Electric cable with improved thermal conductivity

Abstract

A cable is provided having at least one electrically insulating layer obtained from a polymer composition with at least one polypropylene-based thermoplastic polymer material and at least one inorganic filler selected from aluminium oxide, a hydrated aluminium oxide, magnesium oxide, zinc oxide, and a mixture thereof; and a method for making the cable.

Claims

1. Electric cable comprising: at least one elongated electrically conducting element and at least one electrically insulating layer obtained from a polymer composition comprising at least one polypropylene-based thermoplastic polymer material and at least one inorganic filler, wherein the inorganic filler is selected from aluminium oxide, a hydrated aluminium oxide, magnesium oxide, zinc oxide, and a mixture thereof.

2. Electric cable according to claim 1, wherein the inorganic filler is aluminium oxide.

3. Electric cable according to claim 1, wherein the polymer composition comprises at least 1 wt % of inorganic filler, relative to the total weight of the polymer composition.

4. Electric cable according to claim 1, wherein the inorganic filler is in the form of particles ranging in size from 0.01 to 6 m.

5. Electric cable according to claim 1, wherein the polypropylene-based thermoplastic polymer material comprises a propylene copolymer P.sub.1.

6. Electric cable according to claim 5, wherein the propylene copolymer P.sub.1 is a random propylene copolymer or a heterophase propylene copolymer.

7. Electric cable according to claim 1, characterized in that the polypropylene-based thermoplastic polymer material comprises a random propylene copolymer and a heterophase propylene copolymer, or two different heterophase propylene copolymers.

8. Electric cable according to claim 5, wherein the polypropylene-based thermoplastic polymer material further comprises an olefin homopolymer or copolymer P.sub.2.

9. Electric cable according to claim 1, wherein the polymer composition further comprises a dielectric liquid.

10. Electric cable according to claim 1, wherein the electrically insulating layer is a non-crosslinked layer.

11. Electric cable according to claim 1, wherein the electrically insulating layer has a tensile strength before or after ageing of at least 8.5 MPa.

12. Electric cable according to claim 1, wherein the electrically insulating layer has an elongation at break before or after ageing of at least 250%.

13. Electric cable according to claim 1, wherein said electric further comprises: at least one semiconducting layer surrounding the elongated electrically conducting element, and at least one electrically insulating layer surrounding the elongated electrically conducting element, the electrically insulating layer being as defined in any one of the preceding claims.

14. Method of manufacturing an electric cable as defined in claim 1, wherein said method comprises at least one step 1) of extrusion of the polymer composition around the elongated electrically conducting element, to obtain an electrically insulating layer surrounding said elongated electrically conducting element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0188] FIG. 1 shows an electric cable in accordance with one embodiment.

DETAILED DESCRIPTION

Examples

[0189] FIG. 1 shows a schematic view of an electric cable according to a preferred embodiment of the invention.

[0190] For reasons of clarity, only the elements essential for understanding the invention have been shown schematically, and they are not drawn to scale.

[0191] The medium-voltage or high-voltage electric cable 1 according to the first object of the invention, illustrated in FIG. 1, comprises a central elongated electrically conducting element 2, notably made of copper or aluminium. The electric cable 1 further comprises several layers arranged successively and coaxially around this central elongated electrically conducting element 2, namely: a first semiconducting layer 3 called internal semiconducting layer, an electrically insulating layer 4, a second semiconducting layer 5 called external semiconducting layer, a metal screen 6 for earthing and/or protection, and an outer protective sheath 7.

[0192] The electrically insulating layer 4 is a non-crosslinked extruded layer, obtained from the polymer composition as defined in the invention.

[0193] The semiconducting layers 3 and 5 are thermoplastic (i.e. non-crosslinked) extruded layers.

[0194] The presence of the metal screen 6 and outer protective sheath 7 is preferred, but not essential, this cable structure being well known per se by a person skilled in the art.

[0195] Polymer Compositions

[0196] A composition I1 according to the invention, i.e. comprising at least one polypropylene-based thermoplastic polymer material and at least aluminium oxide as inorganic filler, was compared against a comparative composition C1, the composition C1 corresponding to a composition comprising a polypropylene-based thermoplastic polymer material identical to that used for the composition of the invention I1 but comprising kaolin as inorganic filler, instead of aluminium oxide.

[0197] Table 1 below presents polymer compositions, with the amounts of the compounds expressed in percentages by weight, relative to the total weight of the polymer composition.

TABLE-US-00001 TABLE 1 Polymer compositions C1 (*) I1 Heterophase propylene copolymer 15 29 Random propylene copolymer 44 29 High-density polyethylene 21.5 21.5 Inorganic filler: kaolin 15 0 Inorganic filler: aluminium oxide 0 15 Dielectric liquid 4.5 4.5 Antioxidant 1.0 1.0 (*) Comparative composition not forming part of the invention

[0198] The origin of the compounds in Table 1 is as follows: [0199] high-density polyethylene marketed under the reference Eltex A4009MFN1325 by the company Ineos and the density of which is 0.960 g/cm.sup.3 according to standard ISO 1183A at a temperature of 23 C. (MFI=0.9), and the elastic modulus is 1700 MPa; [0200] heterophase propylene copolymer marketed by the company Basell Polyolefins under the reference Adflex Q 200F; [0201] random propylene copolymer marketed by the company Borealis under the reference Bormed RB 845 MO; [0202] dielectric liquid comprising 95 wt % of a mineral oil marketed by the company Nynas under the reference Nytex 810, and 5 wt % of benzophenone marketed by the company Sigma-Aldrich under the reference B9300, relative to the total weight of the dielectric liquid; [0203] antioxidant marketed by the company Ciba under the reference Irganox B 225, which comprises an equimolar mixture of Irgafos 168 and Irganox 1010; [0204] chalk as inorganic filler marketed under the reference Omya EXH1, and [0205] aluminium oxide as inorganic filler marketed under the reference P122SB or Timal-12.

[0206] 2. Preparation of the Non-Crosslinked Layers

[0207] The compositions presented in Table 1 are used as follows.

[0208] The following constituents: mineral oil, antioxidant and benzophenone of compositions C1 and I1 referred to in Table 1, for each layer to be considered, are metered and mixed with stirring at about 75 C., to form a liquid mixture comprising the dielectric liquid.

[0209] The liquid mixture is then mixed with the following constituents: heterophase propylene copolymer, random propylene copolymer, high-density polyethylene compositions C1 and I1 referred to in Table 1, for each polymer layer to be considered, in a vessel. Then the resultant mixture and the inorganic filler, for each polymer layer to be considered, are homogenized using a twin-screw extruder (Berstorff twin screw extruder) at a temperature of about 145 to 180 C., and then melted at about 200 C. (screw speed: 80 rev/min).

[0210] The homogenized and melted mixture is then granulated.

[0211] The granules were then pressed hot to form layers in the form of plates.

[0212] Each of the polymer compositions C1 and I1 was prepared in this way in the form of layers with a thickness of 1 mm for evaluating their mechanical properties as well as layers with a thickness of 8 mm for carrying out the measurements of thermal conductivity.

[0213] These compositions C1 and I1 were then compared from the standpoint of their mechanical properties (tensile strength/elongation at break before and after ageing at 135 C. for 240 hours) and their thermal conductivity.

[0214] The tests of tensile strength (TS) and elongation at break (EB) were carried out on the materials according to standard NF EN 60811-1-1, using an instrument marketed under the reference 3345 by the company Instron.

[0215] The results corresponding to each of these tests are reported in Table 2 (mechanical properties) below:

TABLE-US-00002 TABLE 2 Properties C1 (*) I1 TS (MPa) 13.5 19.5 EB (%) 445 692 TS after ageing (MPa) 15.1 19.2 EB after ageing (%) 372.75 613.93 (*) Comparative composition not forming part of the invention

[0216] All these results show that incorporating an inorganic filler as defined in the invention in a polypropylene matrix improves the mechanical properties of the electrically insulating layer, notably in terms of tensile strength and elongation at break, including after ageing.

[0217] The tests of thermal conductivity were carried out on the materials according to the familiar method known by the English term Transient Plane Source or TPS, using an instrument marketed under the reference HOT DISK TPS 2500S by the company THERMOCONCEPT.

[0218] The results corresponding to these tests are reported in Table 3 (thermal conductivity) below:

TABLE-US-00003 TABLE 3 Properties C1 (*) I1 Thermal conductivity 0.28 0.31 at 40 C. (W/m .Math. K) (*) Comparative composition not forming part of the invention

[0219] The results for thermal conductivity show that the presence of an inorganic filler as defined in the invention in a polypropylene matrix leads to an electrically insulating layer having a thermal conductivity greater than that of an electrically insulating layer in which the inorganic filler is chalk.