DC power cable insulation material

Abstract

The invention relates to 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 in formula (I) wherein R1 = H or C1-C10 alkyl; x = O or NH; r2 is a C1-C40 alkyl group; r1, R2, R3, R4 = H or C1-C10 alkyl, which can be linked by a cyclic structure, R5 = H or C1-C20, in formula (II) wherein R6 = H or C1-C10 alkyl, preferably wherein ethylene copolymer (Z) is obtained by copolymerizing a polyethylene and an ion pair compound consisting of a cation of formula (I) and an anion of formula (II). # imgabs0 #

Description

[0001] The present invention relates to an insulating material for DC (direct current) power cables and a DC power cable comprising such an insulating material for DC power cables.

[0002] For many years, due to the need to transmit DC current along long distances with less loss, research in the field of insulating materials has experienced an incredible acceleration. Today, the demand for high-performance materials in the field of DC transmission is becoming increasingly stringent. For cables with an extruded insulating layer, the most commonly used cable insulating base material is polyethylene, thanks to its good electrical properties and ease of processing. However, the low melting temperature of LDPE makes crosslinking necessary. Crosslinked polyethylene (XLPE) can be obtained by a free radical reaction using dicumyl peroxide (DCP) as an initiator.

[0003] It is advantageous if the insulating material of the DC power cable exhibits low DC conductivity and high melt strength. For example, XLPE prepared by free radical crosslinking of LDPE with dicumyl peroxide is disclosed in US 10,679,769, CE2 in Table 3, which has a DC conductivity of 11.1 fS / m. In addition, XLPE has a relatively high melt strength.

[0004] Furthermore, WO2011057927A1 discloses a crosslinkable polymer composition comprising a polyolefin and a crosslinking agent such as dicumyl peroxide for producing an insulating layer of a crosslinkable DC power cable.

[0005] The free radical reaction using an initiator such as dicumyl peroxide results in the formation of some by-products such as water, methane, and α-methylstyrene. They have a very negative impact on the electrical properties of the insulating film, as explained, for example, in A. Smedberg; T. Hjertberg; B. Gustafsson; Polymer, 1997, 38, 4127.

[0006] WO2019224334 describes a crosslinkable polyolefin composition comprising a first olefin polymer (A) and a second olefin polymer (B), the first olefin polymer (A) comprising a first comonomer (which comprises an epoxy group), and the second olefin polymer (B) comprising a second comonomer (which comprises a carboxylic acid group and / or its precursor). According to WO2019224334, this composition can be crosslinked without the need for a curing agent that can produce by-products. Although the solution proposed in the patent application is promising, the crosslinked material produces a certain amount of waste by-products and the resulting material can be difficult to recycle.

[0007] The object of the present invention is to provide a DC power cable insulating material which solves the above-mentioned and / or other problems. Another object of the present invention is to improve the insulating and mechanical properties of the DC power cable insulating material while maintaining processability without crosslinking the material.

[0008] Accordingly, the present invention provides a DC power cable insulating material 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

[0009]

[0010] wherein R1 = H or C1-C 10 alkyl; X = O or NH; R2 = C1-C 40 alkyl; R3, R4 = H or C1-C 10 alkyl, which may be connected by a cyclic structure, R5 = H or C1-C 20 ,

[0011]

[0012] wherein R6 = H or C1-C 10 alkyl, preferably wherein the ethylene copolymer (Z) is obtained by copolymerizing ethylene and an ion pair compound consisting of a cation of formula (I) and an anion of formula (II).

[0013] Particularly preferably, the composition is not crosslinked, and preferably the ethylene copolymer (Z) is not a crosslinked ethylene copolymer. Advantageously, the ethylene copolymer (Z) can be used in the DC power cable insulating material with a desired combination of low DC conductivity, melt strength and storage modulus without crosslinking the ethylene copolymer (Z).

[0014] In some embodiments, the composition comprises a mixture of monomers comprising ethylene, an ion pair compound and an α-olefin having 3 to 8 carbon atoms. In a preferred aspect, the composition consists of ethylene and an ion pair compound.

[0015] Preferably, the present invention provides a DC power cable insulating material comprising an ethylene copolymer (Z) obtained by copolymerizing ethylene and an ion pair compound consisting of a cation of formula (I) and an anion of formula (II), wherein

[0016]

[0017] wherein R1 = H or C1-C 10 alkyl; X = O or NH; R2 = C1-C40 alkyl; R3, R4 = H or C1-C 10 alkyl, which may be linked by a cyclic structure, R5 = H or C1-C 20 ,

[0018]

[0019] wherein R6 = H or C1-C 10 alkyl.

[0020] Preferably, based on the ethylene copolymer (Z), the amount of units derived from the ionic pair compound in the ethylene copolymer (Z) ≤ 25.0% by weight, and the amount of units derived from ethylene in the ethylene copolymer (Z) ≥ 75.0% by weight.

[0021] Preferably, based on the ethylene copolymer (Z), the amount of units derived from the ionic pair compound in the ethylene copolymer (Z) ≤ 15.0% by weight, and the amount of units derived from ethylene in the ethylene copolymer (Z) ≥ 85.0% by weight.

[0022] Preferably, based on the ethylene copolymer (Z), the amount of units derived from the ionic pair compound in the ethylene copolymer (Z) ≤ 10.0% by weight, and the amount of units derived from ethylene in the ethylene copolymer (Z) ≥ 90.0% by weight.

[0023] Preferably, based on the ethylene copolymer (Z), the amount of units derived from the ionic pair compound in the ethylene copolymer (Z) ≤ 5.0% by weight, and the amount of units derived from ethylene in the ethylene copolymer (Z) ≥ 95.0% by weight.

[0024] It is believed that when the amount of units derived from the ionic pair compound in the ethylene copolymer (Z) ≤ 25.0% by weight, preferably ≤ 15.0% by weight, preferably ≤ 10.0% by weight, it exhibits the desired properties of low DC conductivity, melt strength and storage modulus.

[0025] The ethylene copolymer (Z) used according to the present invention advantageously avoids the formation of by-products formed in the case of crosslinked polyethylene. It has surprisingly been found that the ethylene copolymer (Z) has a high melt strength and a very low DC conductivity. This makes it advantageous as a material to be used for the preparation of DC power cable insulation materials. One of the key parameters for viewing whether a material is suitable for use as a DC power cable is the measurement of DC conductivity.

[0026] A suitable measure for evaluating the melt strength is by determining the number average molecular weight. The present invention provides an ethylene copolymer having a suitable high molecular weight without even chemical crosslinking, making it have improved processability for DC power cable applications, while minimizing waste generation and other adverse side effects associated with the use of crosslinking agents.

[0027] The ethylene copolymer (Z) has suitable mechanical and rheological properties represented by a sufficiently high storage modulus. Advantageously, the composition maintains a high storage modulus even at elevated temperatures (e.g., at 150 °C).

[0028] It is noted that WO2021009274 discloses a copolymer of ethylene and an ion pair compound shown below, wherein the comonomer content is 2.17 wt%.

[0029]

[0030] Figure 2 of WO2021009274 shows the relationship between the resistivity (reciprocal of conductivity) of the copolymer and the frequency, but does not show the DC conductivity. WO2021009274 provides information on the electrical response at different frequencies (but not at zero frequency, which cannot be inferred from the provided data). It will be understood that information on the DC conductivity cannot be obtained from this disclosure. In addition, WO2021009274 does not mention the melt strength of the copolymer disclosed therein.

[0031] The molar ratio of the cation (I) to the anion (II) in the ion pair compound is 1:1.

[0032] The DC power cable insulation material according to the present invention comprises the ethylene copolymer (Z) in an amount of preferably 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% based on the power cable insulation material.

[0033] Preferably, the DC power cable insulation material according to the present invention comprises the ethylene copolymer (Z) in 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% based on the power cable insulation material.

[0034] Preferably, the DC power cable insulation material according to the present invention comprises the ethylene copolymer (Z) in an amount of at least 99.9 wt%, at least 99.99 wt% or 100 wt% based on the power cable insulation material.

[0035] Preferably, the DC power cable insulation material comprises the ethylene copolymer (Z) in an amount of 95 wt% to 100 wt% based on the power cable insulation material. Preferably, the DC power cable insulation material according to the present invention comprises additives in an amount of 0 to 5 wt% based on the power cable insulation material. Non-limiting examples of the additives include, for example, antioxidants, stabilizers, color pigments.

[0036] Preferably, the DC power cable insulating material according to the present invention comprises an ethylene copolymer (Z) in an amount of 95% to 99.99% by weight, based on the power cable insulating material, and an additive in an amount of 0.01% to 5% by weight, based on the power cable insulating material.

[0037] The DC power cable insulating material according to the present invention may comprise an additive such as an antioxidant, a stabilizer, a color pigment in an amount of 0.01% to 5% by weight, based on the power cable insulating material.

[0038] The present invention also provides a DC power cable comprising a conductor and an inner semiconductive layer, an insulating layer, and an outer semiconductive layer sequentially surrounding the conductor, wherein the insulating layer comprises the DC power cable insulating material according to the present invention or the insulating layer is the DC power cable insulating material according to the present invention. The DC power cable may further comprise one or more additional layers, such as a shield, a sheath layer, or other protective layers, which may surround the outer semiconductive layer.

[0039] The DC power cable according to the present 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 present invention is an HV or EHV DC power cable.

[0040] The present invention also provides a method for producing a DC power cable according to the present invention, which comprises applying an inner semiconductive layer, an insulating layer, and an outer semiconductive layer on a conductor by coextrusion.

[0041] The present invention also provides the use of an ethylene copolymer (Z) for producing an insulating layer in a power cable, the power cable comprising a conductor and an inner semiconductive layer, an insulating layer, and an outer semiconductive layer sequentially surrounding the conductor.

[0042] Preparation of ionic pair compounds

[0043] The ion pair compound (Z) used according to the present invention can be prepared according to the method described in lines 21 of page 4 to line 26 of page 12 of patent publication WO2021009274.

[0044] Specifically, the ion pair compound can be prepared a) from an alkali salt and an acid salt; b1) from an alkali salt and a free acid; b2) from a free base and an acid salt; or c) from a free base and a free acid.

[0045] Preferably, R5 = H.

[0046] Preferably, R1 = H or CH3.

[0047] Preferably, X = O.

[0048] Preferably, R2 = CH2-CH2.

[0049] Preferably, R3 = R4 = R5 = H; R3 = R4 = CH3, R5 = H; R3 = R4 = Et, R5 = H; R3 = tert-butyl, R4 = R5 = H or R3 = R4 = R5 = CH3.

[0050] Preferably, the free base is selected from the following:

[0051] 2-(Dimethylamino)ethyl acrylate

[0052] 2-(Diethylamino)ethyl acrylate

[0053] 2-(Diethylamino)ethyl methacrylate

[0054] 2-(Dimethylamino)ethyl methacrylate

[0055] 2-(tert-Butylamino)ethyl methacrylate

[0056] N-[3-(Hexahydro-1H-azepin-1-yl)-1,1-dimethylpropyl]-2-propenamide -1-yl)-1,1-dimethylpropyl]-2-propenamide

[0057] N-[2-(Tetrahydro-1,4-oxazin-4(5H)-yl)ethyl]-2-propenamide -4(5H)-yl)ethyl]-2-propenamide

[0058] N-[2-[Methyl(tetrahydro-2H-pyran-4-yl)amino]ethyl]-2-propenamide

[0059] N-[3-(Hexahydro-4-methyl-1H-1,4-diazepin-1-yl)propyl]-2-propenamide -1-yl)propyl]-2-propenamide

[0060] N-[1-Methyl-2-(methylamino)propyl]-2-propenamide,

[0061] N-[2-(Methylamino)propyl]-2-propenamide,

[0062] N-[2-Methyl-2-(methylamino)propyl]-2-propenamide

[0063] N-[1-Methyl-2-(methylamino)ethyl]-2-propenamide

[0064] N-[1-Methyl-3-(methylamino)butyl]-2-propenamide

[0065] N-[1-Methyl-2-(methylamino)propyl]-2-propenamide.

[0066] In a particularly preferred embodiment, the free base is selected from the following:

[0067] 2-(Dimethylamino)ethyl acrylate

[0068] 2-(Diethylamino)ethyl acrylate

[0069] 2-(Diethylamino)ethyl methacrylate, 2-(dimethylamino)ethyl methacrylate and 2-(tert-butylamino)ethyl methacrylate.

[0070] These free bases can be easily in a liquid state and are readily available.

[0071] Preferably, the base salt is a halide salt of the above-mentioned free base.

[0072] Preferably, cation (I) is the quaternization of the above-mentioned free base.

[0073] Preferably, R6 is H or CH3.

[0074] Preferably, the free acid is selected from acrylic acid and methacrylic acid.

[0075] Preferably, the acid salt is an alkali metal salt of the above-mentioned free acid.

[0076] Preferably, anion (II) is the deprotonated form of the above-mentioned free acid.

[0077] Preferably, the ion pair compound is a compound represented by one of the following formulas:

[0078]

[0079] More preferably, the ion pair compound is a compound represented by the following formula:

[0080]

[0081] Even more preferably, the ion pair compound is a compound represented by any one of the following formulas:

[0082]

[0083] Copolymerization method

[0084] The ethylene copolymer (Z) used in this application is obtained by copolymerizing ethylene and an ion pair compound composed of a cation of formula (I) and an acid anion of formula (II).

[0085] The ion pair compound according to the present invention can be dissolved in various common polar organic solvents such as isopropanol, acetonitrile, and ethyl acetate and injected into the polymerization reactor as a solution.

[0086] The copolymerization can be carried out by known methods.

[0087] Preferably, the ethylene copolymer according to the present invention is produced in a high-pressure free-radical polymerization process. The advantage of polymerization in such a high-pressure free-radical process is that polymerization can be carried out without the need for the presence of a catalyst. This allows the use of some 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) due to interference with such catalysts.

[0088] Another advantage of preparing the ethylene copolymer according to the present invention in a high-pressure free-radical polymerization process is that such polymerization results in an ethylene copolymer having a certain degree of long-chain branching. To meet some applications, including extrusion coating applications, it is required that the ethylene copolymer has 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. Therefore, it is preferred to prepare the ethylene copolymer according to the present invention via a high-pressure free-radical polymerization process.

[0089] The pressure in such a high-pressure free-radical polymerization process is preferably in the range of ≥180 MPa and ≤350 MPa, preferably ≥200 MPa and ≤300 MPa. The temperature in such a high-pressure free-radical polymerization process is preferably in the range of ≥100 and ≤350 °C, preferably ≥150 and ≤310 °C, preferably ≥190 and ≤260 °C, more preferably ≥200 and ≤250 °C.

[0090] Such a high-pressure free-radical polymerization process can be carried out, for example, in a tubular reactor. Such a tubular reactor can be, for example, a reactor described in Nexant PERP Report 2013-2, “Low Density Polyethylene”, pages 31-48.

[0091] Such a tubular reactor can operate, for example, at a pressure in the range of 150 - 300 MPa. The tubular reactor can have, for example, a tube length of ≥ 1000 m and ≤ 5000 m. The tubular reactor can have, for example, a length-to-inner diameter ratio of ≥ 1000:1, optionally ≥ 10000:1, optionally ≥ 25000:1, for example ≥ 10000:1 and ≤ 50000:1, optionally ≥ 25000:1 and ≤ 35000:1. The residence time in the tubular reactor can be, for example, ≥ 30 s and ≤ 300 s, optionally ≥ 60 s and ≤ 200 s. Such a tubular reactor can have, for example, an inner tube diameter of ≥ 0.01 m and ≤ 0.20 m, optionally ≥ 0.05 m and ≤ 0.15 m. The tubular reactor can have, for example, one or more inlets and one or more outlets. The feed composition can be fed, for example, to the inlet of the tubular reactor. The stream leaving the tubular reactor from the outlet can, for example, contain an ethylene copolymer. The stream leaving the tubular reactor from the outlet can, for example, contain unreacted feed composition. Such unreacted feed composition can be recycled back to the tubular reactor via one or more inlets.

[0092] A high-pressure free radical polymerization process is carried out in the presence of one or more free radical initiators. Preferably, the free radical initiator is selected from organic peroxides and / or azo compounds.

[0093] Suitable organic peroxides can, for example, include diacyl peroxides, dialkyl peroxides, peroxy monocarbonates, peroxy dicarbonates, peroxyketals, peroxy esters, cyclic peroxides, hydroperoxides. Suitable azo compounds can, for example, include 2,2'-azobis(isobutyronitrile), 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(hexahydrobenzyl cyanide).

[0094] Examples of suitable diacyl peroxides are diisobutyryl peroxide, bis(3,5,5-trimethylhexanoyl) peroxide, dilauroyl peroxide, didecanoyl peroxide, dibenzoyl peroxide.

[0095] Examples of suitable dialkyl peroxides are dicumyl peroxide, bis(tert-butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, tert-butyl cumyl peroxide, 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexyne, di-tert-butyl peroxide, diisononanoyl peroxide, di-tert-amyl peroxide, didecanoyl peroxide.

[0096] Examples of suitable peroxy monocarbonates are tert-amyl peroxy 2-ethylhexyl carbonate, tert-butyl peroxy isopropyl carbonate, tert-butyl peroxy 2-ethylhexyl carbonate.

[0097] Examples of suitable peroxydicarbonates are bis(3-methoxybutyl) peroxydicarbonate, di-sec-butyl peroxydicarbonate, diisopropyl peroxydicarbonate, bis(4-tert-butylcyclohexyl) peroxydicarbonate, bis(2-ethylhexyl) peroxydicarbonate, dibutyl peroxydicarbonate, diacetyl peroxydicarbonate, dimyristyl peroxydicarbonate, dicyclohexyl peroxydicarbonate.

[0098] Examples of suitable peroxyketals are 1,1-bis(tert-butylperoxy)-3,5,5-trimethylcyclohexane, 1,1-bis(tert-amylperoxy)cyclohexane, 1,1-bis(tert-butylperoxy)cyclohexane, 2,2-bis(tert-butylperoxy)butane, butyl 4,4-bis(tert-butylperoxy)valerate, n-ethyl 4,4-bis(tert-butylperoxy)valerate, ethyl 3,3-bis(tert-butylperoxy)butyrate, ethyl 3,3-bis(tert-amylperoxy)butyrate.

[0099] Examples of suitable peroxyesters are cumyl peroxyneodecanoate, 1,1,3,3-tetramethylbutyl peroxyneodecanoate, cumyl peroxyneodecanoate, tert-amyl peroxyneodecanoate, tert-butyl peroxyneodecanoate, tert-butyl peroxyisononanoate, tert-butyl permaleate, tert-butyl peroxy diethylisobutyrate, 1,1,3,3-tetramethylbutyl peroxyneopentanoate, tert-butyl peroxyneoheptanoate, tert-amyl peroxyneopentanoate, tert-butyl peroxyneopentanoate, 2,5-dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane, 1,1,3,3-tetramethylbutyl peroxy-2-ethylhexanoate, tert-amyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy diethylacetate, tert-butyl peroxyisobutyrate, tert-amyl peroxyacetate, tert-butyl peroxy-3,5,5-trimethylhexanoate, tert-amyl peroxybenzoate, tert-butyl peroxyacetate, tert-butyl peroxybenzoate.

[0100] Examples of suitable cyclic peroxides are 3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxynonane, 3,3,5,7,7-pentamethyl-1,2,4-trioxepane, 3,3,6,6,9,9-hexamethyl-1,2,4,5-tetraoxonane.

[0101] Examples of suitable hydroperoxides are cumene hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, cumyl hydroperoxide, tert-butyl hydroperoxide, tert-amyl hydroperoxide, methyl isobutyl ketone hydroperoxide, diisopropyl hydroxyperoxide.

[0102] Preferably, the free radical initiator composition is selected from 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, tert-butyl peroxyneopentanoate (t-BPP) and / or tert-butyl peroxybenzoate (t-BPB).

[0103] Such an initiator can be fed, for example, in pure form or as a solution in a solvent into a tubular reactor. As the solvent, for example, C2-C 20 n-alkanes or C2-C 20 isoalkanes can be used. For example, such a solution can contain ≥2.0% and ≤65.0% by weight of the initiator, alternatively ≥5.0% and ≤40.0% by weight, alternatively ≥10.0% and ≤30.0% by weight, based on the total weight of the solution.

[0104] Such an initiator can be introduced into the polymerization reactor, for example, in an amount of ≤300 ppm, preferably ≤200 ppm, based on the total weight of the material fed to the polymerization reactor.

[0105] In addition, an additional modifier can be fed into the tubular reactor. Examples of such modifiers can include initiators, scavengers, and / or chain transfer agents, such as alcohols, aldehydes, ketones, and aliphatic hydrocarbons. Such a modifier can be fed, for example, in pure form or as a solution in a solvent into the tubular reactor.

[0106] Examples of suitable chain transfer agents include cyclopropane, methane, tert-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, tert-butyl acetate, methyl formate, ethyl acetate, butane, triphenylphosphine, methylamine, methyl benzoate, ethyl benzoate, N,N-diisopropylethylamide, 2,2,4-trimethylpentane, n-hexane, isobutane, dimethoxymethane, ethanol, n-heptane, n-butyl acetate, cyclohexane, methylcyclohexane, 1,2-dichloroethane, acetonitrile, N-ethylethylamide, propylene, n-decane, N,N-diethylethylamide, cyclopentane, acetic anhydride, n-tridecane, n-butyl benzoate, isopropanol, toluene, acetone, 4,4-dimethylpentene-1, trimethylamine, N,N-dimethylacetamide, isobutene, n-butyl isocyanate, methyl butyrate, n-butylamine, N,N-dimethylformamide, diethyl sulfide, diisobutene, tetrahydrofuran, 4-methylpentene-1, p-xylene, p-di Alkanes, trimethylamine, butene-2, 1-bromo-2-chloroethane, octene-1, 2-methylbutene-2, cumene, butene-1, methyl vinyl sulfide, n-butyl cyanide, 2-methylbutene-1, ethylbenzene, n-hexadecene, 2-butanone, n-butyl isothiocyanate, methyl 3-cyanopropionate, tri-n-butylamine, 3-methyl-2-butanone, isobutyl cyanide, 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.

[0107] Preferably, the polymerization is carried out in the presence of a chain transfer agent selected from methanol, propionaldehyde, n-heptane, propane, isopropanol and acetone.

[0108] The amount of the chain transfer agent is preferably in the range of 0.01 and 2 mol% relative to the total weight of the materials fed to the polymerization reactor.

[0109] No neutralization step is required after the copolymerization step.

[0110] Ethylene copolymer

[0111] Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) is 0.01-10.00 mol%, 0.02-5.00 mol%, 0.03-1.50 mol%, 0.05-1.00 mol% or 0.15-1.00 mol% based on the ethylene copolymer.

[0112] Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer is 0.10-25.00 wt%, 0.25-10.00 wt%, 0.50-7.50 wt% or 1.00-5.00 wt% based on the ethylene copolymer.

[0113] Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) is 1.00-5.00 wt%, 2.00-4.00 wt% based on the ethylene copolymer (Z).

[0114] Preferably, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 0.10-25.00 wt% based on the ethylene copolymer, preferably 0.25-15.00 wt%, preferably 0.25-10.00 wt%, preferably 0.50-7.50 wt%, preferably 1.00-7.50 wt%, preferably 1.00-5.00 wt%, preferably 1.50-5.00 wt%, preferably 2.50-5.00 wt%, preferably 2.00-4.00 wt%.

[0115] Preferably, the amount of units derived from ethylene in the ethylene copolymer (Z) ranges from 75.00 to 99.90% by weight, preferably from 85.00 to 99.75% by weight, preferably from 90.00 to 99.75% by weight, preferably from 92.50 to 99.50% by weight, preferably from 92.50 to 99.00% by weight, preferably from 95.00 to 99.00% by weight, preferably from 95.00 to 98.50% by weight, preferably from 95.00 to 97.50% by weight, based on the ethylene copolymer.

[0116] Preferably, based on the ethylene copolymer, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 0.10 to 25.00% by weight, and the amount of units derived from ethylene in the ethylene copolymer ranges from 75.00 to 99.90% by weight.

[0117] Preferably, based on the ethylene copolymer, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 0.25 to 15.00% by weight, and the amount of units derived from ethylene in the ethylene copolymer ranges from 85.00 to 99.75% by weight.

[0118] Preferably, based on the ethylene copolymer, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 0.25 to 10.00% by weight, and the amount of units derived from ethylene in the ethylene copolymer ranges from 90.00 to 99.75% by weight.

[0119] Preferably, based on the ethylene copolymer, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 0.50 to 7.50% by weight, and the amount of units derived from ethylene in the ethylene copolymer ranges from 92.50 to 99.50% by weight.

[0120] Preferably, based on the ethylene copolymer, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 1.00 to 7.50% by weight, and the amount of units derived from ethylene in the ethylene copolymer ranges from 92.50 to 99.00% by weight.

[0121] Preferably, based on the ethylene copolymer, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 1.00 to 5.00% by weight, and the amount of units derived from ethylene in the ethylene copolymer ranges from 95.00 to 99.00% by weight.

[0122] Preferably, based on the ethylene copolymer, the amount of units derived from the ion pair compound in the ethylene copolymer (Z) ranges from 1.50 to 5.00% by weight, and the amount of units derived from ethylene in the ethylene copolymer ranges from 95.00 to 98.50% by weight.

[0123] Preferably, relative to the ethylene copolymer, the amount of the unit derived from the ionic pair compound in the ethylene copolymer (Z) ranges from 2.50 to 5.00% by weight, and the amount of the unit derived from ethylene in the ethylene copolymer ranges from 95.00 to 97.50% by weight.

[0124] Preferably, the ethylene copolymer has an Mn of 1 - 100 kg / mol, more preferably 5 - 60 kg / mol, and even more preferably 10 - 50 kg / mol. This results in a relatively high melt strength, which is beneficial to the preparation process of the DC power cable insulating material. Mn is measured according to ASTM D6474 - 12 (Standard Test Method for Determination of Molecular Weight Distribution and Molecular Weight Averages of Polyolefins by High Temperature Gel Permeation Chromatography). Mn represents the number average molecular weight.

[0125] It should be noted that the present invention relates to all possible combinations of the features described herein, and particularly preferred are those combinations of the features present in the claims. Thus, it will be understood that all combinations of the features related to the compositions according to the present invention are described herein; all combinations of the features related to the methods according to the present invention, and all combinations of the features related to the compositions according to the present invention and the features related to the methods according to the present invention.

[0126] It should also be noted that the term "comprising / including / containing" does not exclude the presence of other elements. However, it is also understood that the description of a product / composition containing some components also discloses a product / composition consisting of these components. The product / composition consisting of these components may be advantageous because it provides a simpler and more economical method for preparing the product / composition. Similarly, it is also understood that the description of a method including some steps also discloses a method consisting of these steps. The method consisting of these steps may be advantageous because it provides a simpler and more economical method.

[0127] When referring to the values of the lower and upper limits of a parameter, it is also understood that the ranges formed by the combination of the lower limit value and the upper limit value are disclosed.

[0128] The present invention is now illustrated by the following examples, however, it is not limited thereto.

[0129] CEx 1

[0130] Measure the properties of LDPE available commercially as SABIC LDPE2101N0W (MFR 0.85 dg / min at 190 °C, 2.16 kg; density 921 kg / m 3 ) and show them in Table 1.

[0131] CEx 2

[0132] XLPE containing a crosslinked polyethylene composition was prepared as follows. The ground LDPE was dispersed in a solution of DCP in methanol at 40 °C and stirred for 1 h, and then the solvent was evaporated. In a hot press, the resulting ground LDPE impregnated with 1 wt% DCP was melt-pressed at 120 °C under a pressure of up to 3750 kPa for 5 min. Then the temperature was raised to 180 °C, where the sample was crosslinked for 10 min before cooling. Finally, this XLPE sample was degassed in a vacuum oven at 50 °C. The properties were measured and are shown in Table 1.

[0133] CEx 3

[0134] Measure the properties of commercially available Surlyn TM 8920 (ionomer of ethylene acid copolymer) and are shown in Table 1.

[0135] Sample CEx 1, CEx 2 and CEx 3 Samples showing no units prepared from ion pair compounds as shown in the present invention.

[0136] Ex 1-7

[0137] Various ethylene copolymers were produced using the following polymerization parameters by continuously stirring an autoclave reactor, and their properties were measured and are shown in Table 1.

[0138] Polymerization parameters:

[0139] · Pressure 2000 bar

[0140] · Ethylene flow rate fixed at about 4 kg / h (residence time ~ 50 s)

[0141] · Impeller speed fixed at 1540 rpm

[0142] · Peroxide: Luperox 11M75

[0143] · Comonomer stream: solution of comonomer in isopropanol

[0144] The comonomer type, comonomer concentration and polymerization temperature are shown in Table 1.

[0145] Table 1

[0146]

[0147]

[0148] IPC1

[0149]

[0150] IPC2

[0151]

[0152] IPC3

[0153]

[0154] IPC4

[0155]

[0156] Storage modulus

[0157] The relationship between the storage modulus and temperature was measured by dynamic mechanical analysis (DMA). DMA was performed using a TA Q800 DMA in tensile mode on 13×11 mm pieces cut from 0.7 mm thick melt-pressed films. Variable temperature measurements were completed at a heating rate of 3 °C / min, using a preload force of 0.01 N, a strain of 0.05%, and a frequency of 1 Hz.

[0158] The results of CEx 2 and Ex 4 are shown in Figure 1.

[0159] DC conductivity

[0160] The relationship between DC conductivity and time was measured. The test cell consisted of a three-electrode setup 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; measuring electrodes with diameters of D = 28 or 59 mm were used. A DC voltage of V = 4 to 5 kV was applied across the L = 0.14 to 0.16 mm thick specimen film for 18 h, generating an electric field of approximately 30 kV / mm, and then the voltage was turned off for 6 h. Subsequently, the same voltage was applied again for an additional 18 h. The reported value of σ -1 corresponds to the apparent conductivity value obtained at the end of the second 18 h period. The volume leakage current was recorded using a Keithley 6517B electrometer and dynamically averaged. In addition, a low-pass filter was added to the circuit on the high voltage side to limit the current in the event of specimen breakdown and to filter out high frequency noise. DC

[0161] The apparent conductivity σ was calculated according to the following

[0162] σ = I / V -1 ·4L / (πD 2 )

[0163] where L is the distance between the measuring electrode and the high voltage electrode (i.e., the sample thickness), D is the diameter of the measuring electrode, V is the applied voltage, and I is the current at 70 °C and 30 kV / mm -1 ​The leakage current recorded below, the intermittent step is 6 hours, during which the applied voltage is turned off.

[0164] The results of CEx 2 and Ex 4 are shown in Figure 2.

[0165] Mn

[0166] Mn is measured according to ASTM D6474-12.

[0167] It is understandable that the ethylene copolymers of Ex 1-7 have a high storage modulus and a low conductivity, which makes them used as substitutes for the ethylene copolymers of CEx2 for DC power cable insulation materials.

[0168] It can also be seen from the above table that the DC conductivity of the non-crosslinked ethylene copolymers of Ex 1-7 is lower than that of the crosslinked system of CEx 2, indicating that the copolymers of the present invention are even more suitable than crosslinked polymer products (such as XLPE). In addition, the inventors believe that the copolymers of the present invention and especially the polymers from samples IE1-7 can impart the desired improved heat distortion tolerance at high temperatures and the desired dissipation factor performance, even without crosslinking any samples of the present invention.

[0169] The measured Mn of the ethylene copolymers of Ex 1-7 shows that they have a relatively high melt strength, which is beneficial to the preparation process of DC power cable insulation materials.

[0170] In addition, the ethylene copolymers of Ex 1-7 exhibit improved properties (a combination of low conductivity, high storage modulus and high molecular weight) compared to CEx 3 (also an ethylene-based ionomer material), making the ethylene copolymers of the present invention particularly suitable for DC power cable insulation material properties.

[0171] In fact, for CEx3, at a high temperature of 150 °C, the storage modulus of the Surlyn ionomer is lower compared to the copolymers prepared from Ex 1-7. This clearly shows that a DC power cable containing the ionomeric ethylene copolymer according to the present invention can impart the desired properties, even exceeding commercial vinyl ionomer materials.

[0172] Therefore, in summary, it is obvious that the ethylene copolymers derived from the ion pair compounds of the type shown in the present invention can exhibit the desired properties suitable for DC power cables. From CEx 1 (which is ordinary LDPE) and CEx 2 (based on crosslinked ethylene polymers), the lower conductivity of the ethylene copolymers of the present invention makes them more suitable for DC power insulation applications.

[0173] Surprisingly, the properties of the copolymers of the present invention are superior to those of other ethylene ionomer copolymers such as CEx 3, making the ethylene copolymers derived from the ion pair compounds of the present invention more suitable for DC electrical insulation applications.

Claims

1. A DC power cable insulating material comprising an ethylene copolymer (Z) obtained by copolymerizing a composition comprising ethylene and an ion pair compound, the ion pair compound consisting of a cation of formula (I) and an anion of formula (II), wherein wherein R1 = H or C1-C 10 alkyl; X = O or NH; R2 = C1-C 40 alkyl; R3, R4 = H or C1-C 10 alkyl, which may be connected by a cyclic structure, R5 = H or C1-C 20 , wherein R6 = H or C1-C 10 alkyl, preferably wherein the ethylene copolymer (Z) is obtained by copolymerizing ethylene and an ion pair compound composed of a cation of formula (I) and an anion of formula (II).

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

3. The DC power cable insulating material according to any one of the preceding claims, wherein R1 = H or CH3; X = O; R2 = CH2-CH2; and / or R3 = R4 = R5 = H; R3 = R4 = CH3, R5 = H; R3 = R4 = Et, R5 = H; R3 = tert-butyl, R4 = R5 = H or R3 = R4 = R5 = CH3.

4. The DC power cable insulating material according to any one of the preceding claims, wherein R6 is H or CH3.

5. The DC power cable insulating material according to any one of the preceding claims, wherein the cation (I) is a quaternization of a free base (IB) selected from the following: 2-(Dimethylamino)ethyl acrylate 2-(Diethylamino)ethyl acrylate 2-(Diethylamino)ethyl methacrylate 2-(Dimethylamino)ethyl methacrylate 2-(tert-Butylamino)ethyl methacrylate N-[3-(Hexahydro-1H-azepin-1-yl)-1,1-dimethylpropyl]-2-propenamide ​ N-[2-(tetrahydro-1,4-oxazin-4(5H)-yl)ethyl]-2-propenamide ​ N-[2-[Methyl(tetrahydro-2H-pyran-4-yl)amino]ethyl]-2-propenamide N-[3-(Hexahydro-4-methyl-1H-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 insulating material according to any one of the preceding claims, wherein the anion (II) is selected from acrylic acid and methacrylic acid.

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

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

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

10. The DC power cable insulating material according to any one of the preceding claims, wherein the DC power cable insulating material comprises the ethylene copolymer (Z) in 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% based on the power cable insulating material, preferably wherein the DC power cable insulating material comprises the ethylene copolymer (Z) in an amount of 95 wt% to 100 wt% based on the power cable insulating material.

11. A DC power cable comprising a conductor and an inner semiconductive layer, an insulating layer, and an outer semiconductive layer sequentially surrounding the conductor, wherein the insulating layer comprises the DC power cable insulating material according to any one of the preceding claims or the insulating layer is the DC power cable insulating material according to any one of the preceding claims.

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

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

14. A method for producing a DC power cable according to any one of claims 11-13, which comprises applying the inner semiconductive layer, the insulating layer, and the outer semiconductive layer on the conductor by coextrusion.

15. Use of the ethylene copolymer defined in any one of claims 1-10 for producing an insulating layer of a power cable, the power cable comprising a conductor and an inner semiconductive layer, an insulating layer, and an outer semiconductive layer sequentially surrounding the conductor.

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