CABLE COMPRISING AN EASILY PEELABLE SEMI-CONDUCTIVE LAYER

Abstract

An electrical cable has a cross-linked semi-conductive layer obtained from a cross-linkable polymer composition based on at least one non-polar olefin polymer, and at least 15% by weight of a butene polymer, with respect to the total weight of the cross-linkable polymer composition.

Claims

1. Electrical cable comprising: at least one elongated electrically conducting element; an electrically insulating layer surrounding the elongated electrically conducting element; and a semi-conductive layer surrounding the electrically insulating layer, wherein the semi-conductive layer is a cross-linked layer obtained from a cross-linkable polymer composition comprising at least one non-polar olefin polymer and at least 15% by weight of a butene polymer, with respect to the total weight of the cross-linkable polymer composition.

2. Electrical cable according to claim 1, wherein the butene polymer is a copolymer of 1-butene.

3. Electrical cable according to claim 1, wherein the butene polymer is a copolymer of 1-butene and an olefin selected from ethylene, propylene and C.sub.5-C.sub.12 olefins.

4. Electrical cable according to claim 1, wherein the butene polymer is a homopolymer of 1-butene.

5. Electrical cable according to claim 1, wherein the butene polymer has a molecular weight of at least 10000 g/mol.

6. Electrical cable according to claim 1, wherein the cross-linkable polymer composition comprises at least 6% by weight of a conductive filler, with respect to the total weight of the cross-linkable polymer composition.

7. Electrical cable according to claim 1, wherein the non-polar olefin polymer is selected from homopolymers and copolymers of ethylene.

8. Electrical cable according to claim 1, wherein the non-polar olefin polymer is selected from a linear very low density polyethylene, a linear low density polyethylene, a low density polyethylene, a copolymer of ethylene and octene, and a blend thereof.

9. Electrical cable according to claim 1, wherein the cross-linkable polymer composition comprises at least 20% by weight of butene polymer, with respect to the total weight of the cross-linkable polymer composition.

10. Electrical cable according to claim 1, wherein the cross-linkable polymer composition comprises at least 15% by weight of non-polar olefin polymer, with respect to the total weight of the cross-linkable polymer composition.

11. Electrical cable according to claim 1, wherein the cross-linkable polymer composition comprises at least 50% by weight of polymer(s), with respect to the total weight of the cross-linkable polymer composition.

12. Electrical cable according to claim 1, wherein the cross-linkable polymer composition comprises at most 30% by weight of polar polymer(s), with respect to the total weight of polymer(s) in the cross-linkable polymer composition.

13. Electrical cable according to claim 1, wherein the cross-linkable polymer composition further comprises at least one cross-linking agent.

14. Electrical cable according to claim 1, wherein the cross-linkable polymer composition further comprises at least one antioxidant.

15. Electrical cable according to claim 1, wherein the electrically insulating layer comprises at least one olefin polymer.

16. Electrical cable according to claim 1, wherein said electrical cable additionally comprises a further semi-conductive layer surrounding the elongated electrically conducting element which is in turn surrounded by the electrically insulating layer.

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0122] Other characteristics and advantages of the present invention will become apparent in light of the description of a non-limiting example of an electrical cable in accordance with the invention, made with reference to FIG. 1 which shows a diagrammatic perspective sectional view of an electrical cable according to a preferred embodiment in accordance with the invention.

[0123] For the purposes of clarity, only the elements which are essential to comprehension of the invention are shown in a diagrammatic manner, and not to scale.

DETAILED DESCRIPTION

[0124] The cable is a medium or high voltage power cable 1, illustrated in FIG. 1, and it comprises a central elongated electrically conducting element 2, in particular formed from copper or aluminium and, successively and coaxially around this element 2, it comprises a first semi-conductive layer 3 termed the inner semi-conductive layer, an electrically insulating layer 4, a second semi-conductive layer 5 termed the outer semi-conductive layer, a metallic screen 6 of the cylindrical tube type, and an outer protective sheath 7.

[0125] The outer semi-conductive layer 5 of FIG. 1 is a semi-conductive layer in accordance with the invention.

[0126] In the cable of FIG. 1, the layers 3, 4 and 5 are typically extruded polymeric layers.

[0127] The presence of the metallic screen 6 and the outer protective sheath 7 is preferred, but is not essential; this type of structure of a cable per se is well known to a person skilled in the art.

Example

1. Preparation of Semi-Conductive Layers

[0128] In order to demonstrate the properties of the invention, a semi-conductive layer was produced from a cross-linkable polymer composition in accordance with the invention, i.e. comprising at least one non-polar olefin polymer, and at least 15% by weight of a butene polymer, and was compared with a semi-conductive layer produced from a comparative cross-linkable polymer composition C1 of the prior art comprising no butene polymer.

[0129] Table 1 below brings together the cross-linkable polymer compositions I1 and C1, wherein the quantities of the compounds are expressed as percentages by weight with respect to the total weight of the composition.

TABLE-US-00001 TABLE 1 Composition C1 (*) I1 Non-polar olefin polymer 0.0 42.6 Butene polymer 0.0 25.5 EVA 36.0 0.0 NBR 17.9 0.0 Polyethylene wax 3.8 0.0 Chalk 6.0 0.0 Stearyl erucamide 0.9 0.0 Antioxidant 1.0 1.5 Conductive filler 33.8 29.8 Organic peroxide 0.6 0.6 Total ingredients in composition 100 100 (*) Comparative composition not forming part of the invention

[0130] The origins of the compounds of Table 1 were as follows: [0131] non-polar olefin polymer: linear low density polyethylene sold by Versalis with the reference Clearflex FGH B0; [0132] butene polymer: butene homopolymer sold by Lyondellbasell with the reference PB 0300M; [0133] EVA: copolymer of ethylene and vinyl acetate sold by Exxon Mobil Chemicals with the reference Escorene Ultra UL00728; [0134] NBR: nitrile rubber sold by Arlanxeo with the reference Baymod N 34.82; [0135] polyethylene wax sold by Clariant with the reference Licowax PE 520; [0136] chalk sold by Omya with the reference Omcarb EXH-1 SP; [0137] stearyl erucamide sold by PMC with the reference Biogenix Kemamide E-180; [0138] antioxidant: polymerized 2,2,4-trimethyl-1,2 dihydroquinoline (TMQ) sold by Lanxess with the reference Vulkanox HS/LG Low Salt; [0139] conductive filler: carbon black sold by Orion with the reference Printex MV (composition I1) or by Birla with the reference Conductex 7095 Ultra (composition C1); and [0140] organic peroxide: tert-butyl cumyl peroxide sold by NOURYON with the reference Trigonox T.

[0141] With the exception of the liquid organic peroxide, all of the constituents of the various compositions of Table 1 could be mixed in a continuous mixture of the BUSS co-kneader type, twin screw extruder type or another type of mixer which is appropriate for rubber blends or filled thermoplastic blends.

[0142] As is conventional, the polymer materials were fused in the continuous mixer, the other constituents of the mixture were introduced and dispersed in the fused polymeric matrix, and the mixture was homogenized.

[0143] The mixture was then extruded in the form of rods. The rods were then cooled and dried in order to be formed into granules. These granules were then impregnated with liquid organic peroxide.

[0144] The process for the impregnation of the granules means that in particular, the peroxide is added after forming the granules, thereby preventing the peroxide from starting to cross-link the fused polymeric matrix and forming a polymer gel (a phenomenon which is known as scorching). In particular, the peroxide is preferably introduced into a phase in the process where the temperatures are not too high.

[0145] These impregnated granules were placed in suitable moulds in order to be able to form cross-linked sheets by compression and by heat treatment. In order to form these circular cross-linked sheets, a DK 60 automatic heating press was used under the conditions shown in Table 2 below, steps 1 to 4 being carried out successively.

[0146] Typically, a moulding cycle comprised the following steps: [0147] the granules were placed in the mould provided with non-stick Teflon films, [0148] the mould was closed by its cover and was placed on the press, pre-heated to 60 C., in order to bring it up to temperature, [0149] when the mould was at 60 C., the press was placed under pressure, [0150] the mould was heated to 130 C. in order to allow cross-linking, [0151] the mould was cooled and the cross-linked sheets were unmoulded.

TABLE-US-00002 TABLE 2 Steps Step 1 Step 2 Step 3 Step 4 Temperature of 130 130 130 50 press ( C.) Pressure of 6 12 12 12 press (bar) Duration 4 1 0.5 4 (minutes)

[0152] Once step 4 had been carried out, the circular sheet was unmoulded then cooled to ambient temperature. It had a thickness of between 0.5 and 1.5 mm.

2. Preparation of Electrically Insulating Layers

[0153] Two electrically insulating compositions termed reference, 12 and 13, were prepared using a process like that described above with a continuous mixer, then the granules were formed using techniques which are well known to a person skilled in the art.

[0154] The composition 12 comprised 98.23% by weight of a low density polyethylene sold by INEOS with the reference BPD 2000, 0.22% by weight of 4,6-bis(octylthiomethyl)-o-cresol sold by BASF with the reference Irgastab KV 10 as the antioxidant, and 1.55% by weight of tert-butyl cumyl peroxide sold by NOURYON with the reference Trigonox T as the organic peroxide, with respect to the total weight of the composition.

[0155] The composition 13 was a composition sold by Borealis with the reference Borlink LH4201R, comprising a low density polyethylene and a copolymer of ethylene and a polar co-monomer.

[0156] These granules were then pressed in order to form circular sheets with the aid of a DK60 heating press under the conditions brought together in Table 3 below, steps 1 to 4 being carried out successively:

TABLE-US-00003 TABLE 3 Steps Step 1 Step 2 Step 3 Step 4 Temperature of 130 130 130 50 press ( C.) Pressure of 3 12 12 12 press (bar) Duration 4 1 0.5 4 (minutes)

[0157] Once step 4 had been carried out, the circular sheet was unmoulded then cooled to ambient temperature. It had a thickness of between 0.5 and 1.5 mm.

3. Preparation of Double Layers

[0158] A first circular sheet of electrically insulating layer and a second circular sheet of semi-conductive layer obtained as above were positioned on top of each other in order to form a double layer and placed in a DK 60 heating press under the conditions brought together in Table 4 below; steps 1 to 3 being carried out successively:

TABLE-US-00004 TABLE 4 Steps Step 1 Step 2 Step 3 Temperature of 190 190 50 press ( C.) Pressure of 2 3 3 press (bar) Duration 5 2 4 (minutes)

[0159] During the preparation of the double layer, before placing it in the press, a polyester film was positioned between a portion of the two layers in order to be able to carry out the peelability test subsequently by easily separating the portion of the two layers separated by said film.

[0160] Once step 3 had been carried out, the double layer was unmoulded then cooled to ambient temperature. It had a thickness or between 1 and 3 mm.

[0161] The peelability test was carried out with the aid of an Alpha Technologies T2000 tensile testing machine, comprising the following steps: [0162] rectangles of 200 mm by 10 mm were cut out from the circular double layer, [0163] approximately 25 mm in length of the semi-conductive layer out of the 200 mm of the double layer was separated from the electrically insulating layer because of the polyester film which had been positioned in a portion of the double layer beforehand in order to obtain separation of the double layer in this portion, [0164] the end of the separated portion of the semi-conductive layer was fixed to a stationary gripping means of the tensile testing machine, while the end of the portion of the separated electrically insulating layer was fixed to a movable gripping means of the tensile testing machine, and [0165] the force necessary to separate the semi-conductive layer from the electrically insulating layer was measured over a separation distance of at least 100 mm with an angle of 180 and a tensile speed of 25050 mm/min.

[0166] Five specimens were tested per double layer formed.

[0167] The peelability results (in Newton) are brought together in Table 5 below.

TABLE-US-00005 TABLE 5 Double Double Double Double layer layer layer layer c1/i2 (*) c1/i3 (*) i1/i2 i1/i3 Peelability 5.79 19.13 7.87 4.27 (N) (*) Comparative double layers which do not form part of the invention

[0168] This set of results shows that the semi-conductive layer of the cable of the invention has very good peelability, irrespective of whether the nature of the subjacent electrically insulating layer is polar or non-polar; this is in contrast to that of the prior art.