Polymer composition for cable insulation

Abstract

A polymer composition comprising (I) a propylene copolymer of propylene and ethylene having: (a) a melt flow rate MFR2 (230 °C), measured according to ISO 1133, in the range of from 0.1 to 2.0 g / 10min; (b) a soluble fraction (SF) content 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)) of from 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) at least one α-nucleating agent.

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] 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.

[0007] Another important aspect is the rate at which the crystallization process occurs in the insulation layer. In general, the higher the crystallization rate the less sensitive to processing conditions the material will be. This ensures the achievement of similar mechanical and electrical performance of cables made of the same insulation material using different cable extrusion processes. Furthermore, a higher crystallization temperature allows reduction of the time necessary for cooling the cable prior to turning points on the extrusion line, potentially resulting in shorter cooling segments on the line or in higher line speed.

[0008] WO 2022 / 200395 A1 and WO 2022 / 200396 A1 disclose flexible polypropylene compositions suitable for cable insulation, which both comprise a flexible heterophasic copolymer of propylene and ethylene. Said compositions show a superior balance of properties in regard of flexibility, impact strength and electric properties, which makes these compositions especially suitable for medium voltage cables.

[0009] The inventors have now found that a-nucleation gives multiple advantages when added to random heterophasic polypropylene copolymers as insulation material for HVDC power cables.

[0010] The impact resistance at room and at low temperature is improved by a- nucleation agent to low MFR random heterophasic polypropylene copolymers. The improvement in impact resistance is achieved whilst keeping strain at break and tensile strength at an acceptable level and above the requirements of CIGRE TB852. Moreover, the DC conductivity at 100°C is in the same range as for cross-linked LDPE insulation materials for HVDC power cable application. Remarkably, the addition of the polyvinylcyclohexane (PVCH(-based nucleation allowed a DC conductivity lower than for the pure polymers.

[0011] Finally, a-nucleation increases the crystallization temperature of the random heterophasic polypropylene copolymers more than p-nucleation, which is beneficial during the cable extrusion process to reduce the cooling time after the exit of the material from the extrusion die.

[0012] Summary of the invention

[0013] Viewed from one aspect the invention provides a polymer composition comprising

[0014] (I) a propylene copolymer of propylene and ethylene having:

[0015] (a) a melt flow rate MFR2(230 °C), measured according to ISO 1133, in the range of from 0.1 to 2.0 g / 10min;

[0016] (b) a soluble fraction (SF) content 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^

[0017] (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

[0018] (e) a C2 content of the soluble fraction (C2(SF)) of from 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

[0019] (II) at least one a-nucleating agent.

[0020] 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.

[0021] 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.

[0022] Definitions

[0023] 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.

[0024] The semi-crystalline matrix phase is a random copolymer of propylene and at least one alpha-olefin comonomer such as ethylene. 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 comonomer-rich block structure and a propylene-rich block structure. Ideally therefore, the propylene copolymer does not contain therefore a homopolymer matrix phase (so as to form a block heterophasic copolymer).

[0025] A random heterophasic polypropylene copolymer is differentiated from one- phasic propylene copolymers 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. 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 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 an alpha nucleating agent.

[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 from 0.1 to 2.0 g / 10min;

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

[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 from 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

[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 90 wt.%, such as at least 92 wt.%, more preferably at least 95 wt.%, especially at least 97 wt.% based on the total weight of the polymer composition. Ideally, the propylene copolymer is the only non-additive polyolefin component present in the polymer composition. However, it will be appreciated that some additives, including the alpha-nucleating agents mentioned herein, may be carried on a polyolefin as a masterbatch or the alpha nucleating agent may itself be polymeric. Such additives are not excluded. 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 low density polyethylene, 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.

[0045] 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. In one embodiment, the propylene copolymer has a total comonomer content, such as total ethylene content, of 11 .5 wt% or less, such as 7.0 to 11 .0 wt.-% based on the total weight of the propylene copolymer and determined by FT-IR spectroscopy calibrated by quantitative13C-NMR spectroscopy.

[0046] 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).

[0047] 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. The propylene copolymer 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 higher than 1 .10 g / 1 Omin to 2.0 g / 10min, such as 1 .20 to 1 .75 g / 10 min, or 1 .30 to 1 .70 g / 1 Omin. In one embodiment, the propylene copolymer has a melt flow rate MFR2 of 1 .35 to 2.0 g / 10 min, such as 1 .45 to 2.0 g / 1 Omin. In another embodiment, the propylene copolymer has a melt flow rate MFR2 of 1.1 g / 10 min or less, such as 0.5 to 1.1 g / 10 min. MFRs within this range may offer good processability which, combined with an alpha nucleated composition, can result in reduced DC conductivity.

[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). 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.

[0053] 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.

[0054] 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% based on the total weight amount of the propylene copolymer. In one embodiment, the propylene copolymer has a soluble fraction (SF) content, determined by crystallization extraction (CRYSTEX), of less than 25 wt%, preferably in the range from 10 to 24 wt%, or 13 to 24 wt% based on the total weight amount of the propylene copolymer.

[0055] In one embodiment therefore the propylene copolymer of propylene and ethylene has a melt flow rate MFR2(230 °C), measured according to ISO 1133, in the range of from 0.1 to 2.0 g / 10min, such as 0.5 to 1.1 g / 10min and a soluble fraction (SF) content, determined by crystallization extraction (CRYSTEX), in the range of from 10 to 24 wt% based on the total weight of the propylene copolymer.

[0056] 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). In one embodiment, said soluble fraction (SF) has an ethylene content (C2(SF)), determined by FT-IR spectroscopy calibrated by quantitative13C-NMR spectroscopy more than 30 wt% based on the total amount of monomer units in the soluble fraction (SF).

[0057] 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). In one embodiment, the propylene copolymer has a soluble fraction (SF) content, determined by crystallization extraction (CRYSTEX), in the range from 30 to 45 wt%, more preferably from 33 to 42 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 11 .5 to 30 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, still more preferably from 2.3 to 3.5 dL / g and most preferably from 2.5 to 2.8 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, still more preferably from 2.5 to 3.5 dL / g, most preferably 2.7 to 3.3 dL / g.

[0060] It is preferred that the crystalline fraction (OF) 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, (ISO1183).

[0063] In one embodiment, the propylene copolymer has:

[0064] (a) a melt flow rate MFR2(230 °C), measured according to ISO 1133, in the range of 0.5 to 1 .8 g / 10min;

[0065] (b) a soluble fraction (SF) determined by crystallization extraction (CRYSTEX) in the range of from 15 to 40 wt.%;

[0066] (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; (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.

[0071] 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.

[0072] 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.

[0073] A further suitable slurry-gas phase process is the Spheripol® process of LyondellBasell. 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.

[0079] 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 of from up to 5000 ppm, more preferably up to 4000 ppm, based on the total amount of the polymer composition.

[0080] 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:

[0081] (i) salts of monocarboxylic acids and polycarboxylic acids, e.g. sodium benzoate or aluminum tert-butylbenzoate; (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). In one embodiment, the alpha-nucleating agent is not a BNT alpha-nucleating agent. 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] Polymer Composition

[0088] The polymer composition can comprise additional polymeric components different from the propylene copolymer such as in an amount of preferably 0.0 to 10.0 wt.% based on the total weight of the polymer composition although as noted above, it is preferred if the propylene copolymer is the only polymer component present other than any polymers used as carriers in a masterbatch.

[0089] The polymer composition can comprise one or more additives (in addition to the alpha nucleating agent) 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, beta 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.

[0090] 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.

[0091] In one embodiment, the polymer composition does not contain beta nucleating agents. In one embodiment, the polymer composition contains less than 1 .0 wt.% inorganic filler, such as free of inorganic filler.

[0092] Preferably, the polymer composition is prepared by melt blending the propylene copolymer, the nucleating agent and optional further additives. The polymer composition is preferably not subjected to vis-breaking.

[0093] In one embodiment the polymer composition does not comprise, i.e. is free of a dielectric fluid, such as e.g. described in EP 2 739 679.

[0094] The polymer composition may have a melt flow rate MFR2 of from 0.5 to 3.0 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.

[0095] The polymer composition may have a Ky of at least 0.35, such as 0.40 to 0.75. Ky is a measure of the relative content of gamma crystals within the crystallised polymer composition.

[0096] It is preferred if the polymer composition has a Kp of no more than 0.1 , such as less than 0.05. Kp is a measure of the relative content of beta crystals within the crystallised polymer composition.

[0097] The polymer composition preferably has a Charpy notched impact strength at 23°C of at least 65 kJ / m2, more preferably from 65 to 90 kJ / m2and most preferably from 70 to 85 kJ / m2, determined according to ISO 179-1 / 1 eA.

[0098] The polymer composition preferably has a Charpy notched impact strength at -20°C of at least 3.0 kJ / m2, such as 3.0 to 15 kJ / m2, more preferably from 3.5 to 12 kJ / m2determined according to ISO 179-1 / 1 eA.

[0099] The polymer composition preferably has a strain at break of at least 500% at 23°C, such as at least 700%, such as 700 to 950% determined according to I EC 60811-501 at 23°C before ageing. Ageing is carried out according to IEC 60811-401 for any embodiment where ageing is required.

[0100] The polymer composition preferably has a strain at break of at least 180% at -20°C, such as at least 200%, such as 200 to 500% determined according to IEC 60811-501 at 23°C before ageing.

[0101] The polymer composition preferably has a tensile strength of at least 20 MPa at 23°C, such as at least 25 MPa, preferably 25 to 40 MPa (IEC 60811-501)

[0102] The polymer composition preferably has an E-modulus of less than 850 MPa at 23°C, such as 400 to 850 MPa (IEC 60811-501).

[0103] The polymer composition preferably has a Tc at 1 °C / min cooling rate of at least 121 °C, such as 122 to 127 °C.

[0104] The polymer composition preferably has a Tc at 3°C / min cooling rate of at least 118°C, such as 118 to 124°C.

[0105] The polymer composition preferably has a Tc at 10 °C / min cooling rate of at least 113°C, such as 113 to 120°C.

[0106] The polymer composition preferably has a Tc at 30°C / min cooling rate of at least 106°C, such as 106 to 115°C.

[0107] The polymer composition preferably has a Tc at 100°C / min cooling rate of at least 102°C,suchas 102 to 112 °C.

[0108] 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 102°C, especially also having a having a Tc at 3°C / min cooling rate of at least 118°C and having a Tc at 30°C / min cooling rate of at least 106°C.

[0109] The polymer composition preferably has a REact value of 2400 or more such as 2400 to 4500 (+ / - 350). This parameter is measured according to the method described in the methods section below and reflects the slope of Tc as function of the applied cooling rate.

[0110] The polymer composition preferably has a DC conductivity at 100 °C of less than 40 fS / m, preferably less than 30 fS / m, especially less than 15 fS / m, especially wherein the DC conductivity is lower than that of the non-nucleated propylene copolymer itself. The DC conductivity at 100 °C may be in the range of 1 .0 to 30 fS / m. The polymer composition may have an activation energy of less than 0.5 eV wherein activation energy is fitted against and calculated using the formula: where o is the DC conductivity, Eais the activation energy, kBis the Boltzmann constant, T is the temperature and Tref is 70 °C.

[0111] Article

[0112] 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.

[0113] 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.

[0114] It is preferred that the article meets all CIGRE TB852 recommendations for HVDC cable systems.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] 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.

[0122] Through alpha nucleation of the defined low MFR propylene copolymer, the invention therefore provides a polymer composition for the insulation layer in a cable with multiple advantages, especially for HVDC power cables. The impact resistance at room and at low temperature is improved without damaging the strain at break or tensile strength. The polymer composition offers remarkably low DC conductivity at 100 °C and is in the same range as for cross-linked LDPE insulation materials for HVDC power cable applications. When adding PVCH-based nucleation the DC conductivity was even lower than for the pure polymers. Moreover, a-nucleation increases the crystallization temperature of the propylene copolymer further than when using p-nucleation, which is beneficial during the cable extrusion process to reduce the cooling time after the exit of the material from the extrusion die.

[0123] Viewed from another aspect, the invention provides a HVDC cable 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 hereinbefore defined, in particular, a polymer composition comprising:

[0124] (I) a propylene copolymer of propylene and ethylene having:

[0125] (a) a melt flow rate MFR2(230 °C), measured according to ISO 1133, in the range of from 0.1 to 2.0 g / 10min, such as 1.10 g / 10min to 2.01 g / 10min, e.g.

[0126] 1.20 to 1.75 g / 10 min, or 1.30 to 1.70 g / 10min or an MFR2of 1.35 to 2.0 g / 1 Omin, such as 1 .45 to 2.0 g / 1 Omin;

[0127] (b) a soluble fraction (SF) content determined by crystallization extraction (CRYSTEX) in the range of from 10 to 45 wt.% based on the total weight of the propylene copolymer;

[0128] (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^

[0129] (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

[0130] (e) a C2 content of the soluble fraction (C2(SF)) of from 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

[0131] (II) at least one a-nucleating agent.

[0132] In particular, the invention provides a HVDC cable operating at a voltage of at least 30 kV. Viewed from another aspect the invention provides a process for transmitting power between a source of DC power (such as a transformer associated with an energy generator) and a receiver of DC power (such as a transformer associated with an energy substation) said process comprising connecting said source of DC power and said receiver of DC power via a cable as hereinbefore defined and allowing high voltage power at a voltage of 30 kV or more to flow through said cable.

[0133] Suitable power generators include wind turbines that can be located on or offshore.

[0134] The invention will now be described with reference to the following non-limiting examples and figures.

[0135] Brief Description of the Figures:

[0136] Figure 1a / b show DSC heating scans after cooling at 10 °C / min of PP1 -based compounds (1a) and PP2-based compounds (1 b).

[0137] Figure 2a / b show the temperature of crystallization (Tc) as function of cooling rate via DSC for PP1 based compounds (2a) and PP2-based compounds (2b.)

[0138] Determination Methods

[0139] 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.

[0140] Measurement methods a) Melt Flow Rate (MFR?)

[0141] 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.

[0142] The melt flow rate MFR? of propylene copolymer and the polymer composition is measured at 230°C with a load of 2.16 kg according to ISO 1133.

[0143] The melt flow rate MFR? of any ethylene based polymers is measured at 190°C with a load of 2.16 kg according to ISO 1133. b) Density 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

[0144] 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.

[0145] 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}.

[0146] 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.

[0147] 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:

[0148] E = 0.5 (Spp + Spy + Sp6 + 0.5( Sap + Say))

[0149] Through the use of this set of sites the corresponding integral equation becomes:

[0150] 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.

[0151] The mole percent comonomer incorporation was calculated from the mole fraction:

[0152] E [mol%] = 100 * fE

[0153] 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:

[0154] 1) Busico, V., Cipullo, R., Prog. Polym. Sci. 26 (2001) 443.

[0155] 2) Busico, V., Cipullo, R., Monaco, G., Vacatello, M., Segre, A.L., Macromolecules 30 (1997) 6251.

[0156] 3) Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha, A., Baugh, D. Winniford, B., J. Mag. Reson. 187 (2007) 225.

[0157] 4) Busico, V., Carbonniere, P., Cipullo, R., Pellecchia, R., Severn, J., Talarico, G., Macromol. Rapid Commun. 2007, 28, 1128.

[0158] 5) Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev. 2000, 100, 1253.

[0159] 6) Wang, W-J., Zhu, S., Macromolecules 33 (2000), 1157.

[0160] 7) Cheng, H. N., Macromolecules 17 (1984), 1950.

[0161] 8) Singh, G., Kothari, A., Gupta, V., Polymer Testing 28 5 (2009), 475.

[0162] 9) Kakugo, M., Naito, Y., Mizunuma, K., Miyatake, T. Macromolecules 15 (1982) 1150.

[0163] 10) Randall, J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1989, C29, 201.

[0164] 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):

[0165] These parameters are 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. 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.

[0166] 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

[0167] 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.

[0168] Determination of Crystalline and soluble fractions and their respective properties (IV and Ethylene content)

[0169] 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

[0170] 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

[0171] Characterization, 25:8, 581-596).

[0172] 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.

[0173] 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 13 mg / 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. 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)

[0182] The reduced viscosity (also known as viscosity number), r)red, and intrinsic viscosity, IV, are determined according to ISO 1628-3: “Determination of the viscosity of polymers in dilute solution using capillary viscometers”.

[0183] 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).

[0184] 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]

[0185] Reduced viscosity (nreci) 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

[0186] 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) Strain at break, Tensile strength and E-modulus

[0187] 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 results were calculated as median of at least 5 measurements. Ageing is carried out according to IEC 60811-401. i) Indentation in pressure test

[0188] 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. j) DC Conductivity test

[0189] The DC electrical conductivity was tested for all the studied random heterophasic polypropylene copolymers 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.

[0190] After placing the compression moulded plaques in test cell, annealing at 115 °C for 6 h (PP1) or 24 h (PP2) with applied voltage was performed on PP1 and on PP2 based compounds, respectively. 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.

[0191] 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

[0192] I = Measured current

[0193] U = applied voltage a = Area of the measuring electrode d = Plaque thickness

[0194] Besides the absolute DC conductivity value, the sensitivity of DC conductivity to the temperature can also be assessed. This is described by a single parameter called the activation energy. This parameter was calculated from the coefficient obtained via exponential fit of the normalized DC conductivity data with the following equation: where a is the DC conductivity, Eais the activation energy, ks is the Boltzmann constant, T is the temperature and Tref is 70 °C. k) Crystallinity by WAXS, beta phase and gamma phase content

[0195] The degree of crystallinity of the iPP samples was studied by carrying out WAXS measurements in reflection mode with a Bruker Discover D8 diffractometer equipped with a two-dimensional GADDS detector and a Ni-filtered CuKa X-rays. Three measurements were performed on each sample and the corresponding results were averaged. The amorphous halo obtained from an atactic-PP sample (D. Tranchida, L. Resconi L., Influence of 2, 1-erythro regiodefects on the crystallization behavior of isotactic polypropylene, Polymer Crystallization 1 (2018) e 10022) was properly scaled and subtracted and a crystallinity index (Xc) was quantified according to: where Atot is the area under the total pattern and Ac is the area after subtraction of the amorphous halo. Also, the relative content of the p-modification was calculated from the intensities of specific reflections after subtraction of the amorphous halo according to Turner-Jones et al. (A.T. Jones, J.M. Aizlewood, D. Beckett, Crystalline forms of isotactic polypropylene, Makromol. Chem.: Macromol. Chem. Phys. 75 (1964) 134-158):

[0196] 7)3(300) Kp~ 7a(110) + 7a(040) + 7a(130) + 7)3(300)

[0197] The relative content of the y-modification was calculated using the method developed by Pae (Pae K. D.: y-a Solid-solid transition of isotactic polypropylene. Journal of Polymer Science A-2: Polymer Physics, 6, 657-663 (1968)):

[0198] Where the intensities of the diagnostic reflections y(1 17) and a(130) are calculated from a baseline drawn under these two reflections on the diffraction pattern after subtraction of the amorphous halo.

[0199] I) REact parameter calculation

[0200] 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. The evaluation of the REact parameter was performed according to the description in “Tranchida, D., Gloger, D., & Gahleitner, M. (2017). Journal of Thermal Analysis and Calorimetry, 129(2), 1057-1064”.

[0201] Examples - Polymer composition

[0202] The following resins were used for the preparation of the polymer compositions of the examples:

[0203] Preparation of the random heterophasic propylene copolymer

[0204] 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.

[0205] 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.

[0206] 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:

[0207] The properties of the propylene copolymer obtained in Table 1 are reported in Table 2: Table 2

[0208]

[0209] The two propylene copolymers were then compounded in a Coperion ZSK18 intermeshing, co-rotating twin screw 18 mm L / D 40 extruder together with an additive package and different nucleating agents (a and p type), as shown in Table 3 and Table 4. The additives used are Irganox 1010 sold by BASF and DHT-4V supplied by Kisuma / Kyowa (synthetic hydrotalcite).

[0210] The p-nucleating agents are the SIPAX NAB-82, a metal salt of Cyclohexene- 1 ,2-dicarboxylic acid produced by GCH Co. Ltd., and the CGNA-7588, quinacridonequinone produced by Sun Chemicals.

[0211] The a-nucleating agents are 1 ,3: 2,4 Bis(3,4-dimethylbenzylidene) sorbitol (CAS135861-56-2). 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).

[0212] Table 3

[0213] Table 4

[0214] Discussion

[0215] The calculated total crystallinity and the relative fraction of p-phase and of y- phase are reported in Table 3 / 4. For the pure random heterophasic propylene copolymers, the crystalline phase is made by a- or y-crystals (Kp values equal or below 0.03 are due to noise and must be considered as no p-phase). When adding the p- nucleating agents, the amount of y-crystals decreases to maximum 10% of the total crystallinity (Ky = 0.1) and the majority of the crystals are p-type (Kp = 0.8-0.9). Instead, when adding the a-nucleating agents no p-crystals are formed and the majority crystalline phase is a and y.

[0216] The mechanical properties of polymer compositions are shown in Table 5.

[0217]

[0218] The addition of a-nucleation does not affect strain and tensile strength at room temperature for both PP1 and PP2 materials. The stiffness is below the targeted value of 1000 MPa.

[0219] Nucleation affects the impact strength, although in a different manner for the two base polymers: for PP1 the BNT a-nucleating agent increases the impact strength at low temperature, while for PP2 the impact strength at low temperature is not affected, but at room temperature it is increased by about 50% when adding either DMDBS or BNT.

[0220] The pressure test according to method 1 in the CIGRE TB852 was passed with 0% indentation for the pure polymers already. The addition of a-nucleation is seen only positively in this respect and was, therefore, not measured. This is due to the fact that the higher crystallization temperature achieved with a-nucleation will result in a narrower melting distribution or even in fractions of crystals melting at higher temperature (see Figure 1) and in higher crystallinity (see Table 3 and Table 4). Therefore, the a-nucleated polymer is expected to have higher resistance to indentation at high temperature compared to the non-nucleated polymer.

[0221] P-crystals have a lower thermal stability than a-crystals. At the temperature at which the pressure test method 1 is carried out (130 °C), a significant fraction of the p-phase in both random heterophasic propylene copolymers is melting (see Figure 1). This results in higher indentation values compared to the pure polymers.

[0222] 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 6.

[0223] Table 6

[0224] 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 14 °C higher Tc when compared to the pure polymer and the P-nucleated polymer. This is beneficial because it allows shortening of the time needed for the material to solidify after exiting the extrusion die.

[0225] A second approach to describe the increase of crystallization rate is to evaluate the REact parameter. This parameter reflects the slope of Tc as function of the applied cooling rate.

[0226] As observable in Figure 2a / b, the slope of a logarithmic fitting line decreases for the compounds containing a-nucleating agent compared to the pure polymers, which means that the material is less sensitive to cooling and it results in a higher REact parameter for the a-nucleated compounds. REact of p-nucleated compounds (CE2, CE3, CE5) is higher than REact for the BNT-nucleated compounds (IE1 , IE3), although BNT-nucleation gives higher Tc. On the contrary, the DM DBS-nucleated compound (IE2) has the highest REact parameter, due to the fact that the Tc decrease with increasing the cooling rate is the smallest achieved.

[0227] DC Electrical Conductivity and Activation Energy

[0228] DC conductivity and activation energy data is presented in table 7.

[0229]

[0230] Table 7 shows the comparison of DC conductivity values obtained at 100 °C for comparative and inventive examples. For the PP1 -based compounds, the lowest DC conductivity values are achieved with CGNA (CE3) or BNT (IE1). However, the activation energy with CGNA is higher than with BNT, pointing towards a higher sensitivity of DC conductivity to temperature variations. Furthermore, CGNA gives a yellow / red coloration to the material, while BNT does not affect the original colour of the polymer.

[0231] For the PP2-based compounds, BNT-nucleation (IE3) shows again lower DC conductivity than SIPAX-nucleation (CE5). DMDBS-nucleation (IE2) shows higher DC conductivity than the pure polymer.

[0232] The two random heterophasic polypropylene copolymers show the lowest achieved values of the activation energy.

[0233] In conclusion, the addition of a-nucleating agents to low MFR random heterophasic polypropylene copolymers shows improved impact resistance at room temperature and at low temperature, while maintaining strain at break and tensile strength above the lower required limit of 350% and 8.5 MPa, respectively.

[0234] The indentation performance at high temperature (HV pressure test method 1) is seen as positively affected (i.e. reduced) due to the higher thermal resistance while for p-nucleated compounds this increases significantly compared to the pure polymer due to the lower thermal resistance of (3-rich random heterophasic polypropylene copolymers.

[0235] Moreover, the crystallization temperature is significantly increased when adding the two a-nucleating agents, even more than with p-nucleation.

[0236] DC conductivity values are still lower than 100 fS / m with all nucleation types (a and P), but PVCH-based nucleation shows the lowest values of DC conductivity, remarkably even lower than the pure polymer. The addition of quinacridonequinone also gives lower DC conductivity than the pure polymer.

Claims

Claims1 . A polymer composition comprising(I) a propylene copolymer of propylene and ethylene having:(a) a melt flow rate MFR2(230 °C), measured according to ISO 1133, in the range of from 0.1 to 2.0 g / 10min;(b) a soluble fraction (SF) content 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)) of from 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) at least one a-nucleating agent.

2. A polymer composition as claimed in claim 1 wherein the polymer composition has a Ky of at least 0.35 (Wide angle X-ray scattering).

3. A polymer composition as claimed in any preceding claim wherein the at least one a-nucleating agent is a sorbitol derivative or a polymeric a -nucleating agent such as polyvinyl cyclohexane.

4. A polymer composition as claimed in any preceding claim wherein the at least one a -nucleating agent is present in an amount of from 0.00005 to 1 .0 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 from 8.0 to 14 wt.%, 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 from 2.3 to 3.0 dL / g;(e) C2 content of 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.

6. A polymer composition as claimed in any preceding claim having an intrinsic viscosity (IV) determined according to ISO 1628 / 3 of the propylene copolymer of from 2.0 to 4.0 dL / g; and / or having a melt flow rate MFR2(230 °C), measured according to ISO 1133, in the range of 0.5 to 3.0 g / 10min;.

7. A polymer composition as claimed in any preceding claim wherein the propylene copolymer forms at least 92 wt.% of the polymer composition such as at least 95 wt.%%.

8. A polymer composition as claimed in any preceding claim wherein the polymer composition is free of beta nucleating agents and / or comprising less than 1 .0 wt.% inorganic filler based on the total weight of the polymer composition.

9. A polymer composition as claimed in any preceding claim wherein the propylene copolymer has a crystalline fraction (CF) determined by crystallisation extraction (CRYSTEX) in the range of from 90 to 55 wt.%, 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)) in the range of from 3.0 to 8.0 wt.%, determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy and based on the total weight of the crystalline fraction.

10. A polymer composition as claimed in any preceding claim having a strain at break of at least 500% at 23°C, such as at least 700% (IEC 60811-501) and / or having a tensile strength of at least 20 MPa at 23°C, such as at least 25 MPa (IEC 60811-501).11 . A polymer composition as claimed in any preceding claim having an E-modulus of less than 850 MPa at 23°C (IEC 60811-501).

12. A polymer composition as claimed in any preceding claim having a Charpy notched impact strength at 23°C of at least 65 kJ / m2and / or having a Charpy notched impact strength at -20°C of at least 3.0 kJ / m2determined according to ISO 179-1 / 1 eA.

13. A polymer composition as claimed in any preceding claim having 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 102°C, especially also having a having a Tc at 3°C / min cooling rate of at least 118°C and having a Tc at 30°C / min cooling rate of at least 106°C; and / or having a REact value of 2400 or more such as 2400 to 4500.

14. A polymer composition as claimed in any preceding claim having a DC conductivity at 100 °C of less than 40 fS / m, especially less than 15 fS / m, especially wherein the DC conductivity is lower than that of the non-nucleated propylene copolymer itself; and / or wherein the polymer composition has an activation energy of less than 0.5 wherein activation energy is calculated using the formula:where a is the DC conductivity, Eais the activation energy, kBis the Boltzmann constant, T is the temperature and Tref is 70 °C.

15. A polymer composition as claimed in any preceding claim wherein said propylene copolymer has a soluble fraction (SF) content, determined by crystallization extraction (CRYSTEX), from 10 to 24 wt% based on the total weight amount of the propylene copolymer.

16. A polymer composition as claimed in any preceding claim wherein said propylene copolymer has a melt flow rate MFR2(230 °C), measured according to ISO 1133 of from higherthan 1 .10 g / 10min to 2.0 g / 10min, such as 1 .20 to 1 .75 g / 10 min, or 1.30 to 1.70 g / 10min or an MFR2of 1.35 to 2.0 g / 10min, such as 1.45 to 2.0 g / 10min.

17. A polymer composition as claimed in claim 1 to 15 wherein said propylene copolymer has a melt flow rate MFR2(230 °C), measured according to ISO 1133 of 1 .1 g / 10 min or less, such as 0.5 to 1 .1 g / 1 Omin.

18. An article comprising the polymer composition of any of claims 1 to 17, preferably a cable, in particular a power cable, especially a HV cable, and most especially an HVDC cable.

19. An article as claimed in claim 18 which is a cable 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 17.

20. Use of a polymer composition as claimed in any of claims 1 to 17 as cable insulation, e.g. for high voltage DC cables such as HVDC cables operating atvoltages of at least 30kV, such as 40 kV or higher, 50 kV or higher, 60 kV or higher or 80 kV or higher.21 . A process for transmitting power between a source of DC power (such as a transformer associated with an energy generator) and a receiver of DC power (such as a transformer associated with an energy substation) said process comprising connecting said source of DC power and said receiver of DC power via a cable 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 a polymer composition comprising(I) a propylene copolymer of propylene and ethylene having:(a) a melt flow rate MFR2(230 °C), measured according to ISO 1133, in the range of from 0.1 to 2.0 g / 10min, such as higher than 1.10 g / 10min to 2.0 g / 10min, such as 1.20 to 1.75 g / 10 min, or 1.30 to 1.70 g / 10min or an MFR2of 1 .35 to 2.0 g / 1 Omin, such as 1 .45 to 2.0 g / 1 Omin;(b) a soluble fraction (SF) content 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)) of from 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) at least one a-nucleating agent; and allowing high voltage power at a voltage of 30 kV or more to flow through said cable.