RU2850497C1 - Cable - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: the cable comprises a conductor surrounded by at least an inner semiconductive layer, an insulating layer and an outer semiconductive layer in that order. The insulation layer comprises at least 60% by weight of low-density polyethylene homopolymer or low-density polyethylene copolymer with at least one polyunsaturated comonomer and, optionally, one or more additional comonomers. In addition, the insulating layer comprises from 10 to 35 wt% of a low-density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate.

EFFECT: improved electrical breakdown strength of the cable in a saline aqueous environment, which is maintained during long-term operation.

18 cl, 2 tbl

Description

The invention relates to a wet-type cable for saltwater environments, in particular to a wet-type power cable for saltwater environments, comprising an insulating layer in which water treeing is inhibited using a polymeric water tree retarder. Specifically, the insulating layer of the cable according to the invention comprises a combination of an LDPE homopolymer or LDPE copolymer and a polymeric water tree retarder. The invention also relates to a method for manufacturing such a cable and to its use in saltwater environments.

State of the art

A standard power cable consists of a conductor surrounded by an inner semiconducting layer (also called the conductor shield), an insulating layer, and an outer semiconducting layer (also called the insulation shield), in that order. The cable may also be provided with additional layers, such as a sheath layer, as is well known in the art.

It is known that in cases where a cable is used underwater or buried underground, the cable should be enclosed in a sheath of a waterproof layer (water-proof layer), usually a metallic waterproof layer, to protect the cable from problems caused by water ingress.

This is called a dry cable design. Dry cable design, or dry cable construction, refers to a cable construction in which a moisture-impermeable layer is present around the cable core (the core is defined as the conductor, an inner semiconducting layer, an insulating layer, and an outer semiconducting layer). If necessary, other layers, such as a shielding layer, may also be present outside the cable core. The moisture-impermeable layer is a layer that water cannot penetrate.

Years of field experience have shown that extruded metal sheaths, such as extruded lead/lead alloy sheaths, act as moisture barriers. When a cable design includes a moisture barrier layer other than the extruded metal sheath, tests to determine whether the design is dry or not can be found in Cigre TB722. Therefore, the moisture barrier layer is preferably metallic.

A so-called wet-type design or wet-construction is defined as a cable construction that lacks a moisture-proof layer. Therefore, such a design does not contain a moisture-proof layer, such as an extruded metal moisture-proof layer.

Thus, in dry cable designs, a moisture-proof layer prevents water ingress, but it significantly increases the cost of cable raw materials and is also expensive to use. Furthermore, conventional moisture-proof layers are often made of lead, which has significant environmental implications.

It is necessary to determine the cable material system (i.e., the materials for the insulating and semiconducting layers of the cable) that can be used in a wet-type cable. In a wet-type cable, the outer semiconducting layer may be uncoated, or the outer layer may be a shield or sheath layer. Such cables offer advantages in terms of raw material and manufacturing costs, but cables without a water-resistant layer (a moisture-proof layer) must be thoroughly tested to demonstrate that the cable has sufficient water resistance.

Therefore, in wet cables, care must be taken to maximize the water tree retardant (WTR) properties in the cable insulation layer.

A limitation of polyethylene is its tendency to develop bush-shaped defects, known as water dendrites, in the presence of water and under the influence of electric fields. This can lead to reduced breakdown strength and, possibly, failure of electrical equipment. This tendency is influenced by the presence of inhomogeneities, microcavities, and impurities in the material.

In electrically stressed polymeric materials exposed to water, processes known as "water treeing" can occur. Insulated cables are known to experience reduced service life when installed in environments where the polymer is exposed to water, such as underground or in high-humidity locations.

In principle, two types of water dendrites can be distinguished:

"Vented dendrites" which have their origin on the surface of the semiconducting shields and extend into the cable insulation layer, and "bow tie dendrites" which are initiated inside the cable insulation layer.

The structure of the water dendrite represents a localized damage, leading to a decrease in dielectric strength.

Various water tree retarders are well known in the art. There are numerous publications on adding water tree retarders to cables to minimize water dendrites. EP 1731566 describes the combination of an unsaturated polyolefin with a specific vinyl content and a polar copolymer to improve aging properties in humid conditions.

WO2010/112333 describes a cable comprising a conductor surrounded by a semi-conductive layer and an insulating layer, wherein the semi-conductive layer consists of a composition (A) containing a polar copolymer (a) and carbon black, and the insulating layer consists of a composition (B) containing a polar copolymer, wherein the difference between the melting temperature of said polar copolymer (a) and the melting temperature Tm (b2) of said polar copolymer (b2) is less than 25°C.

WO 85/05216 describes an insulating composition comprising polyethylene and 10 to 40 wt.% of a (meth)acrylate polymer such as ethylene butyl acrylate.

JP H08 319381 describes certain mixtures based on ethylene methyl acrylate copolymer and LDPE and test sheets made from them, which are subjected to water treeing tests in a very strong salt solution (2 mol/l).

The inventors of this invention are particularly concerned about aging in humid conditions in salt water, particularly seawater. Salt water typically exacerbates the problem of water treeing in cables, making the design of cables that may be exposed to salt water challenging.

In addition to sodium chloride, salt water may contain magnesium, calcium, potassium, and sulfate ions. These dissolved salts can affect water treeing in wet-type cables. Salt water, like seawater, is a more aggressive environment than, for example, freshwater. Therefore, just because a material can operate in freshwater environments doesn't mean it can withstand exposure to seawater.

The inventors of this invention have now discovered that the insulation layer of wet-type cables suitable for saltwater environments can comprise a combination of a homo- or copolymer of LDPE with a polyunsaturated comonomer and a copolymer of LDPE with a polar comonomer. This copolymer of LDPE with a polar comonomer acts as a polymeric water tree retarder.

Wet-performance cables were discussed in Johansson et al., 8th International Conference on Insulated Power Cables, "Influence of Subsea Conditions on the Long-Term Performance of AC XLPE Cables," Jicable'11, June 19-23, 2011, Versailles, France. The cables tested used a highly effective water treeing retarder—cross-linked polyethylene (XLPE) copolymer.

In Jicable '19, Paris 23-27 June 2019, Featherstone et al., "Full scale wet age testing of XLPE insulated power cables in salt water" report on salt solution testing on XLPE cables in a wet condition.

However, the combination of specific polymers in the insulating and semiconducting layers defined in this description and their ability to resist dielectric breakdown in a salt water environment is not disclosed in the prior art.

Brief description of the invention

In one aspect of the invention, there is provided the use of a cable, for example for transmitting power, in a salt water environment, such as in or under the sea;

wherein said cable comprises a conductor which is surrounded by at least an inner semiconducting layer, an insulating layer and an outer semiconducting layer in the said order;

wherein the said insulating layer contains:

(i) at least 60 wt.% of a low-density polyethylene homopolymer or a low-density polyethylene copolymer with at least one polyunsaturated comonomer and, optionally, one or more additional comonomers; and

(ii) from 10 to 35 wt.% of a low-density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate.

In another aspect of the invention, there is provided the use of a cable, for example for transmitting power, in a salt water environment, for example in or under the sea;

wherein said cable comprises a conductor which is surrounded by at least an inner semiconducting layer, an insulating layer and an outer semiconducting layer in the said order;

wherein the said insulating layer contains:

(i) at least 60 wt.% of a low-density polyethylene homopolymer or a low-density polyethylene copolymer with at least one polyunsaturated comonomer and, optionally, one or more additional comonomers; and

(ii) from 10 to 35 wt.% of a low-density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate, or vinyl acetate; and

wherein the said inner and outer semiconducting layers independently contain:

(a) a low density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate; and

(b) carbon black.

In one aspect of the invention, a cable is provided comprising a conductor that is surrounded by at least an inner semiconducting layer, an insulating layer, and an outer semiconducting layer, in that order;

wherein the said insulating layer contains:

(i) at least 60 wt.% of a low-density polyethylene homopolymer or a low-density polyethylene copolymer with at least one polyunsaturated comonomer and, optionally, one or more additional comonomers; and

(ii) from 10 to 35 wt.% of a low-density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate, or vinyl acetate; and

wherein the said inner and outer semiconducting layers independently contain:

(c) a low density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate; and

(d) carbon black.

The cable according to the invention is ideally a wet cable.

The cable according to the invention is cross-linkable or cross-linked. Therefore, in another embodiment, the insulating layer contains peroxide and is cross-linkable. In another embodiment, the inner and/or outer semiconductive layer contain peroxide and are cross-linkable. In another embodiment, the insulating layer and the inner and outer semiconductive layers contain peroxide and are cross-linkable. When exposed to cross-linking conditions, the cross-linkable cable can be cross-linked. The peroxide decomposes and generates free radicals, which initiate a cross-linking reaction in the composition.

In another aspect, the invention provides a cross-linked cable obtained by cross-linking a cross-linkable cable as defined above. In another aspect, the invention provides the use of a cross-linked cable as defined above, for example, for power transmission in a saltwater environment, such as at sea or under the sea.

From another aspect, the invention provides a method for producing a cable comprising a conductor surrounded by at least an inner semiconducting layer, an insulating layer and an outer semiconducting layer in that order, the method comprising the steps of:

- extrusion, such as co-extrusion, on the conductor of an inner semiconducting layer, an insulating layer and an outer semiconducting layer and

- crosslinking one or more of the said internal semiconductive layer, insulating layer and external semiconductive layer;

wherein the said insulating layer contains:

(i) at least 60 wt.% of a low-density polyethylene homopolymer or a low-density polyethylene copolymer with at least one polyunsaturated comonomer and, optionally, one or more additional comonomers; and

(ii) from 10 to 35 wt.% of a low-density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate, or vinyl acetate; and

wherein the said inner and outer semiconducting layers independently contain:

(a) a low density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate; and

(b) carbon black.

Detailed description of the invention

The invention provides a cable for use in a saltwater environment, such as a cross-linked cable or a cross-linked cable, for example, a cross-linked or cross-linked power cable, comprising a conductor surrounded by at least an inner semiconductive layer, an insulating layer, and an outer semiconductive layer. The invention also relates to a cross-linked cable comprising a conductor surrounded by at least an inner semiconductive layer, an insulating layer, and an outer semiconductive layer.

The term "salt water" or "salt aqueous medium" as used herein refers to water containing dissolved sodium chloride (NaCl) having a NaCl content of 1.0 wt.% or more, preferably 2.0 wt.% or more, more preferably 3.0 wt.% or more and up to 10 wt.% or less, preferably 8 wt.% or less, more preferably 6 wt.% or less, based on the total amount of water.

The cable according to the invention preferably has a wet design, as described above. Ideally, the cable does not contain a moisture-impermeable layer to prevent water ingress, such as a metallic waterproofing layer. Despite their wet design, the cables according to the invention exhibit excellent resistance to water in salt water environments. This is demonstrated by analyzing the electrical breakdown strength after aging under humid conditions in salt water. Thus, the invention provides cables with improved electrical breakdown strength in salt water environments.

Some experts consider the presence of a polymer shell over an outer semiconductive layer to be a "semi-wet" design, as the shell layer will limit the rate of water vapor penetration. The authors of this paper consider this solution to be a wet design, as the shell layer is not impermeable to water or moisture.

Insulating layer

The cable according to the invention comprises an insulating layer containing at least 60 wt. % of a low-density polyethylene homopolymer or a low-density polyethylene copolymer with at least one polyunsaturated comonomer and, possibly, one or more additional comonomers, and

from 10 to 35 wt.% of a low-density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate.

A low density polyethylene homopolymer or copolymer with at least one polyunsaturated comonomer and optionally one or more additional comonomers

Component (i) of the insulating layer is an LDPE homopolymer or an LDPE copolymer with at least one polyunsaturated comonomer and, optionally, one or more additional comonomers. In this description, this is referred to as the LDPE polymer component (i).

Although the term LDPE is an abbreviation for low-density polyethylene, the term is not understood to limit the density range, but rather encompasses similar high-density polyethylenes (HDPE). The term "LDPE" describes and distinguishes only the nature of HDPE, with its typical features, such as a different branching architecture compared to polyethylene produced in the presence of an olefin polymerization catalyst.

It should be understood that the term "LDPE homopolymers" generally refers to low-density polyethylene polymers consisting essentially of ethylene monomers. Therefore, ideally, an LDPE homopolymer does not contain any comonomer. However, small amounts of comonomers other than ethylene that do not significantly affect the properties of the LDPE may be acceptable, since such a material is still essentially a homopolymer. In this regard, a small amount of comonomer may mean less than 3 wt.%, for example, less than 1 wt.%, less than 0.5 wt.%, or less than 0.1 wt.% of non-polar or polar comonomers other than ethylene.

Preferably, the LDPE polymer component (i) is a copolymer of LDPE with at least one polyunsaturated comonomer.

In one embodiment, the low-density polyethylene copolymer with at least one polyunsaturated comonomer and, optionally, one or more additional comonomers of the insulating layer contains less than 5 wt.% of polar comonomers selected from alkyl acrylate, alkyl methacrylate or vinyl acetate. In one embodiment, the LDPE copolymer component (i) of the insulating layer contains less than 3.0 wt.%, preferably less than 2.0 wt.%, especially less than 1.0 wt.% of such polar comonomers.

Preferably, the LDPE copolymer component (i) is a binary copolymer of ethylene and only one polyunsaturated comonomer.

The polyunsaturated comonomer preferably consists of a linear carbon chain with at least 8 carbon atoms and at least 4 carbon atoms between non-conjugated double bonds, of which at least one is terminal. The polyunsaturated comonomer is preferably a diene, for example a diene that contains at least eight carbon atoms, wherein the first carbon-carbon double bond is terminal and the second carbon-carbon double bond is not conjugated with the first, for example a diene that is selected from a C ₈ -C ₁₄ non-conjugated diene or mixtures thereof, for example selected from 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene, 7-methyl-1,6-octadiene, 9-methyl-1,8-decadiene or mixtures thereof, for example from 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene or any mixture thereof.

If another comonomer is present, it may be a C ₃ -C ₁₀ alpha-olefin.

The LDPE copolymer component (i) preferably comprises from 0.001 to 40 wt.%, preferably from 0.05 to 40 wt.%, more preferably from 0.05 to 30 wt.%, even more preferably from 1.0 to 30 wt.%, even more preferably from 1.0 to 20 wt.% of one or more comonomers in total (i.e. all comonomers). In one embodiment, the LDPE copolymer comprises from 0.05 to 20 wt.%, preferably from 0.05 to 15 wt.%, such as from 1.0 to 10 wt.%, especially from 0.05 to 5.0 wt.%, such as from 1.0 to 5.0 wt.%, more particularly from 0.05 to 3.0 wt.%, such as from 1.0 to 3.0 wt.% of comonomer in total.

The content of polyunsaturated comonomers is preferably from 0.001 to 10 wt.%, preferably from 0.01 to 10 wt.%, more preferably from 0.01 to 5.0 wt.%, even more preferably from 0.01 to 3.0 wt.%, especially from 0.01 to 2.0 wt.%, more particularly from 0.1 to 2.0 wt.%. In some embodiments, the only comonomer present is a polyunsaturated comonomer.

The LDPE polymer component (i) is preferably unsaturated. It preferably has a total number of carbon-carbon double bonds of more than 0.4/1000 carbon atoms, preferably more than 0.5/1000 carbon atoms, such as more than 0.6/1000 carbon atoms, especially more than 0.7/1000 carbon atoms, for example more than 0.8/1000 carbon atoms. The upper limit of the number of carbon-carbon double bonds present in the polyolefin is not limited and may preferably be less than 5.0/1000 carbon atoms, preferably less than 3.0/1000 carbon atoms.

In some embodiments, the total number of carbon double bonds originating from vinyl groups, vinylidene groups and trans-vinylene groups, if present, in the LDPE polymer component (i) is above 0.40/1000 carbon atoms, preferably above 0.50/1000 carbon atoms, more preferably above 0.60/1000 carbon atoms, even more preferably above 0.70/1000 carbon atoms, even more preferably 0.75/1000 carbon atoms, especially 0.8/1000 carbon atoms. Preferably, the total number of carbon-carbon double bonds originating from vinyl groups, vinylidene groups and trans-vinylene groups is less than 5.0/1000 carbon atoms, preferably less than 3.0/1000 carbon atoms.

In some embodiments, the LDPE polymer component (i) contains at least vinyl groups, and the total amount of vinyl groups is preferably greater than 0.05/1000 carbon atoms, even more preferably greater than 0.08/1000 carbon atoms, and most preferably greater than 0.11/1000 carbon atoms.

In some embodiments, the LDPE polymer component (i) comprises at least vinyl groups, and the total amount of vinyl groups is preferably greater than 0.15/1000 carbon atoms, such as above 0.20/1000 carbon atoms, more preferably above 0.25/1000 carbon atoms, especially above 0.3/1000 carbon atoms, more particularly above 0.35/1000 carbon atoms, most particularly above 0.40/1000 carbon atoms, such as above 0.45/1000 carbon atoms or 0.50/1000 carbon atoms.

Preferably, the total number of vinyl groups is less than 4.0/1000 carbon atoms. More preferably, the LDPE polymer component (i) before crosslinking contains vinyl groups in a total number of more than 0.20/1000 carbon atoms, even more preferably more than 0.30/1000 carbon atoms and most preferably more than 0.40 vinyl groups/1000 carbon atoms, for example, above 0.45 vinyl groups/1000C, especially above 0.50/1000 carbon atoms.

Preferably, the LDPE polymer component (i) has a melt flow rate index MFI 2.16/190°C of 0.1 to 50 g/10 min, preferably of 0.3 to 20 g/10 min, more preferably of 0.3 to 15 g/10 min, even more preferably of 0.50 to 15 g/10 min or of 0.60 to 10 g/10 min. In some embodiments, the MFR2 is from 0.50 to 8.0 g/10 min, such as from 0.60 to 6.0 g/10 min, preferably from 0.70 to 5.5 g/10 min, such as from 0.80 to 5.0 g/10 min, more preferably from 0.90 to 4.75 g/10 min, even more preferably from 1.0 to 4.5 g/10 min, but even more preferably from 1.1 to 4.25 g/10 min, most preferably from 1.2 to 4.0 g/10 min, such as from 1.2 to 3.0 g/10 min.

Any LDPE polymer component (i) may have a density of from 905 to 935 kg/ ^m3 , preferably from 910 to 935 kg/ ^m3 , such as from 910 to 928 kg/ ^m3 .

The insulating layer preferably does not contain carbon black.

The insulating layer may comprise at least 60% by weight of the LDPE polymer component (i), for example at least 60 to 90% by weight, especially 70 to 85% by weight. A mixture of LDPE polymers may be used as component (i). If a mixture of LDPE polymers is used, this percentage refers to the sum of the LDPE polymers present.

The LDPE polymer component (i) usually makes up the rest of the layer after the other components have been calculated.

Polymeric water tree retarder: a low-density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate.

The insulating layer also contains a low-density polyethylene copolymer made of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate, or vinyl acetate (component ii). This component is called a polymeric water tree retarder. A mixture of these compounds may be used.

A low-density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate as a polymeric water tree retarder may be the same as or different from the low-density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate used in the inner and outer semiconductive layers. In general, the definition of a low-density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate, proposed below with respect to the inner and outer semiconductive layer, is applicable to component (ii) - the polymeric water tree retarder of the insulating layer.

The polar comonomer(s) of the low-density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate are preferably selected from _C1 - _C6 alkyl acrylates, _C1 - _C6 alkyl methacrylates or vinyl acetate. Even more preferably, the LDPE copolymer used is a copolymer of ethylene with _C1 - _{C6 alkyl} acrylates, such as methyl, ethyl, propyl or butyl acrylate, or vinyl acetate.

It is preferable to use an ethylene-methyl acrylate copolymer (EMA), an ethylene-methyl methacrylate copolymer (EMMA), an ethylene-ethyl acrylate copolymer (EEA), an ethylene-ethyl methacrylate copolymer (EEMA), an ethylene-butyl methacrylate copolymer (EBMA), an ethylene-butyl acrylate copolymer (EBA), or an ethylene-vinyl acetate copolymer (EVA).

The use of a copolymer of ethylene and methyl acrylate (EMA), ethylene and butyl acrylate (EBA), or ethylene and ethyl acrylate (EEA) is particularly preferred.

The low-density polyethylene copolymer of ethylene and at least one polar comonomer of the insulating layer preferably contains from 0.001 to 40 wt.%, more preferably from 0.05 to 40 wt.%, even more preferably from 1 to 30 wt.%, of one or more comonomers. The content of the polar comonomer is more preferably from 5 to 30 wt.%, from 5 to 25 wt.%, from 5 to 20 wt.%, for example from 7 to 25 wt.%, especially from 7 to 20 wt.%, from 10 to 25 wt.% or from 10 to 30 wt.%.

Preferably, the low-density polyethylene copolymer of ethylene and at least one polar comonomer of the insulating layer has a melt flow index MFR 2.16/190°C of from 0.1 to 50 g/10 min, more preferably from 1.0 to 30 g/10 min, even more preferably from 2.0 to 25 g/10 min and most preferably from 2.0 to 22 g/10 min. In some embodiments, the low-density polyethylene copolymer of ethylene and at least one polar comonomer of the insulating layer has a melt flow index MFR 2.16/190°C of from 0.1 to 20 g/10 min, more preferably from 0.5 to 12 g/10 min. In even more preferred embodiments, the polyethylene copolymer of low density ethylene and at least one polar comonomer of the insulating layer has a MFI 2.16/190°C of from 2.0 to 20.0 g/10 min, such as from 2.0 to 17.0 g/10 min, preferably from 2.0 to 15 g/10 min, for example from 2.0 to 13.5 g/10 min, from 2.0 to 13.0 g/10 min, from 2.5 to 12.5 g/10 min or from 2.5 to 12.0 g/10 min.

The LDPE copolymer may have a density of 910 to 940 kg/ ^m3 , preferably 915 to 940 kg/ ^m3 , such as 920 to 940 kg/ ^m3 .

The insulating layer may contain from 10 to 35 wt.% of a low-density polyethylene copolymer (ii) of ethylene and at least one polar comonomer, for example from 10 to 30 wt.% or from 12 to 35 wt.%, especially from 15 to 30 wt.%. If a mixture of these polymers is used in component (ii), this percentage refers to the total amount of low-density polyethylene copolymer of ethylene and at least one polar comonomer present.

Peroxide - insulating layer

The insulating layer may be crosslinkable or non-crosslinked. In such crosslinkable embodiments, the crosslinkable insulating layer preferably contains peroxide. Preferably, the crosslinkable insulating layer contains peroxide after the cable core structure has been formed. Mixtures of peroxides may also be used.

The preferred crosslinking agent is an organic peroxide. Non-limiting examples are organic peroxides such as di-tert-amyl peroxide, 2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne, 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, tert-butylcumyl peroxide, di(tert-butyl)peroxide, dicumyl peroxide, butyl 4,4-bis(tert-butylperoxy)valerate, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butyl peroxybenzoate, dibenzoyl peroxide, bis(tert-butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, 1,1-di(tert-butylperoxy)cyclohexane, 1,1-di(tert-amylperoxy)cyclohexane or any mixtures thereof. Preferably, the peroxide is selected from 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, di(tert-butylperoxyisopropyl)benzene, dicumyl peroxide, tert-butylcumyl peroxide or mixtures thereof. Most preferably, the peroxide is dicumyl peroxide.

The peroxide is preferably present in the insulating layer in an amount of less than 3.0 wt.%, more preferably 0.1-2.5 wt.%, and even more preferably 0.3-2.5 wt.%, based on the weight of the insulating layer. If a mixture of peroxides is used, this percentage refers to the sum of the peroxides present.

Inner and outer semiconducting layers

The inner and outer semiconducting layers may be the same or different, but preferably the same. This means that the chemical compositions of the inner and outer semiconducting layers are identical before crosslinking. The inner and outer semiconducting layers differ from the insulating layer.

The semiconducting properties of the semiconducting layers are a result of the presence of a conductive component contained in the semiconducting layer, i.e. carbon black.

Preferably, both the inner and outer semiconductive layers comprise a LDPE copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate, or vinyl acetate, and carbon black. Preferably, both the inner and outer semiconductive layers comprise a LDPE copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate, or vinyl acetate, carbon black, peroxide, and optionally an antioxidant. The discussion below may relate to one or both semiconductive layers.

A low density polyethylene copolymer (LDPE) of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate

The said LDPE copolymer of the inner and/or outer semiconductive layer contains a polar comonomer(s) selected from the group of alkyl acrylates, alkyl methacrylate, or vinyl acetate, or mixtures thereof. It is also possible to use a mixture of such LDPE copolymers.

Further preferably, said polar comonomer(s) are(are) selected from _C1 - _C6 alkyl acrylates, _C1 - _C6 alkyl methacrylates, or vinyl acetate. Even more preferably, the LDPE copolymer used in the inner and/or outer semiconductive layer is a copolymer of ethylene with a _C1 - _C6 alkyl acrylate, such as methyl, ethyl, propyl, or butyl acrylate, or vinyl acetate.

It is preferable to use an ethylene-methyl acrylate copolymer (EMA), an ethylene-methyl methacrylate copolymer (EMMA), an ethylene-ethyl methacrylate copolymer (EEMA), an ethylene-butyl methacrylate copolymer (EBMA), an ethylene-ethyl acrylate copolymer (EEA), an ethylene-butyl acrylate copolymer (EBA), or an ethylene-vinyl acetate copolymer (EVA).

The use of a copolymer of ethylene and methyl acrylate (EMA), ethylene and butyl acrylate (EBA), or ethylene and ethyl acrylate (EEA) is particularly preferred.

The LDPE copolymer of the inner and/or outer semiconductive layer preferably contains from 0.001 to 40 wt.%, more preferably from 0.05 to 40 wt.%, even more preferably from 1 to 30 wt.%, of one or more comonomers. The content of the polar comonomer is more preferably from 5 to 30 wt.%, from 5 to 25 wt.%, from 5 to 20 wt.%, for example from 7 to 20 wt.%.

Preferably, the LDPE copolymer of the inner and/or outer semiconductive layer has a melt flow rate index MFI 2.16/190°C of from 0.1 to 50 g/10 min, more preferably from 1.0 to 30 g/10 min, even more preferably from 2.0 to 25 g/10 min, for example from 3.0 to 20 g/10 min or from 4.0 to 20 g/10 min, and most preferably from 4.0 to 22 g/10 min, for example from 5.0 to 20 g/10 min.

The LDPE copolymer may have a density of 910 to 940 kg/ ^m3 , preferably 915 to 940 kg/ ^m3 , such as 920 to 940 kg/ ^m3 .

The inner and/or outer semiconductive layer may comprise at least 50 wt.% of LDPE copolymer, for example at least 55 wt.%. If a mixture of LDPE copolymers is used, this percentage refers to the sum of the LDPE copolymers present.

In some embodiments, the inner and/or outer semiconductive layer contains at least 60% by weight of LDPE copolymer. LDPE typically forms the remainder of the layer after the other components of the semiconductive layer have been selected. The inner and/or outer semiconductive layer preferably contains no more than 90% by weight of LDPE copolymer.

Any LDPE homopolymer or copolymer described in this invention can be prepared by any conventional polymerization process. Preferably, it is prepared by radical polymerization, such as high-pressure radical polymerization. High-pressure polymerization can be carried out in a tubular reactor or an autoclave reactor. Preferably, it is a tubular reactor. In general, the pressure can be 1200-3500 bar (120-350 MPa), and the temperature can be 150°C-350°C. More detailed information on high-pressure radical polymerization is given in Encyclopedia of Polymer Science and Engineering, Vol. 6 (1986), pp. 383-410 and in Encyclopedia of Materials: Science and Technology, Elsevier Science Ltd.: "Polyethylene: High-pressure", R. Klimesch, D. Littmann and 7181-7184. 2001, which are incorporated herein by reference.

It is well known that, for example, propylene can be used as a comonomer or as a chain transfer agent (CTA), or both, and can contribute to the total number of C-C double bonds, preferably to the total number of vinyl groups. For the purposes of this invention, when a compound that can also act as a comonomer, such as propylene, is used during polymerization as a CTA to provide double bonds, said copolymerizable comonomer is not taken into account when calculating the comonomer content.

Carbon black

According to the present invention, the inner and outer semiconducting layers also contain carbon black.

The semiconductive properties result from the addition of carbon black. The amount of carbon black is thus at least such that a semiconductive layer is obtained. Preferably, the inner and/or outer semiconductive layer contains from 10 to 60 wt.%, preferably 15-48 wt.% carbon black. In other preferred embodiments, the amount of carbon black is 10-45 wt.%, for example 20-45 wt.%, preferably 25-45 wt.%, more preferably 25-40 wt.%, or especially 30-41 wt.%, based on the weight of the semiconductive layer.

Any carbon black that is electrically conductive can be used. Examples of suitable carbon blacks include furnace blacks and acetylene blacks. Mixtures can also be used. If a mixture of carbon blacks is used, the percentage is based on the sum of the carbon blacks present.

The carbon black may have a nitrogen adsorption specific surface area (BET) of from 5 to 400 ^m2 /g, such as from 10 to 300 ^m2 /g, such as from 30 to 200 ^m2 /g, when determined in accordance with ASTM D3037-93. In addition, the carbon black may have one or more of the following properties:

i) a primary particle size of at least 5 nm, which is determined as the average particle diameter in accordance with ASTM D3849-95a,

ii) an iodine adsorption number (IAN) of at least 10 mg/g, such as 10 to 300 mg/g, such as 30 to 200 mg/g, when determined in accordance with ASTM D-1510; and/or

iii) a DBP (dibutyl phthalate) absorption number (= oil number) of at least 30 ^cm3 /100 g, such as 60 to 300 ^cm3 /100 g, such as 70 to 250 ^cm3 /100 g, such as 80 to 200 ^cm3 /100 g, such as 90 to 180 ^cm3 /100 g, when measured according to ASTM D 2414.

In addition, carbon black may have one or more of the following properties:

a) a primary particle size of at least 5 nm, which is determined as the average particle diameter in accordance with ASTM D3849-95a;

b) an iodine value of at least 30 mg/g according to ASTM D1510;

c) an oil absorption number of at least 30 ml/100 g, as measured in accordance with ASTM D2414.

One group of suitable furnace blacks has an average primary particle size of 28 nm or less. The average primary particle size is defined as the number-average particle diameter, measured in accordance with ASTM D3849-95a. Particularly suitable furnace blacks in this category may have an iodine value of 60 to 300 mg/g in accordance with ASTM D1510. Additionally, an oil absorption value (for this category) of 50 to 225 ml/100 g, for example, 50 to 200 ml/100 g, is acceptable, and is measured in accordance with ASTM D2414.

Another group of equally suitable furnace blacks has an average primary particle size greater than 28 nm. Average primary particle size is defined as the number-average particle diameter according to ASTM D3849-95a. Suitable furnace blacks in this category have an iodine value of 30 to 200 mg/g according to ASTM D1510. Furthermore, the oil absorption value (for this category) ranges from 80 to 300 ml/100 g, measured according to ASTM D2414.

Other suitable types of carbon black may be prepared by any other method or may be further processed.

Suitable carbon blacks for semiconductive cable layers are suitably characterized by their purity. Therefore, suitable carbon blacks have an ash content of less than 0.2 wt%, measured in accordance with ASTM D1506, a 325 mesh sieve residue of less than 30 ppm, according to ASTM D1514, and a total sulfur content of less than 1 wt%, according to ASTM D1619.

Furnace black is a generally accepted term for a well-known type of carbon black produced in a furnace-type reactor. Examples of carbon blacks, their preparation methods, and reactors include EP-A-0629222 Cabot, US 4,391,789, US 3,922,335, and US 3,401,020. Examples of commercially available carbon blacks include grades described in ASTM D 1765-98b, including N351, N293, and N550. Furnace blacks are traditionally distinguished from acetylene blacks, which are another type of carbon black suitable for the semiconductive layer. Acetylene blacks are produced by the process for producing acetylene black by reacting acetylene and unsaturated hydrocarbons, such as described in US 4,340,577.

In particular, acetylene blacks may have an average particle size greater than 20 nm, for example, from 20 to 80 nm. The average primary particle size is defined as the number-average particle diameter in accordance with ASTM D3849-95a. Suitable acetylene blacks of this category have an iodine value of 30 to 300 mg/g, for example, from 30 to 150 mg/g according to ASTM D1510. In addition, the oil absorption number (for this category) is, for example, from 80 to 300 ml/100 g, for example, from 100 to 280 ml/100 g, and is measured in accordance with ASTM D2414. Acetylene black is a generally recognized term and is very well known, and is supplied, for example, by Denka.

In yet another embodiment, the invention describes a semiconductive layer in which the conductive component comprises or consists of a conductive carbon black, such as a carbon black with one or more, such as all, of the following properties:

a primary particle size of at least 5 nm, which is determined as the number average particle diameter in accordance with ASTM D3849-95a;

an iodine adsorption value (IAV) of at least 10 mg/g, such as 10 to 300 mg/g, when determined in accordance with ASTM D-1510; or

DBP (dibutyl phthalate) absorption number (= oil absorption number) of at least 30 ^cm3 /100 g, such as 60 to 300 ^cm3 /100 g, when measured in accordance with ASTM D 2414.

Peroxide

The inner and/or outer semiconductive layer is crosslinkable or crosslinked. Preferably, the crosslinkable inner and/or outer semiconductive layer contains peroxide in an amount of less than 3.0 wt.%, more preferably 0.1-2.5 wt.%, even more preferably 0.3-2.5 wt.%, based on the weight of the semiconductive layer. In some embodiments, the peroxide is present in an amount of 0.4-2.5 wt.%, preferably from 0.4 to 2.0 wt.%, based on the weight of the semiconductive layer. If a mixture of peroxides is used, this percentage refers to the sum of the peroxides present.

The preferred crosslinking agent is an organic peroxide. Non-limiting examples are organic peroxides such as di-tert-amyl peroxide, 2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne, 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, tert-butylcumyl peroxide, di(tert-butyl)peroxide, dicumyl peroxide, butyl 4,4-bis(tert-butylperoxy)valerate, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butyl peroxybenzoate, dibenzoyl peroxide, bis(tert-butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, 1,1-di(tert-butylperoxy)cyclohexane, 1,1-di(tert-amylperoxy)cyclohexane or any mixtures thereof. Preferably, the peroxide is selected from 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, di(tert-butylperoxyisopropyl)benzene, dicumyl peroxide, tert-butylcumyl peroxide or mixtures thereof. Most preferably, the peroxide is bis(tert-butylperoxyisopropyl)benzene.

Antioxidant

Any cable layer can contain an antioxidant. Examples of antioxidants include sterically hindered or semi-hindered phenols, aromatic amines, aliphatic sterically hindered amines, organic phosphates, thio compounds, polymerized 2,2,4-trimethyl-1,2-dihydroquinoline, and mixtures thereof.

More preferably, the antioxidant is selected from the group consisting of 4,4'-bis(1,1'-dimethylbenzyl)diphenylamine, para-oriented styrenated diphenylamines, 4,4'-thiobis(2-tert-butyl-5-methylphenol), polymerized 2,2,4-trimethyl-1,2-dihydroquinoline or derivatives thereof.

More preferably, the antioxidant is selected from the group (but not limited to) 4,4'-bis(1,1'-dimethylbenzyl)diphenylamine, para-oriented styrenated diphenylamines, 4,4'-thiobis(2-tert-butyl-5-methylphenol), 2,2'-thiobis(6-tert-butyl-4-methylphenol), distearyl thiodipropionate, 2,2-thio-diethyl-bis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, polymerized 2,2,4-trimethyl-1,2-dihydroquinoline or derivatives thereof. Of course, not only one of the above-described antioxidants can be used, but also any mixture of them.

The amount of antioxidant, possibly a mixture of two or more antioxidants, may be from 0.005 to 2.5 wt.%, for example from 0.01 to 2.5 wt.%, preferably from 0.01 to 2.0 wt.%, more preferably from 0.03 to 2.0 wt.%, especially from 0.03 to 1.5 wt.%, more specifically from 0.05 to 1.5 wt.% or from 0.1 to 1.5 wt.% based on the weight of the semiconducting layer.

In some embodiments, the amount of antioxidant is from 0.05 to 1.5 wt.%, preferably from 0.05 to 1.0 wt.%, more preferably from 0.05 to 0.8 wt.%, especially from 0.05 to 0.6 wt.%, more particularly from 0.05 to 0.5 wt.% based on the weight of the insulating layer.

Other components

The inner and/or outer semiconductive layer or insulating layer may contain additional additives. Possible additives include vulcanization retarders, crosslinking accelerators, stabilizers, processing aids, flame retardants, acid scavengers, inorganic fillers, voltage stabilizers, or mixtures thereof.

A "scorch retarder" is defined as a compound that reduces premature crosslinking, or scorching, during extrusion. In addition to its scorch retarder properties, a scorch retarder can also have other effects, such as enhancing crosslinking properties. Using a scorch retarder in the insulating layer is particularly beneficial.

Useful scorch retarders can be selected from substituted or unsubstituted diphenylethylene, quinone derivatives, hydroquinone derivatives such as 2,5-di-tert-butylhydroquinone, monofunctional vinyl-containing esters and ethers, or mixtures thereof. More preferably, the scorch retarder is selected from substituted or unsubstituted diphenylethylene or mixtures thereof. A highly preferred embodiment is 2,4-diphenyl-4-methyl-1-pentene.

Preferably, the amount of scorch retarder is from 0.005 to 1.0 wt.%, more preferably from 0.01 to 0.8 wt.%, based on the weight of the layer in question. Other preferred ranges are from 0.03 to 0.75 wt.%, from 0.05 to 0.50 wt.%, from 0.05 to 0.70 wt.% and from 0.10 to 0.50 wt.%, based on the weight of the layer in question.

The crosslinking accelerator may be a compound containing at least 2 unsaturated groups, such as an aliphatic or aromatic compound, an ester, an ether, an amine, or a ketone that contains at least 2 unsaturated groups, such as cyanurate, isocyanurate, phosphate, orthoformate, an aliphatic or aromatic ether, or an allyl ester of benzenetricarboxylic acid. Examples of esters, ethers, amines and ketones are compounds selected from the general groups of diacrylates, triacrylates, tetraacrylates, triallyl cyanurate, triallyl isocyanurate, 3,9-divinyl-2,4,8,10-tetra-oxaspiro[5,5]-undecane (DVS), triallyl trimellitate (TATM) or N,N,N',N',N'',N''-hexaallyl-1,3,5-triazine-2,4,6-triamine (HAT AT A) or any mixtures thereof. The crosslinking accelerator may be added in an amount of such crosslinking of less than 2.0 wt. %, such as less than 1.5 wt. %, such as less than 1.0 wt. %, such as less than 0.75 wt. %, such as less than 0.5 wt. %, and the lower limit thereof is, for example, at least 0.05 wt. %, for example at least 0.1 wt. %, based on the weight of the polymer composition or based on the weight of the layer in question.

In another embodiment of the invention, the insulation layer does not contain water tree retarders other than the polymeric water tree retarder discussed herein.

Conductor

The cable according to the invention comprises a conductor. The conductor may be made of any suitable conductive metal, such as copper or aluminum.

Cable

A power cable is defined as a power transmission cable operating at any voltage, typically operating at voltages above 1 kV. The voltage supplied to the power cable may be alternating current (AC), direct current (DC), or transient (pulsed). Furthermore, the cable is very preferably an alternating current power cable, such as a cable operating at voltages of 1-525 kV, 6-525 kV, 6-275 kV, 6-220 kV, 6-150 kV, 6-72 kV, or a power cable from 6 to 60 kV (root mean square voltage, the voltage between any two conductors in a three-phase cable). In some embodiments, the cable is an alternating current power cable operating at voltages above 1 kV, preferably above 6 kV. In some embodiments, the cable is an AC power cable operating at voltages below 525 kV, preferably below 400 kV, more preferably below 380 kV, especially below 275 kV, below 220 kV, or even below 150 kV.

As is well known, a cable can contain additional layers if necessary, such as layers surrounding the outer semiconducting layers, such as the sheath layer. It is preferable to avoid a moisture-impermeable layer to prevent water ingress, meaning the cable is considered a wet-type cable.

The cable can be obtained by a method comprising the steps of: (a)

- providing and mixing, for example, melt mixing in an extruder, a crosslinkable first semiconductive composition for the inner semiconductive layer,

- providing and mixing, for example, melt mixing in an extruder, a crosslinkable insulating composition for the insulating layer,

- providing and mixing, for example, melt mixing in an extruder, a second semiconductive composition for the outer semiconductive layer,

(b) applied to a conductor, for example by co-extrusion,

- a molten mixture of the first semiconducting composition obtained from step (a), to form an internal semiconducting layer,

- a molten mixture of the insulating layer composition obtained in step (a) to form an insulating layer, and

- a molten mixture of the second semiconducting composition obtained from step (a), to form an outer semiconducting layer, and

(c) crosslinking is possible under crosslinking conditions of one or more of the insulating layer, the inner semiconducting layer and the outer semiconducting layer of the resulting cable.

Preferably, if peroxide is used in the cable layer, the layer is cross-linked. This makes the cable cross-linkable.

The first semiconductive composition for the inner semiconductive layer, the crosslinkable insulating composition for the insulating layer and the second semiconductive composition for the outer semiconductive layer contain components necessary for forming the corresponding inner semiconductive, insulating and outer semiconductive layers of the cable.

The desired polymer composition can be produced in several ways using several different production technologies, such as closed mixers like Banbury or Boiling, continuous single-screw devices like BUSS, or continuous twin-screw devices like Farrel or Werner & Pfleiderer. The type of mixer and the selected operating conditions for preparing the semiconductive compound directly impact the melt quality and influence the final compound properties, such as melt flow index, volume resistivity, and surface smoothness. Particularly useful is the technology using a co-mixer (BUSS, X-compounds). When preparing semiconductive layers, the conductive filler can be added to the polymer in the molten state with complete control of the production temperature. Using this technology, one skilled in the art can produce a mixture with sufficiently developed dispersive and distributive mixing.

Preferably, all layers are cross-linked. The invention therefore further provides a cross-linked cable obtained by cross-linking the cables defined in this description.

The crosslinking procedure can be performed at elevated temperatures, such as above 150°C, for example from 160 to 350°C.

Melt blending means blending above the melting point of at least the major polymer component(s) of the resulting mixture, and is typically carried out at a temperature of at least 10-15°C above the melting or softening point of the polymer component(s).

The term "coextrusion" as used herein means that all layers or a portion of a layer(s) are produced simultaneously using one or more extrusion dies. For example, triple extrusion can be used to form three layers.

In other embodiments of the present invention, the cross-linked cable according to the invention has a Weibull Eb value, measured on a 20 kV cable (nominal insulation thickness 5.5 mm) after 1 year of wet aging in salt water, as described below in the determination methods section, of at least 55 kV/mm, such as from 55 to 75 kV/mm.

In addition, said first and second semiconducting compositions may, for example, be identical.

The thickness of the insulating layer of a power cable, such as an AC cable, is typically 2 mm or more, such as at least 2.5 mm, at least 3 mm, such as from at least 3.5 to 50 mm, such as from 4 to 50 mm, preferably from at least 4.5 to 35 mm, such as from 5 to 30 mm when measured from the cross-section of the insulating layer of the cable.

The thickness of the inner and/or outer semi-conductive layer of a power cable may typically be 0.5 mm or more, such as from 0.7 mm to 5.0 mm when measured from the cross-section of the layer.

In another aspect, the invention provides a cable comprising a conductor that is surrounded by at least an inner semiconducting layer, an insulating layer and an outer semiconducting layer, in that order;

wherein said internal and/or external semiconducting layer independently includes:

at least 50 wt.% of a copolymer of LDPE with a polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate,

25-48 wt.% carbon black; and

0.1-2.5 wt. % peroxide; And

wherein said insulating layer contains

(i) at least 60 wt.% of a low density polyethylene copolymer with at least one polyunsaturated comonomer and possibly one or more additional comonomers;

(ii) from 10 to 35 wt.% of a low density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate, preferably alkyl acrylate; and

(iii) 0.1-2.5 wt. % peroxide.

Application

The cable according to the invention is specifically adapted for use in saltwater environments. Thus, the cable can be laid underwater or located in the sea or on the seabed. Cables laid in the ground on land, but close to the shore, can also be exposed to the saltwater environment underground. Cables used in tidal estuaries are also exposed to saltwater. There are also saltwater lakes and saltwater seas where cables according to the invention could be used, for example, in or under such bodies of water. The cables according to the invention are suitable for use in any saltwater environment.

In one embodiment, the invention relates to a method comprising laying a cable, as described hereinbefore, in the ground under the sea.

In one embodiment, the invention relates to a method involving laying a cable, as previously described, on the seabed. The cable can then be retrieved from its reel. This process may involve a cable-laying vessel.

In one embodiment, the cables according to the invention can be used to connect an offshore power generation system to shore, so the cable lies on the seabed. Thus, in another aspect, the invention provides an electrical system comprising:

(A) a marine power generator such as a wind turbine;

(B) a cable, as described herein, connecting said offshore power generator to a substation located on land and/or offshore through the seabed.

Offshore wind turbines are typically located on offshore platforms and connected by cables to an offshore substation located on a separate platform and/or an onshore substation. These cables are submarine cables, and therefore the cables according to the invention, which operate exclusively in the presence of seawater, are ideal for this application.

Thus, cables according to the invention can generally connect a marine device to a land-based device or another marine device. Therefore, a cable according to the invention can connect these devices through the seabed.

In one embodiment, the cables according to the invention can be used to distribute energy generated in an offshore power generation system and to connect said system to a substation or collection system located offshore and/or onshore, through the seabed.

The invention is further described with reference to the following non-limiting examples.

Methods of determination

Unless otherwise stated in the description or experimental section, the following methods were used to determine the properties.

Mass %: % by weight

Melt Flow Index (MFI) is determined according to ISO 1133 and is expressed in g/10 min. MFI is an indicator of the flowability and, therefore, the processability of the polymer. The higher the MFI, the lower the viscosity of the polymer. MFI is determined at 190°C for polyethylene and can be determined at various loads, such as 2.16 kg (MFI ₂ ) or 21.6 kg (MFI ₂₁ ).

Density

Density was measured according to ISO 1183-1/method A. Sample preparation was carried out by direct compression in accordance with ISO 17855-2:2016.

Comonomer content

a) Quantitative assessment of alpha-olefin content in low-density polyethylenes by NMR spectroscopy:

Comonomer content was determined by quantitative ^13C nuclear magnetic resonance (NMR) spectroscopy after assignment of the major bands (J. Randall JMS - Rev. Macromol. Chem. Phys., C29(2&3), 201-317 (1989)). Experimental parameters were adjusted to ensure measurement of quantitative spectra for this specific task.

Specifically, solution-state NMR spectroscopy was used using a Bruker Avancell 400 spectrometer. Homogeneous samples were prepared by dissolving approximately 0.200 g of polymer in 2.5 mL of deuterated tetrachloroethylene in 10 mm tubes using a heat block and a rotating tube furnace at 140 °C. ¹³ C single-pulse proton-coupled nuclear Overhauser effect (NOE) (on) NMR spectra were recorded using the following acquisition parameters: 90 degree flip angle, 4 blank scans, 4096 transitions with an acquisition time of 1.6 s, 20 kHz spectral width, 125 °C temperature, WALTZ two-level proton decoupling scheme, and 3.0 s relaxation delay. The obtained free induction decay (FID) signal was processed using the following processing parameters: zero padding to 32k data points and apodization using a Gaussian window function; automatic zero- and first-order phase correction; and automatic baseline correction using a fifth-order polynomial limited to the region of interest.

The values were calculated using simple corrected ratios of signal integrals of representative areas based on methods well known in the art.

b) Determination of the content of polar comonomers in low-density polyethylene

The comonomer content (wt %) was determined by a known method based on Fourier transform infrared spectroscopy (FTIR) calibrated using quantitative nuclear magnetic resonance (NMR) spectroscopy.

The films were pressed using a Specac film press at 150°C with a load of approximately 5 tons for 1-2 minutes, and then cooled uncontrolled with cold water. The exact thickness of the resulting film samples was measured.

Following FTIR analysis, baselines were drawn in absorption mode for peak analysis. The comonomer absorption peak was normalized using the polyethylene absorption peak. The FTIR peak height ratio was correlated with the polar comonomer content of the reference materials, determined by NMR. The NMR calibration procedure was performed using a conventional method well known in the literature.

Quantitative assessment of polar comonomer content in polymers using NMR spectroscopy

The polar comonomer content was determined by quantitative nuclear magnetic resonance (NMR) spectroscopy after assignment of the main bands (e.g., "NMR Spectra of Polymers and Polymer Additives," A.J. Brandolini and D.D. Hills, 2000, Marcel Dekker, Inc. New York). Experimental parameters were adjusted to ensure measurement of quantitative spectra for this specific task (e.g., "200 and More NMR Experiments: A Practical Course," S. Berger and S. Braun, 2004, Wiley-VCH, Weinheim). Values were calculated using simple adjusted ratios of the signal integrals of the representative sites in a manner known in the art.

Below are examples of determining the polar comonomer content in a copolymer of ethylene with ethyl acrylate, ethylene with butyl acrylate, and ethylene with methyl acrylate.

Wt.% can be converted to mol.% by calculation. This is well known in the literature.

(1) Ethylene copolymers containing butyl acrylate

For FTIR measurements, polymer film samples were prepared: 0.5-0.7 mm thick was used for ethylene-butyl acrylate copolymer with butyl acrylate content >6 wt %, and 0.05 to 0.12 mm thick was used for ethylene-butyl acrylate copolymer with butyl acrylate content <6 wt %. After FT-IR analysis, the absorbance value for the baseline at 3510 cm ^-1 (A _{butyl acrylate} - A ₃₅₁₀ ) was subtracted from the maximum absorbance peak for the butyl acrylate content >6 wt % at 3450 cm ^-1 . Then, the absorbance value for the baseline at 2120 cm ^-1 (A ₂₀₂₀ - A ₂₁₂₀ ) was subtracted from the maximum absorbance peak for the polyethylene peak at 2020 cm ^-1 . The ratio between (A _{butyl acrylate} - A ₃₅₁₀ ) and (A ₂₀₂₀ - A ₂₁₂₀ ) was then calculated in the usual way, which is well known in the literature.

From the maximum absorption value for the peak for the butyl acrylate comonomer content <6 wt.% at 1165 cm ^-1 , the absorbance value for the baseline at 1865 cm (A _{butyl acrylate} - A ₁₈₆₅ ) was subtracted. Then, from the maximum absorption value for the polyethylene peak at 2660 cm ^-1 , the absorbance value for the baseline at 1865 cm ^-1 (A ₂₆₆₀ - A ₁₈₆₅ ) was subtracted. Then, the ratio between (A _{butyl acrylate} - A ₁₈₆₅ ) and (A ₂₆₆₀ - A ₁₈₆₅ ) was calculated.

(2) Ethylene copolymers containing ethyl acrylate

For FTIR measurements, film samples of polymers were prepared: for the ethylene-ethyl acrylate copolymer, a thickness of 0.5 mm was used.

Following FT-IR analysis, the maximum absorption peak for ethyl acrylate was determined at 3450 cm ^-1 with a linear baseline correction applied between approximately 3205 and 3295 cm ^-1 (A _{ethyl acrylate} ). Then, the maximum absorption peak for polyethylene was determined at 2020 cm ^-1 with a baseline correction applied between approximately 1975 and 2120 cm ^-1 (A ₂₀₂₀ ). Then, the ratio between (A _{ethyl acrylate} ) and (A ₂₀₂₀ ) was calculated using a standard method well known in the literature.

(3) Ethylene copolymers containing methyl acrylate

For FTIR measurements, polymer film samples were prepared: 0.1 mm thickness was used for ethylene-methyl acrylate copolymer with methyl acrylate content >8 wt.%, and 0.05 mm thickness was used for ethylene-methyl acrylate copolymer with methyl acrylate content <8 wt.%. After analysis, the maximum peak of absorption index for methyl acrylate content >8 wt.% at 3455 cm ^-1 was subtracted by the absorbance value for the baseline at 3510 cm ^-1 (A _{methyl acrylate} - A ₃₅₁₀ ). Then, the maximum peak of absorption index for the polyethylene peak at 2675 cm ^-1 was subtracted by the absorbance value for the baseline at 2450 cm ^-1 (A ₂₆₇₅ - A ₂₄₅₀ ). The ratio between (A _{methyl acrylate} - A ₃₅₁₀ ) and (A ₂₆₇₅ - A ₂₄₅₀ ) was then calculated in the usual way, which is well known in the literature.

From the maximum absorption value for the peak for the methyl acrylate comonomer content <8 wt.% at 1164 cm ^-1 , the absorbance value for the baseline at 1850 cm ^-1 (A _{methyl acrylate} - A ₁₈₅₀ ) was subtracted. Then, from the maximum absorption peak for the polyethylene peak at 2665 cm ^-1 , the absorbance value for the baseline at 1850 cm ^-1 (A ₂₆₆₅ - A ₁₈₅₀ ) was subtracted. Then, the ratio between (A methyl acrylate - A ₁₈₅₀ ) and (A ₂₆₆₅ - A ₁₈₅₀ ) was calculated.

ASTM D3124-98 and ASTM D6248-98 methods for determining the amount of double bonds in a polymer, i.e., in polyethylene

ASTM D3124-98 and ASTM D6248-98 are used to determine double bonds in LDPE Component (i). LDPE Component (i) is referred to as the "polymer" in this method description.

ASTM D3124-98 and ASTM D6248-98 methods include, on the one hand, a procedure for determining the number of double bonds/1000 C atoms based on ASTM D3124-98. ASTM D3124-98 method provides a detailed description for the determination of vinylidene groups/1000 C atoms based on 2,3-dimethyl-1,3-butadiene. ASTM D6248-98 method provides detailed descriptions for the determination of vinyl and trans-vinylene groups/1000 C atoms based on 1-octene and trans-3-hexene, respectively. The sample preparation procedures described therein were applied here to determine vinyl groups/1000 C atoms, vinylidene groups/1000 C atoms, and trans-vinylene groups/1000 C atoms in the present invention. ASTM D6248-98 allows for the possible inclusion of the bromination procedure of ASTM D3124-98, but the samples according to this invention were not brominated. To determine the extinction coefficient for these three types of double bonds, the following three compounds were used: 1-decene for vinyl, 2-methyl-1-heptene for vinylidene, and trans-4-decene for trans-vinylene, and the procedures described in ASTM D3124-98 and ASTM D6248-98 were followed with the above exception.

The total number of vinyl bonds, vinylidene bonds and trans-vinylene double bonds of the "polymer" was analyzed by IR spectrometry and reported as the number of vinyl bonds, vinylidene bonds and trans-vinylene bonds per 1000 carbon atoms.

The polymer being analyzed was pressed into thin films 0.5–1.0 mm thick. The actual thickness was measured. FT-IR analysis was performed on a Perkin Elmer Spectrum One. Two scans were recorded at a resolution of 4 cm ^-1 .

1) Polymer compositions containing polyethylene homopolymers and copolymers or polyethylene homopolymers and copolymers, with the exception of polyethylene copolymers with >0.4 wt. % polar comonomer

For polyethylenes, three types of C=C containing functional groups were quantified, each with a characteristic absorption, and each was calibrated against its model compound, resulting in individual extinction coefficients:

- vinyl (R-CH=CH2) at 910 cm ^-1 based on 1-decene [dec-1-ene], which gives E = 13.13 l⋅mol ^-1 ⋅mm ^-1 ,

- vinylidene (RR'C=CH2) at 888 cm ^-1 based on 2-methyl-1-heptene [2-methylhept-1-ene], which gives E = 18.24 l⋅mol ^-1 ⋅mm ^-1 ,

- trans-vinylene (R-CH=CH-R') at 965 cm ^-1 based on trans-4-decene [(E)-dec-4-ene], which gives E = 15.14 l⋅mol ^-1 ⋅mm ^-1 .

For polyethylene homopolymers or copolymers with <0.4 wt% polar comonomer, a linear base correction was applied between approximately 980 and 840 cm ^-1 .

2) Polymer compositions containing polyethylene copolymers, or polyethylene copolymers with >0.4 wt. % polar comonomer

For polyethylene copolymers with >0.4 wt.% polar comonomer, two types of C=C containing functional groups were quantified, each with a characteristic absorption and each calibrated against its model compound, resulting in individual extinction coefficients:

- vinyl (R-CTNCH2) at 910 cm ^-1 based on 1-decene [dec-1-ene], which gives E = 13.13 l⋅mol ^-1 ⋅mm ^-1 ,

- vinylidene (RR'C=CH2) at 888 cm ^-1 based on 2-methyl-1-heptene [2-methylhept-1-ene], which gives E = 18.24 l⋅mol ^-1 ⋅mm ^-1 ,

EBA:

For ethylene butyl acrylate copolymer (EBA) systems, a linear baseline correction was applied between approximately 920 and 870 cm ^-1 .

EEA:

For ethylene ethyl acrylate (EEA) copolymer systems, a linear baseline correction was applied between approximately 920 and 825 cm ^-1 .

EMA:

For ethylene ethyl acrylate (EMA) copolymer systems, a linear baseline correction was applied between approximately 930 and 870 cm ^-1 .

ASTM D3124-98 and ASTM D6248-98 methods, on the other hand, also include a procedure for determining the molar extinction coefficient. At least three 0.18 mol⋅L ^-1 solutions in carbon disulfide ( _CS2 ) were used, and the average molar extinction coefficient was used.

The number of vinyl groups originating from the polyunsaturated comonomer per 1000 carbon atoms was determined and calculated as follows.

The polymer to be analyzed and the reference polymer were produced in the same reactor, using essentially the same conditions (i.e., similar peak temperatures, pressures, and production rates), with the only difference being that the polyunsaturated comonomer was added to the polymer to be analyzed and not to the reference polymer. The total amount of vinyl groups in each polymer was determined using FT-IR measurements, as described herein. It was then assumed that the baseline level of vinyl groups—that is, those formed during the process and from chain transfer agents leading to vinyl groups (if present)—was the same for the reference polymer and the polymer to be analyzed, with the only exception being that the polyunsaturated comonomer was also added to the reactor for the polymer to be analyzed. This baseline is then subtracted from the measured amount of vinyl groups in the polymer being analyzed, resulting in the number of vinyl groups/1000 C atoms that are produced by the polyunsaturated comonomer.

Wet aging test in salt water

The wet aging properties of cables in salt water are assessed using the Mode A procedure described in Cigre technical brochure 722 "Recommendations for additional testing for submarine cables from 6 kV (Um=7.2 kV) up to 60 kV (Um=72.5 kV)", issued in April 2018.

Preliminary preparation:

The cable is immersed in a tank of water for 500 hours at a temperature of 55°C. The NaCl content in the water is 3.5 wt.%.

Aging

The cables are subjected to electrical aging in a water tank. The water temperature is 40°C, and the applied voltage at 50 Hz is 38.5 kV, which equals a conductor strain of 9.1 kV/mm. The NaCl content in the water is 3.5 wt.%.

AC breakdown test: AC breakdown tests after 1 year of aging were carried out in accordance with Cigre Technical Brochure 722, Section 3.6.4.1, and HD 605 5.4.15.3.4 (b). The cable was therefore cut into six test specimens with an active length of 10 meters (in addition to the end sections). The specimens were tested for breakdown by stepwise application of an alternating current of 50 Hz for 72 hours after removal from the aging tank, according to the following procedure:

- Start at 36 kV for 5 minutes;

- Increase voltage in 12 kV steps every 5 minutes until breakdown occurs.

The calculation of the Weibull parameters for a data set of six breakdown values follows the least squares regression procedure described in IEC 62539 (2007).

Experimental part

The following materials were used in these examples:

EEA 1 is a copolymer of LDPE, ethylene, and 15 wt.% ethyl acrylate comonomer (i.e., a polar comonomer) produced in a high-pressure process. The MFR 2 is approximately 7 g/10 min.

LDPE1 is a copolymer of LDPE m3 ethylene and 1,7-octadiene comonomer (i.e. polyunsaturated comonomer) with a vinyl group content of approximately 0.55 vinyl/1000 C and a MFI2 of approximately 2 g/10 min, obtained in a high pressure process.

DCP: dicumyl peroxide

Two 20 kV cables with the following dimensions (described below) were extruded on a pilot CCV line at a line speed of 2.79 m/min and the following heating zone temperatures: 460/400/385/375°C in the vulcanizing tube, followed by water-cooling of the cable core. The same semiconducting layers were used in the two 20 kV cables (insulation thickness of approximately 5.5 mm) for both the inventive example and the comparative example.

The inner and outer semiconductive layers include LE0595, supplied by Borealis (which contains carbon black and peroxide). The cables are available in the following sizes:

^150mm2 aluminum conductor;

Inner semiconductive layer: thickness approximately 1mm;

Insulation layer: thickness 5.5-5.6 mm;

Outer semiconductive layer: thickness approximately 0.8 mm.

The insulating layer in the comparative example is LE4212, a crosslinkable water tree retardant (WTR) additive supplied by Borealis. The dielectric strength was determined for 20 kV cables according to the invention and comparison cables after one year of wet aging in salt water, as described above.

The one-year test results after the electrical breakdown (Eb) test show a clear advantage of using a polymer composite containing a polymeric water tree retarder (WTR) compared to a polymer composite containing a WTR additive. In a cable with polymeric WTR insulation, the Weibull Eb (63.2%) value is 64.1 kV/mm after one year of aging in humid conditions.

In a cable with WTR-added insulation, the Weibull Eb value (63.2%) is 51.2 kV/mm after one year of wet aging. These breakdown strength values refer to the electrical voltage at the conductor's breakdown voltage. This clearly demonstrates the advantage of the WTR polymer composite in long-term wet aging tests in a saltwater environment. Table 2 below shows the measured Eb values.

Claims (37)

1. Application of cable for transmission of electric power in salt water environment in the sea or under the sea; wherein said cable comprises a conductor which is surrounded by at least an inner semiconducting layer, an insulating layer and an outer semiconducting layer in the said order; wherein said insulating layer includes (i) at least 60% by weight of a low density polyethylene homopolymer or a low density polyethylene copolymer with at least one polyunsaturated comonomer and possibly one or more additional comonomers and (ii) from 10 to 35 wt.% of a low-density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate. 2. A cable for transmitting electric power at sea or under the sea, comprising a conductor which is surrounded by at least an inner semi-conducting layer, an insulating layer and an outer semi-conducting layer, in that order; wherein said insulating layer contains (i) at least 60% by weight of a low density polyethylene homopolymer or a low density polyethylene copolymer with at least one polyunsaturated comonomer and possibly one or more additional comonomers and (ii) from 10 to 35 wt.% of a low density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate; and wherein said inner and outer semiconducting layers independently include: (a) a low density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate; and (b) carbon black. 3. The cable according to claim 2, wherein the inner semiconductive layer has the same chemical composition as the outer semiconductive layer. 4. A cable according to claim 2 or 3, wherein the inner and/or outer semiconducting layer comprises an ethylene-alkyl acrylate copolymer or an ethylene-vinyl acetate copolymer. 5. A cable according to any one of paragraphs 2-4, in which component (ii) of the insulating layer is a copolymer of ethylene and alkyl acrylate. 6. A cable according to claim 4 or 5, wherein the ethylene-alkyl acrylate copolymer is an ethylene-methyl acrylate copolymer, an ethylene-ethyl acrylate copolymer, or an ethylene-butyl acrylate copolymer. 7. A cable according to any one of claims 2 to 5, wherein the polyunsaturated comonomer of the LDPE copolymer component (i) is a linear carbon chain with at least 8 carbon atoms and at least 4 carbon atoms between non-conjugated double bonds, of which at least one is terminal. 8. The cable of claim 7, wherein the polyunsaturated comonomer of the LDPE copolymer component (i) is a C ₈ - C ₁₄ non-conjugated diene, preferably selected from 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene, 7-methyl-1,6-octadiene, 9-methyl-1,8-decadiene or mixtures thereof. 9. A cable according to any one of paragraphs 2-8, wherein the conductor comprises aluminum. 10. A cable according to any of paragraphs 2-9, which does not contain a moisture-proof layer. 11. A cable according to any of paragraphs 2-10, which is stitchable. 12. The cable of claim 11, wherein the insulating layer, inner and outer semiconducting layer comprise a peroxide. 13. The cable according to claim 12, wherein said inner and/or outer semiconductive layer independently include: (a) at least 50% by weight of a copolymer of LDPE with a polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate, (b) 25-48 wt.% carbon black and (c) 0.1-2.5 wt.% peroxide, and wherein said insulating layer includes (i) at least 60% by weight of a low density polyethylene homopolymer or a low density polyethylene copolymer with at least one polyunsaturated comonomer and possibly one or more additional comonomers; (ii) from 10 to 35 wt.% of a low density polyethylene copolymer of ethylene and at least one polar comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate or vinyl acetate, and (iii) 0.1-2.5 wt.% peroxide. 14. A cable according to any of paragraphs 2-10, which is cross-linked. 15. The cable according to item 14, which is an AC power cable. 16. Use of a cable according to any of paragraphs 2-15 for transmitting electrical energy in a salt water environment at sea or under the sea. 17. A method for connecting a marine device to a land device or another marine device, which comprises laying a cable in the ground according to any of paragraphs 2-15 under the sea, or laying a cable according to any of paragraphs 2-15 on the seabed and transmitting electrical energy from the said marine device to the said land device. 18. An electric power generating system, including: (A) a marine power generator such as a wind turbine; (B) a cable according to any of paragraphs 2 to 15, transmitting electrical energy from the said marine generator to a substation located on land and/or at sea, through the seabed.