Note: Descriptions are shown in the official language in which they were submitted.
48~
I~Method of manufacturing a longitudinally watertight telecommunication cable,
and longitudinally watertight telecommunication cable obtained by this
method".
The invention relates ~o a method of manufacturing a longitudinal_
ly watertight telecommunication cable having a core comprising a plurality of
conductors each covered with a synthetic insulating material, the inter-
stices between the conductors and the space between the core and the sheath
being filled with a compound which prevents water from penetrating into the
cable core in the direction of length of the cable.
Cables of this type are known. In cables laid in the ground water
may penetrate into the cable core through defects of the sheath due to mechan-
ical damage. Such defects may be due to subsidence of the ground or to mechan-
` ical force. In the core of a cable composed of conductors or bundles of
conductors which are twisted together, as generally is the case in telecommuni-
cation cables, water which has made its way into the cable may spread in
the direction of length thereof through the interstices between the conductors
and the space between the core and the sheath. When the conductors are
individually insulated with a synthetic resin, the presence of water becomes
manifest only after the water has penetrated over a comparatively large dis-
tance in that the electric properties of the cable deteriorate, which
deterioration can be made good only with great difficulty. Penetration of
vater.can be prevented in that highly viscous compounds or a foamed syn-
thetic resin are applied throughout the length of the cable or in blocks,
i,e. at regular interrals in the direction of length of the cable over a
~given distance. The highly viscous compounds proposed for this purpose
usually are paraffin-like substances, petroleum waxes, petroleum jelly and
the like. These substances are molten or heated till their viscosity is low
and thereafter are pressed into the interstices between the conductors and
; into the space between the core and the sheath. ~fter solidification
~o~
the compounds introduced into the cable form a seal against water. A disad-
vantage of these substances is, however, that they become liquid again when
the cable temperature rises. This may occur, for example, when the cable
wound on a drum is exposed to irradiation by the sun for a long time. By
the action of gravity the molten sealing compound tilen flows to the lower
parts of the cable on the drum, causing poorly filled interstices or voids
to be left in the remaing parts of the cable. This is particularly detri-
mental when the sealing compound has been applied in blocks. Another disad-
vantage is that the quality of conductor insulations of polyethylene when
brought into contact with some paraffin-like substances deteriorates in
electrical and mechanical respects in the long run.
When the interstices between the conductors and the space bet-ween
the core and the sheath are filled with a foamed synthetic material~ diffi-
culties of another nature arise. Foaming is a process which is not readily
controllable and which usually starts immediately after the foam-forming
mixture has left the container. Foam-forming may have stopped before the
interstices between the conductors and the space between the core and the
sheath have sufficiently been filled. Another disadvantage is that as a
rule a cable in which the core contains a foamed synthetic material as a
sealing means will be stiff, which may cause difficulties when the cable is
wound on a drum and is laid in the ground.
When manufacturing power cables having a small number of conduc-
tors of circular cross-section it is known to fill the interstices between,
and the space around, the conductors to a circular cross-section with a
silicone rubber which is spontaneously vulcanized at room temperature.
The vulcanizing agent is added to the filler material immediately before
the application thereof.
A disadvantage of this method is that when the manufacturing
process comes to a standstill conduits containing the material are likely
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to become clogged.
For this reason the use of such a spontaneously vulcanized filler
material in manufacturing telecommunication cables, in particular when the
material is applied in blocks and is intended to prevent penetration of water,
gives rise to difficulties.
It is an object of the present invention to provide a method of
manufacturing a permanently flexible longitudinally watertight cable in
which the sealing compound does not flow upon heating and does not attack
the conductor-insulation.
In accordance with this invention, there is provided a method
of manufacturing a longitudinally watertight telecommunication cable, which
comprises providing an initial multiplicity of longitudinally extending syn-
thetic resin-insulated conductors, applying about said initial multiplicity
of conductors in longitudinally separated regions first layers of a moisture-
vulcanizable silicone rubber, positioning a second multiplicity of longitud-
inally extending synthetic resin-insulated conductors about said first con-
ductor-silicone rubber combination, applying about said second multiplicity
of conductors second layers of a moisture-vulcanizable silicone rubber
at the same longitudinally separated regions as the first layers of moisture-
vulcanizable silicone rubber, repeating said sequential steps of positioning
a multiplicity of longitudinally extending synthetic resin-insulated conduc-
tors and applying layers of a moisture-vulcanizable silicone rubber until a
cable core having the desired diameter is obtained, and thereafter applying
a sheath about said cable core before the silicone rubber has been complete-
ly vulcanized.
A silicone rubber which is vulcanized by the action of the moisture
in the air is distinguished from a silicone rubber which vulcanizes at
ambient temperature in that in the former vulcanization starts only when
the rubber is exposed to a moisture-containing atmosphere, whereas in the
~V44~
latter vulcanization starts immediately when the constituents are mixed and
is not influenced by the ambient atmoshpere.
Silicone rubbers which vulcanize at ambient temperature by the
action of the air moisture are known and commercially available. They usual-
ly consist of a mixture of a diorganopolysiloxane, a filler such as silicon
dioxide, a substance which causes crosslinking such, for example, as ethyl
silicate, and a vulcanisation catalyser. Moisture curing compositions of
this kind are described for example in United States Patent No. 3.661.817.
The commercial product may be used as such or mixed with a silicone oil or
a rapidly evaporating solvent. The vulcanized~compound is permanently
rubber-elastic. It was found that for the purpose concerned the said sili-
cone rubbers sufficiently adhere to the commonly used conductor insulations
consisting, for example, of polyethylene and polyvinyl chloride, even if
the cable is repeatedly bent, but can readily be removed by hand from the
conductor insulation.
Another advantage of the said silicone rubbers ~hich vulcanize at
ambient temperature by the action of the air moisture is that the process
of vulcanisation is comparatively slow. Hence the cable core can be pro-
vided with a sheet consisting of an overlappingly wou~d foil before termina-
tion of the vulcanizing process, which results in thorough filling of allthe interstices between the conductors and of the space between the core
and this sheath. During the process of encasing the cable core in the
sheath the compound which has not yet completely vulcanized and is still
plastic is pressed into all the spaces between the cable core and the
sheath. Under nonmal conditions the moist~re of the air enclosed in the
core after the provision of the sheath is sufficient in the vulcanizing
process to produce complete vulcanisation of the silicone rubber. When the
silicone rubber was applied in blocks it proved possible to build up the
blocks from layers. In the case of a core composed of conductors twisted
~_
lV~
together, for example in the form of star-quads or pairs, the said layered
provision may be effected in that when a layer of conductors or bundles of
conductors is applied around the preceding layer, at the same time a layer
is provided of the silicone rubber which vulcanized at ambient temperature
by the action of the air moisture. ~fter vulcanization the amounts of
silicone rubber provided successively at the same location but at a later
instant fo~n a single coherent barrier impenetrable to water. The same applies
when the core is built up by twisting together conductor bundles. In this
building-up process, during the formation of the cable core the unvulcanized
silicone rubber is applied in and round the bundles at locations such that
after assembly of the bundles the amounts of silicone rubber will form a
single mass in the core at each block location.
Telecommunication cables manufactured by the method according to
the invention are permanently flexible and provide no difficulty when being
wound on drums or being installed. It was found that during installation the
silicone rubber can simply be removed by hand from the conductors.
Example
The core of a telephone cable comprising 150 star~quads of conduc-
tors which each consist of a copper wire of diameter 0.5 mm coated with an
insulating layer of polyethylene 0.32 mm thick was built up by winding on a
core consisting of 3 star-quads layers of successively 9, 15, 21, 27, 34 and
41 star-quads~alternately in left and right helices.
Around each layer, except the outermost one, a foil of a linear
polyester is wound in an open helix.
On the core and on each successive layer of star_quads silicone
rubber which vulcanizes at ambient temperature by the action of the air
moisture was provided at regular intervals (of 1 meter) through a length of
about 10 cm in an amount such that the interstices between the conductors
were completely filled. A material used for this purpose was a product
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which is marketed under the Silastic (trademark) 738 RTV by Dow Corning
Corporation ard which according to the manufacturer is a silicone rubber
which vulcanizes at room temperature by the action of the air moisture.
A foil consisting of a linear polyester was overlappingly wound
around the cable core and subsequently a sheet was extruded onto the core.
To determine the effect of the sealing, a length of the resulting
cable laid horizontally was connected to a vertically arran8ed tube which
contained water to a height of t m above the cable. After 6 weeks it was
found that the water had penetrated only to the first barrier of silicone
rubber.
