Electric cable resistant to water penetration

1. An electric cable comprising at least one conductor, an insulating covering, a metallic screen positioned externally to said insulating covering and at least one water swellable layer arranged in a position radially external to said metallic screen, said at least one water swellable layer comprising a continuous layer of pre-expanded polymeric material uniformly distributed along the longitudinal direction of said cable in which a water swellable material is embedded, said pre-expanded polymeric material having a degree of expansion between 5% and 500%.

2. An electric cable according to claim 1 , comprising at least two conductors, each of which is provided with an insulating covering and with a metallic screen positioned externally to said insulating covering, a filler arranged externally to each metallic screen to give said cable a structure of circular cross section, and at least one water swellable layer comprising an pre-expanded polymeric material in which a water swellable material is embedded and arranged in a position radially external to said structure.

3. An electric cable according to claim 1 , wherein said degree of expansion is between 10% and 200%.

4. An electric cable according to claim 1 , wherein said degree of expansion is between 10% and 50%.

5. An electric cable according to claim 1 , wherein said at least one water swellable layer has a thickness between 0.3 mm and 6 mm.

6. An electric cable according to claim 5 , wherein said thickness is between 1 mm and 4 mm.

7. An electric cable according to claim 1 , wherein said at least one water swellable material is a polymer possessing hydrophilic groups along the polymeric chain.

8. An electric cable according to claim 7 , wherein the amount of said at least one water swellable material is between 1 phr and 120 phr.

9. An electric cable according to claim 8 , wherein said amount is between 5 phr and 80 phr.

10. An electric cable according to claim 1 , wherein said pre-expanded polymeric material comprises an expandable thermoplastic polymer selected from polyolefins, copolymers of various olefins, olefins/unsaturated esters copolymers, polyesters, polycarbonates, polysulphones, phenolic resins, urea resins, and their mixtures.

11. An electric cable according to claim 10 , wherein said pre-expanded polymeric material is a polyolefinic polymer or copolymer based on ethylene, propylene or ethylene and propylene.

12. An electric cable according to claim 11 , wherein said pre-expanded polymeric material is selected from:

(a) copolymers of ethylene with an ethylenically unsaturated ester, in which the amount of unsaturated ester is between 5 wt. % and 80 wt. %;

(b) elastomeric copolymers of ethylene with at least one C 3 –C 12 α-olefin, and optionally a diene, having the following composition: 35–90 mol. % of ethylene, 10–65 mol. % of α-olefin, 0–10 mol. % of diene;

(c) copolymers of ethylene with at least one C 4 –C 12 α-olefin, and optionally a diene, generally with a density of between 0.86 and 0.90 g/cm 3 ; and

(d) polypropylene modified with ethylene/C 3 –C 12 α-olefin copolymers, where the weight ratio between polypropylene and the ethylene/C 3 –C 12 α-olefin copolymer is between 90/10 and 30/70.

13. An electric cable according to claim 12 , wherein the amount of unsaturated ester in the copolymers of ethylene with an ethylenically unsaturated ester is between 10 wt. % and 50 wt. %.

14. An electric cable according to claim 12 , wherein the weight ratio between polypropylene and the ethylene/C 3 –C 12 α-olefin copolymers is between 50/50 and 30/70.

15. An electric cable according to claim 1 , wherein said at least one water swellable layer is obtained by extrusion.

16. An electric cable according to claim 15 , wherein expansion of said at least one water swellable layer is effected during said extrusion by adding an expanding agent.

17. An electric cable according to claim 16 , wherein said expansion is obtained by injecting a gas at high pressure.

18. An electric cable according to claim 16 , wherein after expansion, said pre-expanded polymeric material is submitted to cross-linking.

19. An electric cable according to claim 1 , further comprising a tubular covering arranged in a position radially external to said at least one water swellable layer.

20. An electric cable according to claim 19 , wherein said tubular covering is a thin foil comprising aluminum, lead or copper.

21. An electric cable according to claim 1 , further comprising a metallic armor arranged in a position radially external to said at least one water swellable layer.

22. A method of blocking the longitudinal flow of water that has accidentally penetrated to the interior of an electric cable, in a portion of the latter that is radially external to at least one metallic screen possessed by said cable, comprising providing a water swellable layer in a position radially external to said at least one metallic screen, said water swellable layer comprising a pre-expanded polymeric material in which a water swellable material is embedded, said pre-expanded polymeric material having a degree of expansion between 5 and 500%.

23. A method according to claim 22 , wherein said water swellable layer is distributed continuously and uniformly.

24. A method according to claim 22 , wherein said water swellable layer is obtained by extrusion.

25. A method according to claim 24 , wherein said water swellable layer is co-extruded with an outer polymeric covering possessed by said cable.