KR20120103498A - Medium-tension or high-tension electrical cable - Google Patents

Abstract

PURPOSE: A middle-pressure or high-pressure electric cable including a cross-linking layer is provided to enhance water treeing resistance. CONSTITUTION: A middle-pressure or high-pressure electric cable(1) comprises electrical conductors(2), a first semi-conductive layer(3) which surrounds the electrical conductors, a second electrical insulating layer which surrounds the first layer(4), and a third semi-conductive layer(5) which surrounds the second layer. One or more of the three layers(3,4,5) is a crosslinked layer obtained from the crosslinkable composition including one or more polyolefin and organic peroxide as a crosslinking agent. The composition additionally includes aliphatic peroxide as a crosslink bonding agent. The aliphatic peroxide is selected from aliphatic peroxycarbonate, aliphatic peroxide of tertiary dialkyl, aliphatic peroxyacetal and aliphatic peroxyester or a mixture thereof.

Description

Medium or high voltage electrical cable {MEDIUM-TENSION OR HIGH-TENSION ELECTRICAL CABLE}

The present invention relates to an electrical cable. The present invention is typically applied to medium voltage (particularly 6 to 45-60 kV) power cables or high voltage (particularly above 60 kV, which may range up to 800 kV) regardless of whether it is a DC cable or an AC cable. This is not restrictive.

Medium or high voltage power cables typically comprise a central electrical conductor, a semiconductive inner layer having a common axis continuously around the electrical conductor, an electrically insulating intermediate layer and a semiconducting outer layer, the three layers being provided to a person skilled in the art. Crosslinking is via well known techniques.

Typically, the three crosslinked layers are obtained from a composition based on a polyethylene polymer matrix in combination with an organic peroxide such as dicumyl peroxide or tert-butylcumyl peroxide (ie cumyl peroxide). While the compositions are crosslinking, this type of peroxide decomposes to form crosslinking byproducts, in particular methane, acetophenone, cumyl alcohol, acetone, tert-butanol, α-methylstyrene and / or water. The last two by-products are formed by dehydration of cumyl alcohol.

When the cable is arranged to run, the production of water from cumyl alcohol is relatively slow and can occur months, or even years later. Thus, the risk of breakdown of the crosslinked layer is significantly increased.

In addition, unless the methane formed in the crosslinking step is removed from the crosslinked layer, the risks associated with the explosiveness of the methane and its flammability cannot be ignored. The gas may also cause damage when using the cable. Generally for medium and high voltage cables, when the semiconducting outer layer is surrounded by a metal shielding layer, the gas is longitudinally along the cable until it reaches the junction and terminal of the electrical installation (ie, the electrical component). Can only spread. Accordingly, methane can accumulate and exert pressure on electrical components, which can cause electrical shock. There are methods to limit the presence of methane in the cable, such as heat treatment of the cable to promote the release of methane out of the cable, but these methods are time consuming and expensive when the insulation layer is thick.

The document US Pat. No. 5,252, 676 discloses a method for limiting crosslink byproducts generated from crosslinkers in an electrical power cable. To this end, the document proposes to reduce the amount of crosslinker in order to limit the amount of gas and water generated during the decomposition of the crosslinker, while at the same time tert-butyl as crosslinker in the preparation of the three crosslinked layers. It is suggested to continue using cumyl peroxide (IPC). Nevertheless, a significant amount of water remains, and excessive reductions in the amount of peroxide tend to degrade the thermomechanical properties of the crosslinked layer.

It is an object of the present invention to overcome the shortcomings of the prior art by proposing a medium or high voltage electrical cable comprising a crosslinking layer, wherein the creation of the crosslinking layer significantly reduces the presence of crosslinking by-products such as methane and / or water. While limiting, at the same time it provides optimum thermomechanical properties such as hot creep, which is characterized by precise crosslinking of the layer.

An aspect of the invention is an electrical cable comprising an electrical conductor, a first semiconducting layer surrounding the electrical conductor, a second electrically insulating layer surrounding the first layer, and a third semiconducting layer surrounding the second layer, At least one of the three layers is characterized in that it is a crosslinked layer obtained from a crosslinkable composition comprising at least one polyolefin, and the composition further comprises an aliphatic peroxide as a crosslinking agent.

The crosslinkers of the present invention have the advantage of not forming cumyl alcohol as a crosslinking byproduct during crosslinking of the crosslinkable composition, thus making it possible to significantly limit the presence of water in the crosslinked layer and at the same time Maintains good thermomechanical properties. The thermomechanical properties for the crosslinked layer according to the invention are stresses according to standard NF EN 60811-2-1, which ranges from 100% or less, preferably 80% or less, particularly preferably in the range from 60% to 80%. It can be advantageously represented by the maximum thermal elongation under.

In addition, the amount of methane formed during the decomposition of aliphatic peroxide is less than the amount of methane formed during the decomposition of cumyl peroxide, whereby the presence of methane in the crosslinking by-products is advantageously reduced. For substantially the same thermomechanical properties, the amount of aliphatic peroxide required is less than the amount of cumyl peroxide.

Aliphatic peroxides of the invention are peroxides which do not contain any aromatic groups. In particular, aliphatic peroxides can be aliphatic peroxides comprising one or more tertiary alkyl groups.

Aliphatic peroxides of the present invention include:

Aliphatic peroxycarbonates such as tert-amylperoxy 2-ethylhexyl carbonate, tert-amylperoxy 2-ethylhexyl carbonate, tert-butylperoxy isopropylcarbonate;

Aliphatic peroxides of tertiary dialkyl, such as 1,1-bis (tert-butylperoxy) cyclohexane, 2,5-dimethyl-2,5-bis (tert-butylperoxy) -3-hexyne, 2, 5-dimethyl-2,5-bis (tert-butylperoxy) -3-hexane, di-tert-amyl peroxide, di-tert-butyl peroxide, cyclic peroxides such as 3,6,9-tree Ethyl-3,6,9-trimethyl-1,4,7-triperoxonan;

Aliphatic peroxyacetals such as butyl 4,4-bis- (tert-butylperoxy) valerate; And

Aliphatic peroxyesters such as tert-butyl peroxyacetate, tert-amyl peroxyacetate.

Among the peroxides mentioned above, aliphatic peroxides of tertiary dialkyls will preferably be used. The reason is that while using the composition, this type of peroxide has a very good advantage between the rate of crosslinking and the risk of burn-out or pre-crosslinking.

Peroxide-path crosslinking of the crosslinkable compositions according to the invention can be carried out under the action of heat and pressure, such as using vulcanized tubes under nitrogen pressure, and such crosslinking techniques are well known to those skilled in the art.

The crosslinkable compositions of the present invention comprise up to 3.00 parts by weight of aliphatic peroxide with respect to 100 parts by weight of polymer (s) in the composition; Preferably up to 1.50 parts by weight of aliphatic peroxide in 100 parts by weight of the polymer (s) in the composition; Preferably up to 1.25 parts by weight of aliphatic peroxide with respect to 100 parts by weight of the polymer (s) in the composition; And particularly preferably up to 1.10 parts by weight of aliphatic peroxide with respect to 100 parts by weight of polymer (s) in the composition.

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

The term "polyolefin" generally means by itself an olefin homopolymer or copolymer. The term may especially mean thermoplastics or elastomers.

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

Examples of ethylene polymers that may be mentioned include linear low density polyethylene (LLDPE), low density polyethylene (LDPE), medium density polyethylene (MDPE), copolymers of high density polyethylene (HDPE), ethylene and vinyl acetate (EVA), ethylene and butyl Copolymers of acrylate (EBA), copolymers of ethylene and methyl acrylate (EMA), copolymers of 2-hexylethyl acrylate (2HEA), copolymers of ethylene and α-olefins, for example polyethylene-octene ( PEO), polyethylene-butene (PEB), copolymers of ethylene and propylene (EPR), such as ethylene-propylene-diene (EPDM) terpolymers and mixtures thereof.

Low density polyethylene (LDPE) is preferred to use low density polyethylene (LDPE) because it has excellent rheological properties and very good thermomechanical and electrical properties, especially in use by extrusion.

The term "low density" means in particular a density which can be included in the range of 0.910 to 0.940 g / cm 3, preferably in the range of 0.910 to 0.930 g / cm 3, according to the standard ISO 1183 (at a temperature of 23 ° C.).

Typically, low density polyethylene (LDPE) can be obtained through a polymerization process in a high pressure tubular reactor or an autoclave reactor.

The crosslinkable composition comprises more than 50.0 parts by weight of polyolefin with respect to 100 parts by weight of polymer (s) (ie polymer matrix) in the composition, preferably at least 70 parts by weight of polyolefin, in particular with respect to 100 parts by weight of polymer in the composition Preferably, the composition may include at least 90 parts by weight of polyolefin based on 100 parts by weight of polymer.

In a particularly advantageous manner, the crosslinkable composition comprises a polymer matrix composed of polyolefins alone or a mixture of polyolefins.

The crosslinkable compositions of the present invention may further comprise one or more crosslinking aids of the multifunctional type. The crosslinking adjuvant comprises two or more reactive functional groups which are the same or different, the functional groups can first be grafted to the polyolefin, and secondly the crosslinking (ie crosslinked) of the polyolefin in the presence of the aliphatic peroxide of the invention Formation of three-dimensional structures of polyolefins).

Preferably, at least two reactive functional groups of the crosslinking aid are unsaturated functional groups.

Especially preferably, at least one of said reactive functional groups is a vinyl functional group, in particular an ethylenic functional group in the form of CH ₂ = CH-.

More preferably, the two reactive functional groups are vinyl functional groups, in particular ethylenic functional groups of the form CH ₂ = CH-.

Crosslinking aids can particularly significantly reduce the proportion of peroxides used in crosslinkable compositions, while at the same time maintaining satisfactory crosslinking rates and thermomechanical properties such as good thermal creep.

Preferably, as the boiling point is sufficiently high, auxiliaries which do not evaporate during the use of the crosslinkable composition, in particular during the step of use by extrusion, will be used.

For example, crosslinking aids include 1,3-hexadiene; 1,4-hexadiene; 1,5-hexadiene; 2,3-dimethyl-1,3-butadiene; 2,4-hexadiene; 2-methyl-1,4-pentadiene; 3-methyl-1,3-pentadiene; 4-methyl-1,3-pentadiene; 1,6-heptadiene; 2,4-dimethyl-1,3-pentadiene; 2-methyl-1,5-hexadiene; 4-vinyl-1-cyclohexene; 1,7-octadiene; 2,5-dimethyl-1,5-hexadiene; 2,5-dimethyl-2,4-hexadiene; 5-ethylidene-2-norbornene; 5-vinyl-2-norbornene; 1,8-nonadiene; 7-methyl-1,6-octadiene; 1,5,9-decatriene; 2,6-dimethyl-2,4,6-octatriene; Dipentene; 7-methyl-3-methylene-1,6-octadiene; 1,9-decadiene; 3,9-divinyl-2,4,8,10-tetraoxaspiro [5.5] undecane; 1,2,4-trivinylcyclohexane; 1,13-tetradecadiene; 2,3-diphenyl-1,3-butadiene; Trans, trans-1,4-diphenyl-1,3-butadiene; 1,15-hexadecadiene; 1,6-diphenyl-1,3,5-hexatriene; 2,3-dibenzyl-1,3-butadiene; And polybutadiene; Or mixtures thereof.

The concentration of said adjuvant is preferably limited so as not to interfere with the extrusion process of the crosslinkable composition of the present invention. For example, the crosslinkable composition may comprise up to 3 parts by weight of crosslinking aid with respect to 100 parts of the polymer (s) in the crosslinkable composition. It will be desirable for the crosslinkable composition to use from 0.5 to 2 parts by weight of adjuvants relative to 100 parts by weight of the polymer (s) in the crosslinkable composition.

The crosslinkable composition of the present invention may further comprise an aromatic compound comprising a single reactive functional group capable of grafting to the polyolefin and one or more aromatic nuclei. Preferably, the reactive functional group of the aromatic compound is a vinyl functional group. Thus, the aromatic compound, when present in the crosslinkable composition, does not participate in crosslinking of the polyolefin in contrast to the crosslinking aid.

The crosslinked layer obtained from the crosslinkable composition has enhanced and persistent properties in the field of electrical cables and provides better water tree resistance. In particular it relates to resistance to electrical breakdown, in particular the ability to dissipate the space charge that accumulates in the high voltage cable under direct current.

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

Examples of styrene derivatives mentioned may include compounds having the general formula:

Wherein X is hydrogen, an alkyl group or an aryl group; R is hydrogen, an alkyl group or an aryl group. In particular, mention may be made of 4-methyl-2,4-diphenylpentane and triphenylethylene.

In the present invention, styrene derivatives of the polycyclic aromatic hydrocarbon (PAH) type may also be considered. In particular, examples of vinyl naphthalene include 2-vinyl naphthalene; Examples of vinylanthracene include 9-vinylanthracene or 2-vinylanthracene; And as examples of vinylphenanthrene, 9-vinylphenanthrene may be mentioned.

Grafting of the aromatic compounds to the polyolefin polymer chains is typically performed during the crosslinking step of the polyolefin according to radical addition mechanisms well known to those skilled in the art in the presence of the aliphatic tertiary alkyl peroxides of the present invention.

The crosslinkable composition according to the present invention may further comprise a protective agent such as one or more antioxidants. Antioxidants protect the composition against thermal constraints that occur during the manufacture of the cable or during the operation of the cable.

Antioxidants are preferably selected from:

Tetrakis-methylene (3,5-di- t -butyl 4-hydroxyhydrocinnamate) methane, a sterically hindered phenolic antioxidant, octadecyl 3- (3,5-di- t -butyl-4 -Hydroxyphenyl) propionate , 2,2'-thiodiethylenebis [3- (3,5-di- t -butyl-4-hydroxyphenyl) propionate], 2,2'-thiobis (6- t -butyl-4-methylphenol), 2,2'-methylenebis (6- t -butyl-4-methylphenol) , 1,2-bis (3,5-di- t -butyl-4 -Hydroxyhydrocinnamoyl) hydrazine, [2,2'-oxamidobis (ethyl 3- (3,5-di- t -butyl-4-hydroxyphenyl) propionate) and 2,2'-oxa Midobis- [ethyl 3- ( t -butyl-4-hydroxyphenyl) propionate];

Thioesters such as 4,6-bis (octylthiomethyl) -o-cresol, bis [2-methyl-4- {3-n- (C12 or C14) alkylthiopropionyloxy} -5- t -butyl Phenyl] sulfide and thiobis [2- t -butyl-5-methyl-4,1-phenylene] bis [3- (dodecylthio) -propionate];

Sulfur-based antioxidants such as dioctadecyl 3,3'-thiodipropionate or dididodecyl 3,3'-thiodipropionate;

Phosphorus-based antioxidants such as phosphites or phosphonates, for example tris (2,4-di- t -butylphenyl) phosphite or bis (2,4-di- t -butylphenyl) pentaerythritol Diphosphites; And

Antioxidants of the amine type, such as polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ), the latter type of antioxidants are particularly preferred in the compositions of the invention.

TMQ can have several grades, that is;

A “standard” grade with low degree of polymerization, ie, a grade having more than 1 wt% residual monomer and at least 100 ppm to 800 ppm (parts per million by mass) residual NaCl;

A high degree of polymerization "high polymerization" grade, ie, a grade having less than 1% by weight of residual monomers and from 100 ppm to 800 ppm or more of residual NaCl;

A "low residual salt containing" grade, ie a grade having a residual NaCl in a content of less than 100 ppm.

The type of stabilizer and its content in the crosslinkable composition are typically selected as a function of the maximum temperature the polymer receives during the production of the mixture and by extrusion to the cable, which is also at the maximum exposure time at that temperature. Depends.

The crosslinkable composition may typically comprise 0.1 to 2 weight percent antioxidant (s). In particular when the antioxidant is TMQ, it will preferably comprise up to 0.7% by weight of antioxidant (s).

Impact retardants; Processing aids such as lubricants or waxes; Compatibilizers; Binder; UV stabilizers; Nonconductive fillers; Other additives and / or fillers well known to those skilled in the art, such as conductive fillers and / or semiconducting fillers, may additionally be added to the crosslinkable compositions of the present invention.

According to one preferred embodiment, the crosslinked layer of the invention is an electrically insulating layer (ie a second layer). In the case of an electrically insulating layer, the crosslinkable composition does not include any (electrically) conductive fillers and / or semiconducting fillers.

In particular, at least two of the three layers of the cable are crosslinked layers, preferably three layers of the cable are crosslinked layers.

When a crosslinkable composition is used in the manufacture of a semiconducting layer (first layer and / or third layer), the crosslinkable composition is characterized in that the composition capable of crosslinking one or more (electrically) conductive fillers or one or more semiconducting fillers is semiconducting. Include in an amount sufficient to be this.

In particular, it is considered that the semiconductor is semiconducting when the electrical conductivity of the layer is greater than 0.001 Sm ⁻¹ (siemens per meter).

The crosslinkable composition used to obtain the semiconducting layer may comprise 0.1 to 40% by weight of (electrically) conductive filler, preferably at least 15% by weight of conductive filler, more preferably at least 25% by weight of conductive filler. have.

The semiconducting filler can be advantageously selected from carbon black, carbon nanotubes and graphite, or mixtures thereof.

Regardless of whether it is a first semiconducting layer, a second electrically insulating layer and / or a third semiconducting layer, at least one of the three layers is an extrusion layer, preferably two of the three layers are extrusion layers, more preferably These three layers are extruded layers.

In one particular embodiment, generally in accordance with electrical cables well known in the field of application of the present invention, the first semiconducting layer, the second electrical insulation layer and the third semiconducting layer constitute a three layer insulator. That is, the second electrically insulating layer is in direct physical contact with the first semiconducting layer and the third semiconductive layer is in direct physical contact with the second electrically insulating layer.

The electrical cable of the present invention may further comprise a metal shielding layer surrounding the third semiconducting layer.

The metal shielding layer comprises a "wire" shielding layer consisting of an assembly of copper or aluminum conductors arranged along a length around a third semiconducting layer, and one or more conductive metal strips helically spiraling around the third semiconducting layer. It may be a "strip" shield layer that is constructed, or a "leaktight" shield layer, such as a metal tube surrounding the third semiconducting layer. This type of shielding layer makes it possible to form a barrier, in particular against moisture, which tends to penetrate the electrical cable in the radial direction.

All types of metal shielding layers can serve to ground the electrical cable, thus delivering a short circuit in the event of a short circuit in the relevant communication network, for example.

The electrical cable of the invention may also comprise an outer protective sheath surrounding the third semiconducting layer or optionally surrounding the metal shielding layer, especially when a metal shielding layer is present. The outer protective sheath is typically a suitable thermoplastic material such as HDPE, MDPE or LLDPE; Or optionally made of a flame retardant material or fire retardant material. In particular, if the latter material does not contain halogen, the sheath is referred to as HFFR (Halogen Free Flame Retardant) type.

Other layers, such as a layer that expands in the presence of moisture, may be added between the third semiconducting layer and the metal shielding layer in the presence of the metal shielding layer, and / or the metal shielding layer and the metal shielding layer and in the presence of the outer sheath. It can be added between the outer sheaths and these layers provide longitudinal and / or transverse leakage protection of the electrical cable with respect to water. The electrical conductor of the cable of the invention may further comprise a material which expands in the presence of moisture to obtain a "leaktight core".

Further features and advantages of the present invention will be described in terms of a non-limiting embodiment of the electric cable according to the invention, and FIG. 1 schematically showing a perspective view and a cross-sectional view of the electric cable according to a preferred embodiment according to the invention. This is referred to.

For the sake of clarity, only those elements which are essential for understanding the present invention have been shown schematically, which is not to scale.

The medium or high voltage power cable 1 shown in FIG. 1 comprises an elongated central conductive element 2, in particular made of copper or aluminum. With a continuous common axis around the conductive element 2, the power cable 1 is also the first semiconducting layer 3, also known as the "internal semiconducting layer", the second electrically insulating layer 4, the "external semiconductor". A third semiconducting layer 5, known as a "layer layer", a ground and / or protective metal shielding layer 6, and an outer protective sheath 7, wherein the layers 3, 4 and 5 are compositions according to the invention. Can be obtained from The layers 3, 4 and 5 are extruded and crosslinked layers.
The presence of the metal shielding layer 6 and the protective outer sheath 7 is preferred, but not essential, and the structure of the cable itself is well known to those skilled in the art.

Crosslinkable  Preparation of the composition

Crosslinkable Composition C1 C2 C3 C4 Polyolefin 100 100 100 100 BCP 1.42 - 1.27 - DTBH - 1.25 - 1.05 Antioxidant 0.18 0.18 0.18 0.18 TVCH - - 1.00 1.00

In Table 1, the compositions C1 and C3 are referred to as comparative examples, and compositions C2 and C4 are referred to as compositions according to the invention.

The amounts of the components of Compositions C1 to C4 described in Table 1 are expressed in parts by weight (pcr) relative to 100 parts by weight of polymer in the crosslinkable composition.

The sources of the various components of Compositions C1 to C4 of Table 1 are detailed below:

"Polyolefin" is a low density polyethylene sold under the trade name BPD 2000 by the company Ineos;

"BCP" is tert-butylcumyl peroxide sold under the name Luperox 801 by the company Arkema;

"DTBH" is 1,1-bis (tert-butylperoxy) cyclohexane (tertiary dialkyl aliphatic peroxide) sold by the company Arkema under the trade name Luperox 101;

"TVCH" is a crosslinking adjuvant 1,2,4-trivinylcyclohexane sold by BASF under the trade name TVCH.

To sufficiently impregnate the polyethylene particles, compositions C1 to C4 are prepared by mixing the polyethylene particles with additive peroxides, antioxidants and optionally an adjuvant for 3 hours in a closed container located in a roll mixer. do. The polyethylene particles were preheated to 60 ° C. before being impregnated.

The mixture was then left at 40 ° C. for 16 hours before sealing and storing.

Of the composition Characterization

ratio- Crosslinked On a plate Characterization

Kinetics and levels of crosslinking

An MDR rheometer (Moving Die Rheometer, Alpha Technologies) makes it possible to monitor the crosslinking / vulcanization of the material by measuring a change in the viscosity of the material (DIN 53529 (1983)).

The chamber containing the sample is formed from two heating plates. The lower plate of the arc applies a vibration of a constant frequency (100 cycles / mn, ie 1.67 Hz) of amplitude ± 0.5 °; The top plate measures the material's response, ie the resistance of the material to the applied stress. The unit of measurement is torque, expressed in dN.m.

Samples are made from impregnated polyethylene particles, which are cast into plates of 3 mm thickness at 120 ° C. in a hydraulic press following a cycle of two minutes under pressure of 100 bar for two minutes without pressure, and Then cooled.

Two discs of 35 mm diameter for complete lining of the chamber are cut out of the plate using a punch and then placed between two sheets of polyester terpene® located in the rheometer chamber.

The measurement is carried out at a temperature of 190 ° C., which indicates tube vulcanization conditions. After the initial drop in torque due to premelting of the material, the viscosity of the material and the resulting torque increase, which is a signal that crosslinking has occurred.

The parameter of interest is MH, which corresponds to the measured maximum torque. This is the plateau value obtained when the entire system reacts and when crosslinking reaches the maximum value available. For a given material, a good correlation between MH and crosslink density that controls the thermomechanical properties after the crosslinking step is noted.

● Shock time ( Breakdown time )

Mooney Viscometer (Monsanto MV2000) makes it possible to measure the viscosity of a material or to monitor its change over time in the case of crosslinkable materials (standard ASTM D1646 (2005)).

It consists of two jaws that form a cylindrical hole for placing the sample to be tested. At the center of the chamber is a metal disk that rotates at a constant speed of 2 rpm. In the case of the present invention, the "large" of the two available size rotors is used.

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

The sample is made from impregnated polyethylene particles, which are cast into a 3 mm thick plate at 120 ° C. temperature in a hydraulic press according to one cycle of three minutes under pressure of 100 bar for 2 minutes without pressure and thereafter Is cooled.

Four disks 50 mm in diameter are cut out of the plate using a punch. Two of them are drilled with a 12 mm diameter hole in the center to be sewn onto the rotor stem; The other two will be stored intact and placed on the rotor. The whole is then placed between two sheets of polyester terpene® installed in the viscometer chamber.

The resistance of the material to the rotation of the rotor is measured. Measurements are expressed in Mooney (MU), in arbitrary units. The parameter of interest is the minimum viscosity value ML, measured at time t0 (minutes). ML + 1 is the viscosity value corresponding to ML increased by 1 Mooney unit. This is measured at time t1 (minutes). ML + 2 is the value corresponding to ML increased by 2 Mooney units. This is measured at time t2 (minutes).

Measurement of volatiles (ie methane) by the Sievert technique;

The amount of volatiles generated during the polyethylene crosslinking step and then removed is determined by Sievert method using PCT Pro 2000 (HY-ENERGY, SETARAM).

The sample is made from impregnated polyethylene particles, which are cast into a 1 mm thick plate at 120 ° C. temperature in a hydraulic press according to one cycle of 3 minutes under pressure of 100 bar for 2 minutes without pressure and then cooled.

The disc, 6 mm in diameter, is then cut out of the plate using a punch and then accurately weighed to within mg (total mass = 300-350 mg).

The sample is placed in the chamber of the instrument under pressure (helium). The chamber is connected to the 5 ml reservoir by a valve which is itself under pressure. At the beginning of the test, the pressure in the chamber and the reservoir is the same. During the temperature cycle, the valve opens and closes intermittently, which results in a new equilibrium when the valve is opened and allows the new pressure in the reservoir to be measured when the valve is closed. The cause of the pressure change is partly due to the release of methane and partly due to the change in size of the chamber with temperature. Accordingly, real-time recording of methane emissions requires pre-calibration by applying temperature cycles envisioned in the chamber.

Simulating the crosslinking conditions for the various polyethylene layers in the vulcanization tube, the instrument provides a controlled temperature gradient of 1 ° C / s.

The cycle is expected to be heated from room temperature to 250 ° C.

The difference between final pressure and initial pressure at the same temperature allows access to methane emissions. The amount of volatiles (ie methane) is expressed in μmol / g of crosslinked polyethylene.

Crosslinked On a plate Characterization

Crosslink density measured by thermal creep

1 mm thick plates are cast from impregnated polyethylene particles. The molding is carried out at 120 ° C. temperature in the press according to one cycle of three minutes at 100 bar then two minutes without pressure. The plate is then cooled at a pressure of 100 bar.

The crosslinking step is carried out in a press at a temperature of 190 ° C., a pressure of 100 bar and lasts for 10 minutes. The mold is preheated to 190 ° C. The cooling step is carried out while maintaining a pressure of 100 bar.

Thermal creep measurements of materials under mechanical stress are determined according to standard NF EN 60811-2-1.

This test is commonly referred to as a hot set test (HST), which involves ballasting one end of an H2 dumbbell type specimen having a mass corresponding to a stress application equivalent to 0.2 MPa, and 15 Positioning the assembly in a heated oven at 200 ± 1 ° C. for minutes.

After this time, the maximum thermal elongation under stress of the specimen, expressed as a%, is recorded.

The suspended mass is then removed and the specimen is held in the oven for an additional 5 minutes.

Before expressed as a%, the residual permanent elongation, also known as residual magnetic (or residual magnetic elongation), is measured.

It is recalled that the more crosslinked the material, the lower the value of residual magnetism and maximum elongation under stress.

It is also noted that the test results are logically considered to be a failure if the specimen is broken during testing under the combined action of mechanical stress and temperature.

It should be noted above that if the elongation does not exceed 100% the value is considered to comply with the requirements. If the value is exceeded, the test action will be considered noncompliant, as ruptured.

All results regarding the characterization of non-crosslinked plates and crosslinked plates derived from crosslinkable compositions C1 to C4 are shown in Table 2 below.

Crosslinkable Composition C1 C2 C3 C4 MH
(dN.m) 3.2 2.9 3.4 3.2 ML at 130 ℃
(Mooney unit) 9.4 9.7 9.7 7.5 t0 (ML + 0)
(min) 15 17 22 31 t1 (ML + 1)
(min) 45 49 52 69 t2 (ML + 2)
(min) 66 79 74 102 CH ₄ Sievert (μmol / g XLPE) 113 104 101 69 Hot set at 200 ° C Elongation
(%) 70-75 70-95 50-65 60-65 Residual magnetism
(%) 0 0 0 0

For equivalent thermomechanical properties (results of maximum elongation and residual magnetism-see thermal setting test (HST) at 200 ° C.), the amount of aliphatic peroxide used in the crosslinkable composition C2 of the present invention is crosslinkable composition C1. Less than the amount used for (1.42 for C1 versus 1.25 for C2).

Comparing the crosslinkable compositions C1 and C2, the amount of methane released during crosslinking decreases from 113 μmol / g XLPE to 104 μmol / g XLPE.

In addition, cumyl alcohol as a crosslinking byproduct in composition C2 during crosslinking is replaced by the formation of tert-butanol. Of course, tert-butanol, such as any tertiary alcohol, can be dehydrated to form isobutene and water. However, since isobutene is significantly less stabilized than [alpha] -methylstyrene formed in the dehydration of cumyl alcohol, this reaction is significantly less desirable and the formation of water is also slowed.

The difference is more significant when comparing Compositions C3 and C4. Even at equivalent thermomechanical properties, significantly reduced amounts of methane are observed in composition C4 of the present invention as compared to composition C3. In addition, the burn-out time value is much better in composition C4 than in composition C3. Therefore, since composition C4 does not contain any cumyl peroxide, the formation of water in the composition is significantly slowed.

Claims (11)

An electrical conductor 2, a first semiconducting layer 3 surrounding the electrical conductor 2, a second electrical insulation layer 4 surrounding the first layer 3 and a third surrounding the second layer 4 In the electric cable (1) comprising a semiconducting layer (5),
At least one of the three layers (3, 4, 5) is characterized in that it is a crosslinked layer obtained from a crosslinkable composition comprising at least one polyolefin, said composition further comprising an aliphatic peroxide as a crosslinking agent. Characterized in that, the electrical cable (1). The method of claim 1,
And said aliphatic peroxide is selected from aliphatic peroxycarbonates, aliphatic peroxides of tertiary dialkyls, aliphatic peroxyacetals and aliphatic peroxyesters, or mixtures thereof. The method according to claim 1 or 2,
And said polyolefin is an ethylene polymer. The method of claim 3,
And wherein said ethylene polymer is low-density polyethylene (LDPE). 5. The method according to any one of claims 1 to 4,
And wherein the crosslinkable composition comprises more than 50.0 parts by weight of polyolefin based on 100 parts by weight of polymer in the composition. The method according to any one of claims 1 to 5,
Wherein said crosslinkable composition further comprises one or more crosslinking aids comprising two or more of the same or different reactive functional groups capable of first grafting to the polyolefin and secondly participating in crosslinking of the polyolefin. Electric cables. The method of claim 6,
At least two reactive functional groups of said adjuvant are vinyl functional groups. 8. The method according to any one of claims 1 to 7,
Wherein the crosslinkable composition further comprises an aromatic compound comprising a single reactive functional group capable of grafting to the polyolefin and at least one aromatic nucleus. The method of claim 8,
An electric cable, wherein the single reactive functional group of the aromatic compound is a vinyl functional group. 10. The method according to any one of claims 1 to 9,
And wherein said crosslinked layer is an electrical insulation layer. 11. The method according to any one of claims 1 to 10,
The three layers of the cable are crosslinked layers.

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