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AU2012201230A1 - Medium-Tension or High-Tension Electrical Cable - Google Patents

Medium-Tension or High-Tension Electrical Cable Download PDF

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Publication number
AU2012201230A1
AU2012201230A1 AU2012201230A AU2012201230A AU2012201230A1 AU 2012201230 A1 AU2012201230 A1 AU 2012201230A1 AU 2012201230 A AU2012201230 A AU 2012201230A AU 2012201230 A AU2012201230 A AU 2012201230A AU 2012201230 A1 AU2012201230 A1 AU 2012201230A1
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Prior art keywords
layer
cable according
polyolefin
composition
crosslinking
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AU2012201230A
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Jerome Alric
Yannick Goutille
Jean-Michel Marty
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Nexans SA
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Nexans SA
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    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01BCABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B3/00Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties
    • H01B3/18Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances
    • H01B3/30Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes
    • H01B3/44Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes vinyl resins; acrylic resins
    • H01B3/441Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes vinyl resins; acrylic resins from alkenes

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  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Compositions Of Macromolecular Compounds (AREA)
  • Organic Insulating Materials (AREA)

Abstract

MEDIUM-TENSION OR HIGH-TENSION ELECTRICAL CABLE Abstract of the disclosure The present invention relates to an electrical cable (1) comprising an electrical conductor (2), a first semiconductive layer (3) surrounding the electrical conductor (2), a second electrically insulating layer (4) surrounding the first layer (3), and a third semi conductive layer (5) surrounding the second layer (4), characterized in that at least one of these three layers (3, 4, 5) is a crosslinked layer obtained from a crosslinkable composition comprising at least one polyolefin, characterized in that the composition also comprises an aliphatic peroxide as crosslinking agent. Figure to be published : Figure 1. .... .... .. 5 3. . . .. . . . 2 Fig.1..

Description

1 Medium-tension or high-tension electrical cable The present invention relates to an electrical cable. It applies typically, but not exclusively, to the 5 fields of medium-tension (especially from 6 to 45-60 kV) or high-tension (especially greater than 60 kV, which may be up to 800 kV) power cables, whether they are DC or AC cables. Medium-tension or high-tension power cables 10 typically comprise a central electrical conductor and, successively and coaxially around this electrical conductor, a semiconductive inner layer, an electrically insulating intermediate layer and a semiconductive outer layer, these three layers being crosslinked via 15 techniques that are well known to those skilled in the art. Conventionally, these three crosslinked layers are obtained from a composition based on a polyethylene polymer matrix combined with an organic peroxide such as 20 dicumyl peroxide or tert-butylcumyl peroxide (i.e. cumyl peroxide) . During the crosslinking of said compositions, this type of peroxide decomposes and forms crosslinking byproducts especially such as methane, acetophenone, cumyl alcohol, acetone, tert-butanol, a-methylstyrene 25 and/or water. These last two byproducts are formed by the dehydration reaction of cumyl alcohol. The formation of water from cumyl alcohol is relatively slow and may take place after several months, or even several years, once the cable is in operational 30 configuration. The risk of breakdown of the crosslinked 'Layers is thus significantly increased. Furthermore, if the methane formed during the crosslinking step is not removed from the crosslinked 2.ayers, risks associated with the explosiveness of 35 methane and its flammability cannot be ignored. This gas 2 may also cause damage once the cable comes into service. When the semiconductive outer layer is surrounded by a metal shield, which is generally the case in the structure of medium-tension and high-tension cables, said 5 gas can only diffuse longitudinally along the cable up to the junctions and terminals of the electrical installation (i.e. power accessories). The methane may thus accumulate and exert a pressure on the power accessories, which may lead to an electrical breakdown. 10 Although solutions exist for limiting the presence of methane within a cable, for instance heat-treating the cable in order to accelerate the diffusion of methane out of the cable, they become long and expensive when the crosslinked layers are thick. 15 Document US 5 252 676 presents a solution for limiting the crosslinking by-products originating from the crosslinking agents, in electrical power cables. To do this, it recommends decreasing the amount of crosslinking agent in order for the amount of gas and 20 water released during the decomposition of the crosslinking agent not to be too great, while at the same time continuing to use tert-butylcumyl peroxide (IPC) as crosslinking agent in the manufacture of the three crosslinked layers. Nevertheless, the amount of water 25 remains substantially large, and an excessive decrease in the amount of peroxide tends to degrade the thermomechanical properties of the crosslinked layers. The aim of the present invention is to overcome the drawbacks of the prior art by proposing a medium-tension 30 or high-tension electrical cable, comprising a crosslinked layer whose manufacture significantly limits the presence of crosslinking byproducts, such as for example methane and/or water, while at the same time providing optimum thermomechanical properties, such as 35 the hot creep, which are characteristic of correct 3 crosslinking of said layer. One subject of the present invention is an electrical cable comprising an electrical conductor, a first semiconductive layer surrounding the electrical 5 conductor, a second electrically insulating layer surrounding the first layer, and a third semiconductive layer surrounding the second layer, characterized in that at least one of these three layers is a crosslinked layer obtained from a crosslinkable composition comprising at 10 least one polyolefin, characterized in that the composition also comprises an aliphatic peroxide as crosslinking agent. The crosslinking agent of the invention has the advantage of not forming cumyl alcohol as crosslinking 15 byproduct during the crosslinking of the crosslinkable composition, and thus makes it possible to significantly limit the presence of water in the crosslinked layer, while at the same time maintaining very good thermomechanical properties. The thermomechanical 20 properties for the crosslinked layer according to the invention may advantageously be reflected by a maximum hot elongation under stress according to standard NF EN 60811-2-1 of not more than 100%, preferably not more than 80% and particularly preferably ranging from 60% to 80%. 25 In addition, during the decomposition of the aliphatic peroxide, the amount of methane formed is smaller than that formed during the decomposition of cumyl peroxide: the presence of methane in the crosslinking by-products is thereby advantageously 30 decreased. For substantially identical thermomechanical properties, the amount of aliphatic peroxide necessary is smaller than the amount of cumyl peroxide. The aliphatic peroxide of the invention is a peroxide which does not comprise any aromatic groups. The 35 aliphatic peroxide may be, in particular, an aliphatic 4 peroxide comprising at least one tertiary alkyl group. The aliphatic peroxides of the invention that may be mentioned include: - aliphatic peroxycarbonates, for instance tert 5 amylperoxy 2-ethylhexyl carbonate, tert-amyl peroxy 2-ethylhexyl carbonate, tert-butylperoxy isopropyl carbonate; - aliphatic peroxides of tertiary dialkyl, for instance 1,1-bis(tert-butylperoxy)cyclohexane, 10 2,5-dimethyl-2,5-bis(tert-butylperoxy)-3-hexyne, 2,5-dimethyl-2,5-bis(tert-butylperoxy)-3-hexane, di-tert-amyl peroxide, di-tert-butyl peroxide, cyclic peroxides such as 3,6,9-triethyl-3,6,9 trimethyl-1,4,7-triperoxonane; 15 - aliphatic peroxyacetals, for instance butyl 4,4-bis(tert-butylperoxy)valerate; and - aliphatic peroxyesters, for instance tert-butyl peroxyacetate, tert-amyl peroxyacetate. Among the mentioned peroxides, aliphatic peroxides 20 of tertiary dialkyl will preferably be used. Peroxide of this type has a very good compromise between rate of crosslinking and risk of burn-out or of precrosslinking during the implementation of the composition. The peroxide-route crosslinking of the 25 crosslinkable composition according to the invention may be performed under the action of heat and pressure, for example using a vulcanization tube under nitrogen pressure, this crosslinking technique being well known to those skilled in the art. 30 The crosslinkable composition of the invention may comprise not more than 3.00 parts by weight of aliphatic peroxide per 100 parts by weight of polymer(s) in the composition; preferably 1.50 parts by weight of aliphatic peroxide per 100 parts by weight of polymer(s) in the 35 composition; preferably 1.25 parts by weight of aliphatic 5 peroxide per 100 parts by weight of polymer (s) in the composition; and particularly preferably 1.10 parts by weight of aliphatic peroxide per 100 parts by weight of polymer(s) in the composition. 5 Preferably, the crosslinkable composition does not comprise any aromatic peroxide, especially such as dicumyl peroxides or derivatives thereof. The term "polyolefin" per se generally means olefin homopolymer or copolymer. It may especially denote a 10 thermoplastic polymer or an elastomer. Preferably, the olefin polymer is an ethylene homopolymer or an ethylene copolymer. Examples of ethylene polymers that may be mentioned include linear low-density polyethylene (LLDPE), low 15 density polyethylene (LDPE), medium-density polyethylene (MDPE), high-density polyethylene (HDPE), copolymers of ethylene and vinyl acetate (EVA), copolymers of ethylene and butyl acrylate (EBA), of methyl acrylate (EMA), of 2 hexylethyl acrylate (2HEA), copolymers of ethylene and of 20 a-olefins, for instance polyethylene-octenes (PEO), polyethylene-butenes (PEB), copolymers of ethylene and of propylene (EPR), for instance ethylene-propylene-diene terpolymers (EPDM), and mixtures thereof. It will be preferred to use a low-density 25 polyethylene (LDPE) since it has good rheological properties for its implementation, especially by extrusion, and very good thermomechanical and electrical properties. The term "low density" means a density that may 30 range especially from 0.910 to 0.940 g/cm2, and which may preferably range from 0.910 to 0.930 g/cm 3 according to standard ISO 1183 (at a temperature of 23 0 C). Typically, the low-density polyethylene (LDPE) may be obtained via a polymerization process in a high 35 pressure tubular reactor or in an autoclave reactor.
6 The crosslinkable composition may comprise more than 50.0 parts by weight of polyolefin per 100 parts by weight of polymer(s) (i.e. polymer matrix) in the composition, preferably at least 70 parts by weight of 5 polyolefin per 100 parts by weight of polymer(s) in said composition, and particularly preferably at least 90 parts by weight of polyolefin per 100 parts by weight of polymer(s) in said composition. In a particularly advantageous manner, the 10 crosslinkable composition comprises a polymer matrix that is composed solely of a polyolefin or a mixture of polyolefins. The crosslinkable composition of the invention may also comprise at least one crosslinking coagent of 15 multifunctional type. This crosslinking coagent comprises at least two reactive functions, which are identical or different, which are capable firstly of grafting to the polyolefin, and secondly of participating in the crosslinking of the polyolefin (i.e. the formation of the 20 three-dimensional network of the crosslinked polyolefin), in the presence of the aliphatic peroxide of the invention. Preferably, said at least two reactive functions of the crosslinking coagent are unsaturated functions. 25 Particularly preferably, at least one of said reactive functions is a vinyl function, especially an ethylenic function of the type CH 2 =CH-. Even more preferably, said two reactive functions are vinyl functions, especially ethylenic functions of 30 the type CH 2 =CH-. The crosslinking coagent can especially significantly reduce the proportion of peroxide to be used in the crosslinkable composition, while at the same time maintaining good theromechanical properties such as 35 the hot creep, and also a satisfactory rate of 7 crosslinking. Preferably, a coagent whose boiling point is sufficiently high, such that it does not evaporate during the step of implementation of the crosslinkable 5 composition, especially by extrusion, will be used. By way of example, the crosslinking coagent may be chosen from 1,3-hexadiene; 1,4-hexadiene; 1,5-hexadiene; 2,3-dimethyl-1,3-butadiene; 2,4-hexadiene; 2-methyl-1,4 pentadiene; 3-methyl-1,3-pentadiene; 4-methyl-1,3 10 pentadiene; 1,6-heptadiene; 2,4-dimethyl-1,3-pentadiene; 2-methyl-1,5-hexadiene; 4-vinyl-l-cyclohexene; 1,7 octadiene; 2,5-dimethyl-1,5-hexadiene; 2,5-dimethyl-2,4 hexadiene; 5-ethylidene-2-norbornene; 5-vinyl-2 norbornene; 1,8-nonadiene; 7-methyl-1,6-octadiene; 1,5,9 15 decatriene; 2,6-dimethyl-2,4,6-octatriene; dipentene; 7 methyl-3-methylene-1,6-octadiene; 1,9-decadiene; 3,9 divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane; 1,2,4 trivinylcyclohexane; 1,13-tetradecadiene; 2,3-diphenyl 1,3-butadiene; trans,trans-1,4-diphenyl-1,3-butadiene; 20 1,15-hexadecadiene; 1,6-diphenyl-1,3,5-hexatriene; 2,3 dibenzyl-1,3-butadiene; and polybutadiene; or a mixture thereof. The coagent concentration is preferably limited so as not to disrupt the process of extrusion of the 25 crosslinkable composition of the invention. For example, the crosslinkable composition may comprise not more than 3 parts by weight of crosslinking coagent per 100 parts of polymer(s) in the crosslinkable composition. It will be preferred to use from 0.5 to 2 parts by weight of 30 coagent per 100 parts by weight of polymer(s) in the crosslinkable composition. The crosslinkable composition of the invention may also comprise an aromatic compound comprising at least one aromatic nucleus, and a single reactive function 35 capable of grafting to the polyolefin. Preferably, said 8 reactive function of the aromatic compound is a vinyl function. As a result, this aromatic compound does not participate in the crosslinking of the polyolefin, in contrast with the crosslinking coagent, when it is 5 present in the crosslinkable composition. The crosslinked layer obtained from this crosslinkable composition has reinforced and durable properties in the field of electrical cables, offering better resistance to water treeing. More particularly, 10 this concerns the resistance to electrical breakdown, and especially the capacity to dissipate the space charges that accumulate especially in high-tension cables under direct current. The aromatic compound may be chosen from styrene, 15 styrene derivatives and isomers thereof. Examples of styrene derivatives that may be mentioned include the compounds having the following general formula: X R 20 in which X is a hydrogen, an alkyl group or an aryl group; and R is either a hydrogen, an alkyl group or an aryl group. More particularly, mention may be made of 4-methyl-2,4-diphenylpentane, and triphenylethylene. In the context of the present invention, styrene 25 derivatives of the polycyclic aromatic hydrocarbon (PAH) type may also be considered. More particularly, mention may be made of vinylnaphthalenes, for instance 2-vinyl naphthalene; vinylanthracenes, for instance 9-vinyl anthracene or 2-vinylanthracene; and vinylphenanthrenes, 30 for instance 9-vinylphenanthrene. The grafting of these aromatic compounds onto the polymer chain of the polyolefin is typically performed 9 during the phase of crosslinking of the polyolefin, according to a radical addition mechanism that is well known to those skilled in the art, in the presence of the tertiary aliphatic alkyl peroxide of the invention. 5 The crosslinkable composition according to the invention may also comprise at least one protective agent such as an antioxidant. Antioxidants protect the composition against the thermal constraints generated during the manufacturing steps of the cable or during the 10 functioning of the cable. The antioxidants are preferably chosen from: - sterically hindered phenolic antioxidants such as tetrakismethylene(3,5-di-t-butyl 4-hydroxyhydro cinnamate)methane, octadecyl 3-(3,5-di-t-butyl-4-hydroxy 15 phenyl)propionate, 2,2'-thiodiethylenebis[3-(3,5-di-t butyl-4-hydroxyphenyl) propionate], 2,2'-thiobis(6-t butyl-4-methylphenol), 2,2'-methylenebis(6-t-butyl 4-methylphenol), 1,2-bis(3,5-di-t-butyl-4-hydroxyhydro cinnamoyl)hydrazine, [2,2'-oxamidobis(ethyl 3-(3,5-di-t 20 butyl-4-hydroxyphenyl)propionate) and 2,2'-oxamidobis [ethyl 3-(t-butyl-4-hydroxyphenyl)propionate]; - thioethers such as 4,6-bis(octylthiomethyl) o-cresol, bis[2-methyl-4-{3-n-(C12 or C14)alkylthio propionyloxy}-5-t-butylphenyl] sulfide and thiobis[2-t 25 butyl-5-methyl-4,1-phenylene]bis[3-(dodecylthio) propionate]; - sulfur-based antioxidants such as dioctadecyl 3,3'-thiodipropionate or didodecyl 3,3'-thiodipropionate; - phosphorus-based antioxidants such as phosphites 30 or phosphonates, for instance tris(2,4-di-t-butylphenyl) phosphite or bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite; and - antioxidants of amine type such as polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ), the latter 35 type of antioxidant being particularly preferred in the 10 composition of the invention. TMQs may be of different grades, namely: - a "standard" grade with a low degree of polymerization, i.e. with a residual monomer content of 5 greater than 1% by weight and with a residual NaCl content that may range from 100 ppm to more than 800 ppm (parts per million by mass); - a "high polymerization degree" grade with a high degree of polymerization, i.e. with a residual monomer 10 content of less than 1% by weight and with a residual NaCl content that may range from 100 ppm to more than 800 ppm; - a "low residual salt content" grade with a residual NaCl content of less than 100 ppm. 15 The type of stabilizer and its content in the crosslinkable composition are conventionally chosen as a function of the maximum temperature to which the polymers are subjected during the production of the mixture and during the implementation by extrusion on the cable, and 20 also depending on the maximum exposure time at this temperature. The crosslinkable composition may typically comprise from 0.1% to 2% by weight of antioxidant(s). Preferably, it may comprise not more than 0.7% by weight 25 of antioxidant(s), especially when the antioxidant is TMQ. Other additives and/or fillers that are well known to those skilled in the art may also be added to the crosslinkable composition of the invention, such as 30 breakdown retardants; processing aids such as lubricants or waxes; compatibilizers; couplers; UV stabilizers; non conductive fillers; conductive fillers; and/or semiconductive fillers. According to one preferred embodiment, the 35 crosslinked layer of the invention is the electrically 11 insulating layer (i.e. the second layer). In the case of the electrically insulating layer, the crosslinkable composition does not comprise any (electrically) conductive filler and/or does not comprise any semi 5 conductive filler. More particularly, at least two of the three layers of the cable are crosslinked layers, and preferably the three layers of the cable are crosslinked layers. When the crosslinkable composition is used for the 10 manufacture of semiconductive layers (first layer and/or third layer), the crosslinkable composition also comprises at least one (electrically) conductive filler or one semiconductive filler, in an amount that is sufficient to make the crosslinkable composition semi 15 conductive. It is more particularly considered that a layer is semiconductive when its electrical conductivity is at least 0.001 S.m 1 (siemens per meter). The crosslinkable composition used to obtain a 20 semiconductive layer may comprise from 0.1% to 40% by weight of (electrically) conductive filler, preferably at least 15% by weight of conductive filler, and even more preferentially at least 25% by weight of conductive filler. 25 The semiconductive filler may be advantageously chosen from carbon blacks, carbon nanotubes and graphites, or a mixture thereof. Whether it is the first semiconductive layer, the second electrically insulating layer and/or the third 30 semiconductive layer, at least one of these three layers is an extruded layer, preferably two of these three layers are extruded layers, and even more preferentially these three layers are extruded layers. In one particular embodiment, generally in 35 accordance with the electrical cable that is well known 12 in the field of application of the invention, the first semiconductive layer, the second electrically insulating layer and the third semiconductive layer constitute a three-layer insulation. In other words, the second 5 electrically insulating layer is directly in physical contact with the first semiconductive layer, and the third semiconductive layer is directly in physical contact with the second electrically insulating layer. The electrical cable of the invention may also 10 comprise a metal shield surrounding the third semi conductive layer. This metal shield may be a "wire" shield, composed of an assembly of copper or aluminum conductors arranged around and along the third semiconductive layer, and a 15 "strip" shield composed of one or more conductive metal strips laid spirally around the third semiconductive layer, or a "leaktight" shield such as a metal tube surrounding the third semiconductive layer. This type of shield makes it possible especially to form a barrier to 20 the moisture that has a tendency to penetrate the electrical cable in the radial direction. All the types of metal shield may serve for earthing the electrical cable and may thus transport fault currents, for example in the case of a short 25 circuit in the network concerned. In addition, the electrical cable of the invention may comprise an outer protective sheath surrounding the third semiconductive layer, or alternatively more particularly surrounding said metal shield, when it 30 exists. This outer protective sheath may be conventionally made from suitable thermoplastic materials such as HDPE, MDPE or LLDPE; or alternatively flame propagation-retardant materials or fire-propagation resistant materials. In particular, if the latter 35 materials do not contain halogen, this sheath is referred 13 to as being of HFFR type (Halogen Free Flame Retardant). Other layers, such as layers that swell in the presence of moisture, may be added between the third semiconductive layer and the metal shield when it exists, 5 and/or between the metal shield and the outer sheath when they exist, these layers providing longitudinal and/or transverse leaktightness of the electrical cable to water. The electrical conductor of the cable of the invention may also comprise materials that swell in the 10 presence of moisture to obtain a "leaktight core". Other characteristics and advantages of the present invention will emerge in the light of the description of a nonlimiting example of an electrical cable according to the invention, given with reference to figure 1 showing a 15 schematic view in perspective and in cross section of an electrical cable according to one preferred embodiment in accordance with the invention. For reasons of clarity, only the elements that are essential for understanding the invention have been 20 schematically represented, and without being drawn to scale. The medium-tension or high-tension power cable 1, illustrated in figure 1, comprises an elongated central conductive element 2, especially made of copper or 25 aluminum. Successively and coaxially around this conductive element 2, the power cable 1 also comprises a first semiconductive layer 3 known as the "inner semi conductive layer", a second electrically insulating layer 4, a third semiconductive layer 5 known as the "outer 30 semiconductive layer", an earthing and/or protective metal shield 6, and an outer protective sheath 7, layers 3, 4 and 5 possibly being obtained from a composition according to the invention. Layers 3, 4 and 5 are extruded and crosslinked layers. 35 The presence of the metal shield 6 and of the 14 protective outer sheath 7 is preferential, but not essential, this cable structure being, per se, well known to those skilled in the art. 5 Examples Preparation of crosslinkable compositions Crosslinkable C1 C2 C3 C4 composition Polyolefin 100 100 100 100 BCP 1.42 - 1.27 DTBH - 1.25 - 1.05 Antioxidants 0.18 0.18 0.18 0.18 TVCH - - 1.00 1.00 Table 1 10 In Table 1, the compositions C1 and C3 make reference to comparative examples, whereas the compositions C2 and C4 make reference to compositions according to the invention. 15 The amounts of the constituents of compositions Cl to C4, detailed in Table 1, are expressed in parts by weight (pcr) per 100 parts by weight of polymer in the crosslinkable composition. The origin of the various constituents of 20 compositions Cl to C4 of Table 1 is detailed as follows: - "polyolefin" is a low-density polyethylene sold by the company In6os under the reference BPD 2000; - "BCP" is tert-butylcumyl peroxide, sold by the 25 company Arkema under the reference Luperox 801; - "DTBH" is 1, 1-bis (tert-butylperoxy)cyclohexane (tertiary dialkyl aliphatic peroxide), sold by 15 the company Arkema under the reference Luperox 101; and - "TVCH" is the crosslinking coagent 1,2,4-tri vinylcyclohexane, sold by the company BASF 5 under the reference TVCH. The compositions Cl to C4 are prepared by mixing the polyethylene granules and the additives such as the peroxide, the antioxidants and optionally the coagent, in a closed jar placed on a roll mixer for 3 hours, so as to 10 fully impregnate the polyethylene granules. The polyethylene granules were preheated to 60*C before impregnation. Next, the mixture was placed at 40"C for 16 hours, before being hermetically stored. 15 Characterizations of the compositions Characterizations on non-crosslinked plate 20 * Kinetics and level of crosslinking The MDR rheometer (Moving Die Rheometer, Alpha Technologies) makes it possible to monitor the cross linking/vulcanization of a material by measuring the change in its viscosity (DIN 53529 (1983)). 25 The chamber containing the sample is formed from two hotplates. The lower plate applies an oscillation of constant frequency (100 cycles/mn, i.e. 1.67 Hz) of amplitude ± 0.50 of arc; the upper plate measures the response of the material, i.e. its resistance to the 30 applied stress. The unit of measurement is that of a torque, expressed in dN.m. The sample is prepared from the impregnated polyethylene granules, molded in a hydraulic press into a plate 3 mm thick at a temperature of 120'C, according to 35 a cycle of 2 min without pressure followed by 3 min under 16 a pressure of 100 bar, before being cooled. Two disks 35 mm in diameter for fully lining the chamber are cut out of the plate using a punch, and are then placed between two sheets of polyester terphane*, to 5 be placed in the rheometer chamber. The measurement is performed at a temperature of 190'C, which is representative of tube vulcanization conditions. After an initial drop in the torque due to the premelting of the material, the viscosity of the 10 material and the resulting torque increase, which is a sign that crosslinking takes place. A parameter of interest is the MH, which corresponds to the maximum torque measured. This is a plateau value, obtained when the whole system has reacted 15 and when the maximum accessible level of crosslinking is reached. For a given material, a good correlation between MH and crosslinking density, which govern the thermo mechanical properties after the crosslinking step, is noted. 20 - Breakdown time The Mooney viscometer (Monsanto MV2000) makes it possible to measure the viscosity of a material, or, in the case of crosslinkable materials, to monitor their change over time (standard ASTM D1646 (2005)). 25 It is composed of two jaws forming a cylindrical cavity into which is placed the sample to be tested. The chamber has at its center a metal disk that is rotated at a constant speed of 2 rpm. In our case, of the two available normalized rotors, it is the "large" one that 30 is used. During the measurement, the jaws and the chamber are maintained under pressure and at a temperature of 130 0 C. The sample is prepared from the impregnated 35 polyethylene granules, molded in a hydraulic press into a 17 plate 3 mm thick at a temperature of 120*C, according to a cycle of 2 min without pressure, followed by 3 min at a pressure of 100 bar, before being cooled. Four disks 50 mm in diameter are cut out of the 5 plate using a punch. Two of them are pierced at their center with a hole 12 mm in diameter, enabling them to be threaded onto the rotor stem, under the latter; the other two are stored intact and will be placed above the rotor. The whole is then placed between two sheets of polyester 10 terphane*, to be positioned in the viscometer chamber. It is the resistance of the material to the rotation of the rotor that is measured. The measurement is expressed in arbitrary units, Mooney (MU) . The parameters of interest are: the ML, minimal viscosity 15 value, measured at time tO (min) . ML+l, the viscosity value corresponding to the ML increased by one Mooney unit. This is measured at time tl (min). The ML+2, the value corresponding to the ML increased by two Mooney units. This is measured at time t2 (min). 20 * Measurement of the volatiles (i.e. methane) by the Sievert technology The determination of the amount of volatiles produced during the polyethylene crosslinking phase, and then desorbed, is made via the Sievert method, using the 25 PCT Pro 2000 (HY-ENERGY, SETARAM). The sample is prepared from the impregnated polyethylene granules, molded in a hydraulic press into a plate 1 mm thick at a temperature of 120'C, according to a cycle of 2 min without pressure, followed by 3 min at a 30 pressure of 100 bar, before being cooled. Disks 6 mm in diameter are then cut out of the plate using a punch, and then weighed accurately, to within a mg (total mass = 300-350 mg). The sample is placed in the chamber of the machine, 35 and placed under pressure (helium) . This chamber is 18 connected by means of a valve to a 5 ml reservoir, which is itself also under pressure. At the start of the test, the pressures in the chamber and in the reservoir are identical. During the temperature cycle, the valve opens 5 and closes intermittently, allowing the establishment of a new equilibrium when it is open, and then the measurement of the new pressure in the reservoir, when it is closed. The change in pressure arises partly from the release of methane, and partly from the size variation of 10 the chamber with the temperature. A real-time reading of the amount of methane released thus necessitates a precalibration, by subjecting the chamber to the envisioned temperature cycle. The equipment allows controlled temperature ramps 15 of 1"C/s, simulating the crosslinking conditions for the various polyethylene layers in a vulcanization tube. The cycle envisages heating from room temperature to 250 0 C. The difference between the final and initial 20 pressure measurements, at identical temperature, gives access to the amount of methane given off. The amount of volatiles (i.e. methane) is expressed in pmol/g of crosslinked polyethylene. 25 Characterizations on crosslinked plate e Crosslinking density by measurement of the hot creep Plates 1 mm thick are molded from the impregnated 30 polyethylene granules. The molding is performed in a press at 120*C, according to a cycle of 2 minutes without pressure followed by 3 minutes at 100 bar. The plates are then cooled at a pressure of 100 bar. The crosslinking step is performed in a press, at a 35 temperature of 190 0 C at a pressure of 100 bar and lasts 19 for 10 minutes. The molds are preheated to 190 0 C. The cooling step takes place under a pressure maintained at 100 bar. The measurement of the hot creep of a material 5 under mechanical stress is determined according to standard NF EN 60811-2-1. This test is commonly referred to as the Hot Set Test (HST) and consists in ballasting one end of a specimen of H2 dumbbell type with a mass corresponding to 10 the application of a stress equivalent to 0.2 MPa, and in placing the assembly in an oven heated at 200±1C for a period of 15 minutes. After this period, the maximum hot elongation under stress of the specimen, expressed as a %, is recorded. 15 The suspended mass is then removed, and the specimen is maintained in the oven for a further 5 minutes. The remaining permanent elongation, also known as the remanence (or remanent elongation), is then measured 20 before being expressed as a %. It is recalled that the more a material is cross linked, the lower will be the values of maximum elongation under stress and of remanence. It is moreover pointed out that, in the case where 25 a specimen breaks during the test, under the combined action of the mechanical stress and the temperature, the test result would then logically be considered a failure. In the case that is of interest here, an elongation value will be considered as being in compliance with the 30 requirements if it does not exceed 100%. Beyond this value, in the same respect as a rupture, the test behavior will be considered as noncompliant. All the results concerning the characterization of the non-crosslinked and crosslinked plates derived from 35 the crosslinkable compositions Cl to C4 are collated in 20 Table 2 below. Crosslinkable compositions Cl C2 C3 C4 MH 3.2 2.9 3.4 3.2 (dN.m) ML at 130*C 9.4 9.7 9.7 7.5 (Mooney units) tO (ML+0) 15 17 22 31 (min) tl (ML+1) 45 49 52 69 (min) t2 (ML+2) 66 79 74 102 (min)
CH
4 Sievert (pmol/g XLPE) 113 104 101 69 Hot set at 200 0 C Maximum elongation 70-75 70-95 50-65 60-65 (%)__ _ _ __ _ _ __ _ _ Remanence 0 0 0 0 (%)__ _ _ __ _ _ __ _ _ __ _ _ Table 2 5 For equivalent thermomechanical properties (see the results of the maximum elongation and of the remanence cf. Hot Set Test at 200 0 C), the amount of aliphatic peroxide used in the crosslinkable composition C2 of the invention is smaller than that used in the crosslinkable 10 composition C1 (1.25 for C2 versus 1.42 for Cl). By comparing the crosslinkable compositions Cl and C2, the amount of methane given off during the crosslinking falls from 113 to 104 pmol/g of XLPE. In addition, the cumyl alcohol as a crosslinking by 15 product is replaced by the formation of tert-butanol in composition C2, during the crosslinking. tert-Butanol, like any tertiary alcohol, may of course be dehydrated to 21 form isobutene and water. However, since isobutene is markedly less stabilized than the a-methylstyrene formed in the dehydration reaction of cumyl alcohol, this reaction is markedly unfavored, and the formation of 5 water is slowed down. These differences are all the more significant when compositions C3 and C4 are compared. Again for equivalent thermomechanical properties, a significant decrease in the amount of methane is observed for composition C4 of 10 the invention relative to composition C3. In addition, the burn-out time values are much better for composition C4 than for composition C3. Finally, since composition C4 does not comprise any cumyl peroxide, the formation of water is markedly slowed down in this composition. 15

Claims (11)

1. Electrical cable (1) comprising an electrical conductor (2), a first semiconductive layer (3) 5 surrounding the electrical conductor (2), a second electrically insulating layer (4) surrounding the first layer (3), and a third semiconductive layer (5) surrounding the second layer (4), characterized in that at least one of these three layers (3, 4, 5) is a 10 crosslinked layer obtained from a crosslinkable composition comprising at least one polyolefin, characterized in that the composition also comprises an aliphatic peroxide as crosslinking agent.
2. Cable according to claim 1, characterized in 15 that the alphatic peroxide is chosen from aliphatic peroxycarbonates, aliphatic peroxides of tertiary dialkyl, aliphatic peroxyacetals and aliphatic peroxyesters, or a mixture thereof.
3. Cable according to claim 1 or 2, characterized 20 in that the polyolefin is an ethylene polymer.
4. Cable according to claim 3, characterized in that the ethylene polymer is a low-density polyethylene (LDPE).
5. Cable according to any one of the preceding 25 claims, characterized in that the crosslinkable composition comprises more than 50.0 parts by weight of polyolefin per 100 parts by weight of polymer in the composition.
6. Cable according to any one of the preceding 30 claims, characterized in that the crosslinkable composition also comprises at least one crosslinking coagent comprising at least two reactive functions, which are identical or different, which are capable firstly of grafting to the polyolefin, and secondly of participating 35 in the crosslinking of the polyolefin. 23
7. Cable according to claim 6, characterized in that the at least two reactive functions of the coagent are vinyl functions.
8. Cable according to any one of the preceding 5 claims, characterized in that the crosslinkable composition also comprises an aromatic compound comprising at least one aromatic nucleus and a single reactive function capable of grafting onto the polyolefin. 10
9. Cable according to claim 8, characterized in that the single reactive function of the aromatic compound is a vinyl function.
10. Cable according to any one of the preceding claims, characterized in that the crosslinked layer is 15 the electrically insulating layer.
11. Cable according to any one of the preceding claims, characterized in that the three layers of the cable are crosslinked layers.
AU2012201230A 2011-03-08 2012-02-29 Medium-Tension or High-Tension Electrical Cable Abandoned AU2012201230A1 (en)

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EP2498263A1 (en) 2012-09-12
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CN102682904B (en) 2017-05-17
EP2498263B1 (en) 2014-11-19
CN102682904A (en) 2012-09-19
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ES2530362T3 (en) 2015-03-02
KR20180061101A (en) 2018-06-07

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