 min. After the addition agitation was continued for a further
hour
at 80 C until the liquid was completely absorbed in the polymer granules.
After this first stage, the resultant material was kneaded in a laboratory
double-screw Brabender PlasticorderTM PL2000 at a temperature of 185 C to
complete homogenization. The material left the double-screw mixer in the form
of granules.
Measurement of dielectric strength (DS)
The dielectric strength of the polymer compositions obtained was
evaluated on test-pieces of insulating material having the geometry proposed
by the EFI (Norwegian Electric Power Research Institute) in the publication
"The EFI Test Method for Accelerated Growth of Water Trees" (IEEE
International Symposium on Electrical insulation, Toronto, Canada, June 3-6
1990). In this method, the cable is simulated with glass-shaped test pieces of
insulating material having their base coated on both sides with a
semiconductive material coating.
The glass-shaped test-pieces were formed by moulding discs of
insulating material at 160-170 C from a plate of thickness 10 mm obtained by

CA 02425382 2003-03-13
WO 02/27731 PCT/EP01/09700
14
compressing granules at about 190 C.
The inner and outer surfaces of the base, which had a thickness of
about 0.40-0.45 mm, were coated with a semiconductive coating. The DS
measurement was made by applying to these specimens, immersed in silicone
oil at 20 C, an alternating current at 50 Hz starting with a voltage of 25 kV
and
increasing in steps of 5 kV every 30 minutes until perforation of the test-
piece
occurred. Each measurement was repeated on 10 test-pieces. The values given
in Table 1 are the arithmetic mean of the individual measured values.
TABLE 1
Ex. Polymer Dielectric % dielectric DS (mean)
liquid liquid by weight
1* Rexflex -- -- 92
WL 105
2* Rexflex Baysilone 5 90
WL 105 PD5
3* Rexflex Flexon 641 5 94
WL 105
4 Rexflex Baylectrol 6 140
WL 105 4900
5 Rexflex Baylectrol 13 152
WL 105 4900
6 Rexflex Neovac SY 10 145
WL 105
7* Hifax -- -- 90
KS081
8 Hifax Baylectrol 13 140
KS081 4900
* comparison
The dielectric strength values given in Table 1 highlight the
improvement in electrical performance deriving from the dielectric liquids of
the invention, compared to that of the base polymer as such or when mixed
with the comparison dielectric liquids.
Tests on cables
Cable production:
The composition of the insulating layer and of the semiconductive
layers is described in Table 2 below.

CA 02425382 2003-03-13
WO 02/27731 PCT/EP01/09700
TABLE 2
Reference cable Cable of the
(composition invention
Ex 1) (composition
Ex 5)
Insulation Inner and Insula- Inner
outer tion and
semicond. outer
layer semicond
. layer
Phr Phr Phr Phr
Rexflex WL105 100 87
Ba lectrol 4900 13 10
Hifax KS081 100 100
Nero 55 55
Y-200
Ir anox 1330 0.3 0.3
Nero Y-200: acetylene carbon black of the firm SN2A with specific
surface of 70 m2/g; IrganoxR 1330: 1,3,5-trimethyl-2,4,6-tris (3,5-di-
tertbutyl-
4-hydroxy-benzyl)benzene (Ciba Geigy).
5 The process used for manufacturing the cable was the following. The
ReflexR WL105 and the BaylectrolR 4900, this latter with previously added
IrganoxR 1330, were fed into a double-screw extruder (T=180 C); the mixture
formed in this manner was then passed into a single-screw extruder (T=190 C,
screw cross-section 150 mm2) where the filtered mixture (50 micron) feeds
10 another extruder (screw cross-section 150 mmz, 190 C). After subsequent
filtration (80 micron) the material was fed into triple head and deposited
simultaneously with the semiconductive layers to form a triple layer on the
metal conductor of copper plait (cross-section 400 mm2).
The cable leaving the extrusion head was fed into a tube containing
15 silicone oil at 100 C and then into water where it was cooled to ambient
temperature.
The finished cable consisted of a copper conductor (cross-section 400
mm2), an inner semiconductive layer of about 2 mm, an insulating layer of
about 5.5 mm and finally an outer semiconductive layer of about 2 mm.
Under similar conditions, using the materials indicated in Table 2, a
reference cable was produced.
Partial discharges:
Partial discharges were measured at 20 kV/mm without encountering
currents exceeding 5 pico Columb (pC) (in accordance with IEC 60-502).

CA 02425382 2003-03-13
WO 02/27731 PCT/EP01/09700
16
Dielectric strength:
100 metres of each of the two cables produced as described above were
subjected to dielectric strength measurement based on ENEL DC4584 using
alternating current at ambient temperature. Starting from 30 kV/mm the
gradient applied to the cables was increased by 5 kV/mm every 30 minutes
until the cables perforated. The perforation gradient considered is that on
the
conductor.
Table 3 summarizes the data relative to the cables and the results of the
electrical tests.
TABLE 3
Ex. Polymer Dielectric % dielectric DS (relative)
liquid liquid by weight
1* Rexflex -- -- 100
WL 105
5 Rexflex 13 180
WL 105 Baylectrol
4900
The results obtained indicate that the cable with additives shows a DS
increase of 80% over the cable without additives.

Representative Drawing