Electrical cable comprising a polypropylene-based thermoplastic insulating layer including gas bubbles

Abstract

The present invention relates to an electrical cable comprising at least one electrically conductive elongated element and an electrically insulating layer in direct contact with the electrically conductive elongated element, wherein: - the electrically insulating layer is obtained from a thermoplastic polymer composition comprising at least 50% by mass of polypropylene, in the form of a homopolymer, a copolymer, or a mixture and in homophasic, heterophasic or mixture form; and - the electrically insulating layer comprises gas bubbles.

Description

[0001] The present invention relates to the field of electrical cables and more specifically to the electrically insulating polymer layers present in these cables. The invention further relates to an insulating layer that possesses good electrical and mechanical performance, and in particular good flexibility and ease of stripping.

[0002] The invention typically applies to electrical cables intended for the transport of energy, and in particular to low voltage power cables (i.e. carrying a direct or alternating current typically having a voltage less than or equal to 6 kV), and which can for example be used in the fields of aerial, submarine, terrestrial, or aeronautical electricity transport.

[0003] A low-voltage power transmission cable typically includes: an elongated electrically conductive element (for example a wire or a set of wires twisted together), in particular made of copper or aluminum; an electrically insulating layer surrounding said elongated electrically conductive element; and optionally an electrically insulating protective sheath surrounding said electrically insulating layer.

[0004] Historically, the insulating layers of such cables were made of polyethylene, particularly cross-linked polyethylene (XLPE), which has the disadvantage of being difficult to recycle. More recently, non-cross-linked thermoplastic insulating layers have been proposed, typically based on recyclable thermoplastic blends of polyethylene and polypropylene.

[0005] In this context, application WO2022 / 129814 described, in particular, non-crosslinked thermoplastic polymer compositions well-suited for the formation of insulating layers for low-voltage cables, comprising (i) at least 50% by weight of a homophasic propylene polymer; (ii) at least one homophasic ethylene polymer present in a proportion lower than that of the homophasic propylene polymer and having an elastic modulus of more than 300 MPa; and (iii) optionally, up to a maximum of 10% by weight of a heterophasic propylene copolymer. WO2022 / 129814 indicates that, when present, the heterophasic propylene copolymer provides improved compatibility between the homophasic propylene polymer and the homophasic ethylene polymer.

[0006] Although they are very well suited for the formation of insulating layers in low voltage cables that can operate at temperatures above 70 °C and exhibit very good thermomechanical and electrical performance, the compositions of WO2022 / 129814 nevertheless exhibit less performance in terms of flexibility.

[0007] An object of the present invention is to provide thermoplastic polymer compositions well suited for the constitution of insulating layers of low voltage cables, and exhibiting in particular adequate thermomechanical and electrical performance, and which also exhibit better flexibility and greater ease of stripping than the layers described in the above documents.

[0008] To this end, the present invention proposes to use an electrically insulating layer of the expanded type, namely an insulating layer which includes gas bubbles trapped inside a thermoplastic polymer matrix.

[0009] More specifically, the present invention relates to an electrical cable comprising at least one electrically conductive elongated element and an electrically insulating layer in direct contact with the electrically conductive elongated element, where the electrical cable is a low voltage cable and where said electrically insulating layer is obtained from a thermoplastic polymer composition comprising at least 50% by mass of polypropylene and where said electrically insulating layer comprises gas bubbles.

[0010] According to another aspect, the present invention relates to a method of applying a thermoplastic insulating layer around at least one elongated electrically conductive element and in direct contact with the elongated electrically conductive element, said method comprising an application of a thermoplastic polymer composition as described above, in particular by extrusion, and the formation of gas bubbles within the composition during this application.

[0011] The insulating polymer layer containing gas bubbles is also referred to in this description as an "expanded polymer layer," which is based on a material known as an "expanded polymer material." Expanded polymer materials are polymers whose density is reduced because a gas has been introduced into the polymer in its plasticized or molten state. This gas, which can be introduced chemically or physically, produces bubbles within the material, resulting in the formation of voids. A material containing these voids generally exhibits desirable properties such as reduced weight and the ability to provide more uniform damping than a material without voids. Adding a large amount of gas results in a much lighter material, but adding too much gas can reduce the material's mechanical integrity.Expanded polymer materials are polymer materials whose density is reduced because gas has been introduced into the polymer matrix. This gas, which can be introduced chemically (by generating gas from precursors) or physically (by gas injection), is present in the form of bubbles ("voids") inside the material.

[0012] To the inventors' knowledge, expanded materials of this type have not been considered for use in cable construction with a polypropylene-based insulating layer in direct contact with the electrically conductive elongated element. Among the few applications considered in the field of cables, WO2007 / 011350 describes a power cable with an outer sheath surrounding an insulated conductor, where the outer sheath can be obtained from a composition including a chemical expander. The use of such an expander is not envisaged in other cable layers, and the document does not suggest any interest in layers other than the outer sheath.

[0013] According to a first advantageous embodiment of the invention, the insulating layer of the cable according to the invention (in direct contact with the electrically conductive elongated element) is obtained from a thermoplastic polymer composition comprising at least 50% by mass of polypropylene relative to the total mass of polymer in said composition. The polypropylene present in the composition may be in the form of at least one homopolymer of propylene, at least one copolymer of propylene and at least one other co-monomer distinct from propylene, or a mixture of several of these homo- and / or co-polymers. The aforementioned mass percentage refers to the total quantity of homo- and / or co-polymers when the polypropylene used is a mixture. The polypropylene used may be homophasic or heterophasic.If the polypropylene composition is heterophasic, the mixture may contain either only heterophasic polypropylene, or a mixture of homophasic and heterophasic polypropylene.

[0014] In one particular embodiment, the polypropylene comprises at least one homophasic polypropylene (which may be homopolymer or copolymer). In this embodiment, the polymer composition typically comprises the homophasic polypropylene at a level of at least 85% by weight, preferably at least 90% by weight, relative to the total weight of polypropylene. In another particular variant, the polypropylene composition comprises the homophasic polypropylene as the sole polypropylene (at a level of 100% by weight relative to the total weight of polypropylene).

[0015] The work carried out by the inventors in the context of the present invention has now made it possible to demonstrate that an expanded polypropylene-based insulating layer as used according to the invention is particularly well suited in a low voltage cable, and that the expanded insulating layer has interesting electrical and thermomechanical properties, with further good performance in terms of flexibility, lower density, ease of stripping, lower production cost, optimization of the mechanical properties of the insulator such as tensile strength and elongation at break.

[0016] Optionally, the polypropylene composition comprises at least 80% by weight of homophasic propylene polymer relative to the total weight of polypropylene.

[0017] Optionally, the polypropylene composition includes between 1 and 10% by weight of heterophasic polypropylene relative to the total weight of polypropylene.

[0018] Optionally, the mass ratio between heterophasic polypropylene and homophasic polypropylene present in the polypropylene composition is between 0 / 1 and 1 / 6.

[0019] Optionally, these gas bubbles contain N2 or CO2.

[0020] Optionally, a chemical expander is introduced at a content of between 0.1 and 10% by weight relative to the total weight of the thermoplastic polymer composition.

[0021] Optionally, a gas is injected during extrusion.

[0022] Various features of the invention and different possible variants are described in more detail below. Homophasic polypropylene

[0023] For the purposes of this description, a "homophasic polymer" is defined as a polymer existing in a single phase, generally a substantially homogeneous phase. Such a homophasic polymer, as defined herein, is distinct from heterophasic polymers, which comprise multiple phases. A homophasic polymer, as defined herein, therefore excludes, for example, heterophasic propylene copolymers such as Adflex Q200F copolymers.

[0024] According to one possible embodiment, homophasic polypropylene is a homopolymer of polypropylene.

[0025] Alternatively, according to another conceivable mode, homophasic polypropylene is a propylene copolymer.

[0026] Examples of homophasic propylene copolymers usable according to the invention include copolymers of propylene and an olefin other than propylene, this olefin being able to be chosen in particular from ethylene or an α olefin different from propylene.

[0027] The homophasic propylene copolymer is advantageously a statistical copolymer.

[0028] As an example of a useful homophasic statistical copolymer of propylene according to the invention, we can cite that marketed by the company Total Energies under the reference PPR 3221, that marketed by the company Sabic under the reference PP 620P, or that marketed by the company Repsol Isplen under the reference RC530S2E.

[0029] A homophasic polypropylene useful according to the present invention can, in particular, be characterized by its elastic modulus. This elastic modulus (or Young's modulus, also known by the English term "Tensile Modulus") is well known to those skilled in the art and can be easily determined according to ISO 527-1, -2 (2012). ISO 527 has a first part, designated "ISO 527-1", and a second part, designated "ISO 527-2", specifying the test conditions relating to the general principles of the first part of ISO 527.

[0030] A useful homophasic propylene homopolymer according to the invention preferably has an elastic modulus ranging from 1250 to 1600 MPa.

[0031] A useful homophasic propylene copolymer according to the invention preferably has an elastic modulus ranging from 600 to 1200 MPa, and particularly preferably ranging from 800 to 1100 MPa.

[0032] Furthermore, a useful homophasic polypropylene according to the invention advantageously has a melting temperature above 130 °C, particularly preferably above 135 °C, and more particularly preferably ranging from approximately 140 °C to 175 °C.

[0033] A homophasic polypropylene useful according to the invention may also have an enthalpy of fusion ranging from approximately 20 to 100 J / g. When it is a homopolymer of homophasic propylene, its enthalpy of fusion preferably ranges from 80 to 90 J / g. When it is a copolymer of homophasic propylene, it preferably has an enthalpy of fusion ranging from 40 to 90 J / g, and particularly preferably from 50 to 85 J / g.

[0034] A useful homophasic polypropylene according to the invention may also have a melt flow index ranging from 0.5 to 3.5 g / 10 min, preferably ranging from 1.0 to 2.8 g / 10 min, and particularly preferably ranging from 1.2 to 2.5 g / 10 min; in particular determined at approximately 230 °C with a charge of approximately 2.16 kg according to ASTM D1238-00, or ISO 1133-1.

[0035] A useful homophasic polypropylene according to the invention may also have a density ranging from approximately 0.81 to 0.92 g / cm³, preferably ranging from 0.85 to 0.91 g / cm³, and particularly preferably ranging from 0.87 to 0.91 g / cm³; in particular determined according to ISO 1183A (at a temperature of 23 °C). Heterophasic polypropylene

[0036] The polymer composition from which the insulating layer of the cable of the invention is formed may comprise a heterophasic propylene copolymer. This propylene copolymer, referred to as "heterophasic" (or also "heterophasic"), comprises two distinct phases, typically: a continuous thermoplastic phase based on polypropylene (this is a thermoplastic matrix based on at least one homo- or co-polymer of propylene); and an elastomeric phase (usually both elastomeric and thermoplastic) dispersed in this continuous thermoplastic phase (matrix).

[0037] This heterophasic mixture can be introduced into the composition in the form of a preformed heterophasic mixture (typically in the form of a commercial two-phase mixture, generally referred to as a "heterophasic copolymer"; in this case, the mixture can, for example, be introduced into an extruder in the form of granules, each consisting of this heterophasic mixture).Alternatively, the two phases of the heterophasic mixture can be formed in situ following the mixing of their constituent polymers in proportions leading to the dispersion of the elastomer phase in the continuous thermoplastic phase based on polypropylene (for example by introducing into an extruder (i) granules comprising the first homophasic polymer (or a homophasic mixture of polymers) constituting the continuous thermoplastic phase, and (ii) other granules not comprising the first polymer and comprising a second homophasic polymer (or a second homophasic mixture of polymers) constituting the continuous phase).

[0038] The heterophasic copolymer used according to the invention comprises at least two distinct phases: one comprising a polymer matrix, and the other typically comprising particles or nodules dispersed within this polymer matrix. This type of polymer can be easily identified by techniques well known to those skilled in the art, such as scanning electron microscopy (SEM). More specifically, at a magnification of x 10,000, it is common to observe said particles or nodules dispersed within said polymer matrix, said particles having an average size ranging from 200 nm to 10 µm, for example, between 500 nm and 2 µm.

[0039] The thermoplastic phase constituting the matrix of the heterophasic propylene copolymer generally represents between 50 and 85% by weight, advantageously between 55 and 80% by weight and for example between 55 and 75% by weight relative to the total weight of the heterophasic propylene copolymer.

[0040] The dispersed phase of the heterophasic propylene copolymer advantageously represents at least 15% by weight, preferably at least 20% and for example at least 30% and generally less than 50% by weight, for example less than 45% by weight relative to the total weight of the heterophasic propylene copolymer.

[0041] According to a first possible embodiment, the elastomeric phase which is dispersed in a continuous thermoplastic phase based on polypropylene comprises an elastomeric copolymer of ethylene and an α olefin other than ethylene, in particular an elastomeric copolymer of ethylene and propylene.

[0042] The heterophasic propylene copolymer used according to this first embodiment preferably has an elastic modulus ranging from about 50 to 1200 MPa, and particularly preferably: either an elastic modulus ranging from about 50 to 550 MPa, and more particularly preferably ranging from about 50 to 300 MPa.

[0043] The heterophasic propylene copolymer used according to this first embodiment is generally introduced into the polymer composition as a preformed heterophasic mixture. Examples of preformed heterophasic propylene copolymers useful according to this first embodiment include the heterophasic propylene copolymers marketed by LyondellBasell under the references Adflex®< Q 200 F, Hifax CA 10A, or Hifax CA 12A.

[0044] Heterophase polypropylene according to the first mode described above preferably has a melting temperature above about 140 °C, particularly preferably above about 145 °C, and more particularly preferably from about 150 to 175 °C.

[0045] Heterophasic polypropylene according to the first mode described above can have an enthalpy of fusion ranging from about 20 to 100 J / g, and preferably from about 20 to 50 J / g.

[0046] Heterophasic polypropylene according to the first mode described above can have a melt flow index ranging from 0.5 to 5 g / 10 min, and preferably from approximately 0.6 to 2 g / 10 min; in particular determined at approximately 230°C with a charge of approximately 2.16 kg according to ASTM D1238-00, or ISO 1133-1.

[0047] Heterophasic polypropylene according to the first mode described above can have a density of approximately 0.81 to 0.92 g / cm³, preferably of 0.85 to 0.91 g / cm³, and particularly preferably of 0.87 to 0.91 g / cm³; in particular determined according to ISO 1183A (at a temperature of 23 °C).

[0048] According to a second possible embodiment, the elastomeric phase dispersed in a continuous thermoplastic polypropylene-based phase comprises an ethylene elastomeric copolymer and at least one comonomer bearing a polar group. In this embodiment, the elastomeric phase preferably represents at most 45% by weight (typically between 25% and 45%) of the total weight of the heterophasic propylene copolymer, with the matrix representing at least 55% by weight (typically between 55% and 75%).

[0049] The heterophasic propylene copolymer used according to this first embodiment may optionally be introduced into the polymer composition as a preformed heterophasic mixture. Alternatively, according to this second embodiment, the polymer composition may be obtained by mixing (i) the homo- or copolymer of propylene constituting the continuous phase based on polypropylene (typically introduced into an extruder as first granules) and (ii) the ethylene elastomeric polymer and at least one co-monomer bearing a polar group (typically introduced into an extruder as second granules distinct from the first), in which case the heterophasic mixture is formed in situ in the composition (typically by melting and mixing the granules in the extruder).

[0050] The ethylene elastomer copolymer and at least one polar group-bearing monomer used in the second mode described above may in particular have a melting temperature below 110 °C, for example less than or equal to 100 °C.

[0051] The elastomeric copolymer of ethylene and at least one monomer bearing polar groups used in the second mode may, in particular, be a polymer bearing one or more polar functional groups selected from acetate, acrylate, hydroxyl, nitrile, carboxyl, carbonyl, ether, ester, silane, and mixtures thereof. A homophasic ethylene polymer with useful polar characteristics according to the invention may, in particular, be: an ethylene-vinyl acetate (EVA) copolymer; an ethylene-butyl acrylate (EBA) copolymer; an ethylene-ethyl acrylate (EEA) copolymer; an ethylene-methyl acrylate (EMA) copolymer; an ethylene-acrylic acid (EAA) copolymer; or a mixture of these compounds. Obtaining the expanded polymer material that constitutes the insulating layer

[0052] The formation of bubbles within the polymer matrix of the insulating layer is generally achieved by generating or injecting a gas into the polymer composition used to produce the layer during the extrusion phase. This generation or introduction of gas can be done Chemically: namely, by introducing a chemical expander (also called a "blowing agent") into the polymer composition. This expander is a compound or mixture of compounds that generates gas under extrusion conditions (through thermal decomposition or a chemical reaction between two components of the mixture, for example) upstream of the polymer layer formation. Typically, this chemical expander is introduced before the composition is fed into the extruder, for example, in the form of a "foaming masterbatch." However, the chemical expander can also be co-injected separately into the extruder, and / or physically: that is, by injecting gas into the polymer composition, typically at high pressure and typically directly at the inlet or within the extruder. When a polymer material is expanded using a chemical expander, gas bubbles are formed and trapped within the resulting expanded polymer material. The imprint of these gas bubbles within the polymer material constitutes what are known as the "cells" of the foam; the cells are made up of the polymer surrounding the "voids" left in the material by the gas.

[0053] The use of a "foaming masterbatch" allows users simpler, safer handling and more precise dosing of the compounds used to generate gas bubbles in the final material.

[0054] Examples of suitable expanders include azodicarbonamide (which generates a gas by thermal decomposition), or mixtures comprising at least (i) a carbonate and / or bicarbonate (e.g., sodium bicarbonate) and (ii) acids in solid form, typically organic acids such as citric acid, with components (i) and (ii) reacting together to release CO₂. Such mixtures also usually include a polymeric base (iii), for example, a thermoplastic polymeric base and, for example, an ethylene-based polymeric base. The ethylene in the polymeric base may be in the form of at least one ethylene homopolymer, at least one ethylene copolymer and at least one other distinct ethylene comonomer, or a mixture of several of these homopolymers and / or copolymers. The polymeric base may thus be EVA or LDPE.It should be noted that the polymer base allows users simpler, safer handling and more precise dosing of components (i) and (ii) that play a role in bubble formation. The polymer base itself does not play a role in the actual bubble formation. It should also be noted that the mixture formed from components (i), (ii), and optionally (iii) allows for the creation of a "foaming masterbatch," as previously mentioned.

[0055] The foaming masterbatch may alternatively include a chemical expander designed to generate a gas through thermal decomposition. This decomposition reaction can be endothermic, exothermic, or hybrid. Chemical expanders of this type react to the heat generated by the extrusion process or release a gas. Endothermic chemical expanders absorb heat during their chemical reaction and generally produce carbon dioxide, a low-pressure gas, and small cells in the resulting insulating layer. Exothermic chemical expanders release heat and generally produce nitrogen, a higher-pressure gas, resulting in a higher gas yield and larger cells in the final insulating layer.Hybrid chemical expanders, a family of chemical expanders containing mixtures of endothermic and exothermic chemical expanders, combine the fine, uniform cellular structure of endothermics with higher gas pressure from the exothermic component.

[0056] The choice of an endothermic, exothermic, or hybrid chemical expander depends on its compatibility with the polymer material incorporated in the expanded sleeving layer, the profiles and extrusion processes, the desired amount of foam, the size and structure of the foam cells, and other design considerations specific to the cable being produced and apparent to those skilled in the art. Generally, for similar amounts of active ingredients, exothermic chemical expanders will reduce density the most and produce foam with larger, more uniform cells. Endothermic chemical expanders produce foams with a finer cell structure. Although an exothermic foam layer is used in a preferred embodiment, other chemical expanders can provide satisfactory cell structures.A closed-cell structure is preferable, particularly to ensure good mechanical resistance and a uniform surface texture of the expanded insulating layer.

[0057] The formation of gas bubbles by physical means can typically be achieved by injecting, generally at high pressure and for example into the extrusion cylinder, a gas chosen from nitrogen, carbon dioxide, air or even low boiling point hydrocarbons (i.e. gaseous under extrusion conditions) such as propane or butane.

[0058] The electrically insulating layer, including the gas bubbles, has a density between 0.5 and 1 (mass density between 0.5 and 1 g / cm³), typically less than 0.9. The expanded electrically insulating layer obtained according to the invention includes gas bubbles forming "voids," namely spaces not occupied by the polymer matrix. The percentage of these voids in the expanded electrically insulating layer obtained according to the invention (i.e., the ratio of the volume of the "voids" to a given volume of the layer's constituent material) is expressed as an "Expansion Rate," defined as follows: Expansion Rate = ((density of the polymer composition used to form the layer - density of the polymer layer) / density of the polymer composition used to form the layer). It is desirable to obtain as high an expansion rate as possible while maintaining the desired properties of the cable.In particular, a higher expansion rate will reduce material costs by increasing the space occupied by voids. Applicants have found that appropriate expansion rates, or density reduction rates, are generally between 0.02 and 0.50, i.e., between 2% and 50%, typically between 10% and 30%, although higher expansion rates are achievable.

[0059] As noted above, foaming can provide a homogeneous and reproducible expansion rate along the entire length of the cable. The chosen chemical expander must be capable of producing consistent cable dimensions. Chemical expanders that prove particularly effective in the preferred embodiment include, for example, endothermic chemical expanders and / or mixtures comprising (i) a bicarbonate (e.g., sodium bicarbonate) and (ii) an organic acid (e.g., citric acid) and, optionally, (iii) an ethylene-based polymer. For example, for 100 parts by weight of the chemical expander composition, the chemical expander composition comprises between 10 and 50 parts by weight of the polymer base, and for example, between 20 and 40 parts by weight, and for example, between 25 and 35 parts by weight.

[0060] Such a mixture is marketed for example by the company Avient under the name Hydrocerol CT3274 (with a polymeric base including EVA) and / or Hydrocerol BM70 (with a polymeric base including PE).

[0061] The electrically insulating layer including gas bubbles has a thickness between 0.5 mm and 2 mm. additional homophasic ethylene polymer

[0062] According to one possible embodiment, the thermoplastic polymer composition may comprise a homophasic ethylene polymer,

[0063] The polymer composition may possibly result from a mixture of the previously described polypropylenes with a homophasic ethylene polymer typically present at less than 45% by weight relative to the total weight of the polymers in the thermoplastic polymer composition.

[0064] This homophasic ethylene polymer, when present in the composition, is typically a homopolymer or copolymer of ethylene. It preferably comprises at least about 80 mole percent of ethylene, particularly preferably at least about 90 mole percent of ethylene, and more particularly preferably at least about 95 mole percent of ethylene, relative to the total number of moles of monomers in the ethylene polymer.

[0065] When such a homophasic ethylene polymer is present, it typically has an elastic modulus of at least 300 MPa, preferably an elastic modulus of at least 325 MPa, and particularly preferably of at least 350 MPa. This elastic modulus generally remains less than or equal to 600 MPa, for example less than or equal to 500 MPa.

[0066] When a homophasic ethylene polymer is present in the composition, its melt flow index advantageously ranges from 0.5 to 5 g / 10 min, and preferably from 1 to 3 g / 10 min (the melt flow index referred to here is that determined at approximately 230 °C with a charge of 2.16 kg according to ASTM D1238-00, or ISO 1133-1).

[0067] When a homophasic ethylene polymer is present in the composition, it may, for example, be low-density polyethylene, linear low-density polyethylene, medium-density polyethylene, or high-density polyethylene; in particular according to ISO 1183A (at a temperature of 23 °C).

[0068] In this description, the term "low density polyethylene" refers to a homophasic ethylene polymer having a density of approximately 0.91 to 0.925 g / cm³, said density being measured according to ISO 1183A (at a temperature of 23 °C).

[0069] In this description, the term "medium density polyethylene" refers to a homophasic ethylene polymer having a density ranging from approximately 0.926 to 0.940 g / cm³, said density being measured according to ISO 1183A (at a temperature of 23 °C).

[0070] In this description, the term "high-density polyethylene" refers to a homophasic ethylene polymer having a density ranging from 0.941 to 0.965 g / cm³, said density being measured according to ISO 1183A (at a temperature of 23 °C).

[0071] When a homophasic ethylene polymer is present in the composition, it may advantageously be a polyethylene having a density of not more than 0.936 g / cm³, particularly preferably of not more than 0.930 g / cm³, and more particularly preferably of not more than 0.925 g / cm³; said density being measured in particular according to ISO 1183A (at a temperature of 23 °C), and the homophasic ethylene polymer is then advantageously a low-density polyethylene, a linear low-density polyethylene, or a medium-density polyethylene, and particularly preferably a linear low-density polyethylene.

[0072] Examples of linear low-density polyethylene include that marketed by Ineos under reference BPD 6342, that marketed by ExxonMobil under reference LL 1004YB, that marketed by Sabic under reference 318BJ, and that marketed by Versalis under reference Flexirene CL 10.

[0073] Homophasic ethylene polymer, when present, is typically present at a content between 25 and 45%, for example between 30 and 40% by weight relative to the total weight of polymers in the thermoplastic polymer composition.

[0074] When the composition includes a homophasic ethylene polymer, it generally also systematically includes a homophasic polypropylene, with a mass proportion (i.e., a quantity by weight) of the homophasic polypropylene that preferably remains strictly greater than the mass proportion (i.e., the quantity by weight) of the homophasic ethylene polymer, relative to the total weight of polymers in the polymer composition. Possible other polymers

[0075] The polymer composition from which the insulating layer of the cable of the invention is formed may optionally include other polymers in addition to the aforementioned polymers.

[0076] However, according to a preferred embodiment of the invention, the polymer composition comprises less than 20% (for example, less than 15%, or even less than 10%) by weight of polymer other than polypropylene (homophasic or heterophasic) and homophasic ethylene polymer relative to the total weight of polymers in the composition.

[0077] Furthermore, the polymer composition from which the insulating layer of the cable of the invention is formed is a thermoplastic polymer composition. It is therefore neither crosslinkable nor crosslinked.

[0078] In particular, the polymer composition from which the insulating layer of the cable of the invention is formed does not include crosslinking agents, silane-type coupling agents, peroxides and / or additives that enable crosslinking.

[0079] Furthermore, it is preferable that the polymer composition from which the insulating layer of the cable of the invention is formed does not include olefin polymers grafted with crosslinkable functions, such as, for example, vinyl silane olefin polymers.

[0080] Given its thermoplastic nature, the insulating layer of the cable of the invention generally has the advantage of being recyclable. Optional Add-ons

[0081] The polymer composition from which the insulating layer of the cable of the invention is formed may optionally include one or more additives in addition to the aforementioned polymers.

[0082] These additives can be chosen from among agents that promote implementation such as lubricants, compatibilizing agents, coupling agents, antioxidants, anti-UV agents, antioxidants, anti-copper agents, anti-water treeing agents, pigments, and mixtures thereof.

[0083] The polymer composition preferably includes at least one antioxidant and / or at least one anti-copper agent (also called a metal deactivator). The polymer composition may typically include approximately 0.01 to 5% by weight, and preferably approximately 0.1 to 2% by weight, of additives, relative to the total weight of the polymer composition.

[0084] Antioxidants help to protect the polymer composition from thermal stresses generated during the cable manufacturing or operation stages.

[0085] Antioxidants are preferably chosen from among hindered phenols, thioesters, sulfur-based antioxidants, phosphorus-based antioxidants, amine-type antioxidants, and mixtures thereof.

[0086] Examples of sterically hindered phenols include 1,2-bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyl)hydrazine (Irganox® < MD 1024), pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) (Irganox® < 1010), octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (Irganox® < 1076), 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene (Irganox® < 1330), and 4,6-bis(octylthiomethyl)-o-cresol (Irgastab® < KV10 or Irganox® < 1520), 2,2'-thiobis(6-tert-butyl-4-methylphenol) (Irganox ®< 1081), 2,2'-thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] (Irganox ®< 1035), tris(3,5-di-tert-butyl-4-hydroxybenzyl) isocyanurate (Irganox ®< 3114), 2,2'-oxamido-bis(ethyl-3(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) (Naugard XL-1), or 2,2'-methylenebis(6-tert-butyl-4-methylphenol).

[0087] Examples of sulfur-based antioxidants include thioethers such as didodecyl-3,3'-thiodipropionate (Irganox ®< PS800), distearyl thiodipropionate or dioctadecyl-3,3'-thiodipropionate (Irganox ®< PS802), bis[2-methyl-4-{3-n-alkyl (C12 or C14) thiopropionyloxy}-5-tert-butylphenyl]sulfide, thiobis-[2-tert-butyl-5-methyl-4,1-phenylene]bis[3-(dodecylthio)propionate], or 4,6-bis(octylthiomethyl)-o-cresol (Irganox ®< 1520 or Irgastab ®< KV10).

[0088] Examples of phosphorus-based antioxidants include tris(2,4-di-tert-butyl-phenyl) phosphite (Irgafos ®< 168) or bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite (Ultranox ®< 626).

[0089] Examples of amine-type antioxidants include phenylenediamines (e.g., paraphenylenediamines such as 1PPD or 6PPD), diphenylamine styrene, diphenylamines, 4-(1-methyl-1-phenylethyl)-N-[4-(1-methyl-1-phenylethyl)phenyl]aniline (Naugard 445), mercaptobenzimidazoles, or polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ).

[0090] Examples of usable antioxidant mixtures according to the invention include Irganox B 225, which comprises an equimolar mixture of Irgafos 168 and Irganox 1010 as described above.

[0091] The metal deactivator can be chosen from nitrogen-containing aromatic heterocycles and aromatic compounds containing at least one -NH-C(=O)- functional group, and preferably from aromatic compounds containing at least one -NH-C(=O)- functional group. The presence of oxygen in the metal deactivator is important for the long-term immobilization of metal ions.

[0092] The metal deactivator is preferably different from a hindered amine. In other words, the metal deactivator preferably does not include one or more tetramethylpiperidine groups.

[0093] Examples of nitrogen-containing aromatic heterocyclics include quinoline derivatives such as polymerized 2,2,4-trimethyl-1,2-dihydroquinolines (TMQ).

[0094] Examples of aromatic compounds comprising at least one -NH-C(=O)- function include those comprising two -NH-C(=O)- functions, preferably two covalently linked -NH-C(=O)- functions, and more particularly preferred ones comprising a divalent -NH-C(=O)-C(=O)-NH- or -C(=O)-NH-NH-C(=O)- group, such as 2,2'-oxamidobis-[ethyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (Naugard XL-1), 2'-,3-bis[[3-[3,5-di-tert-butyl-4-hydroxyphenyl]propionyl]]propionohydrazide, or 1,2-bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyl)hydrazine (Irganox ® < 1024 or Irganox ®< MD 1024), or oxalyl bis(benzylidenehydrazide) (OABH).

[0095] Some metal deactivators are also antioxidants.

[0096] In particular, since the insulating layer of the cable of the invention is intended for a low voltage application, it is preferable that the polymer composition from which the insulating layer is formed be free of dielectric liquid, in particular liquids selected from a mineral oil (e.g. naphthenic oil, paraffinic oil or aromatic oil), a vegetable oil (e.g. soybean oil, linseed oil, rapeseed oil, corn oil or castor oil), a synthetic oil such as an aromatic hydrocarbon (alkylbenzene, alkylnaphthalene, alkylbiphenyl, alkyldiarylethylene, for example), a silicone oil, an ether oxide, an organic ester, and an aliphatic hydrocarbon. The electrically insulating layer

[0097] The electrically insulating layer of the cable of the invention, obtained from the polymer composition defined in the preceding paragraphs, is a thermoplastic layer. It is preferably non-crosslinked.

[0098] For the purposes of this description, the term "non-crosslinked coating" means a coating whose gel content according to ASTM D2765-01 (xylene extraction) is preferably not more than about 30%, preferably not more than about 20%, particularly preferred not more than about 10%, more particularly preferred not more than 5%, and even more particularly preferred not more than 0%.

[0099] The electrically insulating layer of the cable of the invention, preferably non-crosslinked, has a tensile strength (RT) of at least 8.5 MPa, preferably of at least about 10 MPa, and particularly preferably of at least about 15 MPa, before aging (according to standard HD 603).

[0100] According to a particular embodiment, the electrically insulating layer, preferably non-crosslinked, has an elongation at break (ER) of at least about 250%, preferably of at least about 300%, and particularly preferably of at least about 350%, before aging (according to standard HD 603).

[0101] In a particular embodiment, the electrically insulating layer, preferably non-crosslinked, has a tensile strength (RT) of at least 8.5 MPa, preferably of at least about 10 MPa, and particularly preferably of at least about 15 MPa, after aging (according to standard HD 603).

[0102] In a particular embodiment, the electrically insulating layer, preferably non-crosslinked, has an elongation at break (ER) of at least about 250%, preferably about at least about 300%, and particularly preferably about at least about 350%, after aging (according to standard HD 603).

[0103] The tensile strength (RT) and elongation at break (ER) (before or after aging) can be carried out according to Standard NF EN 60811 1-1, in particular using a device marketed under reference 3345 by the company Instron.

[0104] Aging is generally carried out at 135°C for 240 hours (or 10 days).

[0105] The electrically insulating layer of the cable of the invention is preferably a recyclable layer.

[0106] The electrically insulating layer of the invention can be an extruded layer, in particular by processes well known to those skilled in the art.

[0107] The electrically insulating layer has a variable thickness depending on the type of cable. In particular, since the cable according to the invention is a low-voltage cable, the thickness of the electrically insulating layer is generally approximately 1 to 2 mm. The aforementioned thicknesses depend on the size of the elongated electrically conductive element.

[0108] In the present invention, "electrically insulating layer" means a layer whose electrical conductivity can be at most 1.10 8< S / m (siemens per meter), preferably at most 1.10 9< S / m, and particularly preferably at most 1.10 10< S / m, measured at approximately 25 °C in direct current.

[0109] The electrically insulating layer of the invention may comprise at least homophasic polypropylene, homophasic ethylene polymer, possibly heterophasic polypropylene, and possibly the aforementioned ingredients as defined in the invention.

[0110] The proportions of the different ingredients in the electrically insulating layer can be identical to those described in the invention for these same ingredients in the polymer composition.

[0111] The cable of the invention relates more particularly to the field of electrical cables operating in direct current (DC) or alternating current (AC). The cable employing the insulating layer of the invention

[0112] Preferably, the electrically insulating layer of the invention, obtained from the polymer composition defined in the preceding paragraphs, surrounds the elongated electrically conductive element.

[0113] The elongated electrically conductive element is typically positioned in the center of the cable.

[0114] The elongated electrically conductive element can be a single-core conductor such as a metal wire or a multi-core conductor such as a plurality of twisted (stranded together) or untwisted metal wires.

[0115] The elongated electrically conductive element may be made of aluminium, aluminium alloy, copper, copper alloy, or a combination thereof.

[0116] The cable may also include an outer protective sheath surrounding the electrically insulating layer.

[0117] The outer protective sheath can be in direct physical contact with the electrically insulating layer.

[0118] The outer protective sheath can be an electrically insulating sheath.

[0119] According to a particularly preferred embodiment of the invention, the electrically insulating layer is in direct physical contact with the elongated electrically conductive element.

[0120] The cable of the invention is a low-voltage cable. A cable according to the invention does not generally include a semiconductor layer. Method for applying a thermoplastic coating layer

[0121] According to another aspect, the present invention relates to a method of applying a thermoplastic coating layer around at least one elongated electrically conductive element, said method comprising the application, in particular by extrusion, of a thermoplastic polymer composition as described above and the addition of gas bubbles during extrusion.

[0122] According to a first embodiment, a chemical expander is added to the thermoplastic polymer composition before or during extrusion. The chemical expander is added in an amount of between 0.1 and 10% by weight relative to the total weight of the electrically insulating layer.

[0123] According to a second embodiment, a gas is injected during extrusion. Brief description of the drawings

[0124] There figure 1 represents a cable conforming to the invention.

[0125] For the sake of clarity, only the essential elements for understanding the invention have been represented schematically, and this without regard to scale.

[0126] There Figure 1represents a low-voltage electrical cable 1 according to the invention, comprising a central elongated electrically conductive element 2, in particular made of copper or aluminum or one of their alloys. The electrical cable 1 further comprises an electrically insulating layer 3, and optionally an outer protective sheath 4.

[0127] The electrically insulating layer 3 is an extruded, non-crosslinked layer, obtained from a polymer composition according to the invention.

[0128] The presence of the outer protective sheath 4 is preferable, but not essential according to the invention.

[0129] There figure 2 represents a photograph of the electrical cable according to the invention. Example Example 1: Formulation and method of applying a thermoplastic coating layer around the elongated electrically conductive element according to the invention

[0130] All compounds, according to Table 1, are introduced simultaneously into the extruder after being pre-mixed to obtain a homogeneous blend. Extrusion was carried out on a Mapré single-screw extruder (D = 38 mm, L = 26 D) with a barrier screw (compression ratio = 1.8), a rotation speed of 20 rpm, with a temperature increase profile from 155 °C at the inlet to 180 °C at the extrusion head to obtain a 1.5 mm thick strip on a conductive wire. Table 1 Tests 1 2 3 4 5 6 Polymer blend >99% >99% >98% >98% >98% >98% Antioxidant agent <1% <1% <1% <1% <1% <1% Chemical expander 1 0 0 0,6 0 0,6 0 Chemical expander 2 0 0 0 0,6 0 0,6 Polymer blend : mixture of 60 to 70% statistical polypropylene copolymer, less than 10% heterophasic polypropylene (only for examples 5 and 6) and 30 to 40% LLDPE. Chemical expander 1: Hydrocerol CT3274 marketed by the company Avient Chemical expander 2: Hydrocerol BM70 is marketed by the company Avient. Example 2: Measurement of the density of an electrical cable according to the invention.

[0131] The density was measured by immersion in a Sartorius laboratory balance according to ISO 1183. The results are presented in Table 2. Table 2 Tests 1 2 3 4 5 6 Density 0,90 0,90 0,70 0,66 0,69 0,74 Example 3: Measurement of the expansion rate of the insulating layer according to the invention

[0132] The expansion rate is calculated as follows: The results are presented in Table 3. Table 3 Tests 1 2 3 4 5 6 Expansion rate / / 0,228 + / -0,003 0,282 + / -0,003 0,238 + / -0,002 0,178 + / -0,002 Example 4: Measurement of the mechanical properties of the electrical cable according to the invention

[0133] Mechanical properties are measured in the initial state and after 10 days at 135 °C in accordance with EN 60881-401 and EN 60881-501 standards on H2 dumbbell test specimens with an Instron tensile testing device at a tensile speed of 25 mm / min.

[0134] The measurement of Young's modulus is carried out on the Instron tensile test bench according to the ISO 527-2 standard.

[0135] The determination of the flexural modulus is carried out in accordance with ISO 178 using the Instron tensile test bench on specimens of the following dimensions (length: 80 ± 2 mm; width: 10.0 ± 0.2 mm; thickness: 4.0 ± 0.2 mm).

[0136] The results are presented in Table 4. Table 4 Tests 1 2 3 4 5 6 At t0 Tensile strength at break (MPa) 39 36 22 21 18 17 Elongation at break (%) 926 933 676 652 578 604 Young's modulus (MPa) 413 372 350 305 199 330 Flexural modulus (MPa) / 852 398 451 356 412 At t=10 J 135 °C Tensile strength at break (MPa) 33 36 18 19 17 22 Elongation at break (%) 824 854 460 504 446 576

[0137] The obtaining of an expanded layer exhibiting porosity was verified by observation under a Leica optical microscope on 100 µm thick slides cut with a Leica microtome.

Claims

1. An electrical cable comprising at least one electrically conductive elongated element and an electrically insulating layer in direct contact with the electrically conductive elongated element, the electrical cable being a low-voltage cable characterized in that - said electrically insulating layer is obtained from a thermoplastic polymer composition comprising at least 50% by mass of polypropylene, in the form of a homopolymer, a copolymer, or a mixture and in homophasic, heterophasic or mixture form; and - said electrically insulating layer comprises gas bubbles.

2. Electrical cable according to claim 1, characterized in that the polypropylene composition includes at least one homophasic polypropylene.

3. Electrical cable according to claim 2, characterized in that the polypropylene composition comprises at least 80% by weight of homophasic propylene polymer relative to the total weight of polypropylene.

4. Electrical cable according to any one of the preceding claims, characterized in that the polypropylene composition includes between 1 and 10% by weight of heterophasic polypropylene relative to the total weight of polypropylene.

5. Electrical cable according to any one of the preceding claims, characterized in that the mass ratio between heterophasic polypropylene and homophasic polypropylene present in the polypropylene composition is between 0 / 1 and 1 / 6.

6. Electric cable according to any one of the preceding claims, characterized in that the thermoplastic polymer composition comprises thermoplastic polyethylene, preferably LLDPE.

7. Electrical cable according to any one of the preceding claims, characterized in that the electrically insulating layer including gas bubbles has a thickness between 0.5 mm and 2 mm.

8. Electric cable according to any one of the preceding claims characterized in that the electrically insulating layer including the gas bubbles has a density between 0.5 and 1.

9. Electrical cable according to claim 1, characterized in that These gas bubbles contain N2 or CO2.

10. Electrical cable according to any one of the preceding claims, characterized in that The electrical cable includes an outer sheath in contact with the electrically insulating layer.

11. Method of applying a thermoplastic coating layer around at least one elongated electrically conductive element, said method comprising the application, in particular by extrusion, of a thermoplastic polymer composition according to claim and the addition of gas bubbles during extrusion.

12. Method according to claim 11, characterized in thatA chemical expander is added to the thermoplastic polymer composition before extrusion.

13. Method according to claim 12, characterized in that The chemical expander is a composition made up of at least one bicarbonate and one citric acid.

14. Method according to claim 12 or 13, characterized in that The chemical expander is introduced at a content of between 0.1 and 10% by weight relative to the total weight of the thermoplastic polymer composition.

15. Method according to claim 11, characterized in that A gas is injected during extrusion.

16. Method according to claim 11, characterized in that The expansion rate of the insulating layer is between 2% and 50%.

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