EP2784787A1 - Electric cable including a layer with electrical property gradient - Google Patents

Abstract

The cable (1) has a layer (3) with an electrical property gradient surrounding a central lengthened electric driver (2). The layer includes a zone (3c) located between an inner surface (3a) and an outer surface (3b). The zone includes an electrical conductivity and/or a dielectric permittivity lower than electrical conductivities and/or dielectric permitvities of the inner surface and the outer surface. The layer is obtained from a polymeric composition including polymer and electrically conducting filling material e.g. carbonaceous filling material. The carbonaceous filling material is carbon black, carbon fiber, graphite, graphene, fullerene, carbon nanotube, or mixture. An independent claim is also included for a method for manufacturing a layer with an electrical property gradient.

Description

The present invention relates to an electric cable comprising a layer of electrical property gradient based on electrically conductive charges, intended to improve the resistance to breakdown and resistance to aging in a wet environment under electrical voltage.

It applies typically, but not exclusively, to the fields of medium voltage (especially 6 to 45-60 kV) or high voltage (especially greater than 60 kV) and up to 500-600 kV, or up to 800 kV), whether DC or AC.

The discontinuity of electrical properties between an electrically insulating material and a conductive (or semiconductor) material can cause a local reinforcement of the electric field by accumulation of space charges or charged species capable of initiating a tree under action. an electric field.

In particular, the presence of moisture combined with the presence of an electric field with a polymer material promote the progressive degradation of the insulating properties of medium and high voltage power cables.

This degradation mechanism, well known as the term " water treeing ", may typically be present in medium or high voltage power cables, including classically an insulation three-layer internal semiconductor layer / electrically insulating layer / outer semiconductor layer, and optionally surrounded by a metal screen and a protective sheath.

• un premier écran semi-conducteur interne, pouvant être divisé en deux couches élémentaires avec un taux massique de polymère conducteur constant et différent dans ces deux couches, puis
• une couche électriquement isolante, puis
• un second écran semi-conducteur externe pouvant être divisé en deux couches élémentaires avec un taux massique de polymère conducteur constant et différent dans ces deux couches, et enfin
• une gaine de protection.

As such, for example, the document EP-0 645 781 which discloses an energy cable comprising an elongated electrical conductor, and a succession of several layers around said elongate electrical conductor, such as:

• a first internal semiconductor screen, which can be divided into two elementary layers with a constant and different conductive polymer mass content in these two layers, and then
• an electrically insulating layer, then
• a second external semiconductor screen that can be divided into two elementary layers with a constant and different conductive polymer mass content in these two layers, and finally
• a protective sheath.

Said growth of electric trees due to water can thus lead to the breakdown of the electric cable concerned and therefore constitutes a considerable threat to the reliability of the energy transmission network with well-known economic consequences caused by short circuits.

The object of the present invention is to overcome the disadvantages of the prior art techniques by proposing an electric cable, in particular a medium voltage or high voltage power cable, having an electrical breakdown resistance as well as an electrical resistance. aging in a humid environment in the presence of an electric field, significantly improved.

• une conductivité électrique et/ou une permittivité diélectrique respectivement inférieure à la conductivité électrique et/ou la permittivité diélectrique de la surface interne, et
• une conductivité électrique et/ou une permittivité diélectrique respectivement inférieure à la conductivité électrique et/ou la permittivité diélectrique de la surface externe.

A subject of the present invention is an electrical cable comprising an elongated electrical conductor, characterized in that the electrical cable further comprises an electrical property gradient layer surrounding the elongate electrical conductor, said layer comprising an inner surface, an outer surface and a an area between said inner surface and said outer surface, said area having:

• an electrical conductivity and / or a dielectric permittivity respectively lower than the electrical conductivity and / or the dielectric permittivity of the internal surface, and
• an electrical conductivity and / or a dielectric permittivity respectively lower than the electrical conductivity and / or the dielectric permittivity of the outer surface.

The electrical properties of the electrical property gradient layer may be electrical conductivity and / or dielectric permittivity. We can speak of gradient layer of electrical conductivity and / or dielectric permittivity.

• un premier gradient de propriété électrique dont la conductivité électrique et/ou la permittivité diélectrique diminue, notamment graduellement, de la surface interne vers ladite zone, et
• un second gradient de propriété électrique dont la conductivité électrique et/ou la permittivité diélectrique diminue, notamment graduellement, de la surface externe vers ladite zone.

More particularly, the electrical conductivity and / or dielectric permittivity of the inner surface, on the one hand, and the electrical conductivity and / or the dielectric permittivity of the outer surface, on the other hand, decrease towards said area. In other words, the gradient layer of electrical property comprises in particular:

• a first electrical property gradient whose electrical conductivity and / or dielectric permittivity decreases, in particular gradually, from the internal surface towards said zone, and
• a second electrical property gradient whose electrical conductivity and / or dielectric permittivity decreases, in particular gradually, from the outer surface towards said zone.

The first and second gradients are therefore within the same layer of electrical property gradient, said zone being positioned in the thickness of said layer.

The area between the inner surface and the outer surface of the electrical property gradient layer may be located substantially equidistant from the inner surface on the one hand, and from the outer surface of another, in cross-section of the cable electric.

More particularly, this zone may be positioned around the middle of the thickness E of the electrical property gradient layer. The term "middle of the thickness" means the distance defined by half the thickness E of the layer of electrical property gradient, in cross section of the electric cable.

The electrical property gradient layer of the invention preferably has a thickness E substantially constant along the electric cable to ensure homogeneous and repeatable mechanical and electrical properties all along the electrical cable.

The Applicant has surprisingly discovered that the presence of a layer with an electrical property gradient as defined in the invention makes it possible effectively to limit, or even avoid, the degradations associated with electrical trees caused by space charges or charged species induced in particular by the presence of water in this type of electric cable.

In addition, the gradient layer of electrical property according to the invention advantageously improves the resistance to breakdown AC or DC voltage cable of the invention.

Finally, the layer of electrical property gradient of the invention, which is more particularly a "monolayer", advantageously replaces the multilayer insulation, in particular tricouche, well known in the medium or high voltage energy cables, this monolayer thus being easy to implement, in particular by extrusion. The gradient layer of electrical property of the invention is therefore advantageously a single layer.

In other words, said monolayer of the invention is not a superposition of several layers (including extruded layers) with constant electrical properties in each of said layers. The gradient layer of electrical property of the invention thus overcomes the physical interfaces of the multilayer insulation of the prior art.

• la surface externe peut avoir la conductivité électrique d'un matériau électriquement conducteur ou d'un matériau semi-conducteur ou d'un matériau électriquement isolant,
• la surface interne peut avoir la conductivité électrique d'un matériau électriquement conducteur ou d'un matériau semi-conducteur ou d'un matériau électriquement isolant,
• la zone située entre la surface interne et la surface externe de la couche à gradient de propriété électrique a une conductivité électrique inférieure à celle de la surface interne d'une part, et inférieure à celle de la surface externe d'autre part : la conductivité électrique dans ladite zone peut donc être celle d'un matériau électriquement conducteur ou d'un matériau semi-conducteur ou d'un matériau électriquement isolant.

The variation of electrical conductivity in the thickness of the electrical property gradient layer between the inner surface and the outer surface can be defined more particularly as follows:

• the outer surface may have the electrical conductivity of an electrically conductive material or a semiconductor material or an electrically insulating material,
• the inner surface may have the electrical conductivity of an electrically conductive material or a semiconductor material or an electrically insulating material,
• the area between the inner surface and the outer surface of the electrical property gradient layer has a lower electrical conductivity than the inner surface on the one hand, and lower than the outer surface on the other hand: the conductivity electrical in said area may be that of an electrically conductive material or a semiconductor material or an electrically insulating material.

In a particularly preferred embodiment, the electrical conductivity of the outer surface is that of a semiconductor material, the electrical conductivity of the inner surface is that of a semiconductor material, and the electrical conductivity of the zone is that of an electrically insulating material.

The electrical conductivities of the inner surface and the outer surface may be different or the same.

In the present invention, the electrical conductivity of an electrically conductive material may be at least 1.10 ³ S / m.

The electrical conductivity of a semiconductor material may be at least 1.10 ^-9 S / m (siemens per meter), preferably at least 1.10 ^-3 S / m, and preferably may be less than 1.10 ³ S / m.

The electrical conductivity of an electrically insulating material may be at most 1.10 ^-9 S / m.

In the present invention, the electrical conductivity of a material is conventionally determined according to the ASTM D 991 standard. The dielectric permittivity is conventionally determined according to the IEC 60250 standard.

The characteristics of electrical conductivity and / or dielectric permittivity are in particular functions of the nature of the materials directly in physical contact with the inner and outer surfaces of said layer of electrical property gradient.

More particularly, when the electrical property gradient layer is directly in physical contact with the elongated electrical conductor, the electrical conductivity at the inner surface of said layer will preferably be that of an electrically conductive material or a semiconductor material. .

When the electrical property gradient layer is directly in physical contact with a metal screen surrounding said layer, the electrical conductivity at the outer surface of said layer will preferably be that of an electrically conductive material or a semiconductor material.

The electrical conductivity of the zone may be in this case that of a semiconductor material or an electrically insulating material.

By way of example, the electrical conductivity of the inner surface of the electrical property gradient layer may be at least 1.10 ^-9 S / m, preferably at least 1.10 ^-3 S / m, and preferably may be less than 1.10 ³ S / m.

The electrical conductivity of the outer surface of the electrical property gradient layer may be at least 1.10 ^-9 S / m, preferably at least 1.10 ^-3 S / m, and preferably may be less than 1.10 ^3. S / m.

The electrical conductivity of the zone (located between the inner surface and the outer surface) of the electrical property gradient layer may be at most 1.10 ^-9 S / m.

The electrical property gradient layer of the invention is obtained from a polymer composition comprising at least one polymer and electrically conductive charges.

The electrically conductive charges of the invention are preferably conductive carbon charges.

"Conductive carbon charge" is understood to mean any particle, or mixture of particles, mainly consisting of carbon atoms, functionalized or otherwise, grafted or not, and having electrically conductive properties.

By way of examples, the conductive carbonaceous fillers are chosen from carbon blacks, carbon fibers, graphites, graphenes, fullerenes, carbon nanotubes and one of their mixtures.

Carbon nanotubes are preferably used. The term "nanotubes" means nanoparticles of substantially elongate shape, and whose smallest dimension may be between 1 and 100 nm (inclusive) (dimension determined by microscopic analysis such as SEM (Scanning Electron Microscopy), TEM (Transmission Electron Microscopy) or by MFA (Atomic Force Microscopy) Nanotubes typically have a so-called "acicular" shape.

Carbon nanotubes have the advantage of having better compatibility with the polymer of the polymeric composition, compared to other types of conductive carbonaceous fillers mentioned in the present invention.

In addition, carbon nanotubes having a high form factor, especially at least 1000, they achieve the percolation with relatively low amounts of conductive carbonaceous charges compared to other carbonaceous charges.

The form factor is typically the ratio of the smallest dimension of the conductive filler (i.e. the diameter, for the carbon nanotubes) to the largest dimension of said conductive filler (i.e. the length, for the carbon nanotubes).

Thus, through the use of carbon nanotube type charges, the mechanical and electrical properties as well as the adhesion properties of the electrical property gradient layer are optimized.

Carbon nanotubes can be of several types. They may be chosen from single-walled carbon nanotubes, double-walled carbon nanotubes, multiwall carbon nanotubes and one of their mixtures. Multi-walled carbon nanotubes , well known under the Anglicism " multi-walled nanotubes (MWNT)", will preferably be used.

In order to constitute a gradient of electrical conductivity, the quantity of electrically conductive charges in the polymer composition of the invention is in particular sufficient to constitute a percolating network. To constitute a dielectric permittivity gradient, this condition is not necessary.

"Percolating network" means an organization of electrically conductive charges capable of creating one or more electrical paths continuous within the polymeric composition of the electrical property gradient layer.

In the present invention, this percolating network can in particular be achieved by the application of heat treatment (s).

More particularly, the level of electrically conductive fillers is sufficient for the polymeric composition of the invention to achieve a so-called "dynamic" percolation transition.

"Dynamic percolation" is understood to mean an insulator-conductive transition (ie an increase of several orders of magnitude in the electrical conductivity and / or divergence of the dielectric permittivity associated with the mesoscopic scale by the formation and growth of clusters of particles that tend to constituting an infinite cluster of inter-connected charges) observed at constant charge rate and resulting from a microstructural rearrangement of the composite by self-assembly of the conductive particles in the molten polymer. The kinetics of this mechanism is a priori thermally activated.

The polymeric composition of the electrical property gradient layer may comprise at most 30% by weight of electrically conductive fillers, preferably at most 10% by weight of electrically conductive charges, and particularly preferably at most 5% by weight of charges. electrically conductive. Preferably, it comprises at least 0.1% by weight of electrically conductive charges.

The electrical property gradient layer is obtained from a polymer composition comprising at least one polymer in which said electrically conductive charges are incorporated, to form a composite polymer material.

"Polymeric composition" is understood to mean a composition based on one or more polymers, in particular enabling it to be easily shaped by extrusion, injection or molding.

The polymer composition may comprise at least 40% by weight of polymer (s), preferably more than 50% by weight of polymer (s), preferably at least 70% by weight of polymer (s), preferably at least 80% by weight. % by weight of polymer (s), and particularly preferably at least 90% by weight of polymer (s).

The polymeric composition may be a thermoplastic or elastomeric composition, crosslinkable or not.

The polymeric composition of the invention may be a thermoplastic composition, that is to say that it mainly comprises one or more thermoplastic polymers with respect to the polymers constituting the polymeric composition.

The polymeric composition of the electrical property gradient layer may be an elastomeric composition, that is to say that it mainly comprises one or more elastomeric polymers with respect to the polymers constituting the polymeric composition.

When the polymeric composition is crosslinkable, it may further comprise one or more crosslinking agents well known to those skilled in the art.

The polymer of the polymeric composition of the invention may be selected from an organic polymer, an inorganic polymer, and a mixture thereof.

When the polymer of the polymeric composition is an organic polymer, said organic polymer may comprise at least one polyolefin and / or at least one polyepoxide.

The term "polyolefin" as such generally means homopolymer or copolymer of olefin. Preferably, the olefin polymer is a homopolymer of ethylene, or a copolymer of ethylene (i.e., copolymer comprising at least ethylene).

By way of example of ethylene polymers, mention may be made of linear low density polyethylene (LLDPE), very low density polyethylene (VLDPE), low density polyethylene (LDPE) and medium density polyethylene (MDPE). ), high density polyethylene (HDPE), copolymers of ethylene and vinyl acetate (EVA), copolymers of ethylene and butyl acrylate (EBA), methyl acrylate (EMA), 2-hexylethyl acrylate (2HEA), copolymers of ethylene 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), poly (ethylene terephthalate) (PET), or a mixture thereof, and / or a derivative thereof.

It is preferable to use an EVA with a low level of vinyl acetate groups (less than 20% by weight) in order to limit the presence of polar functions, or more advantageously a polyethylene of the VLDPE, LDPE, LLDPE, MDPE or HDPE type.

The polymeric composition of the electrical property gradient layer 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 constituent polymer (s) of the polymer composition of the electrical property gradient layer are only one or more polyolefins. In this case, it will be preferred to use a single type of polymer in the composition such as EVA with a low level of vinyl acetate moieties, or VLDPE, LDPE, LLDPE, MDPE or HDPE.

The term "polyepoxide" (or "epoxide polymer") as such generally means a multi-component polymer obtained by polymerization of epoxide monomers with a crosslinking agent, said crosslinking agent being of the acid anhydride type, phenol, or amine.

As an example of a polyepoxide, mention may be made of DiGlycidylether of Bisphenol A (DGEBA).

When the polymer of the composition is an inorganic polymer, said inorganic polymer may comprise at least one polysiloxane. In the present invention, the inorganic polymers are therefore very different from the organic polymers.

In fact, the polysiloxanes, or silicones, are inorganic compounds formed of a silicon-oxygen chain (... -Si-O-Si-O-Si-O-...) on which groups can be attached to the silicon atoms.

By way of example, mention may be made of poly (dimethylsiloxane).

The electrical cable of the invention may further comprise a metal screen surrounding the electrical property gradient layer.

This metal screen may be a so-called "wired" screen, consisting of a set of copper or aluminum conductors arranged around and along the layer of electrical property gradient, a so-called "striped" screen composed of one or more conductive metal ribbons laid helically around the layer of electrical property gradient, or a so-called "sealed" type metal screen surrounding the gradient layer of electrical property. 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 play the role of grounding the electric cable and can thus 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 also comprise an outer protective sheath surrounding the layer of electrical property gradient, 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 ") .

The electrical cable of the invention may advantageously be a medium or high voltage power cable, which may include said metal screen and said protective sheath.

In a particular embodiment, the electrical property gradient layer is preferably directly in physical contact with the elongated electrical conductor, and / or directly in physical contact with the metal screen when it exists.

In other words, the inner surface of the electrical property gradient layer is preferably directly in physical contact with the elongate electrical conductor, and / or the outer surface of the electrical property gradient layer is preferably directly in contact physical with the metallic screen when it exists.

In a particular embodiment, the electrical property gradient layer of the invention is an extruded layer, the extrusion being a method well known to those skilled in the art.

• réaliser une couche polymérique obtenue à partir d'une composition polymérique comprenant au moins un polymère et des charges électriquement conductrices, ladite couche polymérique comprenant une première surface, une deuxième surface, et une zone située entre ladite première surface et ladite deuxième surface,
• réaliser un premier traitement thermique de ladite couche polymérique en appliquant une première température T1 à la première surface et une deuxième température T2 à la deuxième surface, de sorte à avoir une conductivité électrique et/ou une permittivité diélectrique dans la zone respectivement inférieure à la conductivité électrique et/ou une permittivité diélectrique de la première surface,
• réaliser un deuxième traitement thermique de la couche polymérique en appliquant une troisième température T3 à la première surface et une quatrième température T4 à la deuxième surface, de sorte à avoir une conductivité électrique et/ou une permittivité diélectrique dans la zone respectivement inférieure à la conductivité électrique et/ou une permittivité diélectrique de la deuxième surface,

pour obtenir ladite couche à gradient de propriété électrique.Another subject of the invention relates to a method of manufacturing an electric gradient of property layer, characterized in that it comprises the following steps:

• producing a polymeric layer obtained from a polymeric composition comprising at least one polymer and electrically conductive fillers, said polymeric layer comprising a first surface, a second surface, and an area between said first surface and said second surface,
• performing a first heat treatment of said polymeric layer by applying a first temperature T1 to the first surface and a second temperature T2 at the second surface, so as to have an electrical conductivity and / or a dielectric permittivity in the area respectively less than the electrical conductivity and / or a dielectric permittivity of the first surface,
• performing a second heat treatment of the polymeric layer by applying a third temperature T3 to the first surface and a fourth temperature T4 to the second surface, so as to have an electrical conductivity and / or a dielectric permittivity in the area respectively less than the conductivity electrical and / or dielectric permittivity of the second surface,

to obtain said gradient layer of electrical property.

The electrical property gradient layer of said method is more particularly that defined in the present invention.

Said first heat treatment makes it possible to form a first electrical property gradient from the first surface towards said zone.

Said second heat treatment makes it possible to form a second electrical property gradient from the second surface towards said zone.

The polymeric composition of the above manufacturing process is that described in the present invention. In particular, it will be preferred to use the conductive carbonaceous charges as electrically conductive charges.

As defined in the invention, the zone situated between the first surface and the second surface notably comprises an electrical conductivity and / or a dielectric permittivity respectively lower than the electrical conductivity and / or dielectric permittivity of the first surface, and respectively less than the electrical conductivity and / or dielectric permittivity of the second surface.

The control of the temperatures T1 and T2 on the one hand, and T3 and T4 on the other hand, makes it possible to control the gradient of the percolation kinetics, in particular of dynamic percolation, and consequently the levels of electrical conductivities and / or dielectric permittivity achieved.

• la température T1 soit différente de la température T2 pour former ledit premier gradient de propriété électrique, et
• la température T3 soit différente de la température T4 pour former ledit deuxième gradient de propriété électrique.

Of course, it is necessary that:

• the temperature T1 is different from the temperature T2 to form said first gradient of electrical property, and
• the temperature T3 is different from the temperature T4 to form said second gradient of electrical property.

The difference between the temperatures T1 and T2 may be at least 10 ° C, preferably at least 50 ° C, and particularly preferably at least 100 ° C.

The difference between the temperatures T3 and T4 can be at least 10 ° C, preferably at least 50 ° C, and particularly preferably at least 100 ° C.

The temperature T1 may preferably be greater than the temperature T2.

The temperature T4 may preferably be greater than the temperature T3.

The temperature T1 may preferably be a temperature equal to or greater than the melting temperature Tf or the glass transition temperature Tg of said polymer.

The temperature T2 may preferably be a temperature equal to or less than the melting temperature Tf or the glass transition temperature Tg of said polymer.

The temperature T4 may preferably be a temperature equal to or greater than the melting temperature Tf or of the glass transition temperature Tg of said polymer.

The temperature T3 may preferably be a temperature equal to or less than the melting temperature Tf or of the glass transition temperature Tg of said polymer.

In a particular embodiment, the temperature T1 may be substantially equal to the temperature T4 and / or the temperature T2 may be substantially equal to the temperature T3.

In the present invention, the melting temperature Tf is to be considered when for example the polymer is a semicrystalline or crystalline polymer. The glass transition temperature Tg is to be considered when, for example, the polymer is an amorphous polymer. These types of polymer structures are well known to those skilled in the art.

The heat treatments of the invention may be carried out by techniques well known to those skilled in the art, such as, for example, by convection, conduction and / or irradiation.

The heat treatments (ie heating steps) of the polymeric composition of the invention advantageously make it possible to form gradients of electrical conductivity and / or dielectric permittivity in the thickness of the polymeric layer obtained from said polymeric composition.

Indeed, thanks to the heat treatment, the relaxation of the polymer chains in the molten or viscous state, associated with flocculation forces, cause a modification of the microstructure of the polymeric composition, thereby promoting the formation or improvement of the network. percolating.

The kinetics of this self-arrangement of the electrically conductive charges is particularly dependent on the temperature. In other words, the high temperatures compared with the melting temperature T _f of the semi-crystalline or glass transition polymers T _g of the amorphous polymers result in high load network reinforcement kinetics and lead to high values of electrical conductivity and / or dielectric permittivity of the polymeric layer. On the other hand, at temperatures close to T _f or T _g , the auto-setting rate remains low and the electrical properties of the polymeric layer are only slightly affected. In other words, the completion of the network of electrically conductive charges, associated with the levels of electrical conductivity and / or dielectric permittivity presented by the polymer layer of the invention, is directly correlated with the temperature of the heat treatment called "curing treatment" .

The characteristic temperatures of the physicochemical transitions of a polymeric composition, of the crosslinked or non-crosslinked type, can be classically determined by differential scanning calorimetry (DSC) with a temperature ramp of 10 ° C./min under a nitrogen atmosphere.

In a particular embodiment, when the polymeric layer surrounds an elongate electrical conductor, such as that defined herein. In order to form an electrical cable, said elongated electrical conductor can be used as a heat source for the application of the first and / or second heat treatment (s).

Considering that the first surface of the polymeric layer is the inner surface in contact with the electrical conductor, and the second surface of the polymeric layer is the outer surface, the first heat treatment may be to apply a current through the elongated electrical conductor for heating at a temperature T1, by Joule effect or by induction, the inner surface of the polymeric layer, while the outer surface of the polymeric layer is maintained at a temperature T2 lower than the temperature T1.

The second heat treatment may consist in heating, especially by convection, conduction or irradiation, the outer surface of the polymeric layer, while the inner surface of the polymeric layer is maintained at a temperature T3 lower than the temperature T4.

Of course, if the electric cable further comprises an electrically conductive element surrounding said polymeric layer, such as for example a metal screen, the latter may be used as a means of heating the outer surface of the polymeric layer, by passing a current through this electrically conductive element.

The temperatures T1, T2, T3 and T4 are those defined in the invention.

The heat treatment step is preferably carried out after any implementation step of the polymeric composition of the invention.

More particularly, once the polymeric composition used to form the polymeric layer of the invention, the heat treatments can be applied to said polymeric layer.

• d'une couche extrudée obtenue par extrusion de la composition polymérique de l'invention, et/ou
• d'un élément ou d'une plaque, obtenu par moulage ou par injection de la composition polymérique de l'invention.

The heat treatments can thus be applied to a polymeric layer in the form:

• an extruded layer obtained by extrusion of the polymeric composition of the invention, and / or
• an element or a plate, obtained by molding or by injection of the polymeric composition of the invention.

In addition, if the layer of a polymeric material with a gradient of electrical property is a crosslinked layer, the heat treatment step preferably takes place prior to the crosslinking of said layer.

The method of the invention may further comprise an additional step, between the first heat treatment and the second heat treatment.

This additional step consists in cooling the polymer layer to a temperature below the Tf or glass transition temperature Tg of said polymer, once the first heat treatment has been performed, and before carrying out the second heat treatment.

• La figure 1 représente une vue schématique en coupe transversale d'un câble électrique selon un mode de réalisation préféré conforme à l'invention.
• La figure 2 représente l'évolution de la conductivité électrique dans l'épaisseur E de la couche à gradient de propriété électrique, dans un mode de réalisation préféré conforme à l'invention.

Other features and advantages of the present invention will appear in light of the examples which follow with reference to the annotated figures, said examples and figures being given for illustrative and not limiting.

• The figure 1 is a schematic cross-sectional view of an electric cable according to a preferred embodiment according to the invention.
• The figure 2 represents the evolution of the electrical conductivity in the thickness E of the gradient layer of electrical property, in 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.

• une couche 3 à gradient de propriété électrique selon la présente invention, et
• un écran métallique 4 entourant ladite couche 3.

Medium or high voltage power cable 1, shown in FIG. figure 1 comprises an elongate central electrical conductor 2, in particular made of copper or aluminum, and, successively and coaxially around this central electrical conductor 2, is:

• a gradient layer 3 of electrical property according to the present invention, and
• a metal screen 4 surrounding said layer 3.

Layer 3 is an extruded layer, which may or may not be crosslinked. This layer 3 is in direct physical contact with the electrical conductor 2 and with the metal screen 4.

This layer comprises a thickness E delimited by an internal surface 3a and an external surface 3b, this thickness E being substantially constant around the electrical conductor 2.

This layer further comprises an area 3c located between the inner surface 3a and the outer surface 3c. This zone 3c is substantially at equidistance of the inner surface 3a and the outer surface 3b, and is in the middle of said layer, around E / 2.

The electrical property gradient in the thickness E of the layer 3 is such that the electrical conductivity and / or dielectric permittivity of the inner surface 3a and the electrical conductivity and / or dielectric permittivity of the outer surface 3b are substantially identical and are respectively greater than the electrical conductivity and / or dielectric permittivity of the zone 3c.

The metal screen 4 is of the cylindrical tube type, but does not limit the invention. An outer protective sheath (not shown) may also be positioned around the metal screen 4.

The figure 2 illustrates the evolution of the electrical conductivity within the layer of electrical property gradient in a preferred embodiment according to the invention.

Starting from the internal surface 3a where the electrical conductivity is equal to σ1, the electrical conductivity σ1 will decrease in the direction of the zone 3c, this zone being situated around the middle of the thickness E of the layer 3, to reach a conductivity electrical equal to σ2, σ2 being less than σ1.

Then the electrical conductivity σ2 of the zone 3c will increase, within the same layer, to reach an electrical conductivity substantially equal to σ1 at the outer surface 3b of the gradient layer of electrical property.

The variation profile of the electrical conductivity in the thickness E of the electrical property gradient layer of the invention is a "U" shaped profile in the embodiment shown in FIG. figure 2 .

The same curve profile as that of the figure 2 (see the references to electrical conductivities being replaced by those of dielectric permittivities) can also be obtained for the evolution of the dielectric permittivity in the thickness E of the gradient layer of electrical property.

Examples

A polymeric layer having an electrical property gradient according to the invention was made by applying a first temperature gradient and then a second temperature gradient, on a composite polymer sample comprising 5% by weight of multi-walled conductive carbon nanotubes.

This polymer sample is obtained by dispersing, in a "molten" bis-screw extruder, a masterbatch containing 30% by weight of ARKEMA Graphistrength C100 multi-walled carbon nanotubes supported on a polyethylene base in a matrix. VLDPE polymer ("Very Low Density Polyethylene") having a MFI ("Melt Flow Index") of 7.5 (determined according to ASTM D1238), a density of 0.9 (determined according to ASTM D1505), and a melting temperature of 113 ° C.

Once the homogenization has been carried out, the polymeric composition is shaped by hot pressing at 120 ° C., to obtain a plate 8 mm thick (substantially constant thickness), with a lower surface (ie internal surface) and a outer surface (ie outer surface).

This plate is then placed in a hot compression press consisting of an upper plate and a lower plate.

• une température T1 de 300°C est appliquée au plateau supérieur de la presse, le plateau supérieur de la presse étant en contact physique avec la surface supérieure de la plaque, et
• une température T2, régulée pour ne pas dépasser 60°C, est appliquée au plateau inférieur de la presse, le plateau inférieur de la presse étant en contact physique avec la surface inférieure de la plaque.

A first temperature gradient (ie first heat treatment) was established as follows:

• a temperature T1 of 300 ° C. is applied to the upper plate of the press, the upper plate of the press being in physical contact with the upper surface of the plate, and
• a temperature T2, regulated to not exceed 60 ° C, is applied to the lower plate of the press, the lower plate of the press being in physical contact with the lower surface of the plate.

This first heat treatment took place for 6 minutes.

At the end of the first heat treatment, said plate was cooled to a temperature of 25 ° C.

A second temperature gradient (ie second heat treatment) was then applied, using the same hot compression press, and under the same conditions as the first heat treatment, except that for this second heat treatment, the plate has been returned.

• une température T1 de 300°C appliquée au plateau supérieur de la presse, le plateau supérieur de la presse étant en contact physique avec la surface inférieure de la plaque, et
• une température T2, régulée pour ne pas dépasser 60°C, appliquée au plateau inférieur de la presse, le plateau inférieur de la presse étant en contact physique avec la surface supérieure de la plaque.

We then have:

• a temperature T1 of 300 ° C applied to the upper plate of the press, the upper plate of the press being in physical contact with the lower surface of the plate, and
• a temperature T2, regulated to not exceed 60 ° C, applied to the lower plate of the press, the lower plate of the press being in physical contact with the upper surface of the plate.

This second heat treatment took place for 6 minutes.

At the end of the second heat treatment, said plate was cooled to a temperature of 25 ° C.

Next, a cylindrical sample is cut from the core of said plate by means of a 16 mm punch (i.e. sample of 16 mm diameter with a thickness of 8 mm).

Said sample is itself cut, using a precision saw equipped with a diamond wheel, in two equal parts parallel to its outer surface and its inner surface, at its center to obtain two half-samples having each a diameter of 16 mm and a thickness of 4 mm.

Thus, the first half-sample comprises the external surface of the sample and the so-called "core" surface of the sample, and the second half-sample comprises the internal surface of the sample and the so-called "core" surface. of the sample.

• Résistance électrique surfacique de la surface supérieure (i.e. surface externe) : 15 kilo-ohm, mesurés entre deux lames de cuivre distantes de 5 mm appliquées sur la surface supérieure de l'échantillon (i.e. surface supérieure du premier demi-échantillon), soit une conductivité électrique (surfacique) équivalente de l'ordre de 5.10^-3 S/m ;
• Résistance électrique surfacique de la surface inférieure (i.e. surface interne) : 15 kilo-ohm, mesurés entre deux lames de cuivre distantes de 5 mm, appliquées sur la surface inférieure de l'échantillon (i.e. surface inférieure du deuxième demi-échantillon), soit une conductivité électrique (surfacique) équivalente de l'ordre de 5.10^-3 S/m ; et
• Résistance électrique surfacique de la surface de coeur (du premier dem i-échantillon ou du deuxième dem i-échantillon) : très supérieure à 10 giga-ohm (non mesurable avec l'équipement employé), mesurés entre deux lames de cuivre distantes de 5 mm appliquées sur la surface de coeur du premier ou du deuxième demi-échantillon, soit une conductivité électrique (surfacique) équivalente inférieure à 1.10^-9 S/m.

Finally, measurements of the surface electrical resistance are made using an ohm-meter and the following results are obtained:

• Surface electrical resistance of the upper surface (ie external surface): 15 kilo-ohm, measured between two copper plates 5 mm apart applied to the upper surface of the sample (ie upper surface of the first half-sample), ie equivalent electrical conductivity (surface area) of the order of 5.10 ^-3 S / m;
• Surface electrical resistance of the lower surface (ie internal surface): 15 kilo-ohm, measured between two blades of copper 5 mm apart, applied on the lower surface of the sample (ie lower surface of the second half-sample), equivalent electrical (surface) conductivity of the order of 5.10 ^-3 S / m; and
• Surface electrical resistance of the core area (first half i-sample or second half i-sample): much greater than 10 giga-ohm (not measurable with the equipment used), measured between two copper blades spaced 5 mm applied on the core surface of the first or second half-sample, equivalent electrical (surface) conductivity less than 1.10 ^-9 S / m.

It is important to emphasize that the amplitude of the electrical conductivity gradients formed in the material, as well as the maximum and minimum electrical conductivity values can be modulated by controlling the profile of the applied temperatures, the durations of the heat treatments, the mass composition in electrically conductive charges as well as the nature and / or the form factor of said charges. Finally, the notion of "charge affinity polymer matrix", well known to those skilled in the art, can also play a major role in obtaining and controlling said gradient of electrical property whether at the level of the values of the electrical conductivity or dielectric permittivity values.

Claims (26)

An electric cable (1) comprising an elongated electrical conductor (2), characterized in that the electric cable further comprises a layer (3) of electrical property gradient surrounding the elongated electrical conductor (2), said layer comprising an inner surface ( 3a), an outer surface (3b) and an area (3c) between said inner surface (3a) and said outer surface (3b), said area (3c) having: an electrical conductivity and / or a dielectric permittivity respectively lower than the electrical conductivity and / or the dielectric permittivity of the internal surface (3a), and an electrical conductivity and / or a dielectric permittivity respectively lower than the electrical conductivity and / or the dielectric permittivity of the external surface (3b). Electrical cable according to claim 1, characterized in that the conductivity gradient layer (3) comprises: a first electrical property gradient whose electrical conductivity and / or a dielectric permittivity decreases from the internal surface (3a) towards said zone (3c), and a second electrical property gradient whose conductivity decreases and / or a dielectric permittivity of the external surface (3b) towards said zone (3c). Electrical cable according to claim 1 or 2, characterized in that said layer (3) with a conductivity gradient is obtained from a polymer composition comprising at least one polymer and electrically conductive charges. Electrical cable according to claim 3, characterized in that the electrically conductive charges are conductive carbon charges. Electrical cable according to claim 4, characterized in that the conductive carbonaceous fillers are selected from carbon blacks, carbon fibers, graphites, graphenes, fullerenes, carbon nanotubes, and a mixture thereof. Electrical cable according to claim 5, characterized in that the carbon nanotubes are chosen from single-walled carbon nanotubes, double-walled carbon nanotubes, multiwall carbon nanotubes, and a mixture thereof. Electrical cable according to any one of the preceding claims, characterized in that the internal surface electrical conductivity (3a) is at least 1.10 ^-9 S / m. Electrical cable according to any one of the preceding claims, characterized in that the electrical conductivity of the outer surface (3b) is at least 1.10 ^-9 S / m. Electrical cable according to any one of the preceding claims, characterized in that the electrical conductivity of the zone (3c) is at most 1.10 ^-9 S / m. Electrical cable according to any one of claims 3 to 9,
characterized in that the polymeric composition of the electrical property gradient layer comprises at most 30% by weight of electrically conductive charges. Electrical cable according to any one of the preceding claims,
characterized in that the electrical property gradient layer (3) is in direct physical contact with the electrical conductor (2). Electrical cable according to any one of the preceding claims,
characterized in that it further comprises a metal screen (4) surrounding the conductivity gradient layer (3). Electrical cable according to claim 12, characterized in that the
Electric property gradient layer (3) is in direct physical contact with the electrical conductor (2) and with the metal screen (4). Electrical cable according to any one of the preceding claims,
characterized in that the electrical property gradient layer (3) is an extruded layer. Electrical cable according to any one of the preceding claims,
characterized in that the electrical property gradient layer (3) is a monolayer. Method of manufacturing a gradient property layer (3)
characterized in that it comprises the following steps: producing a polymeric layer obtained from a polymeric composition comprising at least one polymer and electrically conductive fillers, said polymeric layer comprising a first surface (3a), a second surface (3b), and a zone situated between said first surface (3a) and said second surface (3b), - Performing a first heat treatment of said polymeric layer by applying a first temperature T1 to the first surface (3a) and a second temperature T2 to the second surface (3b), so as to have an electrical conductivity and / or a dielectric permittivity in the zone (3c) respectively lower than the electrical conductivity and / or a dielectric permittivity of the first surface (3a), - Performing a second heat treatment of the polymeric layer by applying a third temperature T3 to the first surface (3a) and a fourth temperature T4 to the second surface (3b), so as to have a conductivity electrical and / or dielectric permittivity in the zone (3c) respectively lower than the electrical conductivity and / or a dielectric permittivity of the second surface (3b), to obtain said layer (3) with gradient of electrical property. Process according to Claim 16, characterized in that the temperature T1 is greater than the temperature T2. Process according to Claim 16 or 17, characterized in that the temperature T4 is greater than the temperature T3. Process according to any one of Claims 16 to 18, characterized in that the temperature T1 is a temperature equal to or greater than the melting temperature Tf or the glass transition temperature Tg of said polymer. Process according to any one of Claims 16 to 19, characterized in that the temperature T2 is a temperature equal to or less than the melting temperature Tf or the glass transition temperature Tg of said polymer. Process according to any one of Claims 16 to 20, characterized in that the temperature T4 is a temperature equal to or greater than the melting temperature Tf or the glass transition temperature Tg of said polymer. Process according to any one of Claims 16 to 21, characterized in that the temperature T3 is a temperature equal to or lower than the melting temperature Tf or of the glass transition temperature Tg of said polymer. Process according to any one of Claims 16 to 22, characterized in that the temperature T1 is substantially equal to the temperature T4. Process according to any one of Claims 16 to 23, characterized in that the temperature T2 is substantially equal to the temperature T3. Method according to any one of claims 16 to 24, characterized in that , when the polymeric layer surrounds an elongated electrical conductor (2), said electrical conductor (2) is used as a heat source for the application of the first and / or or the second heat treatment (s). Method according to any one of claims 16 to 25, characterized in that the electrically conductive charges are conductive carbon charges.

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