Note: Descriptions are shown in the official language in which they were submitted.
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
CABLE WITH IMPACT-RESISTANT COATING
The present invention relates to a coating for
cables which is capable of protecting the cable from
accidental impacts.
Accidental impacts on a cable, which may occur,
for example, during their transportation, laying etc.,
may cause a series of structural damage to the cable,
including deformation of the insulating layer, detach-
ment of the insulating layer from the semiconductive
layer, and the like; this damage may cause variations
in the electrical gradient of the insulating coating,
with a consequent decrease in the insulating capacity
of this coating.
In the cables which are currently commercially
available, for example in those for low- or medium-
tension power transmission or distribution, metal armor
capable of withstanding such impacts is usually applied
in order to protect cables from possible damages caused
by accidental impacts. This armor may be in the form of
tapes or wires (generally made of steel), or alter-
natively in the form of a metal sheath (generally made
of lead or aluminum); this armor is, in turn, usually
clad with an outer polymer sheath. An example of such a
cable structure is described in US patent 5,153,381.
The Applicant has observed that the presence of
the abovementioned metal armor has a certain number of
drawbacks. For example, the application of the said
armor includes one or more additional phases in the
- 30 processing of the cable. Moreover, the presence of the
metal armor increases the weight of the cable consider-
ably, in addition to posing environmental problems
since, if it needs to be replaced, a cable constructed
in this way is not easy to dispose of.
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 2 -
The Japanese patent published under the number
(Kokai) 7-320550 describes a domestic cable with an
impact-resistant coating 0.2-1.4 mm in thickness,
placed between the insulator and the outer sheath. This
impact-resistant coating is a non-expanded polymer
material containing a polyurethane resin as main
component.
On the other hand, use of expanded polymeric
materials in cables' construction is known for a
variety of purposes.
For instance, German patent application no.
P 15 15 709 discloses the use of an intermediate layer
between the outer plastic sheath and the inner metallic
sheath of a cable, in order to increase the resistance
of the outer plastic sheath to low temperatures. No
mention is made in such document about protecting the
inner structure of the cable with said intermediate
layer. As a mattter of fact, such intermediate layer
should compensate for elastic tensions generated in the
outer plastic sheath due to temperature's lowering and
may consist of loosely disposed glass fibers or of a
material which may either be expanded or incorporating
hollow glass spheres.
Another document, German utility model
no. G 81 03 947.6, discloses an electric cable for use
in connections inside apparatuses and machines, having
particular mechanical resistance and flexibility. Said
cable is specifically designed for passing on a pulley
and is sufficiently flexible in order to recover its
straight structure after the passage on said pulley.
Accordingly, this kind of cable is specifically aimed
to resist to mechanical loads of the static type (such
as those generated during the passage onto a pulley),
and its main feature is the flexibility. It is readily
apparent to those skilled in the art that this kind of
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 3 -
cable substantially differs from low- or medium-tension
power transmission or distribution having a metal armor
which, rather to be flexible, should be capable of
withstanding dynamic loads due to impacts of a certain
strength onto the cable.
In addition, in signal transmission cables of
the coaxial or twisted pair type, it is known to use
expanded materials in order to insulate a conductive
metal. Coaxial cables are usually intended to carry
high-frequency signals, such as coaxial cables for TV
(CATV) (10-100 MHz), satellite cables (up to 2 GHz),
coaxial cables for computers (above 1 MHz); traditional
telephone cables usually carry signals with frequencies
of about 800 Hz.
The purpose of using an expanded insulator in
such cables is to increase the transmission speed of
the electrical signals, in order to approach the ideal
speed of signal transmission in an aerial conductive
metal (which is close to the speed of light). The
reason for this is that, compared with non-expanded
polymer materials, expanded materials generally have a
lower dielectric constant (K), which is proportionately
closer to that of air (K=1) the higher the degree of
expansion of the polymer.
For example, US patent 4,711,811 describes a
signal transmission cable having an expanded fluoro-
polymer as insulator (thickness of 0.05-0.76 mm) clad
with a film of ethylene/tetrafluoroethylene or
ethylene/chlorotrifluoroethylene copolymer (thickness
of 0.013-0.254 mm). As described in that patent, the
purpose of the expanded polymer is to insulate the
conductor, while the purpose of the film of non-
expanded polymer which clads the expanded polymer is to
improve the mechanical properties of the insulation, in
particular by imparting the necessary compression
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 4 -
strength when two insulated conductors are twisted to
form the so-called "twisted pair".
Patent EP 442,346 describes a signal trans-
mission cable with an insulating layer based on
expanded polymer, placed directly around the conductor;
this expanded polymer has an ultramicrocellular
structure with a void volume of greater than 75%
(corresponding to a degree of expansion of greater than
3000). The ultramicrocellular structure of this polymer
should be such that it is compressed by at least 100
under a load of 6.89 x 109 Pa and recovers at least 500
of its original volume after removal of the load; these
values correspond approximately to the typical
compression strength values which the material needs to
have in order to withstand the compression during
twisting of the cables.
In International patent application
WO 93/15512, which also relates to a signal trans-
mission cable with an expanded insulating coating, it
is stated that by coating the expanded insulator with a
layer of non-expanded insulating thermoplastic polymer
(as described, for example, in the abovementioned
US patent 4,711,811) the required compression strength
is obtained, this however reducing the speed of
propagation of the signal. The said patent application
WO 93/15512 describes a coaxial cable with a double
layer of insulating coating, where both the layers
consist of an expanded polymer material, the inner
layer consisting of microporous polytetrafluoroethylene
(PTFE) and the outer layer consisting of a closed-cell
expanded polymer, in particular perfluoroalkoxytetra-
fluoroethylene (PFA) polymers. The insulating coating
based on expanded polymer is obtained by extruding the
PFA polymer over the inner layer of PTFE insulator,
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 5 -
injecting Freon 113 gas as expanding agent. According
to the details given in the description, this closed-
cell expanded insulator makes it possible to maintain a
high speed of signal transmission. It is moreover
defined in that patent application as being resistant
to compression, although no numerical data regarding
this compression strength are given. The description
emphasizes the fact that conductors clad with such a
double-layer insulator can be twisted. Moreover,
according to that patent application, the increase in
void volume in the outer expanded layer makes it
possible to obtain an increase in the speed of
transmission, thereby giving rise to small variations
in the capacity of this coating to oppose the compres-
sion of the inner expanded layer.
As is seen from the abovementioned documents,
the main purpose of using "open cell" expanded polymer
materials as insulating coatings for signal transmis-
sion cables is to increase the speed of transmission of
the electrical signal; however, these expanded coatings
have the drawback of having an insufficient compression
strength. A few expanded materials are also generically
defined as "resistant to compression", since they have
to ensure not only a high speed of signal transmission
but also a sufficient resistance to the compression
forces which are typically generated when two
conductors coated with the abovementioned expanded
insulation are twisted together; accordingly, also in
this case, the applied load is substantiantially of
static type.
Thus, while, on the one hand, it is necessary
for these insulating coatings made of expanded polymer
material for signal transmission cables to have
characteristics such that they can bear a relatively
modest compression load (such as that which arises when
CA 02289748 1999-11-15
p ' - .-- --_
.. - -
PC711 - 6 -
two cables are twisted together), on the other hand, no
mention is made in any document known to the Applicant
of any type of impact strength which may be provided by
an expanded polymer coating. Moreover, although such an
expanded insulating coating promotes a higher speed of
signal transmission, this is considered to be less
advantageous than a coating made of a similar non-
expanded material as regards the compression strength,
..as reported in the abovementioned patent application
WO 93/15512.
The Applicant has now found that by inserting
into the structure of a power transmission cable a
suitable coating made of expanded polymer material of
adequate thickness and flexural modulus, preferably in
contact with the sheath of outer polymer coating, it is
possible to obtain a cable having a high impact
strength, thereby making it possible to avoid the use
of the abovementioned protective metal armor in the
structure of this cable. In particular, the Applicant
has observed that the polymer material should be
selected in order to have a sufficiently high flexural
modulus, measured before its expansion, so to achieve
the desired impact resistant properties and avoid
possible damages of the inner structure of the cable
due to undesired impacts on the outer surface of it. In
the present description, the term "impact" is intended
to encompass all those dynamic loads of a certain
energy capable to produce substantial damages to the
structure of conventional unarmored cables, while w
having negligible effects on the structure of
conventional armored cables. As an indication, such an
impact may be considered an impact of about 20-30 joule
produced by a V-shaped rounded-edge punch, having a
curvature radius of about l mm, onto the outer sheath
of the cable.
AME~!~JE9 S~aE~'i
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
The Applicant has moreover observed that,
surprisingly, an expanded polymer material used as a
coating for cables according to the invention makes it
possible to obtain an impact strength which is better
than that obtained using a similar coating based on the
same polymer which is not expanded.
A cable with a coating of this type has various
advantages over a conventional cable with metal armor
such as, for example, easier processing, a reduction in
the weight and dimensions of the finished cable and a
reduced environmental impact as regards recycling of
the cable once its working cycle is over.
One aspect of the present invention thus
relates to a power transmission cable comprising
a) a conductor;
b) at least one layer of compact insulating
coating,
c) a coating made of expanded polymer material,
wherein said polymer material has predetermined
mechanical strength properties and a predetermined
degree of expansion so as to impart impact resistant
properties to said cable.
According to a preferred aspect of the present
invention, the expanded polymer material is obtained
from a polymer material which has, before expansion, a
flexural modulus at room temperature, measured accord-
ing to ASTM standard D790, higher than 200 MPa,
preferably between 900 MPa and 1500 MPa, values of
between 600 MPa and 1300 MPa being particularly
preferred.
According to a preferred aspect, said polymer
material has a degree of expansion of from abuot 20o to
about 30000, preferably from about 30% to about 5000, a
degree of expansion of from about 50o to about 200%
being particularly preferred.
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
_ g _
According to a preferred embodiment of the
present invention, the coating of expanded polymer
material has a thickness of at least 0.5 mm, preferably
between 1 and 6 mm, in particular between 2 and 4 mm.
According to a preferred aspect of the present
invention, this expanded polymer material is chosen
from polyethylene (PE), low density PE (LDPE), medium
density PE (MDPE), high density PE (HDPE) and linear
low density PE (LLDPE); polypropylene (PP); ethylene-
propylene rubber (EPR), ethylene-propylene copolymer
(EPM), ethylene-propylene-dime terpolymer (EPDM);
natural rubber; butyl rubber; ethylene/vinyl acetate
(EVA) copolymer; polystyrene; ethylene/acrylate
copolymer, ethylene/methyl acrylate (EMA) copolymer,
ethylene/ethyl acrylate (EEA) copolymer, ethylene/butyl
acrylate (EBA) copolymer; ethylene/a-olefin copolymer;
acrylonitrile-butadiene-styrene (ABS) resins; halogena-
ted polymer, polyvinyl chloride (PVC); polyurethane
(PUR); polyamide; aromatic polyester, polyethylene
terephthalate (PET), polybutylene terephthalate (PBT);
and copolymers or mechanical mixtures thereof.
According to a further preferred aspect, this
polymer material is a polyolefin polymer or copolymer
based on PE and/or PP, preferably modified with
ethylene-propylene rubber, in which the PP/EPR weight
ratio is between 90/10 and 50/50, preferably between
85/15 and 60/40, in particular about 70/30.
According to a further preferred aspect, this
polyolefin polymer or copolymer based on PE and/or PP
contains a predetermined amount of vulcanized rubber in
powder form, preferably between loo and 600 of the
weight of the polymer.
According to a further preferred aspect, this
cable moreover comprises an outer polymer sheath, which
- w -AUG. 1. 2000- 8:27AM KI~RBY EADES 613 237 0045 N0, 1190 P. 2
WO 98/521f7 ~CT/EP98/02698
_ g
is preferably in contact with the expanded polymer
coating, this sheath preferably having a thickness of
~''"' at least 0.5 mm, pzeferably between 1 and 5 mm.
Another aspect of the present invention relates
to a method foz imparting impact strength to a cable,
which comprises coatzng this cable with a coating made
of expanded polyrnex material.
P.ccording to a prefezred aspect, this method
for imparting impact strength to a cable moreover
comprises coating this expanded coating with an outer
protective sheath.
A further aspect of the present invention
relates to the use of an expanded polymer material in
order to impart impact strength to a power transmission
cable.
A fuxther aspect of the present invention
relates to a method for evaluating the impact strength
of a cable comprising at least one insulating coating,
this method consisting in
a) measuring the average peel stxength of the
said insulating layer:
b) subjecting the cable to an impact of pre-
determined energy:
c) measuring the peel strength of the said
insulating layer at the point of impact;
d) checking that the difference between the
average peel strength and the peel strength measured at
the point of impact is less than a pxEdetermined value
for the said cable xelative to the average peel
strength.
According to a preferred aspect, this peel
stzength is measured between the layer.of insulating
coating and the outer layer of semiconductive coating.
CA 02289748 2000-07-05
CA 02289748 2002-09-19
9a
In accordance with one aspect 0:E tree present
invention there is provided a cable comx:~rising: an
inner structure; and a coating l..ayer disposed to
surround said inner structure, wherein raid coating
layer provides impact resistance and is made from an
expanded polymer material having a degree of expansion
of from about 200 to about 300C)~ and a f::lexural modulus
of at least 200 MPa before expansion of said polymer,
wherein said coating of expanded polymer material has a
thickness of between 1. and 6 mm.
In accordance with another aspect c>f the present
invention there is provided use of an expanded polymer
material for imparting impact strength to a power
transmission cable comprising: an inner structure; and
a coating layer disposed tc surround said inner
structure, wherein said coating layer provides impact
resistance and is made from; saz.d exp<~nded polymer
material having a degree of expansion c>:ffrom 200 to
about 30000 and a flexural modulus of at least 200 MPa
before expansion of said polymer.
In the present description, the term "degree of
expansion of the polymer" is understood to refer to the
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 10 -
expansion of the polymer determined in the following
way:
G (degree of expansion) - (do/de - 1) ~ 100
where do indicates the density of the non
expanded polymer (that is to say the polymer with a
structure which is essentially free of void volume) and
de indicates the apparent density measured .for the
expanded polymer.
For the purposes of the present description,
the term "expanded" polymer is understood to refer to a
polymer within the structure of which the percentage of
void volume (that is to say the space not occupied by
the polymer but by a gas or air) is typically greater
than 100 of the total volume of this polymer.
In the present description, the term "peel"
strength is understood to refer to the force required
to separate (peel) a layer of coating from the
conductor or from another layer of coating; in the case
of separation of two layers of coating from each other,
these layers are typically the insulating layer and the
outer semiconductive layer.
Typically, the insulating layer of power
transmission cables has a dielectric constant (K) of
greater than 2. Moreover, in contrast with signal
transmission cables, in which the "electrical gradient"
parameter does not assume any importance, electrical
gradients ranging from about 0.5 kV/mm for low tension,
up to about 10 kV/mm for high tension, are applied in
power transmission cables; thus, in these cables, the
presence of inhomogeneity in the insulating coating
(for example void volumes), which could give rise to a
local variation in the dielectric rigidity with a
consequent decrease in the insulating capacity, tends
to be avoided. This insulating material will thus
typically be a compact polymer material, in which, in
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 11 -
the present description, the term "compact" insulator
is understood to refer to an insulating material which
has a dielectric rigidity of at least 5 kV/mm,
preferably greater than 10 kV/mm, in particular greater
than 40 kV/mm for medium-high tension power
transmission cables. In contrast with an expanded
polymer material, this compact material is
substantially free of void volume within its structure;
in particular, this material will have a density of
0.85 g/cm3 or more.
In the present description, the term low
tension is understood to refer to a tension of up to
1000 V (typically greater than 100 V), the term medium
tension is understood to refer to a tension from about
1 to about 30 kV and the term high tension is under-
stood to refer to a tension above 30 kV. Such power
transmission cables typically operate at nominal
frequencies of 50 or 60 Hz.
Although, in the course of the description, the
use of the expanded polymer coating is illustrated in
detail with reference to power transmission cables, in
which this coating may advantageously replace the metal
armor currently used in such cables, it is clear to
those skilled in the art that this expanded coating may
advantageously be used in any type of cable for which
it might be desired to impart suitable impact protec-
tion to such a cable. In particular, the definition of
power transmission cables includes not only those
specifically of the type for low and medium tension but
also cables for high-tension power transmission.
The invention may be further understood with
the aid of the following figures:
Figure 1 shows a power transmission cable
according to the state of the art, of the tripolar type
with metal armor.
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 12 -
Figure 2 shows a first embodiment of a cable
according to the invention of tripolar type.
Figure 3 shows a second embodiment of a cable
according to the invention of unipolar type.
Fig. 1 is the cross-sectional diagram of a
medium-tension power transmission cable according to
the state of the art, of the tripolar type with metal
armor. This cable comprises three conductors (1), each
clad with an inner semiconductive coating (2), an
insulating layer (3), an outer semiconductive layer (4)
and a metal screen (5); for simplicity, this semi-
finished structure will be defined in the rest of the
description as the "core". The three cores are roped
together and the star-shaped areas between them are
filled with a filling material (9) (generally elasto-
meric mixtures, polypropylene fibers and the like) in
order to make the cross-sectional structure circular,
the whole in turn being coated with an inner polymer
sheath (8), an armor of metal wires (7) and an outer
polymer sheath (6).
Fig. 2 is the cross-sectional diagram of a
cable according to the invention, also of the tripolar
type for medium-tension power transmission. This cable
comprises the three conductors (1), each clad with an
inner semiconductive coating (2), an insulating layer
(3), an outer semiconductive layer (4) and a metal
screen (5); the star-shaped areas between the cores are
filled in this case with an impact-resistant expanded
polymer material (10) which is, in turn, coated with an
outer polymer sheath (&). In the expanded polymer
coating (10), a circular rim (10a) which corresponds to
the minimum thickness of expanded polymer coating, in
proximity to the outer surface of the cores, is also
indicated (by means of a dotted line).
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 13 -
Fig. 3 is the cross-sectional diagram of a
cable according to the invention, of unipolar type for
medium-tension power transmission. This cable comprises
a central conductor (1), clad with an inner semi-
s conductive coating (2), an insulating layer (3), an
outer semiconductive layer (9), a metal screen (5), a
layer of expanded polymer material (10) and an outer
polymer sheath (6). In the case of this unipolar cable
represented in Fig. 3, since the core has a circular
cross-section, the circular rim (10a) indicated in the
case of the tripolar cable coincides with the layer of
expanded polymer material (10).
These figures obviously only show a few of the
possible embodiments of cables in which the present
invention may advantageously be used. It is clear that
suitable modifications known in the art may be made to
these embodiments without any limitations to the
application of the present invention being implied
thereby. For example, with reference to Fig. 2, the
star-shaped areas between the cores may be filled
beforehand with a conventional filling material, thus
obtaining a semi-processed cable of cross-section
corresponding approximately to the circular cross-
section contained within the circular rim (10a); it is
then advantageously possible to extrude over this semi-
processed cable of cross-sectional area the layer of
expanded polymer material (10), in a thickness corres-
ponding approximately to the circular rim (10a), and
subsequently the outer sheath (6). Alternatively, cores
may be provided with a cross-sectional sector, in such
a way that when these cores are joined together a cable
of approximately circular cross-section is formed,
without the need to use the filling material for the
star-shaped areas; the layer of impact-resistant
expanded polymer material (10) is then extruded over
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 14 -
these cores thus joined together, followed by the outer
sheath (6).
In the case of cables for low-tension power
transmission, the structure of these cables will
usually comprise the only insulating coating placed
directly in contact with the conductor, which is in
turn coated with the coating of expanded polymer
material and with the outer sheath.
Further solutions are well known to a person
skilled in the art, who is capable of evaluating the
most convenient solution, based on, for example, the
costs, the type of positioning of the cable (aerial,
inserted in pipes, buried directly into the ground,
inside buildings, under the sea, etc.), the operating
temperature of the cable (maximum and minimum
temperatures, temperature ranges of the environment)
and the like.
The impact-resistant expanded polymer coating
may consist of any type of expandable polymer such as,
for example, polyolefins, polyolefin copolymers, ole-
fin/ester copolymers, polyesters, polycarbonates, poly-
sulfones, phenolic resins, ureic resins and mixtures
thereof. Examples of suitable polymers are polyethylene
(PE), in particular low density PE (LDPE), medium
density PE (MDPE), high density PE (HDPE) and linear
low density PE (LLDPE); polypropylene (PP); ethylene-
propylene rubber (EPR), in particular ethylene-
propylene copolymer (EPM) or ethylene-propylene-dime
terpolymer (EPDM); natural rubber; butyl rubber;
ethylene/vinyl acetate (EVA) copolymer; polystyrene;
ethylene/acrylate copolymer, in particular
ethylene/methyl acrylate (EMA) copolymer,
ethylene/ethyl acrylate (EEA) copolymer, ethylene/butyl
acrylate (EBA) copolymer; ethylene/a-olefin copolymer;
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 15 -
acrylonitrile-butadiene-styrene (ABS) resins; halogena-
ted polymers, in particular polyvinyl chloride (PVC);
polyurethane (PUR); polyamides; aromatic polyesters,
such as polyethylene terephthalate (PET) or
polybutylene terephthalate (PBT); and copolymers or
mechanical mixtures thereof. Preferably, polyolefin
polymers or copolymers are used, in particular those
based on PE and/or PP mixed with ethylene-propylene
rubbers. Advantageously, polypropylene modified with
ethylene-propylene rubber (EPR) may be used, the PP/EPR
weight ratio being between 90/10 and 50/50, preferably
between 85/15 and 60/40, a weight ratio of about 70/30
being particularly preferred.
According to a further aspect of the present
invention, the Applicant has moreover observed that it
is possible to mix mechanically the polymer material
which is subjected to the expansion, in particular in
the case of olefin polymers, specifically polyethylene
or polypropylene, with a predetermined amount of rubber
in powder form, for example vulcanized natural rubber.
Typically, these powders are formed from
particles with sizes of between 10 and 1000 um,
preferably between 300 and 600 um. Advantageously,
vulcanized rubber rejects derived from the processing
of tires may be used. The percentage of rubber in
powder form may range from 10°s to 60o by weight
relative to the polymer to be expanded, preferably
between 30% and 500.
The polymer material to be expanded, which is
either used without further processing or which is used
as an expandable base in a mixture with powdered
rubber, will have to have a rigidity such that, once it
is expanded, it ensures a certain magnitude of desired
impact resistance, so as to protect the inner part of
the cable (that is to say the layer of insulator and
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 16 -
the semiconductive layers which may be present) from
damage following accidental impacts which may occur. In
particular, this material will have to have a
sufficiently high capacity to absorb the impact energy,
so as to transmit to the underlying insulating layer an
amount of energy which is such that the insulating
properties of the underlying coatings are not modified
beyond a predetermined value. The reason for this, as
illustrated in greater detail in the description which
follows, is that the Applicant has observed that in a
cable subjected to an impact, a difference is observed,
between the average value and the value measured at the
point of impact, of the peel strength of the underlying
insulating coatings; advantageously, this peel strength
may be measured between the insulating layer and the
outer semiconductive layer. The difference in this
strength is proportionately greater the greater the
impact energy transmitted to the underlying layers; in
the case where the peel strength is measured between
the insulating layer and the outer semiconductive
layer, it has been evaluated that the protective
coating offers a sufficient protection to the inner
layers when the difference in peel strength at the
point of impact, relative to the average value, is less
than 25a.
The Applicant has observed that a polymer
material chosen from those mentioned above is parti-
cularly suitable for this purpose, this material
having, before expansion, a flexural modulus at room
temperature of greater than 200 MPa, preferably of at
least 400 MPa, measured according to ASTM standard
D790. On the other hand, since excessive rigidity of
the expanded material may make the finished product
difficult to handle, it is preferred to use a polymer
material which has a flexural modulus at room tempera-
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 17 -
ture of less than 2000 MPa. Polymer materials which are
particularly suitable for this purpose are those which
have, before expansion, a flexural modulus at room
temperature of between 400 and 1800 MPa, a polymer
material with a flexural modulus at room temperature of
between 600 and 1500 MPa being particularly preferred.
These flexural modulus values may be charac-
teristic of a specific material or may result from the
mixing of two or more materials having different
moduli, mixed in a ratio such as to obtain the desired
rigidity value for the material. For example, poly-
propylene, which has a flexural modulus of greater than
1500 MPa, may be appropriately modified with suitable
amounts of ethylene-propylene rubber (EPR), having a
modulus of about 100 MPa, for the purpose of lowering
its rigidity in a suitable manner.
Examples of commercially available polymer
compounds are:
low density polyethylene: Riblene FL 30
(Enichem);
high density polyethylene: DGDK 3369 (Union
Carbide);
polypropylene: PF 814 (Montell);
polypropylene modified with EPR: Moplen
EP-S 30R, 33R and 81R (Montell); Fina-Pro 56606, 46606,
2660S and 3660S (Fina-Pro).
The degree of expansion of the polymer and the
thickness of the coating layer will have to be such
that they ensure, in combination with the outer polymer
sheath, resistance to typical impacts which occur
during the handling and laying of the cable.
As mentioned previously, the "degree of expan-
sion of the polymer" is determined in the following
way:
G (degree of expansion) - (do/de - 1)-100
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 18 -
where do indicates the density of the non-
expanded polymer and de indicates the apparent density
measured for the expanded polymer.
The Applicant has observed that, insofar as the
maintenance of the desired impact-resistance charac-
teristics allows, for an equal thickness of the
expanded layer, it is preferable to use a polymer
material having a high degree of expansions since, in
this way, it is possible to limit the amount of polymer
material used, with advantages in terms of both economy
and reduced weight of the finished product.
The degree of expansion is very variable, both
as a function of the specific polymer material used and
as a function of the thickness of the coating which it
is intended to use; in general, this degree of
expansion may range from 20o to 30000, preferably from
30o to 5000, a degree of expansion of between SOo and
200% being particularly preferred. The expanded polymer
generally has a closed-cell structure.
The Applicant has observed that beyond a
certain degree of expansion, the capacity of the
polymer coating to give the required impact strength
decreases. In particular, it has been observed that the
possibility of obtaining high degrees of expansion of
the polymer by maintaining a high efficacy of protec-
tion against impacts may be correlated with the value
of the flexural modulus of the polymer to be expanded.
The reason for this is that the Applicant has observed
that the modulus of the polymer material decreases as
the degree of expansion of this material increases,
approximatly according to the following formula:
Ez/E1= (Pz/pi) z
wherein:
CA 02289748 1999-11-15
' , ' ._
~ °_." '..' ,.'
PC711 - 19 - -
Y
EZ represents the flexural modulus of the pol;~mer at the
higher degree of expansion; y
E1 represents the flexural modulus of the polymer at the
lower degree of expansion
p2 represents the apparent density of the polymer at the
higher degree of expansion; and
p1 represents the apparent density of the polymer at the
lower degree of expansion;
As a guidance, for a polymer with a flexural modulus of
about 1000 MPa, a variation in the degree of expansion
of from 25o to 100% entails an approximate halving o.f
the value of the flexural modulus for the material.
Polymer materials which have a high flexural modulus
may therefore be expanded to a greater degree than
polymer materials which have low modulus values,
without this prejudicing the ability of the coating to
withstand impacts.
Another variable which is liable to influence
the impact strength of the cable is the thickness of
the expanded coating; the minimum thickness which is
capable of ensuring the impact strength which it is
desired to obtain with such a coating will depend
mainly on the degree of expansion and on the flexural
modulus of this polymer. In general, the Applicant has
observed that, for the same polymer and for the same
degree of expansion, by increasing the thickness of the
expanded coating it is possible to reach higher values
of impact strength. However, for the purpose of using a
limited amount of coating material, thus decreasing
both the costs and the dimensions of the finished
product, the thickness of the layer of. expanded
material will advantageously be the minimum thickness
required to ensure the desired impact strength. In
particular, for cables of the medium tension type, it
o.,~.Lr,~~,jGD S~~~~
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 20 -
has been observed that an expanded coating thickness of
about 2 mm is usually capable of ensuring a sufficient
resistance to the normal impacts to which a cable of
this type is subjected. Preferably, the coating thick-
ness will be greater than 0.5 mm, in particular between
about 1 mm and about 6 mm, a thickness of between 2 mm
and 4 mm being particularly preferred.
The Applicant has observed that it is possible
to define, to a reasonable approximation, the relation-
ship between the coating thickness and the degree of
expansion of the polymer material, for materials with
various flexural modulus values, such that the thick-
ness of the expanded coating is suitably dimensioned as
a function of the degree of expansion and of the
modulus of the polymer material, in particular for
thicknesses of the expanded coating of about 2-4 mm.
Such a relationship may be expressed as follows:
V ~ de >_ N
where
V represents the volume of expanded polymer
material per linear meter of cable (m3/m), this volume
being relative to the circular rim defined by the
minimum thickness of expanded coating, corresponding to
the circular rim (10a) of Fig. 2 for multipolar cables,
or to the coating (10) defined in Fig. 3 for unipolar
cables;
de represents the apparent density measured for
the expanded polymer material (kg/m3); and
N is the result of the product of the two
abovementioned values, which will have to be greater
than or equal to:
0.03 for materials with a modulus > 1000 MPa,
0.04 for materials with a modulus of 800-
1000 MPa,
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 21 -
0.05 for materials with a modulus of 400-
800 MPa,
0.06 for materials with a modulus < 900 MPa.
The parameter V is related to the thickness (S)
of the expanded coating by the following relationship:
V = It (2Rl S + S2)
where Ri represents the inner radius of the
circular rim (10a).
The parameter de is related to the degree of
expansion of the polymer material by the previous
relationship:
G = (do/dP - 1 ) ~ 100
Based on the abovementioned relationship, for
an expanded coating about 2 mm in thickness, placed on
a circular section of cable with a diameter of about
22 mm, for various materials having different flexural
moduli at room temperature (Mf), it is found that this
coating will have to have a minimum apparent density of
about:
0.90 g/cm3 for LDPE (Mf of about 200);
0.33 g/cm3 for a 70/30 PP/EPR mixture (Mf of
about 800);
0.26 g/cm3 for HDPE (Mf of about 1000);
0.20 g/cm3 for PP (Mf of about 1500).
These values of apparent density of the
expanded polymer correspond to a maximum degree of
expansion of about:
130% for LDPE (do = 0.923)
180% for the PP/EPR mixture (do = 0.890)
260% for HDPE (d~ = 0.945)
350% for PP (do = 0.900) .
Similarly, for a thickness of the expanded
coating of about 3 mm placed on a cable of identical
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98I02698
- 22 -
dimensions, the following values of minimum apparent
density are obtained:
0.25 g/cm3 for LDPE;
0.21 g/cm' for the PP/EPR mixture;
0.17 g/cm3 for HDPE;
0.13 g/cm3 for PP;
corresponding to a maximum degree of expansion of
about:
270% for LDPE;
3200 for the PP/EPR mixture;
9600 for HDPE;
6000 for PP.
The results shown above indicate that in order
to optimize the impact strength characteristics of an
expanded coating of predetermined thickness, both the
mechanical strength characteristics of the material (in
particular its flexural modulus)) and the degree of
expansion of said material should be taken in account.
However, the values determined by applying the above
relationship should not be considered as limiting the
scope of the present invention. In particular, the
maximum degree of expansion of polymers which have
flexural modulus values close to the upper limits of
the intervals defined for the variation of the number N
(that is to say 400, 800 and 1000 MPa) may in reality
be even greater than that calculated according to the
above relationship; thus, for example, a layer of
PP/EPR about 2 mm in thickness (with Mf of about
800 MPa) will still be able to provide the desired
impact protection even with a degree of expansion of
about 2000.
The polymer is usually expanded during the
extrusion phase; this expansion may either take place
chemically, by means of addition of a suitable
"expanding" compound, that is to say one which is
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 23 -
capable of generating a gas under defined temperature
and pressure conditions, or may take place physically,
by means of injection of gas at high pressure directly
into the extrusion cylinder.
Examples of suitable chemical "expanders" are
azodicarboamide, mixtures of organic acids (for example
citric acid) with carbonates and/or bicarbonates (for
example sodium bicarbonate).
Examples of gases to be injected at high
pressure into the extrusion cylinder are nitrogen,
carbon dioxide, air and low-boiling hydrocarbons such
as propane and butane.
The protective outer sheath which clads the
layer of expanded polymer may conveniently be of the
type normally used. Materials for the outer coating
which may be used are polyethylene (PE), in particular
medium-density PE (MDPE) and high-density PE (HDPE),
polyvinyl chloride (PVC), mixtures of elastomers and
the like. MDPE or PVC is preferably used. Typically,
the polymer material which forms this outer sheath has
a flexural modulus of between about 400 and about
1200 MPa, preferably between about 600 MPa and about
1000 MPa.
The Applicant has observed that the presence of
the outer sheath contributes towards providing the
coating with the desired impact strength characteris-
tics, in combination with the expanded coating. In
particular, the Applicant has observed that this
contribution of the sheath towards the impact strength,
for the same thickness of expanded coating, increases
as the degree of expansion of the polymer which forms
this expanded coating increases. The thickness of this
outer sheath is preferably greater than 0.5 mm, in
particular between 1 and 5 mm, preferably between 2 and
9 mm.
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 24 -
The preparation of a cable with an impact
strength according to the invention is described with
reference to the cable structure diagram of Figure 2,
in which, however, the star-shaped spaces between the
cores to be coated are filled, not directly with the
expanded polymer (10) but rather with a conventional
filler; the expanded coating is then extruded over this
semi-processed cable, to form a circular rim (10a)
around this semi-processed cable and is subsequently
clad with the outer polymer sheath (2). The preparation
of the cable cores, that is to say the assembly of the
conductor (4), inner semiconductive layer (9),
insulator (5), outer semiconductive layer (8) and metal
screen (4), is carried out as known in the art, for
example by means of extrusion. These cores are then
roped together and the star-shaped spaces are filled
with a conventional filling material (for example
elastomeric mixtures, polypropylene fibers and the
like), typically by means of extrusion of the filler
over the roped cores, so as to obtain a semi-processed
cable with a circular cross-section. The coating of
expanded polymer (10) is then extruded over the filling
material. Preferably, the die of the extruder head will
have a diameter slightly smaller than the final
diameter of the cable with expanded coating, in order
to allow the polymer to expand outside the extruder.
It has been observed that, under identical
extrusion conditions (such as spin speed of the screw,
speed of the extrusion line, diameter of the extruder
head and the like) the extrusion temperature is one of
the process variables which has a considerable
influence on the degree of expansion. In general, for
extrusion temperatures below 160°C, it is difficult to
obtain a sufficient degree of expansion; the extrusion
temperature is preferably at least 180°C, in particular
CA 02289748 1999-11-15
PC711 - 25 - -
about 200°C. Usually, an increase in the extrusion
temperature corresponds to a higher degree of
expansion.
Moreover, it is possible to control to some
extent the degree of expansion of the polymer by acting
on the rate of cooling since, by appropriately slowing
down or speeding up the cooling if the polymer which
forms the expanded coating at the extruder outlet, it
is possible to increase or decrease the degree of
expansion of the said polymer.
As mentioned, the Applicant has observed that
it is possible to determine quantitatively the effects
of an impact on a cable coating by means of measuring
the peel strength of the cable coating layers, dif-
ferences between the average value of this peel
strength and the value measured at the point of impact
being evaluated. In particular, for cables of the
medium-tension type, with a structure comprising an
inner semiconductive layer, an insulating layer and an
outer semiconductive layer, the peel strength (and the
relative difference) may advantageously be measured
between the layer of outer semiconductive material and
the insulating layer.
The Applicant has observed that the effects of
the particularly severe impacts ~ to which a
cable may be subjected, in particular an armored
medium-tension cable, may be reproduced by means of an
impact test based on the French standard HN 33-S-52,
relating to armored cables for high-tension power
transmission, which allows for an energy of impact on
the cable of about 72 joules (J).
The peel strength of the coating layer may be
measured according to the French standard HN 33-S-52,
according to which the force needed to be applied to
separate the outer semiconductive layer from the
P~EI~IOE~ S~EE~
CA 02289748 1999-11-15
PC711 - 26 -
insulating layer is measured. The Applicant has
observed that by measuring this force continuously, at
the points at which the impact takes place, force peaks
are measured which indicate a variation in the cohesive
force between the two layers. It was observed that
these variations are generally associated with a
decrease in the insulating capacity of the coating. The
variation will be proportionately larger the smaller
the impact strength provided by the outer covering
(which, in the case of the present invention; consists
of the expanded coating and the outer sheath). The size
of the variation of this force measured at the points
of impact, relative to the average value measured along
the cable, thus provides an indication of the degree of
protection provided by the protective coating. In
general, variations in the peel strength of up to
20-25% relative to the average value are considered to
be acceptable.
The characteristics of the expanded coating
(material, degree of expansion, thickness), which may
advantageously be used together with a suitable
protective outer polymer sheath, may be appropriately
sel e~~C~ ~...a
according to the impact protection which it is
intended to- provide to the underlying cable structure,
and also depending on the characteristics of the
specific material used as insulator and/or
semiconductor, such as hardness of the material,
density and the like.
As it can be appreciated throughout the present
description, the cable of the invention is particularly
suitable to replace conventional armored cables, due to
the advantageous properties of the expanded polymer
coating-with respect to metal armoring. However, its
use should not be limited to such a specific
application. As a matter of fact, the cable of the
~~,~~~DE~ S~'~FE't
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 27 -
invention may advantageously be employed in all those
application wherein a cable having enhanced impact-
resistant properties would be desirable. In particular,
the impact-resistant cable of the invention may replace
conventional unarmored cables in all those application
wherein, up to now, use of armored cables would have
been advantageous but has been discouraged due to the
drawbacks of the metal armoring.
A few illustrative examples are given herein-
below in order to describe the invention in further
detail.
c~rn~T_~!
Preparation of the cable with expanded coatin
In order to evaluate the impact strength of an
expanded polymer coating according to the invention,
various test pieces were prepared by extruding variable
thicknesses of a few polymers with various degrees of
expansion over a core composed of a multi-wire
conductor about 14 mm in thickness coated with a layer
of 0.5 mm of semiconductive material, a layer of 3 mm
of an insulating mixture based on EPR and a further
layer of 0.5 mm of "easy stripping" semiconductive
material based on EVA supplemented with carbon black,
for a total core thickness of about 22 mm.
Low density polyethylene (LDPE), high density
polyethylene (HDPE), polypropylene (PP) a 70/30 by
weight mechanical mixture of LDPE and finely powdered
vulcanized natural rubber (particle size of 300-600 um)
(PE-powder), PP modified with EPR rubber (PP-EPR as a
70/30 by weight mixture) were used as polymer materials
to be expanded; these materials are identified in the
following text by the letters A to E and are described
in detail in the following table:
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 28 -
Material Brand name and manufacturer Modulus
(MPa)
A LDPE Riblene FL 30 - Enichem 260
B HDPE DGDK 3364 - Union Carbide 1000
C PP PF 814 - Montell 1600
D PP-EPR FINA-PRO 36605 1250
E PE/powder Riblene FL 30
The polymer was expanded chemically,
alternatively using two different expanding compounds
(CE), these being identified as follows:
Compound Brand name and manufacturer
CEl azodicarboamide Sarmapor PO - Sarma
CE2 carboxylic acid- Hydrocerol CF 70 - Boehringer
bicarbonate Ingelheim
The polymer to be expanded and the expanding
compound were loaded (in the ratios indicated in
Table 2) into an 80 mm - 25 D single-screw extruder
(Bandera); this extruder is equipped with a threaded
transfer screw characterized by a depth in the final
zone of 9.6 mm. The extrusion system consists of a male
die capable of providing a smooth throughput of the
core to be coated (generally with a diameter which is
about 0.5 mm greater than the diameter of the core to
be coated), and a female die in which the diameter is
chosen so as to have a size about 2 mm less than the
diameter of the cable with the expanded coating; in
this way, the extruded material expands on exiting the
extrusion head rather than inside this head or inside
the extruder. The throughput speed of the core to be
coated (speed of the extrusion line) is set as a
function of the desired thickness of expanded material
(see Table 2). At a distance of about S00 mm from the
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 29 _
extrusion head is a cooling pipe (containing cold
water) in order to stop the expansion and to cool down
the extruded material. The cable is then wound on a
bobbin.
The composition of the polymer material/
expander mixture and the extrusion conditions (speed,
temperature) were varied appropriately, as described in
Table 2 below.
Table 2: Expanding mixture and extrusion conditions
Cable Material Extruder "'Extruder Line
+
o
No. and type of speed temp. (C) speed
expander (rev/min) (m/min)
1 A 2oCE1 6.4 165 3
+
2 A 2%CE1 11.8 190-180 2
+
3 A 2sCEl 5.5 190-180 3
+
4 A 2oCE1 6.8 190-180 2
+
5 A 2$CE1 6.4 165 1.5
+
6 A 0.8%CE2 5.7 225-200 2
+
7 C 0.8oCE2 3.7 200 2
+
8 C 0.8~CE2 6.3 200 2
+
9 E 0.8~CE2 4.9 225-200 1.8
+
10 B l.2oCE2 8.2 225-200 2
+
11 D 2~CE2 8 225-200 2
+
'1': The extrusion temperature relates to the cylinder
and extrusion head. When only one value is given, these
temperatures are identical. In the initial zone of the extruder,
the temperature is about 150°C.
Sample 1 did not undergo expansion, presumably
because the temperature of the extruder was too low
(165°C), and likewise, for the same reason, Sample 5
underwent limited expansion (only 50).
The cable with the expanded coating was then
subsequently coated with a conventional sheath of MDPE
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 30 -
(CE 90 - Materie Plastiche Bresciane) of variable
thickness (see Table 3) by means of conventional
extrusion methods, thus obtaining cable samples with
the characteristics defined in Table 3; cable No. 1, in
which the polymer has not undergone expansion, was
taken as comparative non-expanded polymer coating.
Table 3 also gives, for comparative purposes, the
characteristics of a cable lacking the expanded filling
and coated with only the outer sheath (cable No. 0).
Table 3: Characteristics of the coating
Cable Degree of Thickness of Sheath
No. expansion of the filling thickness
the filling (~) (mm) (mm)
0 - 0 3
1 0 1 3
2 31 4.3 3
3 61 1 3
4 48 2.5 3
5 5 3 3
6 35 2 2
7 52 2 2
8 29 3 2.2
9 23 2.5 2
10 78 4 2
11 82 4 2
In a similar manner to that described above,
using an expanded polymer coating with a flexural
modulus of about 600 MPa consisting of a polypropylene
modified with about 30°s of an EPR rubber, another 6
cable samples were prepared, as reported in Table 4
(Examples 12-17); Table 4 also gives two comparative
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02b98
- 31 -
examples of cables with expanded coating but lacking
the outer sheath (Examples 16a and 17a).
Table 9: Characteristics of the coating
Cable Degree of Thickness of Sheath
No. expansion of the filling thickness
the filling ( (mm) (mm)
o)
12 71 3 1.9
13 22 2 2
14 167 3 1.8
15 124 2 2
16 56 2 2
16a 56 2 -
17 84 2 2
17a 84 2 -
EXAMPLE 2
Impact strength tests
In order to evaluate the impact strength of the
cables prepared according to Example 1, impact tests
were carried out on the cable with subsequent evalua-
tion of the damage. The effects of the impact were
evaluated both by means of visual analysis of the cable
and by means of measuring the variation in peel
strength of the layer of semiconductive material at the
point of impact. The impact test is based on the French
standard HN 33-S-52, which provides for an energy of
impact on the cable of about 72 joules (J), which is
obtained by dropping a 27 kg weight from a height of
27 cm. For the present test, such energy of impact has
been produced by a 8 kg weight dropped from a height of
97 cm. The impact-end of the weight is provided with a
V-shaped rounded-edge (1 mm curvature radius) punching
head. For the purposes of the present invention, the
impact strength was evaluated on a single impact. For
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 32 -
samples 6-12, the test was repeated a second time at a
distance of about 100 mm from the first.
The peel strength was measured according to the
French standard HN 33-S-52, according to which the
force needed to be applied in order to separate the
outer semiconductive layer from the insulating layer is
measured. By measuring this force continuously, force
peaks are measured at the points at which the impact
occurred. For each test piece, at the point of impact,
a "positive" force peak was measured, corresponding to
an increase in the force (relative to the average
value) required to separate the two layers, and a
"negative" force peak (decrease relative to the average
value). From the difference between the maximum (Fmax)
and minimum (Fmin) of the force peaks measured, the
maximum variation in the peel strength at the point of
impact is obtained.
The variation in the peel strength is thus
calculated by determining the percentage ratio between
the abovementioned difference (Fmax-Fmin) and the
average peel strength value measured for the cable
(F<>), according to the following relationship:
variation = 100 (Fmax-Fmin)/F<>
The size of the variation of this force
measured at the points of impact, relative to the
average value measured along the cable, thus gives an
indication of the degree of protection provided by the
expanded coating. In general, variations of up to 20-
250 are considered to be acceptable. Table 5 gives the
values of the variation in the peel strength for
samples 0-17a.
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 33 -
Table 5: % variation in the peel strength
Cable 1st test 2nd test
0 62 78
1 40 -
2 18 -
3 27 -
4 13 -
21 -
6 17 23
7 9 12
8 4 5
9 19 15
9.8 12.5
11 4.3 2.5
12 7 14
13 16 17
19 14 12
10 10
16 1& 18
16a 30 55
17 15.5 13
17a 116 103
As is seen in Table 3, for sample 1 (for which
no expansion was obtained), the percentage variation in
5 peel strength is extremely high; this indicates that a
non-expanded polymer has a decidedly lower capacity to
absorb impacts than a layer of identical thickness of
the same polymer which is expanded (see sample 3, with
61o expanded coating). Sample 3 shows a variation in
10 the peel strength which is slightly above the 250 limit
value; the limited impact strength provided by the
sample may be attributed mainly to the thickness, of
only 1 mm, of the expanded coating, relative to the 2-
3 mm thicknesses of the other samples.
15 Sample 5, with an expanded coating thickness of
3 mm, has a high value of peel strength on account of
the low degree of expansion of the polymer (5%), thus
demonstrating the limited impact strength provided by a
coating with a low degree of expansion. Sample 4,
although having a thickness of expanded material which
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 34 -
is less than that of sample 5 (2.5 mm as opposed to
3 mm), nevertheless has a higher impact strength, with
a variation in the peel strength of 13% compared with
21o for sample S, thereby demonstrating the fact that a
higher degree of expansion affords a higher impact
strength.
By comparing sample 13 with sample 15, it is
seen how an increase in the degree of expansion of the
polymer (from 22 to 1240), for the same thickness of
the layer of expanded material and of the outer sheath,
entails an increase in the impact strength of the
coating (going from 16-17o to loo of variation in the
peel strength). This trend is confirmed by comparing
sample 16 with sample 17. However, by comparing samples
16a and 17a (without outer sheath) with the respective
samples 16 and 17, it may be seen how the contribution
provided by the outer sheath towards the impact
protection increases as the degree of expansion
increases.
L~VTIITfT L~ 7
Impact strength comparison test with armored cable
Cable no. 10 has been tested versus a
conventional armored cable, in order to verify the
impact strength efficiency of the expanded coating
layer.
The armored cable has the same core as cable no. 10
(i.e. a multi-wire conductor about 14 mm in thickness
coated with a layer of 0.5 mm of semiconductive
material, a layer of 3 mm of an insulating mixture
based on EPR and a further layer of 0.5 mm of "easy
stripping" semiconductive material based on EVA
supplemented with carbon black, for a total core
thickness of about 22 mm). Said core is encircled, from
the inside towards the outside of the cable by:
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02G98
- 35 -
a) a layer of rubber-based filling material of about
0.6 mm thickness;
b) a sheath of PVC of about 0.6 mm thickness;
c) 2 armoring steel tapes of about 0.5 mm thickness
each;
d) an outer sheath of MDPE of about 2 mm thickness.
For the comparison test, a dynamic machine of the
"falling weight" type (LEAST, mod. 6758) has been
employed. Two sets of tests has been carried out, by
dropping a 11 kg weight from a height of 5C cm (energy
impact of about 59 joule) and 20 cm (energy impact of
about 21 joule), respectively; the weight is provided
at its impacting end with a semispheric head of about
10 mm radius.
The resulting deformation of the cables is shown in
figg. 4 and 5 (50 cm and 20 height, respectively),
wherein the cable according to the invention is
indicated with a), while the conventional armored cable
is indicated with b).
The deformation of the core has been measured, in order
to evalute the damages of the cable structure. As a
matter of fact, higher deformations of the
semiconductive-insulating-semicondutive sheath are more
likely to cause electric defects in the insulating
properties of the cable. The results are reported in
table 6
CA 02289748 1999-11-15
WO 98/52197 PCT/EP98/02698
- 36 -
Table 6:o reduction of the thickness of the
semiconductive layer after impact
In conventional In Cable no. 10
armored cable
50 cm height 410 26.50
impact
20 cm height 4.90 2.90
impact
As apparent from the results reported in table
6, the cable of the invention shows even better impact
strength performances than a conventional armored
cable.
