WO2024110328A1

WO2024110328A1 - Dc power cable insulation

Info

Publication number: WO2024110328A1
Application number: PCT/EP2023/082185
Authority: WO
Inventors: Jerome VACHON; Peter Neuteboom; Silvia D'AURIA; Enrico Dalcanale; Roberta Pinalli; Christian Müller; Amir Masoud POURRAHIMI
Original assignee: SABIC Global Technologies BV
Current assignee: SABIC Global Technologies BV
Priority date: 2022-11-23
Filing date: 2023-11-17
Publication date: 2024-05-30
Anticipated expiration: 2025-05-23
Also published as: CN120225575A; EP4623014A1

Abstract

The invention relates to a DC power cable insulation comprising an ethylene copolymer (Z) obtained by copolymerizing composition comprising ethylene and an ion pair compound consisting of a cation of formula (I) and an anion of formula (II), wherein Formula (I), where R1 = H or C1-C10 alkyl; X = O or NH; R2=C1-C40 alkyl; R3, R4 = H or C1-C10 alkyl which can be connected through a cyclic structure, R5 = H or C1-C20, Formula (II), where R6 = H or C1-C10 alkyl, preferably wherein the ethylene copolymer (Z) is obtained by copolymerizing ethylene and the ion pair compound consisting of a cation of formula (I) and an anion of formula (II).

Description

DC POWER CABLE INSULATION

The present invention relates to a DC (direct current) power cable insulation and a DC power cable comprising such DC power cable insulation.

Over the years, the research in the field of insulating materials has undergone an incredible acceleration due to the need to transport DC current along long distances with few losses. Today, the demand for high-performance materials in the field of direct current transport is becoming more and more stringent. The most used cable insulation based material, for cables with an extruded insulation layer, is polyethylene thanks to its good electrical properties and the ease of being processed. However, the low melting temperature of LDPE makes crosslinking necessary. Crosslinked polyethylene (XLPE) can be obtained by a radical reaction using dicumyl peroxide (DCP) as initiator.

It is advantageous if the insulation material of a DC power cable displays a low DC electrical conductivity and a high melt strength. For example, an XLPE prepared by radical crosslinking of LDPE with dicumyl peroxide is disclosed in US 10,679,769, CE2 of Table 3, having a DC electrical conductivity of 11.1 fS/m. Further, XLPE has a relatively high melt strength.

Further, WO2011057927A1 discloses a crosslinkable polymer composition comprising a polyolefin and a crosslinking agent such as dicumyl peroxide for producing an insulation layer of a crosslinkable direct current power cable.

The radical reaction using initiators such as dicumyl peroxide results in the formation of some byproducts, like water, methane and a-methyl styrene. They have a very negative impact on the electrical properties of the insulation film, as explained e.g. in A. Smedberg; T. Hjertberg; B.Gustafsson; Polymer, 1997, 38, 4127.

WO2019224334 describes a cross-linkable polyolefin composition comprising a first olefin polymer (A) comprising a first comonomer comprising epoxy groups, and a second olefin polymer (B) comprising a second comonomer comprising carboxylic acid groups and/or precursor thereof. According to WO2019224334, this composition may be crosslinked without the need for curing agents that may generate by-products. Although the solution proposed in the patent application is promising, cross-link material generates certain degree of waste byproducts and the resultant material may be difficult to recycle.

It is an objective of the present invention to provide a DC power cable insulation in which the above-mentioned and/or other problems are solved. It is also an objective of the present invention to improve insulation and mechanical property of a DC power cable insulation material while retaining processability, without the need for cross-linking the material.

Accordingly, the present invention provides a DC power cable insulation comprising an ethylene copolymer (Z) obtained by copolymerizing a composition comprising ethylene and an ion pair compound consisting of a cation of formula (I) and an anion of formula (II), wherein

where Ri = H or C1-C10 alkyl; X = O or NH; R2 = C1-C40 alkyl; R3, R4 = H or C1-C10 alkyl which can be connected through a cyclic structure, R5 = H or C1-C20,

where Re = H or C1-C10 alkyl, preferably wherein the ethylene copolymer (Z) is obtained by copolymerizing ethylene and the ion pair compound consisting of a cation of formula (I) and an anion of formula (II).

It is particularly preferred that the composition is free of cross-linking, preferably the ethylene copolymer (Z) is not a cross-linked ethylene copolymer. Advantageously, the ethylene copolymer (Z) can be used in DC power cable insulation with desired combination of low DC conductivity, melt strength and storage modulus without the need of cross-linking the ethylene copolymer (Z). In some embodiments, the composition comprises a mixture of monomers comprising ethylene, the ion pair compound, and an alpha-olefin having 3-8 carbon atoms. In a preferred aspect, the composition consists of ethylene and the ion pair compound.

Preferably, the present invention provides a DC power cable insulation comprising the ethylene copolymer (Z) is obtained by copolymerizing ethylene and the ion pair compound consisting of the cation of formula (I) and the anion of formula (II), wherein

where Ri = H or C1-C10 alkyl; X = O or NH; R2 = C1-C40 alkyl; R3, R4 = H or Ci-C alkyl which can be connected through a cyclic structure, R₅ = H or C1-C20,

where R₆ = H or C1-C10 alkyl.

Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) is < 25.0 wt% and the amount of units derived from ethylene in the ethylene copolymer (Z) > 75.0 wt.%, with respect to the ethylene copolymer (Z).

Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) is < 15.0 wt% and the amount of units derived from ethylene in the ethylene copolymer (Z) > 85.0 wt.%, with respect to the ethylene copolymer (Z).

Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) is < 10.0 wt% and the amount of units derived from ethylene in the ethylene copolymer (Z) > 90.0 wt.%, with respect to the ethylene copolymer (Z). Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) is < 5.0 wt% and the amount of units derived from ethylene in the ethylene copolymer (Z) > 95.0 wt.%, with respect to the ethylene copolymer (Z).

It is believed that when the amount of units derived from the ion pair compound in the ethylene copolymer (Z) is < 25.0 wt%, preferably < 15.0 wt%, preferably < 10.0 wt%, the desired properties of low DC conductivity, melt strength and storage modulus is demonstrated.

The ethylene copolymer (Z) used according to the invention advantageously avoids the formation of byproducts formed in the case with a cross-linked polyethylene. It was surprisingly found that the ethylene copolymer (Z) has a high melt strength as well as a very low DC conductivity. This makes it advantageous as a material to be used for making a DC power cable insulation. One of the key parameters to see if a material is suitable for use as a DC power cable is the measurement of the DC conductivity.

One suitable metric to evaluate melt strength is by determining the number average molecular weight. The present invention provides for ethylene copolymer having suitably high molecular weight even without chemical cross-linking, rendering it with improved processability for DC power cable application, while minimizing waste generation and other adverse side effects associated with using cross-linking agents.

The ethylene copolymer (Z) has suitable mechanical and rheological properties indicated with sufficiently high storage modulus. Advantageously, the composition retains high storage modulus even at an elevated temperature, for example at 150 °C.

It is noted that WO2021009274 discloses a copolymer of ethylene and an ion pair compound shown below, where the comonomer content is 2.17 wt%.

The resistivity (inverse of conductivity) of this copolymer is shown in relation to the frequency in Figure 2 of WO2021009274, but not the DC conductivity. W02021009274 provides information about the electrical response at different frequencies but not zero- frequency, which cannot be extrapolated from the provided data. It will be appreciated that no information on the DC conductivity can be derived from this disclosure. Further, WO2021009274 does not mention melt strength of the copolymer disclosed therein.

The molar ratio of the cation (I) and the anion (I I) in the ion pair compound is 1 :1.

The DC power cable insulation according to the invention comprises the ethylene copolymer (Z) preferably at an amount of at least 50 wt%, at least 70 wt%, at least 80 wt%, at least 90 wt%, at least 95 wt%, at least 98 wt%, at least 99 wt%, at least 99.9 wt%, at least 99.99 wt% or 100 wt% with respect to the power cable insulation.

It is preferred that the DC power cable insulation according to the invention comprises the ethylene copolymer (Z) at an amount of at least 95 wt%, at least 98 wt%, at least 99 wt%, at least 99.9 wt%, at least 99.99 wt% or 100 wt% with respect to the power cable insulation.

It is preferred that the DC power cable insulation according to the invention comprises the ethylene copolymer (Z) at an amount of at least 99.9 wt%, at least 99.99 wt% or 100 wt% with respect to the power cable insulation.

Preferably the DC power cable insulation comprises the ethylene copolymer (Z) at an amount of 95 wt% to 100 wt.%, with respect to the power cable insulation. Preferably, the DC power cable insulation according to the invention comprises additives at an amount of 0 to 5 wt.% with respect to the power cable insulation. Non-limiting examples of additives include such as anti-oxidants, stabilisers, color pigments.

Preferably, the DC power cable insulation according to the invention comprises the ethylene copolymer (Z) at an amount of 95 wt.% to 99.99 wt.%, with respect to the power cable insulation and additives at an amount of 0.01 to 5 wt.% with respect to the power cable insulation.

The DC power cable insulation according to the invention may comprise additives such as anti-oxidants anti-oxidants, stabilisers, color pigments at an amount of 0.01 to 5 wt% with respect to the power cable insulation. The present invention further provides a DC power cable comprising a conductor and an inner semiconductive layer, an insulation layer and an outer semiconductive layer which surround the conductor in that order, wherein the insulation layer comprises or is the DC power cable insulation according to the invention. The DC power cable may further comprise one or more further layers, such as screen(s), a jacketing layer(s) or other protective layer(s) which may surround the outer semiconductive layer.

The DC power cable according to the invention may be selected from low voltage (LV), medium voltage (MV), high voltage (HV) or extra high voltage (EHV) DC power cables. Preferably, the DC power cable according to the invention is an HV or EHV DC power cable.

The present invention further provides a process for producing the DC power cable according to the invention, comprising applying the inner semiconductive layer, the insulation layer and the outer semiconductive layer on the conductor by co-extrusion.

The present invention further provides use of the ethylene copolymer (Z) for producing an insulation layer in a power cable comprising a conductor and an inner semiconductive layer, the insulation layer and an outer semiconductive layer which surround the conductor in that order.

The ion pair compound (Z) used according to the invention may be prepared according to the method described in p.4, 1.21 to p.12, I. 26 of the patent publication WO2021009274.

In particular, the ion pair compound may be prepared a) from a base salt and an acid salt; b1) from a base salt and a free acid, b2) from a free base and an acid salt or c) from a free base and a free acid.

Preferably, Rs = H.

Preferably, Ri = H or CH3.

Preferably, X = O.

Preferably, R2 = CH2-CH2. Preferably, R₃ = R4 = Rs = H; R₃ = R4 = CH₃, R5 = H; R₃ = R4 = Et, R5 = H; R₃ = tButyl, R4 = Rs ⁼ H or R₃ = R4 = R5 = CH₃.

Preferably, the free base is selected from the group consisting of:

2-(Dimethylamino)ethyl acrylate

2-(Diethylamino)ethyl acrylate

2-(Diethylamino)ethyl methacrylate

2-(Dimethylamino)ethyl methacrylate

2-(tert-Butylamino)ethyl methacrylate

N-[3-(hexahydro-1 H-azepin-1-yl)-1 ,1-dimethylpropyl]- 2-Propenamide

N-[2-(tetrahydro-1 ,4-oxazepin-4(5H)-yl)ethyl]- 2-Propenamide

N-[2-[methyl(tetrahydro-2H-pyran-4-yl)amino]ethyl]- 2-Propenamide

N-[3-(hexahydro-4-methyl-1 H-1 ,4-diazepin-1-yl)propyl]- 2-Propenamide

N-[1-methyl-2-(methylamino)propyl]- 2-Propenamide,

N-[2-(methylamino)propyl]- 2-Propenamide,

N-[2-methyl-2-(methylamino)propyl]- 2-Propenamide

N-[1-methyl-2-(methylamino)ethyl]- 2-Propenamide

N-[1-methyl-3-(methylamino)butyl]- 2-Propenamide N-[1-methyl-2-(methylamino)propyl]- 2-Propenamide.

In particularly preferred embodiments, the free base is selected from the group consisting of:

2-(Dimethylamino)ethyl acrylate

2-(Diethylamino)ethyl acrylate

2-(Diethylamino)ethyl methacrylate

2-(Dimethylamino)ethyl methacrylate and 2-(tert-Butylamino)ethyl methacrylate.

These free bases can be easily brought to a liquid state and are readily available.

Preferably, the base salt is a halogen salt of the free base mentioned above.

Preferably, the cation (I) is a quaternarization of the free base mentioned above.

Preferably, R₆ is H or CH₃. Preferably, the free acid is selected from the group consisting of acrylic acid and methacrylic acid.

Preferably, the acid salt is an alkali metal salt of the free acid mentioned above.

Preferably, the anion (II) is a deprotonated version of the free acid mentioned above.

Preferably, the ion pair compound is a compound represented by one of the following formula:

More preferably, the ion pair compound is a compound represented by the following formula:

5 Even more preferably the ion pair compound is the ion pair compound is a compound represented by any one of the following formula:

Copolymerization process The ethylene copolymer (Z) used in the present application is obtained by copolymerizing ethylene and an ion pair compound consisting of a cation of formula (I) and an acid anion of formula (II).

The ion pair compound according to the invention can be dissolved in various types of common polar organic solvents such as isopropanol, acetonitrile, ethyl acetate and injected in the polymerization reactor as a solution.

The copolymerization may be performed under known processes.

Preferably, the ethylene copolymer according to the invention are produced in a high- pressure free-radical polymerisation process. An advantage of polymerisation in such high-pressure free-radical process is that the polymerisation may be performed without the need for a catalyst being present. This allows for the use of certain comonomers such as polar comonomers which are not suitable as comonomers in the production of ethylene copolymers via catalytic processes such as using Ziegler-Natta type catalysts because of the interference with such catalyst.

A further advantage of preparation of the ethylene copolymer according to the invention in a high-pressure free-radical polymerisation process is that such polymerisation results in ethylene copolymers having a certain degree of long-chain branching. In order to qualify for certain applications, including extrusion coating application, ethylene copolymers are required to have a certain degree of such long-chain branching. The presence of such long-chain branching is understood to contribute to the desired melt processing properties. Accordingly, it is preferred that the ethylene copolymer according to the present invention is prepared via a high-pressure free-radical polymerisation process.

The pressure in such high-pressure free-radical polymerisation process preferably is in the range of > 180 MPa and < 350 MPa, preferably > 200 MPa and < 300 MPa. The temperature in such high-pressure free-radical polymerisation process preferably is in the range of > 100 and < 350 °C, preferably > 150 and < 310 °C, preferably > 190 and < 260 °C, more preferable > 200 and < 250 °C. Such high-pressure free-radical polymerisation process may for example be performed in a tubular reactor. Such tubular reactor may for example be a reactor such as described in Nexant PERP Report 2013-2, ’Low Density Polyethylene’, pages 31-48.

Such tubular reactor may for example be operated at pressures ranging from 150 to 300 MPa. The tubular reactor may have a tube length of for example > 1000 m and < 5000 m. The tubular reactor may for example have a ratio of length to inner diameter of > 1000:1 , alternatively > 10000:1 , alternatively > 25000:1 , such as > 10000:1 and < 50000: 1 , alternatively > 25000: 1 and < 35000: 1. The residence time in the tubular reactor may for example be > 30 s and < 300 s, alternatively > 60 s and < 200 s. Such tubular reactors may for example have an inner tubular diameter of > 0.01 m and < 0.20 m, alternatively > 0.05 m and < 0.15 m. The tubular reactor may for example have one or more inlet(s) and one or more outlet(s). The feed composition may for example be fed to the tubular reactor at the inlet of the tubular reactor. The stream that exits the tubular reactor from the outlet may for example comprise the ethylene copolymer. The stream that exits the tubular reactor from the outlet may for example comprise unreacted feed composition. Such unreacted feed compositions may be recycled back into the tubular reactor via one or more inlet.

The high-pressure free-radical polymerisation process is performed in the presence of one or more free-radical initiator. Preferably, the free-radical initiator is selected from organic peroxides and/or azo compounds.

Suitable organic peroxides may for example include diacyl peroxides, dialkyl peroxides, peroxymonocarbonates, peroxydicarbonates, peroxyketals, peroxyesters, cyclic peroxides, hydroperoxides. Suitable azo compounds may for example include 2,2’- azodi(isobutyronitrile), 2,2’-azodi(2-methylbutyronitrile), 1 , T- azodi(hexahydrobenzonitrile).

Examples of suitable diacyl peroxides are diisobutyryl peroxide, di(3,5,5- trimethylhexanoyl) peroxide, dilauroyl peroxide, didecanoyl peroxide, dibenzoyl peroxide.

Examples of suitable dialkyl peroxides are dicumyl peroxide, di(tert- butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, tert-butyl cumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne, di-tert-butyl peroxide, di- isononanoyl peroxide, di-tert-amyl peroxide, didecanoyl peroxide.

Examples of suitable peroxymonocarbonates are tert-amylperoxy 2-ethylhexyl carbonate, tert-butylperoxy isopropyl carbonate, tert-butylperoxy 2-ethylhexyl carbonate.

Examples of suitable peroxydicarbonates are di(3-methoxybutyl)peroxydicarbonate, di- sec-butyl peroxydicarbonate, diisopropyl peroxydicarbonate, di(4-tert-butylcyclohexyl) peroxydicarbonate, di(2-ethylhexyl) peroxydicarbonate, dibutyl peroxydicarbonate, diacetyl peroxy dicarbonate, dimyristyl peroxydicarbonate, dicyclohexyl peroxydicarbonate.

Examples of suitable peroxyketals are 1 ,1 -di (tert- butyl peroxy)-3,5,5- trimethylcyclohexane, 1 ,1-di(tert-amyl peroxy)cyclohexane, 1 , 1 -di (tert-butyl peroxy)cyclohexane, 2, 2-di (tert- butyl peroxy) butane, butyl 4,4-di(tert-butyl peroxy)valerate, n-ethyl-4,4-di-(tert-butylperoxy)valerate, ethyl-3,3-di(tert- butylperoxy)butyrate, ethyl-3,3-di(tert-amylperoxy)butyrate.

Examples of suitable peroxyesters are cumyl peroxyneodecanoate, 1 , 1 ,3,3, - tetramethylbutylperoxyneodecanoate, cumyl peroxyneoheptanoate, tert-amyl peroxyneodecanoate, tert-butyl peroxyneodecanoate, tert-butyl peroxyisononanoate, tert-butyl permaleate, tert-butyl peroxydiethylisobutyrate, 1 ,1 ,3,3-tetramethylbutyl peroxypivalate, tert-butyl peroxyneoheptanoate, tert-amyl peroxypivalate, tert-butyl peroxypivalate, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, 1 , 1 , 3, 3- tetram ethyl butyl peroxy-2-ethylhexanoate, tert-amyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxydiethylacetate, tert-butyl peroxyisobutyrate, tert-amyl peroxyacetate, tert-butyl peroxy-3,5,5-trimethylhexanoate, tert-amyl peroxybenzoate, tert-butyl peroxyacetate, tert-butyl peroxybenzoate.

Examples of suitable cyclic peroxides are 3,6,9-triethyl-3,6,9-trimethyl-1 ,4,7- triperoxononane, 3,3,5,7,7-pentamethyl-1 ,2,4-trioxepane, 3,3,6,6,9,9,-hexamethyl- 1 ,2,4,5-tetraoxacyclononane.

Examples of suitable hydroperoxides are isopropylcumyl hydroperoxide, 1 ,1 ,3,3- tetramethylbutyl hydroperoxide, cumyl hydroperoxide, tert-butyl hydroperoxide, tert-amyl hydroperoxide, methyl isobutyl ketone hydroperoxide, di-isopropyl hydroxyperoxide. Preferably the free radical initiator composition is selected from 2,5-dimethyl-2,5-di(tert- butylperoxy)hexane, t-butyl peroxy pivalate (t-BPP) and/or t-butyl peroxy benzoate (t- BPB).

Such initiators may for example be fed to the tubular reactor in a pure form or as a solution in a solvent. As solvent, for example a C2-C20 normal paraffin or C2-C20 isoparaffin may be used. For example, such solution may comprise > 2.0% and < 65.0 % by weight of initiator, alternatively >5.0% and <40.0% by weight, alternatively >10.0% and <30.0% by weight, compared to the total weight of the solution.

Such initiators may for example be introduced into the polymerisation reactor in quantities of < 300 ppm, preferably < 200 ppm, compared to the total weight of the materials fed to the polymerisation reactor.

In addition, further modifiers may be fed to the tubular reactor. Examples of such modifiers may include inhibitors, scavengers and/or chain transfer agents, such as alcohols, aldehydes, ketones and aliphatic hydrocarbons. Such modifiers may for example be fed to the tubular reactor in a pure form or as a solution in a solvent.

Examples of suitable chain transfer agents include cyclopropane, methane, t-butanol, perfluoropropane, deuterobenzene, ethane, ethylene oxide, 2,2-dimethylpropane, benzene, dimethyl sulfoxide, vinyl methyl ether, methanol, propane, 2-methyl-3-buten-2- ol, methyl acetate, t-butyl acetate, methyl formate, ethyl acetate, butane, triphenylphosphine, methylamine, methyl benzoate, ethyl benzoate, N,N- diisopropylacetamide, 2,2,4-trimethylpentane, n-hexane, isobutane, dimethoxymethane, ethanol, n-heptane, n-butyl acetate, cyclohexane, methylcyclohexane, 1 ,2- dichlorethane, acetronitrile, N-ethylacetamide, propylene, n-decane, N,N- diethylacetamide, cyclopentane, acetic anhydride, n-tridecane, n-butyl benzoate, isopropanol, toluene, acetone, 4,4-dimethylpentene-1 , trimethylamine, N,N- dimethylacetamide, isobutylene, n-butyl isocyanate, methyl butyrate, n-butylamine, N,N- dimethylformamide, diethyl sulfide, diisobutylene, tetrahydrofuran, 4-methylpentene-1 , p-xylene, p-dioxane, trimethylamine, butene-2, 1-bromo-2-chlorethane, octene-1 ,2- methylbutene-2, cumene, butene-1 , methyl vinyl sulfide, n-butyronitrile, 2-methylbutene- 1 , ethylbenzene, n-hexadecene, 2-butanone, n-butyl isothiocyanate, methyl 3- cyanopropionate, tri-n-butylamine, 3-methyl-2-butanone, isobutyronitrile, di-n- butylamine, methyl chloroacetate, 3-methylbutene-1 , 1 ,2-dibromoethane, dimethylamine, benzaldehyde, chloroform, 2-ethylhexene-1 , propionaldehyde, 1 ,4- dichlorobutene-2, tri-n-butylphosphine, dimethylphosphine, methyl cyanoacetate, carbon tetrachloride, bromotrichloromethane, di-n-butylphosphine, acetaldehyde , hydrogen and phosphine.

Preferably, the polymerization is performed in the presence of a chain transfer agent selected from the group consisting of methanol, propionaldehyde, n-heptane, propane, isopropanol and acetone.

The quantity of the chain transfer agent is preferably in the range between 0.01 and 2 mole %, compared to the total weight of the materials fed to the polymerisation reactor.

No neutralization step is required after the copolymerization step.

Preferably, the amount of the units derived from the ion pair compound in the ethylene copolymer (Z) is 0.01 to 10.00 mol%, 0.02 to 5.00 mol%, 0.03 to 1.50 mol%, 0.05 to 1.00 mol% or 0.15 to 1 .00 mol%, with respect to the ethylene copolymer.

Preferably, the amount of the units derived from the ion pair compound in the ethylene copolymer is 0.10 to 25.00 wt%, 0.25 to 10.00 wt%, 0.50 to 7.50 wt% or 1.00 to 5.00 wtl%, with respect to the ethylene copolymer.

Preferably the amount of units derived from the ion pair compound in the ethylene copolymer (Z) is 1.00 to 5.00 wt%, 2.00 to 4.00 wt%, with respect to the ethylene copolymer (Z).

Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 0.10 to 25.00 wt%, preferably from 0.25 to 15.00 wt.%, preferably from 0.25 to 10.00 wt%, preferably from 0.50 to 7.50 wt%, preferably from 1.00 to 7.50 wt%, preferably from 1.00 to 5.00 wt%, preferably from 1.50 to 5.00 wt.%, preferably from 2.50 to 5.00 wt.%, preferably from 2.00 to 4.00 wt%, with respect to the ethylene copolymer.

Preferably the amount of units derived from ethylene in the ethylene copolymer (Z) ranges from 75.00 to 99.90 wt.%, preferably from 85.00 to 99.75 wt.%, preferably from 90.00 to 99.75 wt.%, preferably from 92.50 to 99.50 wt.%, preferably from 92.50 to 99.00 wt.%, preferably from 95.00 to 99.00 wt.%, preferably from 95.00 to 98.50 wt.%, preferably from 95.00 to 97.50 wt.%, with respect to the ethylene copolymer.

Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 0.10 to 25.00 wt% and the amount of units derived from ethylene in the ethylene copolymer ranges from 75.00 to 99.90 wt.%, with respect to the ethylene copolymer.

Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 0.25 to 15.00 wt.% and the amount of units derived from ethylene in the ethylene copolymer ranges from 85.00 to 99.75 wt.%, with respect to the ethylene copolymer.

Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 0.25 to 10.00 wt% and the amount of units derived from ethylene in the ethylene copolymer ranges from 90.00 to 99.75 wt.%, with respect to the ethylene copolymer.

Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 0.50 to 7.50 wt% and the amount of units derived from ethylene in the ethylene copolymer ranges from 92.50 to 99.50 wt.%, with respect to the ethylene copolymer.

Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 1.00 to 7.50 wt% and the amount of units derived from ethylene in the ethylene copolymer ranges from 92.50 to 99.00 wt.%, with respect to the ethylene copolymer.

Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 1.00 to 5.00 wt% and the amount of units derived from ethylene in the ethylene copolymer ranges from 95.00 to 99.00 wt.%, with respect to the ethylene copolymer.

Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 1.50 to 5.00 wt.% and the amount of units derived from ethylene in the ethylene copolymer ranges from 95.00 to 98.50 wt.%, with respect to the ethylene copolymer.

Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 2.50 to 5.00 wt.% and the amount of units derived from ethylene in the ethylene copolymer ranges from 95.00 to 97.50 wt.%, with respect to the ethylene copolymer.

Preferably, the ethylene copolymer has a Mn of 1 to 100 kg/mol, more preferably 5 to 60 kg/mol, more preferably 10 to 50 kg/mol. This results in a relatively high melt strength, which is beneficial for the preparation process of a DC power cable insulation. Mn are measured according to ASTM D6474-12 (Standard Test Method for Determining Molecular Weight Distribution and Molecular Weight Averages of Polyolefins by High Temperature Gel Permeation Chromatography). Mn stands for the number average molecular weight.

It is noted that the invention relates to all possible combinations of features described herein, preferred in particular are those combinations of features that are present in the claims. It will therefore be appreciated that all combinations of features relating to the composition according to the invention; all combinations of features relating to the process according to the invention and all combinations of features relating to the composition according to the invention and features relating to the process according to the invention are described herein.

It is further noted that the term ‘comprising’ does not exclude the presence of other elements. However, it is also to be understood that a description on a product/com position comprising certain components also discloses a product/com position consisting of these components. The product/com position consisting of these components may be advantageous in that it offers a simpler, more economical process for the preparation of the product/composition. Similarly, it is also to be understood that a description on a process comprising certain steps also discloses a process consisting of these steps. The process consisting of these steps may be advantageous in that it offers a simpler, more economical process. When values are mentioned for a lower limit and an upper limit for a parameter, ranges made by the combinations of the values of the lower limit and the values of the upper limit are also understood to be disclosed.

The invention is now elucidated by way of the following examples, without however being limited thereto.

CEx 1

Properties of an LDPE commercially available as SABIC LDPE2101 N0W (MFR 0.85 dg/min at 190 °C, 2.16 kg; density 921 kg/m³) were measured and are shown in Table 1.

CEx 2

XLPE comprising cross-linked polyethylene composition was prepared as follows.

Milled LDPE was dispersed in a solution of DCP in methanol at 40 °C and stirred for 1 h followed by solvent evaporation. The resulting milled LDPE infused with 1 wt % DCP was melt-pressed at 120 °C for 5 min at a pressure of up to 3750 kPa in a hot press. The temperature was then raised to 180 °C, where the sample was left to cross-link for 10 min before cooling. This XLPE sample was finally degassed in a vacuum oven at 50 °C. Properties were measured and are shown in Table 1.

CEx 3

Properties of commercially available Surlyn™ 8920 - an ionomer of ethylene acid copolymer, were measured and are shown in Table 1.

The samples CEx 1 , CEx 2, and CEx 3, show samples which do contain units prepared from the ion pair compounds as shown in the present invention.

Ex 1-7

Various ethylene copolymers were produced by a continuous stirred autoclave reactor using the following polymerization parameters and their properties were measured and are shown in Table 1.

Polymerization parameters:

• Pressure 2000 bars

• Ethylene flow rate was fixed at around 4 kg/h (residence time ~ 50 s) • The impeller velocity was fixed at 1540 rpm

• Peroxide: Luperox 11M75

• comonomer flow: solution of comonomer in isopropanol

The comonomer type, comonomer concentration and polymerization temperature are as shown in Table 1.

Table 1

IPC1

Storage modulus

The storage modulus in relation to the temperature was measured by Dynamic mechanical analysis (DMA). DMA was carried out using a TA Q800 DMA in tensile mode on 13x11 mm pieces cut from 0.7 mm thick melt-pressed films. Variable-temperature measurements were done at a heating rate 3 °C/min, with a preload force of 0.01 N, a strain of 0.05% and a frequency of 1 Hz.

The results for CEx 2 and Ex 4 are shown in Figure 1.

DC conductivity

The DC conductivity in relation to time was measured. The test cell consisted of a three-electrode setup that was placed in an oven at 70 °C and connected to a high- voltage power supply (Glassman FJ40P03). The high-voltage electrode had a diameter of 60 mm; a measuring electrode with a diameter of D = 28 or 59 mm was used. A DC voltage of V = 4 to 5 kV was applied across L = 0.14 to 0.16 mm thick specimen films for 18 h, yielding an electric field of about 30 kV mm^-1, and then the voltage was switched-off for 6 h. The same voltage was subsequently reapplied for another 18 h. The reported values for ODC correspond to the apparent conductivity values obtained at the end of the second 18 h period. The volume leakage current was recorded with a Keithley 6517B electrometer, and dynamically averaged. In addition, a low pass filter was added into the circuit at the high voltage side for limiting the current in case of specimen breakdown and for filtering out high frequency noise. The apparent conductivity o is calculated according to

where L is the distance between the measuring and high voltage electrode (i.e. the sample thickness), D the diameter of the measuring electrode, V the applied voltage and I the leakage current recorded at 70 °C and 30 kV mm^-1 with an intermittent step of 6 h during which the applied voltage was turned off.

The results for CEx 2 and Ex 4 are shown in Figure 2.

Mn

Mn was measured according to ASTM D6474-12.

It can be understood that the ethylene copolymers of Ex 1-7 has a high storage modulus and a low conductivity, which allows them to be used as an alternative to the ethylene copolymer of CEx 2 for a DC power cable insulation.

From the above table it can also be seen that the DC conductivity of the ethylene copolymers of Ex 1-7 having no cross-linking, was lower than that of the cross-linked system of CEx 2, indicative that the copolymers of the present invention are even better suited over cross-linked polymeric products such as XLPE. Moreover, the present inventors believe that copolymer of the present invention and in particular polymers from the samples IE1-7, is able to impart the desired improved resistance against thermal deformation under high temperature and desired dissipation factor performance even without the need of cross-linking any of the inventive samples.

The measured Mn of the ethylene copolymers of Ex 1-7 indicates that they have a relatively high melt strength, which is beneficial for the preparation process of a DC power cable insulation.

Further, the ethylene copolymers of Ex 1-7 demonstrated improved properties (combination of low conductivity, high storage modulus, and high molecular weight) over that of CEx 3 - also an ethylene based ionomeric material - rendering the ethylene copolymer of the present invention particularly suitable for DC power cable insulation properties. In fact, for CEx3, at high temperature of 150 °C, storage modulus of the Surlyn ionomer is low as compared to the copolymers prepared from Ex 1-7. This clearly shows that a DC power cable containing the ionomeric ethylene copolymer in accordance with the present invention, can impart the desired performance even over commercial ethylene based ionomeric material.

Therefore, in conclusion it is evident that ethylene copolymers derived from ion pair compound of the type shown in the present invention can demonstrate the desired properties suitable for DC power cable. From CEx 1 , which is ordinary LDPE and CEx 2, based out of cross-linked ethylene polymers, the lower conductivity of the present ethylene copolymers make it better suited for DC power insulation application.

Surprisingly, the properties of the copolymers of the present invention over that of other ethylene ionomer copolymers such as CEx 3, makes the ethylene copolymers derived from the ion pair compound of the present invention, better suited for DC power insulation application.

Claims

1. A DC power cable insulation comprising an ethylene copolymer (Z) obtained by copolymerizing a composition comprising ethylene and an ion pair compound consisting of a cation of formula (I) and an anion of formula (II), wherein

where Ri = H or C1-C10 alkyl; X = O or NH; R2 = C1-C40 alkyl; R3, R4 = H or C1-C10 alkyl which can be connected through a cyclic structure, R₅ = H or C1-C20,

where R₆ = H or C1-C10 alkyl, preferably wherein the ethylene copolymer (Z) is obtained by copolymerizing ethylene and the ion pair compound consisting of a cation of formula (I) and an anion of formula (II).

2. The DC power cable insulation according to claim 1 , wherein R₅ is H.

3. The DC power cable insulation according to any one of the preceding claims, wherein R1 = H or CH₃;

X = O;

R2 = CH2-CH2; and/or

R₃ = R4 = Rs = H; R3 = R4 = CH3, R5 = H; R3 = R4 = Et, R5 = H; R3 = tButyl, R4 = R5 = H or R3 = R4 = R5 = CH3.

4. The DC power cable insulation according to any one of the preceding claims, wherein Re is H or CH3.

5. The DC power cable insulation according to any one of the preceding claims, wherein the cation (I) is a quaternarization of a free base (IB) selected from the group consisting of:

2-(Dimethylamino)ethyl acrylate

2-(Diethylamino)ethyl acrylate

2-(Diethylamino)ethyl methacrylate

2-(Dimethylamino)ethyl methacrylate

2-(tert-Butylamino)ethyl methacrylate

N-[3-(hexahydro-1 H-azepin-1-yl)-1 ,1-dimethylpropyl]- 2-Propenamide N-[2-(tetrahydro-1 ,4-oxazepin-4(5H)-yl)ethyl]- 2-Propenamide N-[2-[methyl(tetrahydro-2H-pyran-4-yl)amino]ethyl]- 2-Propenamide

N-[3-(hexahydro-4-methyl-1 H-1 ,4-diazepin-1-yl)propyl]- 2-Propenamide

N-[1-methyl-2-(methylamino)propyl]- 2-Propenamide,

N-[2-(methylamino)propyl]- 2-Propenamide,

N-[2-methyl-2-(methylamino)propyl]- 2-Propenamide

N-[1-methyl-2-(methylamino)ethyl]- 2-Propenamide N-[1-methyl-3-(methylamino)butyl]- 2-Propenamide N-[1-methyl-2-(methylamino)propyl]- 2-Propenamide.

6. The DC power cable insulation according to any one of the preceding claims, wherein the anion (II) is selected from the group consisting of acrylic acid and methacrylic acid.

7. The DC power cable insulation according to any one of the preceding claims, wherein the ion pair compound is a compound represented by one of the following formula:

8. The DC power cable insulation according to any one of the preceding claims, wherein the amount of units derived from the ion pair compound in the ethylene copolymer (Z) is 0.10 to 25.00 wt%, 0.25 to 10.00 wt%, 0.50 to 7.50 wt% or 1.00 to 5.00 wt%, with respect to the ethylene copolymer, preferably wherein the amount of units derived from the ion pair compound in the ethylene copolymer (Z) is 1.00 to 5.00 wt%, 2.00 to 4.00 wt%, with respect to the ethylene copolymer (Z).

9. The DC power cable insulation according to any one of the preceding claims, wherein the ethylene copolymer (Z) has a Mn of 1 to 100 kg/mol, more preferably 5 to 60 kg/mol, more preferably 10 to 50 kg/mol according to ASTM D6474-12.

10. The DC power cable insulation according to any one of the preceding claims, wherein the DC power cable insulation comprises the ethylene copolymer (Z) at an amount of at least 50 wt%, at least 70 wt%, at least 80 wt%, at least 90 wt%, at least 95 wt%, at least 98 wt%, at least 99 wt%, at least 99.9 wt%, at least 99.99 wt% or 100 wt% with respect to the power cable insulation, preferably wherein the DC power cable insulation comprises the ethylene copolymer (Z) at an amount of 95 wt.% to 100 wt.%, with respect to the power cable insulation.

11. A DC power cable comprising a conductor and an inner semiconductive layer, an insulation layer and an outer semiconductive layer which surround the conductor in that order, wherein the insulation layer comprises or is the DC power cable insulation according to any one of the preceding claims.

12. The DC power cable according to claim 11 , wherein the DC power cable is low voltage (LV), medium voltage (MV), high voltage (HV) or extra high voltage (EHV) DC power cables.

13. The DC power cable according to claim 11 or 12, wherein the DC power cable is HV or EHV DC power cable.

14. A process for producing the DC power cable according to any one of claims 11-13, comprising applying the inner semiconductive layer, the insulation layer and the outer semiconductive layer on the conductor by co-extrusion.

15. Use of the ethylene copolymer defined in any one of claims 1-10 for producing an insulation layer of a power cable comprising a conductor and an inner semiconductive layer, an insulation layer and an outer semiconductive layer which surround the conductor in that order.