BR112018076291B1 - CABLE WITH IMPROVED ELECTRICAL PROPERTIES - Google Patents

Abstract

It is a cable comprising a conductor surrounded by at least an inner semiconductor layer, an insulating layer and an outer semiconducting layer, in that order, wherein the insulating layer is not crosslinked or is crosslinked and comprises at least 90% by weight of a polymer composition, said polymer composition comprising: (I) 95.5 to 99.9% by weight of an LDPE; and (II) 0.1 to 4.5% by weight of an HDPE having a density of at least 940 kg/m3.

Description

[0001] This invention relates to the use of blends comprising low density polyethylene polymers (LDPE) in the insulation layers of cables such as DC cables. In particular, the invention relates to the combination of LDPEs with small amounts of a secondary HDPE polymer in order to surprisingly make possible the formation of cable insulation layers with extremely low conductivity.

[0002] The mixtures of the invention can be used in crosslinked or non-crosslinked form, in the latter case avoiding the need for a crosslinking agent to be present and avoiding the need for a subsequent crosslinking degassing procedure to remove crosslinking agent by-products.

[0003] Whether cross-linked or not cross-linked, the conductivity of the resulting composition and hence the conductivity of the insulating layer is lower than using the corresponding LDPE alone regardless of very low levels of HDPE being employed.

Background

[0004] Polyolefins produced in a high pressure (HP) process are widely used in stringent polymer applications, where polymers must meet high mechanical and/or electrical demands. For example, in power cable applications, particularly in medium voltage (MV) and especially in high voltage (HV) and extra high voltage (EHV) cable applications, the electrical properties of the polymer composition used in the cable are of significant importance. Furthermore, electrical properties of importance may differ in different cable applications, as is the case between alternating current (AC) and direct current (DC) cable applications.

[0005] A typical power cable comprises a conductor surrounded by at least an inner semiconductor layer, an insulating layer and an outer semiconductor layer, in that order. Cables are commonly produced by extruding layers into a conductor.

[0006] The polymer material in one or more of said layers is generally cross-linked to improve, for example, heat and deformation resistance, displacement properties, mechanical strength, chemical resistance and abrasion resistance. During the crosslinking reaction, crosslinks (bridges) are mainly formed. Crosslinking can be effected using, for example, a free radical generating compound which is typically incorporated into the layer material prior to extrusion of the layer (or layers) onto a conductor. After forming the cable in layers, the cable is then subjected to a cross-linking step to initiate radical formation and thus the cross-linking reaction.

[0007] Peroxides are very commonly used as free radical generating compounds. The decomposition products resulting from peroxides can include volatile by-products which are generally unwanted as, for example, they can have a negative influence on the electrical properties of the cable. Therefore, volatile decomposition products such as methane are conventionally reduced to a minimum or removed after a cooling and cross-linking step. Such a removal step, generally known as a degassing step, is time consuming and energy consuming causing extra costs. It will be appreciated that a cross-linked polyethylene material is thermostable.

[0008] LDPE is also an ideal cable forming material from a cleaning point of view. LDPE can be manufactured in very pure form without impurities. In contrast, low pressure polymers generally contain more catalyst residues and gels, which can lead to cable defects.

[0009] In order to increase the power transmission capacity of extruded high voltage direct current (HVDC) cables, the voltage needs to be increased. In HVDC cables, the insulation is heated by the leakage current. Heating is proportional to insulation conductivity x voltage2. Thus, if the voltage is increased, more heat will be generated. This can lead to thermal breakdown followed by electrical outage. Thus, in order to increase the power transmission capacity, insulation material with very low electrical conductivity is needed. In one embodiment, the voltage can be increased from the day's highest level of 320 kV to 640 kV or more.

[0010] The present inventors are now investigating the possibility of reducing conductivity despite the combination of LDPE with a secondary polymer. The secondary polymer, however, is typically one produced using an olefin polymerization catalyst, and consequently the content of catalyst residue can be high. This leads to a higher risk of mechanical disruption compared to pure XLPE. However, the inventors have surprisingly found that the combination of small amounts of HDPE and LDPE leads to considerable conductivity reduction in crosslinked and thermoplastic insulation layers even at very low levels of HDPE.

[0011] Thermoplastic LDPE offers several advantages as cable insulation compared to a thermostable crosslinked PE. As the polymer is not crosslinked, there is no possibility of peroxide initiated abrasion. Furthermore, in the degassing step it is necessary to remove peroxide decomposition products. Eliminating crosslinking and degassing steps can lead to faster, less complicated and more cost-effective cable production. However, the absence of a cross-linked material can lead to reduced temperature resistivity and hence significant problems with displacement. Therefore, better thermomechanical properties are needed in order to provide a polymer material that can be used without crosslinking in a cable insulation layer.

[0012] The present inventors have now found that combining an LDPE with a low amount of an HDPE can provide a mixture, such as a thermoplastic mixture, which is ideally suited for cable manufacture. Surprisingly, these blends have much lower conductivity than the corresponding LDPE alone and do not suffer from dielectric breakdown. Furthermore, as the HDPE content is very low, this leads to a reduced risk of mechanical disruption caused by the presence of the less pure HDPE with the LDPE.

[0013] Furthermore, the inventors have found that certain LDPEs can be combined with small amounts of HDPE to form a mixture that has extremely low conductivity in crosslinked form.

[0014] The LDPE used in the invention, by itself, is not new and has been previously proposed in the literature. Furthermore, the possibility of using non-crosslinked LDPE in the insulation layer of a cable is not new. In WO2011/113685, LDPE of density 922 kg/m3 and MFR2 of 1.90 g/10 min is suggested for use in the insulation layer of a cable. WO2011/113685 also suggests the use of other polymers individually in the non-crosslinked insulation layer of a cable.

[0015] In WO2011/113686, a mixture of LDPE and HDPE is used to manufacture a cross-linked polymer composition that can be used in the insulation layer of a cable, however, the amounts of HDPE taught are relatively high (minimum 5% by weight).

[0016] In document US2013/0175068 there is a disclosure of the use of HDPE and LDPE to improve resistance to disruption in thermoplastic cables. 20% by weight HDPE is exemplified in the examples.

[0017] The blends of the invention are therefore ideal for use in the insulation layer in a direct current (DC) power cable or AC power cable and the blends enable the cables to operate at higher voltages than is currently possible.

summary of invention

[0018] Viewed from one aspect, the invention provides a cable comprising one or more conductors surrounded by at least an inner semiconductor layer, an insulating layer and an outer semiconducting layer, in that order, wherein the insulating layer is uncrosslinked or is crosslinked and comprises at least 90% by weight of a polymer composition, wherein said polymer composition comprises:

[0019] (I) 95.5 to 99.9% by weight of an LDPE; It is

[0020] (II) 0.1 to 4.5% by weight of an HDPE having a density of at least 940 kg/m3.

[0021] Viewed from one aspect, the invention provides a cable comprising one or more conductors surrounded by at least an inner semiconductor layer, an insulating layer and an outer semiconducting layer, in that order, wherein the insulating layer is crosslinked and comprises at least 90% by weight of a polymer composition, wherein said polymer composition comprises:

[0022] (I) 95.5 to 99.9% by weight of an LDPE; It is

[0023] (II) 0.1 to 4.5% by weight of an HDPE having a density of at least 940 kg/m3.

[0024] Viewed from another aspect, the invention provides a cable comprising one or more conductors surrounded by at least an inner semiconductor layer, an insulating layer and an outer semiconducting layer, in that order, wherein the insulating layer is not crosslinked and comprises at least 90% by weight of a polymer composition, wherein said polymer composition comprises:

[0025] (I) 95.5 to 99.9% by weight of an LDPE; It is

[0026] (II) 0.1 to 45% by weight of an HDPE, preferably a multimodal HDPE, having a density of at least 940 kg/m3.

[0027] When the insulation layer is not crosslinked, the layer should be free of any crosslinking agent such as a peroxide.

[0028] In one embodiment, the HDPE(II) component is bimodal, especially where the insulating layer is not crosslinked.

[0029] In particular, the cable of the invention is a direct current (DC) power cable, preferably operating at or capable of operating at 320 kV or more such as 650 kV or more.

[0030] Viewed from another aspect, the invention provides a process for producing a cable comprising the steps of:

[0031] - apply to one or more conductors, preferably by (co)extrusion, an inner semiconductor layer, an insulating layer and an outer semiconducting layer, in that order, in which the insulating layer comprises at least 90% by weight of a polymer composition, as previously defined in this document, and optionally crosslinking the layer.

[0032] Viewed from another aspect, the invention provides the use of a composition, as previously defined in this document, in the insulation layer of a cable.

[0033] In all embodiments of the invention, it is preferred if the polymer composition of the insulation layer has a conductivity of 1.5 fS/m or less when measured according to the DC conductivity method as described under "Method A of Determination".

[0034] In all embodiments, it is preferred if the conductivity of the polymer composition of the insulation layer is 3.5 x 10-17 S/cm or less when measured according to the DC conductivity method as described under “B Methods of Determination”.

Definitions

[0035] Cable layers or non-crosslinked polymer compositions are considered as thermoplastics.

[0036] The polymer composition of the invention may also be called a polymer blend herein. These terms are used interchangeably.

[0037] The low density polyethylene, LDPE, of the invention is a polyethylene produced in a high pressure process. Typically, the polymerization of ethylene and optionally additional comonomer (or comonomers) in a high pressure process is carried out in the presence of an initiator (or initiators). The meaning of the term LDPE is well known and documented in the literature. The term LDPE describes and distinguishes a high pressure polyethylene from polyethylenes produced in the presence of an olefin polymerization catalyst. LDPEs have certain typical features, such as different branching architecture.

Detailed description of the invention

[0038] The present invention necessitates the use of a particular polymer composition comprising low density polyethylene (LDPE) and small amounts of HDPE in the insulation layer of a cable, especially a power cable such as a direct current (DC) power cable. Unexpectedly, the combination of small amounts of HDPE with LDPE makes it possible to form an insulating layer that has reduced electrical conductivity, i.e. low electrical conductivity. "Reduced" or "low" electrical conductivity as used interchangeably herein means that the value obtained from measuring DC conductivity, as defined below under "Methods of Determination" is low, ie reduced. Low electrical conductivity is beneficial for minimizing unwanted heat build-up in a cable's insulating layer.

[0039] Furthermore and unexpectedly, some of the LDPE compositions of the invention and consequently the insulation layer of the cable of the invention have low electrical conductivity without the need for crosslinking. Furthermore, the insulation still has the necessary mechanical properties for an insulating layer of a cable, preferably a DC power cable, such as low bending modulus, good tensile modulus, and good stress slip.

[0040] It is also considerable that such low levels of conductivity are achievable at such low levels of HDPE addition. Due to the very low amount of HDPE added, the mixture is “cleaner”. LDPE can be manufactured in very pure form without impurities, but as soon as LDPE is mixed with a low pressure polymer like HDPE, more catalyst residues and gels are introduced, which can lead to defects in the composition. It is therefore preferred if the addition of the HDPE is kept to a minimum to maximize the purity of the insulation layer. However, it was realized that low levels of HDPE addition would not lead to clear improvements in conductivity.

[0041] Notably, it has been observed that at very low levels of HDPE addition, conductivity is comparable to or sometimes better (ie, lower) than the conductivity achieved at higher HDPE loading. Consequently, one can minimize the conductivity and prepare a defect-free, purer insulation layer.

LDPE

[0042] The low density polyethylene, LDPE, of the invention is a polyethylene produced in a high pressure process. Typically, the polymerization of ethylene and optionally additional comonomer (or comonomers) in a high pressure process is carried out in the presence of an initiator (or initiators). The meaning of the term LDPE is well known and documented in the literature. The term LDPE describes and distinguishes a high pressure polyethylene from polyethylenes produced in the presence of an olefin polymerization catalyst. LDPEs have certain typical features, such as different branching architecture.

LDPE homopolymer or copolymer

[0043] The LDPE used in the composition of the invention may have a density of 915 to 940 kg/m3, preferably 918 to 935 kg/m3, especially 920 to 932 kg/m3, such as about 922 to 930 kg/m3.

[0044] The LDPE polymer of the invention may be one having a high density. The LDPE polymer density is preferably 927 to 940 kg/m3, preferably 928 to 935 kg/m3, especially 929 to 932 kg/m3, such as about 930 kg/m3. In particular, this higher density range is employable with crosslinked multimodal HDPE polymer.

[0045] The MFR2 (2.16 kg, 190 °C) of the LDPE polymer is preferably 0.05 to 30.0 g/10 min, more preferably it is 0.1 to 20 g/10 min, and most preferably it is 0.1 to 10 g/10 min, especially 0.1 to 5.0 g/10 min. In a preferred embodiment, the MFR2 of LDPE is 0.1 to 4.0 g/10 min, especially 0.5 to 4.0 g/10 min, especially 1.0 to 3.0 g/10 min.

[0046] LDPE may have a tensile modulus (1 mm/min ISO527-2) of at least 300 MPa, such as at least 325 MPa. Values up to 600 MPa are possible.

[0047] LDPE may have a bending modulus (ISO178) of at least 300 MPa, such as at least 320 MPa. Values up to 600 MPa are possible.

[0048] It is possible to use a mixture of LDPEs in the polymer composition of the invention, however, it is preferred if a single LDPE is used.

[0049] The LDPE can be a low density homopolymer of ethylene (hereinafter called LDPE homopolymer) or a low density copolymer of ethylene with one or more comonomer (or comonomers) (herein called LDPE copolymer). The one or more comonomers of the LDPE copolymer are preferably selected from polar comonomer (or comonomers), non-polar comonomer (or comonomers) or from a mixture of polar comonomer (or comonomers) and non-polar comonomer(s). Furthermore, said LDPE homopolymer or LDPE copolymer may optionally be unsaturated.

[0050] As a polar comonomer for LDPE copolymer, comonomer (or comonomers) containing hydroxyl group (or groups), alkoxy group (or groups), carbonyl group (or groups), carboxyl group (or groups), ether group (or groups) or ester group (or groups), or a mixture thereof, can be used. More preferably, comonomer (or comonomers) containing ester and/or carboxyl group (or groups) are used as said polar comonomer. Even more preferably, the polar comonomer (or comonomers) of LDPE copolymer is selected from the groups of acrylate (or acrylates), methacrylate (or methacrylates) or acetate (or acetates), or any mixtures thereof.

[0051] If present in said LDPE copolymer, the polar comonomer (or comonomers) is preferably selected from the group of alkyl acrylates, alkyl methacrylates or vinyl acetate, or a mixture thereof. Still preferably, said polar comonomers are selected from C1- to C6-alkyl acrylates, C1- to C6-alkyl methacrylates or vinyl acetate. Even more preferably, said LDPE copolymer is a copolymer of ethylene with C1- to C4-alkyl acrylate, such as methyl, ethyl, propyl or butyl acrylate, or vinyl acetate, or any mixture thereof.

[0052] As the non-polar comonomer (or comonomers) for the LDPE copolymer, comonomer (or comonomers) other than the polar comonomers defined above can be used. Preferably, non-polar comonomers are other than comonomer (or comonomers) that contain hydroxyl group (or groups), alkoxy group (or groups), carbonyl group (or groups), carboxyl group (or groups), ether group (or groups) or ester group (or groups). A preferred non-polar comonomer (or comonomers) group preferably comprises monounsaturated (= one double bond) comonomer (or comonomer) comonomer (or comonomer) preferably olefins, preferably alpha-olefins, most preferably C3 to C10 alpha-olefins such as propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, styl rene, 1-octene, 1-nonene; polyunsaturated comonomer (or comonomers) (= more than one double bond); a silane group containing comonomer (or comonomers); or any mixtures thereof. The polyunsaturated comonomer (or comonomers) is further described below in relation to unsaturated LDPE copolymers.

[0053] If the LDPE polymer is a copolymer, it preferably comprises from 0.001 to 35% by weight, even more preferably less than 30% by weight, most preferably less than 25% by weight, of one or more comonomers. Preferred ranges include 0.5 to 10% by weight, such as 0.5 to 5% by weight of comonomer.

[0054] The LDPE polymer may optionally be unsaturated, i.e. it may comprise carbon-carbon double bonds (-C=C-). Preferred "unsaturated" LDPEs contain carbon-carbon/1000 carbon double bonds in a total amount of at least 0.4/1000 carbon atoms. If an uncrosslinked LDPE is used in the final cable, then the LDPE is typically unsaturated as defined above. Unsaturated means that the C=C content is preferably less than 0.2/1000 carbon atoms, such as 0.1/1000 C atoms or less.

[0055] As is well known, unsaturation can be provided to the LDPE polymer by means of comonomers, a low molecular weight (Mw) additive compound such as a crosslink enhancer, abrasion retardant additive or CTA, or any combination thereof. The total amount of double bonds herein means double bonds added by any means. If two or more above sources of double bonds are chosen to be used to supply the unsaturation, then the total amount of double bonds in the LDPE polymer means the sum of the double bonds present. Any double bond measurements are performed before optional crosslinking.

[0056] The term "total amount of carbon-carbon double bonds" refers to the combined amount of double bonds that originate from vinyl groups, vinylidene groups and trans-vinylene groups, if present.

[0057] If an LDPE homopolymer is unsaturated, then unsaturation may be provided, for example, by a chain transfer agent (CTA) such as propylene, and/or by polymerization conditions. If an LDPE copolymer is unsaturated, then the unsaturation can be provided by one or more of the following means: by a chain transfer agent (CTA), by one or more polyunsaturated comonomers, or by polymerization conditions. It is well known that selected polymerization conditions such as pressure and peak temperatures can have an influence on the level of unsaturation. In the case of an unsaturated LDPE copolymer, it is preferably an ethylene unsaturated LDPE copolymer with at least one polyunsaturated comonomer and optionally with another comonomer (or comonomers), such as polar comonomer (or comonomers) which is preferably selected from acrylate or acetate comonomer (or comonomers). More preferably, an unsaturated LDPE copolymer is an ethylene unsaturated LDPE copolymer with at least polyunsaturated comonomer (or comonomers).

[0058] The polyunsaturated comonomer suitable for the second unsaturated polyolefin (b) preferably consists of a linear carbon chain with at least 8 carbon atoms and at least 4 carbons between the non-conjugated double bonds, of which at least one is terminal, more preferably, said polyunsaturated comonomer is a diene, preferably a diene comprising at least eight carbon atoms, the first carbon-carbon double bond being terminal and the second carbon-carbon double bond is unconjugated with respect to the first. Preferred dienes are selected from C8 to C14 unconjugated dienes or mixtures thereof, more 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. Even more preferably, the diene is selected from 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene, or any mixture thereof, however without limitation to the above dienes.

[0059] It is well known that, for example, propylene can be used as a comonomer or as a chain transfer agent (CTA), or both, thus it can contribute to the total amount of carbon-carbon double bonds, preferably to the total amount of vinyl groups. In the present document, when a compound which can also act as a comonomer, such as propylene, is used as CTA to provide double bonds, then said copolymerizable comonomer is not calculated for comonomer content.

[0060] If LDPE polymer is unsaturated, then it preferably has a total amount of carbon-carbon double bonds, which originates from vinyl groups, vinylidene groups and trans-vinylene groups, if present, of more than 0.4/1000 carbon atoms, preferably more than 0.5/1000 carbon atoms. The upper limit of the amount 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.

[0061] In some embodiments, for example, where higher crosslinking level with low peroxide content is desired, the total amount of carbon-carbon double bonds, which originate from vinyl groups, vinylidene groups and trans-vinylene groups, if present, in the unsaturated LDPE, is preferably greater than 0.40/1000 carbon atoms, preferably greater than 0.50/1000 carbon atoms, preferably greater than 0.60/1000 carbon atoms.

[0062] If the LDPE is unsaturated LDPE as defined above, it preferably 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. Preferably, the total amount of vinyl groups is less than 4.0/1000 carbon atoms. More preferably, the second polyolefin (b), before crosslinking, contains vinyl groups in total amount of more than 0.20/1000 carbon atoms, even more preferably more than 0.30/1000 carbon atoms.

[0063] It is, however, preferred if the LDPE of the invention is unsaturated and has less than 0.2 C=C/1000 C atoms, preferably less than 0.1 C=C/1000 C atoms. It is also preferred if the LDPE is a homopolymer. As the polymer composition of the invention is not designed for crosslinking, the presence of unsaturation within the LDPE is not necessary or desired.

[0064] The LDPE polymer can have a high melting point, which can be of importance especially for a thermoplastic insulation material. Melting points of 112°C or higher are predicted, such as 114°C or higher, especially 116°C or higher, such as 112 to 125°C.

[0065] The LDPE polymer is produced at high pressure by free radical initiated polymerization (called high pressure radical (HP) polymerization). The HP reactor can be, for example, a well known autoclave or tube reactor or a mixture thereof, preferably a tube reactor. High pressure (HP) polymerization and adjustment of process conditions to further adjust the other properties of the polyolefin depending on the desired end application are well known and described in the literature, and can be readily used by one skilled in the art. Suitable polymerization temperature range up to 400 °C, preferably 80 to 350 °C and pressure of 70 MPa, preferably 100 to 400 MPa, more preferably 100 to 350 MPa. The pressure can be measured at least after the compression stage and/or after the tubular reactor. The temperature can be measured at several points during all stages.

[0066] After separation, the obtained LDPE is typically in a form of a molten polymer which is usually mixed and pelletized in a pelletizing section such as a pelletizing extruder arranged in connection with the HP reactor system. Optionally, additive (or additives) such as antioxidant (or antioxidants) can be added to this mixer in a known manner.

[0067] Further details of the production of ethylene (co)polymers by high pressure radical polymerization can be found, for example, in the Encyclopedia of Polymer Science and Engineering, Vol. 6 (1986), pp 383-410 and Encyclopedia of Materials: Science and Technology, 2001 Elsevier Science Ltd.: “Polyethylene: High-pressure, R.Klimesch, D.Littmann and F.-O. Mahling pp. 7181-7184.

[0068] When an ethylene-unsaturated LDPE copolymer is prepared, then, as is well known, the carbon-carbon double bond content can be adjusted by polymerizing ethylene, for example, in the presence of one or more polyunsaturated comonomer, chain transfer agents, or both, with the use of the desired feed ratio between monomer, preferably ethylene, and polyunsaturated comonomer and/or chain transfer agent, depending on the nature and amount of C-C double bonds desired for the unsaturated LDPE copolymer. For example, WO 9308222 describes a high pressure radical polymerization of ethylene with polyunsaturated monomers. As a result, unsaturation can be evenly distributed along the polymer chain in a random copolymerization manner.

[0069] The polymer composition of the invention preferably comprises from 95.5 to 99.8% by weight of the LDPE. Preferably, the composition comprises from 96.0 to 99.5% by weight, such as from 96.5 to 99.5% by weight of the LDPE, especially from 97.0 to 99.0% by weight.

High density polyethylene component

[0070] The composition of the invention may include a high density polyethylene component which may be unimodal or multimodal. When the insulation layer is thermoplastic, a multimodal HDPE is preferably used. When the insulation layer is crosslinked, either a unimodal insulation layer or a multimodal insulation layer can be used, preferably a unimodal HDPE. The polymer is one that has a density of at least 940 kg/m3.

[0071] The term "multimodal" herein means, unless otherwise indicated, multiple modalities with respect to molecular weight distribution and therefore includes a bimodal polymer. In general, a polyethylene composition, which comprises at least two polyethylene fractions, which were produced under different polymerization conditions resulting in different molecular weight distributions and molecular weights (average weight) for the fractions, is called "multimodal". The prefix "multi" refers to the number of different polymer fractions present in the polymer. Thus, for example, multimodal polymer includes so-called "bimodal" polymer consisting of two fractions. The shape of the molecular weight distribution curve, i.e., the appearance of the graph of the polymer weight fraction as a function of its molecular weight, of a multimodal polymer will show two or more maxima or is typically magnified differently compared to the curves for the individual fractions. For example, if a polymer is produced in a sequential multistage process, using reactors coupled in series and using different conditions in each reactor, the polymer fractions produced in the different reactors will each have their own molecular weight distribution and weight average molecular weight. When the molecular weight distribution curve of such a polymer is plotted, the individual curves of these fractions typically form together an expanded molecular weight distribution curve for the total resulting polymer product.

[0072] A unimodal polymer, unless otherwise noted, is unimodal with respect to molecular weight distribution and therefore contain a single peak present is GPC curve.

[0073] The HDPE component (II) of the mixture of the invention is preferably present in an amount of from 0.2 to 4.5% by weight, such as from 0.5 to 4.0% by weight, preferably from 0.5 to 3.5% by weight, such as from 1.0 to 3.0% by weight.

[0074] The HDPE preferably has a density according to ISO 1183 at 23°C of at least 940 kg/m3, preferably at least 945 kg/m3. The upper limit for density can be by 980 kg/m3, preferably 975 kg/m3, especially 970 kg/m3. A highly preferred density range is 945 to 965 kg/m3, such as 954 to 965 kg/m3.

[0075] The MFR2 according to ISO 1133 of HDPE is preferably in the range from 0.1 to 40 g/10 min, preferably from 0.25 to 20 g/10 min. Preferably, the HDPE has an MFR2 of 0.3 to 15 g/10 min. An especially preferred range is 0.4 to 15 g/10 min.

[0076] In another embodiment, the HDPE may have an MFR21 according to ISO 1133 HDPE is preferably in the range of 8 to 30 g/10 min, preferably 10 to 20 g/10 min.

[0077] In some embodiments of the invention, it is preferred if the HDPE is a multimodal polyethylene comprising at least (i) a lower weight average molecular weight ethylene polymer or homopolymer component, and (ii) a higher weight average molecular weight ethylene polymer or homopolymer component. Preferably, at least one of said LMW and HMW components is a copolymer of ethylene with at least one comonomer. It is preferred that at least said HMW component is an ethylene copolymer. Alternatively, if one of said components is a homopolymer, then said LMW is preferably the homopolymer.

[0078] Said multimodal polymer LMW component preferably has an MFR2 of at least 5 g/10 min, preferably at least 50 g/10 min, more preferably at least 100 g/10 min.

[0079] The LMW component density of said multimodal polymer may be in the range of 950 to 980 kg/m3, for example, 950 to 970 kg/m3.

[0080] The LMW component of said multimodal polymer may form 30 to 70% by weight, for example 40 to 60% by weight of the multimodal polymer, wherein the HMW component forms 70 to 30% by weight, for example 60 to 40% by weight. In one embodiment, said LMW component forms 50% by weight or more of the multimodal polymer as defined above or below. Typically, the LMW component forms 45 to 55% and the HMW component forms 55 to 45% of the multimodal polymer.

[0081] The HMW component of said HDPE has a lower MFR2 than the LMW component.

[0082] The HDPE may be an ethylene homopolymer or copolymer. Ethylene homopolymer means a polymer that is formed essentially only of ethylene monomer units, i.e., is 99.9% by weight ethylene or greater. It will be noted that minor traces of other monomers may be present due to industrial ethylene which contains trace amounts of other monomers.

[0083] The HDPE can also be a copolymer (and is preferably a copolymer) and can therefore be formed from ethylene with at least one other comonomer, for example C3-20 olefin. Preferred comonomers are alpha-olefins, especially with 3-8 carbon atoms. Preferably, the comonomer is selected from the group consisting of propene, 1-butene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1,7-octadiene and 7-methyl-1,6-octadiene. The use of 1-hexene or 1-butene is more preferred.

[0084] The HDPE may comprise one monomer or two monomers or more than 2 monomers. The use of a single comonomer is preferred. If two comonomers are used, it is preferred if one is a C3-8 alpha-olefin and the other is a diene as defined earlier herein.

[0085] The amount of comonomer is preferably such that it comprises from 0 to 3% by mol, more preferably from 0.1 to 2.0% by mol, and most preferably from 0.1 to 1.5% by mol of the HDPE. Values under 1.0 mol% are also predicted, for example from 0.1 to 1.0 mol%. They can be determined by NMR.

[0086] However, it is preferred if the ethylene polymer of the invention comprises an LMW homopolymer component and an ethylene HMW copolymer component, for example, an ethylene hexene copolymer or an ethylene butene copolymer.

[0087] For the preparation of HDPE, polymerization methods well known to the person skilled in the art can be used. It is within the scope of the invention for a multimodal, i.e. at least bimodal, polymers to be produced by mixing each of the components in situ during their polymerization process (called in-situ process) or, alternatively, by mechanically mixing two or more separately produced components in a manner known in the art.

[0088] The polyethylenes useful in the present invention are preferably obtained by in-situ mixing in a multistage polymerization process. Consequently, polymers are obtained by in-situ mixing in a multistage polymerization process, i.e., two or more stages, including solution, slurry, and gas phase process, in any order. While it is possible to use different single-site catalysts at each stage of the process, it is preferred if the catalyst employed is the same at both stages.

[0089] In a highly preferred embodiment, the HDPE polymer of the invention is prepared by single-site catalyzed polymerization. The use of a single site catalyzed ethylene polymer provides better conductivity than, for example, a Ziegler Natta based catalyst. Additionally, the use of single site catalyzed polymer allows a smaller amount of crosslinking agent to be used to achieve the desired degree of crosslinking than, for example, a Ziegler Natta based catalyst. Most importantly, HDPE from SSC is a “cleaner” polymer, which means there is less impurity in the cable and less chance of defects. HDPE as defined herein can be produced using any conventional single site catalysts including metallocenes and non-metallocenes as well known in the field.

[0090] Preferably, said catalyst is one comprising a metal coordinated by one or more h-link ligands. Such h-linked metals are typically Group 3 to 10 transition metals, for example Zr, Hf or Ti, especially Zr or Hf. The h-linker linker is typically an h5-cyclic linker, i.e. a homo- or heterocyclic cyclopentadienyl group optionally with pendant or fused substituents. Such single-site procatalysts, preferably metallocene, have been widely described in the scientific and patent literature for about twenty years. Pro-catalysts herein refer to said transition metal complex.

[0091] The metallocene procatalyst may have a formula II: (Cp)mRnMXq (II)

[0092] in which:

[0093] each Cp independently is a fused and/or substituted or unsubstituted homo- or heterocyclopentadienyl linker, for example, substituted or unsubstituted cyclopentadienyl, substituted or unsubstituted indenyl, or substituted or unsubstituted fluorenyl;

[0094] The optional one or more substituent (or substituents) is independently selected preferably from halogen, hydrocarbyl (e.g. C1-C20-alkyl, C2-C20-alkenyl, C2-C20-alkynyl, C3-C12-cycloalkyl, C6-C20-aryl or C7-C20-arylalkyl), C3-C12-cycloalkyl containing 1, 2, 3 or 4 heteroatoms in the chemical ring moiety, C6-C20-heteroaryl, C1-C20-haloalkyl, -SiR"3, -OSiR"3, -SR", -PR"2 or -NR"2,

[0095] each "R" is independently a hydrogen or hydrocarbyl, for example, C1-C20-alkyl, C2-C20-alkenyl, C2-C20-alkynyl, C3-C12-cycloalkyl or C6-C20-aryl; or, for example, in the case of -NR"2, the two R" substituents may form a ring, for example a five- or six-membered ring, together with the nitrogen atom to which they are attached;

[0096] R is a bridge of 1-7 atoms, for example, a bridge of 1-4 C-atoms and 0-4 heteroatoms, where the heteroatom (or heteroatoms) may be, for example, atom (or atoms) of Si, Ge and/or O, where each of the bridge atoms may independently bear substituents, such as substituents of C1-20-alkyl, tri(C1-20-alkyl)silyl, tri( C1-20-alkylsiloxy or C6-20-aryl); or a bridge of 1-3, for example one or two, heteroatoms, such as silicon, germanium and/or oxygen atom (or atoms), for example -SiR12-, where each R1 is independently a residue of C1-20-alkyl, C6-20-aryl or tri(C1-20-alkyl)silyl-, such as trimethylsilyl;

[0097] M is a transition metal of Group 3 to 10, preferably of Group 4 to 6 such as Group 4, for example Ti, Zr or Hf, especially Hf;

[0098] each X is independently a sigma ligand, such as H, halogen, C1-20-alkyl, C1-20-alkoxy, C2-C20-alkenyl, C2-C20-alkynyl, C3-C12-cycloalkyl, C6-C20-aryl, C6-C20-aryloxy, C7-C20-arylalkyl, C7-C20-aryl kenyl, -SR", -PR"3, -SiR"3, -OSiR"3, -NR"2, or -CH2-Y, where Y is C6-C20-aryl, C6-C20-heteroaryl, C1-C20-alkoxy, C6-C20-aryloxy, NR"2, -SR", -PR"3, -SiR"3, or -OSiR"3;

[0099] each of the above-mentioned chemical ring moieties alone or as part of another chemical moiety, as the substituent for Cp, X, R" or R1 may be further substituted, for example by C1-C20-alkyl which may contain Si and/or O atoms;

[0100] n is 0, 1 or 2, for example, 0 or 1,

[0101] m is 1, 2 or 3, for example, 1 or 2,

[0102] q is 1, 2 or 3, for example, 2 or 3,

[0103] where m+q is equal to the valency of M.

[0104] Suitably, in each X as -CH2-Y, each Y is independently selected from C6-C20-aryl, NR"2, -SiR"3 or -OSiR"3. Most preferably, X as -CH2-Y is benzyl. C7-C20-arylalkenyl or -NR"2 as defined above, for example -N(C1-C20-alkyl)2.

[0105] Preferably, q is 2, each X is halogen or -CH2-Y, and each Y is independently as defined above.

[0106] Cp is preferably cyclopentadienyl, indenyl, tetrahydroindenyl or fluorenyl, optionally substituted as defined above.

[0107] In a suitable subgroup of the compounds of formula II, each Cp independently bears 1, 2, 3 or 4 substituents as defined above, preferably 1, 2 or 3, as 1 or 2 substituents, which are preferably selected from C1-C20-alkyl, C6-C20-aryl, C7-C20-arylalkyl (wherein the aryl ring alone or as part of an additional chemical moiety may be further substituted as indicated above), -OSiR"3, where R" is as indicated above, preferably C1-C20-alkyl.

[0108] R, if present, is preferably a methylene, ethylene or a silyl bridge, whereby the silyl may be substituted as defined above, for example a (dimethyl)Si=, (methylphenyl)Si= or (trimethylsilylmethyl)Si=; n is 0 or 1; m is 2 and q is two. Preferably, R" is other than hydrogen.

[0109] A specific subgroup includes the well-known metallocenes of Zr, Hf, and Ti with two n-5-linkers which may be bridged or unbridged cyclopentadienyl linkers optionally substituted by, for example, siloxy, or alkyl (for example, C1-6-alkyl) as defined above, or with two bridged or unbridged indenyl linkers optionally substituted on any of the ring chemical moieties with, for example, siloxy or alkyl 1a as defined above, for example at the 2-, 3-, 4- and/or 7 positions. Preferred bridges are ethylene or -SiMe2.

[0110] The preparation of the metallocenes can be carried out according to or analogously to methods known from the literature and is within the ability of a person skilled in the art in the field. Thus, for preparation see, for example, EP-A-129 368, examples of compounds in which the metal atom bears a -NR"2 ligand see, for example, WO-A-9856831 and WO-A-0034341. , WO-A-9849208, WO-A-9912981, WO-A-9919335, WO-A-9856831, WO-A-00/34341, EP-A-423 101 and EP-A-537 130.

[0111] Alternatively, in a further subgroup of metallocene compounds, the metal bears a Cp group as defined above and additionally an n1 or n2 ligand, wherein said ligands may or may not bridge each other. Such compounds are described, for example, in WO-A-9613529, the contents of which are incorporated herein by reference.

[0112] Additional preferred metallocenes include those of formula (I) Cp'2M'X'2

[0113] wherein each X' is halogen, C1-6 alkyl, benzyl or hydrogen;

[0114] M' is Hf or Zr;

[0115] Cp' is a cyclopentadienyl or indenyl group optionally substituted by a C1-10 hydrocarbyl group or groups and is optionally bridged, for example, via an ethylene or dimethylsilyl linkage.

[0116] Especially preferred catalysts are bis-(n-butyl cyclopentadienyl) dibenzyl hafnium, and bis-(n-butyl cyclopentadienyl) zirconium dichloride.

[0117] Metallocene pro-catalysts are generally used as part of a catalyst system that also includes a catalyst activator, also called a cocatalyst. Useful activators are, among others, aluminum compounds, such as alkoxy aluminum compounds. Suitable aluminum alkoxy activators are, for example, methylaluminoxane (MAO), hexaisobutylaluminoxane and tetraisobutylaluminoxane. In addition, boron compounds (for example, a fluoroboron compound such as triphenylpentafluoroboro or triphenylcarbenium tetraphenylpentafluoroborate ((C6H5)3B+B-(C6F5)4)) can be used as activators. Cocatalysts and activators and the preparation of such catalyst systems are well known in the field. For example, when an aluminum alkoxy compound is used as an activator, the molar ratio Al/M of the catalyst system (Al is the aluminum of the activator and M is the transition metal of the transition metal complex) is suitable from 50 to 500 mol/mol, preferably from 100 to 400 mol/mol. Ratios below or above these ranges are also possible, but the ranges above are generally the most useful.

[0118] If desired, the procatalyst, procatalyst/cocatalyst mixture or a procatalyst/cocatalyst reaction product can be used in supported form (for example, on a silica or aluminum carrier), unsupported form, or it can be precipitated and used as such. One possible way to produce the catalyst system is based on emulsion technology, in which no external support is used, but the solid catalyst is formed by solidification of dispersed catalyst droplets in a continuous phase. The solidification method and additional possible metallocenes are described, for example, in WO03/051934 which is incorporated herein by reference.

[0119] It is also possible to use combinations of different activators and pro-catalysts. Furthermore, additives and modifiers and the like can be used, as is known in the art.

[0120] Any catalytically active catalyst system including the pro-catalyst, eg metallocene complex, is referred to herein as a metallocene catalyst (system) or single site.

[0121] Therefore, ideally, the HDPE used in the mixture of the invention is produced in at least two-stage polymerization using a single-site catalyst catalyst. Thus, for example, two slurry reactors or two gas phase reactors, or any combination thereof, in any order may be employed. Preferably, however, the polyethylene is produced using a slurry polymerization in a loop reactor followed by a gas phase polymerization in a gas phase reactor.

[0122] A loop reactor - gas phase reactor system is well known as Borealis technology, ie as a BORSTARTM reactor system. Such a multistage process is disclosed, for example, in EP517868.

[0123] The conditions used in such a process are well known. For slurry reactors, the reaction temperature will generally be in the range 60 to 110 °C, e.g. 85 to 110 °C, the reactor pressure will generally be in the range 0.5 to 8 MPa (5 to 80 bar), e.g. 5 to 6.5 MPa (50 to 65 bar), and the residence time will generally be in the range 0.3 to 5 hours, e.g. 0.5 to 2 hours. The diluent used will generally be an aliphatic hydrocarbon having a boiling point in the range of -70 to +100 °C, for example propane. In such reactors, polymerization can, if desired, be carried out under supercritical conditions. Slurry polymerization can also be carried out in bulk, where the reaction medium is formed from the monomer that is polymerized.

[0124] For gas phase reactors, the reaction temperature used will generally be in the range of 60 to 115 °C, for example, from 70 to 110 °C, the reactor pressure will generally be in the range of 1.0 to 2.5 MPa (10 to 25 bar), and the residence time will generally be from 1 to 8 hours. The gas used will commonly be a non-reactive gas such as nitrogen or a low boiling point hydrocarbon such as propane together with a monomer, eg ethylene.

[0125] The concentration of ethylene in the first reactor, preferably loop reactor, may be about 5 to 15 mol%, for example, 7.5 to 12 mol%.

[0126] In the second reactor, preferably gas phase reactor, the ethylene concentration is preferably much higher, for example at least 40 mol%, such as 45 to 65 mol%, preferably 50 to 60 mol%.

[0127] Preferably, the first polymer fraction is produced in a continuously operating loop reactor, in which ethylene is polymerized in the presence of a polymerization catalyst as indicated above and a chain transfer agent such as hydrogen. The diluent is typically an inert aliphatic hydrocarbon, preferably isobutane or propane. The reaction product is then preferably transferred to a continuously operating gas phase reactor. The second component can then be formed in a gas phase reactor preferably using the same catalyst.

[0128] HDPE is a commercial product and can be purchased from several suppliers.

Polymer composition

[0129] The polymer composition for use in the insulation layer of the invention comprises HDPE and LDPE mixed in ratios, as previously defined in this document. The polymer composition preferably consists essentially of components (I) and (II). The term is essentially to imply that there are no other polymer components present in the composition. It will be appreciated that the polymer composition may contain standard polymer additives discussed in more detail below. The term is essentially used to exclude the presence of other polymer components, but is intended to allow for the option of additives being present.

[0130] During the manufacture of the composition, the components can be mixed and mixed homogeneously, for example, blended by melting in an extruder.

Conductivity

[0131] In all embodiments of the invention, it is preferred if the conductivity of the polymer composition or the conductivity of the insulating layer is 1.5 fS/m or less when determined using method A.

[0132] As demonstrated in the examples, in a cross-linked insulation layer, where LDPE is combined with a low amount of HDPE, such as 1 to 3% by weight, the resulting mixture has an extremely low conductivity, for example, 1.0 fS/m measured using CC method A. In contrast, when 5% by weight is used a much higher value is measured (see document WO2011/113686). Low HDPE content is therefore surprisingly associated with lower conductivity in crosslinked materials. Lower HDPE also means less impurity and consequently less defect change in the insulation layer.

[0133] In thermoplastic applications, lower HDPE offers comparable conductivity with higher percentages of HDPE and better purity. When HDPE is multimodal, lower conductivity is observed in thermoplastic modalities.

[0134] In all embodiments, it is preferred if the conductivity of the polymer composition of the insulation layer is 3.5 x 10-17 S/cm or less when measured according to the DC conductivity method as described under “Methods B of Determination”, as 3.0 x 10-17 S/cm or less.

[0135] It was also found that in the context of a thermoplastic mixture, the use of a bimodal material such as HDPE leads to lower conductivity.

Cables

[0136] The cable of the invention is preferably a DC cable. A DC power cable is defined as a DC cable that transfers power that operates at any voltage level, typically operating at voltages greater than 1 kV. The DC power cable may be a low voltage (LV), medium voltage (MV), high voltage (HV) or extra high voltage (EHV) DC cable, which terms, as is well known, indicate the operating voltage level. The polymer is even more preferred used in the insulation layer for a DC power cable that operates at voltages greater than 36 kV, such as an HV DC cable. For HV DC cable, the operating voltage is defined in this document as the electrical voltage between ground and the conductor of the high voltage cable.

[0137] Preferably, the HV DC power cable of the invention is one that operates at voltages of 40 kV or greater, even at voltages of 50 kV or greater. Most preferably, the HV DC power cable operates at voltages of 60 kV or greater. The invention is also highly operable in very complex cable applications and additionally cables of the invention are HV DC power cables operating at voltages greater than 70 kV. Voltages of 100 kV or more are targeted, such as 200 kV or more, more preferably 300 kV or more, especially 400 kV or more, more especially 500 kV or more. Voltages of 640 KV or more such as 700 kV are also foreseen. The upper limit is not limited. The practical upper limit can be up to 1500 kV or up to 1100 kV. The cables of the invention operate well, therefore, in extra complex HV DC power cable applications operating at 400 to 850 kV, such as 650 to 850 kV.

[0138] A cable, such as a DC cable, comprises an inner semiconductor layer comprising a first semiconductor composition, an insulating layer comprising the polymer composition of the invention, and an outer semiconductor layer comprising a second semiconductor composition, in that order.

[0139] The polymer composition of the invention is used in the insulation layer of the cable. Ideally, the insulation layer comprises at least 95% by weight, such as at least 98% by weight, of the polymer composition of the invention, such as at least 99% by weight. Therefore, it is preferred if the polymer composition of the invention has the only non-additive component used in the insulation layer of the cables of the invention. Therefore, it is preferred if the insulating layer essentially consists of the composition of the invention. The term is essentially used herein to mean that the only polymer composition present is the one defined. It will be noted that the insulation layer may contain standard polymer additives such as abrasion retardants, irrigation retardants, antioxidants and so on. They are not excluded by the term "consists essentially of". Also note that these additives may be added as part of a masterbatch subsequently transported on a polymer carrier. The use of masterbatch additives is not excluded by the term essentially consists of.

[0140] The insulation layer can have a beneficial low electrical conductivity when it is crosslinked with a crosslinking agent. The insulating layer of the cables of the invention can thus optionally be crosslinkable.

[0141] The term crosslinkable means that the insulation layer can be crosslinked using a crosslinking agent before use. The insulating layer will need to comprise a crosslinking agent in order to be crosslinkable, typically a free radical generating agent. The crosslinked polymer composition has a typical network, for example, interpolymer crosslinks (bridges), as well known in the field.

[0142] If the insulation layer is crosslinked, any other parameter of the insulation layer besides conductivity is ideally measured before crosslinking, unless otherwise indicated. In embodiments, where the insulating layer comprises no crosslinking agent, the electrical conductivity as described under the "Method of Determination" is measured from a sample of polymer forming the insulating layer that is uncrosslinked (i.e., does not contain a crosslinking agent and has not been crosslinked with a crosslinking agent). In embodiments, where the insulation layer is crosslinked with a crosslinking agent, then the electrical conductivity is measured from a sample of the crosslinked polymer (i.e., a sample of the polymer is first crosslinked with the initially present crosslinking agent and then the electrical conductivity is measured from the obtained crosslinked sample). Conductivity measurement of a sample of cross-linked or non-crosslinked polymer composition is described under "Methods of Determination".

[0143] The amount of crosslinking agent used, if present, can preferably vary within the ranges given below. Preferably, a peroxide is used in an amount of 0 to 110 mmol -O-O-/kg of polymer composition of the insulation layer, preferably from 0 to 90 mmol -O-O-/kg of polymer composition (corresponds to 0 to 2.4% by weight of dicumyl peroxide based on the polymer composition), preferably from 0 to 37 mmol -O-O-/kg of polymer composition, preferably from 0 to 35 mmol - O-O-/kg of polymer composition, preferably from 0 to 34 mmol -O-O-/kg of polymer composition, preferably from 0 to 33 mmol -O-O-/kg of polymer composition, more preferably from 0 to 30 mmol -O-O-/kg of polymer composition, more preferably from 0 to 20 mmol -O-O-/kg of polymer composition, more preferably from 0 to 10.0 mmol -O-O -/kg of polymer composition, more preferably from 0 to 7.0 mmol -O-O-/kg of polymer composition, more preferably less than 5.0 mmol -O-O-/kg of polymer composition, most preferably, the polymer composition comprises no crosslinking agent (= 0% by weight of crosslinking agent added). The insulation layer is thus ideally free of peroxide decomposition by-products.

[0144] The lower limit of the crosslinking agent, if present, is not limited and may be at least 0.1 mmol -O-O-/kg of polymer composition in the insulation layer, preferably at least 0.5 mmol -O-O-/kg of polymer composition, more preferably at least 5.0 mmol -O-O-/kg of polymer composition. The lower peroxide content can decrease the required degassing step of the produced and crosslinked cable, if desired.

[0145] The unit “mmol -O-O-/kg of polymer composition” means herein the content (mmol) of peroxide functional groups per kg of polymer composition, when measured from the polymer composition before crosslinking. For example, the polymer composition of 35 mmol -O-O-/kg corresponds to 0.95% by weight of the well-known dicumyl peroxide based on the total amount (100% by weight) of the polymer composition.

[0146] Such a polymer composition may comprise one type of peroxide or two or more different types of peroxide, wherein the amount (in mmol) of -O-O-/kg of polymer composition, as defined above, below or in the claims, is the sum of the amount of -O-O-/kg of polymer composition of each type of peroxide. As non-limiting examples of suitable organic peroxides, di-tert-amylperoxide, 2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexine, 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, tert-butylcumylperoxide, di(tert-butyl)peroxide, dicumylperoxide, butyl-4,4-bis(tert-butyl peroxy)-valerate, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butylperoxybenzoate, dibenzoylperoxide, bis(tert-butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, 1,1-di(tert-butylperoxy)cyclohexane, 1,1-di(tert) and amylperoxy)cyclohexane, or any mixtures thereof, may be mentioned. Preferably, the peroxide is selected from 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, di(tert-butylperoxyisopropyl)benzene, dicumylperoxide, tert-butylcumylperoxide, di(tert-butyl)peroxide, or mixtures thereof. Most preferably, the peroxide is dicumylperoxide.

[0147] When the insulation layer is not crosslinked, it is preferred that the insulation layer comprises no crosslinking agent. The prior art disadvantages related to the use of a cable layer crosslinking agent can therefore be avoided. Of course, the non-crosslinked embodiment also simplifies the cable production process. As no cross-linking agent is required, raw material costs are lower. In addition, it is generally necessary to degas a crosslinked layer of cable to remove peroxide by-products after crosslinking. When the material is not crosslinked, no such degassing step is required.

[0148] The insulation layer may contain, in addition to the mixture of LDPE and the optional peroxide, additional component (or components) such as additives (such as any of antioxidants (or antioxidants), abrasion retardant (or retardants) (SR), crosslinking enhancer (or enhancers), stabilizer (or stabilizers), processing aid (or aids), flame retardant additive (or additives), irrigation retardant additive (or additives), acid or ion scavenger (or scavengers) , inorganic filler (or fillers) and stress stabilizer (or stabilizers) as known in the polymer field.

[0149] The insulation layer may therefore comprise additive (or additives) conventionally used for W&C applications, such as one or more antioxidants (or antioxidants) and optionally one or more abrasion retardant (or retardants), preferably at least one or more antioxidants (or antioxidants). The amounts of additives used are conventional and well known to a person skilled in the art, for example 0.1 to 1.0% by weight.

[0150] As non-limiting examples of antioxidants, for example, sterically hindered or semi-hindered phenols, aromatic amines, sterically hindered aliphatic amines, organic phosphites or phosphonites, thio compounds, and mixtures thereof, may be mentioned.

[0151] Preferably, the insulating layer does not comprise a carbon black. Furthermore, preferably, the insulation layer does not comprise flame retardant additive (or additives), for example a metal hydroxide containing additives in flame retardant amounts.

[0152] The cable of the invention also contains inner and outer semiconductor layers. They may be produced from any conventional material suitable for use in these layers. The inner and outer semiconductor compositions can be different or identical and can comprise a polymer (or polymers) which is preferably a polyolefin or a mixture of polyolefins and a conductive filler, preferably carbon black. Suitable polyolefins are, for example, polyethylene produced in a low pressure process or a polyethylene produced in an HP process (LDPE). The carbon black can be any conventional carbon black used in the semiconductor layers of a DC power cable, preferably in the semiconductor layer of a DC power cable. Preferably, the carbon black has one or more of the following properties: a) a primary particle size of at least 5 nm which is defined as the number average particle diameter in accordance with ASTM D3849-95a, dispersion procedure D b) iodine number of at least 30 mg/g in accordance with ASTM D1510, c) oil absorption number of at least 30 ml/100 g which is measured in accordance with ASTM D2414. Non-limiting examples of carbon blacks are, for example, acetylene carbon black, furnace carbon black and Ketjen carbon black, preferably furnace carbon black and acetylene carbon black. Preferably, the polymer composition comprises 10 to 50% by weight carbon black, based on the weight of the semiconductor composition.

[0153] In a preferred embodiment, the outer semiconductor layer is crosslinked. In another preferred embodiment, the inner semiconductor layer is preferably non-crosslinked. In general, therefore, it is preferred if the inner semiconductor layer and the insulating layer remain uncrosslinked where the outer semiconductor layer is crosslinked. A peroxide crosslinking agent can therefore be provided on the outer semiconductor layer only.

[0154] The cable comprises one or more conductors, for example wires. Preferably, the conductor is an electrical conductor and comprises one or more metal wires. Cu wire is preferred.

[0155] As is well known, the cable may optionally further comprise layers, for example, fabric (or fabrics), a coating layer (or layers), another protective layer (or layers) or any combination thereof.

Cable Manufacturing

[0156] The invention also provides a process for producing a cable comprising the steps of - applying to a conductor, preferably by (co)extrusion, an inner semiconductor layer, an insulating layer and an outer semiconducting layer, in that order, in which the insulating layer comprises the composition of the invention.

[0157] The process may optionally comprise crosslinking steps in one or both of the inner semiconductor layer or outer semiconductor layer, without crosslinking the insulating layer. Preferably, however, the layer is crosslinked. The handle of the invention is ideally thermoplastic. More preferably, a cable is produced, the process comprising the steps of

[0158] (a) - supplying and mixing, preferably molten mixture in an extruder, a first optionally crosslinkable semiconductor composition comprising a polymer, a carbon black and, optionally, additional component (or components) for the inner semiconductor layer,

[0159] - supplying and mixing, preferably molten mixture in an extruder, the polymer composition of the invention; It is

[0160] - supplying and mixing, preferably molten mixture in an extruder, a second semiconductor composition that is optionally crosslinkable and comprises a polymer, a carbon black and optionally additional component (or components) for the outer semiconductor layer,

[0161] (b) apply to one or more conductors, preferably by coextrusion,

[0162] - a molten mixture of the first semiconductor composition obtained from step (a) to form the inner semiconductor layer,

[0163] - a molten mixture of polymer composition of the invention obtained from step (a) to form the insulation layer, and

[0164] - a molten mixture of the second semiconductor composition obtained from step (a) to form the outer semiconductor layer, and

[0165] (c) optionally, crosslinking under crosslinking conditions one or both of the first semiconductor composition of the inner semiconductor layer and the second semiconductor composition of the outer semiconductor layer, of the obtained cable, without crosslinking of the insulating layer.

[0166] Preferably, in step (c), the second polymer semiconductor composition of the outer semiconductor layer is crosslinked. Furthermore, preferably, the second polymer semiconductor composition of the outer semiconductor layer is crosslinked without crosslinking of the insulating layer or the first semiconductor composition of the inner semiconductor layer.

[0167] Melt mixing means mixing above the melting point of at least the largest polymer component (or components) of the mixture obtained and is carried out, for example, without limitation, at a temperature of at least 15 °C above the melting or flexibilization point of the polymer component (or components).

[0168] The term "(co)extrusion" means herein that in the case of two or more layers, said layers may be extruded in separate steps, or at least two or all of said layers may be co-extruded in the same extrusion step, as is well known in the art. The term "(co)extrusion" herein also means that all or part of the layer (or layers) are formed simultaneously using one or more extrusion heads. For example, a triple extrusion can be used to form three layers. In case a layer is formed using more than one extrusion head, then, for example, the layers can be extruded using two extrusion heads, the first head to form the inner semiconductor layer and the inner part of the insulating layer, and the second head to form the outer insulating layer and the outer semiconducting layer.

[0169] As is well known, the polymer composition of the invention and the preferred and optional first and second semiconductor compositions can be produced before or during the cable production process.

[0170] Preferably, the polymers needed to manufacture the cable of the invention are supplied to the cable production process in the form of powder, grain or pellets. Pellets herein generally mean any polymer product that is formed from reactor-produced polymer (obtained directly from the reactor) by post-reactor modification to solid polymer particles.

[0171] Consequently, the LDPE and component material (II) can be pre-mixed, for example, melt-blended and pelletized, before mixing. Alternatively and preferably these components can be supplied in separate pellets to the mixing (melting) step (a) where the pellets are mixed.

[0172] The mixing (melting) step (a) of the provided polymer composition of the invention and the preferred first and second semiconductor compositions is preferably carried out in a cable extruder. Step a) of the cable production process can optionally comprise a separate mixing step, for example in a mixer arranged in connection with and preceding the cable extruder of the cable production line. Mixing in the foregoing separate mixer can be performed mixing with or without external heating (heating with an external source) of the component (or components).

[0173] Any crosslinking agent can be added before the cable production process or during the mixing (melting) step (a). For example, and preferably, the crosslinking agent and also the optional additional component (or components) such as additive (or additives) can already be present in the used polymers. The crosslinking agent is added, preferably impregnated, into the solid polymer particles, preferably pellets.

[0174] It is preferred that the molten mixture of the polymer composition obtained from the mixing (melting) step (a) consists of the LDPE (I) and second component (II) as the only polymer component (or components). The optional and preferred additive (or additives) can be added to the polymer composition as such or as a blend with a carrier polymer, i.e. in a so-called masterbatch form.

[0175] Crosslinking of other layers can be carried out at increased temperature which is chosen, as well known, depending on the type of crosslinking agent. For example, temperatures above 150 °C, such as 160 to 350 °C, are typical, however, without limitation thereto.

[0176] Processing temperatures and devices are well known in the art, for example conventional mixers and extruders such as twin or single screw extruders are suitable for the process of the invention.

[0177] The nature of the cooling process is important when managing the properties of the insulation layer. Once the polymer material has been extruded, the polymer composition needs to cool. This can be done quickly or slowly depending on the conditions applied to the cable.

[0178] The inventors found that rapid quenching maximizes the formation of co-crystals, while slow cooling largely favors segregation of LDPE and HDPE and, thereby, the formation of pure crystalline domains. Improved displacement properties are generally seen with slower cooling. It is therefore a further object of the invention if the insulating layer composition is cooled at a rate lower than 5°C/min.

[0179] The thickness of the insulation layer of the cable, more preferably of the DC power cable as HV DC power cable, is typically 2 mm or more, preferably at least 3 mm, preferably at least 5 to 100 mm, more preferably 5 to 50 mm, and conventionally 5 to 40 mm, for example 5 to 35 mm, when measured from a cross section of the cable insulation layer.

[0180] The thickness of the inner and outer semiconductor layers is typically less than that of the insulation layer, and in HV DC power cables it may be, for example, greater than 0.1 mm, such as from 0.3 to 20 mm, 0.3 to 10 of inner semiconductor layer and outer semiconductor layer. The thickness of the inner semiconductor layer is preferably 0.3 to 5.0 mm, preferably 0.5 to 3.0 mm, preferably 0.8 to 2.0 mm. The thickness of the outer semiconductor layer is preferably 0.3 to 10 mm, such as 0.3 to 5 mm, preferably 0.5 to 3.0 mm, preferably 0.8 to 3.0 mm. It is apparent to and within the skill of one skilled in the art that the thickness of the DC cable layers depends on the intended voltage level of the final application cable and can be chosen accordingly.

[0181] The preferred embodiments of the invention may be combined with each other in any way to further define the invention.

[0182] The invention will now be described with reference to the appended non-limiting examples.

Determination Methods

[0183] Unless otherwise indicated in the description or experimental part, the following methods were used for property determinations.

[0184] % by weight: % by weight

melt flow rate

[0185] The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the flowability and therefore the processability of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer. MFR is determined at 190°C for polyethylene and 230°C for polypropylene. MFR can be determined at different loads as 2.16 kg (MFR2) or 21.6 kg (MFR21).

Molecular weight

[0186] Mz, Mw, Mn, and MWD are measured by Gel Permeation Chromatography (GPC) according to the following method:

[0187] The weight average molecular weight Mw and molecular weight distribution (MWD = Mw/Mn where Mn is the number average molecular weight and Mw is the weight average molecular weight; Mz is the z average molecular weight) is measured according to ISO 16014-4:2003 and ASTM D 6474-99. A Waters GPCV2000 instrument, equipped with refractory index detector and online viscometer was used with 2 x GMHXL-HT and 1x G7000HXL-HT TSK-gel columns available from Tosoh Bioscience and 1,2,4-trichlorobenzene (TCB, stabilized with 250 mg/l 2,6-Di tert-butyl-4-methyl-phenol) as solvent at 140 °C and at a constant flow rate of 1 ml/min. 209.5 μl of sample solution was injected per analysis. The column fit was calibrated using universal calibration (according to ISO 16014-2:2003) with at least 15 Narrow MWD (PS) polystyrene standards in the range of 1 kg/mol to 12,000 kg/mol. Mark Houwink constants were used as stated in ASTM D 6474-99. All samples were prepared by dissolving 0.5 to 4.0 mg of polymer in 4 ml (at 140 °C) of stabilized TCB (same as in the mobile phase) and kept for a maximum of 3 hours at a maximum temperature of 160 °C with continuous gentle agitation before sampling on the GPC instrument.

comonomer content

[0188] a) Comonomer content in polypropylene random copolymer:

[0189] Quantitative Fourier transform infrared spectroscopy was used to quantify the amount of comonomer. Calibration was achieved by correlation with comonomer content determined by quantitative nuclear magnetic resonance (NMR) spectroscopy.

[0190] The calibration procedure based on results obtained from quantitative 13C NMR spectroscopy was performed in the conventional manner well documented in the literature.

[0191] The amount of comonomer (N) was determined as percent by weight (% by weight) using: N = k1 (A / R) + k2

[0192] where A is the defined maximum absorbance of the comonomer band, R the maximum absorbance defined as peak height of the reference peak and with k1 and k2 the linear constants obtained by calibration. The band used for ethylene content quantification is selected depending on whether the ethylene content is random (730 cm-1) or block-like (as in heterophasic PP copolymer) (720 cm-1). The absorbance at 4324 cm-1 was used as a reference band.

[0193] b) Quantification of alpha-olefin content in linear low density polyethylenes and low density polyethylenes by NMR spectroscopy:

[0194] The comonomer content was determined by quantitative 13C nuclear magnetic resonance (NMR) spectroscopy after basic assignment (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.

[0195] Specifically, solution state NMR spectroscopy was employed using a Bruker AvanceIII 400 spectrometer. Homogeneous samples were prepared by dissolving approximately 0.200 g of polymer in 2.5 ml of deuterated tetrachloroethene in 10 mm sample tubes using a heat block and rotating tube furnace at 140 °C. NOE-decoupled (gate-energized) proton 13C single-pulse 13C NMR spectra were recorded using the following acquisition parameters: a transfer angle of 90 degrees, 4 dummy scans, 4096 pass an acquisition time of 1.6 s, a spectral width of 20 kHz, a temperature of 125 °C, a bilevel WALTZ proton decoupling scheme, and a delay of 3.0 s relaxation. The resulting FID was processed using the following processing parameters: zero padding for 32k data points and apodization using a Gaussian window function; automatic first- and zeroth-order phase correction and automatic baseline correction using a fifth-order polynomial constrained to the region of interest.

[0196] Quantities were calculated using simple corrected ratios of signal integrals from representative locations based on methods well known in the art.

[0197] c) Comonomer content of polar comonomers in low density polyethylene

[0198] (1) Polymers containing > 6% by weight of polar comonomer units

[0199] Comonomer content (% by weight) was determined in a known manner based on Fourier transform infrared spectroscopy (FTIR) determination calibrated with quantitative nuclear magnetic resonance (NMR) spectroscopy. Below is an example of the determination of the polar comonomer content of ethylene ethylacrylate, ethylene butylacrylate and ethylene methylacrylate. Film samples of the polymers were prepared for FTIR measurement: 0.5 to 0.7 mm thick was used for ethylene butyl acrylate and ethylene ethyl acrylate and 0.10 mm thin film for ethylene methyl acrylate in amounts of >6% by weight. The films were compressed using a Specac film press at 150 °C, approximately 5 tons, 1 to 2 minutes, and then cooled with ice water in an uncontrolled manner. The precise thickness of the obtained film samples was measured.

[0200] After analysis with FTIR, baselines in absorbance mode were taken for the peaks to be analyzed. The absorbance peak for the comonomer was normalized with the absorbance peak for polyethylene (eg, the peak height for butyl acrylate or ethyl acrylate at 3450 cm-1 was divided with the peak height for polyethylene at 2020 cm-1). The NMR spectroscopy calibration procedure was carried out in the conventional manner that is well documented in the literature, explained below.

[0201] For the determination of the methyl acrylate content, a sample of film thickness of 0.10 mm was prepared. After the analysis, the maximum absorbance for the peak for methylacrylate at 3455 cm-1 was subtracted with the absorbance value for the baseline at 2475 cm-1 (Amethylacrylate - A2475). Then, the maximum absorbance peak for the polyethylene peak at 2660 cm-1 was subtracted with the absorbance value for the baseline 2475 cm-1 (A2660 -A2475). The ratio between (Amethylacrylate-A2475) and (A2660-A2475) was then calculated in the conventional manner that is well documented in the literature.

[0202] The wt% can be converted to mole % by calculation. It is well documented in the literature.

[0203] Quantification of copolymer content in polymers by NMR spectroscopy

[0204] The comonomer content was determined by quantitative nuclear magnetic resonance (NMR) spectroscopy after basic assignment (eg, "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 (eg, “200 and More NMR Experiments: A Practical Course”, 5. Berger and S. Braun, 2004, Wiley-VCH, Weinheim). Quantities were calculated using simple corrected ratios of the signal integrals of representative sites in a manner known in the art.

[0205] (2) Polymers containing 6% by weight or less of polar comonomer units

[0206] Comonomer content (% by weight) was determined in a known manner based on Fourier transform infrared spectroscopy (FTIR) determination calibrated with quantitative nuclear magnetic resonance (NMR) spectroscopy. Below is an example of the determination of the polar comonomer content of ethylene butylacrylate and ethylene methylacrylate. For FT-IR measurement, 0.05 to 0.12 mm thick film samples were prepared as described above under method 1). The precise thickness of the obtained film samples was measured.

[0207] After the analysis with FT-IR baselines in absorbance mode, were removed for the peaks to be analyzed. The maximum absorbance for the peak for the comonomer (eg for methylacrylate at 1164 cm-1 and butylacrylate at 1165 cm-1) was subtracted with the absorbance value for the baseline at 1850 cm-1 (A polar comonomer - A1850). Then, the maximum absorbance peak for the polyethylene peak at 2660 cm-1 was subtracted from the absorbance value for the baseline at 1850 cm-1 (A2660 - A1850). The ratio between (Acomonomer-A1850) and (A2660-A1850) was then calculated. The NMR spectroscopy calibration procedure was carried out in the conventional manner which is well documented in the literature, as described above under method 1).

[0208] The wt% can be converted to mole % by calculation. It is well documented in the literature.

[0209] Below is an example of how the polar comonomer content obtained from the above method (1) or (2), depending on the amount thereof, can be converted to micromol or mmol per g of polar comonomer as used in the definitions in the text and claims:

[0210] Millimol (mmol) calculations and micro mole calculations were performed as described below.

[0211] For example, if 1 g of poly(ethylene-co-butylacrylate) polymer contains 20% by weight of butylacrylate, then this material contains 0.20/Mbutylacrylate (128 g/mol) = 1.56 x 10-3 mol. (=1,563 micromol).

[0212] The content of polar comonomer units in the polar copolymer Polar comonomer is expressed in mmol/g (copolymer). For example, a polar poly(ethylene-co-butylacrylate) polymer containing 20% by weight of butyl acrylate comonomer units has a polar Comonomer of 1.56 mmol/g.

[0213] The molecular weights used are: Mbutylacrylate = 128 g/mol, Methylacrylate = 100 g/mol, Mmethylacrylate = 86 g/mol).

Density:

[0214] Low density polyethylene (LDPE): The density was measured according to ISO 1183-2. Sample preparation was carried out in accordance with Table 3 Q of ISO 1872-2 (compression molding).

[0215] Low pressure process polyethylene: Polymer density was measured according to ISO 1183 / 1872-2B.

Method for determining the amount of double bonds in the polymer composition or in the polymer A) Quantification of the amount of carbon-carbon double bonds by IR spectroscopy

[0216] Quantitative infrared (IR) spectroscopy was used to quantify the amount of carbon-carbon (C=C) doubles. Calibration was achieved before determining the molar extinction coefficient of C=C functional groups in representative low molecular weight model compounds of known structure.

[0217] The amount of each of these groups (N) was determined as the number of carbon-carbon double bonds per thousand of total carbon atoms (C=C/1,000 C) using: N = (A x 14 ) / (E x L x D)

[0218] where A is the maximum absorbance defined as peak height, E is the molar extinction coefficient of the group in question (l •mol-1 •mm-1), L is film thickness (mm) and D is material density (g^cm-1).

[0219] The total amount of C=C bonds per thousand of total carbon atoms can be calculated by summing N for the individual components that contain C=C.

[0220] For polyethylene samples, solid state infrared spectra were recorded using an FTIR spectrometer (Perkin Elmer 2000) on compression molded thin films (0.5 to 1.0 mm) at a resolution of 4 cm-1 and analyzed in absorption mode.

1) Polymer compositions comprising homopolymers and copolymers of polyethylene, except polyethylene copolymers with > 0.4% by weight of polar comonomer

[0221] For polyethylenes, three types of functional groups containing C=C were quantified, each with a characteristic absorption and each calibrated for a different model compound resulting in individual extinction coefficients:

[0222] • vinyl (R-CH=CH2) through 910 cm-1 based on 1-decene [dec-1-ene] giving E = 13.13 l •mol-1 •mm- 1

[0223] • vinylidene (RR'C=CH2) through 888 cm-1 based on 2-methyl-1-heptene [2-methyl-hept-1-ene] yielding E = 18.24 l •mol-1 •mm-1

[0224] • trans-vinylene (R-CH=CH-R') through 965 cm-1 based on trans-4-decene [(E)-dec-4-ene] giving E = 15.14 l •mol-1 •mm-1

[0225] For polyethylene homopolymers or copolymers with < 0.4% by weight of polar comonomer linear baseline correction was applied between approximately 980 and 840 cm-1.

2) Polymer compositions comprising polyethylene copolymers with >0.4% by weight of polar comonomer

[0226] For polyethylene copolymers with > 0.4% by weight of polar comonomer, two types of functional groups containing C=C were quantified, each with a characteristic absorption and each calibrated for a different model compound resulting in individual extinction coefficients:

[0227] • vinyl (R-CH=CH2) through 910 cm-1 based on 1-decene [dec-1-ene] giving E = 13.13 l •mol-1 •mm- 1

[0228] • vinylidene (RR'C=CH2) through 888 cm-1 based on 2-methyl-1-heptene [2-methyl-hept-1-ene] yielding E = 18.24 l •mol-1 •mm-1

[0229] EBA:

[0230] For poly(ethylene-co-butylacrylate) (EBA), linear baseline correction of systems was applied between approximately 920 and 870 cm-1.

[0231] EMA:

[0232] For poly(ethylene-co-methylacrylate) (EMA), linear baseline correction of systems was applied between approximately 930 and 870 cm-1.

3) Polymer compositions comprising low molecular weight unsaturated molecules

[0233] For systems containing low molecular weight, direct calibration of species containing C=C using the molar extinction coefficient of absorption of C=C in the low molecular weight species itself was performed.

B) Quantification of molar extinction coefficients by IR spectroscopy

[0234] The molar extinction coefficients were determined according to the procedure provided in ASTM D3124-98 and ASTM D6248-98. Solution-state infrared spectra were recorded using an FTIR spectrometer (Perkin Elmer 2000) equipped with a liquid cell of 0.1 mm path length at a resolution of 4 cm-1.

[0235] The molar extinction coefficient (E) was determined as l •mol-1 •mm-1 through:

[0236] E = A / (L x W)

[0237] where A is the maximum absorbance defined as peak height, C the concentration (mol^l-1) and L the cell thickness (mm).

[0238] At least three solutions of 0.18 mol^l-1 in carbon disulfide (CS2) were used and the main value of the molar extinction coefficient determined.

DC conductivity methods Method A

[0239] The plates are compression molded from pellets of the test polymer composition. End boards are 1 mm thick and 200 x 200 mm.

[0240] The conductivity measurement can be performed with the use of a test polymer composition that does not comprise or comprises the optional cross-linking agent. In the case of no crosslinking agent, the conductivity is measured from an uncrosslinked plate sample using the procedure below. If the test polymer composition comprises the cross-linking agent, then cross-linking takes place during the preparation of the plate samples, in this way, the conductivity is then measured according to the procedure below on the resulting cross-linked plate sample. The crosslinking agent, if present in the polymer composition prior to crosslinking, is preferably a peroxide, as herein.

[0241] The plates are press molded at 130 °C for 12 min while the pressure is gradually increased from 2 to 20 MPa. Subsequently, the temperature is increased and reaches 180 °C after 5 min. The temperature is then held constant at 180 °C for 15 min during which time the plate becomes fully cross-linked via the peroxide, if present in the test polymer composition. Finally, the temperature is reduced using a cooling rate of 15 °C/min until ambient temperature is reached when the pressure is released. The plates are, immediately after the release of pressure, wrapped in metallic foil in order to prevent loss of volatile substances.

[0242] If the plate has to be degassed (that is, after crosslinking), it is placed in a ventilated furnace at atmospheric pressure for 24 h at 70 °C. Subsequently, the plate is wrapped again in metallic foil in order to further prevent the exchange of volatile substances between the plate and the surrounding environment.

[0243] A high voltage source is connected to the upper electrode to apply voltage to the test sample. The resulting current through the sample is measured with an electrometer. The measuring cell is a three-electrode system with brass electrodes. The brass electrodes are equipped with heating pipes connected to a heating circulator to facilitate elevated temperature measurements and provide uniform test sample temperature. The measuring electrode diameter is 100 mm. Silicone rubber skirts are placed between the brass electrode edges and the test specimen to prevent sparking from the round edges of the electrodes.

[0244] The applied voltage was DC of 30 kV meaning a main electric field of 30 kV/mm. The temperature was 70 °C. The current through the plate was recorded for all complete experiments lasting 24 hours. The current after 24 hours was used to calculate the insulation conductivity.

[0245] This method and a schematic depiction of the measurement setup for conductivity measurements were fully described in a publication presented at the Nordic Insulation Symposium 2009 (Nord-IS 09), Gothenburg, Sweden, June 15-17, 2009, pages 55-58: Olsson et al, “Experimental determination of DC conductivity for XLPE insulation”.

DC conductivity method b (values in s/cm) Broadband Dielectric Spectroscopy (BDS)

[0246] The samples (40 x 100 mm) were produced by hot compaction at 250 °C and 100 kN of gripping force. Spacers with a thickness of 0.1 mm were used to control the thickness. The disc-shaped samples were then cut from the plates.

[0247] All measurements were performed on disc-shaped samples with 40 mm in diameter and ~0.1 mm thick. Conductivity measurements were obtained using a dielectric spectrometer.

[0248] Broadband Dielectric Spectroscopy (BDS) was performed using a Novocontrol alpha spectrometer in a frequency range from 10-2 to 107 Hz, at different temperatures in the range -20 °C to 110 °C (253 to 383 K) with an error of ± -273 °C (0.1 K), at atmospheric pressure and under nitrogen atmosphere. For selected temperatures, frequency sweeps were also performed to investigate local and ion dynamics. The sample cell consisted of two silver-coated electrodes 40 mm in diameter and the sample having a thickness of about 0.1 mm. The complex dielectric permittivity ε*=ε'-iε'', where ε' is the real part and ε'' is the imaginary part, is usually a function of frequency, a, temperature T, and pressure P1, although in the present document only the frequency and temperature dependencies have been investigated. The complex dielectric conductivity o* can also be calculated from the complex dielectric function ε* as o*=iaεfε*, (εf is the free space permittivity, 8.854 pF/m), where conductivity can also be analyzed into a real part and an imaginary part: o*=o'+io''. This means that conductivity data is effectively an alternative representation of permittivity, regardless of focusing on features other than dielectric behavior as will be discussed below. The analysis was produced using the empirical equation of Havriliak and Negami2

[0249] where T HN(T,P) is the characteristic relaxation time in this equation, Δε(T,P) is the resistance to relaxation of the process under investigation, ε¥ is the dielectric permittivity in the high frequency limit, and a, Y (0 < a, aY ^ 1) describe, respectively, the symmetric and asymmetric widening of the distribution of relaxation times. The relaxation times at minimum loss (tmax) presented in this document were obtained in a way analysis by fitting the relaxation spectra with the following Havriliak-Negami (HN) equation:

Experimental part

[0250] The following materials are used in the examples:

[0251] LDPE1 - LDPE homopolymer that has the properties in table 1: Table 1

[0252] LDPE 2 - a copolymer of LDPE with octadiene of density 922 kg/m3 and MFR2 of 2.0 g/10 min.

[0253] HDPE1: A bimodal high density polyethylene produced using Ziegler Natta catalyst, density 946 kg/m3 and MFR2 045 g/10 min.

[0254] HDPE2: A unimodal high-density polyethylene produced using a Ziegler Natta catalyst, density 962 kg/m3 and MFR2 of 12 g/10 min, butene comonomer.

[0255] Formation of the polymer compositions: Each polymer component of the composition was added as separate pellets to another pilot scale extruder (Prism TSE 24TC), in addition to the crosslinking agent. The mixture obtained was melt blended under the conditions given in the table below and extruded into pellets in a conventional manner.

[0256] The crosslinking agent, if present, was added to the pellets and the resulting pellets were used for the experimental part. Example 1 - thermoplastic mixtures Table 2

[0257] Surprisingly, at 2 wt% HDPE, conductivity is lower with the bimodal grid.

Example 2 - cross-linked polymers

[0258] The mixtures in Table 3 were prepared as described above. Table 3

Claims (13)

1. Cable characterized in that it comprises one or more conductors surrounded by at least an inner semiconductor layer, an insulating layer and an outer semiconducting layer, in that order, wherein the insulating layer is not crosslinked or is crosslinked and comprises at least 90% by weight of a polymer composition, wherein said polymer composition comprises: (I) 95.5 to 99.9% by weight of an LDPE; and (II) 0.1 to 4.5% by weight of an HDPE having a density of at least 940 kg/m3, wherein the polymer composition of the insulating layer has a conductivity of 1.5 fS/m or less when measured according to the DC conductivity method as described under “Method A of Determination”, and 3.5 x 10-17 S/cm or less when measured according to the DC conductivity method as described under “Method A” s B for Determination”. 2. Cable characterized by the fact that it comprises one or more conductors surrounded by at least an inner semiconductor layer, an insulating layer and an outer semiconducting layer, in that order, in which the insulating layer is not crosslinked and comprises at least 90% by weight of a polymer composition, wherein said polymer composition comprises: (I) 95.5 to 99.9% by weight of an LDPE; and (II) 0.1 to 4.5% by weight of a multimodal HDPE having a density of at least 940 kg/m 3 . 3. Cable according to any one of claims 1 or 2, characterized in that it is a power cable such as a direct current (DC) power cable, preferably operating at or capable of operating at 320 kV or more, such as 650 kV or more. 4. Cable according to any one of claims 1 to 3, characterized in that the LDPE has an MFR2 (2.16 kg, 190 °C) from 0.1 to 10 g/10 min, especially from 0.1 to 5.0 g/10 min. 5. Cable according to any one of claims 1 to 4, characterized in that LDPE is a homopolymer. 6. Cable according to any one of claims 1 to 5, characterized in that the HDPE has a density of 945 to 965 kg/m3. 7. Cable, according to any one of claims 1 to 6, characterized in that the LDPE has a density of 920 to 932 kg/m3. 8. Cable according to any one of claims 1 to 7, characterized in that the HDPE has an MFR2 (2.16 kg, 190 °C) of 0.1 to 40 g/10 min. 9. Cable according to any one of claims 1 to 8, characterized in that the composition comprises 1.0 to 3.0% by weight of HDPE. 10. Cable according to claim 1 or 3 to 9, characterized in that the insulation layer is reticulated and the HDPE is unimodal. 11. Cable according to any one of claims 1 to 10, characterized in that the HDPE is an HDPE produced in a single site. 12. Cable according to any one of claims 1 to 11, characterized in that the insulating layer comprises 99% by weight or more of the polymer composition. 13. Process for producing a cable as defined in any one of claims 1 to 12, characterized in that it comprises the steps of: - applying one or more conductors, preferably by (co)extrusion, an inner semiconductor layer, an insulating layer and an outer semiconducting layer, in that order, wherein the insulating layer comprises at least 90% by weight of a polymer composition, as defined in any one of claims 1 to 12, and is optionally cross-linked.