Polymer composition for cable insulation

Abstract

A polymer composition comprising (I) at least 50 wt% based on the total weight of the composition of a propylene copolymer (A) of propylene and ethylene, said propylene copolymer 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 10 to 45 wt% based on the total weight of the propylene copolymer, and (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)) 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; and (II) 15 to 50 wt%, based on the total weight of the composition, of a low density polyethylene polymer (B) having a density in the range of from 920 to 935 kg / m3 determined according to ISO 1183-1; and wherein the sum of the weight percentage of the soluble fraction (SF) of the polymer composition and the absolute C2 content of the crystalline fraction of the polymer composition (absolute C2(CF)) is less than 60 wt% based on the total weight of the polymer composition.

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] WO2012 / 130793 describes polypropylene compositions for bottles. The composition comprises an alpha nucleated heterophasic propylene copolymer and a polyethylene component. The composition is useful for extrusion blow moulding and provides bottles with high gloss and transparency at low haze.

[0008] 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 insulation materials for the same application.

[0009] Furthermore, the use of a pure propylene copolymer for the insulation layer of power cable suffers from high thermal resistivity (low thermal conductivity). This is a crucial aspect to take into consideration during operation of the cable at high temperature, since a poor exchange of the heat generated from the conductor towards the outside of the cable causes higher electrical losses. Therefore, a decrease of thermal resistivity in the thermoplastic insulation materials is required for power cable application. Moreover, this should ideally be achieved at low MFR in order to provide better dimension stability when extruded around the conductor during the cable extrusion process.

[0010] The inventors have now found that thermal conductivity of random heterophasic propylene-ethylene copolymers (Raheco PP) can be significantly improved by the addition of high pressure polyethylene (i.e. low density polyethylene (LDPE)), while fulfilling the mechanical requirements suggested by the CIGRE TB852 for the insulation of HVDC power cables.

[0011] The mechanical properties of the blends of the invention fulfil the requirements of tensile strength and elongation to break at room temperature before and after ageing at 135°C for 10 days. Also at -20°C the elongation to break for the blends of the invention were found to be similar or higher than the pure propylene copolymer. This was achieved without the use of any added external compatibilizer to improve the adhesion at the interface between the two immiscible polymers.

[0012] The DC conductivity of the blends of the invention was on a similar acceptable level to that of the neat propylene copolymer.

[0013] Furthermore, the inventors have now found that the addition of a-nucleation to the blends increases the crystallization temperature, while not significantly affecting the DC conductivity and the mechanical properties of the composition. Higher crystallization temperature is beneficial during the cable extrusion process to reduce the cooling time after the exit of the material from the extrusion die. 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 of a propylene copolymer (A) of propylene and ethylene, said propylene copolymer 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 / 1 Omin,

[0017] (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

[0018] (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,

[0019] (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

[0020] (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; and

[0021] (II) 15 to 50 wt% based on the composition as a whole of a low density polyethylene polymer (B) having a density of from 920 to 935 kg / m3according to ISO 1183-1 ; and wherein the sum of the weight percentage of the soluble fraction (SF) of the polymer composition and the absolute C2 content of the crystalline fraction of the polymer composition (absolute C2(CF)) is less than 60 wt%; wherein absolute C2(CF) is determined as C2(CF)*CF / 100, the C2 content of the crystalline fraction (C2(CF)) of the polymer composition is determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy and based on the total weight of the crystalline fraction and the crystalline fraction (CF) of the polymer composition is determined by crystallisation extraction (CRYSTEX).

[0022] 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.

[0023] 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.

[0024] Definitions

[0025] 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.

[0026] 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.

[0027] 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. 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 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 an LDPE.

[0035] Propylene copolymer of propylene and ethylene

[0036] The polymer composition of the invention comprises a propylene copolymer of propylene and ethylene having:

[0037] (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 / 1 Omin,

[0038] (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

[0039] (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,

[0040] (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

[0041] (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.

[0042] It is also required that the sum of the weight percentage of the soluble fraction (SF) of the polymer composition and the absolute C2 content of the crystalline fraction of the polymer composition (C2(CF)) is less than 60 wt%. The absolute value of the C2(CF) is determined as C2(CF)*CF / 100.

[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. The propylene copolymer is preferably present in the polymer composition in an amount of at least 50 wt%, such as at least 55 wt%, more preferably 55 to 85 wt%, based on the total weight of the polymer composition. Ideally, the propylene copolymer and the LDPE 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. In particular the alpha nucleating agent mentioned herein may be carried on a carrier. 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.

[0044] 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.

[0045] 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).

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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).

[0052] 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).

[0053] 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.

[0054] 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.

[0055] 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. 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).

[0056] 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).

[0057] 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.

[0058] 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.

[0059] 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.

[0060] It is preferred that the crystalline fraction (CF) and the soluble fraction (SF) make up 100 wt% of the propylene copolymer.

[0061] 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.

[0062] The density of the propylene copolymer is preferably in the range of 880 to 905 kg / m3(I S01183).

[0063] In one embodiment, the propylene copolymer has: (a) a melt flow rate MFR2(230 °C), measured according to ISO 1133, in the range of 0.5 to 1 .8 g / 10min;

[0064] (b) a soluble fraction (SF) determined by crystallization extraction (CRYSTEX) in the range of 15 to 40 wt%;

[0065] (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;

[0066] (d) an 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; and

[0067] (e) C2 content of soluble fraction (C2(SF)) of 20 to 40 wt% determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy.

[0068] Manufacture

[0069] 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.

[0070] 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. 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.

[0071] 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.

[0072] A further suitable slurry-gas phase process is the Spheripol® process of LyondellBasell.

[0073] Suitable sequential polymerization processes for polymerizing the propylene copolymer, preferably the random heterophasic polypropylene copolymer, are e.g. disclosed in WO 2015 / 117948.

[0074] 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.

[0075] The propylene copolymer is preferably not subjected to a visbreaking step as e.g. described in WO 2013 / 092620 A1.

[0076] 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.

[0077] Nucleating Agent

[0078] The polymer composition of the invention comprises at least one alpha nucleating agent. The alpha nucleating agent may be present in an amount of 0.00005 to 2.0 wt.%, such as 0.0001 to 1 .0 wt.% of the polymer composition as a whole. These numbers refer to the content of the actual nucleating agent. If the nucleating agent is carried on a support then the weight of that support is not counted in these figures. Alternatively viewed, alpha nucleating agent may be present in an amount of 0.5 ppm or more such as 0.5 to 20,000 ppm. Alternatively viewed, the polymer composition contains from 0.001 to 2.5 wt.%, of the alpha-nucleating agent based on the total amount of the polymer composition (including in this option the weight of any carrier used). The amount of pure alphanucleating agent in the polymer composition (without the optional masterbatch carrier polymer) is preferably in the range up to 5000 ppm, more preferably up to 4000 ppm, based on the total amount of the polymer composition.

[0079] The alpha-nucleating agent is preferably selected from soluble alpha-nucleating agents and particulate alpha-nucleating agents. The alpha-nucleating agent is preferably selected from the group consisting of:

[0080] (i) salts of monocarboxylic acids and polycarboxylic acids, e.g. sodium benzoate or aluminum tert-butylbenzoate;

[0081] (ii) dibenzylidenesorbitol (e.g. 1 ,3 : 2,4 dibenzylidenesorbitol) and C1-8-alkyl- substituted dibenzylidenesorbitol derivatives, such as methyldibenzylidenesorbitol, ethyldibenzylidenesorbitol or dimethyldibenzylidenesorbitol (e.g. 1 ,3 : 2,4 di(methylbenzylidene) sorbitol), or substituted nonitol-derivatives, such as 1 ,2,3- trideoxy-4,6:5,7-bis-0-[(4- propylphenyl)methylene]-nonitol;

[0082] (iii) salts of diesters of phosphoric acid, e.g. sodium 2,2'-methylenebis (4,6-di- tertbutylphenyl) phosphate or aluminium-hydroxy-bis[2,2'-methylene-bis(4,6-di- tbutylphenyl)phosphate];

[0083] (iv) vinylcycloalkane polymer and vinylalkane polymer (as discussed in more detail below); and

[0084] (v) mixtures thereof.

[0085] The alpha-nucleating agent is preferably selected from the group consisting of dibenzylidenesorbitol (e.g. 1 ,3 : 2,4 dibenzylidene sorbitol), dibenzylidenesorbitol derivative, preferably dimethyldibenzylidenesorbitol (e.g. 1 ,3 : 2,4 di(methylbenzylidene) sorbitol), or substituted nonitol-derivatives, such as 1 ,2,3- trideoxy-4,6:5 ,7-bis-O-[(4- propylphenyl)methylene]-nonitol, vinylcycloalkane polymer, vinylalkane polymer, and mixtures thereof.

[0086] Especially preferred are vinylcycloalkane polymers such as e.g. vinylcyclohexane (VCH) polymers. Such polymers can be added e.g. using Borealis Nucleation Technology (BNT). The alpha-nucleating agent can be added to the polypropylene composition as an isolated raw material or in a mixture with a carrier polymer, i.e. in a so-called master batch. The use of a vinylcycloalkane polymer as nucleating agent may allow a reduction in the DC electrical conductivity of the composition.

[0087] Low density Polyethylene polymer (B)

[0088] The low density polyethylene polymer, LDPE, of the invention is a polyethylene produced in a high pressure process. Typically the polymerization of ethylene in a high pressure process is carried out in the presence of an initiator(s). 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 polymerisation catalyst. LDPEs have certain typical features, such as different branching architecture. A typical density range for an LDPE is 0.920 to 0.935 g / cm3.

[0089] The low density polyethylene (LDPE) used in the composition of the invention may have a density of 920 to 932 kg / m3, such as about 920 to 931 kg / m3.

[0090] The MFR2 (2.16 kg, 190°C) of the LDPE polymer is preferably from 0.05 to 10 g / 10 min, more preferably is from 0.05 to 5.0 g / 10min, and most preferably is from 0.1 to 3.0 g / 10min.

[0091] The LDPE may have a PDI of 5 to 15, such as 8 to 14. Still further, the low density polyethylene preferably has a melting temperature Tm of from 105 to 125°C, preferably from 107 to 120°C, more preferably from 109 to 120°C, determined by to DSC analysis according to ISO 11357 I part 3 / method C2.

[0092] Furthermore, the low density polyethylene preferably has a Vicat A50 softening temperature of from 85 to 105°C, preferably from 87 to 102°C, more preferably from 90 to 100°C, determined according to ISO 306 at 10 N.

[0093] 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.

[0094] The low density polyethylene (LDPE) is preferably an ethylene homopolymer. If a comonomer is present in the LDPE, it is preferred if that comonomer is non-polar and is therefore free of any polar groups such as carboxyl groups or hydroxyl groups. A non-polar comonomer preferably consists of hydrogen and carbon atoms only.

[0095] The LDPE polymer may be unsaturated. Unsaturation can be provided e.g. by a chain transfer agent (CTA), such as propylene, and / or by polymerization conditions. It is well known that selected polymerisation conditions such as peak temperatures and pressure, can have an influence on the unsaturation level.

[0096] The LDPE may be present in the polymer composition in any amount of 15 to 45 wt%, such as 18 to 40 wt%. If a blend of LDPEs is present this figure refers to the total amount of all LDPEs.

[0097] It is particularly preferred if component (II) is 15 to 45 wt% based on the composition as a whole of a low density polyethylene polymer (B) having a density of from 920 to 935 kg / m3according to ISO 1183-1 and an MFR2 (190°C) of 0.05 to 5.0 g / 10min.

[0098] The low density polyethylene is polymerized in a high pressure polymerization process by means of free radical polymerization, preferably a tubular high pressure polymerization process. Said high pressure polymerization processes, especially tubular high pressure polymerization processes are well known in the art.

[0099] Low density polyethylene resins suitable as low density polyethylene are also commercially available. These resins may already be additivated with stabilizer packages. Thus, when using commercially available resins as low density polyethylene the addition of additives as described above might have to be adjusted to the already present additives.

[0100] In case of a commercially available low density polyethylene the above stated properties can be measured using a common measurement method or verified by the technical documentation provided by the supplier.

[0101] Polymer Composition

[0102] The polymer composition can comprise additional polymeric components different from the propylene copolymer and the LDPE 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 LDPE are the only polymer components present other than any polymers used as carriers in a masterbatch. 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.

[0103] 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.

[0104] Preferably, the polymer composition is prepared by melt blending the propylene copolymer, the LDPE and optional further additives. The polymer composition is preferably not subjected to vis-breaking.

[0105] 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.

[0106] The polymer composition may have a soluble fraction (SF), determined by crystallization extraction (CRYSTEX), in the range of 8.0 to 35 wt%.

[0107] 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%.

[0108] The polymer composition may have an intrinsic viscosity (IV) determined according to ISO 1628 / 3 of 2.0 to 4.0 dL / g.

[0109] The polymer composition may have a C2 content of the crystalline fraction (C2(CF)) of 25 to 50 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy.

[0110] The polymer composition may have a C2 content of the soluble fraction (C2(SF)) of 20 to 40 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy.

[0111] 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).

[0112] The polymeric part of the polymer composition preferably consists of propylene monomer units and ethylene monomer units.

[0113] 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.

[0114] 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.

[0115] The polymer composition preferably has a Charpy notched impact strength at 23°C of at least 65 kJ / m2, or at least 75 kJ / m2, more preferably from 65 to 95 kJ / m2and most preferably from 75 to 95 kJ / m2, determined according to ISO 179- 1 / 1 eA.

[0116] The polymer composition preferably has a Charpy notched impact strength at -20°C of at least 4.0 kJ / m2, or of at least 5.0 kJ / m2, such as 4.5 to 15 kJ / m2, more preferably from 5.0 to 12 kJ / m2determined according to ISO 179-1 / 1 eA.

[0117] The polymer composition preferably has a flexural modulus of less than 700 MPa, such as less than 650 MPa, e.g. 250 to 650 MPa, determined according to ISO 178.

[0118] The polymer composition preferably has an elongation to break at 23°C of at least 600%, such as 600 to 850 % (before ageing). After ageing at 135°C for 10 days (IEC 60811-401) values are preferably still at least 450%. Ageing is carried out according to IEC 60811-401 for any embodiment where ageing is required.

[0119] The polymer composition may have an elongation at break at -20°C of at least 200%, such as 200 to 600%. The elongation to break is determined according to IEC 60811-501.

[0120] The polymer composition may have a tensile strength at 23°C of at least 15 MPa at 23°C (before), such as 15 to 25 MPa. The polymer composition may have a tensile strength of at least 15 MPa at 23°C after ageing at 135°C for 10 days (IEC 60811-401), such as 15 to 25 MPa. The tensile strength is determined according to IEC 60811-501. The polymer composition may have an indentation pressure test (as described in HV method 1) 130°C 1 h from CIGRE TB852) of less than 25 %, such as less than 15%.

[0121] The polymer composition may have a tensile strength of at least 20 MPa at -20°C (before), such as 20 to 40 MPa, as determined according to IEC 60811-501 .

[0122] 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.

[0123] 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).

[0124] The polymer composition preferably has a DC conductivity at 100 °C of less than 40 fS / m, preferably less than 30 fS / m, especially 5-30 fS / m.

[0125] Importantly, the sum of the weight percentage of the soluble fraction (SF) of the polymer composition and the absolute value of the C2 content of the crystalline fraction of the polymer composition (absolute C2(CF)) is less than 60 wt%, such as less than 55 wt%. The absolute value of the C2(CF) is determined as 2(CF)*CF / 100.

[0126] The polymer composition preferably has a Tc at 1 °C / min cooling rate of at least 121 °C, such as 122 to 127 °C.

[0127] The polymer composition preferably has a Tc at 3°C / min cooling rate of at least 118°C, such as 118 to 124°C.

[0128] The polymer composition preferably has a Tc at 10 °C / min cooling rate of at least 113°C, such as 113 to 120°C.

[0129] The polymer composition preferably has a Tc at 30°C / min cooling rate of at least 106°C, such as 106 to 115°C.

[0130] The polymer composition preferably has a Tc at 100°C / min cooling rate of at least 100°C, such as 101 to 112 °C.

[0131] In one embodiment, the polymer composition may have a Tc at 1 °C / min cooling rate of at least 121 °C, a Tc at 10 °C / min cooling rate of at least 113°C and a Tc at 100°C / min cooling rate of at least 100°C, especially also having a having a Tc at 3°C / min cooling rate of at least 118°C and having a To at 30°C / min cooling rate of at least 106°C.

[0132] Article

[0133] 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.

[0134] 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.

[0135] It is preferred that the article meets all CIGRE TB852 recommendations for HVDC cable systems.

[0136] 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.

[0137] 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.

[0138] 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.

[0139] 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.

[0140] 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.

[0141] 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.

[0142] 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.

[0143] The invention will now be described with reference to the following non-limiting examples and figure.

[0144] Brief Description of the Figure:

[0145] Figure 1 shows the Indentation from pressure test at 130°C for 1 h (method 1

[0146] HVDC) as function of the sum of the soluble fraction (SF) and of the C2 content in the crystalline fraction (C2(CF)). The data were taken from CE1 , CE2, CE3, IE1 , IE2, IE3, CE4, CE5, CE6, IE4, IE5.

[0147] Determination Methods

[0148] 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.

[0149] Measurement methods a) Melt Flow Rate (MFR2)

[0150] 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.

[0151] 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 1 133.

[0152] 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

[0153] 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

[0154] Comonomer content quantification of poly(propylene-co-ethylene) copolymers Quantitative13C{1H} NMR spectra were recorded in the solution-state using a Bruker A vance 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-cfe) 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.

[0155] 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}.

[0156] 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.

[0157] 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:

[0158] E = 0.5 (Spp + Spy + Sp6 + 0.5( Sap + Say))

[0159] Through the use of this set of sites the corresponding integral equation becomes: E = 0.5 (lH+IG + 0.5(lc+ ID)) using the same notation used in the article of Wang et al. {6}. Equations used for absolute propylene content were not modified.

[0160] The mole percent comonomer incorporation was calculated from the mole fraction: E [mol%] = 100 * fE

[0161] 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:

[0162] 1) Busico, V., Cipullo, R., Prog. Polym. Sci. 26 (2001) 443.

[0163] 2) Busico, V., Cipullo, R., Monaco, G., Vacatello, M., Segre, A.L., Macromolecules 30 (1997) 6251.

[0164] 3) Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha, A., Baugh, D. Winniford, B., J. Mag. Reson. 187 (2007) 225.

[0165] 4) Busico, V., Carbonniere, P., Cipullo, R., Pellecchia, R., Severn, J., Talarico, G., Macromol. Rapid Commun. 2007, 28, 1128.

[0166] 5) Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev. 2000, 100, 1253.

[0167] 6) Wang, W-J., Zhu, S., Macromolecules 33 (2000), 1157.

[0168] 7) Cheng, H. N., Macromolecules 17 (1984), 1950.

[0169] 8) Singh, G., Kothari, A., Gupta, V., Polymer Testing 28 5 (2009), 475.

[0170] 9) Kakugo, M., Naito, Y., Mizunuma, K., Miyatake, T. Macromolecules 15 (1982) 1150.

[0171] 10) Randall, J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1989, C29, 201.

[0172] 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):

[0173] 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.

[0174] 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.

[0175] 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 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.

[0176] Determination of Crystalline and soluble fractions and their respective properties (IV and Ethylene content)

[0177] 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 technique and the method can be found in literature (Ljiljana Jeremie, Andreas Albrecht, Martina Sandholzer & Markus

[0178] 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

[0179] Characterization, 25:8, 581-596).

[0180] 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: Cone = a + b*Abs(CH) + c*(Abs(CH))2+ d*Abs(CH3) + e*(Abs(CH3)2+ f*Abs(CH)*Abs(CH3) (Equation 1) CH3 / 1000C = a + b*Abs(CH) + c* Abs(CH3) + d * (Abs(CH3) / Abs(CH)) + e * (Abs(CH3) / Abs(CH))2(Equation 2)

[0181] The constants a to f for equation 1 and a to e for equation 2 were determined by using least square regression analysis.

[0182] 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)

[0183] 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.

[0184] 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.

[0185] 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.

[0186] 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.

[0187] 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)

[0188] The reduced viscosity (also known as viscosity number), rjred, and intrinsic viscosity, IV, are determined according to ISO 1628-3: “Determination of the viscosity of polymers in dilute solution using capillary viscometers”.

[0189] 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).

[0190] 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: r ■■ [dime—nsionlessn]

[0191] Reduced viscosity (rjred) is calculated using the equation: m where C is the polymer solution concentration at 135°C: C= — , Vy 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). 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) Charpy notched impact strength

[0192] The Charpy notched impact strength was determined acc. to ISO 179-1 / 1 eA on notched 80 mm x 10 mm x 4 mm compression moulded specimens. Testing temperatures were 23±2° C or -20±2° C. The results were calculated as median of at least 5 measurements. h) Elongation to break and tensile strength

[0193] The tensile specimens were compression moulded 5A dogbones prepared according to IEC 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 IEC 6081 1 -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. i) Flexural modulus

[0194] Flexural modulus was measured at +23°C on 80x10x4 mm compression moulded specimens according to ISO 178. The results were calculated as median of at least 5 measurements. j) Indentation in pressure test

[0195] 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. k) DC Conductivity test

[0196] The DC electrical conductivity was tested for all the studied examples 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.

[0197] 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 test 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.

[0198] 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

[0199] I = Measured current

[0200] U = applied voltage a = Area of the measuring electrode d = Plaque thickness l) 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. m) Crystallization temperature at different cooling rates

[0201] A TA Instruments Q2500 Differential Scanning Calorimeter calibrated with Indium, Zinc, and Tin and operating under 50 mL / min of nitrogen flow was used. The employed thermal program consisted of a first heating step from 0 to 225°C to erase the previous thermal history and a cooling step at 3, 10, 30, 100 °C / min. The crystallization temperatures were taken as the peak values from the cooling scan.

[0202] Examples - Polymer composition

[0203] The following resins were used for the preparation of the polymer compositions of the examples:

[0204] Preparation of the random heterophasic propylene copolymer

[0205] 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-containing catalyst on a solid support as described in EP491566. As donor diisobutylphthlate was used. As co-catalyst triethyl-aluminium (TEAL) and as external donor dicyclo pentyl dimethoxy silane (D-donor) was used.

[0206] For PP-2 a phthalate free Ziegler Natta catalyst was prepared 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.

[0207] 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.

[0208] Table 1 : Polymerization conditions of the random heterophasic propylene copolymer powder: The properties of the propylene copolymer obtained in Table 1 are reported in Table 2: Table 2

[0209] The LDPEs used in the examples of the invention are explained in table 3. They are homopolymers.

[0210] Table 3 Compositions were prepared on a Coperion ZSK18 intermeshing, co-rotating twin screw 18 mm L / D 40 extruder. Irganox 1010 sold by BASF and DHT-4V supplied by Kisuma / Kyowa (synthetic hydrotalcite) were added in the same amount in each composition. Alpha-nucleation via BNT was achieved by adding 2 wt% of a propylene homopolymer with an MFR2(230°C) of 8.0 g / 10 min and a melting temperature of 162 °C, which is produced with a Ziegler-Natta type catalyst in the Borealis nucleation technology (BNT), comprising a polymeric a-nucleating agent, and is distributed by Borealis AG (Austria). Prepared compositions are disclosed in Table 4 and 5:

[0211] Table 4

[0212] Table 5

[0213] Discussion

[0214] The MFR (2.16kg / 230°C) of all inventive and comparative examples is low and in the optimal range for the cable extrusion process. However, the indentation from the pressure test is higher than 50% for PP-2 blends containing LDPE amounts from 50 wt% and above (CE2, CE3), and for PP-1 blends containing LDPE amounts from 35 wt% and above (CE5, CE6). Thus, no further analysis of other properties of CE2, CE3, CE5 and CE6 was carried out. Mechanical properties of the other examples was analysed and the data is presented in Table 6: Table 6

[0215] Table 6

[0216] In Table 6, mechanical properties, thermal and DC conductivity are shown for the inventive examples and some selected comparative examples. As can be observed, the tensile strength at 23°C requirement before ageing (> 8.5 MPa) is fulfilled by all inventive examples. The tensile properties at -20°C for the inventive examples are similar as for the comparative examples, showing high elongation to break. Furthermore, the impact performance of the neat propylene copolymer is improved when adding LDPE. The most important improvement caused by the addition of LDPE is the increase of thermal conductivity (or decrease of thermal resistivity).

[0217] The DC conductivity level at 100°C for IE1 and IE2 is slightly higher than for CE1 , but still in a similar range as cross-linked LDPE insulation materials for HVDC power cable. For IE4 and IE5, the DC conductivity at 100°C is essentially the same as for CE4. In conclusion, the addition of LDPE in the blends leaves the DC conductivity at 100°C unchanged or does not cause a significant increase.

[0218] The addition of a-nucleation strongly affects the rate of crystallization. The crystallization temperatures (Tc) at different cooling rates measured via Differential Scanning Calorimetry (DSC) are reported in Table 7. A range of cooling rates between 1 and 100 °C / min was chosen based on the typical range that the insulation layer of a HVDC power cable experiences across its thickness during the cable extrusion process. As can be seen, a-nucleation gives up to 12 °C higher Tc when compared to the pure polymer. This is beneficial because it allows shortening of the time needed for the material to solidify after exiting the extrusion die.

[0219] Table 7

[0220] ‘nucleated The Crystex results showing the C2 content, the soluble fraction and the intrinsic viscosities (IV) of some inventive and comparative examples are reported in tables 8 and 9.

[0221]

[0222] Discussion

[0223] It is possible to find a relation between the indentation achieved during the pressure test at 130°C and some parameters obtained from the Crystex analysis of the materials. From a physical point of view, the resistance to indentation at 130°C depends on the amount of mobile phase at that temperature, which will be the sum of the soluble fraction and of crystallizable part of the material that is molten at 130°C, i.e., the crystals belonging to the LDPE component or to the crystallizable C2 chain segments belonging to the PP component (C2(CF)).

[0224] As shown in Figure 1 , PP-2-based and PP-1 -based blends show a similar data trend when plotting the indentation as a function of the sum of soluble fraction (SF) and of the crystallizable C2 content in absolute value. There is no indentation occurring up to about 50 wt% of SF+absolute C2(CF) for both PP-2 and PP-1 based materials. The data corresponding to indentation > 0% were linearly fitted and the following equation was obtained:

[0225] Indentation = 4.10 * (SF + absolute C2(CF)) - 194.29

[0226] From the equation one can calculate that an indentation of 50% is achieved when SF+absolute C2(CF) is ~60 wt%.

Claims

Claims1 . A polymer composition comprising(I) at least 50 wt% based on the total weight of the composition of a propylene copolymer (A) of propylene and ethylene, said propylene copolymer 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 / 1 Omin,(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(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)) 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; and(II) 15 to 50 wt%, based on the total weight of the composition, of a low density polyethylene polymer (B) having a density in the range of from 920 to 935 kg / m3determined according to ISO 1183-1 ; and wherein the sum of the weight percentage of the soluble fraction (SF) of the polymer composition and the absolute C2 content of the crystalline fraction of the polymer composition (absolute C2(CF)) is less than 60 wt% based on the total weight of the polymer composition; wherein absolute C2(CF) is determined as C2(CF)*CF / 100, the C2 content of the crystalline fraction (C2(CF)) of the polymer composition and is determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy and based onthe total weight of the crystalline fraction and the crystalline fraction (CF) of the polymer composition is determined by crystallisation extraction (CRYSTEX).

2. A polymer composition as claimed in claim 1 wherein the low density polyethylene polymer (B) has an MFR2(ISO1133, 190°C) of 0.05 to 5.0 g / 10min, preferably from 0.1 to 3.0 g / 10min.

3. A polymer composition as claimed in any preceding claim wherein the low density polyethylene polymer (B) is present in an amount in the range of from 18 to 40 wt% based on the total weight of the polymer composition..

4. 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 from 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 soluble fraction.

5. 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 from 8.0 to 35 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 in the range of from 25 to 50 wt% based on the total weight of the polymer composition; and / or the polymer composition has a melt flow rate MFR2 of from 0.25 to 4.0 g / 10 min, determined according to ISO 1133 at 230°C and 2.16 kg.

6. A polymer composition as claimed in any preceding claim wherein the polymer composition has at least one of: an intrinsic viscosity (IV) determined according to ISO 1628 / 3 in the range of from 2.0 to 4.0 dL / g; a C2 content of the crystalline fraction (C2(CF)) in the range of from 25 to 50 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy and based on the total weight of the crystalline fraction; a C2 content of the soluble fraction (C2(SF)) in the range of from 20 to 40 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.

7. 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 from 90 to 55 wt%, based on the total weight of the propylene copolymer, such as from 85 to 60 wt%, preferably wherein said crystalline fraction has an intrinsic viscosity (IV(CF)), determined according to ISO 1628 / 3, in the range of from 2.0 to 4.0 dL / g and / or a C2 content (C2(CF)) of3.0 to 8.0 wt%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy based on the total weight of the crystalline fraction..

8. A polymer composition as claimed in any preceding claim wherein the polymer composition has a notched charpy impact strength of at least 75 kJ / m2at 23°C, and / or a has a notched charpy impact strength of at least 5.0 kJ / m2at -20°C (ISO179-1)9. A polymer composition as claimed in any preceding claim wherein the polymer composition has a flexural modulus of less than 700 MPa (ISO178).

10. A polymer composition as claimed in any preceding claim wherein the polymer composition has an elongation at break at 23°C of at least 600 % (IEC 60811- 501) and / or an elongation at break after ageing at 135°C for 10 days (IEC 60811- 401) of at least 450%. .11 . A polymer composition as claimed in any preceding claim wherein the polymer composition has a tensile strength of at least 15 MPa at 23°C before ageing (IEC 60811-501) and / or having a tensile strength of at least 15 MPa at 23°C after ageing (IEC 60811-501 , IEC60811-401)).

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 1 h from CIGRE TB852) of less than 25 %.

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 4.5 Km / W or less especially wherein the thermal resistivity of the polymer composition is lower than that of the propylene copolymer itself.

14. A polymer composition as claimed in any preceding claim wherein the composition is devoid of further polymer components than the propylene copolymer (A) and low density polyethylene polymer (B).

15. A polymer composition as claimed in any preceding claim further comprising an alpha nucleating agent.

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

17. A cable as claimed in claim 16 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 15.

18. Use of a polymer composition as claimed in any of claims 1 to 15 as cable insulation, e.g. for high voltage DC cables.

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