EP2498262B1 - High or middle tension electric cable - Google Patents

Description

The present invention relates to an electric cable. It applies typically, but not exclusively, to the fields of medium-voltage (in particular 6 to 45-60 kV) or high-voltage (in particular greater than 60 kV, and up to 800 kV) energy cables. , whether DC or AC.

The medium or high voltage energy cables typically comprise a central electrical conductor and, successively and coaxially around this electrical conductor, a semiconducting inner layer, an electrically insulating intermediate layer, a semiconducting outer layer, these three layers being crosslinked by techniques well known to those skilled in the art.

Conventionally, these three crosslinked layers are obtained from a composition based on a polyethylene polymer matrix associated with an organic peroxide of the dicumyl peroxide or tert-butyl peroxide cumyl type. During the crosslinking of said compositions, this type of peroxide decomposes and forms cross-linking by-products such as in particular methane, acetophenone, cumyl alcohol, acetone, tertiobutanol, alpha Styrene methyl and / or water. These latter two by-products 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 related to the explosiveness of methane and its ability to ignite should not be ignored. This gas can also cause damage once the cable is put into service. When the semiconducting outer layer is surrounded by a metal screen, which is generally the case in the medium and high voltage cable structure, it can spread only longitudinally along the cable to the junctions and terminations of the electrical installation (ie energy accessories). As a result, methane can accumulate and exert pressure on the energy accessories, which can lead to electrical breakdown. Even if solutions exist to limit the presence of methane within the cable, such as for example treating the cable thermally to accelerate the diffusion of methane outside the cable, they become long and expensive when the thickness of the insulating layers is important.

The document US 5,252,676 has a three-layer insulation for power cable. This trilayer insulation is obtained from compositions comprising an ethylene copolymer, an organic peroxide as crosslinking agent, and an additive of the isopropenyl benzene type or a derivative thereof. In order to limit the amount of gas released during the decomposition of the crosslinking agent, this document recommends reducing the amount of crosslinking agent.

However, the crosslinkable compositions used in this document are not optimized to limit the amount of crosslinking by-products, while ensuring satisfactory thermomechanical properties once the crosslinked compositions.

The object of the present invention is to overcome the disadvantages of the prior art techniques by proposing a medium or high voltage electrical cable, comprising a crosslinked layer whose manufacture significantly limits the presence of cross-linking by-products, such as methane, while ensuring optimum thermomechanical properties, such as hot creep, characteristics of the good crosslinking of said layer.

The present invention relates to an electrical cable comprising an electrical conductor, a first semiconductor layer surrounding the electrical conductor, a second electrically insulating layer surrounding the first layer, and a third semiconductor 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, characterized in that the composition further comprises a crosslinking coagent comprising at least two unsaturations, one of these two unsaturations being a vinyl function of the type CH ₂ = CH-.

The crosslinking coagent of the invention, which is different from the crosslinking agent, is of the multifunctional type since it comprises at least two unsaturations.

The at least two unsaturations are more particularly reactive functions of the carbon-carbon double bond type, which are capable on the one hand of grafting to the polyolefin and, on the other hand, of participating in the crosslinking of the polyolefin (ie on the formation of the three-dimensional network of the cross-linked polyolefin).

The crosslinking coagent advantageously makes it possible to significantly reduce the proportion of organic peroxide to be used in the crosslinkable composition, and thus to reduce the amount of methane derived from crosslinking by-products from said peroxide, while maintaining good thermomechanical properties such as than the hot creep, as well as a satisfactory crosslinking rate. The thermomechanical properties for the crosslinked layer according to the invention can advantageously result in a maximum stress elongation under the standard NF EN 60811-2-1 of at most 100%, preferably at most 80%, and in a particularly preferred manner ranging from 60 to 80%.

Preferably, a coagent whose boiling temperature is sufficiently high so that it does not volatilize during the stage of implementation of the crosslinkable composition, in particular by extrusion, will be used.

By way of example, the crosslinking agent may be chosen from 1,3-hexadiene; 1,4-hexadiene; 1,5-hexadiene; 2,3-dimethyl-1,3-butadiene; 2-methyl-1,4-pentadiene; 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; Vinyl-2 norbornene; 1,8-nonadiene; 7-methyl-1,6-octadiene; 1,4,9-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; 1,15-hexadecadiene; 1,6-diphenyl-1,3,5-hexatriene ; 2,3-dibenzyl-1,3-butadiene; and polybutadiene; or a mixture thereof.

In a particularly preferred embodiment, the at least two unsaturations of the crosslinking agent are vinyl functions, especially ethylenic functions of the CH ₂ = CH- type.

In this case, the crosslinking agent may be selected from 1,5-hexadiene; 1,6-heptadiene; 1,7-octadiene; 1,8-nonadiene; 1,4,9-decatriene; 1,9-decadiene; 3,9-divinyl-2,4,8,10-tetraoxaspiro [5.5] undecane; 1,2,4-trivinylcyclohexane; 1,13-tetradecadiene; 1,15-hexadecadiene; .

The concentration of the coagent is preferably limited so as not to disturb the extrusion process of the crosslinkable composition of the invention. For example, the crosslinkable composition may comprise at most 3 parts by weight of crosslinking agent per 100 parts of polymer (s) in the crosslinkable composition. It is preferred to use from 0.5 to 2 parts by weight of coagent per 100 parts by weight of polymer (s) in the crosslinkable composition.

The organic peroxide according to the invention may be chosen from organic peroxides well known to those skilled in the art, whether they be aliphatic or aromatic peroxides.

As an example of aromatic peroxide, mention may be made of dicumyl peroxide or tert-butyl peroxide cumyl.

• les peroxycarbonates aliphatiques tels que par exemple le tert-amylperoxy 2-ethylhexyl carbonate, le tert-amylperoxy 2-ethylhexyl carbonate, le tert-butylperoxy isopropyl carbonate ;
• les peroxydes aliphatiques de dialkyle tertiaire tels que par exemple le 1,1-di(tert-butylperoxy)cyclohexane, 2,5-dimethyl-2,5-di(tert-butylperoxy)-3-hexyne, 2,5-dimethyl-2,5-di(tert-butylperoxy)-3-hexane, le di-tert-amyl peroxyde, le di-tert-butyl peroxyde, les peroxydes cycliques tels que le 3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane ;

• les peroxyacetals aliphatiques tels que par exemple le butyl 4,4-di(tert-butylperoxy)valerate ; et
• les peroxyesters aliphatiques tels que par exemple le tert-butyl peroxyacétate, le tert-amyl peroxyacétate.

The aliphatic peroxide may be an aliphatic peroxide comprising at least one tertiary alkyl group. As an example of aliphatic peroxide, mention may be made of:

• aliphatic peroxycarbonates such as for example tert-amylperoxy 2-ethylhexyl carbonate, tert-amylperoxy 2-ethylhexyl carbonate, tert-butylperoxy isopropyl carbonate;
• tertiary dialkyl aliphatic peroxides such as, for example, 1,1-di (tert-butylperoxy) cyclohexane, 2,5-dimethyl-2,5-di (tert-butylperoxy) -3-hexyne, 2,5-dimethyl- 2,5-di (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;

• aliphatic peroxyacetals such as butyl 4,4-di (tert-butylperoxy) valerate; and
• aliphatic peroxyesters such as for example tert-butyl peroxyacetate, tert-amyl peroxyacetate.

The aliphatic peroxides, in comparison with aromatic peroxides, have the advantage of not forming cumyl alcohol as cross-linking by-products during the crosslinking of the crosslinkable composition, and thus makes it possible to significantly limit the presence of water at within the reticulated layer, while keeping very good thermomechanical properties.

Among the aliphatic peroxides mentioned, it will be preferred to use tertiary dialkyl aliphatic peroxides. Indeed, this type of peroxides has a very good compromise between crosslinking speed and risk of roasting or pre-crosslinking during the implementation of the composition.

Preferably, the crosslinkable composition does not comprise any aromatic peroxide, such as in particular dicumyl peroxides or their derivatives.

The peroxidic crosslinking of the crosslinkable composition according to the invention can be carried out under the action of heat and pressure, for example by means of a vulcanization tube under a nitrogen atmosphere, this crosslinking technique being well known to those skilled in the art.

The crosslinkable composition of the invention may comprise at most 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 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" as such generally means homopolymer or copolymer of olefin. It may in particular designate a thermoplastic polymer or an elastomer.

Preferably, the olefin polymer is an ethylene homopolymer or an ethylene copolymer.

By way of example of ethylene polymers, mention may be made of linear low density polyethylene (LLDPE), low density polyethylene (LDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), copolymers of d ethylene and vinyl acetate (EVA), copolymers of ethylene and butyl acrylate (EBA), methyl acrylate (EMA), 2-hexylethyl acrylate (2HEA), ethylene copolymers and alpha-olefins such as for example polyethylene-octene (PEO), polyethylene-butene (PEB), copolymers of ethylene and propylene (EPR) such as for example terpolymers of ethylene propylene diene (EPDM) ), and their mixtures.

It will be preferred to use a low density polyethylene (LDPE) since it has good rheological properties for its implementation, in particular by extrusion, and very good thermomechanical and electrical properties.

The term "low density" a density that can range from 0.910 to 0.940 g / cm ³ , and preferably ranging from 0.910 to 0.930 g / cm ³ according to ISO 1183 (at a temperature of 23 ° C).

Typically, low density polyethylene (LDPE) can be obtained by 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) (ie polymer matrix) in the 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.

Particularly advantageously, the crosslinkable composition comprises a polymer matrix which is composed solely of a polyolefin or a mixture of polyolefins.

The crosslinkable composition of the invention may further comprise an aromatic compound comprising at least one aromatic ring, 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 participate in the crosslinking of the polyolefin, unlike the crosslinking coagent, when it is present in the crosslinkable composition.

The crosslinked layer obtained from this crosslinkable composition has enhanced and durable properties in the field of electric cables, offering better resistance to water trees. More particularly, this relates to the resistance to electrical breakdown, and in particular the ability to dissipate the space charges that accumulate especially in high voltage cables under direct current.

The aromatic compound may be selected from styrene, styrene derivatives, and their isomers.

dans laquelle X est un hydrogène, un groupement alkyle ou un groupement aryle ; et R est soit un hydrogène, un groupement alkyle ou un groupement aryle. Plus particulièrement, on peut citer le 4-methyl-2,4-diphenyl pentene, et le triphenyl éthylène.By way of example of styrene derivatives, mention may be made of the compounds of the following general formula:

wherein X is 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-diphenyl pentene and triphenyl ethylene.

In the context of the present invention, it is also possible to consider the styrene derivatives of the polycyclic aromatic hydrocarbon (PAH) type. More particularly, mention may be made of vinyl naphthalene, for example 2-vinyl naphthalene; vinyl anthracene such as 9-vinyl anthracene or 2-vinyl anthracene; and vinyl phenanthrene such as 9-vinyl phenanthrene.

The grafting of these aromatic compounds onto the polymer chain of the polyolefin is typically carried out during the crosslinking phase of the polyolefin, according to a radical addition mechanism well known to those skilled in the art, in the presence of tertiary alkylperoxide. aliphatic of the invention.

The crosslinkable composition according to the invention may further comprise at least one protective agent such as an antioxidant. Antioxidants help to protect the composition of thermal stresses generated during the cable manufacturing or cable operation steps.

• les antioxydants phénoliques à encombrement stérique tels que le tetrakismethylene(3,5-di-t-butyl-4-hydroxy-hydrocinnamate) méthane, le octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, le 2,2'-thiodiethylene bis[3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate], le 2,2'-Thiobis(6-t-butyl-4-methylphenol), le 2,2'-methylenebis(6-t-butyl-4-methylphenol), le 1,2-Bis(3,5-di-t-butyl-4-hydroxyhydrocinnamoyl)hydrazine, le [2,2'-oxamido-bis(ethyl 3(3,5-di-t-butyl-4-hydroxyphenyl)propionate), et le 2,2'-oxamido-bis[ethyl 3-(t-butyl-4-hydroxyphenyl)propionate] ;
• les thioethers tels que le 4,6-bis(octylthiomethyl)-o-cresol, le Bis[2-methyl-4-{3-n-alkyl (C12 ou C14)thiopropionyloxy}-5-t-butylphenyl]sulfide et le Thiobis-[2-t-butyl-5-methyl-4,1-phenylene] bis [3-(dodecyltio)propionate] ;
• les antioxydants à base de soufre tels que le Dioctadecyl-3,3'-thiodipropionate ou le Didodecyl-3,3'-thiodipropionate ;
• les antioxydants à base de phosphore tels que les phosphites ou phosphonates comme par exemple le Tris(2,4-di-t-butyl-phenyl)phosphite ou le Bis(2,4-di-t-butylphenyl) pentaerythritol diphosphite ; et
• les antioxydants de type amine tels que le 2,2,4-trimethyl-1,2 dihydroquinoline polymérisé (TMQ), ce dernier type d'antioxydant étant particulièrement préféré dans la composition de l'invention.

The antioxidants are preferably chosen from:

• hindered phenolic antioxidants such as tetrakismethylene (3,5-di- t- butyl-4-hydroxy-hydrocinnamate) methane, octadecyl 3- (3,5-di- t- butyl-4-hydroxyphenyl) propionate, 2,2'-thiodiethylene bis [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-hydroxyhydrocinnamoyl) hydrazine, [2,2'-oxamido-bis] (ethyl 3 (3,5-di- t- butyl-4-hydroxyphenyl) propionate), and 2,2'-oxamido-bis [ethyl 3- ( t- butyl-4-hydroxyphenyl) propionate];
• thioethers such as 4,6-bis (octylthiomethyl) -o-cresol, bis [2-methyl-4- {3-n-alkyl (C 12 or C 14) thiopropionyloxy} -5- t- butylphenyl] sulfide and the Thiobis- [2- t -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 or phosphonates such as, for example, Tris (2,4-di- t- butyl-phenyl) phosphite or bis (2,4-di- t- butylphenyl) pentaerythritol diphosphite; and
• amine antioxidants 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.

• un grade dit « standard » avec un faible degré de polymérisation, c'est-à-dire avec un taux de monomère résiduel supérieur à 1% en poids et ayant une teneur en NaCl résiduelle pouvant aller de 100 ppm à plus de 800 ppm (parties par million massiques) ;
• un grade dit « à haut degré de polymérisation » avec un haut degré de polymérisation, c'est-à-dire avec un taux de monomère résiduel inférieur à 1% en poids et ayant une teneur en NaCl résiduelle pouvant aller de 100 ppm à plus de 800 ppm ;
• un grade dit « à faible teneur en sel résiduel » avec une teneur en NaCl résiduelle inférieure à 100 ppm.

TMQs can have different grades, namely:

• a so-called "standard" grade with a low degree of polymerization, that is to say with a residual monomer level greater than 1% by weight and having a residual NaCl content ranging from 100 ppm to more than 800 ppm ( parts per million mass);
• a so-called "high degree of polymerization" grade with a high degree of polymerization, that is to say with a residual monomer content of less than 1% by weight and having a residual NaCl content ranging from 100 ppm to more 800 ppm;
• a so-called "low residual salt" grade with a residual NaCl content of less than 100 ppm.

The type of stabilizer and its level in the crosslinkable composition are conventionally chosen according to the maximum temperature experienced by the polymers during the production of the mixture and during the implementation by extrusion on the cable as well as according to the maximum duration of exposure to this temperature.

The crosslinkable composition may typically comprise from 0.1% to 2% by weight of antioxidant (s). Preferably, it may comprise at most 0.7% by weight of antioxidant (s), especially when the antioxidant is TMQ.

Other additives and / or other fillers well known to those skilled in the art may also be added to the crosslinkable composition of the invention such as scorch retarders; agents promoting implementation such as lubricants or waxes; compatibilizing agents; coupling agents; UV stabilizers; non-conductive charges; conductive charges; and / or semiconductor charges.

According to a preferred embodiment, the crosslinked layer of the invention is the electrically insulating layer (ie second layer). In the case of the electrically insulating layer, the crosslinkable composition may not to understand charge (electrically) conductive and / or may not include semiconductor charge.

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 manufacture of the semiconductor layers (first layer and / or third layer), the crosslinkable composition further comprises at least one (electrically) conductive filler or a semiconductor filler, in an amount sufficient to to make the composition crosslinkable semiconductor.

It is more particularly considered that a layer is semiconductive when its electrical conductivity is at least 0.001 Sm ^-1 (Siemens per meter).

The crosslinkable composition used to obtain a semiconductor 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 preferably at least 25% by weight. conductive charge weight.

The conductive filler may advantageously be chosen from carbon blacks, carbon nanotubes, and graphites, or a mixture thereof.

Whether it is the first semiconductor layer, the second electrically insulating layer and / or the third semiconductor layer, at least one of these three layers is an extruded layer, preferably two of these three layers are extruded layers, and more preferably, these three layers are extruded layers.

In a particular embodiment, generally in accordance with the electrical cable well known in the field of application of the invention, the first semiconductor layer, the second electrically insulating layer and the third semiconductor layer constitute a three-layer insulation. In other words, the second electrically insulating layer is directly in physical contact with the first semiconductor layer, and the third semiconductor layer is directly in physical contact with the second electrically insulating layer.

The electrical cable of the invention may further comprise a metal screen surrounding the third semiconductor layer.

This metal screen may be a "wired" screen, consisting of a set of copper or aluminum conductors arranged around and along the third semiconductor layer, a so-called "ribbon" screen composed of one or more ribbons conductive metal laid helically around the third semiconductor layer, or a so-called "sealed" type metal screen surrounding the third semiconductor layer. This last type of screen makes it possible in particular to provide a moisture barrier that tends to penetrate the electrical cable radially.

All types of metal screens can act as grounding of the electric cable and can carry fault currents, for example in the event of a short circuit in the network concerned.

In addition, the electrical cable of the invention may comprise an outer protective sheath surrounding the third semiconductor layer, or more particularly surrounding said metal screen when it exists. This outer protective sheath can be made conventionally from suitable thermoplastic materials such as HDPE, MDPE or LLDPE; or materials retarding the propagation of the flame or resisting the spread of fire. In particular, if the latter materials do not contain halogen, it is called cladding type HFFR (for the Anglicism " Halogen Free Flame Retardant ").

Other layers, such as swelling layers in the presence of moisture, may be added between the third semiconductor layer and the metal screen when it exists, and / or between the metal screen and the outer sheath when they exist, these layers making it possible to ensure the longitudinal and / or transverse sealing of the electric cable with water. The electrical conductor of the cable of the invention may also comprise swelling materials in the presence of moisture to obtain a "sealed core".

Other characteristics and advantages of the present invention will appear in the light of the description of a non-limiting example of an electric cable according to the invention made with reference to the figure 1 showing a schematic perspective view in section of an electric cable according to a preferred embodiment according to the invention.

For the sake of clarity, only the essential elements for understanding the invention have been shown schematically, and this without respect of the scale.

Medium or high voltage power cable 1, shown in FIG. figure 1 comprises an elongate central conductive element 2, in particular made of copper or aluminum. Successively and coaxially around this conductive element 2, the energy cable 1 further comprises a first semiconductor layer 3 called "internal semiconductor layer", a second electrically insulating layer 4, a third so-called semiconductor layer 5 "External semiconductor layer", a metal screen 6 for grounding and / or protection, and an outer protective sheath 7, the layers 3, 4 and 5 being obtainable from a composition according to invention. Layers 3, 4 and 5 are extruded and crosslinked layers.

The presence of the metal screen 6 and the outer protective sheath 7 is preferred, but not essential, this cable structure being as such well known to those skilled in the art.

Examples Preparation of crosslinkable compositions

Table 1 Crosslinkable composition C1 C2 C3 C4 C5 C6 C7 C8 C9 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 - - - - - - - 1.00 2.00

In Table 1, crosslinkable compositions C1, C4, C5, C8 and C9 refer to comparative examples, while compositions C2, C3, C6 and C7 refer to compositions according to the invention.

The amounts of the components of the compositions C1 to C9, detailed in Table 1, are expressed in parts by weight (phr) per 100 parts by weight of polymer in the crosslinkable composition.

• « Polyoléfine » est un polyéthylène basse densité commercialisé par la société Inéos sous la référence BPD 2000 ;
• « BCP » est le peroxyde de tert-butyl cumyl, commercialisé par la société Arkema sous la référence Luperox 801 ;
• « DTBH » est le 1,1-di(tert-butylperoxy) cyclohexane (peroxyde aliphatique de dialkyle tertiaire), commercialisé par la société Arkema sous la référence Luperox 101 ;
• « TVCH » est le coagent de réticulation 1,2,4-trivinylcylclohexane, commercialisé par la société BASF sous la référence TVCH ;
• « DVB » est le coagent de réticulation divinyl benzène, commercialisé par la société Sigma-Aldrich sous la référence divinyl benzène ; et
• « MDIB » est le coagent m-diisopropenyl benzène, commercialisé par la société Sigma-Aldrich sous la référence 1,3-diisopropenyl benzène.

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 Inéos under the reference BPD 2000;
• "BCP" is tert-butyl cumyl peroxide, sold by Arkema under the reference Luperox 801;
• "DTBH" is 1,1-di (tert-butylperoxy) cyclohexane (tertiary dialkyl aliphatic peroxide), sold by Arkema under the reference Luperox 101;
• "TVCH" is 1,2,4-trivinylcyclohexane crosslinking coagent, marketed by BASF under the reference TVCH;
• "DVB" is the divinyl benzene crosslinking coagent, marketed by Sigma-Aldrich under the reference divinyl benzene; and
• "MDIB" is the coagent m-diisopropenyl benzene, sold by Sigma-Aldrich under the reference 1,3-diisopropenyl benzene.

Compositions C1 to C9 are prepared by mixing the polyethylene granules and additives such as peroxide, antioxidants, and optionally the coagent, in a closed jar placed on a roller mixer for 3 hours to thoroughly impregnate the granules. polyethylene. The polyethylene granules were preheated to 60 ° C before impregnation.

Then, the mixture was placed at 40 ° C for 16 hours, before being stored tightly.

Characterizations of the compositions Characterizations on non-crosslinked plate ● Kinetics and crosslinking level

The MDR rheometer (Moving Die Rheometer, Alpha Technologies) makes it possible to follow the crosslinking / vulcanization of a material by measuring the evolution of its viscosity (DIN 53529 (1983)).

The chamber containing the sample consists of two heating trays. The lower plate applies a constant oscillation frequency (100 cycles / min, 1.67Hz) and amplitude ± 0.5 ° arc; the upper plate measures the response of the material, ie its resistance to the stress applied. The unit of measure is that of a pair, 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., following a cycle of 2 minutes without pressure followed by 3 minutes under a pressure of 100 bars. before being cooled.

Two washers 35 mm in diameter to completely fill the chamber, are cut in the plate with a punch, then placed between two sheets of terphane polyester ^® , to be positioned in the rheometer chamber.

The measurement is made at a temperature of 190 ° C., representative of the tube vulcanization conditions. After an initial fall of the couple due to pre-melting of the material, the viscosity of the material and the resulting torque increase, sign that the crosslinking takes place.

A parameter of interest is the MH which corresponds to the maximum measured torque. This is a plateau value, obtained when the entire system has reacted and the maximum accessible crosslinking level is reached. For a given material, there is a good correlation between MH and crosslinking density that govern the thermomechanical properties after the crosslinking step.

● Toasting time

The Mooney viscometer (Monsanto MV2000) is used to measure the viscosity of a material, or in the case of crosslinkable materials, to monitor its evolution over time (ASTM D1646 (2005)).

It consists of two jaws forming a cylindrical cavity in which is placed the sample to be tested. The chamber has in its center a metal disk which is rotated at a constant speed of 2 rpm. In our case, of the two standard rotors available, the "big" is used.

During the measurement, the jaws and the chamber are maintained under pressure and at a temperature of 130 ° C.

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., following a cycle of 2 minutes without pressure followed by 3 minutes under a pressure of 100 bars. before being cooled.

Four washers 50 mm in diameter are cut from the plate with a punch. Two of them are drilled in their center with a hole of 12mm in diameter to put them in the rotor shaft, under it; the other two are kept intact and will be placed above the rotor. The whole is then placed between two sheets of terphane ^® polyester, to be positioned in the chamber of the viscometer.

It is the resistance of the material to the rotation of the rotor which is measured. The measure is expressed in arbitrary unit, the Mooney (MU). Parameters of interest are: ML, minimum viscosity value, measured at time t0 (Min). ML + 1, viscosity value corresponding to ML increased by one Mooney unit. This is measured at time t1 (min). ML + 2, viscosity value corresponding to ML increased by two Mooney units. This is measured at time t2 (min).

● Measurement of volatiles (see methane) by Sievert technology

The determination of the amount of volatiles produced during the crosslinking phase of the polyethylene, then desorbed, is made by the Sievert method, using the 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., following a cycle of 2 minutes without pressure followed by 3 minutes under a pressure of 100 bars. before being cooled.

Washers 6mm in diameter are then cut into the plate using a punch, and weighed exactly, to the nearest mg (total mass = 300-350 mg).

The sample is placed in the chamber of the apparatus, pressurized (helium). This chamber is connected by means of a valve, to a tank of 5ml also under pressure. At the beginning of the test, the pressures in the chamber and in the tank are identical. During the temperature cycle, the valve opens and closes intermittently, allowing a new equilibrium to be established when it is open, then measuring the new pressure in the tank when it is closed. The evolution of the pressure comes partly from the release of the methane, and partly from the dimensional variation of the chamber with the temperature. A real-time reading of the amount of methane released therefore requires a prior calibration by subjecting the chamber to the temperature cycle envisaged.

The equipment allows controlled temperature ramps of 1 ° C / s, simulating the crosslinking conditions of the different layers of polyethylene vulcanization tube.

The cycle provides heating from room temperature up to 250 ° C.

The difference between the final and initial pressure measurements, at the same temperature, makes it possible to access the level of methane released. The amount of volatiles (i.e. methane) is expressed in μmol / g of cross-linked polyethylene.

Characterizations on reticulated plate ● Cross-linking density by hot creep measurement

1mm thick plates are molded from the impregnated polyethylene granules. The molding is done in a press at 120 ° C, following a cycle of 2 minutes without pressure and then 3 minutes under 100 bars. The plates are then cooled under a pressure of 100 bar.

The crosslinking step is carried out in a press at a temperature of 190 ° C. under 100 bar pressure and lasts 10 minutes. The molds are preheated to 190 ° C. The cooling step is carried out under pressure maintained at 100 bars.

The measurement of the hot creep of a material under mechanical stress, as for it, is determined according to the standard NF EN 60811-2-1.

This test is commonly referred to as the Anglicism Hot Set Test (HST) and consists of ballasting one end of a dumbbell type H2 specimen with a mass corresponding to the application of a stress equivalent to 0.2MPa, and to place the whole in an oven heated to 200 +/- 1 ° C for a period of 15 minutes.

At the end of this time, the maximum heat stress elongation of the test piece, expressed in%, is recorded.

The suspended mass is then removed, and the test piece is kept in the oven for another 5 minutes.

The remaining permanent elongation, also called remanence (or remanent elongation), is then measured before being expressed in%.

It is recalled that the more a material is crosslinked, the higher the values of maximum elongation under stress and remanence will be low.

It is also specified that in the case where a test piece 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 this case, an elongation value will be considered as meeting the requirements if it does not exceed 100%. Beyond this value, in the same way as a break, the test will be considered as non-compliant.

● Mechanical properties and thermal aging

1mm thick plates are molded from the impregnated granules. The molding is done in a press at 120 ° C, following a cycle of 2 minutes without pressure and then 3 minutes under 100 bars. The plates are then cooled under a pressure of 100 bar.

The crosslinking step is carried out in a press at a temperature of 190 ° C. under 100 bar pressure and lasts 10 minutes. The molds are preheated to 190 ° C. The cooling step is carried out under pressure maintained at 100 bars.

The mechanical properties (stress and elongation at break), are measured on specimens of the dumbbell type H2, according to standard NF EN 60811-1-1.

The specimens, whose thickness is precisely measured, are tested after a minimum period of 16 hours at room temperature. The pulling speed is 200 mm / min. The initial mechanical property values are thus determined.

Specimens, the thickness of which is precisely measured, are also placed in an oven for accelerated thermal aging (7 days, at 135 ° C.) and then characterized in the same way. The variations in stress and elongation at break are thus determined. The variations are considered satisfactory when they are between + 25% and -25%, whether for the stress at break or for elongation at break.

The set of results concerning the characterizations of the non-crosslinked and crosslinked plates from the crosslinkable compositions C1 to C9 is collated in Table 2 below. <b><u> Table 2 </ u></b> Crosslinkable compositions C1 C2 C3 C4 C5 C6 C7 C8 C9 MH (dN.m) 3.2 3.4 3.2 3.0 2.2 3.2 2.8 1.9 1.2 ML at 130 ° C (Mooney units) 9.4 9.7 9.7 9.7 9.1 7.5 7.5 8.8 8.8 t0 (ML + 0) (min) 15 22 13 17 19 31 25 22 15 t1 (ML + 1) (min) 45 52 15 49 55 69 64 66 84 t2 (ML + 2) (min) 66 74 16 79 91 102 104 115 154 CH ₄ Sievert (μmol / g XLPE) 113 101 93 104 58 69 67 - - Hot set at 200 ° C Maximum elongation (%) 70-75 50-65 60-80 70-95 breaking 60-65 65-80 breaking breaking Remanence (%) 0 0 0 0 - 0 0 - - Accelerated thermal aging for 7 days at 135 ° C Variation (%) of the stress at break -33 0 6 -33 -23 -16 0 3 3 Variation (%) of elongation at break -17 6 5 -16 -13 -4 10 3 -1

The addition of a coagent according to the invention makes it possible to substantially reduce the level of peroxide necessary for the crosslinking, and thus to reduce the amount of volatiles (methane) evolved, while maintaining the desired crosslinking density. This is given by the MH and the elongation at the Hot Set Test at 200 ° C.

Comparative examples of compositions C1 (1.42pcr BCP) and C4 (1.25 phr DTBH)) have an MH of the order of 3.0-3.2dNm, and an HST elongation in the range 60-80%.

In the case of cumyl peroxide (PCO), the addition of 1.0 pcr of TVCH makes it possible to lower the peroxide level to 1.27 pcr (composition C2) without impacting the properties mentioned above. The same conclusions apply to DVB of composition C3 for which the addition of 1.0 pcr of DVB allows a decrease to 1.12 pcr of the peroxide level. At the same time, the level of methane released during crosslinking increases from 113 to 101 μmol / g XLPE (TVCH) and to 93 μmol / g XLPE (DVB). The roasting times t1 and t2 and the resistance to thermal aging at 135 ° C. are also improved for the composition C2 relative to the composition C1.

In the case of aliphatic peroxide (DTBH), compared with Comparative Example C4, the addition of 1.0 phr (composition C6) or 2.0 phr (composition C7) of TVCH makes it possible to reduce the level of peroxide to 1, 05pcr, without impacting the crosslinking properties. The level of methane released during the crosslinking is greatly reduced, from 104 to 69 .mu.mol / g XLPE (composition C6) and 67 .mu.mol / g XLPE (composition C7). Again, the roasting times and the resistance to thermal aging at 135 ° C are greatly improved.

Such a reduction without addition of coagent (composition C5) leads to a subcrosslinked system (rupture with HST).

Under the same conditions, (1.0 pcr TVCH, composition C8 - 2.0 pcr TVCH, composition C9), the use of m-diisopropenyl benzene (MDIB) does not lead to a sufficiently crosslinked network to pass the HST.

Claims (11)

1. An electric cable (1) comprising an electric conductor (2), a first semiconducting layer (3) surrounding the electric conductor (2), a second electrically insulating layer (4), surrounding the first layer (3), and a third semiconducting layer (5) surrounding the second layer (4), at least one of these three layers (3, 4, 5) being a crosslinked layer obtained from a crosslinkable composition comprising at least one polyolefin and an organic peroxide as a crosslinking agent, characterized in that the composition further comprises a co-crosslinking agent comprising at least two unsaturations, one of these two unsaturations being a vinyl function of the CH₂=CH- type.
2. The cable according to claim 1, characterized in that said at least two unsaturations of the co-crosslinking agent are vinyl functions.
3. The cable according to claim 1 or 2, characterized in that the polyolefin is a polymer of ethylene.
4. The cable according to claim 3, characterized in that the polymer of ethylene is a low density polyethylene (LDPE).
5. The cable according to any of the preceding claims, characterized in that the crosslinkable composition comprises more than 50.0 parts by weight of polyolefin for 100 parts by weight of polymer in the composition.
6. The cable according to any of the preceding claims, characterized in that the organic peroxide is an aliphatic peroxide.
7. The cable according to claim 6, characterized in that the organic peroxide is a tertiary aliphatic dialkyl peroxide.
8. The cable according to any of the preceding claims, characterized in that the crosslinkable composition further comprises an aromatic compound comprising at least one aromatic ring and a single reactive function capable of being grafted to the polyolefin.
9. The cable according to claim 8, characterized in that the single reactive function of the aromatic compound is a vinyl function.
10. The cable according to any of the preceding claims, characterized in that the crosslinked layer is the electrically insulating layer.
11. The cable according to any of the preceding claims, characterized in that the three layers of the cable are crosslinked layers.

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