Electric cable with improved temperature ageing resistance

Abstract

An electric cable has at least one semi-conductive layer obtained from a polymer composition having at least one polypropylene-based thermoplastic polymer material, at least one first antioxidant and at least one metal deactivator.

Claims

1. Electric cable comprising: at least one elongated electrically conductive element; and at least one semi-conductive layer surrounding said elongated electrically conductive element, wherein the semi-conductive layer is obtained from a polymer composition comprising at least one polypropylene-based thermoplastic polymer material, at least one first antioxidant, and at least one metal deactivator, wherein the semi-conductive layer is a non-crosslinked layer, wherein the polymer composition further comprises a dielectric liquid wherein the polypropylene-based thermoplastic polymer material comprises an amount of less than 10% by weight of polar polymer(s), with respect to the total weight of the polypropylene-based thermoplastic polymer material.

2. Electric cable according to claim 1, wherein the first antioxidant is selected from hindered phenols, aromatic amines, and nitrogen-containing aromatic heterocyclics.

3. Electric cable according to claim 1, wherein the polymer composition comprises at least 0.3% by weight of the first antioxidant, based on the total weight of the polymer composition.

4. Electric cable according to claim 1, wherein the metal deactivator is selected from nitrogen-containing aromatic heterocycles, and aromatic compounds comprising at least one function —NH—C(═O)—.

5. Electric cable according to claim 1, wherein the polymer composition comprises at least 0.2% by weight of the metal deactivator, based on the total weight of the polymer composition.

6. Electric cable according to claim 1, wherein the polymer composition comprises at least 6% by weight of conductive filler, based on the total weight of the polymer composition.

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

8. Electric cable according to claim 7, wherein the polypropylene-based thermoplastic polymer material comprises a random propylene copolymer or a heterophasic propylene copolymer, as propylene copolymer P.sub.1.

9. Electric cable according to claim 1, wherein the polypropylene-based thermoplastic polymer material comprises a random propylene copolymer and a heterophasic propylene copolymer, or two different heterophasic propylene copolymers.

10. Electric cable according to claim 1, wherein the polypropylene-based thermoplastic polymer material can further comprise an olefin homopolymer or copolymer P.sub.2.

11. Electric cable according to claim 1, wherein the polypropylene-based thermoplastic polymer material comprises at least 50% by weight of propylene polymer(s), based on the total weight of the polypropylene-based thermoplastic polymer material.

12. Electric cable according to claim 1, wherein the polymer composition further comprises a second antioxidant different from the first antioxidant.

13. Electric cable according to claim 12, wherein the polymer composition comprises at least 0.2% by weight of the second antioxidant, based on the total weight of the polymer composition.

14. Electric cable according to claim 1, wherein the semi-conductive layer has a reduction in tensile strength after temperature ageing of at most 40%.

15. Electric cable according to claim 1, wherein the semi-conductive layer has a reduction in elongation at break after temperature ageing of at most 50%.

16. Electric cable according to claim 1, wherein said electric cable further comprises an electrically insulating layer.

17. Electric cable according to claim 1, wherein the dielectric liquid includes a mineral oil.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic view of an electric cable according to a preferred embodiment in accordance with the invention.

(2) For reasons of clarity, only the elements essential for understanding the invention have been represented schematically, and this not to scale.

DETAILED DESCRIPTION

(3) The medium- or high-voltage electric cable 1 conforming to the first subject of the invention, shown in FIG. 1, comprises a central elongated electrically conductive element 2, in particular of copper or aluminium. The electric cable 1 further comprises several layers arranged successively and coaxially around this central elongated electrically conductive element 2, namely: a first semi-conductive layer 3 known as the “internal semi-conductive layer”, an electrically insulating layer 4, a second semi-conductive layer 5 known as the “external semi-conductive layer”, a metal shield 6 for earthing and/or protection, and an outer protective sheath 7.

(4) The electrically insulating layer 4 is an extruded thermoplastic (i.e. non-crosslinked) layer.

(5) The semi-conductive layers 3 and 5 are thermoplastic (i.e. non-crosslinked) extruded layers obtained from the polymer composition as defined in the invention.

(6) The presence of the metal shield 6 and the outer protective sheath 7 is preferential, but not essential, as this cable structure per se is well known to the skilled person.

EXAMPLES

(7) 1. Polymer Compositions

(8) Compositions I1 and I2 according to the invention, i.e. comprising at least one polypropylene-based thermoplastic polymer material, at least one first antioxidant, and at least one metal deactivator, were compared to comparative compositions C1, C2 and C3, the composition C1 corresponding to a composition comprising a polypropylene-based thermoplastic polymer material, but not comprising a first antioxidant, and a metal deactivator; and the compositions C2 and C3 comprising a polypropylene-based thermoplastic polymer material and, a metal deactivator or a first antioxidant.

(9) Table 1 below lists the above-mentioned polymer compositions in which the amounts of the compounds are expressed as percentages by weight, based on the total weight of the polymer composition.

(10) TABLE-US-00001 TABLE 1 Polymer compositions C1 (*) C2 (*) C3 (*) I1 I2 random propylene 51.61 51.06 51.06 51.06 50.79 copolymer heterophasic propylene 10.75 10.64 10.64 10.64 10.58 copolymer linear low-density 10.75 10.64 10.64 10.64 10.58 polyethylene conductive filler 26.89 26.60 26.60 26.60 26.46 metal deactivator 0.00 0.00 1.06 0.35 0.53 first antioxidant 0.00 0.00 0.00 0.71 0.53 second antioxidant 0.00 1.06 0.00 0.00 0.53 (*) Comparative compositions not forming part of the invention

(11) The origin of the compounds in Table 1 is as follows: random propylene copolymer marketed by Borealis with the product name Bormed RB 845 MO; heterophasic propylene copolymer marketed by LyondellBasell Industries with the product name Adflex® Q 200F; linear low-density polyethylene marketed by ExxonMobil Chemicals with the product name LLDPE 1002 YB; conductive filler: carbon black marketed by Cabot with the product name Vulcan XC-500; metal deactivator: 1,2-bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyl) hydrazine (Irganox® 1024 or Irganox® MD 1024) marketed by BASF; first antioxidant: pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) (Irganox® 1010) marketed by BASF; and second antioxidant: polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ) marketed by Addivant with the product name Naugard Super Q for the composition C2; and tris(2,4-di-tert-butyl-phenyl)phosphite (Irgafos® 168) marketed by BASF for the compositions I1 and I2.

(12) 2. Preparation of Non-Crosslinked Layers and Cables

(13) The compositions listed in Table 1 are implemented as follows.

(14) The polymer components for forming the polypropylene-based thermoplastic polymer material as defined in the invention are metered for each of the compositions described above by loss-in-weight feeders in a continuous mixer. The polymer components are melted and then the conductive filler is added to the mixture of molten polymer components. The continuous mixer can be of the single-screw oscillating rotary co-kneader (“BUSS”) type, a twin-screw extruder, or any other mixer allowing a good dispersion and distribution of the conductive filler within the polypropylene-based thermoplastic polymer material.

(15) The resulting mixture in molten form is then extruded into rods, which are cooled, for example, in an elongated tank containing cold water. The rods, once cooled, are transformed into granules.

(16) For each of the compositions C2, C3, I1, and I2, the granules obtained previously are then introduced into a single-screw extruder equipped with a loss-in-weight feeder to meter them; and a very-high-precision loss-in-weight feeder to meter the first antioxidant, the second antioxidant, and/or the metal deactivator. The various components are melted, then the resulting mixture in the molten form is extruded into rods, which are cooled. The rods, once cooled, are transformed into granules.

(17) These granules for each of the compositions C1, C2, C3, C3, I1 and I2 can then be transformed into compression-moulded plates using a suitable mould and a heated hydraulic press, or transformed into strips using a single-screw extruder, typically with a thickness of about 1 mm, and a width of about 15 mm. Once cooled, these strips can be used to cut H2 dumbbell samples with a punch. The dumbbell samples are then used to test the mechanical properties, using a traction machine well known in the prior art, of the compositions as described above in their initial state, or after thermal ageing in air, for example in an oven. Each result represents the average value of at least 5 individual results each from a tested H2 sample. The tensile speed during mechanical tests is 25 mm/minute.

(18) The thermal ageing conditions selected are as follows: duration of about 240 hours (10 days), and isothermal and constant temperature of about 135° C.

(19) Tensile strength (TS) and elongation at break (EB) tests were carried out on materials in accordance with NF EN 60811-1-1, using an apparatus marketed under the product number 3345 by Instron.

(20) The results of each of these tests are reported in Table 2 (mechanical properties) below:

(21) TABLE-US-00002 TABLE 2 Properties C1 (*) C2 (*) C3 (*) I1 I2 TS (MPa) 21.7 21.4 22.0 22.0 20.1 EB (%) 421 449.5 435.6 417.4 411.5 TS after ageing (MPa) 6.4 15.8 18.3 16.9 14.7 EB after ageing (%) 6.7 160.3 126.1 287.5 277.6 (*) Comparative compositions not forming part of the invention

(22) All these results show that the presence of a first antioxidant and a metal deactivator in a polypropylene-based semi-conductive layer improves temperature ageing resistance. Without this combination of a first antioxidant and a metal deactivator, tensile strength and elongation at break drop during thermal ageing.