Polymer composition for cable insulation

Abstract

A polymer composition comprising (I) at least 50 wt%, based on the total weight of the composition, as a whole of a propylene copolymer (A) of propylene and ethylene, said propylene copolymer (A) having: (a) a melt flow rate MFR2 (230 °C) measured according to ISO 1133 in the range of more than 0.1 to 2.5 g / 10min, (b) a soluble fraction (SF), determined by crystallization extraction (CRYSTEX), in the range of from 10 to 45 wt% based on the total weight of the propylene copolymer; (c) a comonomer content in the range of from 7.0 to 15 wt% based on the total weight of the propylene copolymer and determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy, (d) an intrinsic viscosity, determined according to ISO 1628 / 3, of the soluble fraction (IV(SF)) fraction of the propylene copolymer is in the range of 2.0 to 4.0 dL / g; and (e) a C2 content of the soluble fraction (C2(SF)) in the range of from 15 to 60 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy based on the total weight of the soluble fraction; and (II) in the range of from 15 to 50 wt% based on the total weight of the composition as a whole of a polyethylene homo- or copolymer with a C4-C10 alpha olefin, the polyethylene homo- or copolymer having a density in the range of from 935 to 965 kg / m3 determined according to ISO 1183-1.

Description

[0001] Polymer composition for cable insulation

[0002] The present invention relates to a polymer composition, an article comprising said polymer composition, preferably a cable comprising an insulation layer comprising said polymer composition and the use of said polymer composition as cable insulation for medium and high voltage cables, especially for high voltage (HV) cable, extra high voltage (EHV) cables and ultra-high voltage (UHV) cables.

[0003] Background

[0004] Crosslinkable LDPE products are used as insulation and semiconducting shields for extruded medium, high and extra high voltage cables, as a result of for e.g. their easy processability and their beneficial electrical properties. Cables insulated with crosslinked LDPE based polymer normally have a targeted operating temperature of up to 90°C. Thermoplastic ethylene polymer products (PE and HDPE) according to IEC 62895 are limited to 80°C as the maximum operating temperature. .

[0005] Attempts have been made to use thermoplastic propylene polymers as insulation material for medium, high and extra high voltage (MV, HV and EHV) cables. Propylene polymers offer a benefit especially for high and extra high voltage cables due to their higher melting point allowing for a higher maximum operating temperature.

[0006] WO2024 / 068577 discloses flexible polypropylene compositions for articles such as cables. The polypropylene composition contains a majority heterophasic polypropylene copolymer component and a minor ethylene polymer component. The compositions show a balance of flexibility, mechanical properties, impact properties and electrical breakdown strength.

[0007] An insulation material for HVDC power cables should combine good mechanical and electrical properties. According to the recommendations of the CIGRE TB852 , strain to break and tensile strength before and after ageing at 135°C for 10 days must be > 350% and > 8.5 MPa, respectively, and the indentation during the HV pressure test at 130°C for 1 h (method 1) must be below 50%. In general, cable manufacturers prefer insulation materials with low stiffness (< 1000 MPa), in order to allow collection of the cable on the drum without crack formation, and high resistance to impact, which avoids formation of cracks during the installation or maintenance. For the electrical properties of thermoplastic HVDC power cables, the DC conductivity must be low and preferably in the range of cross-linked LDPE (XLPE) insulation materials for the same application.

[0008] For both the mechanical and the thermal resistivity performance, the use of the low MFR propylene copolymer alone offered insufficient strain to break after ageing and the thermal resistivity was too high. The present inventors have now found that a blend of high density polyethylene (HDPE) and propylene copolymer improves the strain to break after ageing and reduces thermal resistivity while keeping the indentation from a pressure test at 130°C for 1 h below 50 % and having a level of DC conductivity in small scale specimens close to XLPE materials for HVDC application.

[0009] It was observed that, when blending an amount of high-density polyethylene into low MFR propylene copolymer of the invention strain to break at room temperature after ageing at 135°C for 10 days is fulfilling the requirement of at least 350% and is even higher compared to strain to break before ageing. This was achieved without the use of any added external compatibilizer to improve the adhesion at the interface between the two immiscible polymers.

[0010] Furthermore, the thermal resistivity at 100°C for the blend with 35 wt% HDPE was found to be 0.8 units lower than for the neat propylene copolymer, and therefore closer to thermal resistivity values of current XLPE insulation at its highest possible temperature of operation. The solution of this invention however can be used at higher operation temperature than XLPE insulation material due to its intrinsically higher thermal resistance.

[0011] These improvements were achieved while keeping the indentation from pressure test at 130°C for 1 h (method 1) below the 50% limit and the tensile strength still above the 8.5 MPa limit required by the CIGRE TB852.

[0012] Finally, the DC conductivity values at 100°C have the same order of magnitude as for the neat propylene copolymer and for XLPE insulation materials for HVDC power cables.

[0013] Summary of the invention

[0014] Viewed from one aspect the invention provides a polymer composition comprising

[0015] (I) at least 50 wt%, based on the total weight of the composition, as a whole of a propylene copolymer (A) of propylene and ethylene, said propylene copolymer (A) having:

[0016] (a) a melt flow rate MFR2 (230 °C) measured according to ISO 1133 in the range of more than 0.1 to 2.5 g / 10min,

[0017] (b) a soluble fraction (SF), determined by crystallization extraction (CRYSTEX), in the range of from 10 to 45 wt% based on the total weight of the propylene copolymer; (c) a comonomer content in the range of from 7.0 to 15 wt% based on the total weight of the propylene copolymer and determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy,

[0018] (d) an intrinsic viscosity, determined according to ISO 1628 / 3, of the soluble fraction (IV(SF)) fraction of the propylene copolymer is in the range of 2.0 to 4.0 dL / g; and

[0019] (e) a C2 content of the soluble fraction (C2(SF)) in the range of from 15 to 60 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy based on the total weight of the soluble fraction; and

[0020] (II) in the range of from 15 to 50 wt% based on the total weight of the composition as a whole of a polyethylene homopolymer or copolymer with a C4-C10 alpha olefin, the polyethylene homo- or copolymer having a density in the range of from 935 to 965 kg / m3determined according to ISO 1183-1.

[0021] Viewed from another aspect the invention provides an article comprising the polymer composition as herein described. Preferably, said article is a cable comprising an insulation layer comprising the polymer composition as described above or below.

[0022] Viewed from another aspect the invention provides use of the polymer composition as herein described as cable insulation for medium voltage cables, high voltage cables, extra high voltage cable and ultra-high voltage cables, preferably for high voltage cables and extra high voltage cables, more preferably high voltage DC (HVDC) cables and extra high voltage DC (EHVDC) cables.

[0023] Definitions

[0024] The propylene copolymer of propylene and ethylene will be called the propylene copolymer herein. The propylene copolymer is ideally a random heterophasic polypropylene copolymer with a semi-crystalline copolymeric matrix phase, which can be a random copolymer of propylene and at least one alpha-olefin comonomer such as ethylene, and an elastomeric phase dispersed therein. The elastomeric phase can be a propylene copolymer with a high amount of comonomer such as ethylene, which is ideally not randomly distributed in the polymer chain but is distributed in a comonomerrich block structure and a propylene-rich block structure.

[0025] A heterophasic polypropylene copolymer is typically differentiated from a one- phasic random copolymer in that it shows two distinct glass transition temperatures Tgwhich are attributed to the matrix phase and the elastomeric phase.

[0026] A propylene random copolymer is a copolymer of propylene monomer units and comonomer units in which the comonomer units are distributed randomly over the polypropylene chain. Accordingly, the propylene random copolymer does not contain an elastomeric polymer phase dispersed therein.

[0027] The propylene copolymer comprises at least two propylene polymer fractions (components), which have been produced under different polymerization conditions resulting in different (weight average) molecular weights and / or different comonomer contents for the fractions, preferably produced by polymerizing in multiple polymerization stages with different polymerization conditions, is referred to as “multimodal”. The prefix “multi” relates to the number of different polymer fractions the propylene polymer is consisting of. As an example of multimodal propylene polymer, a propylene polymer consisting of two fractions only is called “bimodal”, whereas a propylene polymer consisting of three fractions only is called “trimodal”.

[0028] A unimodal propylene polymer only consists of one fraction.

[0029] Thereby, the term “different” means that the propylene polymer fractions differ from each other in at least one property, preferably in the weight average molecular weight - which can also be measured in different melt flow rates of the fractions - or comonomer content or both.

[0030] Vis-breaking is a post reactor chemical process for modifying semi-crystalline polymers such as propylene polymers. During the vis-breaking process, the propylene polymer backbone is degraded, for example by means of peroxides, such as organic peroxides, via beta scission. The degradation is generally used for increasing the melt flow rate and narrowing the molecular weight distribution.

[0031] The propylene copolymer of the invention is ideally suited for use in the insulation layer of a power cable, in particular a high voltage or extra high voltage power cable. According to CIGRE TB852 recommendations for HVDC transmission cable systems high voltage (HV) refers to voltages up to and including 400 kV for DC systems. Extra-high voltage (EHV) refers to voltages above 400 kV to and including 800 kV for DC systems.

[0032] Although not explicitly mentioned in said recommendations, ultra-high voltage (UHV) consequently refers to voltages of more than 800 kV for DC systems.

[0033] In the following, amounts are given in % by weight (wt%) unless it is stated otherwise.

[0034] Detailed description of the invention

[0035] The present invention relates to a polymer composition suitable for use in the insulation layers of power cables. In particular, the polymer composition comprises a low MFR random heterophasic polypropylene copolymer and polyethylene homo- or copolymer with a C4-C10 alpha olefin, the polyethylene homo- or copolymer having a density in the range of from 935 to 965 kg / m3determined according to ISO 1183- 1 . This will be called the HDPE from now one.

[0036] Propylene copolymer of propylene and ethylene

[0037] The polymer composition of the invention comprises a propylene copolymer of propylene and ethylene having:

[0038] (a) a melt flow rate MFR2 (230 °C) measured according to ISO 1133 in the range of more than 0.1 to 2.5 g / 10min,

[0039] (b) a soluble fraction (SF), determined by crystallization extraction (CRYSTEX), in the range of 10 to 45 wt% based on the total weight of the propylene copolymer, and

[0040] (c) a comonomer content in the range of from 7.0 to 15 wt% based on the total weight of the propylene copolymer and determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy,

[0041] (d) an intrinsic viscosity, determined according to ISO 1628 / 3, of the soluble fraction (IV(SF)) fraction of the propylene copolymer is in the range of 2.0 to 4.0 dL / g; and

[0042] (e) a C2 content of the soluble fraction (C2(SF)) of 15 to 60 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy and based on the total weight of the soluble fraction.

[0043] The propylene copolymer of propylene and ethylene may contain one or more C4-C10 alpha olefin comonomers in addition to the propylene and ethylene. Such comonomers might be 1 -butene, 1 -hexene or 1 -octene. If present such monomers would preferably form no more than 10 wt% of the propylene copolymer, such as no more than 5.0 wt%. Ideally however, the propylene copolymer consists of propylene monomer units and ethylene monomer units only.

[0044] The propylene copolymer is preferably present in the polymer composition in an amount of at least 50 wt%, such as 55 to 85 wt%, more preferably 55 to 80 wt%, such as 55 to 79 wt%, especially 55 to 75 wt%, or 60 to 75 wt% based on the total weight of the polymer composition.

[0045] Ideally, the propylene copolymer and the HDPE are the only polyolefin components present in the polymer composition. However, it will be appreciated that some additives mentioned herein, may be carried on a polyolefin as a masterbatch. It is possible therefore that a polyolefin carrier is used in a masterbatch. Preferably, the polymer composition is free of dedicated additional polyolefin components such as a linear low density polyethylene or high density polyethylene. It is also preferred if the propylene copolymer defined herein is the only propylene polymer component in the polymer composition.

[0046] The propylene copolymer has a total comonomer content, such as total ethylene content, of 7.0 to 15 wt%, preferably of 8.0 to 14 wt%, based on the total weight of the propylene copolymer and determined by FT-IR spectroscopy calibrated by quantitative13C-NMR spectroscopy.

[0047] Further, the propylene copolymer preferably has a total propylene content of from 85 to 93.0 wt%, more preferably from 86 to 92.0 wt% based on the total weight of the propylene copolymer and determined by FT-IR spectroscopy calibrated by quantitative13C-NMR spectroscopy from crystallization extraction (CRYSTEX).

[0048] The propylene copolymer preferably consists of propylene and ethylene. Thus, it is preferred that the total ethylene content and the total propylene content make up 100 wt% of the propylene copolymer.

[0049] The propylene copolymer preferably has a melt flow rate MFR2 of from 0.1 to 2.0 g / 10 min, more preferably from 0.25 to 1 .8 g / 10 min, still more preferably from 0.5 to 1 .8 g / 10 min determined according to ISO 1133 at 230°C and 2.16 kg. In one embodiment, the propylene copolymer has a melt flow rate MFR2 of from 0.25 to 1 .25 g / 10 min.

[0050] It is preferred that the propylene copolymer is a random heterophasic polypropylene copolymer. The random heterophasic polypropylene copolymer has a copolymeric matrix phase and an elastomeric phase dispersed in said matrix phase. The matrix phase is preferably a propylene-ethylene random copolymer.

[0051] In the random heterophasic polypropylene copolymer, the matrix phase and the elastomeric phase usually cannot exactly be divided from each other. In order to characterize the matrix phase and the elastomeric phase of a random heterophasic copolymer of propylene and ethylene several methods are known. One method is the CRYSTEX method.

[0052] The propylene copolymer preferably has a crystalline fraction (CF) content, determined by crystallization extraction (CRYSTEX), in the range from 55 to 90 wt%, more preferably 60 to 88 wt%, especially 60 to 85 wt% based on the total weight of the propylene copolymer.

[0053] Said crystalline fraction (CF) preferably has an ethylene content (C2(CF)), determined by FT-IR spectroscopy calibrated by quantitative13C-NMR spectroscopy, in the range from 1 .0 to 10 wt%, more preferably from 2.0 to 10 wt% and most preferably from 3.0 to 8.0 wt%, based on the total amount of monomer units in the crystalline fraction (CF).

[0054] It is preferred that said crystalline fraction (CF) has a propylene content (C3(CF)), determined by FT-IR spectroscopy calibrated by quantitative13C-NMR spectroscopy, in the range from 90 to 99.0 wt.-%, preferably from 90.0 to 98.0 wt.-% and most preferably from 92.0 to 97.0 wt.-%, based on the total amount of monomer units in the crystalline fraction (CF).

[0055] Further, said crystalline fraction (CF) preferably has an intrinsic viscosity (IV(CF)), determined according to ISO 1628-3, in the range 2.0 to 4.0 dL / g, more preferably from 2.25 to 3.75 dL / g and most preferably from 2.5 to 3.5 dL / g.

[0056] In one embodiment, the propylene copolymer has a crystalline fraction (CF) content determined by crystallisation extraction (CRYSTEX) in the range of 90 to 55 wt%, e.g. 85 to 60 wt%, wherein said crystalline fraction has an intrinsic viscosity (IV(CF)), determined according to ISO 1628 / 3, of 2.0 to 4.0 dL / g and / or a C2 content (C2(CF)) of 3.0 to 8.0 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy.

[0057] The propylene copolymer has a soluble fraction (SF) content, determined by crystallization extraction (CRYSTEX), in the range from 10 to 45 wt%, more preferably from 12 to 40 wt%, still more preferably from 15 to 40 wt%, more preferably from 20 to 30 wt%, based on the total weight amount of the propylene copolymer.

[0058] Said soluble fraction (SF) has an ethylene content (C2(SF)), determined by FT-IR spectroscopy calibrated by quantitative13C-NMR spectroscopy, in the range from 15 to 60 wt%, preferably 20 to 50 wt%, more preferably from 20 to 40 wt% based on the total amount of monomer units in the soluble fraction (SF).

[0059] It is preferred that said soluble fraction (SF) has a propylene content (C3(SF)), determined by FT-IR spectroscopy calibrated by quantitative13C-NMR spectroscopy, in the range from 40 to 85 wt.-%, preferably from 50 to 80 wt.-% and most preferably from 60 to 80 wt.-%, based on the total amount of monomer units in the soluble fraction (SF).

[0060] In one embodiment, the propylene copolymer has a soluble fraction (SF) content, determined by crystallization extraction (CRYSTEX), in the range from 15 to 40 wt%, more preferably from 15 to 30 wt% based on the total weight amount of the propylene copolymer and said propylene copolymer has an ethylene content (C2(SF)), determined by FT-IR spectroscopy calibrated by quantitative13C-NMR spectroscopy, in the range from 15 to 40 wt%, such as 30 to 40 wt%, based on the total amount of monomer units in the propylene copolymer.

[0061] Further, said soluble fraction (SF) has an intrinsic viscosity (IV(SF)), determined according to ISO 1628-3, in the range from 2.0 to 4.0 dL / g, more preferably from 2.25 to 3.75 dL / g and most preferably from 2.3 to 3.5 dL / g.

[0062] The propylene copolymer preferably has a total intrinsic viscosity (IV (total)), determined according to ISO 1628-3, in the range from 2.0 to 4.0 dL / g, more preferably from 2.25 to 3.75 dL / g and most preferably from 2.5 to 3.5 dL / g.

[0063] It is preferred that the crystalline fraction (OF) and the soluble fraction (SF) make up 100 wt% of the propylene copolymer.

[0064] It is preferred if the melting point (Tm) of the propylene copolymer is in the range of 132 to 155°C, preferably from 135 to 155°C and most preferably from 140 to 155°C, determined by to DSC analysis according to ISO 113571 part 3 / method C2.

[0065] The density of the propylene copolymer is preferably in the range of 880 to 905 kg / m3(ISO1183).

[0066] It is preferred if the melting point (Tm) of the propylene copolymer is in the range of 132 to 155°C, preferably from 135 to 150°C and most preferably from 140 to 155°C, determined by to DSC analysis according to ISO 113571 part 3 / method C2.

[0067] The density of the propylene copolymer is preferably in the range of 880 to 905 kg / m3(ISO1183).

[0068] In one embodiment, the propylene copolymer has:

[0069] (a) a melt flow rate MFR2 (230 °C), measured according to ISO 1133, in the range of 0.5 to 1.8 g / 1 Omin;

[0070] (b) a soluble fraction (SF) determined by crystallization extraction (CRYSTEX) in the range of 15-40 wt%;

[0071] (c) a comonomer content in the range of 8.0 to 14 wt% based on the total weight of the polymer composition and determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy;

[0072] (d) an intrinsic viscosity, determined according to ISO 1628 / 3, of the soluble fraction (I (SF)) of the propylene copolymer is in the range of 2.3 to 3.0 dL / g; and

[0073] (e) C2 content of soluble fraction (C2(SF)) of 20 to 40 wt% determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy. Manufacture

[0074] The propylene copolymer, such as random heterophasic polypropylene copolymer, can be polymerized in a sequential multistage polymerization process, i.e. in a polymerization process in which two or more polymerization reactors are connected in series. Preferably, in the sequential multistage polymerization process, two or more, more preferably three or more, such as three or four, polymerization reactors are connected in series. The term “polymerization reactor” shall indicate that the main polymerization takes place. Thus in case the process consists of four polymerization reactors, this definition does not exclude the option that the overall process comprises for instance a pre-polymerization step in a pre-polymerization reactor.

[0075] When the propylene copolymer is a random heterophasic polypropylene copolymer, the matrix phase of the random heterophasic polypropylene copolymer is polymerized in first polymerization reactor for producing a unimodal matrix phase or in the first and second polymerization reactor for producing a multimodal matrix phase. The elastomeric phase of the random heterophasic polypropylene copolymer is preferably polymerized in the subsequent one or two polymerization reactor(s) in the presence of the matrix phase for producing a unimodal elastomeric phase or a multimodal elastomeric phase.

[0076] Preferably, the polymerization reactors are selected from slurry phase reactors, such as loop reactors and / or gas phase reactors such as fluidized bed reactors, more preferably from loop reactors and fluidized bed reactors.

[0077] A preferred sequential multistage polymerization process is a “loop-gas phase”- process, such as developed by Borealis A / S, Denmark (known as BORSTAR® technology) described e.g. in patent literature, such as in EP 0 887 379, WO 92 / 12182 WO 2004 / 000899, WO 2004 / 111095, WO 99 / 24478, WO 99 / 24479 or in WO 00 / 68315.

[0078] A further suitable slurry-gas phase process is the Spheripol® process of LyondellBasell.

[0079] Suitable sequential polymerization processes for polymerizing the propylene copolymer, preferably the random heterophasic polypropylene copolymer, are e.g. disclosed in WO 2015 / 117948.

[0080] The propylene copolymer, preferably the random heterophasic polypropylene copolymer can be polymerized in the presence of a Ziegler-Natta catalyst. Suitable Ziegler-Natta catalysts are e.g. disclosed in WO 2015 / 117948.

[0081] The propylene copolymer is preferably not subjected to a visbreaking step as e.g. described in WO 2013 / 092620 A1 . Note that the propylene copolymer is characterized by crystallization extraction (CRYSTEX) using trichlorobenzene (TCB) as a solvent. This method is described below in the determination methods section. The crystalline fraction (CF) contains for the most part the matrix phase and only a small part of the elastomeric phase and the soluble fraction (SF) contains for the most part the elastomeric phase and only a small part of the matrix phase.

[0082] Polyethylene homo- or copolymer

[0083] The polyethylene homo- or copolymer is a high density polyethylene, HDPE. The HDPE can be produced in a low pressure process. The catalyst used to prepare the HDPE may be an Ziegler Natta catalyst or a single site catalyst. The use of an HDPE made using a single site catalyst may offer reduced DC electrical conductivity.

[0084] The high density polyethylene (HDPE) used in the composition of the invention may have a density of 940 to 965 kg / m3, such as about 945 to 960 kg / m3.

[0085] The MFR2 (2.16 kg, 190°C) of the HDPE polymer is preferably from 0.1 to 10 g / 10 min, more preferably is from 0.5 to 5.0 g / 10min, and most preferably is from 0.5 to 3.0 g / 10min.

[0086] The HDPE may be an ethylene homopolymer or an ethylene copolymer with one or more alpha olefin comonomers, such as a C4-C10 alpha olefin comonomer. The use of 1 -butene, 1 -hexene or 1 -octene is preferred, especially 1 -butene or 1 -hexene.

[0087] The HDPE of the invention may be unimodal or multimodal. It is preferred if the HDPE is multimodal. Usually, a polyethylene composition comprising at least two polyethylene fractions, which have been produced under different polymerisation conditions resulting in different (weight average) molecular weights and molecular weight distributions for the fractions, is referred to as "multimodal". Accordingly, in this sense the compositions of the invention are multimodal polyethylenes. The prefix "multi" relates to the number of different polymer fractions the composition is consisting of. Thus, for example, a composition consisting of two fractions only is called "bimodal".

[0088] The form of the molecular weight distribution curve, i.e. the appearance of the graph of the polymer weight fraction as function of its molecular weight, of such a multimodal polyethylene will show two or more maxima or at least be distinctly broadened in comparison with the curves for the individual fractions. For example, if a polymer is produced in a sequential multistage process, utilising 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 recorded, the individual curves from these fractions are superimposed into the molecular weight distribution curve for the total resulting polymer product, usually yielding a curve with two or more distinct maxima.

[0089] In one embodiment, the HDPE can be unimodal. Such a unimodal HDPE may have an MFR and density as disclosed above.

[0090] In a preferred embodiment, the HDPE is a multimodal polyethylene polymer having a lower molecular weight homopolymer component and a higher molecular weight copolymer component, e.g. with a C4-C10 alpha olefin comonomer. Such a bimodal HDPE may also have an MFR and density as disclosed above.

[0091] The HDPE may have a PDI of 5 to 30 such as 8 to 14.

[0092] It is possible to use a mixture of HDPEs in the polymer composition of the invention however it is preferred if a single HDPE is used.

[0093] The HDPE may be present in the polymer composition in any amount of 15 to 45 wt%, such as 20 to 45 wt%, preferably 21 to 45 wt%, especially 25 to 45 wt% or 25 to 40 wt%.

[0094] It is particularly preferred if component (II) of the composition of the invention is present in the range of 25 to 45 wt% based on the composition as a whole. Ideally, it has a density of from 940 to 965 kg / m3according to ISO 1183-1 and an MFR2 of 0.1 to 5.0 g / 10min.

[0095] As noted above, the HDPE preferably comprise a lower molecular weight component (A) and a higher molecular weight component (B). The weight ratio of fraction (A) to fraction (B) in the composition is in the range 30:70 to 70:30, more preferably 35:65 to 65:35, most preferably 40:60 to 60:40. In some embodiments the ratio may be 45 to 55 wt% of fraction (A) and 55 to 45 wt% fraction (B), such as 45 to 52 wt% of fraction (A) and 55 to 48 wt% fraction (B). It has been found however that the best results are obtained when the high molecular weight (HMW) component is present at the same percentage or even predominates, e.g. 50 to 54 wt% of the HMW component (B) and 50 to 46 wt% fraction (A).

[0096] Fractions (A) and (B) may both be ethylene copolymers. At least one of the fractions is an ethylene copolymer. Preferably, the polymer comprises an ethylene homopolymer and an ethylene copolymer component.

[0097] Where one of the components is an ethylene homopolymer, this is preferably the component with the lower weight average molecular weight (Mw), i.e. fraction (A). An ideal HDPE polymer is therefore a lower molecular weight homopolymer component (A) with a higher molecular weight component (B), ideally an ethylene butene higher molecular weight component.

[0098] The lower molecular weight fraction (A) preferably has an MFR2 (190°C) of 10 g / 10m or higher, more preferably of 50 g / 10m or higher, and most preferably 100 g / 10min or higher. Furthermore, fraction (A) preferably, has an MFR2 (190°C) of 1000 g / 10 min or lower, preferably 800 g / 10 min or lower, and most preferably 600 g / 10min or lower.

[0099] Preferably, fraction (A) is an ethylene homo- or copolymer with a density of at least 965 kg / m3.

[0100] Most preferably, fraction (A) is an ethylene homopolymer. If fraction (A) is a copolymer, the comonomer is preferably 1 -butene. The comonomer content of fraction (A), if it is a copolymer, is preferably very low, such as less than 0.2 mol%, preferably less than 0.1 mol%, especially less than 0.05 mol%.

[0101] Preferably, fraction (B) is an ethylene copolymer with a density of less than 965 kg / m3. Preferred ethylene copolymers employ alpha-olefins (e.g. C4-C10 alpha-olefins ) as comonomers. But-1 -ene is an especially preferred comonomer.

[0102] Where herein features of fractions (A) and / or (B) are given, these values are generally valid for the cases in which they can be directly measured on the respective fraction, e.g. when the fraction is separately produced or produced in the first stage of a multistage process. However, the HDPE may also be and preferably is produced in a multistage process wherein e.g. fractions (A) and (B) are produced in subsequent stages. In such a case, the properties of the fractions produced in the second step (or further steps) of the multistage process can either be inferred from polymers, which are separately produced in a single stage by applying identical polymerisation conditions ( e.g. identical temperature, partial pressures of the reactants diluents, suspension medium, reaction time) with regard to the stage of the multistage process in which the fraction is produced, and by using a catalyst on which no previously produced polymer is present. Alternatively, the properties of the fractions produced in a higher stage of the multistage process may also be calculated, e.g. in accordance with B. Hagstrom, Conference on Polymer Processing ( The Polymer Processing Society), Extended Abstracts and Final Programme, Gothenburg, August 19 to 21, 1997, 4: 13.

[0103] A multimodal (e.g. bimodal ) HDPE may be produced by mechanical blending two or more polyethylenes ( e.g. monomodal polyethylenes ) having differently centred maxima in their molecular weight distributions. The monomodal polyethylenes required for blending may be available commercially or may be prepared using any conventional procedure known to the skilled man in the art. Each of the polyethylenes used in a blend and / or the final polymer composition may have the properties hereinbefore described for the lower molecular weight component, higher molecular weight component and the composition, respectively.

[0104] The HDPE is preferably prepared by polymerising ethylene and optionally at least one C4-C10 alpha olefin comonomer so as to form a lower molecular weight component (A); and subsequently polymerising ethylene and at least one C4-C10 alpha olefin comonomer in the presence of component (A) so as to form a higher molecular weight component (B).

[0105] It is preferred if at least one component is produced in a gas-phase reaction. Further preferred, fraction (A), is produced in a slurry reaction, preferably in a loop reactor, and one of the fractions (A) and (B), preferably fraction (B), is produced in a gas-phase reaction.

[0106] Preferably, the multimodal HDPE may be produced by polymerisation using conditions which create a multimodal (e.g. bimodal) polymer product, e.g. using a two or more stage, i.e. multistage, polymerisation process with different process conditions in the different stages or zones (e.g. different temperatures, pressures, polymerisation media, hydrogen partial pressures, etc).

[0107] It is previously known to produce multimodal, in particular bimodal, HDPEs in a multistage process, EP4151677, which is hereby incorporated by way of reference in its entirety, including all its preferred embodiments as described therein, as a preferred multistage process for the production of the polyethylene composition according to the invention.

[0108] The polymerisation catalysts include Ziegler-Natta (ZN), or metallocenes.

[0109] The catalyst may be supported, e.g. with conventional supports including silica, Al- containing supports and magnesium dichloride based supports. HDPEs of use in the invention are commercially available.

[0110] Polymer Composition

[0111] The polymer composition can comprise additional polymeric components different from the propylene copolymer and the HDPE such as in an amount of preferably 0.0 to 10.0 wt% based on the total weight of the polymer composition. It is preferred if the propylene copolymer and HDPE are the only polymer components present other than any polymers used as carriers in a masterbatch.

[0112] The polymer composition can comprise one or more additives in an amount of up to 5.0 wt%, based on the total weight of the polymer composition. The one or more additives are preferably selected from acid scavengers, antioxidants, nucleating agents, etc. Such additives are commercially available and for example described in “Plastic Additives Handbook”, 6thedition 2009 of Hans Zweifel (pages 1141 to 1190). Usually, these additives are added in quantities of 1 to 50,000 ppm for each single component.

[0113] The one or more additives can be added to the polymer composition in a blending step. Thereby, the one or more additives can be added to the polymer composition in form of masterbatches in which one or more additives are blended with a carrier polymer in concentrated amounts.

[0114] Preferably, the polymer composition is prepared by melt blending the propylene copolymer, the HDPE and optional further additives. The polymer composition is preferably not subjected to vis-breaking.

[0115] The polymer composition may have a melt flow rate MFR2 of from 0.25 to 4.0 g / 10 min, more preferably from 0.5 to 3.0 g / 10 min determined according to ISO 1133 at 230°C and 2.16 kg.

[0116] The polymer composition may have a soluble fraction (SF), determined by crystallization extraction (CRYSTEX), in the range of 5.0 to 20 wt%, such as 8.0 to 18 wt%.

[0117] The polymer composition may have a total C2 content determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy of 25 to 50 wt%.

[0118] It is preferred that the polymer composition preferably comprises a total propylene content of from 50 to 75 wt% based on the total weight of the polymer composition and determined by FT-IR spectroscopy calibrated by quantitative13C-NMR spectroscopy from crystallization extraction (CRYSTEX).

[0119] The polymer composition may have an intrinsic viscosity (IV) determined according to ISO 1628 / 3 of 2.0 to 4.0 dL / g.

[0120] The polymer composition may have a C2 content of the crystalline fraction (C2(CF)) of 25 to 55 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy, such as 30 to 50 wt%. The polymer composition may have a C2 content of the soluble fraction (C2(SF)) of 20 to 45 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy, such as 25 to 35 wt%.

[0121] The polymer composition may have a crystalline fraction intrinsic viscosity (IV(CF)), determined according to ISO 1628 / 3, of 2.0 to 4.0 dL / g.

[0122] The polymer composition may have a soluble fraction intrinsic viscosity (IV(SF)), determined according to ISO 1628 / 3, of 2.0 to 4.0 dL / g.

[0123] In one embodiment, the polymer composition has at least one of: an intrinsic viscosity (IV) determined according to ISO 1628 / 3 of 2.0 to 4.0 dL / g; a C2 content of the crystalline fraction (C2(CF)) of 30 to 55 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy; a C2 content of the soluble fraction (C2(SF)) of 25 to 35 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy; a crystalline fraction intrinsic viscosity (IV(CF)), determined according to ISO 1628 / 3, of 2.0 to 4.0 dL / g; a soluble fraction intrinsic viscosity (IV(SF)), determined according to ISO 1628 / 3, of 2.0 to 4.0 dL / g.

[0124] The polymer composition preferably has strain to break of at least 350% at 23°C, such as at least 400% preferably least 450% (before ageing, 23°C) and / or a strain at break after ageing at 135°C for 10 days (I EC 60811-401) of at least 400% at 23°C preferably at least 450% determined according to IEC 60811- 501. Ageing is carried out according to IEC 60811-401 for any embodiment where ageing is required.

[0125] The polymer composition preferably has a tensile modulus of less than 450 MPa at 23°C determined according to IEC 60811-501.

[0126] The polymer composition may have a tensile modulus of less than 3000 MPa at -20°C determined according to IEC 60811-501.

[0127] The polymer composition may have a tensile strength of at least 30 MPa at 23°C before ageing and / or having a tensile strength of at least 30 MPa at 23°C after ageing at 135°C for 10 days (IEC 60811-401), such as 35 to 45 MPa determined according to IEC 60811-501. The polymer composition may have an indentation pressure test (as described in HV method 1) 130°C 1h from CIGRE TB852) of less than 49%, such as less than 10%.

[0128] The polymer composition may have a thermal resistivity at 100°C of 4.5 Km / W or less especially wherein the thermal resistivity is lower than that of the propylene copolymer itself (ISO 22007-2).

[0129] The polymer composition may have a thermal resistivity at 23°C of 4.5 Km / W or less especially wherein the thermal resistivity is lower than that of the propylene copolymer itself (ISO 22007-2).

[0130] The polymer composition preferably has a DC conductivity at 100 °C of less than 40 fS / m, preferably less than 30 fS / m, especially 5-40 fS / m.

[0131] Article

[0132] The present invention further relates to an article comprising the polymer composition as herein defined. The article is preferably a cable, preferably a medium voltage cable voltage cables, high voltage cables, extra high voltage cable or ultra high voltage cable, more preferably a high voltage cable or an extra high voltage cable, even more preferably a high voltage DC (HVDC) cable or an extra high voltage DC (EHVDC) cable, comprising an insulation layer comprising the polymer composition as herein described.

[0133] The insulation layer preferably comprises from 90 to 100 wt%, preferably from 95 to 100 wt%, still more preferably from 99 to 100 wt%, based on the total weight of the insulation layer of the polymer composition. Most preferably the insulation layer consists of the polymer composition.

[0134] It is preferred that the article meets all CIGRE TB852 recommendations for HVDC cable systems.

[0135] The cable usually comprises of at least one conductor and at least one insulation layer comprising the polymer composition as herein described. The term "conductor" means herein above and below that the conductor comprises one or more wires. The wire can be for any use and be e.g. telecommunication or electrical wire. Moreover, the cable may comprise one or more such conductors. Preferably the conductor is an electrical conductor and comprises one or more metal wires. The cable is preferably a power cable.

[0136] A power cable is defined to be a cable capable of transferring energy operating at any voltage, typically operating at voltages higher than 1 kV. The voltage applied to the power cable can be alternating (AC), direct (DC), or transient (impulse). The polymer composition of the invention is very suitable for power cables.

[0137] The term "cable" as used herein, is intended to denote a cable comprising at least one cable core, optionally two cable cores or three cable cores. Each “cable core” as used herein, comprises a conductor surrounded by at least an inner semiconductive layer, an insulation layer and an outer semiconductive layer. The one or more cable cores in the cable may be surrounded by additional layer(s) such as (but not limited to) bedding(s), metallic screen, as well as various protection layer(s) such as jacketing layer(s), armouring layer(s) or any combination thereof. These variations and cable constructions are familiar to the person skilled in the art. The term “DC cable” refers to a direct current (DC) cable generally comprising one or more cable cores, preferably one or two cable cores.

[0138] The cable according to the present invention is very advantageously a DC power cable, which can be e.g. a medium voltage (MV), a high voltage (HV) or an extra high voltage (EHV) or an ultra-high voltage (UHV) DC cable, which terms, as well known, indicate the level of operating voltage.

[0139] The DC power cable may operate at voltages of at least 30kV, such as a HVDC, EHVDC or UHVDC cable. For HVDC, EHVDC or UHVDC cables the operating voltage is defined herein as the electric voltage between ground and the conductor of the high voltage cable.

[0140] The cable may be an HVDC, EHVDC or UHVDC power cable operating at voltages of 40 kV or higher, even at voltages of 50 kV or higher even 60 kV or higher. In some embodiments, the operating voltage may be higher than 80 kV. The upper limit is not limited. A practical upper limit can be up to 1100 kV. In one embodiment, the invention extends to a DC power cable in use, in particular a high voltage DC power cable carrying current and subject to at least 40 kV.

[0141] The present invention also relates to the use of the polymer composition as herein described as cable insulation for medium voltage cables, high voltage cables, extra high voltage cable and ultra-high voltage cables, preferably for high voltage cables and extra high voltage cables, more preferably high voltage DC (HVDC) cables and extra high voltage DC (EHVDC) cables.

[0142] The invention will now be described with reference to the following non-limiting examples.

[0143] Determination Methods The following definitions of terms and determination methods apply for the above general description of the invention as well as to the below examples unless otherwise defined.

[0144] Measurement methods a) Melt Flow Rate (MFR2)

[0145] The melt flow rate is the quantity of polymer in grams which the test apparatus standardized to ISO 1133 extrudes within 10 minutes at a certain temperature under a certain load.

[0146] The melt flow rate MFR2 of propylene copolymer and the polymer composition is measured at 230°C with a load of 2.16 kg according to ISO 1133.

[0147] The melt flow rate MFR2 of any ethylene based polymers is measured at 190°C with a load of 2.16 kg according to ISO 1133. b) Density

[0148] The density is measured according to ISO 1183-1 :2004 Method A on compression moulded specimen prepared according to EN ISO 1872-2 (Feb 2007) and is given in kg / m3. c) Comonomer content

[0149] Comonomer content quantification of poly(propylene-co-ethylene) copolymers Quantitative13C{1H} NMR spectra were recorded in the solution-state using a Bruker Avance NEO 400 NMR spectrometer operating at 400.15 and 100.62 MHz for1H and13C respectively. All spectra were recorded using a13C optimised 10 mm extended temperature probe head at 125°C using nitrogen gas for all pneumatics. Approximately 200 mg of material was dissolved in 3 ml of 7,2-tetrachloroethane-c / 2 (TCE-c ) along with chromium-(lll)-acetylacetonate (Cr(acac)3) resulting in a 60 mM solution of relaxation agent in solvent {8} and with approximately 3 mg BHT (2,6-di-tert-butyl-4- methylphenol CAS 128-37-0) . To ensure a homogenous solution, after initial sample preparation in a heat block, the NMR tube was further heated in a rotatory oven for at least 1 hour. Upon insertion into the magnet the tube was spun at 10 Hz. This setup was chosen primarily for the high resolution and quantitatively needed for accurate ethylene content quantification. Standard single-pulse excitation was employed without NOE, using an optimised tip angle, 1 s recycle delay and a bi-level WALTZ16 decoupling scheme {3, 4}. A total of 6144 (6k) transients were acquired per spectra. Quantitative13C{1H} NMR spectra were processed, integrated and relevant quantitative properties determined from the integrals using proprietary computer programs. All chemical shifts were indirectly referenced to the central methylene group of the ethylene block (EEE) at 30.00 ppm using the chemical shift of the solvent. This approach allowed comparable referencing even when this structural unit was not present. Characteristic signals corresponding to the incorporation of ethylene were observed {7}.

[0150] The comonomer fraction was quantified using the method of Wang et. al. {6} through integration of multiple signals across the whole spectral region in the13C{1H} spectra. This method was chosen for its robust nature and ability to account for the presence of regiodefects when needed. Integral regions were slightly adjusted to increase applicability across the whole range of encountered comonomer contents.

[0151] For systems where only isolated ethylene in PPEPP sequences was observed the method of Wang et al. was modified to reduce the influence of non-zero integrals of sites that are known to not be present. This approach reduced the overestimation of ethylene content for such systems and was achieved by reduction of the number of sites used to determine the absolute ethylene content to:

[0152] E = 0.5 (Spp + Spy + Spb + 0.5( Sap + Say))

[0153] Through the use of this set of sites the corresponding integral equation becomes: E = 0.5 (IH +IG + 0.5(IC+ ID)) using the same notation used in the article of Wang et al. {6}. Equations used for absolute propylene content were not modified.

[0154] The mole percent comonomer incorporation was calculated from the mole fraction: E [mol%] = 100 * fE

[0155] The weight percent comonomer incorporation was calculated from the mole fraction: E [wt%] = 100 * (fE * 28.06 ) / ( (fE * 28.06) + ((1 -fE) * 42.08) ) Bibliographic references:

[0156] 1) Busico, V., Cipullo, R., Prog. Polym. Sci. 26 (2001) 443.

[0157] 2) Busico, V., Cipullo, R., Monaco, G., Vacatello, M., Segre, A.L., Macromolecules 30 (1997) 6251.

[0158] 3) Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha, A., Baugh, D. Winniford, B., J. Mag. Reson. 187 (2007) 225.

[0159] 4) Busico, V., Carbonniere, P., Cipullo, R., Pellecchia, R., Severn, J., Talarico, G., Macromol. Rapid Commun. 2007, 28, 1128.

[0160] 5) Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev. 2000, 100, 1253.

[0161] 6) Wang, W-J., Zhu, S., Macromolecules 33 (2000), 1157.

[0162] 7) Cheng, H. N., Macromolecules 17 (1984), 1950. 8) Singh, G., Kothari, A., Gupta, V., Polymer Testing 28 5 (2009), 475.

[0163] 9) Kakugo, M., Naito, Y., Mizunuma, K., Miyatake, T. Macromolecules 15 (1982) 1150.

[0164] 10) Randall, J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1989, C29, 201.

[0165] 11) Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev. 2000, 100, 1253. d) Differential scanning calorimetry (DSC) analysis, melting temperature (Tm) and crystallization temperature (Tc):

[0166] These parameters were measured with a TA Instrument Q2000 differential scanning calorimetry (DSC) on 5 to 7 mg samples. DSC is run according to ISO 11357 I part 3 / method C2 in a heat I cool / heat cycle with a scan rate of 10°C / min in the temperature range of -30°C to +225°C.

[0167] Crystallization temperature and heat of crystallization (He) are determined from the cooling step, while melting temperature and heat of fusion (Hf) are determined from the second heating step.

[0168] When a sample shows two or more melting temperatures and / or crystallization temperatures only the main melting temperature (at the highest Hf) and main crystallization temperature (at the highest He) are displayed in the accordant table. The difference of melting temperature and crystallization temperature (Tm-Tc) is given for the main melting temperature and the main crystallization temperature. e) Crystallisation extraction (CRYSTEX) analysis

[0169] Note: Crystallization extraction (CRYSTEX) analyses the polymeric part of each component, with non-polymeric parts, such as any fillers or particulate pigments, not contributing to the reported CRYSTEX data presented.

[0170] Determination of Crystalline and soluble fractions and their respective properties (IV and Ethylene content)

[0171] The crystalline (CF) and soluble fractions (SF) of the polypropylene (PP) compositions as well as the comonomer content and intrinsic viscosities of the respective fractions were analyzed by use of the Crystex (crystallisation extraction) method. Potential instruments that can be used are Crystex QC or Crystex 42 (Polymer Char; Valencia, Spain). Details of the technigue and the method can be found in literature (Ljiljana Jeremie, Andreas Albrecht, Martina Sandholzer & Markus

[0172] Gahleitner (2020): Rapid characterization of high-impact ethylene-propylene copolymer composition by crystallization extraction separation: comparability to standard separation methods, International Journal of Polymer Analysis and Characterization, 25:8, 581-596).

[0173] The crystalline and amorphous fractions are separated through temperature cycles of dissolution at 160°C, crystallization at 40°C and re-dissolution in 1 ,2,4- trichlorobenzene at 160°C. Quantification of SF and CF and determination of ethylene content (C2) are achieved by means of an integrated infrared detector (IR4) and for the determination of the intrinsic viscosity (IV) an online 2-capillary viscometer is used. IR4 detector is a multiple wavelength detector measuring IR absorbance at two different bands (CH3 stretching vibration (centred at app. 2960 cm-1) and the CH stretching vibration (2700-3000 cm-1) that are serving for the determination of the concentration and the Ethylene content in Ethylene-Propylene copolymers. IR4 detector is calibrated with series of 8 EP copolymers with known Ethylene content in the range of 2 wt% to 69 wt% (determined by 13C-NMR) and each at various concentrations, in the range of 2 and 13mg / ml. To encounter for both features, concentration and ethylene content at the same time for various polymer concentration expected during Crystex analyses the following calibration equations were applied:

[0174] Cone = a + b*Abs(CH) + c*(Abs(CH))2+ d*Abs(CH3) + e*(Abs(CH3)2+ f*Abs(CH)*Abs(CH3) (Equation 1)

[0175] CH3 / 1000C = a + b*Abs(CH) + c* Abs(CH3) + d * (Abs(CH3) / Abs(CH)) + e * (Abs(CH3) / Abs(CH))2(Equation 2)

[0176] The constants a to f for equation 1 and a to e for equation 2 were determined by using least square regression analysis.

[0177] The CH3 / 1000C is converted to the ethylene content in wt% using following relationship: wt% (Ethylene in EP Copolymers) = 100 - CH3 / 1000TC * 0.3 (Equation 3)

[0178] Amount of Soluble Fraction (SF) and Crystalline Fraction (CF) are correlated through the XS calibration to the “Xylene Cold Soluble” (XCS) quantity and respectively Xylene Cold Insoluble (XCI) fractions, determined according to standard gravimetric method as per ISO16152. XS calibration is achieved by testing various EP copolymers with XS content in the range 2-31 wt%. A linear calibration curve is used.

[0179] Intrinsic viscosity (IV) of the parent EP copolymer and its soluble and crystalline fractions are determined with a use of an online 2-capillary viscometer and are correlated to corresponding IV’s determined by standard method in decalin according to ISO 1628-3. Calibration is achieved with various EP copolymers and PP polymers with IV = 2-4 dL / g. The determined calibration curve is linear.

[0180] The samples to be analyzed are weighed out in concentrations of 10 mg / ml to 20 mg / ml. After automated filling of the vial with 1 ,2,4-TCB containing 250 mg / l 2,6-tert-butyl-4- methylphenol (BHT) as antioxidant, the sample is dissolved at 170°C until complete dissolution is achieved with either constant stirring or gentle shaking. To avoid sample degradation, polymer solution is blanketed with the N2 atmosphere during dissolution.

[0181] For PP composition containing inorganic fillers or pigments or any other non- TCB soluble polymeric substances removal of these is required. This can be done by hot filtration prior injection.

[0182] A defined volume of the polymer solution is injected into the column filled with inert support where the crystallization of the sample and separation of the soluble fraction from the crystalline fraction is taking place. This process is repeated two times. During the first injection the whole sample is measured at high temperature, determining the IV [dl / g] and the C2 [wt%] of the PP composition. During the second injection the soluble fraction (at low temperature) and the crystalline fraction (at high temperature) with the crystallization cycle are measured (wt% SF, wt% C2, IV). f) Intrinsic viscosity (IV)

[0183] The reduced viscosity (also known as viscosity number), qred, and intrinsic viscosity, IV, are determined according to ISO 1628-3: “Determination of the viscosity of polymers in dilute solution using capillary viscometers”.

[0184] Relative viscosities of a diluted polymer solution with concentration of 1 mg / ml and of the pure solvent (decahydronaphthalene stabilized with 200 ppm 2,6-bis(1 , 1 - dimethylethyl)-4-methylphenol) are determined in an automated capillary viscometer (Lauda PVS1) equipped with 4 Ubbelohde capillaries placed in a thermostatic bath filled with silicone oil. The bath temperature is maintained at 135 °C. The sample is dissolved with constant stirring until complete dissolution is achieved (typically within 90 min). The efflux time of the polymer solution as well as of the pure solvent are measured several times until three consecutive readings do not differ for more than 0.2s (standard deviation).

[0185] The relative viscosity of the polymer solution is determined as the ratio of averaged efflux times in seconds obtained for both, polymer solution and solvent: [dimensionless]

[0186] Reduced viscosity (qred) is calculated using the equation: A red m where C is the polymer solution concentration at 135°C: C= — , and m is the polymer mass, V is the solvent volume, and y is the ratio of solvent densities at 20°C and 135°C (y=p2o / pi35=1 .107).

[0187] The calculation of intrinsic viscosity IV is performed by using the Schulz-Blaschke equation from the single concentration measurement: where K is a coefficient depending on the polymer structure and concentration. For calculation of the approximate value for IV, K=0.27. g) Tensile modulus, tensile strength, strain to break

[0188] The tensile specimens were compression moulded 5A dogbones prepared according to I EC 60811-501. The tensile tests were performed at a strain rate of 25 mm / min until break and at +23 °C or -20° C, according to I EC 60811-501 . The tensile tests at +23°C were made before and after ageing at 135°C for 10 days (ageing procedure according to IEC 60811-401). The results were calculated as median of at least 5 measurements. h) Indentation in pressure test

[0189] The indentation properties at high temperature were determined on compression moulded plaques at 130°C for 1 h with a cylindric indentor according to method 1 pressure test in the CIGRE TB852 recommendations. The plaques are made via compression moulding at 210°C with a pressure of 5.6 MPa and cooled down to 40°C at a cooling rate of 15 K / min. The results were calculated as average of at least 3 measurements. i) DC Conductivity test

[0190] The DC electrical conductivity was tested for all the studied materials on 500 pm thick compression moulded plaques with 26 cm diameter. The compression moulding plaques were produced via compression moulding of pellets at 210°C with a pressure of 223 N / cm2and then cooled at 15K / min down to 40°C. A high voltage source is connected to the upper electrode of a three terminal test cell in order to apply voltage over the test sample. The resulting current through the sample is measured with an electrometer / picoammeter. The measurement cell is a three electrodes system placed in a heated oven circulated with dried compressed air to maintain constant humidity level. The diameter of the measurement electrode is 100 mm.

[0191] After placing the compression moulded plaques in test cell, annealing at 115 °C for 6 h with applied voltage was performed on the plaques. This was followed by natural cooling to 70°C and keeping for 4h at each of the testing temperatures (in this order 70, 90, 100, 110, 115 °C). The DC conductivity was calculated in the end of each 4h temperature step. The applied electric field was always 40 kV / mm and calculated as the applied voltage divided by the plaque thickness.

[0192] The DC conductivity o is defined as the current density J divided by the applied electric field E. Then the relation between the measured current I through the sample and DC conductivity o can be expressed in Sl-units as

[0193] I = Measured current

[0194] II = applied voltage a = Area of the measuring electrode d = Plaque thickness j) Thermal conductivity was measured at +23°C and +100°C on 20x20x3 mm compression moulded specimens according to ISO 22007-2. Thermal resistivity is the inverse of thermal conductivity.

[0195] Examples - Polymer composition

[0196] The following resins were used for the preparation of the polymer compositions of the examples:

[0197] Preparation of the random heterophasic propylene copolymer The catalyst used in the polymerization process for the random heterophasic propylene copolymer PP-1 powder was a Ziegler-Natta catalyst produced with a phthalate free Ziegler Natta catalyst according to the example section of WO 2015 / 117948. As co-catalyst triethyl-aluminium (TEAL) and as donor dicyclo pentyl dimethoxy silane (D-donor) was used.

[0198] The propylene copolymer powder was produced in a Borstar™ plant in the presence of the above described polymerization catalyst using one liquid-phase loop reactor and two gas phase reactors connected in series under conditions as shown in Table 1 . The first reaction zone was a loop reactor and the second and third reaction zones were gas phase reactors. The matrix phase was polymerized in the loop and first gas phase reactor and the elastomeric phase was polymerized in the second gas phase reactor. The catalyst as described above was fed into a prepolymerization reactor, which precedes the first reaction zone. Table 1 : Polymerization conditions of the random heterophasic propylene copolymer powder:

[0199] The properties of the propylene copolymer obtained in Table 1 are reported in Table 2:

[0200] Table 2

[0201] The HDPEs used in the examples of the invention are explained in table 4. They are prepared as follows:

[0202] High density Polyethylene 2

[0203] Catalyst preparation

[0204] 130 grams of a metallocene complex bis(1-methyl-3-n-butylcyclopentadienyl) zirconium (IV) dichloride (CAS no. 151840-68-5), and 9.67 kg of a 30% solution of commercial methylalumoxane (MAO) in toluene were combined and 3.18 kg dry, purified toluene was added. The thus obtained complex solution was added onto 17 kg silica carrier Sylopol 55 SJ (supplied by Grace) by very slow uniform spraying over 2 hours. The temperature was kept below 30°C. The mixture was allowed to react for 3 hours after complex addition at 30°C.

[0205] Polymerization

[0206] A Borstar pilot plant with a 2-reactor set-up (loopl - Ioop2) and a prepolymerization loop reactor were used. Details are provided in table 3.

[0207] A1 Prepoly reactor

[0208] HDPE 1 is described in EP-A-4151677 as IE1. Table 4

[0209] Compositions were prepared on a Coperion ZSK18 intermeshing, co-rotating twin screw 18 mm L / D 40 extruder together with different additives like Irganox 1010 sold by BASF, Irgafos 168 sold by BASF and Calcium stearate sold by Baerlocher.

[0210] Prepared compositions are disclosed in Table 5 and 6:

[0211] Table 5

[0212]

[0213] Discussion

[0214] The MFR (2.16kg / 230°C) of the blends is low, as desired for the cable extrusion process, in order to avoid sagging phenomena due to too low melt strength of the molten material.

[0215] The results were calculated as median of at least 5 measurements for the tensile test and as average of 3 measurements for the pressure test, respectively. As can be observed, the neat PP2 (CE1) shows high tensile strength and no indentation, but the strain to break after ageing is below the accepted limit. When adding 65 wt% of HDPE 1 or HDPE 2 (CE2 and CE3), the strain to break before ageing is well above 350 %, but the indentation is too high (above 50 %). At 35 wt% of either HDPE 1 or HDPE 2 in the blend (IE1 and IE2), the strain before and after ageing is above the minimum requirement, while the indentation is way below the upper limit of 50 %. In all cases, tensile strength is above the minimum of 8.5 MPa.

[0216] In addition to the improved mechanical properties after ageing, a higher thermal conductivity was measured for IE1 (blend with 35 wt% HDPE 1) compared to the neat CE1 (PP-1), while having a DC conductivity at high temperature still low enough for a HVDC power cable. Indeed, the DC conductivity value at 100°C for IE1 is in a similar range as cross-linked LDPE insulation materials for HVDC power cables. IE2 (blend with 35 wt% HDPE 2) shows DC conductivity at the same level as CE1 .

[0217] The Crystex results showing the C2 content, the soluble fraction and the intrinsic viscosities (IV) of inventive and comparative examples are reported in Table 7.

Claims

Claims1. A polymer composition comprising(I) at least 50 wt%, based on the total weight of the composition, as a whole of a propylene copolymer (A) of propylene and ethylene, said propylene copolymer (A) having:(a) a melt flow rate MFR2 (230 °C) measured according to ISO 1133 in the range of more than 0.1 to 2.5 g / 10min,(b) a soluble fraction (SF), determined by crystallization extraction (CRYSTEX), in the range of from 10 to 45 wt% based on the total weight of the propylene copolymer;(c) a comonomer content in the range of from 7.0 to 15 wt% based on the total weight of the propylene copolymer and determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy,(d) an intrinsic viscosity, determined according to ISO 1628 / 3, of the soluble fraction (IV(SF)) fraction of the propylene copolymer is in the range of 2.0 to 4.0 dL / g; and(e) a C2 content of the soluble fraction (C2(SF)) in the range of from 15 to 60 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy based on the total weight of the soluble fraction; and(II) in the range of from 15 to 50 wt% based on the total weight of the composition as a whole of a polyethylene homo- or copolymer with a C4-C10 alpha olefin, the polyethylene homo- or copolymer having a density in the range of from 935 to 965 kg / m3determined according to ISO 1183-1.

2. A polymer composition as claimed in claim 1 wherein the polyethylene homo or copolymeris a high density polyethylene homo or copolymer having a density of 945 to 965 kg / m3determined according to ISO 1183-1.

3. A polymer composition as claimed in any preceding claim wherein the polyethylene homo or copolymer has an MFR2 (190°C, ISO1133) of 0.1 to 10 g / 10min, preferably 0.5 to 5.0 g / 10min.

4. A polymer composition as claimed in any preceding claim wherein the polyethylene homo or copolymer is present in an amount of 25 to 45 wt% based on the total weight of the polymer composition.

5. A polymer composition as claimed in any preceding claim wherein the propylene copolymer has:(a) a melt flow rate MFR2 (230 °C), measured according to ISO 1133, in the range of from 0.5 to 1.8 g / 10min;(b) a soluble fraction (SF) determined by crystallization extraction (CRYSTEX) in the range of from 15 to 40 wt% based on the total weight of the propylene copolymer;(c) a comonomer content in the range of 8.5 to 14 wt%, based on the total weight of the propylene copolymer and determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy;(d) the intrinsic viscosity, determined according to ISO 1628 / 3, of the soluble fraction (IV(SF)) of the propylene copolymer is in the range of 2.3 to 3.0 dL / g;(e) C2 content of soluble fraction (C2(SF)) of 20 to 40 wt% determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy based on the total weight of the propylene copolymer.

6. A polymer composition as claimed in any preceding claim wherein the polymer composition has a soluble fraction (SF), determined by crystallization extraction (CRYSTEX), in the range of 8.0 to 18 wt% based on the total weight of the polymer composition; and / or wherein the polymer composition has a total C2 content determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy of 30 to 50 wt% based on the total weight of the polymer composition.

7. A polymer composition as claimed in any preceding claim having at least one of: an intrinsic viscosity (IV) determined according to ISO 1628 / 3 of from 2.0 to4.0 dL / g;a C2 content of the crystalline fraction (C2(CF)) of from 30 to 55 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy based on the total weight of the crystalline fraction of the composition; a C2 content of the soluble fraction (C2(SF)) of 20 to 45 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy and based on the total weight of the soluble fraction ; a crystalline fraction intrinsic viscosity (IV(CF)), determined according to ISO 1628 / 3, in the range of from 2.0 to 4.0 dL / g; a soluble fraction intrinsic viscosity (IV(SF)), determined according to ISO 1628 / 3, in the range of from 2.0 to 4.0 dL / g.

8. A polymer composition as claimed in any preceding claim wherein the propylene copolymer has a crystalline fraction (OF) determined by crystallisation extraction (CRYSTEX) in the range of 90 to 55 wt%, such as 85 to 60 wt%, preferably wherein said crystalline fraction has an intrinsic viscosity (IV(CF)), determined according to ISO 1628 / 3, of from 2.0 to 4.0 dL / g and / or a C2 content (C2(CF)) of 3.0 to 8.0 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy based on the total weight of the crystalline fraction.

9. A polymer composition as claimed in any preceding claim wherein the polymer composition has a strain at break before ageing (I EC 60811-501) of at least 350% at 23°C, such as at least 400% and / or a strain at break after ageing of at least 400% at 23°C, such as at least 450% (I EC 60811-501 , ageing according to IEC60811-401).

10. A polymer composition as claimed in any preceding claim wherein the polymer composition has a tensile modulus of less than 450 MPa at 23°C and / or a tensile modulus of less than 3000 MPa at -20°C (I EC 60811-501).11 . A polymer composition as claimed in any preceding claim wherein the polymer composition has a tensile strength of at least 30 MPa at 23°C before ageing and / or having a tensile strength of at least 30 MPa at 23°C after ageing (I EC 60811-501).

12. A polymer composition as claimed in any preceding claim wherein the polymer composition has an indentation pressure test (as described in HV method 1) 130°C 1h from CIGRE TB852) of less than 49 %.

13. A polymer composition as claimed in any preceding claim wherein the polymer composition has a DC conductivity at 100 °C of less than 40 fS / m and / or a thermal resistivity at 100°C of 4.0 Km / W or less especially wherein the thermal resistivity is lower than that of the propylene copolymer itself.

14. A polymer composition as claimed in any preceding claim wherein the composition is free of low density polyethylene, especially wherein the polymer composition is devoid of further polymer components than the propylene copolymer (A) and the polyethylene homo- or copolymer.

15. An article comprising the polymer composition of any of claims 1 to 14, preferably a cable, such as a power cable, especially a HV cable, and most especially an HVDC cable.

16. A cable as claimed in claim 15 having a conductor surrounded by at least an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order, wherein the insulation layer comprises, or consists of, a polymer composition as defined in any of claims 1 to 14.

17. Use of a polymer composition as claimed in any of claims 1 to 14 as cable insulation, e.g. for high voltage DC cables.

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