Water blocked electric cables

Abstract

An electric cable having a multi-stranded core of insulated conductors and a jacket, in which the spaces between and around the conductors are at least partially filled with a mixture which forms a blockage to the penetration of water. The mixture comprises water repellant treated calcium carbonate and high molecular weight polyacrylamide resin which do not react with each other or when in contact with water. Preferably the individual conductors are coated with a hydrophobic liquid such as a low viscosity paraffinic oil.

Description

long chain fatty acid coated water repellant treated calcium carbonate such as that which has a surface coating of a long chain fatty acid or modified fatty acid such as lauric acid, myristic acid, palmitic acid, stearic acid, or arachidic acid. Suitable such powders are sold under the trade mark marks "Protaxulate" or "Protex E" by Pluess-Stauffer A.G. of France, and the. The hydrophilic powder is a high molecular weight resin, and one such resin is a high molecular weight polyacrylamide such as that sold under the trade mark "Separan" by Dow Chemical Corporation or "Magnifloc" by the Cyanmid Company. Other suitable resins may be a polyacrylamide which has been cross linked by irradiation, an anionic polyacrylamide having an average molecular weight between 2.5×10⁶ and 7×10⁶ and with between 12% and 28% acrylamide groupings which have been chemically converted to acrylic acid groupings. Additionally, the resin may be an irradiated cross-linked polyethylene oxide or a hydrolyzed starch graft polymer of polyacrylonitrile. It should be noted that the manufacturers of such hydrophilic hydrophobic powders specify compacting to achieve water blockage but it will be appreciated that tamping of the core filler in the production of a continuous cable would be very difficult to achieve.

The percentage composition of filler 16 should be between 95% and 70% by weight of the hydrophobic powder and between 5% and 30% by weight of the hydrophilic powder, depending upon the electrical characteristics required of cable 10. The more critical the electrical transmission requirements, the lower will be the percentage by weight of the hydrophilic powder. For instance a telephone cable would require a minimum amount of the hydrophilic powder, i.e. 5%.

Using filler 16 of the invention it is not necessary to substantially fill the voids in claim cable 10. It has been found that in certain instances cable 10 will be water blocked when filled only 33% by volume, because of the nature of the reaction of filler 16 to the ingress of water into the core of the cable. When such ingress occurs in sufficient quantity to break down the surface tension of the coated calcium carbonate the polyacrylamide will dissolve rapidly in the water to increase the viscosity of the water. The solution will travel only a short distance along the cable core, depending on the percentage by volume of the filler present in the core and the pressure of the water itself.

The mixture may fill between 30% and 95% of the total volume of the interstices.

To aid in the production of the filled cable core, and to inhibit the powders of filler 16 from falling out when cable 10 is cut, it is advantageous to wet the surfaces of the insulation of conductors 12 with a hydrophobic liquid which will not produce adverse characteristics to the cable such as stress cracking in a polyethylene jacket. A low viscosity paraffinic oil would be suitable for the purpose preferably with a high analine point.

In the embodiment shown in FIG. 2 of the drawings a shielded cable 20 is formed by applying a core wrap 22 and then a metal sheath 24 between conductors 12 and jacket 14. Before applying core wrap 22, filler 16 is introduced to occupy the voids between conductors 12 as before. After core wrap 22 has been applied a further amount of the same mixture of hydrophobic powder and hydrophilic powder is applied with metal sheath 24 to form a layer 26 interposed between the core wrap and the sheath.

In specific tests three different telecommunications cables consisting of 25 pairs of 19 gauge insulated conductors were filled in the interstices with varying amounts of powder comprising 80% by weight of water repellant treated calcium carbonate and 20% by weight of high molecular weight polyacrylamide resin. The resultant cables had the following characteristics:

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Test Results
Desired Nominal Level
(1)     (2)     (3)
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Filling by bulk
60%         55      57    67
density
Average water flow
35 inches max.
14      26    17
in core after 22
days with 3' head
of water
Average mutual
0.083 ± 0.005
0.080   0.079 0.079
capacitance
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Claims

1. An electric cable having a core of a plurality of insulated conductors and a jacket, the interstices between the conductors and within the jacket being at least partially filled with a mixture of water repellant treated calcium carbonate and at least one high molecular weight resin rapidly hydratable to form a viscous solution.

2. A cable as claimed in claim 1 in which the calcium carbonate is surface coated with a water repellant long chain fatty acid or modified fatty acid selected from the group consisting of lauric acid, myristic acid, palmitic acid, stearic acid and arachidic acid.

3. A cable as claimed in claim 1 in which the resin is an anionic polyacrylamide resin with an average molecular weight between 2.5×10⁶ and 7×10⁶ and with between 12% and 28% acrylamide groupings which have been chemically converted to acrylic acid groupings.

4. A cable as claimed in claim 1 in which the resin is a polyacrylamide resin which has been crosslinked by irradiation.

5. A cable as claimed in claim 1 in which the resin is irradiated cross-linked polyethylene oxide.

6. A cable as claimed in claim 1 in which the resin is a hydrolized starch graft polymer of polyacrylonitrile.

7. A cable as claimed in claim 1 in which the mixture fills between 30% and 95% of the total volume of the interstices.

8. A cable as claimed in claim 1 including a core wrap and a metal sheath located between the conductors and the jacket, the mixture being also interposed between the core wrap and the metal sheath.

9. A cable as claimed in claim 1 in which the conductors are each coated with a hydrophobic liquid.

10. A cable as claimed in claim 9 in which the hydrophobic liquid is a low viscosity paraffinic oil.