U.S. patent application number 10/058004 was filed with the patent office on 2002-08-15 for electrical cable with self-repairing protection.
This patent application is currently assigned to PIRELLI CAVIE SISTEMI S.p.A.. Invention is credited to Balconi, Luca, Bareggi, Alberto, Belli, Sergio, Bosisio, Claudio, Caimi, Luigi, Pozzati, Giovanni.
Application Number | 20020108774 10/058004 |
Document ID | / |
Family ID | 27239026 |
Filed Date | 2002-08-15 |
United States Patent
Application |
20020108774 |
Kind Code |
A1 |
Belli, Sergio ; et
al. |
August 15, 2002 |
Electrical cable with self-repairing protection
Abstract
A cable, in particular a cable for electric power transmission
or distribution, having an inner layer comprising a self-repairing
material with a predetermined cohesiveness and a controlled
flowability, so as to re-establish, upon creation of a
discontinuity in at least one of the cable coating layers, the
continuity in the coating. The discontinuity in the coating can be
caused by mechanical abuses of various types, for example
accidental impact with cutting tools. Infiltration of moisture and
generation of leakage currents, leading to a rapid corrosion of the
conductor, are in this way avoided.
Inventors: |
Belli, Sergio; (Livorno,
IT) ; Caimi, Luigi; (Lomagna, IT) ; Bosisio,
Claudio; (Brembate, IT) ; Bareggi, Alberto;
(Milano, IT) ; Balconi, Luca; (Bresso, IT)
; Pozzati, Giovanni; (Olgiate Olona, IT) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT &
DUNNER LLP
1300 I STREET, NW
WASHINGTON
DC
20005
US
|
Assignee: |
PIRELLI CAVIE SISTEMI
S.p.A.
|
Family ID: |
27239026 |
Appl. No.: |
10/058004 |
Filed: |
January 29, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10058004 |
Jan 29, 2002 |
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09971766 |
Oct 9, 2001 |
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09971766 |
Oct 9, 2001 |
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09261505 |
Mar 3, 1999 |
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60076752 |
Mar 4, 1998 |
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Current U.S.
Class: |
174/120R |
Current CPC
Class: |
Y02A 30/14 20180101;
H01B 3/441 20130101; H01B 7/28 20130101 |
Class at
Publication: |
174/120.00R |
International
Class: |
H01B 007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 4, 1998 |
EP |
98 103 767.4 |
Claims
1. Cable comprising a conductor and at least one coating layer,
characterized in that the said cable comprises an inner layer
comprising a self-repairing material having a predetermined
cohesiveness and a controlled flowability.
2. Cable according to claim 1, comprising an insulating coating
layer and an outer sheath and characterized in that the inner layer
is placed between the insulating layer and the outer sheath.
3. Cable according to claim 1, comprising an insulating coating
layer and characterized in that the inner layer is placed between
the conductor and the insulating layer.
4. Cable according to claim 1, comprising at least two insulating
coating layers and characterized in that the inner layer is placed
between two of the said insulating layers.
5. Cable according to any one of the preceding claims, in which the
conductor is coated with a semiconductive layer.
6. Cable according to any one of the preceding claims, also
comprising an expanded polymer coating.
7. Cable according to claim 6, in which the expanded polymer
coating is placed in direct contact with an outer protective
sheath.
8. Cable according to any one of the preceding claims, in which the
inner layer has a thickness of not less than 0.1 mm.
9. Cable according to claim 8, in which the inner layer has a
thickness of between 0.2 and 2 mm.
10. Cable according to claim 9, in which the inner layer has a
thickness of between 0.3 and 1 mm.
11. Cable according to any one of the preceding claims, in which
the self-repairing material is a dielectric material.
12. Cable according to claim 11, in which the self-repairing
material has a dielectric rigidity under alternating current of
greater than 15 kV/mm and a resistivity of greater than 10.sup.14
.OMEGA..multidot.cm.
13. Cable according to claim 12, in which the self-repairing
material has a dielectric rigidity under alternating current of
greater than 20 kV/mm and a resistivity of greater than 10.sup.16
.OMEGA..multidot.cm.
14. Cable according to any one of the preceding claims, in which
the self-repairing material has a cohesive force at room
temperature of at least 0.05 kg/cm.sup.2.
15. Cable according to claim 14, in which the self-repairing
material has a cohesive force at room temperature of between 0.1
and 4 kg/cm.sup.2.
16. Cable according to claim 15, in which the self-repairing
material has a cohesive force at room temperature of between 0.2
and 2 kg/cm.sup.2.
17. Cable according to any one of claims 14 to 16, in which the
self-repairing material has a cohesiveness which is such that the
force of recohesion measured at room temperature has a value of not
less than 80% relative to the value of the cohesive force measured
on the material as such.
18. Cable according to claim 17, in which the force of recohesion
measured at room temperature has a value of not less than 90%
relative to the value of the cohesive force measured on the
material as such.
19. Cable according to any one of the preceding claims, in which
the self-repairing material has a controlled flowability which is
such that a sample of about 3 grams of self-repairing material,
placed on an aluminium plate inclined at 60.degree. relative to the
horizontal plane and maintained at 60.degree. C. for 24 hours,
shows a displacement of the material front along the inclined plate
of between 0.5 and 400 mm.
20. Cable according to claim 19, in which the displacement of the
front of the self-repairing material sample along the inclined
plate is between 1 and 200 mm.
21. Cable according to claim 20, in which the displacement of the
front of the self-repairing material sample along the inclined
plate is between 50 and 100 mm.
22. Cable according to any one of the preceding claims, in which
the self-repairing material has a saturation water content of less
than 400 ppm.
23. Cable according to claim 22, in which the self-repairing
material has a saturation water content of less than 200 ppm.
24. Cable according to any one of the preceding claims, in which
the self-repairing material has a permeability to water vapour,
measured at room temperature according to ASTM E96, of between
1.2.times.10.sup.-7 and 8.0.times.10.sup.-6
g/(cm.multidot.hour.multidot.mmHg).
25. Cable according to any one of the preceding claims, in which
the self-repairing material comprises an amorphous polymer having
properties of a high-viscosity liquid or of a semi-solid.
26. Cable according to claim 25, in which the amorphous polymer is
selected from: (a) polyisobutene or isobutene copolymers with minor
amounts of different C.sub.4-C.sub.12 .alpha.-olefins; (b) atactic
propylene homopolymers; (c) silicone rubbers, consisting of linear
chains of monomer units of formula --O--SiR.sub.1R.sub.2--, in
which R.sub.1 and R.sub.2 are optionally substituted aliphatic or
aromatic radicals.
27. Cable according to claim 26, in which the amorphous polymer is
polyisobutene having a viscosimetric (Staudinger) average molecular
weight of between 2,000 and 50,000.
28. Cable according to claim 27, in which the amorphous polymer is
polyisobutene having a viscosimetric (Staudinger) average molecular
weight of between 5,000 and 20,000.
29. Cable according to any one of claims 25 to 28, in which the
amorphous polymer is dissolved in a solvent.
30. Cable according to claim 29, in which the solvent is a mineral
oil or a synthetic oil.
31. Cable according to claim 30, in which the solvent is a
paraffinic oil or naphthenic oil.
32. Cable according to claim 29, in which the amorphous polymer is
dissolved in a solvent which is a low molecular weight homologue of
the amorphous polymer.
33. Cable according to claim 32, in which the amorphous polymer is
a polyisobutene according to claim 27 or 28, dissolved in a
polybutenic oil having an osmometric average molecular weight of
between 400 and 1,300.
34. Cable according to claim 32, in which the amorphous polymer is
a silicone rubber, dissolved in a silicone oil having a viscosity
of between 100 and 5,000 mm.sup.2/sec at 25.degree. C.
35. Cable according to any one of claims 29 to 33, in which the
amount of solvent is between 5 and 95% by weight, relative to the
total weight of the mixture.
36. Cable according to claim 35, in which the amount of solvent is
between 50 and 90% by weight, relative to the total weight of the
mixture.
37. Cable according to any one of claims 29 to 36, in which the
self-repairing material also comprises a thickener.
38. Cable according to claim 37, in which the thickener is selected
from: pyrogenic silica, bentonite or mixtures thereof.
39. Cable according to claim 37 or 38, in which the thickener is
added in amounts of between 1 to 20 parts by weight relative to the
total weight of the mixture.
40. Cable according to any one of claims 1 to 24, in which the
self-repairing material comprises a solid polymeric material
dispersed in an oily phase.
41. Cable according to claim 40, in which the oily phase is
selected from: (a) paraffinic or naphthenic oils; (b) polybutene
oils having an osmometric average molecular weight of between 400
and 1,300; (c) polypropylene oils (d) low molecular weight
polyesters; or mixtures thereof.
42. Cable according to any one of claims 40 to 41, in which the
solid polymeric material is a high molecular weight polymer with
elastomeric properties selected from: (i) styrene block copolymers
or terpolymers with different olefins and/or with dienes; (ii)
polyisobutene or copolymers of isobutene with minor amounts of
different C.sub.4-C.sub.12 .alpha.-olefins; (iii) propylene
copolymers with ethylene and/or with C.sub.4-C.sub.12
.alpha.-olefins or with C.sub.4-C.sub.20 dienes; (iv) polyisoprene
or natural rubber; (v) nitrile rubbers; (vi) butyl rubbers; (vii)
amorphous ethylene copolymers; or mixtures thereof.
43. Cable according to any one of claims 40 to 42, in which the
solid polymer material is dispersed in the oily phase in a
subdivided form, in an amount of between 5 and 70% by weight
relative to the total weight of the mixture.
44. Cable according to any one of claims 25 to 43, in which the
self-repairing material also comprises an inorganic filler.
45. Cable according to claim 44, in which the inorganic filler is
selected from: kaolin, calcium carbonate, aluminium hydroxide,
magnesium hydroxide, talc, precipitated silica, or mixtures
thereof.
46. Cable according to claim 44 or 45, in which the inorganic
filler is present in amounts of between 5 and 50 parts by weight
with respect to the total weight of the mixture.
47. Cable according to any one of claims 25 to 46, in which the
self-repairing material also comprises a tackifying agent.
48. Cable according to claim 47, in which the tackifying agent is
selected from: natural or synthetic rosins, or derivatives thereof;
esterified polyalcohols; or mixtures thereof.
49. Cable according to claim 47 or 48, in which the tackifying
agent is present in amounts of between 1 and 20% by weight with
respect to the total weight of the mixture.
50. Method for imparting to a cable comprising a conductor and at
least one coating layer a capacity of self-repairing the coating
layer, the said method comprising providing the cable with an inner
layer comprising a material having the capacity, upon creation of a
discontinuity in the coating layer, of re-establishing the
continuity in the coating layer in a reversible manner.
51. Method according to claim 50, in which the material of the
inner layer is capable of at least partially filling the
discontinuity without leaking from the cable in an uncontrolled
manner.
52. Process for manufacturing a cable having a layer of
self-repairing material, comprising the following steps: (i)
depositing the self-repairing material, maintained in a fluid
state, on a cable core; (ii) forming the said layer of
self-repairing material so as to obtain a uniform layer of a
predetermined thickness.
53. Process according to claim 48, comprising the following steps:
introducing an initial section of the cable core inside an
application head through an inlet hole with a diameter which is
slightly larger than the diameter of the cable core, and an outlet
hole having a diameter which is predetermined according to the
desired thickness of the layer of self-repairing material; feeding
the application head with the self-repairing material maintained in
a fluid state by pre-heating; passing the cable core, through the
application head so as to perform the deposition of the
self-repairing material and the simultaneous forming of the layer
of self-repairing material.
54. Process for manufacturing a cable having a layer of
self-repairing material, in which the self-repairing material is
extruded onto the cable core.
Description
[0001] The present invention relates to a cable, in particular a
cable for electric power transmission or distribution or for
telecommunications. More particularly, the present invention
relates to a cable as defined above comprising at least one coating
layer and having self-repairing protection which is capable of
restoring the continuity of the coating layer after it has been
broken.
[0002] Electrical cables, in particular low- or medium-voltage
cables for the distribution of electric energy for domestic or
industrial use, generally consist of one or more conductors
individually insulated by a polymeric material and coated with a
protective sheath, which is also made of a polymeric material.
These cables, particularly when installed underground, in tunnels
or inside buried pipes, are subject to damage on these layers
caused by various types of mechanical abuses, for example
accidental impact with sharp tools such as shovels or picks, which
exert on the cable both cutting and compression action. This can
lead to partial or total rupture of the outer sheath and possibly
also of the insulating layer, with consequent infiltration of
moisture and generation of leakage currents. If the rupture of the
coating layers reaches the conductor, the combined effect of
leakage currents and moisture leads to a gradual corrosion of the
conductor until, at the utmost, to a complete breakage of the
conductor itself.
[0003] To obtain effective protection against such mechanical
abuses, the cable can be provided with an outer structure capable
of withstanding both cutting and compression, this outer structure
consisting, for example, of a sheath made of a metal or of a
plastic material combined with metal armouring. Besides being
expensive, this solution leads to a considerable increase in cable
dimensions and rigidity, thus making this solution unsuitable for
cables which require easiness of installation and low costs, such
as, in particular, in the case of low-voltage cables.
[0004] In patent application DE-1,590,958 a telecommunications or
high-current cable is described which is protected from mechanical
damage by means of an outer sheath having, on its inside,
microcapsules containing a liquid which is capable of solidifying
rapidly once the microcapsule has been broken. To this purpose, it
is mentioned as preferred the use of the two components commonly
used for manufacturing expanded polyurethane, these components
being microencapsulated separately so that they react together at
the moment the microcapsules are broken, forming an expanded
material which closes the accidental cut. Alternatively, it is
possible to use liquids which solidify when placed in contact with
external agents, for example with moisture.
[0005] According to the Applicant, the solution proposed in the
above-mentioned patent application is difficult to implement in
practice and has many drawbacks. Firstly, it is to be observed that
the possibility of self-repairing is limited to the outer sheath,
and no indications regarding the possibility of restoring integrity
of the insulating layer are provided. Moreover, to obtain an
effective self-repairing effect, it is necessary to introduce a
large amount of microencapsulated material during sheath extrusion,
and this operation can result to be extremely difficult, besides
being expensive. Lastly, it is to be pointed out that the mechanism
of action of the microcapsules is irreversible, consequently the
self-repairing effect can be carried out only once, namely at the
moment the microcapsules are broken. Actually, during the various
stages of the cable life (manufacturing, storage, installation,
use), the coating layers are inevitably subjected to external
mechanical actions of compression and bending and to thermal cycles
of expansion and compression, which can lead to rupture of the
microcapsules with consequent expansion and/or solidification of
the material contained therein. Therefore, this material will no
longer be able to effect the desired self-repairing action when the
sheath should actually be damaged. It is also to be noted that,
even when microcapsules are used containing a liquid material which
solidifies on contact with moisture, accidental rupture of the
microcapsules without any actual damage to the outer sheath
nonetheless leads to solidification of the material because
residual moisture is always present inside the cable.
[0006] The Applicant has now found that, in consequence of a
mechanical damage which creates a discontinuity in at least one of
the cable coating layers, it is possible to obtain effective
self-repairing of the coating by virtue of the presence of an inner
layer, placed, for example, between the insulating layer and the
outer sheath, this inner layer comprising a material having a
predetermined cohesiveness and at the same time a controlled
flowability, which is capable of repairing the damage by restoring
the continuity of the coating layer. After creation of a
discontinuity in the coating, the material "moves" towards the
point of damage and fills up, at least partly, the discontinuity by
forming a substantially continuous layer which is capable of
maintaining the functionality of the cable under the expected
working conditions. The action of the self-repairing material,
which occurs with a revesible mechanism, prevents, among other
things, moisture infiltration and establishment of leakage
currents, and thus a quick corrosion of the conductor.
[0007] The flowability of the material is predetermined so as to
have sufficient fluidity at the working temperature of the cable,
and at the same time so as to prevent the material from draining
from the cable extremities or leaking in an uncontrolled manner
from the coating rupture point.
[0008] In a first aspect, the present invention thus relates to a
cable comprising a conductor and at least one coating layer,
characterized in that the said cable comprises an inner layer
comprising a self-repairing material having a predetermined
cohesiveness and a controlled flowability.
[0009] According to a preferred aspect, the cable according to the
present invention comprises an insulating coating layer and an
outer sheath, and is characterized in that the inner layer is
placed between the insulating layer and the outer sheath.
[0010] According to another embodiment of the present invention,
the inner layer is placed between the conductor and the insulating
layer.
[0011] According to a further embodiment, the cable according to
the present invention comprises at least two insulating coating
layers and is characterized in that the inner layer is placed
between two of the said insulating layers.
[0012] According to a further aspect, the present invention relates
to a method for imparting to a cable comprising a conductor and at
least one coating layer a capacity of self-repairing the coating
layer, characterized in that the said method comprises providing
the cable with an inner layer comprising a material having the
capacity, upon creation of a discontinuity in the coating layer, of
re-establishing the continuity in the coating layer in a reversible
manner.
[0013] In the description hereinbelow and in the claims, the
material which constitutes the inner layer will be referred to, for
simplicity, as the "self-repairing material".
[0014] The term "inner layer" is understood herein to refer to a
layer placed in any position between the conductor and the
outermost coating layer, for example between the conductor and the
insulating layer or, preferably, between the insulating layer and
the outer sheath. Alternatively, when at least two insulating
layers are present, the self-repairing layer can be placed between
two of the said insulating layers.
[0015] The expression "discontinuity in at least one of the coating
layers" is understood herein to refer to a partial or complete
rupture of that layer. In the case of partial rupture only part of
the thickness of the coating layer has been damaged, whereas there
is complete rupture when the layer has been cut throughout its
thickness. Needless to say, a partial rupture may become complete
over time, for example following tractional or flexural mechanical
stresses or alternatively as a result of thermal cycles of
expansion and contraction to which the cable is subjected during
use.
[0016] The expression "re-establishing the continuity" is
understood herein to mean refill, at least partially, a point of
rupture which has been created in the cable coating, so as to
maintain the functionality of the cable at least for a
predetermined period of time, and preferably for the entire period
of the life of the cable, at least under the normal conditions of
use. In other words, the self-repairing material is capable of
preventing or at least slowing down the degradation of the
materials constituting the cable, and in particular of the
conductor, due to the infiltration of external agents through the
point of discontinuity.
[0017] The Applicant has noted that, for the purposes of the
present invention, the desired self-repairing of the cable is
obtained by using a material having a predetermined cohesiveness
and a controlled flowability. Although high cohesiveness values are
considered desirable for the purposes of self-repairing, it is
clear that these high values may conflict with flowability. A
person skilled in the art will be capable of selecting the most
suitable material, in which the desired compromise between cohesion
and fluidity is achieved as a function of the specific cable which
it is desired to manufacture and, above all, as a function of the
conditions of installation and use envisaged for this cable, in
particular in terms of temperature and pressure.
[0018] In a preferred embodiment, the self-repairing material is a
dielectric material which is capable of re-establishing the
electrical insulation of the cable. This property is particularly
important in the case where mechanical damage is such as to cause
in the insulating layer a partial or complete rupture, i.e. up to
reaching the conductor. In general, dielectric rigidity values,
under alternating current, of greater than 15 kV/mm, preferably
greater than 20 kV/mm, and resistivity values of greater than
10.sup.14 .OMEGA..multidot.cm preferably greater than 10.sup.16
.OMEGA..multidot.cm, are sufficient.
[0019] As mentioned above, the self-repairing material has
predetermined cohesiveness which is such that, following the
creation of a discontinuity in this material, for example by the
action of a cutting tool, and once the cause of the discontinuity
has been removed, the molecules which constitute the self-repairing
material are capable of spontaneously recreating intermolecular
bonds that are sufficient to restore continuity of the material.
This phenomenon is of a reversible kind, i.e. the self-repairing
material is capable of effectively carrying out its function an
indefinite number of times.
[0020] For the purposes of the present invention, the expression
"cohesiveness of the self-repairing material" refers both to the
actual cohesive force up to detachment (referred to hereinbelow
more simply as "cohesive force"), i.e. the force per surface area
unit required to cause within the mass of a sample of material a
complete detachment of one part of the material from the remaining
part, and to the force of re-cohesion (or of auto-adhesion), that
is the force required to recreate a complete detachment within the
material once two portions of this material have been placed in
contact for a predetermined time and under predetermined pressure
and temperature conditions. In other words, the cohesiveness of the
self-repairing material must be assessed both as regards the
strength of intermolecular forces which hold the material together
thereby ensuring its integrity, and as regards its capacity to
recreate these intermolecular bonds spontaneously once they have
been broken by the intervention of an external force.
[0021] The cohesive force can be measured according to the method
given in the examples hereinafter. It has been found that cohesive
force values, measured at room temperature, of at least 0.05
kg/cm.sup.2 ensure a sufficient cohesiveness of the self-repairing
material, although values of between 0.1 and 4 kg/cm.sup.2 are
preferred, and even more preferably between 0.2 and 2
kg/cm.sup.2.
[0022] The force of re-cohesion can be evaluated empirically by
placing, one on top of the other, two disks of material of
predetermined dimensions and leaving the two disks in contact for a
predetermined time at room temperature. At the end of this period,
the force required to separate the two disks is measured. The
closer this force is to the intrinsic cohesive force value of the
material as such, the more the material is capable of re-unifying
spontaneously after damage, thus reforming a continuous material.
In practice, the Applicant has found that in the self-repairing
materials according to the present invention, the force of
re-cohesion is preferably substantially identical to the cohesive
force as defined above, and at least has a value not less than 80%,
preferably not less than 90%, relative to the cohesive force
measured on the material as such.
[0023] In the Applicant's perception, another property of the
self-repairing material according to the present invention is its
controlled flowability, i.e. the self-repairing material must be
capable of "moving" so as to migrate towards the point of rupture
of the coating in an amount which is sufficient to repair the
damage.
[0024] On the other hand, as already mentioned above, the
flowability of the self-repairing material must be controlled in
such a way as to avoid loss of material either by drainage from the
extremities of the cable or by leaking from the point of rupture of
the coating. This control of the flowability must be ensured not
only at ambient temperature but also at higher temperatures, for
example at the maximum working temperature envisaged for the cable
(usually 75-90.degree. C.).
[0025] It might be thought that the flowability of the
self-repairing material could be evaluated on the basis of
viscosity measurements. Actually, for the purposes of the present
invention, the Applicant believes that a viscosity measurement is
not significant per se, besides being not easy to carry out, in
particular for materials with semi-solid properties. The Applicant
has therefore found that it is more convenient to evaluate
empirically the flowability of the self-repairing material by means
of a test in which the displacement of a predetermined amount of
material placed on an inclined plane at a predetermined temperature
and for a predetermined period is measured. This test is described
in the technical specification ST/LAB/QFE/06, .sctn. 5.5,
established by France Telecom/CNET (published: January 1994). In
particular, this evaluation can be carried out as follows. About 3
grams of self-repairing material are placed on a smooth aluminium
plate inclined at 60.degree. relative to the horizontal plane. The
material constituting the plate is selected so as to ensure high
adhesion of the test material to the plate itself, thereby
preventing the material from sliding down the plate without
undergoing a substantial deformation. The plate is placed in an
oven thermostatically adjusted to 60.degree. C.; after 24 hours the
material is checked for any demixing of the various components
present therein, and displacement of the material front down the
inclined plane, relative to its initial position, is measured, for
example by means of a gauge, checking that there has been
essentially no sliding of the entire mass.
[0026] In practice, the Applicant has found that the desired
control of the flowability is obtained when the self-repairing
material, subjected to the flow test on an inclined plane at
60.degree. C. for 24 hours described above, shows a displacement of
the front of the material sample on the inclined plane of between
0.5 and 400 mm, preferably between 1 and 200 mm and even more
preferably between 50 and 100 mm.
[0027] Moreover, the Applicant believes that the "movement" of the
self-repairing material towards the point of rupture is promoted by
the action of radial compression exerted on the self-repairing
layer by the other layers constituting the cable, in particular by
the outer sheath. Indeed, the specific volume of plastics decreases
as the temperature decreases, thus during the cooling process
following extrusion the outer sheath contracts to produce a radial
compressing action on underlying layers, with an estimated pressure
of the order of a few bar. In the case of rupture of the cable
coating, this pressure forces the self-repairing material towards
the point of rupture, thereby assisting the self-repairing.
Moreover, the Applicant has observed that the leakage of
self-repairing material from the point of rupture stops rapidly by
virtue of the cohesive properties of this material.
[0028] Another advantageous property of the self-repairing material
is its capacity to exert an effective blocking action against
external moisture which tends to infiltrate the cable through the
point of rupture of the coating. For this purpose, it is
appropriate for the self-repairing material to have a low
saturation water content, with values, measured at room temperature
by Karl-Fisher titration, generally of less than 400 ppm,
preferably less than 200 ppm.
[0029] Moreover, in the case where the self-repairing layer is
placed outside the insulating layer and the latter consists of a
material which is crosslinkable via silanes, it is convenient for
the self-repairing material, although absorbing small amounts of
moisture, to have a sufficient permeability to water vapour since,
as is known, crosslinking via silanes takes place in the presence
of water. Preferred values of permeability to water vapour,
measured at room temperature according to ASTM E96, are generally
between 1.2.times.10.sup.-7 and 8.0.times.10.sup.-6
g/(cm.multidot.hour.multidot.mmHg).
[0030] A further preferred characteristic of the self-repairing
material is a substantial physico-chemical inertness with respect
to the plastic materials with which it is placed in contact. The
reason for this is that it is desirable that the self-repairing
material does not interact, under the working conditions, with the
materials which constitute adjacent layers (generally polyolefins
such as polyethylene and ethylene copolymers, which may or may not
be crosslinked), thereby avoiding swelling phenomena for these
materials with a consequent worsening in their mechanical
properties.
[0031] A first class of materials suitable for making the
self-repairing layer according to the present invention consists of
amorphous polymers having properties of high-viscosity liquids or
of semi-solids, these polymers being selected, for example, from
the following classes of products:
[0032] (a) polyisobutene or isobutene copolymers with minor amounts
of different C.sub.4-C.sub.12 .alpha.-olefins;
[0033] (b) atactic propylene homopolymers;
[0034] (c) silicone rubbers, consisting of linear chains of monomer
units of formula --O--SiR.sub.1R.sub.2--, in which R.sub.1 and
R.sub.2 are optionally substituted aliphatic or aromatic radicals
such as, for example: dimethylsilicone, methylphenylsilicone,
methylvinylsilicone, silicones containing cyanoacrylic or
fluoroalkyl groups, and the like.
[0035] Among the products mentioned above, it is particularly
preferred to use polyisobutene having a viscosimetric (Staudinger)
average molecular weight of between 2,000 and 50,000, preferably
between 5,000 and 20,000, known commercially under the trademarks
Vistanex.RTM. (Esso Chemical), Hycar.RTM. (Goodrich), Oppanol.RTM.
(BASF), and the like.
[0036] The amorphous polymers mentioned above can be used as such
or dissolved in a suitable solvent, for example a mineral oil or a
synthetic oil, in particular a paraffin oil or a naphthenic oil
such as, for example, the oils known by the abbreviations ASTM 103,
104A and 104B. Preferably, low molecular weight products that are
homologues of the amorphous polymer can be used as solvents.
[0037] For example, in the case of polyisobutene, a polybutene oil
with an osmometric average molecular weight of between 400 and
1,300, preferably between 500 and 1,000, which can be obtained by
polymerization of C.sub.4 olefin mixtures containing mainly
isobutene, can advantageously be used as solvent. Products
corresponding to these characteristics can be found on the market
under the trademarks Napvis.RTM. (BP Chemicals) and Indopol.RTM.
(Amoco).
[0038] In the case of silicone rubbers, it is possible to use a
silicone oil with a viscosity generally between 100 and 5,000
mm.sup.2/sec at 25.degree. C. as solvent.
[0039] In general, the amount of solvent is between 5 and 95% by
weight, preferably between 50 and 90% by weight, relative to the
total weight of the mixture.
[0040] In the case where the amorphous polymer is dissolved in a
suitable solvent as mentioned above, a thickener can advantageously
be added to the composition, the main function of this thickener
being to control flowability, thereby reducing the risk of the
self-repairing material uncontrollably leaking from the cable.
[0041] Inorganic products such as pyrogenic silica, bentonite and
the like, or mixtures thereof, can for example be used as
thickeners. The amount of thickener is generally between 1 and 20
parts by weight, preferably between 2 and 10 parts by weight,
relative to the total weight of the mixture.
[0042] The self-repairing material as described above can be
prepared according to standard techniques, for example by
dissolving the amorphous polymer and any additives in the oily
solvent by heating. If a thickener is used, it can be dispersed by
vigorous stirring under heating.
[0043] Another category of materials which are suitable for forming
the self-repairing inner layer according to the present invention
consists of solid polymeric materials dispersed in an oily
phase.
[0044] The oily phase can consist, for example, of:
[0045] (a) paraffinic oils or naphthenic oils, for example the oils
ASTM 103, 104A or 104B;
[0046] (b) polybutene oils with an osmometric average molecular
weight of between 400 and 1,300, preferably between 500 and 1,000,
which can be obtained by polymerization of C.sub.4 olefin mixtures
containing mainly isobutene, for example the commercial products
Napvis.RTM. (BP Chemicals) and Indopol.RTM. (Amoco);
[0047] (c) polypropylene oils;
[0048] (d) low molecular weight polyesters, for example acrylic
acid polyesters, such as the product ECA 7955 from Exxon Chemical
Co.;
[0049] or mixtures thereof.
[0050] The solid polymeric material is generally a high molecular
weight polymer with elastomeric properties, selected, for example,
from:
[0051] (i) styrene block copolymers or terpolymers with different
olefins and/or with dienes, for example with butene, ethylene,
propylene, isoprene, butadiene and the like, and in particular:
styrene-butadiene-styrene (S-B-S), styrene-isoprene-styrene (S-I-S)
and styrene-ethylene/butene-styrene (S-EB-S) triblock polymers;
styrene-ethylene/propylene (S-EP) and styrene-ethylene/butene
(S-EB) diblock polymers; styrene-butadiene or styrene-isoprene
branched polymers; such products are commercially available, for
example, under the trademark Kraton.RTM. (Shell Chemical);
[0052] (ii) polyisobutene or isobutene copolymers with minor
amounts of different C.sub.4-C.sub.12 .alpha.-olefins, having a
viscosimetric (Staudinger) average molecular weight generally of
greater than 40,000, preferably between 50,000 and 200,000;
[0053] (iii) copolymers of propylene with ethylene and/or with
C.sub.4-C.sub.12 .alpha.-olefins (for example 1-butene, isobutene,
1-hexene, and the like), or with C.sub.4-C.sub.20 dienes (for
example 1,3-butadiene, 1,4-hexadiene, 5-ethylidene-2-norbornene,
and the like), known commercially, for example, under the
trademarks Dutral.RTM. (Enichem) or Nordel.RTM. (Dow-Du Pont);
[0054] (iv) polyisoprene or natural rubber;
[0055] (v) nitrile rubbers;
[0056] (vi) butyl rubbers;
[0057] (vii) amorphous ethylene copolymers, for example copolymers
of ethylene with esters having ethylenic unsaturation, such as
ethylene/vinyl acetate (EVA), ethylene/methyl acrylate (EMA),
ethylene/ethyl acrylate (EEA), ethylene/butyl acrylate (EBA)
copolymers, and the like;
[0058] or mixtures thereof.
[0059] The solid polymeric material can be dispersed in the oily
base in a subdivided form, for example in the form of granules or
powder, in amounts generally of between 5 and 70% by weight,
preferably between 10 and 60% by weight, relative to the total
weight of the mixture. A homogeneous dispersion can be obtained by
suitable mixing according to the standard techniques, for example
using an internal mixer of the type with tangential rotors
(Banbury) or interlocking rotors, or alternatively in continuous
mixers of the Ko-Kneader type (Buss) or co-rotating or
counter-rotating twin-screw mixers.
[0060] To avoid an unacceptable reduction in the mobility of the
self-repairing material at low temperatures, the optionally present
oily products generally have a pour point, determined according to
ASTM D97-57, of less than 0.degree. C., preferably less than
-10.degree. C. and even more preferably less than -20.degree.
C.
[0061] Inorganic fillers of various types can be added to the
self-repairing materials described above, these fillers having the
function of improving processability and of controlling
flowability, for example: kaolin, calcium carbonate, aluminium
hydroxide, magnesium hydroxide, talc, precipitated silica, and the
like, or mixtures thereof. The amount of inorganic fillers can vary
within a wide range, generally between 5 and 50 parts by weight,
preferably between 10 and 30 parts by weight, relative to the total
weight of the mixture. The self-repairing material can also contain
additives of various types, such as: stabilizers, antioxidants,
anti-copper products, glass microspheres, and the like.
[0062] To give the anti-repair material greater cohesiveness,
tackifying agents can optionally be added such as: natural or
synthetic rosins (for example the products Polypale.RTM. from
Hercules, or Escorez.RTM. from Esso Chemicals), or derivatives
thereof; esterified polyalcohols (for example the products
Oulupale.RTM. from Veitsiluotooy), or mixtures thereof. The amount
of tackifying agent is generally between 1 and 20% by weight,
preferably between 5 and 10% by weight, relative to the total
weight of the mixture.
[0063] The thickness of the self-repairing material layer according
to the present invention must be sufficient to ensure
self-repairing of the cable, therefore this thickness is selected
mainly as a function of the dimensions of the cable and of the type
of damage which the latter might sustain. In general, thicknesses
of not less than 0.1 mm, preferably between 0.2 and 2 mm and more
preferably between 0.3 and 1 mm, are sufficient.
[0064] For the purpose of giving the electrical cable according to
the present invention impact-strength properties, an expanded
polymer coating can be added, as described in European patent
application No. 97107969.4 filed on 15.05.97 in the name of the
Applicant. This coating is preferably placed in direct contact with
the outer protective sheath. Although the expanded polymer coating
per se does not have any particular resistance to cutting, it has
the capacity of absorbing, at least partly, the energy transmitted
by impact with a cutting tool and thus of reducing the risk of
damage to the layers of cable coating.
[0065] The expanded polymer coating can consist of any type of
expandible polymer such as, for example: polyolefins, olefinic
copolymers, olefin/unsaturated ester copolymers, polyesters,
polycarbonates, polysulphones, phenolic resins, ureic resins, and
mixtures thereof. Preferably, olefinic polymers or copolymers, in
particular based on polyethylene (PE) and/or polypropylene (PP),
mixed with ethylene-propylene rubbers can be used. PP modified with
ethylene-propylene rubbers (EPR), with a PP/EPR weight ratio of
between 90/10 and 50/50, preferably between 85/15 and 60/40, can
advantageously be used. It is also possible to mix prior to
expansion the polymeric material with a predetermined amount of
rubber in powder form, for example vulcanized natural rubber. In
particular, the Applicant has found that a polymer material which
has, prior to expansion, a flexural modulus at room temperature of
greater than 200 MPa, preferably of at least 400 MPa (measured
according to ASTM D790), but not greater than 2,000 MPa, so as not
to excessively increase the rigidity of the finished product, is
particularly suitable for this purpose. The degree of expansion of
the polymer is extremely variable depending on the specific polymer
used and on the thickness of the coating which it is intended to
obtain. In general, the degree of expansion can range between 20%
and 3,000%, preferably between 30% and 500%. The minimum thickness
of the expanded layer which is capable of ensuring the desired
impact strength depends mainly on the degree of expansion and on
the flexural modulus of the polymer. In particular, for medium
voltage cables an expanded-coating thickness of at least 0.5 mm,
preferably of between 1 and 6 mm is sufficient. Further details
regarding the characteristics of this expanded polymer layer are
given in the above-mentioned European patent application No.
97107969.4, whose text constitutes an integral part of the present
description.
[0066] For the purpose of promoting dispersion of the leakage
microcurrents which may be present directly after the cable has
been damaged and before it has been fully self-repaired, this
conductor can advantageously be coated with a layer of polymeric
material having semiconductive properties. By favouring dispersion
of the leakage microcurrents, this material reduces the risk of
triggering of corrosion points onto the conductor. The
semiconductive layer, which is applied to the conductor by, for
example, taping or, preferably, extrusion, generally has a
thickness of at least 0.05 mm, preferably between 0.1 and 0.5
mm.
[0067] FIG. 1 shows schematically the cross-section of an
electrical cable according to the present invention, of unipolar
type, comprising, from the inside outwards, a conductor (1), an
insulating layer (2), a self-repairing layer (3) as described
above, and an outer protective sheath (4).
[0068] FIG. 2 shows a further embodiment of a unipolar electrical
cable according to the present invention, comprising, in addition
to the elements reported above, an expanded polymer layer (5) as
described above, placed between the self-repairing layer (3) and
the outer protective sheath (4), this layer giving the cable high
impact strength.
[0069] FIG. 3 shows schematically the cross-section of the device
used to measure the cohesive force of the self-repairing material,
a detailed description of which is given in the examples.
[0070] The conductor (1) generally consists of metal wires,
preferably made of copper or aluminium, plaited together according
to standard techniques. The insulating layer (2) and the outer
protective sheath (4) consist of a crosslinked or non-crosslinked
polymer composition having as base component a polymer selected,
for example, from: polyolefins (homopolymers or copolymers of
various olefins), olefin/ethylenically unsaturated ester
copolymers, polyesters, polyethers, polyether/polyester copolymers,
and mixtures thereof. Examples of such polymers are: polyethylene
(PE), in particular linear low density PE (LLDPE); polypropylene
(PP); propylene/ethylene thermoplastic copolymers;
ethylene-propylene rubbers (EPR) or ethylene-propylene-diene
rubbers (EPDM); natural rubbers; butyl rubbers; ethylene/vinyl
acetate (EVA) copolymers; ethylene/methyl acrylate (EMA)
copolymers; ethylene/ethyl acrylate (EEA) copolymers;
ethylene/butyl acrylate (EBA) copolymers; ethylene/.alpha.-olefin
thermoplastic copolymers, and the like.
[0071] The abovementioned polymers can be crosslinked according to
known techniques, in particular by heating in the presence of a
radical initiator, for example an organic peroxide such as dicumyl
peroxide. Alternatively, crosslinking can be carried out using
silanes, which involves the use of a polymer such as those
mentioned above, in particular a polyolefin, to which silane units
comprising at least one hydrolysable group, for example
trialkoxysilane groups, in particular trimethoxysilane, have been
covalently attached. The silane units can be introduced by radical
reaction with silane compounds, for example methyltriethoxysilane,
dimethyldiethoxysilane, vinyl-dimethoxysilane and the like. The
crosslinking is carried out in the presence of water and a
crosslinking catalyst, for example an organic titanate or a metal
carboxylate. Dibutyltin dilaurate (DBTL) is particularly
preferred.
[0072] The self-repairing layer can be produced by means of a
process of pultrusion of the self-repairing material on the cable
core, the latter consisting of the conductor alone or, preferably,
of the conductor which has been pre-coated with at least one
insulating layer according to known techniques. This pultrusion
process involves depositing on the cable core a layer of
self-repairing material which is maintained at a sufficient degree
of fluidity, for example by heating, and then forming this layer so
as to obtain the desired final thickness. Further coating layers
(for example the outer protective sheath) can then be applied to
the cable core thus coated, according to known techniques.
[0073] Therefore, in a further aspect, the present invention
relates to a process for manufacturing a cable having a layer of
self-repairing material, this process comprising the following
steps:
[0074] (i) depositing the self-repairing material, maintained in a
fluid state, on a cable core;
[0075] (ii) forming the said layer of self-repairing material so as
to obtain a uniform layer of a predetermined thickness.
[0076] The pultrusion process can be carried out batchwise or,
preferably, continuously.
[0077] Both steps of the pultrusion process can be carried out, for
example, using an application head consisting, for example, of a
hollow cylindrical element having an inlet hole whose diameter is
slightly larger than that of the cable core, and an outlet hole
having a predetermined diameter which depends on the desired
thickness of the self-repairing layer. Once the initial section of
the cable core has been introduced through the abovementioned
holes, the head is fed with the self-repairing material which has
been preheated so as to maintain the self-repairing material at a
temperature such as to obtain a sufficient degree of fluidity. This
temperature is generally between 50.degree. and 200.degree. C., and
is selected essentially as a function of the nature of the
self-repairing material. By running the cable core inside the
application head, the first stage of deposition of the
self-repairing material is carried out. The passing rate of the
cable core through the application head, and thus the time of
immersion in the self-repairing material, can vary within a wide
range, generally between 1 and 1,000 m/min, and is selected mainly
as a function of the type of self-repairing material used.
[0078] The subsequent forming step is carried out by the outlet
hole of the application head, which has a diameter corresponding to
the predetermined diameter value which it is desired to obtain for
the warm cable core coated with the self-repairing material.
[0079] Alternatively, the cable bearing the self-repairing layer
according to the present invention can be made using an extrusion
head of conventional type. The self-repairing material is fed into
the extrusion head under heating so as to obtain a sufficient
fluidity, and is distributed inside the head by means of a suitable
conveyor so as to obtain an outer corona on exit from the extrusion
head, in which the material is uniformly distributed. The extrusion
head can be of the single-layer type or of the multilayer type, so
as to effect co-extrusion of the self-repairing layer and of one or
more of the adjacent layers.
[0080] To describe the invention further, some working examples are
given hereinbelow.
EXAMPLES 1-5
[0081] Various types of self-repairing materials according to the
present invention were prepared, whose compositions are reported in
Table 1 (as parts by weight).
[0082] As to Example 1, the commercial product was used as such,
and the antioxidant was added thereto by dissolution under
heating.
[0083] The materials of Examples 2-4 were prepared by dissolving
under heating (120-150.degree. C.) solid polymeric components and
antioxidant in the oily phase. In the case of Example 2, pyrogenic
silica was dispersed in the thus obtained solution under heating
and with vigorous stirring.
[0084] As regards Example 5, the composition was prepared as
follows. The solid polymeric components were processed in an open
mixer with moderate heating until a continuous and homogeneous
sheet was obtained. The polybutene oil and the antioxidant were
then added, the stirring being continued until the mixture was
fully homogeneous.
[0085] The following measurements were carried out on the
self-repairing materials thus prepared.
[0086] (a) Cohesive force.
[0087] The cohesive force was determined by means of a device which
is shown schematically (in cross-section) in FIG. 3. With reference
to FIG. 3, a cylindrical aluminium container (6) (height 45 mm,
inside diameter 44 mm, which corresponds to a cross-section of 15.2
cm.sup.2), having a movable base (7), also made of aluminium, and a
lid (8), was filled with the test material (9), preheated to about
150.degree. C. so as to obtain sufficient fluidization and thus
homogeneous distribution of the material inside the container,
thereby avoiding the formation of air bubbles. Both the movable
base (7) and the lid (8) have a locking rod (10) which allows a
dynamometer (not shown in FIG. 3) to be attached thereto. The use
of aluminium ensures a high level of adhesion to the test material,
thereby avoiding any detachment of the material from the movable
base and/or from the walls of the cylinder during the test.
[0088] After cooling of the material to room temperature, the
cylinder is closed by the lid (8) (for example by screwing down by
means of a suitable thread, not shown in FIG. 3) and is inserted
into an Instron dynamometer, by means of which an increasing
tensile force is applied to the movable base (7) (pulling rate: 2
mm/min) until the self-repairing material inside the mass "breaks"
with detachment of some of the material, which adheres to the
movable base, from the mass adhering to the walls of the cylinder.
The cohesive force is given by the load "at break" (expressed in
kg) per unit of surface area (in cm.sup.2). For each material, the
measurement was carried out on three samples. In Table 1 the
arithmetic mean value is reported.
[0089] The re-cohesive capacity of the various test materials was
evaluated in the following way. A layer of self-repairing material
of about 1 cm in thickness was deposited on the surface of two
metal disks (made of aluminium) having a 40 mm diameter. The
deposition was carried out under heating and with the aid of a
spatula with rounded edges, so as to obtain a layer which was as
smooth and homogeneous as possible and free of air bubbles. On the
opposite face, each disk was fitted with a locking rod to allow a
dynamometer to be attached thereto. Once the material had cooled to
room temperature, the two disks were placed one on top of the other
with the two faces coated with the self-repairing material coming
together, thus producing an overall thickness of material of about
2 cm. No compressive force was applied to the two disks, therefore
the only force acting on the contact surface between the two layers
of self-repairing material was the weight-force (equal to about 50
grams) exerted by the assembly of the upper disk and of the related
layer of self-repairing material. After about 3 hours, the force
required to separate the material into two distinct parts (without
detaching the disks from this material) was measured using an
Instron dynamometer. It was found that this force was substantially
identical to the cohesive force measured using the cylinder with a
movable base, as described above. In addition, once re-cohesion
took place, it was no longer possible to identify the joining
surface between the two layers.
[0090] (b) Displacement on an inclined plane.
[0091] A smooth aluminium plate (dimensions 400.times.80.times.2
mm) was cleaned thoroughly with alcohol and left to dry. 3 g of
self-repairing material were placed on the upper part of the plate
using a spatula with rounded edges. The material was shaped by
means of the spatula so as to obtain a small uniform mass of
rounded shape, while avoiding the formation of air bubbles. The
material was then left to stand in a horizontal position for about
two hours. The initial position was marked on the edge of the
plate. The plate was then fixed to a support so as to form an angle
of 60.degree. relative to the horizontal plane, and placed in an
oven thermostatically adjusted to 60.degree. C. After 24 hours, the
plate was removed from the oven and left to cool for one hour at
room temperature. The displacement, relative to the initial
position, of the front of the material along the inclined plane was
measured using a gauge. The results are given in Table 1. No
appreciable demixing of the components was observed for any of the
test samples.
[0092] (c) Ageing of crosslinked polyethylene specimens.
[0093] To evaluate inertness of self-repairing materials, prepared
as above, with respect to the polyolefins which normally constitute
the coating layers adjacent to the self-repairing layer, ageing
tests were carried out on samples of silane-crosslinked
polyethylene (Getilan.RTM. ATP 3) kept at 80.degree. C. for 7 and
14 days in the materials of Examples 1 and 2. In particular, the
weight variation relative to the initial weight and the mechanical
properties before and after ageing were determined. The results are
given in Table 2. As can be seen, the tests carried out show the
substantial inertness of the self-repairing materials relative to
crosslinked polyethylene, as demonstrated by the extremely narrow
variations in weight and in mechanical properties of the test
samples.
1TABLE 1 Example 1 2 3 4 5 Vistanex .RTM. LMMH 100 30 -- -- --
Vistanex .RTM. MML80 -- -- -- -- 15 Napvis .RTM. DE10 -- 70 88 86
100 Silica CAB-O-SIL H5 -- 5 -- -- -- Kraton .RTM. G 1702 -- -- 12
14 -- Dutral .RTM. CO 043 -- -- -- -- 85 Irganox .RTM. 1010 0.5 0.5
0.5 0.5 0.5 Cohesive force (kg/cm.sup.2) 0.38 0.70 0.13 0.38 1.00
Displacement on 350 1 10 1 75 inclined plate (mm) Vistanex .RTM.
LMMH (Esso Chem. Co.): polyisobutene with a viscosimetric
(Staudinger) average molecular weight equal to 10,000-11,700;
Vistanex .RTM. MML80 (Esso Chem. Co.): polyisobutene with a
viscosimetric (Staudinger) average molecular weight equal to
64,000-81,000; Napvis .RTM. DE10 (BP Chemicals): polybutene oil
with an osmometric average molecular weight equal to 950; pour
point -7.degree. C. (ASTM D97-57); Silica CAB-O-SIL H5 (Cabot):
pyrogenic silica with a surface area of 325 m.sup.2/g and an
average particle diameter of 0.007 .mu.m; Kraton .RTM. G 1702
(Shell Chemical Co.): styrene-ethylene/propylene diblock copolymer
of average molecular weight 170,000; Dutral .RTM. CO 043 (Enichem
Elastomers): ethylene/propylene elastomeric copolymer; Irganox
.RTM. 1010 (Ciba-Geigy) antioxidant (pentaerythrityl-tetra
[3-(3,5-di-tert-butyl-4-h- ydroxyphenyl)propionate]).
[0094]
2TABLE 2 Material Example 1 Example 2 Ageing time at 0 7 14 0 7 14
80.degree. C. (days) .DELTA. Weight (%) -- -0.54 -0.56 -- +1.48
+1.77 Stress at 21.7 21.3 20.6 21.7 19.7 20.5 break (MPa) (-1.9%)
(-5.1%) (-9.3%) (-5.6%) Elongation at 370 327 320 370 365 335 break
(%) (-11.7%) (-13.6%) (-1.4%) (-9.5%) Modulus (MPa) 499 419 403 499
460 449 (-16%) (-19.3%) (-7.9%) (-10%)
EXAMPLE 6
[0095] (a) Manufacturing of the self-protected cable
[0096] A layer of polypropylene insulating material (commercial
product Moplen.RTM. BT 20 from Montell) with a nominal thickness of
1 mm was deposited on a flexible aluminium conductor of
cross-section equal to 70 mm.sup.2. For this operation, a Bandera
80 mm extruder in configuration 25 D, with a screw of pour-off
thread type and an extrusion head with electrical heating was used,
using the following compression-type; assembly of moulds: tip die
with a diameter of 10.5 mm, ring die with a diameter of 12.0 mm.
The following temperature profile was used during the extrusion
(.degree. C.):
3 draw- zone zone zone zone zone zone hole screw 1 2 3 4 5 6 collar
head 20 neutral 180 190 195 200 210 235 240 250
[0097] The following process conditions were employed:
[0098] Line speed: 2.8 m/min
[0099] Pressure at the extruder end: 60 bar
[0100] Extruder spin speed: 1.74 rpm
[0101] Extruder absorption: 30 Amps
[0102] Nominal diameter of the cold cable: 12.1 mm.
[0103] The cable core was subsequently subjected to a further
processing phase, during which the self-repairing material and the
outer sheath were applied using a tandem-type technique.
[0104] An application head coupled to a component for melting and
pumping the self-repairing material was placed upstream of the
point of application of the outer sheath.
[0105] A Nordson BM 56 device with a pressure plate and a supply
gear pump was used as component for melting the self-repairing
material. This device was coupled to the application head by means
of a supply tube heated by electrical resistance and having a
length of about 3 m.
[0106] The application head consisted of a hollow cylindrical
component having an inlet hole for the cable core with a diameter
slightly larger than that of the core itself, and an outlet hole of
diameter equal to 13 mm.
[0107] A layer of self-repairing material with a nominal thickness
of 0.5 mm, prepared as described above and corresponding to the
composition of Example 1 (see Table 1), was applied using this
application head.
[0108] The pultrusion was carried out using the following
temperature settings:
[0109] Pressure plate: 120.degree. C.
[0110] Supply tube: 120.degree. C.
[0111] Application head: 90.degree. C.
[0112] The outer sheath was applied downstream of the pultrusion
zone, by means of the same Bandera 80 mm-25 D extruder described
above, using Moplen.RTM. BT 20 (Montell) polypropylene as material,
with a nominal thickness of 1 mm. Using the same set-up and the
same temperature profile as indicated above for the insulation
layer, the following mould mounting (in compression) was used: tip
die with a diameter of 13.5 mm, ring die with a diameter of 18.2
mm.
[0113] The process conditions were set as follows:
[0114] Line speed: 2 m/min
[0115] Pressure at the end of the extruder: 20 bar
[0116] Extruder spin speed: 1.75 rpm
[0117] Extruder absorption: 19 Amps
[0118] Nominal diameter of the cold cable: 15.1 mm.
[0119] About 200 m of cable with a self-repairing layer were
produced by means of the process described above.
[0120] (b) Damage tests.
[0121] The cable thus obtained was subjected to tests to effect
various types of damage to the coating layers in a controlled and
reproducible manner.
[0122] To perform cutting, a device consisting of a C-shaped frame
supporting a guide inside which slides a cylindrical shaft was
used. One end of the shaft is threaded so as to allow various types
of cutting tools to be mounted. The following types of damage was
effected using this device:
[0123] (1) Blunt-type damage (based on standard ICEA S-81-570-1996,
.sctn. 6.2.3): the cable was subjected to impact with a steel anvil
with a cutting angle of 92.degree. and a rounded edge with a
curvature radius of 0.6 mm;
[0124] (2) Blade-type damage followed by bending: the cable was
subjected to impact with a steel blade having a thickness of 0.2
mm, and then to bending so as to cause a complete opening of the
cut, using a mandrel with a diameter of 375 mm, equivalent to 25
times the diameter of the cable;
[0125] (3) Shovel-type damage (according to standard DIN 20127):
the cable was subjected to a static load by means of a tool with a
cutting angle of 21.8.degree. and a flat profile at the point of
contact, 0.5 mm in width.
[0126] The impact energy or, in the case of static load (test (3)),
the load required to completely cut through all of the coating
layers until the conductor was reached without damaging it, was
determined for each of the above-mentioned tests. This measurement
was carried out with the aid of an oscilloscope coupled to the
cable, through which a certain amount of current was passed. At the
moment the cutting tool reached the conductor, the oscilloscope
recorded the instantaneous change in the electrical signal caused
by the short-circuit resulting from the contact between the tool
and the conductor.
[0127] For the blunt-type damage (1), the impact energy required to
reach the conductor was 9.2 J, obtained using a mass of 53.7 kg and
a drop height (including the diameter of the cable) of 32.5 mm.
[0128] For the blade-type damage (2), the impact energy required to
reach the conductor was 1 J, obtained using a mass of 26 kg and a
drop height (including the diameter of the cable) of 19 mm.
[0129] For the shovel-type damage (3), the load required to reach
the conductor was 100 kg.
[0130] To qualitatively evaluate the existence of a effect of
radial compression on the layer of self-repairing material by the
outer sheath, the cable was subjected to the following test. Holes
were made in a small length of cable using a pillar drill with
perforation bits of 3 and 5 mm in diameter. The holes were made on
two directrices at 180.degree. relative to each other. The depth of
the holes was such as to cut completely through the thickness of
the sheath until the self-repairing material was reached. The cable
damaged in this way was left in a horizontal position so as to have
one series of holes oriented upwards and the other series oriented
downwards. After 24 hours, it was observed that the self-repairing
material had completely filled all of the holes, leaking out in a
negligible quantity. Since, the test was carried out without
applying any external force, except for the force of gravity, the
leakage of material through the holes facing upwards is a clear
indication of the existence of a radial pressure exerted by the
outer sheath, which assists the movement of the material towards
the points of rupture.
[0131] On the basis of a mathematical model of a cable in which it
is taken account, for the various materials constituting the cable,
of linear thermal dilation coefficient, volume dilation
coefficient, longitudinal tension which maintains congruence
between adjacent layers, elastic modulus and temperature change
which the cable undergoes during the extrusion process, the
Applicant has calculated, for the cable according to the present
example, a radial compression value exerted by the outer sheath on
the self-repairing layer equal to about 3.8 bar. Of course, this
value should be considered only as an approximate evaluation of the
ringing effect of the outer sheath on the self-repairing layer,
bearing in mind that this effect is influenced not only by the
characteristics of the materials used, but also by the specific
conditions under which extrusion and subsequent cooling of the
sheath are carried out.
[0132] (c) Electrical tests.
[0133] To check effectiveness of self-repairing, leakage currents
were measured on small cable lengths damaged according to the
various ways described above under the following conditions.
[0134] Immediately after damage, the cable lengths were connected
to an electrical circuit and immersed in a tank containing tap
water at room temperature. Throughout the period of the test, which
lasted 60 days in total, an a.c. voltage of 150 V at 50 Hz was
applied to the cable lengths, causing a current to flow such as to
bring the temperature of the conductor to about 50.degree. C. in
the part immersed in water, corresponding to about 100.degree. C.
in the part in air, with continuous cycles of 12 hours of heating
and 12 hours of spontaneous cooling.
[0135] The total leakage current (I.sub.L) was measured by means of
a Keithley Mod. 197 type digital multimeter. The current I.sub.L
measured is the result of the vectoral sum of the typical
capacitive current of the undamaged cable
I.sub.c=.omega..multidot.C.andgate.V (where X is the pulsation, C
is the capacitance and V is the applied voltage), and of the
breakdown current (I.sub.B) caused by any occurring damage. In a
damaged but not self-protected cable, the breakdown current is
largely prevailing over the capacitive current, therefore the
measured leakage current is substantially equal to the breakdown
current.
[0136] In Table 3 the results of measurements are reported, as
average value on 5 samples of the same type. For comparative
purposes there are reported the values of leakage currents measured
on an intact, i.e. undamaged, cable having the self-repairing layer
and on an identical cable without the self-repairing layer and
which was subjected to a blade-type damage.
4 TABLE 3 I.sub.L (.mu.A/m) Cable without Cable with self-repairing
layer self-repairing Test time as shovel blade blunt layer (days)
such cut cut cut blade cut 0 12.8 13.0 12.5 13.1 20,000 3 12.9 13.4
12.9 12.7 20,000 9 12.6 12.7 13.1 13.1 8,000 30 13.8 14.5 14.6 14.7
>100,000 60 14.0 13.9 14.5 14.2 --
[0137] As may be noted from the results given Table 3, the cable
with a self-repairing layer according to the present invention and
damaged according to the various methods described above shows very
low leakage currents even after 60 days of immersion in water,
which are substantially identical to those of the intact cable.
Therefore, the breakdown current is essentially null, the leakage
current measured being attributable almost exclusively to the
intrinsic capacitive current of the cable.
[0138] In contrast, the cable without a self-repairing layer and
with a blade-type damage immediately showed high leakage currents
due to the damage, which, after 30 days of standing in water, led
to total corrosion of the conductor, with complete interruption of
the circuit. The slight decrease in the leakage current after 9
days is attributable to the formation of a layer of aluminium
hydroxide as a result of the conductor corrosion, which allowed a
certain degree of electrical insulation to be obtained. As
corrosion progressed, large amounts of aluminium hydroxide formed
which, increasing in volume upon contact with water, led to
complete rupture and opening of the coating layers.
EXAMPLE 7
[0139] (a) Manufacturing of the self-protected cable.
[0140] Following basically the same method as that described for
Example 6, a cable core consisting of a compressed aluminium
conductor (cross-section: 54 mm.sup.2), insulated with a layer of
silane-crosslinked linear low density polyethylene (LLDPE) (product
DFDA 7530 from Union Carbide) having a nominal thickness of 1 mm
was prepared.
[0141] Then, the layer of self-repairing material and the outer
sheath were applied on the cable core using the tandem-type
technique as described in Example 6. The self-repairing material
(nominal thickness: 0.5 mm) had the composition of Example 5 given
in Table 1, whereas the outer sheath consisted of
silane-crosslinked high density polyethylene (HDPE) (product LS
6402-00 from Quantum) (nominal thickness: 1 mm).
[0142] The layer of self-repairing material was applied by means of
the pultrusion process as described in Example 6, under the
following temperature settings:
[0143] Pressure plate: 200.degree. C.
[0144] Supply tube: 200.degree. C.
[0145] Application head: 200.degree. C.
[0146] The outer sheath was applied downstream of the pultrusion
zone, according to the method described in Example 6. The process
conditions were set as follows:
[0147] Line speed: 1.3 m/min
[0148] Extruder spin speed: 3.82 rpm
[0149] Extruder absorption: 61.5 Amps
[0150] Nominal diameter of the cold cable: 14.5 mm.
[0151] About 100 m of cable with a self-repairing layer were
produced by the process described above.
[0152] Lengths of cable were subjected to the same damage tests as
those described in Example 6. The effectiveness of self-repairing
was assessed by measuring the leakage current following the same
procedure as that described in Example 6. Table 4 gives the results
obtained (as average value on 5 samples of the same type).
5 TABLE 4 I.sub.L (.mu.A/m) Cable without Cable with self-repairing
layer self-repairing Test time as shovel blade blunt layer (days)
such cut cut cut blade cut 30 18.7 19.5 20.2 19.5 >100,000 60
19.5 19.8 20.8 20.1 --
[0153] Similarly to the results of Example 6, the cable with the
self-repairing layer according to the present invention and damaged
according to the various methods described above showed very low
leakage currents even after 60 days of immersion in water, which
are substantially identical to those of the intact cable. In
contrast, the cable without the self-repairing layer and with
blade-type damage showed high leakage currents due to the damage,
which, after 30 days of standing in water, led to total corrosion
of the conductor, with complete interruption of the circuit.
* * * * *