Propylene copolymer for cable insulation

Abstract

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 crystalline fraction (CF) content, determined by crystallisation extraction (CRYSTEX), in the range of from 90 to 55 wt% based on the total weight of the propylene copolymer wherein said crystalline fraction has a C2 content (C2(CF)) of 2.5 to 10 wt%, such as 2.5 to 8.0 wt%, based on the total weight of the crystalline fraction; (d) 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; (e) the intrinsic viscosity, determined according to ISO 1628 / 3, of the soluble fraction (IV(SF)) of the propylene copolymer is in the range of 2.0 to 4.0 dL / g; and (f) a C2 content of the soluble fraction (C2(SF)) of 15 to 55 wt.% based on the total weight of the soluble fraction determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy.

Description

[0001] Propylene copolymer for cable insulation

[0002] The present invention relates to a propylene copolymer, an article comprising said propylene copolymer, preferably a cable comprising an insulation layer comprising said propylene copolymer and the use of said propylene copolymer as cable insulation for medium and high voltage cables, especially for high voltage (HV) cables, 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] 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. The inventors have found that certain random heterophasic polypropylene copolymers have low stiffness (flexural modulus < 1000 MPa) and high impact strength at room temperature, while fulfilling the CIGRE TB852 recommendations for thermoplastic HVDC insulation material about elongation to break, tensile strength and indentation during the method 1 of the HV pressure test. Moreover, the DC conductivity of the propylene copolymer of the invention is below 20 fS / m up to 115°C. Moreover, the propylene copolymers of the invention show the lowest activation energy among different types of polypropylene, such as propylene homopolymers, random propylene copolymers and block heterophasic propylene copolymers. The activation energy describes the dependency of DC conductivity on the operating temperature, such that a low activation energy corresponds to a lower sensitivity to temperature and hence a more attractive material.

[0008] Summary of the invention

[0009] Viewed from one aspect the invention provides a propylene copolymer of propylene and ethylene having:

[0010] (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;

[0011] (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;

[0012] (c) a crystalline fraction (CF) content, determined by crystallisation extraction (CRYSTEX), in the range of from 90 to 55 wt% based on the total weight of the propylene copolymer wherein said crystalline fraction has a C2 content (C2(CF)) of from 1 .5 to 10 wt%, such as 2.5 to 8.0 wt% based on the total weight of the crystalline fraction;

[0013] (d) 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^

[0014] (e) the intrinsic viscosity, determined according to ISO 1628 / 3, of the soluble fraction (IV(SF)) of the propylene copolymer is in the range of 2.0 to 4.0 dL / g; and (f) a C2 content of the soluble fraction (C2(SF)) of 15 to 55 wt.% based on the total weight of the soluble fraction determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy.

[0015] Viewed from another aspect the invention provides an article comprising the propylene copolymer as herein described. Preferably, said article is a cable comprising an insulation layer comprising the propylene copolymer as described above or below.

[0016] Viewed from another aspect the invention provides use of the propylene copolymer 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.

[0017] Definitions

[0018] The propylene copolymer of propylene and ethylene will be called the propylene copolymer herein. The propylene copolymer is a random heterophasic polypropylene copolymer with a semi-crystalline matrix phase and an elastomeric phase dispersed therein.

[0019] 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 comonomerrich block structure and a propylene-rich block structure. The propylene copolymer does not contain therefore a homopolymer matrix phase (so as to form a heterophasic block copolymer).

[0020] A random heterophasic polypropylene copolymer preferably shows two distinct glass transition temperatures Tgwhich are attributed to the copolymeric matrix phase and the elastomeric phase.

[0021] 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 so is not heterophasic.

[0022] 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”.

[0023] A unimodal propylene polymer only consists of one fraction.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] Although not explicitly mentioned in said recommendations, ultra-high voltage (UHV) consequently refers to voltages of more than 800 kV for DC systems.

[0028] In the following, amounts are given in % by weight (wt%) unless it is stated otherwise.

[0029] Detailed description of the invention

[0030] The present invention relates to a propylene copolymer suitable for use in the insulation layers of power cables.

[0031] Propylene copolymer of propylene and ethylene

[0032] The propylene copolymer of propylene and ethylene has: (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;

[0033] (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;

[0034] (c) a crystalline fraction (CF) content, determined by crystallisation extraction (CRYSTEX), in the range of from 90 to 55 wt% based on the total weight of the propylene copolymer wherein said crystalline fraction has a C2 content (C2(CF)) of 2.5 to 10 wt%, such as 2.5 to 8.0 wt% based on the total weight of the crystalline fraction;

[0035] (d) 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^

[0036] (e) the intrinsic viscosity, determined according to ISO 1628 / 3, of the soluble fraction (IV(SF)) of the propylene copolymer is in the range of 2.0 to 4.0 dL / g; and

[0037] (f) a C2 content of the soluble fraction (C2(SF)) of 15 to 55 wt.% based on the total weight of the soluble fraction determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy.

[0038] 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.

[0039] 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 1 1 .0 wt.-% based on the total weight of the propylene copolymer and determined by FT-IR spectroscopy calibrated by quantitative13C-NMR spectroscopy.

[0040] 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).

[0041] 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.

[0042] 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.

[0043] 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 are preferred for cable extrusion where the dimensions of the final article are highly determined by the melt strength of the polymer during a crucial intermediate step. A high enough melt strength, provided by the defined MFR, leads to minimal sagging during cable production.

[0044] The propylene copolymer is a random heterophasic polypropylene copolymer. The random heterophasic polypropylene copolymer has a matrix phase and an elastomeric phase dispersed in said matrix phase. The matrix phase is copolymeric. The matrix phase is preferably a propylene-ethylene random copolymer.

[0045] 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.

[0046] The propylene copolymer 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. 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 2.5 to 10 wt%, more preferably from 2.5 to 8.0 wt% and most preferably from 3.0 to 8.0 wt%, based on the total amount of monomer units in the crystalline fraction (CF).

[0047] 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 97.5 wt.-%, preferably from 92.0 to 97.5 wt.-% and most preferably from 92.0 to 97.0 wt.-%, based on the total amount of monomer units in the crystalline fraction (CF).

[0048] 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.

[0049] 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.

[0050] 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.

[0051] Viewed from another aspect the invention provides a propylene copolymer of propylene and ethylene having:

[0052] (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 0.5 to 1.1 g / 10min;

[0053] (b) 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; (c) a crystalline fraction (CF) content, determined by crystallisation extraction (CRYSTEX), in the range of from 90 to 55 wt% based on the total weight of the propylene copolymer wherein said crystalline fraction has a C2 content (C2(CF)) of 2.5 to 10 wt%, such as 2.5 to 8.0 wt%, based on the total weight of the crystalline fraction;

[0054] (d) 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;

[0055] (e) the intrinsic viscosity, determined according to ISO 1628 / 3, of the soluble fraction (IV(SF)) of the propylene copolymer is in the range of 2.0 to 4.0 dL / g; and

[0056] (f) a C2 content of the soluble fraction (C2(SF)) of 15 to 55 wt.% based on the total weight of the soluble fraction determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy.

[0057] 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 55 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).

[0058] 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 45 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).

[0059] 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.

[0060] Further, said soluble fraction (SF) preferably 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.

[0061] 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.

[0062] It is preferred that the crystalline fraction (OF) and the soluble fraction (SF) make up 100 wt% of the propylene copolymer.

[0063] 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.

[0064] The density of the propylene copolymer is preferably in the range of 880 to 905 kg / m3, (ISO1183).

[0065] In one embodiment, the propylene copolymer has:

[0066] (a) a melt flow rate MFR2(230 °C), measured according to ISO 1133, in the range of 0.5 to 1 .8 g / 10min;

[0067] (b) a soluble fraction (SF) determined by crystallization extraction (CRYSTEX) in the range of 15-40 wt%;

[0068] (c) a comonomer content in the range of 8.0 to 14 wt% based on the total weight of the propylene copolymer and determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy;

[0069] (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

[0070] (e) C2 content of soluble fraction (C2(SF)) of 20 to 40 wt% determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy.

[0071] In one embodiment, the propylene copolymer has a crystalline fraction (CF), determined by crystallisation extraction (CRYSTEX), is in the range of 85 to 60 wt%, preferably 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.

[0072] In one embodiment, the propylene copolymer has an elongation to break at 23°C of at least 350% before ageing, such as at least 700%, e.g. 750 to 950% determined according to IEC 60811-501 at 23°C .

[0073] In one embodiment, the propylene copolymer has an elongation to break at 23°C of at least 350% after ageing at 135°C for 10 days (IEC 60811-401), such as at least 700%, e.g. 750 to 950% determined according to IEC 60811-501 at 23°C. Ageing is carried out according to IEC 60811-401 for any embodiment where ageing is required.

[0074] In one embodiment, the propylene copolymer has a tensile strength at 23°C of at least 15 MPa before ageing, especially at least 20 MPa (IEC 60811-501). The upper limit may be 35 MPa.

[0075] In one embodiment, the propylene copolymer has a tensile strength at 23°C of at least 15 MPa after ageing at 135°C for 10 days (IEC 60811-401), especially at least 20 MPa (IEC 60811-501). The upper limit may be 35 MPa.

[0076] In one embodiment, the propylene copolymer has a flexural modulus of less than 800 MPa, preferably less than 750 MPa, such as 250 to 700 MPa, especially 250 to 400 MPa (ISO 178).

[0077] In one embodiment, the propylene copolymer has a Charpy notched impact strength at 23°C of at least 50 kJ / m2, such as 50 to 90 kJ / m2determined according to ISO 179-1 / 1 eA.

[0078] In one embodiment, the propylene copolymer has a tensile strength at -20°C of at least 20 MPa (IEC 60811-501). The upper limit may be 35 MPa.

[0079] In one embodiment, the propylene copolymer has an elongation to break of at least 200% at -20°C (IEC 60811-501). The upper limit may be 450%.

[0080] In one embodiment, the propylene copolymer has a DC conductivity at 100 °C of less than 40 fS / m, especially less than 15 fS / m, such as 1 to 15 fS / m or 5 to 15 fS / m.

[0081] In one embodiment, the propylene copolymer has an activation energy of less than 0.4 eV 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.

[0082] In one embodiment, the activation energy is 0.15 to 0.35 eV, such as 0.20 to 0.30.

[0083] Manufacture

[0084] The propylene copolymer (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.

[0085] 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.

[0086] 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.

[0087] 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. A further suitable slurry-gas phase process is the Spheripol® process of LyondellBasell.

[0088] Suitable sequential polymerization processes for polymerizing the propylene copolymer, preferably the random heterophasic polypropylene copolymer, are e.g. disclosed in WO 2015 / 117948.

[0089] The propylene copolymer can be polymerized in the presence of a Ziegler-Natta catalyst. Suitable Ziegler-Natta catalysts are e.g. disclosed in WO 2015 / 117948.

[0090] The propylene copolymer is preferably not subjected to a visbreaking step as e.g. described in WO 2013 / 092620 A1.

[0091] 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.

[0092] Polymer Composition

[0093] The propylene copolymer of the invention may be combined with another material to form a polymer composition. The polymer composition can comprise additional polymeric components different from the propylene copolymer such as in an amount of preferably 0.0 to less than 10.0 wt% based on the total weight of the polymer composition although, it is preferred if the propylene copolymer is the only polymer component present other than any polymers used as carriers in a masterbatch.

[0094] The propylene copolymer is preferably present in any 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 polyolefin component present in the polymer composition. However, it will be appreciated that some additives may be carried on a polyolefin as a masterbatch. It is possible therefore that a polyolefin carrier is used in a masterbatch. Preferably, the polymer composition is free of dedicated additional polyolefin components such as a 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.

[0095] 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, alpha or betanucleating agents, etc. Such additives are commercially available and for example described in “Plastic Additives Handbook”, 6thedition 2009 of Hans Zweifel (pages 1 141 to 1 190). Usually, these additives are added in quantities of 1 to 50,000 ppm for each single component. It is especially preferred if an antioxidant is present such as a sterically hindered phenol based antioxidant such as Pentaerythritol tetrakis(3-(3,5-di- tert-butyl-4-hydroxyphenyl). It is especially preferred if an acid scavenger is present, such as a hydrotalcite.

[0096] 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.

[0097] In one embodiment, the polymer composition does not contain alpha-nucleating agents. 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.

[0098] Preferably, the polymer composition is prepared by melt blending the propylene copolymer, and additives. The polymer composition is preferably not subjected to visbreaking.

[0099] 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.

[0100] Article

[0101] The present invention further relates to an article comprising the propylene copolymer as herein defined. The article is preferably a cable, preferably a medium voltage cable, high voltage cable, 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 propylene copolymer as herein described.

[0102] The insulation layer preferably comprises at least 90 wt%, at least 95 wt%, still more preferably at least 98 wt% of the propylene copolymer, based on the total weight of the insulation layer. In one embodiment, the insulation layer of a cable consists of a propylene copolymer as defined herein, The term consists of here is intended to exclude the presence of any other polymer component. However, the propylene copolymer used in the insulation layer may contain additives. In a further embodiment therefore, the insulation layer of a cable consists of a propylene copolymer as defined herein and any optional additives.

[0103] It is preferred that the article meets all CIGRE TB852 recommendations for HVDC cable systems.

[0104] The cable usually comprises of at least one conductor and at least one insulation layer comprising the propylene copolymer 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.

[0105] 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 propylene copolymer of the invention is very suitable for power cables.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] The present invention also relates to the use of the propylene copolymer 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.

[0111] The propylene copolymers of the invention show low stiffness (flexural modulus < 1000 MPa) and high impact strength at room temperature, while fulfilling the CIGRE TB852 recommendations for thermoplastic HVDC insulation material relating to elongation to break, tensile strength, indentation during the method 1 of the HV pressure test. Moreover, the DC conductivity of propylene copolymers of the invention is below 20 fS / m up to 115°C. The propylene copolymers of the invention shows the lowest activation energy among different types of polypropylene.

[0112] 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 propylene copolymer of propylene and ethylene having:

[0113] (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.

[0114] 1 .20 to 1 .75 g / 10 min, or 1 .30 to 1 .70 g / 1 Omin or an MFR2of 1 .35 to 2.0 g / 1 Omin, such as 1 .45 to 2.0 g / 1 Omin;

[0115] (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;

[0116] (c) a crystalline fraction (CF) content, determined by crystallisation extraction (CRYSTEX), in the range of from 90 to 55 wt% based on the total weight of the propylene copolymer wherein said crystalline fraction has a C2 content (C2(CF)) of 2.5 to 10 wt%, such as 2.5 to 8.0 wt%, based on the total weight of the crystalline fraction; (d) 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;

[0117] (e) the intrinsic viscosity, determined according to ISO 1628 / 3, of the soluble fraction (IV(SF)) of the propylene copolymer is in the range of 2.0 to 4.0 dL / g; and

[0118] (f) a C2 content of the soluble fraction (C2(SF)) of 15 to 55 wt.% based on the total weight of the soluble fraction determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy.

[0119] In particular, the invention provides a HVDC cable operating at a voltage of at least 30 kV.

[0120] 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.

[0121] Suitable power generators include wind turbines that can be located on or offshore.

[0122] The invention will now be described with reference to the following non-limiting examples and figures.

[0123] Brief Description of the Figures:

[0124] Figure 1a / b show the DC conductivity and normalised DC conductivity as a function of temperature for comparative and inventive examples.

[0125] Determination Methods

[0126] 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.

[0127] Measurement methods a) Melt Flow Rate (MFR2) 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.

[0128] The melt flow rate MFR2 of propylene copolymer is measured at 230°C with a load of 2.16 kg according to ISO 1133.

[0129] 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

[0130] 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

[0131] 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.

[0132] 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}. 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.

[0133] 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:

[0134] E = 0.5 (Spp + Spy + Sp6 + 0.5( Sap + Say))

[0135] Through the use of this set of sites the corresponding integral equation becomes:

[0136] 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.

[0137] The mole percent comonomer incorporation was calculated from the mole fraction: E [mol%] = 100 * fE

[0138] 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:

[0139] 1) Busico, V., Cipullo, R., Prog. Polym. Sci. 26 (2001) 443.

[0140] 2) Busico, V., Cipullo, R., Monaco, G., Vacatello, M., Segre, A.L., Macromolecules 30 (1997) 6251.

[0141] 3) Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha, A., Baugh, D. Winniford, B., J. Mag. Reson. 187 (2007) 225.

[0142] 4) Busico, V., Carbonniere, P., Cipullo, R., Pellecchia, R., Severn, J., Talarico, G., Macromol. Rapid Commun. 2007, 28, 1128.

[0143] 5) Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev. 2000, 100, 1253.

[0144] 6) Wang, W-J., Zhu, S., Macromolecules 33 (2000), 1157.

[0145] 7) Cheng, H. N., Macromolecules 17 (1984), 1950.

[0146] 8) Singh, G., Kothari, A., Gupta, V., Polymer Testing 28 5 (2009), 475.

[0147] 9) Kakugo, M., Naito, Y., Mizunuma, K., Miyatake, T. Macromolecules 15 (1982) 1150.

[0148] 10) Randall, J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1989, C29, 201.

[0149] 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):

[0150] 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.

[0151] 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.

[0152] 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

[0153] 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.

[0154] Determination of Crystalline and soluble fractions and their respective properties (IV and Ethylene content)

[0155] The crystalline (CF) and soluble fractions (SF) of the polypropylene (PP) compositions as well as the comonomer content and intrinsic viscosities of the respective fractions were analyzed by use of the Crystex (crystallisation extraction) method. Potential instruments that can be used are Crystex QC or Crystex 42 (Polymer Char; Valencia, Spain). Details of the technigue and the method can be found in literature (Ljiljana Jeremie, Andreas Albrecht, Martina Sandholzer & Markus Gahleitner (2020): Rapid characterization of high-impact ethylene-propylene copolymer composition by crystallization extraction separation: comparability to standard separation methods, International Journal of Polymer Analysis and Characterization, 25:8, 581- 596).

[0156] 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.

[0157] 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:

[0158] Cone = a + b*Abs(CH) + c*(Abs(CH))2+ d*Abs(CH3) + e*(Abs(CH3)2+ f*Abs(CH)*Abs(CH3) (Equation 1)

[0159] CH3 / 1000C = a + b*Abs(CH) + c* Abs(CH3) + d * (Abs(CH3) / Abs(CH)) + e * (Abs(CH3) / Abs(CH))2(Equation 2)

[0160] The constants a to f for equation 1 and a to e for equation 2 were determined by using least square regression analysis.

[0161] 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)

[0162] 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.

[0163] 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.

[0164] 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. 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.

[0165] 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)

[0166] 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”.

[0167] 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).

[0168] 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]

[0169] Reduced viscosity (r|red) is calculated using the equation: m where C is the polymer solution concentration at 135°C: C= — ,

[0170] 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).

[0171] 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

[0172] 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 at break and Tensile Strength

[0173] 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 60811-501. The tensile tests at +23°C were made before and after ageing at 135°C for 10 days (ageing procedure according to IEC 60811-401). The results were calculated as median of at least 5 measurements. i) Flexural modulus

[0174] 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 The indentation properties at high temperature were determined on compression moulded plaques at 130°C for 1 h with a cylindric indentor according to HV 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

[0175] The DC electrical conductivity were tested for all the studied polymers 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.

[0176] After placing the compression moulded plaques in test cell, annealing at 115 °C for 6 h with applied voltage was performed on the plaques. This was followed by natural cooling to 70°C and keeping for 4h at each of the testing temperatures (in this order 70, 90, 100, 110, 115 °C). The DC conductivity was calculated in the end of each 4h temperature step. The applied electric field was always 30 kV / mm and calculated as the applied voltage divided by the plaque thickness.

[0177] 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

[0178] I = Measured current

[0179] U = applied voltage a = Area of the measuring electrode d = Plaque thickness

[0180] 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, kBis the Boltzmann constant, T is the temperature and Tref is 70 °C.

[0181] Examples - propylene copolymer

[0182] The propylene copolymer was prepared following the protocols described herein.

[0183] Preparation of the random heterophasic propylene copolymer

[0184] 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.

[0185] For PP-2 a phthalate free Ziegler Natta catalyst was prepared according to the example section of WO 2015 / 1 17948. As co-catalyst triethyl-aluminium (TEAL) and as donor dicyclo pentyl dimethoxy silane (D-donor) was used.

[0186] 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.

[0187] Table 1 : Polymerization conditions of the random heterophasic propylene copolymer powder:

[0188] The properties of the propylene copolymer obtained in Table 1 are reported in Table 2:

[0189] Table 2

[0190] 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. The additives used are Irganox 1010 (0.2 wt%) sold by BASF and DHT-4V supplied by Kisuma / Kyowa (synthetic hydrotalcite - 001 wt%)

[0191] The table 2 propylene copolymers are compared to a series of commercial polymers in table 3. The comparative polymers are summarized below with properties presented in table 3.

[0192] CE1 propylene homopolymer

[0193] CE2 ethylene propylene random copolymer

[0194] CE3 ethylene propylene block heterophasic copolymer.

[0195] Table 3

[0196] The mechanical properties of propylene polymers are shown in Table 4.

[0197] Table 4

[0198] Discussion

[0199] The two propylene copolymers of the invention fulfil all the properties recommended by the CIGRE TB852 (tensile strength and elongation to break before and after ageing, pressure test method 1). At the same time, they show flexural modulus below 1000 MPa as well as the highest impact strength at room temperature among the studied polymers. Other than the two propylene copolymers of the invention, the random copolymer CE2 is the only one with flexural modulus below 1000 MPa, but it shows an impact strength of only 10 kJ / m2which corresponds to a brittle fracture type.

[0200] The block heterophasic copolymer CE3 gives a ductile break (impact strength 27 kJ / m2), but is too stiff (flexural modulus > 1000 MPa). The homopolymer CE1 shows the worst combination of mechanical performance, being too stiff and brittle. Moreover, the tensile properties at -20 °C show that the two propylene copolymers of the invention have elongation to break and tensile strength in between the random copolymer CE2 and the heterophasic copolymer CE3. In conclusion, propylene copolymers of the invention can combine low stiffness with very good impact strength and with good tensile properties at low temperature.

[0201] DC Electrical Activity The DC electrical properties were also tested for all the studied polymers.

[0202] As

[0203] Figure la shows, the absolute values of DC conductivity are low for all the PP types (in a similar range as cross-linked PE insulation materials for HVDC power cables), therefore acceptable for the insulation of an HVDC power cable. The random copolymer CE2 stands out due to its higher DC conductivity values at all temperatures, while the other PP types give rather similar values. However, the sensitivity of the DC conductivity to the temperature shows clearer differences between the PP types. This is reported in

[0204] Figure lb as the DC conductivity normalized to the value at 70 °C.

[0205] As can be seen, the homopolymer CE1 shows the highest sensitivity, followed by the random copolymer CE2, the heterophasic copolymer CE3 and the two propylene copolymers of the invention.

[0206] The sensitivity of DC conductivity to the temperature can also be described by a single parameter called the activation energy.

[0207] The two propylene copolymers of the invention show the lowest achieved values of the activation energy.

[0208] Table 5. Calculated activation energy from exponential fit of the normalized DC conductivity.

[0209] Surprisingly, both IE1 and IE2 have a much lower activation energy than CE1 , CE2 and CE3 despite the lower crystallinity and / or the lower melting point. The low DC conductivity and the low change of DC conductivity with temperatur, as quantified by the activation energy, were not predictable. It is expected that a high soluble fraction as in IE1 would lead to higher DC conductivity since a higher content of non-crystallizing polymer chains may provide more mobility of charges in a DC field. However, in Figure 1 b both IE1 and IE2 have the same normalized DC conductivity up until 100°C, despite IE2 having only half of the soluble fraction as IE1 , as can deduced from the data in Table 5 as well. The activation energy data in table 5 and figure 1 b quantifies therefore the change of DC conductivity with temperature and demonstrates that the inventive examples have much lower activation energy.

[0210] In conclusion, two propylene copolymers of the invention are able to fulfil the mechanical recommendations for a thermoplastic insulation for an HVDC power cable and show the lowest stiffness and the lowest temperature sensitivity of DC conductivity to temperature when compared to other polypropylene types like homopolymer, random copolymer and block heterophasic copolymers.

Claims

Claims1 . 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 crystalline fraction (CF) content, determined by crystallisation extraction (CRYSTEX), in the range of from 90 to 55 wt% based on the total weight of the propylene copolymer wherein said crystalline fraction has a C2 content (C2(CF)) of 2.5 to 10 wt%, such as 2.5 to 8.0 wt%, based on the total weight of the crystalline fraction;(d) 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;(e) the intrinsic viscosity, determined according to ISO 1628 / 3, of the soluble fraction (IV(SF)) of the propylene copolymer is in the range of 2.0 to 4.0 dL / g; and(f) a C2 content of the soluble fraction (C2(SF)) of 15 to 55 wt.% based on the total weight of the soluble fraction determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy.

2. A propylene copolymer as claimed in claim 1 having:(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) content, 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% 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; and(e) 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, based on the total weight of the soluble fraction.

3. A propylene copolymer as claimed in any preceding claim having an intrinsic viscosity (IV) determined according to ISO 1628 / 3 in the range of from 2.0 to 4.0 dL / g.

4. A propylene copolymer as claimed in any preceding claim wherein the crystalline fraction (CF), determined by crystallisation extraction (CRYSTEX), is in the range of from 85 to 60 wt% based on the total weight of the propylene copolymer.

5. A propylene copolymer as claimed in any preceding claim 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 based on the total weight of the crystalline fraction.

6. A propylene copolymer as claimed in any preceding claim having an elongation to break at 23°C of at least 350%, as measured according to IEC 60811-501 , before and after ageing at 135°C for 10 days (ageing according to IEC 60811- 401), such as at least 700%.

7. A propylene copolymer as claimed in any preceding claim having a tensile strength at 23°C of at least 15 MPa, as measured according to IEC 60811-501 , before and after ageing at 135°C for 10 days made according to IEC 60811-401 , especially at least 20 MPa.

8. A propylene copolymer as claimed in any preceding claim having a flexural modulus of less than 800 MPa, especially less than 750 MPa (ISO178).

9. A propylene copolymer as claimed in any preceding claim having a Charpy notched impact strength at 23°C of at least 50 kJ / m2(ISO179-1).

10. A propylene copolymer as claimed in any preceding claim having a tensile strength at -20°C of at least 20 MPa (IEC 60811-501) and / or an elongation to break of at least 200% at -20°C (IEC 60811-501).11 . A propylene copolymer 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.

12. A propylene copolymer as claimed in any preceding claim having an activation energy of less than 0.4 eV wherein activation energy is 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.

13. A propylene copolymer as claimed in any preceding claim having 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.

14. A propylene copolymer as claimed in any preceding claim having a melt flow rate MFR2(230 °C), measured according to ISO 1133 of from higher than 1 .10 g / 1 Omin to 2.0 g / 1 Omin, such as 1 .20 to 1 .75 g / 10 min, or 1 .30 to 1 .70 g / 1 Omin or an MFR2of 1 .35 to 2.0 g / 1 Omin, such as 1 .45 to 2.0 g / 1 Omin.

15. A propylene copolymer as claimed in any one of claims 1 to 13 having 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.

16. A polymer composition comprising a propylene copolymer as claimed in any of claims 1 to 15, especially where said propylene copolymer forms at least 90 wt% of the polymer composition.

17. An article comprising the propylene copolymer of claims 1 to 15, preferably a cable, in particular a power cable, especially a HV cable, and most especially an HVDC cable.

18. An article as claimed in claim 17 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 propylene copolymer as defined in claims 1 to 15.

19. Use of a propylene copolymer as claimed in any of claims 1 to 15 as cable insulation, e.g. for high voltage DC cables, such as HVDC cables operating at voltages of at least 30kV, such as 40 kV or higher, 50 kV or higher, 60 kV or higher or 80 kV or higher.

20. 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 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 1.10 g / 10min to 2.0 g / 10min, e.g.1 .20 to 1 .75 g / 10 min, or 1 .30 to 1 .70 g / 10min or an MFR2 of 1 .35 to 2.0 g / 10min, such as 1 .45 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 crystalline fraction (CF) content, determined by crystallisation extraction (CRYSTEX), in the range of from 90 to 55 wt% based on the total weight of the propylene copolymer wherein said crystalline fraction has a C2 content (C2(CF)) of 2.5 to 10 wt%, such as 2.5 to 8.0 wt%, based on the total weight of the crystalline fraction;(d) 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;(e) the intrinsic viscosity, determined according to ISO 1628 / 3, of the soluble fraction (IV(SF)) of the propylene copolymer is in the range of 2.0 to 4.0 dL / g; and(f) a C2 content of the soluble fraction (C2(SF)) of 15 to 55 wt.% based on the total weight of the soluble fraction determined by FT-IR spectroscopy calibrated by quantitative 13C-NMR spectroscopy; and allowing high voltage power at a voltage of 30 kV or more to flow through said cable.

Images