insulating layer for high-voltage electrical cable comprising mineral nanofillers generated in situ during its preparation

Abstract

The present invention relates to a method for preparing an electrical cable comprising a step of applying an insulating layer around at least one elongated electrically conductive element from a polymer composition, wherein mineral precursors are introduced into the polymer composition prior to said application, which are then converted into mineral particles. The invention also relates to cables obtained according to this method that comprise mineral particles within their insulating layer, and which are particularly well suited for the transmission of medium- or high-voltage direct current. Figure for the abstract: NONE

Description

Title of the invention: Insulating layer for high-voltage electrical cable comprising mineral nanofillers generated in situ during its preparation

[0001] The present invention relates to the field of electrical cables for the transmission of electricity (power cable). More specifically, it relates to cables comprising insulating layers based on a polymer matrix including dispersed inorganic materials acting as fillers.

[0002] In this description, for the sake of brevity, this type of dispersed material acting as mechanical fillers is referred to by the generic term "filler". Unless otherwise stated, the term "filler" refers to this type of mechanical filler and is to be distinguished, in particular, from an electrical filler. It typically consists of small mineral particles, known as "mineral fillers".

[0003] Incorporating fillers into the insulating layer of an electrical cable improves the performance of that layer. In particular, they can improve the mechanical and thermal properties of the layer. In power cables intended for the transmission of electricity at high or medium voltage, fillers are typically used to prevent the accumulation of electrical charges (which would otherwise eventually compromise the integrity of the insulating layer, leading to a gradual decrease in its insulating properties over time). The incorporation of mineral fillers into an insulating layer notably reduces electrical conductivity and increases the breakdown voltage of the insulating layer.

[0004] In absolute terms, it is therefore interesting to try to incorporate small particulate charges, in particular mineral charges in the form of particles of nanometric dimensions, into the insulating layer of an electrical cable.

[0005] However, in practice, it generally proves difficult, if not impossible, to properly disperse mineral nanoparticles in a polymer matrix. Small mineral fillers tend to agglomerate with each other, which ultimately does not lead to the desired performance, which requires optimal dispersion of the mineral fillers.

[0006] Furthermore, the handling of small mineral fillers is inherently delicate, particularly when these fillers are in the form of nanometer-sized particles. Small mineral fillers are indeed in the form of very low-density powders, composed of particles with a very high surface area to volume ratio.

[0007] In addition to the practical difficulties involved in their transport and dosage, the handling of small mineral fillers requires taking precautions, particularly because of the negative repercussions that nanoparticles can have on human health.

[0008] An object of the present invention is to provide a means of access to electrical cables containing an insulating layer incorporating small-dimension charges, but avoiding the aforementioned difficulties.

[0009] For this purpose, it is proposed according to the present invention to form in situ mineral fillers within the insulating layer, from mineral precursors introduced before the application of this layer.

[0010] More specifically, according to a first aspect, the present invention relates to a method for preparing an electrical cable comprising a step of applying an insulating layer around at least one elongated electrically conductive element from a polymer composition, characterized in that mineral precursors are introduced into the polymer composition prior to said application, these mineral precursors then being converted into mineral particles within the polymer composition.

[0011] The mineral precursors used according to the invention lead to the formation of mineral particles in the applied insulating layer. This particle formation from the precursors can occur with varying degrees of inertia during or after application.

[0012] Advantageously, the mineral precursors can be introduced into the polymer composition – in liquid form; and / or - in the form of non-nanometric solid particles.

[0013] In such a form, the precursors make it possible to avoid the problems of agglomeration and low dispersion which are encountered with small preformed particulate charges such as nanoparticles.

[0014] Liquid precursors and / or precursors in the form of non-nanometric solid particles also make it possible to avoid having to take the necessary precautions when handling preformed mineral nanoparticles.

[0015] For the purposes of this description, the expression "non-nanometric solid particle" means a solid object whose dimensions are all greater than 500 nm, preferably greater than one micron.

[0016] In the process of the invention, the mineral precursors are introduced into the polymer composition prior to the application of the insulating layer from the polymer composition. To do this, the precursors are generally mixed with polymers of the composition upstream of the application around the elongated electrically conductive element.

[0017] According to a first possible variant, the polymer composition comprising the precursors is first prepared (according to a process typically comprising at least one step of mixing polymers of the composition with the mineral precursors) and then the composition thus obtained is applied around the electrically conductive elongated element.

[0018] According to another possible variant, the step of mixing the polymers of the composition with the mineral precursors can take place just before the application of the insulating layer.

[0019] According to an advantageous embodiment of the process of the invention, the insulating layer can, for example, be applied by extrusion around the elongated electrically conductive element, using an extruder equipped with an extrusion head. In this case, the mineral precursors are typically introduced into the extruder upstream of said extrusion head. The precursors can, for example, be introduced: - by injection into a molten polymer composition stream (which is particularly well suited to precursors in liquid form) and / or - by mixing all or part of the precursors with solid polymer granules upstream of the extruder melting zone ((which is particularly well suited to solid precursors).

[0020] As an example of precursors well suited according to the invention, the following precursors may be cited in particular.

[0021] According to one possible embodiment, the precursors used in the process of the invention are, in whole or in part, metal or silicon alkoxides or metal salts. These alkoxides and metal salts may, in particular, include silicon alkoxides; titanium alkoxides, aluminum alkoxides or zinc alkoxides; and / or magnesium salts, thereby the mineral particles comprising mineral oxides, hydroxides and / or oxyhydroxides.

[0022] In the process of the invention, the mineral precursors can typically be converted into mineral particles by a sol / gel process. In this context, according to a particular method, the mineral precursors can be introduced in solution in water. Alternatively, water can be introduced separately into the polymer composition before the application of the insulating layer, before or after the introduction of the precursors. The mineral precursors can optionally be introduced with a catalyst, introduced concurrently with, or before, or after the introduction of the precursors, for example an acid (such as hydrochloric acid) or a base (such as sodium hydroxide, in particular).

[0023] According to a particular mode, the precursors can be converted in the absence of water by thermal activation.

[0024] Particularly when implementing a sol / gel process, the conversion of mineral precursors into mineral particles can be accompanied by the formation of by-products such as water and / or alcohols, which are typically liquid or gaseous under the process conditions. Such by-products can lead to porosity in the insulating layer. Advantageously, all or part of the by-products are extracted from the polymer composition before the application of the insulating layer (for example, by vacuum extraction upstream of the extrusion head), thereby reducing the porosity of the insulating layer.

[0025] According to a particular embodiment, the precursors used in the process of the invention are in whole or in part silicon alkoxides corresponding to the following formula (I): Si( OR) 4 (I) where each of the R groups (identical or different, and generally identical) is a hydrocarbon group, preferably an alkyl group typically comprising 2 to 4 carbon atoms, whereby the mineral particles are silica-based particles.

[0026] In the process of the invention, whatever the nature of the precursors used, it may prove advantageous for the precursors to be wholly or partly carriers of functional groups not involved in the reaction of formation of mineral particles from the precursors, whereby the mineral particles formed carry these functional groups.

[0027] By way of non-limiting agreement, such optionally present functional groups may, for example, be groups ensuring compatibility between the mineral particles (which are generally hydrophilic) and the polymers of the composition (which are generally hydrophobic). Such compatibilizing functional groups may, for example, be hydrocarbon groups, such as alkyl groups.

[0028] Thus, according to a particular mode, the precursors are wholly or partly functionalized silicon alkoxides corresponding to the following formula (II): Si(OR) 4.n (R')n(H) where n is an integer between 1 and 3, preferably equal to 1 or 2, typically equal to 1 each of the 4-n groups R (identical or different, and generally identical, when there are several) is a hydrocarbon group as defined for the group R of the aforementioned formula (I); Each of the n R' groups (identical or different, and generally identical when there are several) is a functional group, by which the mineral particles are silica-based particles carrying R' functional groups.

[0029] Advantageously, the R' groups of functionalized silicon alkoxides of formula (II) are in whole or in part hydrocarbon groups, in particular alkyl groups comprising between 2 and 10 carbon atoms, which are suitable for ensuring particularly effective compatibility between the particles and the polymers of the insulating layer.

[0030] According to an advantageous embodiment, the process of the invention uses precursors comprising a mixture of functionalized silicon alkoxides having the aforementioned formula (II) and non-functionalized silicon alkoxides having the aforementioned formula (I). In this case, the molar ratio (1) / (11) is preferably between 1 and 10 mol%.

[0031] Whatever the nature of the precursors used in the process of the invention, the polymer composition can, according to a first possible variant, be a non-crosslinkable thermoplastic composition, whereby the applied insulating layer is non-crosslinked.

[0032] According to this first variant, the polymer composition can for example be a thermoplastic composition based on at least one homopolymer or copolymer of propylene.

[0033] Alternatively, according to a second possible variant, regardless of the nature of the precursors used, the polymer composition can be a crosslinkable composition. In this case, the process typically includes a crosslinking step of the crosslinkable composition, generally carried out after the conversion of the mineral precursors into mineral particles, whereby the applied insulating layer is a crosslinked layer including the mineral particles.

[0034] According to this second variant, the polymer composition can for example be a crosslinkable composition based on a homopolymer or copolymer of ethylene, whereby the insulating layer applied is a crosslinked polyethylene (XLPE) based layer.

[0035] According to another aspect, the invention relates to electrical cables as obtained by implementing the process of the invention. A cable as obtained according to the process of the invention comprises an insulating layer around at least one elongated electrically conductive element, said insulating layer being constituted by a polymer composition including, as mineral fillers, mineral particles obtained by conversion of precursors introduced into the polymer composition before the formation of the insulating layer.

[0036] Optionally, an electrical cable that can be obtained according to the process of the invention comprises an insulating layer around at least one elongated electrically conductive element, said insulating layer being made up of a polymer composition including, as mineral fillers, mineral particles obtained by conversion of precursors introduced into the polymer composition before the formation of the insulating layer.

[0037] In a cable such as that obtained according to the process of the invention, the mineral particles present in the insulating layer, obtained by conversion of a mineral precursor, are small particles. These particles typically have at least one dimension less than or equal to 100 nm.

[0038] The process of the invention, because it starts with precursors that are more easily dispersible in the polymer matrix, ultimately allows for optimized dispersion of the mineral fillers in the insulating layer, even when these mineral fillers are very small particles. In this context, the process of the invention makes it possible to avoid the formation of aggregates such as those observed when starting with preformed mineral nanoparticles.

[0039] The particles present in the insulating layer of a cable obtained according to the invention may, for example, be: - particles (advantageously non-agglomerated) contained within a sphere of radius less than 100 nm, and which are advantageously substantially isotropic; - particles with a shape factor greater than 1 (in the form of needles for example), advantageously non-agglomerated, having a circular cross-section or not contained within a circle of diameter less than 100 nm; - particles (advantageously non-agglomerated) of the platelet type, and having a thickness of less than 100 nm.

[0040] Regardless of their exact size and morphology, the mineral particles present in the insulating layer of a cable obtained according to the invention can be oxide-based particles, hydroxides and / or mineral hydroxides, for example based on silica.

[0041] Furthermore, the mineral particles present in the insulating layer of a cable obtained according to the invention can advantageously carry functional groups, in particular hydrocarbon groups, such as alkyls, ensuring compatibility between the mineral particles and the polymers constituting the insulating layer.

[0042] In an electrical cable obtained according to the invention, the insulating layer can be a non-crosslinked thermoplastic layer, preferably based on at least one homopolymer or copolymer of propylene.

[0043] Alternatively, in an electrical cable obtained according to the invention, the insulating layer can be a cross-linked thermoplastic layer, preferably based on XLPE.

[0044] An electrical cable such as obtained according to the invention can in particular be a power cable (cable for the transport of energy), advantageously intended to carry a direct current at medium or high voltage, for example in the fields of aerial, submarine, terrestrial, or aeronautical electricity transport.

[0045] The cable obtained according to the invention may in particular be: - a medium-voltage power cable (also known as an "MV cable"), designed to carry alternating or direct current, typically direct current, with a voltage of 6 to 45-60 kV; or - a high voltage power cable (called "HV cable"), intended to carry alternating or direct current, typically direct, having a voltage greater than 60 kV, and capable of going up to 500-600 kV.

[0046] In particular, when intended for these applications, the cable obtained according to the invention can typically comprise, from the inside out: - an elongated electrically conductive element, in particular made of copper or aluminium; - an internal semiconducting layer surrounding said elongated electrically conductive element; - an electrically insulating layer surrounding said internal semiconductor layer; - an external semiconductor layer surrounding said insulating layer; and - preferably (but not necessarily) an electrically insulating protective sheath surrounding said external semiconductor layer.

[0047] The mineral particles obtained according to the invention from mineral precursors are typically located in the insulating layer sandwiched between the inner and outer semiconductor layers. According to one conceivable embodiment, similar mineral fillers can be incorporated according to the invention into the inner and outer semiconductor layers.

[0048] In the present invention, "semiconductor layer" means a layer whose electrical conductivity can be at least 1.10-9 S / m (siemens per meter), preferably at least 1.10-3 S / m, and preferably can be less than 1.103 S / m (at 25°C).

[0049] The cable as obtained according to the invention in the present invention may optionally comprise, in addition to the aforementioned layers, a metallic screen surrounding the second semiconducting layer. In this case, the electrically insulating sheath generally surrounds said metallic screen. Where appropriate, this metallic screen may be a so-called "wire screen" composed of a set of copper or other conductors. Aluminum arranged around and along the second semiconductor layer, a so-called "ribboned" screen composed of one or more conductive copper or aluminum metal ribbons, possibly laid helix-style around the second semiconductor layer, or a conductive aluminum metal ribbon laid longitudinally around the second semiconductor layer and sealed with adhesive in the overlapping areas of said ribbon, or a so-called "waterproof" screen of the metal tube type surrounding the second semiconductor layer. This last type of screen notably acts as a barrier against moisture that tends to penetrate the power cable radially.

[0050] Other layers may be present in the cable according to the invention, such as swelling layers in the presence of moisture which may for example be added between the second semiconducting layer and the metallic screen, between the metallic screen and the electrically insulating sheath when they exist, these layers making it possible to ensure the longitudinal water tightness of the power cable.

Claims

Demands

1. A method for preparing an electrical cable comprising a step of applying an insulating layer around at least one elongated electrically conductive element from a polymer composition, characterized in that mineral precursors are introduced into the polymer composition prior to said application, these mineral precursors then being converted into mineral particles within the polymer composition

2. A method according to claim 1 wherein the precursors are introduced in the form of liquid and / or non-nanometric solid particles into the polymer composition.

3. A method according to claim 2, wherein the insulating layer is applied by extrusion around the elongated electrically conductive element, using an extruder equipped with an extrusion head, and wherein the mineral precursors are introduced into the extruder upstream of said extrusion head, for example by injection into a stream of polymer composition in the molten state and / or by mixing all or part of the precursors with solid polymer granules upstream of the melting zone of the extruder.

4. A process according to any one of claims 1 to 3, wherein all or part of the precursors are metal alkoxides or silicon alkoxides or metal salts, in particular silicon alkoxides; titanium, aluminum or zinc alkoxides; or magnesium salts, wherein the mineral particles comprise mineral oxides, hydroxides and / or oxyhydroxides.

5. A process according to any one of claims 1 to 4, wherein the precursors are converted into mineral particles by a sol / gel process.

6. A method according to claim 5 wherein the mineral precursors are introduced in solution in water, or with water introduced separately before or after the introduction of the and / or with a catalyst introduced jointly with or before or after the introduction of the precursors, for example an acid or a base.

7. A process according to claim 5 or 6, wherein the gaseous or liquid by-products present at the end of the sol / gel process are extracted from the polymer composition before the application of the insulating layer, for example by vacuum extraction upstream of the extrusion head.

8. A process according to claim 6 wherein all or part of the precursors are silicon alkoxides corresponding to the following formula (I): Si( OR)4(D where each of the R groups (identical or different, and generally identical) is a hydrocarbon group, preferably an alkyl group, whereby the mineral particles are silica-based particles.

9. A process according to any one of claims 1 to 7, wherein all or part of the precursors carry functional groups not involved in the reaction of formation of mineral particles from the precursors, whereby the mineral particles formed carry these functional groups.

10. A process according to claim 8, wherein all or part of the precursors are functionalized silicon alkoxides corresponding to the following formula (II): Si(OR)4-n (R')n(H) where n is an integer between 1 and 3, preferably equal to 1 or 2, typically equal to 1 each of the 4-n R groups (identical or different, and generally identical, when there are several) is a hydrocarbon group as defined for the R group of formula (I) of claim 8 each of the n R' groups (identical or different, and generally identical, when there are several) is a functional group, whereby the mineral particles are silica-based particles bearing R' functional groups.

11. A method according to claim 10, wherein all or part of the R' groups are hydrocarbon groups, in particular alkyl groups.

12. A process according to any one of the preceding claims, wherein the precursors comprise a mixture of functionalized silicon alkoxides of formula (II) as defined in claim 10 and functionalized silicon alkoxides of formula (I) as defined in claim 8.

13. A method according to any one of claims 1 to 12 wherein the polymer composition is a non-crosslinkable thermoplastic composition and wherein the applied insulating layer is non-crosslinked.

14. A method according to claim 13 wherein the polymer composition is a thermoplastic composition based on at least one homopolymer or copolymer of propylene.

15. A method according to any one of claims 1 to 12 wherein the polymer composition is a crosslinkable composition, and wherein the method comprises a crosslinking step of the crosslinkable composition, generally after the conversion of the mineral precursors into mineral particles, whereby the applied insulating layer is a crosslinked layer including the mineral particles.

16. A method according to claim 15 wherein the polymer composition is a crosslinkable composition based on a homopolymer or copolymer of ethylene, wherein the applied insulating layer is a crosslinked polyethylene (XLPE) based layer.

17. An electrical cable that can be obtained according to the process of any one of claims 1 to 16 comprising an insulating layer around at least one elongated electrically conductive element, said insulating layer being made up of a polymer composition including, as mineral fillers, mineral particles obtained by conversion of precursors introduced into the polymer composition before the formation of the insulating layer.

18. Electrical cable according to claim 17, wherein the mineral particles have at least one dimension less than or equal to 100 nm

19. Electric cable according to any one of claims 17 or 18, wherein the mineral particles are oxide-based particles, mineral hydroxides and / or hydroxides, for example silica-based.

20. Electrical cable according to any one of claims 17 to 19, wherein the mineral particles carry functional groups, in particular hydrocarbon groups, such as alkyls, ensuring compatibility between the mineral particles and the polymers constituting the insulating layer.

21. Electric cable according to any one of claims 17 to 20, wherein the insulating layer is a non-crosslinked thermoplastic layer, preferably based on at least one homopolymer or copolymer of propylene.

22. Electric cable according to any one of claims 17 to 20, wherein the insulating layer is a cross-linked thermoplastic layer, preferably based on XLPE.

23. Electric cable according to any one of claims 14 to 19, which is a power cable intended to carry a direct current at medium or high voltage.