Electric cable and manufacturing process thereof
An electric cable for power transmission at low voltage has at least one conductor and an insulating coating surrounding the at least one conductor. The insulating coating has at least two insulating layers. In a radial direction from the inside toward the outside of the electrical cable, the insulating coating has at least one insulating layer made of a non-expanded polymeric material and at least one insulating layer made of an expanded polymeric material.
1. An electric cable comprising a conductor and an insulating coating surrounding said conductor, said insulating coating having a predetermined thickness and comprising at least two insulating layers, said insulating layers, in a radial direction from the inside toward the outside of said electrical cable, comprising:
a. at least one insulating layer made of a non-expanded polymeric material and
b. at least one insulating layer made of an expanded polymeric material, said at least one insulating layer made of an expanded polymeric material being integral with said at least one insulating layer made of a non-expanded polymeric material;
wherein said expanded polymeric material is obtained from a polymeric material that, before expansion, has a flexural modulus at room temperature, measured according to ASTM Standard D790, not greater than 200 MPa.
2. The electric cable according to claim 1 , wherein the thickness of said at least one insulating layer made of a non-expanded polymeric material is at least half of said predetermined thickness of said insulating coating.
3. The electric cable according to claim 2 , wherein the thickness of said at least one insulating layer made of a non-expanded polymeric material is not lower than 70% of said predetermined thickness of said insulating coating.
4. The electric cable according to claim 3 , wherein the thickness of said at least one insulating layer made of a non-expanded polymeric material is not lower than 85% of said predetermined thickness of said insulating coating.
5. The electric cable according to claim 1 , wherein said at least one insulating layer made of an expanded polymeric material is bonded with said at least one insulating layer made of a non-expanded polymeric material.
6. The electric cable according to claim 1 , wherein said at least one insulating layer made of an expanded polymeric material is co-extruded with said at least one insulating layer made of a non-expanded polymeric material.
7. The electric cable according to claim 1 , wherein said at least one insulating layer made a non-expanded polymeric material adheres to said at least one conductor.
8. The electric cable according to claim 1 , wherein said at least one insulating layer made of an expanded polymeric material of said insulating coating is an intermediate layer between an inner insulating layer made of a non-expanded polymeric material and an external insulating layer made of an expanded polymeric material.
9. The electric cable according to claim 1 , wherein said at least one insulating layer made of an expanded polymeric material of said insulating coating is an intermediate layer between an inner insulating layer made of a non-expanded polymeric material and an external insulating layer made of a non-expanded polymeric material.
10. The electric cable according to claim 8 or 9 , wherein said intermediate insulating layer is circumferentially non-continuous in the cross-section.
11. The electric cable according to claim 10 , wherein said intermediate insulating layer presents at least one interruption.
12. The electric cable according to claim 11 , wherein said at least one interruption is located along the external profile of said inner insulating layer.
13. The electric cable according to claim 11 , wherein said at least one interruption is located in proximity of the external profile of said inner insulating layer.
14. The electric cable according to claim 10 , wherein said circumferentially non-continuous intermediate insulating layer comprises at least one semicircular sector.
15. The electric cable according to claim 14 , wherein said at least one semicircular sector is provided within said inner insulating layer.
16. The electric cable according to claim 14 , wherein said at least one semicircular sector is provided within said external insulating layer.
17. The electric cable according to claim 8 or 9 , wherein said intermediate insulating layer is circumferentially continuous in the cross-section.
18. The electric cable according to claim 1 , wherein said flexural modulus is between 20 MPa and 200 MPa.
19. The electric cable according to claim 18 , wherein said flexural modulus is between 10 MPa and 150 MPa.
20. The electric cable according to claim 1 , wherein the polymeric material of said at least one insulating layer is an expandable polymer selected from the group comprising: polyolefins, copolymers of various olefins, olefins/unsaturated esters eopolymers, polyesters, and their mixtures.
21. The electric cable according to claim 20 , wherein said expandable polymer is polyvinyl chloride.
22. The electric cable according to claim 1 , wherein said at least one insulating layer made of a non-expanded polymeric material and said at least one insulating layer made of an expanded polymeric material are made of the same base polymeric material.
23. The electric cable according to claim 1 , wherein said at least one insulating layer made of an expanded polymeric material has an expansion degree between 2% and 500%.
24. The electric cable according to claim 23 , wherein said expansion degree is between 5% and 200%.
25. The electric cable according to claim 24 , wherein said expansion degree is between 10% and 50%.
26. The electric cable according to claim 1 , wherein said at least two insulating layers of said insulating coating present an insulating constant (ki) greater than 750 MOhm*km at 20°C.
27. The electric cable according to claim 1 , wherein said at least two insulating layers of said insulating coating present an insulating constant (ks) greater than 0.3 MOhm*km at 70°C.
28. The electric cable according to claim 1 , wherein said at least one insulating layer made of an expanded polymeric material has a thickness between 0.05 mm and 1.00 mm.
29. The electric cable according to claim 28 , wherein the thickness of said at least one insulating layer made of an expanded polymeric material is between 0.10 mm and 0.50 mm.
30. A process for manufacturing an electric cable, said cable comprising a conductor and an insulating coating surrounding said conductor and comprising, in a radial direction from the inside toward the outside of said electrical cable, at least one insulating layer made of a non-expanded polymeric material and at least one insulating layer made of an expanded polymeric material, comprising the steps of:
feeding said conductor to an extruding machine;
depositing by co-extrusion:
a non-expandable polymeric material in a position radially external to said conductor so as to form said at least one insulating layer made of a non-expanded polymeric material; and
an expandable polymeric material in a position radially external to said at least one insulating layer made of a non-expanded polymeric material so as to form said at least one insulating layer made of an expanded polymeric material; and expanding said expandable polymeric material during said step of depositing by co-extrusion;
wherein said expandable polymeric material is obtained from a polymeric material that, before expansion, has a flexural modulus at room temperature, measured according to ASTM Standard D790, not greater than 200 MPa.
31. The process according to claim 30 , wherein said step of expanding is effected during said step of depositing by co-extrusion by adding an expanding agent.
32. The process according to claim 31 , wherein said step of expanding is effected during said step of depositing by co-extrusion by injecting a gas at a high pressure.