AU2012201176A1 - Medium-Tension or High-Tension Electrical Cable - Google Patents

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), at least one of these three layers (3, 4, 5) being a cross linked layer obtained from a crosslinkable composition comprising at least one polyolefin and an organic peroxide as crosslinking agent, characterized in that the composition also comprises a crosslinking coagent comprising at least two unsaturations, one of these two unsaturations being a vinyl function. Figure to be published : Figure 1. 1 . . . . .. . . . ... .. .. .. 5.. .6 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. 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 and/or water. These last 25 two byproducts are formed by the dehydration reaction of cumyl alcohol. If the methane formed during the crosslinking step is not removed from the crosslinked layers, risks associated with the explosiveness of methane and with its 30 flammability cannot be ignored. This gas 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 gas can only 35 diffuse longitudinally along the cable up to the 2 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. Although solutions exist 5 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 insulating layers are thick. Document US 5 252 676 presents a three-layer 10 insulation for a power cable. This three-layer insulation is obtained from compositions comprising an ethylene copolymer, an organic peroxide as crosslinking agent, and an additive of the type such as isopropenylbenzene or a derivative thereof. In order to limit the amount of gas 15 released during the decomposition of the crosslinking agent, said document recommends reducing the amount of crosslinking agent. However, the crosslinkable compositions used in said document are not optimized for limiting the amount 20 of crosslinking byproducts, while at the same time providing satisfactory thermomechanical properties once the compositions have been crosslinked. The aim of the present invention is to overcome the drawbacks of the prior art by proposing a medium-tension 25 or high-tension electrical cable, comprising a crosslinked layer whose manufacture significantly limits the presence of crosslinking byproducts, such as methane, while at the same time providing optimum thermomechanical properties, such as the hot creep, which are 30 characteristic of correct 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 conductor, a second electrically insulating layer 35 surrounding the first layer, and a third semiconductive 3 layer surrounding the second layer, at least one of these three layers being a crosslinked layer obtained from a crosslinkable composition comprising at least one polyolefin and an organic peroxide as crosslinking agent, 5 characterized in that the composition also comprises a crosslinking coagent comprising at least two unsaturations, one of these two unsaturations being a vinyl function, the vinyl function preferably being an ethylenic function of the type CH 2 =CH-. 10 The crosslinking coagent of the invention, which is different than the crosslinking agent, is of multifunctional type since it comprises at least two unsaturations. The at least two unsaturations are more 15 particularly reactive functions of carbon-carbon double bond type, 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 three-dimensional network of the crosslinked polyolefin). 20 The crosslinking coagent can advantageously significantly reduce the proportion of organic peroxide to be used in the crosslinkable composition, and thus reduce the amount of methane derived from the crosslinking byproducts originating from said peroxide, 25 while at the same time maintaining good thermomechanical properties such as the hot creep, and also a satisfactory rate of crosslinking. The thermomechanical properties for the crosslinked layer according to the invention may advantageously be reflected by a maximum hot elongation 30 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%. Preferably, a coagent whose boiling point is sufficiently high, such that it does not evaporate during 35 the step of implementation of the crosslinkable 4 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-methyl-1,4-pentadiene; 5 3-methyl-1,3-pentadiene; 4-methyl-1,3-pentadiene; 1,6-heptadiene; 2,4-dimethyl-1,3-pentadiene; 2-methyl 1,5-hexadiene; 4-vinyl-1-cyclohexene; 1,7-octadiene; 2,5-dimethyl-1,5-hexadiene; 2,5-dimethyl-2,4-hexadiene; 5-vinyl-2-norbornene; 1,8-nonadiene; 7-methyl-1,6-octa 10 diene; 1,4,9-decatriene; 2,6-dimethyl-2,4,6-octatriene; dipentene; 7-methyl-3-methylene-1, 6-octadiene; 1, 9-deca diene; 3, 9 -divinyl-2,4,8,10-tetraoxaspiro[5.5)undecane; 1,2,4-trivinylcyclohexane; 1,13-tetradecadiene; 2,3-di phenyl-1,3-butadiene; trans,trans-1,4-diphenyl-1,3-buta 15 diene; 1,15-hexadecadiene; 1,6-diphenyl-1,3,5-hexatriene; 2,3-dibenzyl-1,3-butadiene; and polybutadiene; or a mixture thereof. In one particularly preferred embodiment, the at least two unsaturations of the crosslinking coagent are 20 vinyl functions, especially ethylenic functions of the type CH 2 =CH-. In this case, the crosslinking coagent may be chosen from 1,5-hexadiene; 1,6-heptadiene; 1,7-octa diene; 1,8-nonadiene; 1,4,9-decatriene; 1,9-decadiene; 25 3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane; 1,2,4 trivinylcyclohexane; 1,13-tetradecadiene; 1,15-hexa decadiene. The coagent concentration is preferably limited so as not to disrupt the process of extrusion of the 30 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 35 coagent per 100 parts by weight of polymer(s) in the 5 crosslinkable composition. The organic peroxide according to the invention may be chosen from organic peroxides that are well known to those skilled in the art, whether they are aliphatic or 5 aromatic peroxides. Examples of aromatic peroxides that may be mentioned include dicumyl peroxide and tert-butylcumyl peroxide. The aliphatic peroxide may be an aliphatic peroxide 10 comprising at least one tertiary alkyl group. Examples of aliphatic peroxides that may be mentioned include: - aliphatic peroxycarbonates, for instance tert amylperoxy 2-ethylhexyl carbonate, tert-amyl peroxy 2-ethylhexyl carbonate, tert-butylperoxy 15 isopropyl carbonate; - aliphatic peroxides of tertiary dialkyl, for instance 1,1-bis(tert-butylperoxy)cyclohexane, 2,5-dimethyl-2,5-bis(tert-butylperoxy)-3-hexyne, 2,5-dimethyl-2,5-bis(tert-butylperoxy)-3-hexane, 20 di-tert-amyl peroxide, di-tert-butyl peroxide, cyclic peroxides such as 3,6,9-triethyl-3,6,9 trimethyl-1,4,7-triperoxonane; - aliphatic peroxyacetals, for instance butyl 4,4-bis(tert-butylperoxy)valerate; and 25 - aliphatic peroxyesters, for instance tert-butyl peroxyacetate, tert-amyl peroxyacetate. In comparison with aromatic peroxides, aliphatic peroxides have the advantage of not forming cumyl alcohol as crosslinking byproduct during the crosslinking of the 30 crosslinkable composition, and thus make it possible to significantly limit the presence of water in the crosslinked layer, while at the same time maintaining very good thermomechanical properties. Among the mentioned aliphatic peroxides, aliphatic 35 peroxides of tertiary dialkyl will preferably be used.

6 The reason for this is that peroxides of this type have a very good compromise between rate of crosslinking and risk of burn-out or of precrosslinking during the implementation of the composition. 5 Preferably, the crosslinkable composition does not comprise any aromatic peroxide, especially such as dicumyl peroxides or derivatives thereof. The peroxide-route crosslinking of the crosslinkable composition according to the invention may 10 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. The crosslinkable composition of the invention may 15 comprise not more than 2.00 parts by weight of organic peroxide per 100 parts by weight of polymer(s) in the composition; preferably 1.50 parts by weight of organic peroxide per 100 parts by weight of polymer(s) in the composition; preferably 1.25 parts by weight of organic 20 peroxide per 100 parts by weight of polymer(s) in the composition; and particularly preferably 1.10 parts by weight of organic peroxide per 100 parts by weight of polymer(s) in the composition. The term "polyolefin" per se generally means olefin 25 homopolymer or copolymer. It may especially denote a 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 30 include linear low-density polyethylene (LLDPE), low 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 35 hexylethyl acrylate (2HEA), copolymers of ethylene and of 7 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. 5 It will be preferred to use a low-density polyethylene (LDPE) since it has good rheological properties for its implementation, especially by extrusion, and very good thermomechanical and electrical properties. 10 The term "low density" means a density that may range especially from 0.910 to 0.940 g/cm 3 , and which may preferably range from 0.910 to 0.930 g/cm 3 according to standard ISO 1183 (at a temperature of 23'C). Typically, the low-density polyethylene (LDPE) may 15 be obtained via a polymerization process in a high pressure tubular reactor or in an autoclave reactor. 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 20 composition, preferably at least 70 parts by weight of 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. 25 In a particularly advantageous manner, the 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 30 also comprise an aromatic compound comprising at least one aromatic nucleus, and a single reactive function capable of grafting to the polyolefin. Preferably, said reactive function of the aromatic compound is a vinyl function. As a result, this aromatic compound does not 35 participate in the crosslinking of the polyolefin, in 8 contrast with the crosslinking coagent, when it is present in the crosslinkable composition. The crosslinked layer obtained from this crosslinkable composition has reinforced and durable 5 properties in the field of electrical cables, offering better resistance to water treeing. More particularly, this concerns the resistance to electrical breakdown, and especially the capacity to dissipate the space charges that accumulate especially in high-tension cables under 10 direct current. The aromatic compound may be chosen from styrene, styrene derivatives and isomers thereof. Examples of styrene derivatives that may be mentioned include the compounds having the following 15 general formula: X R 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 20 4-methyl-2, 4-diphenylpentane, and triphenylethylene. In the context of the present invention, styrene derivatives of the polycyclic aromatic hydrocarbon (PAH) type may also be considered. More particularly, mention may be made of vinylnaphthalenes, for instance 2-vinyl 25 naphthalene; vinylanthracenes, for instance 9-vinyl anthracene or 2-vinylanthracene; and vinylphenanthrenes, for instance 9-vinylphenanthrene. The grafting of these aromatic compounds onto the polymer chain of the polyolefin is typically performed 30 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 9 tertiary aliphatic alkyl peroxide of the invention. The crosslinkable composition according to the invention may also comprise at least one protective agent such as an antioxidant. Antioxidants protect the 5 composition against the thermal constraints generated during the manufacturing steps of the cable or during the functioning of the cable. The antioxidants are preferably chosen from: - sterically hindered phenolic antioxidants such as 10 tetrakismethylene(3,5-di-t-butyl 4-hydroxyhydro cinnamate)methane, octadecyl 3-(3,5-di-t-butyl-4-hydroxy 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 15 4-methylphenol), 1,2-bis(3,5-di-t-butyl-4-hydroxyhydro cinnamoyl)hydrazine, [2,2'-oxamidobis(ethyl 3-(3,5-di-t butyl-4-hydroxyphenyl)propionate) and 2,2'-oxamidobis [ethyl 3-(t-butyl-4-hydroxyphenyl)propionate]; - thioethers such as 4,6-bis(octylthiomethyl) 20 o-cresol, bis[2-methyl-4-{3-n-(C12 or C14)alkylthio propionyloxy}-5-t-butylphenyl] sulfide and thiobis[2-t butyl-5-methyl-4,1-phenylene]bis[3-(dodecylthio) propionate]; - sulfur-based antioxidants such as dioctadecyl 25 3,3'-thiodipropionate or didodecyl 3,3'-thiodipropionate; - phosphorus-based antioxidants such as phosphites or phosphonates, for instance tris(2,4-di-t-butylphenyl) phosphite or bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite; and 30 - antioxidants of amine type such as polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ), the latter type of antioxidant being particularly preferred in the composition of the invention. TMQs may be of different grades, namely: 35 - a "standard" grade with a low degree of 10 polymerization, i.e. with a residual monomer content of 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); 5 - a "high polymerization degree" grade with a high degree of polymerization, i.e. with a residual monomer content of less than 1% by weight and with a residual NaCl content that may range from 100 ppm to more than 800 ppm; 10 - a "low residual salt content" grade with a residual NaCl content of less than 100 ppm. 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 15 are subjected during the production of the mixture and during the implementation by extrusion on the cable, and also depending on the maximum exposure time at this temperature. The crosslinkable composition may typically 20 comprise from 0.1% to 2% by weight of antioxidant(s). Preferably, it may comprise not more than 0.7% by weight of antioxidant(s), especially when the antioxidant is TMQ. Other additives and/or fillers that are well known 25 to those skilled in the art may also be added to the crosslinkable composition of the invention, such as breakdown retardants; processing aids such as lubricants or waxes; compatibilizers; couplers; UV stabilizers; non conductive fillers; conductive fillers; and/or 30 semiconductive fillers. According to one preferred embodiment, the crosslinked layer of the invention is the electrically insulating layer (i.e. the second layer) . In the case of the electrically insulating layer, the crosslinkable 35 composition does not comprise any (electrically) 11 conductive filler and/or does not comprise any semi conductive filler. More particularly, at least two of the three layers of the cable are crosslinked layers, and preferably the 5 three layers of the cable are crosslinked layers. When the crosslinkable composition is used for the manufacture of semiconductive layers (first layer and/or third layer), the crosslinkable composition also comprises at least one (electrically) conductive filler 10 or one semiconductive filler, in an amount that is sufficient to make the crosslinkable composition semi conductive. It is more particularly considered that a layer is semiconductive when its electrical conductivity is at 15 least 0.001 S.m-1 (siemens per meter). The crosslinkable composition used to obtain a 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 20 preferentially at least 25% by weight of conductive filler. The conductive filler may be advantageously chosen from carbon blacks, carbon nanotubes and graphites, or a mixture thereof. 25 Whether it is the first semiconductive layer, the second electrically insulating layer and/or the third 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 30 these three layers are extruded layers. In one particular embodiment, generally in accordance with the electrical cable that is well known in the field of application of the invention, the first semiconductive layer, the second electrically insulating 35 layer and the third semiconductive layer constitute a 12 three-layer insulation. In other words, the second electrically insulating layer is directly in physical contact with the first semiconductive layer, and the third semiconductive layer is directly in physical 5 contact with the second electrically insulating layer. The electrical cable of the invention may also comprise a metal shield surrounding the third semi conductive layer. This metal shield may be a "wire" shield, composed 10 of an assembly of copper or aluminum conductors arranged around and along the third semiconductive layer, and a "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 15 surrounding the third semiconductive layer. This type of shield makes it possible especially to form a barrier to the moisture that has a tendency to penetrate the electrical cable in the radial direction. All the types of metal shield may serve for 20 earthing the electrical cable and may thus transport fault currents, for example in the case of a short circuit in the network concerned. In addition, the electrical cable of the invention may comprise an outer protective sheath surrounding the 25 third semiconductive layer, or alternatively more particularly surrounding said metal shield, when it exists. This outer protective sheath may be conventionally made from suitable thermoplastic materials such as HDPE, MDPE or LLDPE; or alternatively flame 30 propagation-retardant materials or fire-propagation resistant materials. In particular, if the latter materials do not contain halogen, this sheath is referred to as being of HFFR type (Halogen Free Flame Retardant). Other layers, such as layers that swell in the 35 presence of moisture, may be added between the third 13 semiconductive layer and the metal shield when it exists, 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 5 water. The electrical conductor of the cable of the invention may also comprise materials that swell in the 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 10 a nonlimiting example of an electrical cable according to the invention, given with reference to figure 1 showing a schematic view in perspective and in cross section of an electrical cable according to one preferred embodiment in accordance with the invention. 15 For reasons of clarity, only the elements that are essential for understanding the invention have been schematically represented, and without being drawn to scale. The medium-tension or high-tension power cable 1, 20 illustrated in figure 1, comprises an elongated central conductive element 2, especially made of copper or 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 25 conductive layer", a second electrically insulating layer 4, a third semiconductive layer 5 known as the "outer 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 30 according to the invention. Layers 3, 4 and 5 are extruded and crosslinked layers. The presence of the metal shield 6 and of the protective outer sheath 7 is preferential, but not essential, this cable structure being, per se, well known 35 to those skilled in the art.

14 Examples Preparation of crosslinkable compositions 5 Crosslinkable C1 C2 C3 C4 C5 C6 C7 C8 C9 composition Polyolefin 100 100 100 100 100 100 100 100 100 BCP 1.42 1.27 1.12 - - - - - DTBH - - - 1.25 1.05 1.05 1.05 1.05 1.05 Antioxidants 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 TVCH - 1.00 - - - 1.00 2.00 - DVB - - 1.00 - - - - - MDIB - - - - - - - 1.002.00 Table 1 In Table 1, the crosslinkable compositions C1, C4, C5, C8 and C9 make reference to comparative examples, 10 whereas the compositions C2, C3, C6 and C7 make reference to compositions according to the invention. The amounts of the constituents of compositions Cl to C9, detailed in Table 1, are expressed in parts by weight (pcr) per 100 parts by weight of polymer in the 15 crosslinkable composition. The origin of the various constituents of compositions C1 to C9 of Table 1 is detailed as follows: - "polyolefin" is a low-density polyethylene sold by the company In6os under the reference BPD 20 2000; - "BCP" is tert-butylcumyl peroxide, sold by the company Arkema under the reference Luperox 801; - "DTBH" is 1, 1-bis (tert-butylperoxy)cyclohexane (tertiary dialkyl aliphatic peroxide), sold by 25 the company Arkema under the reference Luperox 15 101; - "TVCH" is the crosslinking coagent 1,2,4-tri vinylcyclohexane, sold by the company BASF under the reference TVCH; 5 - "DVB" is the crosslinking coagent divinyl benzene, sold by the company Sigma-Aldrich under the reference divinylbenzene; and - "MDIB" is the coagent m-diisopropenylbenzene, sold by the company Sigma-Aldrich under the 10 reference 1,3-diisopropenylbenzene. The compositions Cl to C9 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 15 fully impregnate the polyethylene granules. The polyethylene granules were preheated to 60 0 C before impregnation. Next, the mixture was placed at 40 0 C for 16 hours, before being hermetically stored. 20 Characterizations of the compositions Characterizations on non-crosslinked plate 25 - 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)). 30 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 35 applied stress. The unit of measurement is that of a 16 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 5 a cycle of 2 min without pressure followed by 3 min under 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 terphanee, to 10 be placed in the rheometer chamber. The measurement is performed at a temperature of 190 0 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 15 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 20 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. 25 - 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)). 30 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 35 is used.

17 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 5 polyethylene granules, molded in a hydraulic press into a 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 10 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 15 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 20 value, measured at time tO (min) . ML+1, 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). 25 - 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 30 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 1200C, according to a cycle of 2 min without pressure, followed by 3 min at a 35 pressure of 100 bar, before being cooled.

18 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, 5 and placed under pressure (helium). This chamber is 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 10 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 15 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 20 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 2500C. The difference between the final and initial 25 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. 30 Characterizations on crosslinked plate Crosslinking density by measurement of the hot creep Plates 1 mm thick are molded from the impregnated 35 polyethylene granules. The molding is performed in a 19 press at 1200C, 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 5 temperature of 190*C at a pressure of 100 bar and lasts for 10 minutes. The molds are preheated to 1900C. The cooling step takes place under a pressure maintained at 100 bar. The measurement of the hot creep of a material 10 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 15 the application of a stress equivalent to 0.2 MPa, and in placing the assembly in an oven heated at 200±10C for a period of 15 minutes. After this period, the maximum hot elongation under stress of the specimen, expressed as a %, is recorded. 20 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 25 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 30 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 35 requirements if it does not exceed 100%. Beyond this 20 value, in the same respect as a rupture, the test behavior will be considered as noncompliant. - Mechanical and thermal aging properties Plates 1 mm thick are molded using the impregnated 5 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 under a pressure of 100 bar. The crosslinking step takes place in a press, at a 10 temperature of 190 0 C under a pressure of 100 bar and lasts for 10 minutes. The molds are preheated to 190*C. The cooling step takes place under a pressure maintained at 100 bar. The mechanical properties (stress and elongation at 15 break) are measured on specimens of H2 dumbbell type, according to standard NF EN 60811-1-1. The specimens, the thickness of which is measured precisely, are tested after a minimum period of 16 hours at room temperature. The traction speed is 200 mm/min. 20 The values of the initial mechanical properties are thus determined. Specimens, the thickness of which is measured precisely, are also placed in an oven to undergo an accelerated thermal aging (7 days at 135*C) and are then 25 characterized in the same way. The variations in stress and in elongation at break are thus determined. The variations are considered satisfactory when they are between +25% and -25%, whether for the tensile stress or for the elongation at break. 30 All the results concerning the characterization of the non-crosslinked and crosslinked plates derived from the crosslinkable compositions Cl to C9 are collated in Table 2 below.

21 Crosslinkable Cl C2 C3 C4 C5 C6 C7 C8 C9 compositions MH 3.2 3.4 3.2 3.0 2.2 3.2 2.8 1.9 1.2 (dN.m) ML at 1300C (Mooney 9.4 9.7 9.7 9.7 9.1 7.5 7.5 8.8 8.8 units) tO (ML+0) 15 22 13 17 19 31 25 22 15 (min) tl (ML+1) 45 52 15 49 55 69 64 66 84 (min) t2 (ML+2) 66 74 16 79 91 102 104 115 154 (min)

CH

4 Sievert 113 101 93 104 58 69 67 -- - (pmol/g XLPE) Hotsetat_200 0C ____________ 1-ot set at 20000 Maximum 70- 50- 60- 70- 60- 65 elongation 75 65 80 95 Rupture 65 80 Rupture Rupture (%) __ Remanence 0 0 0 0 - 0 0 (% ) I I I I I I I Accelerated thermal aging for 7 days at 1350C Variation (%) -33 0 6 -33 -23 -16 0 +3 +3 of the tensile stress Variation (%) of the -17 +6 5 -16 -13 -4 +10 +3 -1 elongation at break Table 2 The addition of a coagent in accordance with the 22 invention makes it possible to substantially reduce the amount of peroxide necessary for crosslinking, and thus to reduce the amount of volatiles (methane) given off, while at the same time maintaining the desired cross 5 linking density. This is given by the MH and by the elongation in the Hot Set Test at 2000C. The comparative examples of compositions Cl (1.42 pcr BCP) and C4 (1.25 pcr DTBH) have an MH of the order of 3.0-3.2 dN.m, and an HST elongation in the range 10 60-80%. In the case of cumyl peroxide (BCP), the addition of 1.0 pcr of TVCH makes it possible to lower the amount of peroxide to 1.27 pcr (composition C2) without having an impact on the properties mentioned previously. The 15 same conclusions apply to the DVB of composition C3, for which the addition of 1.0 pcr of DVB allows a decrease to 1.12 pcr of the amount of peroxide. At the same time, the amount of methane given off during the crosslinking goes from 113 to 101 pmol/g XLPE (TVCH) and to 93 pmol/g XLPE 20 (DVB). The breakdown times tl and t2 and the thermal aging behavior at 1350C are also improved for composition C2 relative to composition Cl. In the case of the aliphatic peroxide (DTBH), with regard to the comparative example C4, the addition of 25 1.0 pcr (composition C6) or 2.0 pcr (composition C7) of TVCH makes it possible to lower the amount of peroxide to 1.05 pcr, without having an impact on the crosslinking properties. The amount of methane given off during the crosslinking is greatly reduced, going from 104 to 30 69 pmol/g XLPE (composition C6) and 67 pmol/g XLPE (composition C7) . In this case also, the breakdown times and the thermal aging behavior at 1350C are greatly improved. Such a reduction without addition of coagent 35 (composition C5) leads to a crosslinked system (rupture 23 at the HST). Under the same conditions (1.0 pcr TVCH, composition C8 - 2.0 pcr TVCH, composition C9), the use of m-diisopropenylbenzene (MDIB) does not lead to a 5 network that is sufficiently crosslinked to pass the HST.

Claims (12)

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), at least one of these three layers (3, 4, 5) being a crosslinked layer obtained 10 from a crosslinkable composition comprising at least one polyolefin and an organic peroxide as crosslinking agent, characterized in that the composition also comprises a crosslinking coagent comprising at least two unsaturations, one of these two unsaturations being a 15 vinyl function.

2. Cable according to claim 1, characterized in that the at least two unsaturations of the crosslinking coagent are vinyl functions.

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 organic peroxide is an aliphatic peroxide.

7. Cable according to claim 6, characterized in that the organic peroxide is a tertiary dialkyl aliphatic peroxide. 35

8. Cable according to any one of the preceding 25 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 5 polyolefin.

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 10 claims, characterized in that the crosslinked layer is 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. 15

12. Cable according to any one of the preceding claims, characterized in that the vinyl function of the crosslinking coagent is an ethylenic function of the type CH 2 =CH-.