Cable comprising a semiconductive layer with a smooth surface

Abstract

The present invention relates to an electric cable comprising at least one semiconductive layer obtained from a polymer composition comprising at least one homophasic propylene polymer and at least one homophasic copolymer of a C.sub.3-C.sub.6 olefin and ethylene.

Claims

1. An electric cable comprising: at least one elongated electrically conductive element, and at least one semiconductive layer surrounding said elongated electrically conductive element, wherein the semiconductive layer is obtained from a polymer composition comprising at least one homophasic propylene polymer, and at least one homophasic copolymer of a C.sub.3-C.sub.6 olefin and ethylene.

2. The electric according to claim 1, wherein the polymer composition comprises at least 6% by weight of conductive filler, relative to the total weight of the polymer composition.

3. The electric according to claim 1, wherein the homophasic propylene polymer is a copolymer of propylene and ethylene.

4. The electric according to any one of the preceding claim 1, wherein the polymer composition comprises at least 20% by weight of the homophasic propylene polymer, relative to the total weight of the polymer composition.

5. The electric according to claim 1, wherein the polymer composition comprises at most 80% by weight of the homophasic propylene polymer, relative to the total weight of the polymer composition.

6. The electric according to claim 1, wherein the homophasic copolymer of a C.sub.3-C.sub.6 olefin and ethylene is a homophasic copolymer of propylene and ethylene.

7. The electric according to claim 1, wherein the polymer composition comprises at least 10% by weight of the homophasic copolymer of a C.sub.3-C.sub.6 olefin and ethylene relative to the total weight of the polymer composition.

8. The electric according to claim 1, wherein the polymer composition comprises at most 50% by weight of the polymer of the homophasic copolymer of a C.sub.3-C.sub.6 olefin and ethylene, relative to the total weight of the polymer composition.

9. The electric according to claim 1, wherein the homophasic copolymer of a C.sub.3-C.sub.6 olefin and ethylene is obtained by a copolymerization process using a metallocene catalyst.

10. The electric according to claim 1, wherein the homophasic propylene polymer and the homophasic copolymer of a C.sub.3-C.sub.6 olefin and ethylene represent at least 50% by weight, relative to the total weight of polymers in the polymer composition.

11. The electric according to claim 1, wherein the ethylene of the homophasic copolymer of a C.sub.3-C.sub.6 olefin and ethylene represents at most 25 mol % relative to the total number of moles of homophasic copolymer of a C.sub.3-C.sub.6 olefin and ethylene.

12. The electric according to claim 1, wherein the polymer composition also comprises a dielectric fluid.

13. The electric according to claim 1, wherein the semiconductive layer is a non-crosslinked layer.

14. The electric according to claim 1, wherein said electric cable also comprises an electrically insulating layer surrounding the elongated electrically conductive element.

15. The electric according to claim 14, wherein the electrically insulating layer surrounds the semiconductive layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0173] The attached drawings illustrate the invention:

[0174] FIG. 1 schematically represents a structure, in cross section, of a cable in accordance with the invention according to a first embodiment.

[0175] Other characteristics and advantages of the present invention will emerge in the light of the examples that follow with reference to the annotated figures, said examples and figures being given for illustrative purposes and not being in any way limiting.

[0176] FIG. 1 is a schematic view of an electric cable according to a preferred embodiment in accordance with the invention.

[0177] For the sake of clarity, only the elements that are essential for the understanding of the invention have been represented schematically, and are not to scale.

[0178] The medium-voltage or high-voltage electric cable 1 in accordance with the first subject of the invention, illustrated in FIG. 1, comprises a central elongated electrically conductive element 2, notably made of copper or aluminium. The electric cable 1 also comprises several layers arranged successively and coaxially around this central elongated electrically conductive element 2, namely: a first semiconductive layer 3, referred to as the “inner semiconductive layer”, an electrically insulating layer 4, a second semiconductive layer 5, referred to as the “outer semiconductive layer”, an earthing and/or protective metal shield 6, and an outer protective sheath 7.

[0179] The electrically insulating layer 4 is a thermoplastic (i.e. non-crosslinked) extruded layer.

[0180] The semiconductive layer 3 is a thermoplastic (i.e., non-crosslinked) extruded layer obtained from the polymer composition as defined in the invention.

[0181] The semiconductive layer 5 is a thermoplastic (i.e. non-crosslinked) extruded layer.

[0182] The presence of the metal shield 6 and of the outer protective sheath 7 is preferential, but not essential, this cable structure being, per se, well known to those skilled in the art.

EXAMPLE

[0183] Polymer Compositions

[0184] Table 1 below shows a polymer composition in which the amounts of the compounds are expressed as weight percentages relative to the total weight of the polymer composition.

[0185] Composition I1 is a polymer composition in accordance with the invention.

TABLE-US-00001 TABLE 1 Polymer composition I1 Homophasic propylene polymer 36.00 Homophasic copolymer of a C.sub.3-C.sub.6 32.00 olefin and ethylene Conductive filler 30.00 Antioxidant 1.50 Metal deactivator 0.50

[0186] The origin of the constituents collated in table 1 is as follows: [0187] Homophase propylene polymer is a random copolymer of propylene and ethylene, sold by the company Borealis under the reference Bormed® RB 845 MO; [0188] Homophasic copolymer of a C.sub.3-C.sub.6 olefin and ethylene is a copolymer of propylene and ethylene, sold by the company Dow under the reference Versify 2300; [0189] Conductive filler is a furnace black, sold by the company Cabot, under the reference Vulcan XC-500; [0190] Antioxidant is an antioxidant sold under the reference Irganox B225; [0191] Metal deactivator is a metal deactivator sold under the reference Irganox MD1024; and [0192] Dielectric fluid is sold by the company Nynas under the reference Nyflex 210B.

[0193] Preparation of a Strip Obtained from Polymer Composition I1

[0194] A strip 0.3 mm thick was extruded on a single-screw extruder equipped with a flat die to enable a surface state test to be performed. The extrusion temperatures are chosen according to the implementation properties of the polymer matrix and so as to obtain an extruded strip showing practically no deformation coming from the polymer matrix itself (e.g. non-molten matter, gels, particles coming from undesired crosslinking, or particles coming from degradation of one of the polymers of the polymer matrix). In addition, special care is taken to avoid deformations caused by the release of volatile substances that may be contained in the polymer composition. This thus makes it possible to measure protuberances or deformations mainly related to the process of dispersion and distribution of the conductive filler in the polymer matrix.

[0195] Characterization of the Surface State of the Strips

[0196] The test was performed as follows: the extruded strip obtained above is maintained under a constant mechanical tension by a system of rollers at a regulated speed and placed in motion by a winder. The strip thus advances into a measuring zone of an optical detection system consisting of a light source on one side of the measuring zone and a camera on the other side of the measuring zone.

[0197] The orientation of the detection system with respect to the moving strip surface is tangential. The in-line camera coupled to a computer simultaneously records images of the extruded strip surface and performs image analysis. The result is a detailed description of the number of defects present on the surface of the strip, classified by size and shape. The measurement is done by reflection. The results obtained are presented in Table 2 below and indicate the number of defects or protuberances per m.sup.2.

[0198] Results

[0199] The results of the abovementioned surface state test, and of other mechanical and electrical tests, are collated in Table 2 below.

[0200] The tensile strength and elongation at break tests are performed according to the standard NF EN 60811-1-1, using a device sold under the reference 3345 by the company Instron. The tensile strength and the elongation at break are measured by means of a tensile test on an H2 dumbbell specimen, in particular at a tensile speed of 25 mm/min; in the initial state, or after thermal ageing in air, for example in an oven. The thermal ageing conditions chosen are as follows: duration of about 240 hours (10 days), and isothermal and constant temperature of about 135° C.

TABLE-US-00002 TABLE 2 Characteristics I1 Number of protuberances per m.sup.2 6 Tensile strength [MPa] 25.5 Elongation at break [%] 712 Tensile strength [MPa] after 240 hours at 25.8 135° C. in the oven in air Elongation at break [%] after 240 hours at 605 135° C. in the oven in air Volume resistivity [Ohm .Math. m] at 25° C. 6.1 × 10.sup.−2 Volume resistivity [Ohm .Math. m] at 100° C. 2.6 × 10.sup.−1

[0201] These results as a whole show that the semiconductive layer of the invention has a good surface state, notably a smooth appearance and a very low number of protuberances, while at the same time ensuring good mechanical and electrical properties.