CA3008329C - Polymer composition comprising a dielectric liquid having improved polarity - Google Patents

Abstract

The invention relates to a polymer composition comprising at least one thermoplastic polymer material and a dielectric liquid that has improved polarity, a method for producing said polymer composition, a cable comprising at least one electrically insulating layer obtained using said polymer composition, and a method for manufacturing said cable.

Description

5 10 15 20 25 30 CA 03008329 2018-06-13 1 POLYMER COMPOSITION COMPRISING A DIELECTRIC LIQUID HAVING IMPROVED POLARITY The invention relates to a polymer composition comprising at least one thermoplastic polymer material and a dielectric liquid having improved polarity, a method for preparing said polymer composition, a cable comprising at least one electrically insulating layer obtained from said polymer composition, and a method for preparing said cable. The invention typically, but not exclusively, applies to electrical cables intended for power transmission, particularly medium-voltage (especially 6 to 45-60 kV) or high-voltage (especially above 60 kV, and up to 400 kV) power cables, whether direct or alternating current, in the fields of aerial, submarine, terrestrial, or aeronautical power transmission. A medium- or high-voltage power transmission cable generally comprises, from the inside out: - an elongated electrically conductive element, particularly made of copper or aluminum; - an internal semiconducting layer surrounding said elongated electrically conductive element; - an electrically insulating layer surrounding said internal semiconducting layer; - an external semiconducting layer surrounding said insulating layer; and - possibly an electrically insulating protective sheath surrounding said external semiconducting layer. In particular, the electrically insulating layer may be a polymer layer based on a cross-linked polyolefin such as cross-linked polyethylene (XLPE) or a cross-linked ethylene-propylene or ethylene-propylene-diene elastomer. Cross-linking is generally carried out during the extrusion step of the polymer composition around the elongated electrically conductive element. The use of cross-linked polyolefin makes it possible to produce a layer with satisfactory electrical and mechanical properties, resulting in a cable capable of operating at temperatures above 70°C, even up to 90°C. However, several problems arise. Firstly, cross-linked materials cannot be recycled. Secondly, cross-linking (vulcanization) to produce a homogeneous layer requires specific reaction conditions (e.g., in terms of duration and temperature) which reduce the cable manufacturing speed and increase its production cost. Finally, cross-linking can sometimes begin prematurely in the extruder and/or the extruder head, leading to a degradation of the resulting layer quality, particularly its dielectric properties. Alternatives have been proposed, such as a thermoplastic layer of low-density polyethylene (LDPE) or high-density polyethylene (HDPE). However, a cable with such an electrically insulating layer cannot operate at temperatures above approximately 70°C for an LDPE thermoplastic layer and above 80°C for an HDPE thermoplastic layer, thus limiting the power that can be carried in the cable and the manufacturing methods. It is also known to manufacture an electrically insulating layer composed of several paper strips or a paper-polypropylene composite impregnated with a large quantity of dielectric fluid (e.g., oil-impregnated paper cable). The dielectric fluid completely fills the voids in the layer and prevents partial discharges and damage to the cable insulation. However, manufacturing this type of electrically insulating layer is very complex and expensive. The choice of dielectric liquid will depend on the intended application (i.e. the type of electrical equipment) and its compatibility with the solid insulator it is supposed to impregnate and/or with which it forms an intimate mixture. Conventional dielectric liquids include mineral oils (e.g. naphthenic oils, paraffinic oils, aromatic oils5 10 15 20 25 30 CA 03008329 2018-06-13 WO 2017/103509 3 PCT/FR2016/053478 or polyaromatic oils), vegetable oils (e.g. soybean oil, linseed oil, rapeseed oil, corn oil or castor oil) or synthetic oils such as aromatic hydrocarbons (alkylbenzenes, alkylnaphthalenes, alkylbiphenyls, alkyldiarylethylenes, etc.), silicone oils, ethers, organic esters or aliphatic hydrocarbons. In particular, international application WO 02/03398 A1 describes a cable comprising at least one electrical conductor and at least one extruded electrically insulating layer based on a dielectric liquid intimately mixed with a thermoplastic polymer material comprising a homopolymer of propylene or a copolymer of propylene with at least one olefin comonomer selected from ethylene and an olefin other than propylene. The dielectric liquid is an aromatic hydrocarbon comprising at least one alkylaryl hydrocarbon having at least two non-condensed aromatic rings (e.g., dibenzyltoluene or a mixture of 85% by mass of monoxylylxylene and 15% by mass of dixylylxylene). International application WO 02/03398 A1 specifies that the dielectric fluid must be free of polar compounds to minimize water absorption and prevent defects in the insulating layer caused by water vapor during hot extrusion. However, aromatic hydrocarbons (alkylbenzenes, alkylnaphthalenes, alkylbiphenyls, alkyldiarylethylenes) are expensive and generally cause safety and/or environmental problems (e.g., some aromatic hydrocarbons are considered carcinogenic). Mineral oils account for 90 to 95% of the dielectric fluid market due to the low cost of these "natural" products, obtained directly from crude oil refining. The chemical composition of a mineral oil is extremely complex (several thousand different molecules) and can vary considerably. The chemical composition of a mineral oil is therefore generally defined by its paraffinic carbon (Cp) content, its naphthenic carbon (Cn) content, and its aromatic carbon (Ca) content. Mineral oils may also contain a small percentage of hydrocarbon molecules that include heteroatoms such as nitrogen, sulfur, or oxygen (e.g., polar compounds) in their structure. The polar compound content of a mineral oil can be determined according to ASTM D2007-02. However, mineral oils exhibit lower oxidation stability and dielectric strength compared to aromatic hydrocarbons. Furthermore, it is known from International Application WO 2004/066318 A1 that the electrical properties of a cable comprising at least one electrical conductor and at least one extruded electrically insulating layer based on a thermoplastic polymer material intimately blended with a mineral oil as the dielectric fluid can be degraded by the presence of these polar compounds, particularly in terms of dielectric losses. Therefore, there remains a need for electrical cables comprising an electrically insulating layer with electrical properties comparable to those obtained with an XLPE cross-linked layer, while also ensuring good mechanical properties. Thus, the aim of the present invention is to overcome the drawbacks of the prior art and to provide an economical polymer composition comprising recyclable materials, capable of producing an electrically insulating layer with improved dielectric properties, particularly in terms of dielectric strength, while ensuring low dielectric losses. The aim of the present invention is also to provide a cable, particularly for medium or high voltage applications, capable of operating at temperatures above 70°C and exhibiting improved electrical properties, particularly in terms of dielectric strength, while ensuring low dielectric losses. The objectives are achieved by the invention which will be described below. The invention's primary object is a polymer composition comprising at least one thermoplastic polymer material based on polypropylene and a dielectric liquid, characterized in that the dielectric liquid comprises at least one mineral oil and at least one polar compound of the benzophenone, acetophenone, or one of their derivatives type. The combination of a polar compound of the benzophenone, acetophenone, or one of their derivatives type with a mineral oil makes it possible to increase the dielectric strength of the electrically insulating layer of an electrical cable while ensuring low dielectric losses. Furthermore, the presence of a thermoplastic polymer material based on polypropylene makes it possible to increase the operating temperature of the cable to 90°C. 110°C. The dielectric fluid may comprise at least 70% by mass of mineral oil, and preferably at least 80% by mass of mineral oil, relative to the total mass of the dielectric fluid. The mineral oil is generally liquid at approximately 20-25°C. The mineral oil may be selected from naphthenic and paraffinic oils, and preferably from naphthenic oils. The mineral oil is obtained from the refining of crude petroleum. According to a particularly preferred embodiment of the invention, the mineral oil comprises a paraffinic carbon (Cp) content of approximately 45 to 65 atomic percent, a naphthenic carbon (Cn) content of approximately 35 to 55 atomic percent, and an aromatic carbon (Ca) content of approximately 0.5 to 10 atomic percent. In a particular embodiment, the polar compound of type Benzophenone, acetophenone, or one of their derivatives must represent at least 2.5% by mass, preferably at least 3.5% by mass, and even more preferably at least 4% by mass, relative to the total mass of the dielectric fluid. Thanks to this minimum quantity of polar compound, the dielectric strength is improved. The dielectric fluid may comprise at most 30% by mass, preferably at most 20% by mass, and even more preferably at most 15% by mass, of a polar compound of type 5, 10, 15, 20, 25, CA 03008329 2018-06-13 WO 2017/103509 6 PCT/FR2016/053478, benzophenone, acetophenone, or one of their derivatives, relative to the total mass of the dielectric fluid. This maximum quantity ensures minimal losses moderate to low dielectric constants (e.g., less than approximately 10⁻³), and also to prevent migration of the dielectric fluid out of the electrically insulating layer. The amount of polar compounds in the dielectric fluid can be determined according to ASTM D2007-02. The polar compound of the benzophenone, acetophenone, or one of their derivatives type may correspond to the following formula (I): O r1-^\r2 (I) in which R1 and R2, identical or different, are aryl, alkyl, or alkylene-aryl groups, the R1 and R2 groups being able to be linked together by element A representing a single bond or a -(CH2)n- group with n equal to 1 or 2. The aryl group may include one or more aromatic rings, condensed or non-condensed, and preferably non-condensed. The aryl group may include of 5 to 20 carbon atoms, and preferably of 5 to 15 carbon atoms. Each aromatic ring may include one or more heteroatoms such as a nitrogen atom, a sulfur atom, or an oxygen atom. Each aromatic ring may be substituted by one or more substituents X chosen from a halogen atom, an alkyl group, or an alkoxy-O-alkyl group. An alkyl group is preferred. The halogen atom as substituent X on an aromatic ring is preferably a chlorine or fluorine atom, and preferably again a chlorine atom. 5 10 15 20 25 30 CA 03008329 2018-06-13 WO 2017/103509 7 PCT/FR2016/053478 The alkyl group as substituent X on an aromatic ring may have of The alkyl group of the alkoxy group (-O-alkyl) as substituent X on an aromatic ring may have from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, and preferably from 2 to 4 carbon atoms. The aromatic ring(s) do not preferably include practical polar substituents such as an -OH, -SH, -NH, or -NH2 group. The aryl group is preferably a phenyl, naphthyl, or pyridyl group, and preferably a phenyl group. The alkyl group as group R1 and/or R2 may have from 1 to 20 carbon atoms, preferably from 1 to 10 carbon atoms, and preferably from 1 to 5 carbon atoms. In the present invention, the alkyl group may be cyclic, linear, or branched. The alkylene-aryl group is a mixed group comprising an aryl group as defined in the present invention and an alkylene group. The alkylene group of the alkylene-aryl group is directly related to the ketone functional group of the compound of formula (I). The alkylene group may be linear, cyclic, or branched, and preferably linear. In particular, the alkylene group may have from 1 to 5 carbon atoms. Preferably, the alkylene group is a -(CH2)P- group with 1 < 1 < 5; a -(CHR)pi- group with 1 < 1 < 5 and R being an alkyl group, preferably having from 1 to 5 carbon atoms; or a -(CHR)mi-(CH2)m2- group (Le., comprising mi-CH2- and m2-CHR-), with mi + m2 < 5, and R being an alkyl group, preferably having from 1 to 5 carbon atoms; or a statistical group -(CHR)p2-(CH2)p3-(CHR')p4- (i.e., comprising p2-CH2-, p3-CHR-, and p4-CHR'-), with p2 + p3 + p4 ≤ 5, and R and R' being different alkyl groups, each preferably having from 1 to 5 carbon atoms, with preferably 1 < m1 < 4, 1 < m2 < 4, 1 < p2 < 3, 1 < p3 < 3, and 1 ≤ p4 ≤ 3. In the present invention, a statistical group means that the The radicals that constitute it (e.g., -CH2-, -CHR-, and/or -CHR'-) can be randomly positioned within the alkylene group. R (respectively R') can be a methyl, ethyl, propyl, or isopropyl group. When the alkylene group is branched (e.g., presence of at least one of the R or R' groups), it can also be linked via branching to the aryl group of said alkylene-aryl group. The alkylene-aryl group can be a benzyl group (alkylene-aryl group in which the alkylene group is a CH2 and the aryl group is a phenyl group). The compound of formula (I) is of the benzophenone type when the R1 and R2 groups are aryl groups; of the acetophenone type when one of the R1 or R2 groups is an aryl group and the other is an alkyl group; and one of their derivatives when groups R1 and R2 are alkyl groups or one of groups R1 or R2 is an alkylene-aryl group and the other is an alkyl, aryl, or alkylene-aryl group. According to a first embodiment of the invention, at least one of said groups R1 or R2 of the compound of formula (I) is a phenyl group. According to a second embodiment of the invention, groups R1 and R2 of the compound of formula (I) are aryl groups. Preferably, at least one of them comprises a phenyl group, and preferably further, both aryl groups each comprise a phenyl group. According to a third embodiment, groups R1 and R2 of the compound of formula (I) are each phenyl groups, preferably unsubstituted. 5 10 15 20 25 30 CA 03008329 2018-06-13 9 According to a form of In a particularly preferred embodiment of the invention, the compound of formula (I) may be benzophenone, dibenzosuberone, fluorenone, or anthrone, and preferably benzophenone. The ratio of the number of aromatic carbon atoms to the total number of carbon atoms in the dielectric fluid may be less than about 0.3, and preferably even less than about 0.2. Thanks to this low ratio, safety problems related to the toxicity of the dielectric fluid are minimized. The ratio of the number of aromatic carbon atoms to the total number of carbon atoms in the dielectric fluid may be determined according to ASTM D3238. The mineral oil may comprise one or more polar compounds other than the polar compound of the benzophenone, acetophenone, or one of their derivatives, and in particular other than the compound of formula (I). However, it is preferable that these polar compounds be other than the polar compound of Benzophenone, acetophenone, or one of their derivatives, or compounds other than those of formula (I), represent at most approximately 2.5% by mass, and preferably at most approximately 1.5% by mass, relative to the total mass of the dielectric fluid. An oil containing such proportions of polar compounds may be a refined oil. Preferably, the dielectric fluid has a boiling point above approximately 250°C. Thus, the dielectric fluid of the polymer composition of the invention, which combines a mineral oil and a compound of the benzophenone, acetophenone, or one of their derivatives type, can be handled safely at room temperature (low volatility) and at the temperatures required by the process for forming the electrically insulating layer (e.g., extrusion), while ensuring the formation of a homogeneous, intimate mixture with the polymer material of the polymer composition. 30 CA 03008329 2018-06-13 10 ('invention. The polypropylene-based thermoplastic polymer material may comprise at least one homopolymer or copolymer of propylene (Pi), and optionally at least one homopolymer or copolymer of olefin (P2). The combination of Pi and P2 polymers makes it possible to obtain a thermoplastic polymer material exhibiting good mechanical properties, particularly in terms of elastic modulus, and electrical properties. In particular, the propylene Pi copolymer may be a random copolymer of propylene. Examples of propylene Pi copolymers include propylene and olefin copolymers, the olefin being notably chosen from ethylene and an olefin different from propylene. The olefin different from propylene may meet the The formula CH2=CH-R3, in which R3 is a linear or branched alkyl group having from 2 to 10 carbon atoms, may be selected from the following olefins: 1-butene, 1-pentene; 4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, and mixtures thereof. The olefin in the propylene-olefin copolymer preferably represents not more than 15% by mol and preferably not more than 10% by mol of the copolymer. Propylene-ethylene copolymers are preferred as the propylene Pi copolymer. The propylene Pi copolymer preferably has an elastic modulus of approximately 600 to 1200 MPa. The propylene Pi homopolymer preferably has an elastic modulus of approximately 1250 at approximately 1600 MPa. The homopolymer or copolymer of propylene Pi may have a melting point above approximately 130°C, preferably above approximately 140°C, and even more preferably ranging from approximately 140 to 165°C. 5 10 15 20 25 30 CA 03008329 2018-06-13 11 In particular, the homopolymer of propylene Pi may have a melting point of approximately 165°C and the copolymer of propylene Pi may have a melting point ranging from approximately 140 to 150°C. The homopolymer or copolymer of propylene Pi may have an enthalpy of fusion ranging from approximately 30 to 100 J/g. In particular, the homopolymer of propylene Pi may have an enthalpy of fusion ranging from approximately 80 to 90 J/g, and the propylene Pi copolymer can have an enthalpy of fusion ranging from approximately 30 to 70 J/g. The propylene Pi homopolymer or copolymer can have a melt flow index ranging from 0.5 to 3 g/10 min, measured at approximately 230°C with a charge of approximately 2.16 kg according to ASTM D1238-00. According to a preferred embodiment of the invention, the propylene Pi homopolymer or copolymer represents approximately 40 to 70% by mass of the polypropylene-based thermoplastic polymer material. The olefin of the olefin homopolymer or copolymer P2 can correspond to the formula CH2=CH-R4, in which R4 is a hydrogen atom or a linear alkyl group or branched having from 1 to 12 carbon atoms, and in particular being selected from the following olefins: ethylene, propylene, 1-butene, isobutylene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, and mixtures thereof. The olefin α-propylene, 1-hexene, or 1-octene is preferred. The homopolymer or copolymer of olefin α-P2 may be a heterophase copolymer comprising a thermoplastic phase of the propylene type and a thermoplastic elastomer phase of the copolymer type of ethylene and an olefin α-polyethylene, or mixtures thereof. The thermoplastic elastomer phase of the heterophase copolymer may represent at least approximately 20% by mass, and preferably at least 45% by mass. approximately mass, relative to the total mass of the heterophase copolymer. The olefin in the thermoplastic elastomer phase of the heterophase copolymer can be propylene. 5 10 15 20 25 30 CA 03008329 2018-06-13 WO 2017/103509 12 PCT/FR2016/053478 The polyethylene can be a linear low-density polyethylene. In the present invention, the expression "linear low-density polyethylene" means a linear polyethylene having a density ranging from approximately 0.91 to 0.925. According to a preferred embodiment of the invention, the P2 olefin homopolymer or copolymer represents approximately 30 to 60% by mass of the polypropylene-based thermoplastic polymer material. The thermoplastic polymer material The polymer composition of the invention is preferably heterophase (i.e., it comprises several phases). The presence of several phases generally arises from the mixing of two different polyolefins, such as a mixture of polypropylene and a propylene or polyethylene copolymer. The polymer composition of the invention comprises an intimate, homogeneous mixture of the dielectric fluid and the thermoplastic polymer material (e.g., it forms a homogeneous phase). The mass concentration of the dielectric fluid in the polymer composition is preferably less than or equal to the saturation mass concentration of said dielectric fluid in the thermoplastic polymer material. The saturation mass concentration at 20-25°C is generally on the order of approximately 15 to 20%. It can be determined by the liquid absorption method. In particular, plates (e.g., with dimensions of 200 mm x 200 mm x 0.5 mm) made of the Thermoplastic polymer material based on polypropylene, of the polymer composition, is prepared from the corresponding raw materials, notably by molding. Samples of these sheets are weighed (initial weight = Po) and then immersed at approximately 20°C in the dielectric liquid of the polymer composition. The mass saturation concentration is measured by determining the weight change (in percentage) of the samples after different immersion durations (e.g., 3, 6, 9, 12, and 15 days) and after cleaning and drying their surface (final weight = Pf). The absorption of the dielectric liquid is determined according to the following formula: % absorption of dielectric liquid = [(Pf-Po)/Po] x 100. The saturation concentration is reached when Pf shows a Variation of less than 1% with respect to the increase in total weight corresponding to Pf-Po. According to a particular embodiment, the dielectric liquid represents approximately 1% to 20% by mass, preferably approximately 2% to 15% by mass, and preferably approximately 3% to 12% by mass, relative to the total mass of the polymer composition. According to a particular embodiment, the polar compound of the benzophenone, acetophenone, or one of their derivatives represents approximately 0.15% to 1.8% by mass, preferably approximately 0.21% to 1.2% by mass, and preferably approximately 0.24% to 0.9% by mass, relative to the total mass of the polymer composition. According to a particular embodiment, the thermoplastic polymer material based on polypropylene represents approximately 70% to 98% by mass, preferably 80% to 95%. by mass approximately, and preferably approximately 88 to 97% by mass, relative to the total mass of the polymer composition. The polymer composition may further comprise one or more additives. The additives are well known to those skilled in the art and may be selected from antioxidants, UV stabilizers, copper stabilizers, water-arborescence inhibitors, pigments, and mixtures thereof. The polymer composition of the invention may typically comprise approximately 0.01 to 5% by mass, and preferably approximately 0.1 to 2% by mass of additives, relative to the total mass of the polymer composition. 5 10 15 20 25 CA 03008329 2018-06-13 14 More particularly, the antioxidants protect the polymer composition from the thermal stresses generated during the cable manufacturing or operating stages. The antioxidants are selected preferably from among the hindered phenols, thioesters, sulfur-based antioxidants, phosphorus-based antioxidants, amine-type antioxidants, and mixtures thereof. Examples of hindered phenols include Pentaerythritol tetrakils(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), 4,6-bis(octylthiomethyl)-o-cresol (Irgastab® KV10), 2,2'-thiobis(6-fert-butyl-4-methylphenol) (Irganox® 1081), Ie 2,2'- thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] (Irganox® 1035), Ie 2,2'-methylenebis(6-tert-butyl-4-methylphenol), I,2-bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyl)hydrazine (Irganox® MD 1024), or 2,2'-oxamido-bis(ethyl-3(3,5-di-te/tbutyl-4-hydroxyphenyl)propionate). As examples of thioesters, mention may be made of didodecyl-3,3'-thiodipropionate (Irganox® PS800), distearyl thiodipropionate (Irganox® PS802) or 4,6-bis(octylthiomethyl)-o-cresol (Irganox® 1520). Examples of sulfur-based antioxidants include dioctadecyl-3,3'-thiodipropionate or didodecyl-3,3'-thiodipropionate. Examples of phosphorus-based antioxidants include tris(2,4-di-tert-butylphenyl)phosphite (Irgafos® 168) or bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite (Ultranox® 626). Examples of amine-type antioxidants include phenylenediamines (e.g., 1PPD or 6PPD) and diphenylamines. styrene, diphenylamines, mercaptobenzimidazoles and polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ).5 10 15 20 25 30 CA 03008329 2018-06-13 15 As examples of antioxidant mixtures, Irganox B 225 can be cited, which comprises an equimolar mixture of Irgafos 168 and Irganox 1010 as described above. The polymer composition is a thermoplastic polymer composition. It is therefore not cross-linkable. In particular, the polymer composition does not include any t-Af-i /"*1 11 *3t-i /in d/ancint,c da rrumlana do K/na ciliano da narAv\/dac rl'aHHi'HFc ItJui uidt»iwii/ u dyc11tw uc uuuipidyts uic uypc isiid11ts/ uit* pt*i yucis tstyu#ui ui duiuiu11w which allow crosslinking. Indeed, such agents degrade the polypropylene-based thermoplastic polymer material. The polymer composition is preferably recyclable. The invention has as its second object a process for preparing the polymer composition according to the first object, characterized in that it comprises at least one step i) of mixing a polypropylene-based thermoplastic polymer material with a dielectric liquid as defined in the first object of the invention. In particular, the mixing is carried out according to the following sub-steps: i-a) the mixing of a mineral oil and at least one polar compound of the type benzophenone, acetophenone or one of their derivatives (as defined in the first object of the invention), in order to form a dielectric liquid as defined in the first object of the invention, and i-b) the mixing of a thermoplastic polymer material based on polypropylene as defined in the first object of the invention with the dielectric liquid as obtained in the preceding sub-step i-a). The thermoplastic polypropylene-based polymer material of substep i-b) is generally in the form of polymer granules, in particular granules of at least one homopolymer or copolymer of propylene Pi and optionally of at least one homopolymer or copolymer of olefin P2 as defined in the first object of the invention.5 10 15 20 25 30 CA 03008329 2018-06-13 WO 2017/103509 16 PCT/FR2016/053478 The mixing of substep i-a) can be carried out using any apparatus capable of dissolving the polar compound of the benzophenone, acetophenone type or one of their derivatives and optionally the additive(s) (in particular when they are in the form of solid powders) in mineral oil. Substep i-a) is preferably carried out at a temperature ranging from approximately 20 to 100°C, preferably from approximately 50 to 90°C, and preferably still at a temperature of approximately 75°C. Substep i-a) generally lasts from 15 minutes to 1 hour, and preferably from 20 to 30 minutes. At the end of substep i-a), a stable and transparent solution is obtained. The mixture in substep i-a) may further include the additive(s) as defined in the invention (e.g., antioxidant). The mixing of substep i-b) can be carried out by mixing the dielectric liquid obtained in substep i-a) with the polypropylene-based thermoplastic polymer material or the polymer compounds which constitute it, in particular with the aid of an internal mixer, in particular with tangential rotors or with meshing rotors, or a continuous mixer, in particular with a screw or counter-rotating twin screw or a "Buss extruder" type mixer. Substep i-b) can be carried out by impregnating granules of a thermoplastic polymer material based on polypropylene as defined in the first object of the invention with the dielectric liquid, in particular by mixing the polymer granules with the dielectric liquid, heating the resulting mixture so that the granules absorb the dielectric liquid, feeding a single screw extruder with the resulting mixture and homogenizing the mixture obtained in the molten state by mechanical agitation in the extruder. During substep i-b), the polymer composition of the invention can be shaped, in particular into granules. 5 10 15 20 25 CA 03008329 2018-06-13 WO 2017/103509 17 PCT/FR2016/053478 To do this, the temperature within the mixer is chosen to be sufficient to obtain the thermoplastic polymer material in a molten state. Then, the homogeneous mixture can be granulated using techniques well known to those skilled in the art. These granules can then be fed into an extruder to manufacture the cable of the invention according to a process as defined below. The third object of the invention is a cable comprising at least one elongated electrically conductive element and at least one electrically insulating layer obtained from a polymer composition as defined in the first object of the invention. The electrically insulating layer of the invention is a non-crosslinked layer. The electrically insulating layer of the invention is preferably a recyclable layer. The electrically insulating layer of the invention may be an extruded layer, in particular by processes well known to those skilled in the art. In the present invention, "electrically insulating layer" means a layer whose electrical conductivity may be at most 1 x 10⁹ S/m, and preferably at most 1 x 10¹⁰ S/m (siemens per meter) (at 25°C). The cable of the invention relates more particularly to the field of electrical cables operating in direct current (DC) or alternating current (AC). The electrically insulating layer of the invention can surround the elongated electrically conductive element. The elongated electrically conductive element can be a single-core conductor such as, for example, a metallic wire, or a multi-core conductor such as a plurality of twisted or untwisted metallic wires. The elongated electrically conductive element can be made of aluminum, aluminum alloy, copper, copper alloy, and any combination thereof. According to a preferred embodiment of the invention, the electrical cable can comprise: - a first semiconducting layer surrounding the elongated electrically conductive element, - an electrically insulating layer surrounding the first semiconducting layer. electrically insulating layer as defined in the invention, and a second semiconductor layer surrounding the electrically insulating layer. In the present invention, "semiconductive layer" means a layer whose electrical conductivity can be at least 1 x 10⁹ S/m (siemens per meter), preferably at least 1 x 10⁻³ S/m, and preferably less than 1 x 10³ S/m (at 25°C). In a particular embodiment, the first semiconductor layer, the electrically insulating layer, and the second semiconductor layer constitute a three-layer insulation. In other words, the electrically insulating layer is in direct physical contact with the first semiconductor layer, and the second semiconductor layer is in direct physical contact with the electrically insulating layer. The cable may further comprise an electrically insulating sheath surrounding the second semiconducting layer, and may be in direct physical contact with it. The electrical cable may further include a metallic screen surrounding the second semiconducting layer. In this case, the electrically insulating sheath surrounds said metallic screen. This metallic screen may be a so-called "wire" screen composed of a set of copper or aluminum conductors arranged around and along the second semiconducting layer, a so-called "ribbon" screen composed of one or more conductive copper or aluminum metallic ribbons possibly laid helically around the second semiconducting layer, or a conductive aluminum metallic ribbon laid longitudinally around the second semiconducting layer. The cable is made watertight by means of glue in the overlapping areas of the ribbon, or by a so-called "watertight" screen of the metallic tube type, possibly composed of lead or a lead alloy, surrounding the second semiconducting layer. This latter type of screen notably acts as a barrier to moisture that tends to penetrate the electrical cable radially. The metallic screen of the electrical cable of the invention may comprise a so-called "wire" screen and a so-called "watertight" screen, or a so-called "wire" screen and a so-called "ribbon" screen. All types of metallic screens can act as grounding for the electrical cable and can thus carry fault currents, for example, in the event of a short circuit in the network concerned. Other layers, such as layers that swell in the presence of moisture, may be added between the second semiconducting layer and the metallic screen; these layers allow to ensure the longitudinal water tightness of the electrical cable. The invention has as its fourth object a method for manufacturing an electrical cable according to the third object of the invention, characterized in that it comprises at least one step 1) of extruding the polymer composition according to the first object of the invention around an elongated electrically conductive element, to obtain an electrically insulating (extruded) layer surrounding said elongated electrically conductive element. Step 1) can be carried out using techniques well known to those skilled in the art, for example, with an extruder. 5 10 15 20 25 CA 03008329 2018-06-13 20 During step 1), the composition exiting the extruder is said to be "non-crosslinked," the temperature and processing time within the extruder being optimized accordingly. At the extruder outlet, a layer is thus obtained extruded around said electrically conductive element, which may or may not be in direct physical contact with said elongated electrically conductive element. The process does not include a preferred crosslinking step for the layer obtained in step 1). Figure 1 shows a schematic view of an electrical cable according to a preferred embodiment of the invention. Figure 2 shows the curves of the tangent delta (tan(6)) at 90°C as a function of frequency (Hz), for a layer according to the invention and for a comparative layer. For clarity, only the elements essential for understanding the invention have been represented schematically, without regard to scale. The medium- or high-voltage power cable 1, illustrated in Figure 1, comprises an electrically conductive central element 2, made of copper or aluminum. The power cable 1 further comprises several layers arranged successively and coaxially around this central elongated electrically conductive element 2, namely: a first semiconducting layer 3 called the "internal semiconducting layer", an electrically insulating layer 4, a second semiconducting layer 5 called the "external semiconducting layer", a metallic screen 6 for grounding and/or protection, and an external protective sheath 7. The electrically insulating layer 4 is a non-crosslinked extruded layer, obtained from the polymer composition according to the invention. Semiconductor layers 3 and 5 are extruded thermoplastic layers (i.e., non-crosslinked). The presence of the metallic screen 6 and the outer protective sheath 7 is preferable, but not essential, this cable structure being well known to those skilled in the art. Other aspects of the invention are described with reference to any one of the preferred embodiments [1] to [15] defined below.

[1] A cable comprising at least one elongated electrically conductive element, and at least one electrically insulating layer, the electrically insulating layer being obtained from a polymer composition comprising at least one polypropylene-based thermoplastic polymer material and a dielectric liquid, characterized in that the dielectric liquid comprises at least one mineral oil and at least one polar compound selected from the group constitutes compounds corresponding to the following formula (I): in which R1 and R2, identical or different, are aryl, alkyl or alkylene-aryl groups, and optionally R1 and R2 are linked together by the optional element A which, when present, represents a single bond or a -(CHOn-) group with n equal to 1 or 2.

[2] The cable according to embodiment [1], characterized in that the dielectric fluid comprises at least 70% by mass of mineral oil, relative to the total mass of the dielectric fluid.

[3] The cable according to embodiment [1] or [2], is characterized in that the mineral oil is chosen from the group consisting of naphthenic oils and paraffinic oils.

[4] The cable according to any one of the embodiments [1] to [3], characterized in that the polar compound represents at least 2.5% by mass, 5 10 15 20 25 22 with respect to the total mass of the dielectric liquid.

[5] The cable according to any one of the realizations [1] to [4], is characterized in that the polar compound is chosen from the group consisting of benzophenone, dibenzosuberone, fluorenone and anthrone.

[6] Cable I'any of the realizations [1] to [4], is characterized in that the polar compound is chosen from the group constituted by benzophenone and acetophenone.

[7] The cable according to any one of the realizations [1] to [6], is characterized in that the ratio of the number of aromatic carbon atoms to the total number of carbon atoms in the dielectric liquid is less than 0.3.

[8] The cable according to any one of the realizations [1] to [7], characterized in that the dielectric liquid represents from 1% to 20% by mass, relative to the total mass of the polymer composition.

[9] The cable according to any one of the embodiments [1] to [8], characterized in that the thermoplastic polymer material based on polypropylene comprises at least one homopolymer or copolymer of propylene Pi, and at least one homopolymer or copolymer of olefin P2.

[10] The cable according to embodiment [9], is characterized in that the propylene Pi copolymer is a copolymer of propylene and ethylene.

[11] The cable according to embodiment [9] or [10], is characterized in that the homopolymer or copolymer of propylene Pi represents 40 to 70% by mass of the thermoplastic polymer material based on polypropylene.

[12] The cable according to any one of the embodiments [9] to [11], characterized in that the homopolymer or the copolymer of olefin a P2 is a heterophase copolymer comprising a thermoplastic phase of the propylene type and an elastomer phase 5 10 15 20 23 thermoplastic of the copolymer type of ethylene and an olefin a, a polyethylene or a mixture thereof.

[13] The cable according to any one of the realizations [9] to [12], characterized in that the homopolymer or copolymer of olefin at P2 represents 30 to 60% by mass of the thermoplastic polymer material based on polypropylene.

[14] The cable according to any one of the realizations [1] to [13], is characterized in that the electrically insulating layer is a non-crosslinked layer.

[15] A method for manufacturing a cable as defined in any one of embodiments [1] to [14], characterized in that the cable is an electrical cable, and characterized in that the method comprises at least one step 1) of extruding the polymer composition defined in any one of [1] to [14] around the elongated electrically conductive element, to obtain an electrically insulating layer surrounding said elongated electrically conductive element. EXAMPLES 1. Polymer compositions Table 1 below lists polymer compositions in which the quantities of the components are expressed as percentages by weight relative to the total weight of the polymer composition. Compositions C1 and C2 are comparative compositions, and compositions II and II conform to the invention. Polymer Compositions Cl C2 II 12 Propylene Copolymer 100 100 100 100 Linear Low-Density Polyethylene 50 50 50 0 5 10 15 20 24 TABLE 1 Heterophase Propylene Copolymer 50 50 50 100 Mineral Oil 0 12 12 12 Benzophenone 0 0 0.7 0.7 Antioxidant 0.7 0.7 0.7 0.7 The origin of the compounds in Table 1 is as follows: - propylene copolymer marketed by Borealis under the reference BormedMC RB 845 MO; - linear low-density polyethylene marketed by Exxon Mobil Chemicals under the reference LLDPE LL 1002 YB; - heterophase copolymer marketed by Basel! Polyolefins under the reference Adflex™ Q 200F; - mineral oil marketed by Nynas under the reference Nytex™ 810, the oil comprises 10% aromatic carbon atoms, 43% naphthenic carbon atoms and 47% paraffinic carbon atoms; - antioxidant marketed by Ciba under the reference Irganox B 225 which comprises an equimolar mixture of Irgafos 168 and Irganox 1010; and - benzophenone marketed by Sigma-Aldrich under the reference B9300. 2. Preparation of non-crosslinked layers The compositions gathered in Table 1 are implemented as follows. 102 ml of mineral oil, 6 g of antioxidant, and 6 g of benzophenone were mixed with stirring at approximately 75°C to form a dielectric liquid. The dielectric liquid was then mixed with 850 g of propylene copolymer, 425 g of linear low-density polyethylene, and 425 g of heterophase copolymer in a container. The resulting mixture was then homogenized using a twin-screw extruder (Berstorff™ twin screw extruder) at a temperature of approximately 145–180°C and then melted at approximately 200°C (screw speed: 80 rpm). The homogenized and molten mixture was then formed into granules. Cables were manufactured using a laboratory extruder and subjected to electrical characterization. Each cable comprised: - an electrically conductive element with a cross-section of 1.4 mm², - a first semiconducting layer surrounding said electrically conductive element, - an electrically insulating layer obtained from the polymer composition of the invention or a comparable polymer composition surrounding said first semiconducting layer, and - a second semiconducting layer surrounding said electrically insulating layer. The cables had a total outside diameter of approximately 6.1 mm and a total length of approximately 3.64 m. They were stripped of the second semiconducting layer to a thickness of 150 µm and a length of 87 cm. The electrically insulating layer had a thickness of 1.5 mm (with an inner and outer radius of 1.4 mm and 2.9 mm, respectively). The semiconducting layers were thermoplastic layers having the following composition: 48% by mass of static propylene copolymer (Bormed RB 845 MO); 20% by mass of a heterophase copolymer (Adflex Q 200F); 25% by mass of carbon black (VulcanMC XC 500); 6.5% by mass of a mineral oil (Nytex 810) and 0.5% by mass of antioxidant. 5 10 15 20 25 24b 3. Characterization of non-crosslinked layers The dielectric strength of the layers was measured by applying a 50 Hz AC voltage ramp at 2 kV/s to the electrically conductive element of the cable. The second semiconducting layer was connected to ground. Both terminations were immersed in distilled water. A statistical analysis (Weibull model) was performed on 10 experimental values of dielectric strength obtained. The tangent delta (tan6) (or The loss factor at 25°C, 90°C, and 130°C of the layers prepared above was measured by dielectric spectroscopy using an instrument sold under the trade name Alpha-A by Novocontrol Technologies. The tangent of the loss angle indicates the energy dissipated in a dielectric as heat. The tests were carried out on samples approximately 0.5 mm thick, at a frequency range of 40 to 60 Hz with a voltage of 500 V, adjusted according to the thickness of the sample being tested, to apply an electric field of 1 kV/mm. A temperature of 25°C, 90°C, or 130°C was applied during the different tests. Stress bleaching resistance tests were also performed by subjecting Samples with a thickness close to 1 mm underwent a mechanical test such as a sample curvature test at room temperature with a radius of curvature of 5 mm. 4. Results The delta tangent results obtained are summarized in Figures 2A (layer obtained from composition C1) and 2B (layer obtained from composition II). Figure 2 represents the delta tangent (tan(5)) (or loss angle tangent) at 25°C (curves with filled diamonds), at 90°C (curves with filled squares), and at 130°C (curves with filled triangles) as a function of the frequency 24 Hz, for the comparative non-crosslinked layer (Figure 2A) and for the non-crosslinked layer according to the invention (Figure 2B). It is clearly observed that the layer obtained from composition II according to the invention exhibits dielectric losses that are very slightly higher than those obtained with the comparative composition Cl, particularly at 130°C, but which remain very acceptable. The results at 50 Hz are summarized in Table 2 below: Compositions Cl C2 II Tangent delta at 25°C 1.2 x IO'4 9.4 x IO'5 1.2 x IO'4 Tangent delta at 90°C 7.5 x IO5 5.41 x IO'5 8.2 x IO5 Dielectric breaking voltage of the laboratory cable (kV/mm) 103 112 137 TABLE 2 Table 2 shows that the polymer compositions according to the invention exhibit better dielectric properties (i.e., better electrical insulation). In particular, the dielectric stiffness of the layer of the invention II is improved, compared to that of the comparative compositions Cl and C2. WO 2017/103509 CA 03008329 2018-06-13 25 PCT/FR2016/053478 The layer obtained from composition II showed no bleaching under stress, whereas the layer obtained from the comparative composition Cl showed little resistance since a white mark at the bend appeared immediately in response to the applied manual stress.

Claims (15)

26 CLAIMS 1. A cable comprising at least one elongated electrically conductive element, and at least one electrically insulating layer, the electrically insulating layer being obtained from a polymer composition comprising at least one thermoplastic polymer material based on polypropylene and a dielectric liquid, characterized in that the dielectric liquid comprises at least one mineral oil and at least one polar compound selected from the group constitutes compounds corresponding to the following formula (I): o R1^^R2 ^A^ (!) in which R1 and R2, identical or different, are aryl, alkyl, or alkylene-aryl groups, and optionally R1 and R2 are linked together by the optional element A which, when present, represents a single bond or a -(CH2)n- group with n equal to 1 or 2. 2. The cable according to claim 1, characterized in that the dielectric liquid comprises at least 70% by mass of mineral oil, relative to the total mass of the dielectric liquid. 3. The cable according to claim 1 or 2, characterized in that the mineral oil is chosen from the group consisting of naphthenic oils and paraffinic oils. 4. The cable according to any one of claims 1 to 3, characterized in that the polar component represents at least 2.5% by mass, relative to the total mass of the dielectric liquid. 5. The cable according to any one of claims 1 to 4, characterized in that the polar compound is selected from the group consisting of benzophenone, dibenzosuberone, fluorenone, and anthrone. 27 6. The cable of any one of claims 1 to 4, characterized in that the polar component is chosen from the group consisting of benzophenone and acetophenone. 7. The cable according to any one of claims 1 to 6, characterized in that the ratio of the number of aromatic carbon atoms to the total number of carbon atoms in the dielectric liquid is less than 0.3. 8. The cable according to any one of claims 1 to 7, characterized in that the dielectric liquid represents from 1% to 20% by mass, relative to the total mass of the polymer composition. 9. The cable according to any one of claims 1 to 8, characterized in that the thermoplastic polymer material based on polypropylene comprises at least one homopolymer or copolymer of propylene Pi, and at least one homopolymer or copolymer of olefin P2. 10. The cable according to claim 9, characterized in that the propylene Pi copolymer is a propylene and ethylene copolymer. 11. The cable according to claim 9 or 10, characterized in that the homopolymer or copolymer of propylene Pi represents 40 to 70% by mass of the thermoplastic polymer material based on polypropylene. 12. The cable according to any one of claims 9 to 11, characterized in that the homopolymer or copolymer of olefin a P2 is a heterophase copolymer comprising a thermoplastic phase of the propylene type and a thermoplastic elastomer phase of the copolymer type of ethylene and an olefin a, a polyethylene or a mixture thereof. 13. The cable according to any one of claims 9 to 12, characterized in that the homopolymer or copolymer of olefin P2 represents 30 to 60% by mass of the thermoplastic polymer material based on polypropylene. 14. The cable according to any one of claims 1 to 13, characterized in that the electrically insulating layer is a non-crosslinked layer. 28 15. A method for manufacturing a cable as defined in any one of claims 1 to 14, characterized in that the cable is an electrical cable, and characterized in that the method comprises at least one step 1) of extruding the polymer composition defined in any one of claims 1 to 14 around the elongated electrically conductive element, to obtain an electrically insulating layer surrounding said elongated electrically conductive element.