Note: Descriptions are shown in the official language in which they were submitted.
2~)26~4
APPLYING JACKET MATERIAL TO CORRUGATED
METAL SHIELDS OF TELECOMMUNICATIONS CABLE
Backqround of the Invention
Field of the Invention
This invention relates to the application of a
surrounding jacket to a corrugated metal shield in cable
manufacture.
Description of Prior Art
In the manufacture of telecommunications cable, it
is conventional practice to provide a corrugated metal
shield, normally steel, around the cable core which comprises
a plurality of electrical conductors. In one cable type, a
steel shield is tin plated with overlapped longitudinally
extending edge regions of the shield bonded together by a
soldering operation. To provide a moisture barrier between
the shield and a polyolefin cable jacket, a moisture barrier
is provided around the shield by a flooding compound before
the jacket is extruded onto it. Originally flooding compound
was normally asphalt based. However, asphalt flooding
compound is brittle under low temperature environmental
conditions at -20C and this may facilitate crack propagation
in the jacket. It tends to embrittle a polyolefin jacket
which may then disintegrate at such low temperatures par-
ticularly during installation in a manhole when the cable issubjected to twisting, torsional and bending stresses. At
high temperatures, asphalt based flooding compounds have
presented no particular problem. To overcome the low
temperature disadvantage, the asphalt based flooding com-
pounds were replaced by polyisobutylene flooding compoundswhich do not encourage embrittlement of polyolefin jacket
materials. However, at higher temperatures, and wlthin
pressurized cables using polyisobutylene flooding compounds,
there has been a lack of mechanical integrity in a cable and
gas pressures achieved have caused ballooning of the jacket
away from the shield if gas penetration occurs through any
unsoldered areas at the overlapped edge regions of the
shield. In an attempt to prevent such ballooning
occurrences, jacket materials were modified to the use of
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medium densi-ty polyethylenes which are also less crack
susceptible. However, jackets of medium density material are
susceptible to buckling or localized kinking thereby increas-
ing the diameter or width of a cable locally with resultant
jamming of cable in ducting through which it i5 being fed~
The above cable structures using flooding compounds
together with their attendant disadvantages were being
replaced during the 1970's by another type of cable in which
the plating on a steel shield was replaced by a polymeric
coat. The polymeric coat was advantageous in that during
extrusion of jacket material onto a shield, the polymeric
coat was caused to soften and thereby fuse to the jacket
material so that a bonding occurred between the shield and
the jacket. The heat to activate the bonding was supplied by
the jacketing compound itself. The polymeric coating was
sufficiently heat insulating to prevent premature solidifica-
tion of the jack~ting compound. This enabled the use of
jacketing compounds with very low melt indices, i.e. in the
order of 0.2 to 0.5 g/10 min [as measured by the procedure
specified under ASTM D-1238 (condition E)] while such
compounds were able to fill completely the corrugations of a
metal shield. However, with such a polymeric coat, soldering
of the overlapped edges of the shield was impossible and any
bonding between the overlapped edges relied upon a softening
of the contacting coating layers during extrusion so that the
layers became fused together. However, such a bond between
overlapped edges of the shield was not very resistant to
torsion and bending stresses placed upon a resultant cable
whereby shield edge separation could result and the outer
edge of the shield could move outwardly and cut through the
jacket. This action is normally referred to as "zippering".
In an attempt to overcome this problem, plastic filler has
been introduced between the overlapped edges of a shield, but
this has led to undesirable complications during manufacture.
More recently, polyolefin materials modified with
carboxylic acid or anhydride thereof have become available.
It has been found that such materials are useful in jackets
for cables in that they may bond to a metal (preferably
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steel) shield without the use of a polymeric coat on the
shield. As a result, both a polymeric coat and a flooding
compound together with their inherent disadvantages may now
be avoided. However, attempts to apply j acketing materlal
employing polyolefin modified with carboxylic acid to metal
shields by known extrusion techniques have so far proved to
be unsatisfactory. This is because, while the polymeric coat
is not required for bonding, the modified polyolefin contacts
the metal directly which acts to promote rapid heat transfer
from the polyolefin. As a result, the modified polyolefin
commences to solidify at its surface too quickly and cannot
flow to contact intimately the whole of the corrugated outer
surface of the shield. Trapped pockets of air are thus
formed at the bases of the corrugations. While bonding has
been successful at the positions where the jacketing material
actually contacts the surface of the shield, the bonded
regions have been weakened by the presence of adjacent non-
bonded regions. In such a structure, resultant cracking and
disintegràtion of a jacket could occur upon the application
of bending or torsional stresses to the finished cable.
Summary of the Invention
The present invention provides a process and an
apparatus for applying a polyolefin jacket to a metal shield
of a telecommunications cable and wh~ch seeks to eliminate or
minimize the above problem when the polyolefin is modified
with a carboxylic acid or anhydride thereof.
According to one aspect of the present invention,
there is provided a method of applying a surrounding jacket
to a corrugated metal shield covered cable core of insulated
electrical conductors comprising:- passing the shield
covered core along a passline through an extrusion head;
passiny an extrudate through a flow passage in the head, the
extrudate comprising a polyolefin having a modifier provided
by a carboxylic acid or anhydride thereof; extruding the
extrudate onto the outer surface of the shield at an ex-
trusion station so as to form the jacket while reducing the
air pressure on the outside of the shield at the extrusion
station to provide a substantially intimate overall area of
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contact between the jacket and outer surface of the shield;
and effecting a bond between the jacket and thP outer surface
of the shield, throughout the overall area of contact, by the
use of heating applied to the shield, additional to that
provided by the extrudate, to raise the outside surface
temperature of the shield at least to the bonding activation
temperature of the modifier.
To achieve a specified adhesion, certain active
groups are required. For example, to achieve an adhesion
with 15 lbs/inch 'T' pull strength, the active groups in the
formulation may need to be in the order of 0.03 which is the
ratio of the infrared absorption peak of the unsaturated
carboxylic acid or anhydride thereo~ (the reactive or
modifying group) to the infrared absorption peak of CH2 (the
polyolefin chain or group)O The infrared absorption peak of
the unsaturated carboxylic acid or anhydride thereof is 1790
cm~1 while the infrared absorption peak of CH2 is 720 cm~l.
With the inventive method, air pressure reduction
is required to achieve a substantial intimate overall area of
contact between the jacket and the outer surface o~ the
shield as it encourages the extrudate to flow easily into
contact with the shield. Air pockets are thus avoided. This
overall area of contact of course includes unbroken contact
between jacket and shield into the bases of the corruqations.
Preferably, the jacket is cooled after extrusion
sufficiently to cool its surrounding radially outer regions
tn effect radial shrinkage of the jacket and cause it to
apply radial pressure upon the outside surfacs of the shield.
The additional heating is then provided at a heating station
by use of an induction heater and the bond between jacket and
shield is effected during the application of the radial
pressure.
Alternatively, a heating station is provided
upstream along the passline from the extrusion station and
the shield covered core is heated as it passes through the
extrusion station. In this alternative case, the additional
heat coacts with the air pressure reduction to provide a
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substantially intimate overall bonded contact between the
extrudate and shield.
Without the use of preheat and with the additional
heating provided after a cooling step, the extrudate must
have a sufficiently high melt index, to allow it to flow
under vacuum conditions and fill the corrugationsO The melt
index figure is dependent at least partly upon the dimensions
of the corrugations. For instance, in certain cases a melt
index as low as 0.7 g/10 min may suffice, but values above
this are also envisaged. In any other case, and using
preheat, the melt index may be slightly lower.
For temperature control purposes and to ensure the
required temperature, it is essential to have any upstream
heating station as close as possible to the extrusion station
without the interpositioning of a vacuum connection between
these two stations. Such an interpositioning of the vacuum
connection could uncontrollably remove the applied heat and
make temperature control difficult, because this would
encourage outside and cooler air to rush through the heating
station and over the corrugations. With the vacuum applied
upstream from the heating station, however, the gas pressure
is still reduced at the extrusion station while having no
undesirable effect upon the heating step and upon the
temperature control.
In a further alternative process within the scope
of the invention, there are two heating stations, both with
their own heaters, the stations disposed one at a position
upstream of the extrusion station and the other downstream
from a position in which the jacket has been cooled after
extrusion sufficiently to cool its surrounding radial outer
regions to effect radial shrinkage of the jacket and cause it
to apply radial pressure upon the outside surface of the
shield.
In all cases, heating is preferably performed by
induction heating. This should be at such a frequency to
ensure that only the radially outer regions of the shield are
heated. If the shield is the only shield provided, heat at
the inner surface of the shield should be avoided because
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this may deleteriously affect conductor insulation in the
cable core. Alternatively, if the shield is of steel and
surrounds an ~luminum shield for electrical conductivity
purposas longitudinally of the cable, it is normal for such
an aluminum shield to provide a longitudinal gap of about 0.5
inches between its edges. If the frequency of the inductive
heater is too low the induced currents will penetrate through
the steel into the aluminum. As a result, the current will
pass solely along the narrow longitudinal region of the steel
overlying the gap in the aluminum shield. This will overheat
the steel shield. It has been found that an induction heater
operating at a frequency typically 450 kHz or higher will
heat the outer regions of the steel shield as is required to
avoid the above disadvantages.
In a preferred process, the shield comprises a
corrosion resistant metal coating which bonds together
overlapped longitudinally extending edge regions of the
shield. The heat applied to the shield covered core raises
the temperature of the coating to a temperature below its
melting point. The corrosion resistant metal coating may be
for instance tin, copper coated onto the shield by electro-
lytic means, or zinc.
The invention, according to a further aspect, also
provides apparatus for applying and bonding a surrounding
jacket to a corrugated metal shield covered cable core of
insulated electrical conductors, the jacket comprising a
polyolefin having a modifier provided by a carboxylic and/or
anhydride thereof, the apparatus comprising:- an extrusion
head located at an extrusion station and surrounding a
passline for the shiQld covered cabls core; a heating means
surrounding the passline at a heating station for heating the
outPr surface of the shield a~ it moves along the passline to
raise the outside surface temperature of the shield at least
to the bonding activation temperature of the modifier; and an
air pressure reducing means for creating a reduction in air
pressure on the outside of the shield at the extrusion
station from an applied position upstream along the passline
from the extrusion station.
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The apparatus according to the invention is
preferably provided with a jacket cooling means disposed
downstream along the passline from the extrusion station and
with the heating means at the heating station downstream from
the cooling means for inductively heating the out~r surface
of the shield to supply the additional heat.
Alternatively, heating means is disposed at the
heating station disposed upstream fxom the extrusion station
and the air pressure reducing means operates the air pressure
reduction from an applied position upstream along the
passline from the heating station. Conveniently the heating
means is disposed upstream from the extrusion head itself.
Th~s arrangement enables an extrusion head of conventional
design to be used.
The invention further include, according to another
aspect, an electrical cable comprising a cable core of
insulated electrical conductors, a corrugated metal shield
surrounding the core, and a jacket surrounding the metal
shield, the jacket comprising a polyolefin having a modifier
provided by a carboxylic acid or anhydride thereof and the
jacket lying in intimate overall contact with an outer
surface of shield and being bonded by the modifier throughout
the overall contact areas to the outer surface of the shield.
Brief Descri~tion of the Drawings
One embodiment of the pxesent invention will now be
described, by way of example, with reference to the
accompanying drawings, in which:-
Figure 1 is a cross-sectional view through part of
a cable structure having a jacket of a polyethylene modified
with a carboxylic acid or anhydride thereof, the structure
having been made by a conventional cable maXing process;
Figure 2 is a diagrammatical side elevational view
partly in section of an apparatus according to a first
embodiment for applying a surrounding jacket to a corrugated
metal shield covered cable core;
Figure 3 is a view similar to Figure l of part of a
cable having similar jacket material to the structure of
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Figure 1 and in which the jacket has been applied by the
apparatus of Figure 2; and
Figures 4 and 5 are views similar to Figure 2 of
second and third embodiments.
Descri~tiDn of the Preferred Embodiment
As can be seen from ~igure 1, a telecommunications
cable 10 made by a conventional cable making process com-
prises a core 12 formed of a plurality of pairs of in-
dividually insulated conductors (not shown), the core being
surrounded by a core wrap (not shown) around which is
disposed a steel shield 16 of corrugated form. The steel
shield was precoated with a tin based soldering material and
overlapped edges (not shown) of the shield are soldered
together longitudinally of the cable to provide a continuous
water impenetrable seam. Surrounding the shield 16 is a
jacket layer 18 formed from a polyethylene composition which
is modified with a carboxylic acid or anhydride thereof. The
polyethylene composition has a density of 0.925 g/cm3 and a
melt index of about 0.9 g/10 min as measured by the procedure
specified under ASTM D-1238, (conditisn E). The melt index
provided complete filling of corrugations as will be
described, for a corruyation pitch of 0.1 inches and a depth
of 45 mil. The carboxylic acid is either maleic acid or
acrylic acid with any suitable content of the total content
of the jacket material for providing the re~uired adherence
of the jacket to the shield. More specifically, in the
embodiment, the acid is present by volume up to a total of
approximately 2% of the total volume of the jacket.
The jacket 18 was extruded onto the shield 16 by
conventional processes, that is to say, that after location
of the shield around the core and soldering the overlapped
edges of the shield together, the shield covered core,
untreated in any additional fashion, was fed directly through
an extrusion head 11. As a result, it was found that the
extrudate had a reluctance to move across the total outside
surface area of the corrugations so that air pockets 20,
particularly at the bases of the corrugations, were formed.
As a result of the formation of the air pockets 20 in the
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construction of Figure 1, while the jacket material adhered
adequately to the shield 16 in the various regions where
intimate contact was achieved, finished cable was completely
unsatisfactory because of the reduction in total adherence
between jacket and shield. For instance, while local
contacting and bonded regions between jacket and shield
provided substantial localized strength, overall torsion,
bending and peeling strength was weakened by the randomly
positioned air pockets. In addition to this, cracking and
disintegration of the jacket could result upon the applica
tion of torsional or bending stresses to the cable. Further,
the air pockets 20 provided an assistance for moisture
seepage along the outside of the shield 16 after any moisture
access was created through the jacket.
The embodiments to be described avoid a
construction such as described above with reference to Figure
1 together with the substantial elimination of all the
disadvantages inherent in such a construction.
As can be seen from Figure 2, in the first embodi-
ment, an apparatus 22 is provided for applying a jacket to a
corrugated metal shield covered cable core of insulated
electrical conductors. In the apparatus 22 of Figure 2, an
extrusion head 24 of conventional design has an extrusion
orifice 26 surrounding a passline for a shield surrounded
cable core 28 as it moves downstream through the extrusion
head. Tha extrusion orifice 26 is provided at an extrusion
station. Within the extrusion head is disposed a core tube
29 for guiding the core 28 towards and through the orifice
26. Directly upstream from the extrusion head is disposed a
heating means 30 which comprises an induction heater 32
formed by a plurality of turns of wire surrounding and
extending partly along the passline for the shield covered
core 28. The induction heater operates at at least 450 kHz
and for this purpose heaters are available from Pillar
Industries, Lepel or Westinghouse. To provide adequate
control for the temperature at the outside of the shield upon
reaching the extrusion station, it is desirable that the
heating means 30, is disposed in a heating station as closely
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as posslble to the extrusion head. In practice, a distance
of around 12 inches has been found to be desirable between
the downstream end of the heater 32 and the upstream end of
the head.
The apparatus is also provided with a gas pressure
reducing means for creating a reduction in gas pressure on
the outside of the shield at the extrusion station. The gas
pressure reducing means comprises a vacuum creating source 34
which is connected into a chamber 36, surrounding the
passline of the shield covered core at a position upstream
from the heating means 30, so as to be effective for reducing
the air pressure below atmospheric conditions between the
extrusion orifice 26 and the chamber 36. It has been found that
with the vacuum source 34 applied at such an upstream position,
it does not interfere with the control of the heating means 30
in raising the temperature of the outside of the shield to that
required. The apparatus has a conventional water cooling trough
(not shown~ downstream from the extrusion head.
The apparatus shown in Figure 2 is used for applying
the jacket upon the shield covered core 28 to form a telecom-
munications cable 38 shown on the left-hand side of Figure 2 and
in greater detail in Figure 3. The cable of Figure 3 has a core
wrap (not shown) and a corrugated steel shield 42 formed of the
same materials and of the structure of the cable of Figure 1.
While the shield 42 of the cable 38 is coated with a tin based
solder material, in this particular structure, the solder
material has a softening temperature above the melting point of
the jacket material which is the same polyethylene modified with
carboxylic acid or anhydride thereof as described with reference
30 to Figure 1 and has a melt index between 0.25 g/10 min and 0.5
g/10 min as measured by the procedure specified under ASTM
D-1238 (condition E). The composition may be made by taking a
batch of carboxylic acid modified polyethylene and mixing it
with unmodified polyethylene so as to provide a mixture having a
total volumetric content of maleic acid in the region referred
to above, i.e. up to about 2% by volume of the total volume of
the composition. The tin based solder may for instance, be
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chosen from any one of the four following compositions in Table
I parts taken by weight):-
Table I
_
Softening Melting¦
Composition Sn% Sb% Ag% Pb% Point Point
. 1 95 5 232 238
2 96 4 2~1
3 95 5 221 240
4 10 90 224 302
10 10 50 245 246
On the other hand, ~he jacket material has amelting temperature around 200C.
To apply the jacket material to the shield covered
core 28, the core is fed, as shown in Figure 2, through the
chamber 36 to which the pressure reducing source 34 is
applied. The shield covered core 28 then proceeds through
the heating means 30 and out through the extrusion orifice 26
with the whole area enclosed from the chamber 36 to the
extrusion orifice, so that along this length of the feedpath
there is a reduction in air pressure upon the outside of the
shield. The heating means 30 operates to heat the shield to
an outside temperature below the melting point of the solder
and also to maintain that temperature below the melting point
of the solder but at least to the bonding activation tempera--
ture of the modifier when the shield reaches the extrusionstation 26. This is above the melt temperature of the
extrudate. It has been found that the control of the heating
step is not significantly undesirably affected by the
application of the vacuum at the upstream position. Upon the
extrudate contacting the outside surface of the shield 42,
air pressure reduction draws the extrudate down into the
corrugations to achieve a substantial intimate overall area
of contact between the extrudate and the shield, i.e. into
the bases of the corrugations. This action is assisted by
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the high surface temperature of the shield which maintains
the extrudate in a molten condition thereby encouraging it to
flow completely along the total surface of the shield and
down into the corrugations. As a result, the corrugations of
the shield become filled with the extrudate, as can be se~n
from Figure 3. As the shield surface temperature is above
the bond activation temperature of the modifier upon contact
of the extrudate with the shield, overall intimate bonding
contact between the jacket material and the metal of the
shield occurs so as to provide substantial and adequate
torsion, bending and peel strength of the bonded joint. It
follows that no air pockets are created in the structure such
as were seen with the construction of Figure 1 above, and
moisture seepage between the jacket and the shield 42 is
thereby not encouraged. Achievement of the desired strength
requirements ensures that upon the application of bending or
torsional forces to the cable, e.g. during cable laying or
subsequently when in use, the jacket does not separate from
the shield so that cracking and disintegration of the jacket
does not result.
As may be seen therefore, the apparatus and process
described in the embodiment provide a cable having a jacket
of a material which, while normally being reluctant to flow
into corrugations of a metal shield, nevertheless lies in
intimate bonding engagement with the whole outer corrugated
surface of the shield.
In second and third embodiments now to be
described, apparatus having parts similar to those described
in the first emhodiment will bear like reference numerals.
In a second embodiment as shown in Figure 4, the
extrusion head 24 is connected directly to the vacuum
creating source 34 by a passage 44 so as to exclude the
effects of outside air. In this particular embodiment, no
heater is provided upstream from the extrusion head as
described in the first embodiment. Instead, immediately
following the extrusion head there is disposed a water
cooling trough 46 of conventional structure for effecting a
partial cooling of the jacket so as to solidify its outer
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regions only. This cooling trough 46 is followed downstream
along the feedpath by a heater 48 of similar construction to
the heater 32 described in the first embodiment. Immediately
downstream from the heater 48 is disposed a second cooling
trough 50.
In use o~ this embodiment, a shield covered core 28
is moved along a passline through the vacuum tube 44,
extrusion head 24, cooling trough 46, heater 48 and cooling
trough 50. This core 28 is of similar construction to the
core 28 described with reference to the first embodiment.
Upon the core reaching the extrusion orifice 26 it is
contacted by the extrudate 54 issuing from the extrusion
head. The reduction in air pressure directly at thP surface
of the shield at the extrusion orifice as the extrudate moves
into contact with it, desirably draws the extrudate into
intimate contact with the shield. However, because there is
no heating provided upstream from the extrusion head which is
used to assist in the drawing-down of the extrudate into
intimate engagement with the corrugated outer surface of the
shield, then the melt index of the extrudate is needed to be
higher to encourage its flow to achieve the desired intimate
engagement. It is envisaged that a melt index above 0.7 g/10
min is re~uired and in this particular embodiment, the
extrudate has a melt index of approximately 1.6 g/10 min.
The corrugations of the shield have the same pitch and depth
dimensions as in the first embodiment. The cable 52 provided
by the core surrounded by the extruded jacket then passes
from the extrusion head and into the first cooling trough 46,
the length of which taken together with other parameters such
as line speed and temperature of the extrudate, ensure that
upon issue of the cable from the cooling trough 46, the outer
surfaces of the jacket are solidifying and are shrinXing so
as to apply a radial inward pressure at the interface between
jacket and sheath. The cable 52 then passes through the
induction heater 48 with the jacket at the interfacial
regions with the sheath still in a molten condition. The
induction heater 48 induces heat through the jacket and into
the shield so as to raise the temperature of the outer
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surface regions of the shield above the bond activation
temperature of the modi~ier in the extrudate. This raise in
temperature creates a bond at the interface between jacket
and sheath and the achievement of this bond is assisted by
the inward pressure of the jacket material caused by the
shrinking of the solidified matter. Thus, a partial cooling
of the jacket assists in a positive fashion in helping the
bond between the two materials. The cable 52 then proceeds
through the cooling trough 50 so as to completely cool the
jacket. The finished structure of the cable is similar to
that shown in Figure 3.
In a third embodiment as shown in Figure 5, a
combination of the first and second embodiments is used. As
can be seen, the apparatus of Figure 5 incorporates an
extrusion head 24 with the upstream heater 32 and the vacuum
device 34 further upstream from the heater. Downstream from
the extrusion head, the apparatus includes the cooling
troughs 46 and 50 and the heater 48 interposed between them.
With the use of the apparatus of the third embodi-
ment, a shield covered core 28 is passed along the passlineto be preheated before reaching the extrusion orifice. As
described in the first embodiment the preheat applied to the
outer surface of the shield assists in encouraging the
extrudate to move into intimate contact with the surface of
the shield while also creating a bond between the two
materials. The cable 52 formed of the jacket covered core
then moves down to the cooling trough 46 during which the
surface of the jacket is solidified as described in the
second embodiment so as to apply inward pressure to the
bonded interface between jacket and shield. As the cable
moves through the induction heater 48, the temperature of the
shield is again raised above the bond activation temperature
of the modifier in the extrudate and the bonding action is
again initiated so as to increase the bond between jacket and
shield. The cable then proceeds through the cooling trough
50 so as to completely cool the product.
