Polyethylene compositions

Abstract

A polyethylene composition comprising a metallocene catalysed multimodal medium density polyethylene (mMDPE) and 0.5 to 5 weight % carbon black; wherein the metallocene catalysed multimodal medium density polyethylene (mMDPE) comprises: (i) 48 to 55 wt%, based on the total weight of the mMDPE, of a first polyethylene component (A), and (ii) 45 to 52 wt%, based on the total weight of the mMDPE, of a second polyethylene component (B), wherein the first polyethylene component (A) has a density (ISO 1183) in the range of from 950 to 980 kg / m3, and a MFR2 (190°C, 2.16 kg, ISO 1133) in the range of from 20 to 500 g / 10 min; the second polyethylene component (B) has a density (ISO 1183) in the range of from 900 to 925 kg / m3, and wherein the polyethylene composition has a density of 945 to 960 kg / m3; and a MFR5 (190°C, 5.0 kg, ISO 1133) in the range from 0.5 to 3.0 g / 10min.

Description

[0001] Polyethylene Compositions

[0002] The present invention relates to a polyethylene composition suitable for the production of a pipe. The present invention also relates to a pipe or pipe fitting comprising the polyethylene composition, a process for producing the polyethylene composition, and the use of the polyethylene composition for the production of a pipe or pipe fitting.

[0003] Background

[0004] Polyolefin pipes and especially polyethylene pipes are used for the transport of water and gas, as well as industrial liquids and slurries. Due to their versatility, ease of production and installation, and non-corrosivity, their use is constantly increasing.

[0005] New installation techniques, such as trenchless and sand bed-free installation, demand polyethylene pipe resins with higher and higher resistance to slow crack growth. The requirements for slow crack growth are becoming increasingly stringent and many of the existing products fail to consistently meet those requirements. At the same time, there is a need to improve the impact resistance of the HDPE pipe resins in order to avoid pipelines’ failure by rapid crack propagation.

[0006] According to ISO 9080, polyethylene pipes are classified by their minimum required strength, i.e. their capability to withstand different hoop stresses for 50 years at 20 °C without fracturing. According to this standard, pipes withstanding hoop stresses of 8.0 MPa (MRSs.o) are classified as PE80 pipes, and pipes withstanding hoop stresses of 10.0 MPa (MRS-io.o) are classified as PE100 pipes. The service temperature for PE100 is usually within the temperature range from about 0 °C to about 50 °C.

[0007] To meet the PE80 requirements with multimodal resins manufactured by conventional Ziegler-Natta catalysts, the density is typically at least 940 kg / m3 and, to meet PE100 requirements, the density is typically above 945 kg / m3 . However, the density of a polyethylene resin is related to its crystallinity. The higher the crystallinity of a polyethylene resin the lower its slow crack growth resistance. When the density is increased, the resistance to slow crack growth (SCG) decreases. The manufacture of polyethylene materials to be used in pressure pipes is discussed for example in an article by Scheirs et al (Scheirs, Bohm, Boot and Leevers: PE100 Resins for Pipe Applications, TRIP Vol. 4, No 12 (1996) pp. 408- 415).

[0008] WO 00 / 22040 discloses a pipe having good mechanical properties made from a bimodal resin. EP 1 985 660 A1 discloses a pipe or a supplementary pipe article with improved slow crack growth resistance comprising a polyethylene composition comprising a base resin, which comprises a first ethylene homo- or copolymer fraction (A), and a second ethylene homo- or copolymer fraction (B), wherein fraction (A) has a lower average molecular weight than fraction (B), and wherein the base resin has a density in the range of 945 to 949 kg / m3 , an MFRs in the range of 0.2 to 0.4 g / 10 min., a comonomer content of higher than 2.0 wt.- % and a SHI(2.7 / 2io) in the range of 55 to 100.

[0009] There remains a need to provide polyethylene compositions with a desirable balance of mechanical properties including, for example, a desirable balance between tensile strength and resistance to slow crack growth.

[0010] Definitions

[0011] Where the term "comprising" is used in the present description and claims, it does not exclude other non-specified elements. For the purposes of the present invention, the term "consisting of" is considered to be a preferred embodiment of the term "comprising of". If hereinafter a group is defined to comprise at least a certain number of embodiments, this is also to be understood to disclose a group, which preferably consists only of these embodiments.

[0012] Whenever the terms "including" or "having" are used, these terms are meant to be equivalent to "comprising" as defined above.

[0013] Where an indefinite or definite article is used when referring to a singular noun, e.g. "a", "an" or "the", this includes a plural of that noun unless something else is specifically stated.

[0014] Metallocene catalysed medium density polyethylene (mMDPE) is defined in this invention as medium density polyethylene, which has been produced in the presence of a metallocene catalyst.

[0015] For the purpose of the present invention “medium density polyethylene (MDPE) which comprises a first polyethylene component (A) and a second polyethylene component (B)” means that the MDPE is produced in an at least 2-stage sequential polymerization process, wherein first component (A) is produced and component (B) is then produced in the presence of component (A) in a subsequent polymerization step, yielding the MDPE or vice versa, i.e. first component (B) is produced and component (A) is then produced in the presence of component (B) in a subsequent polymerization step, yielding the MDPE. MDPEs produced in a multistage process are also designated as "in-situ" or “reactor” blends. The resulting end-product consists of an intimate mixture of the polymers from the two or more reactors, the different molecular-weight-distribution curves of these polymers together forming a molecular-weight-distribution curve having a broad maximum or two or more maxima, i.e. the end product is a multimodal polymer mixture.

[0016] Term “multimodal” in context of medium density polyethylene (MDPE) means herein multimodality with respect to melt flow rate (MFR) of the at least two polyethylene components, i.e. the two polyethylene components, have different MFR values. The multimodal medium density polyethylene can have in addition or alternatively multimodality between the two polyethylene components with respect to one or more further properties, like density, comonomer type and / or comonomer content, as will be described later below.

[0017] The term “polyethylene homopolymer” may refer to a polyethylene homopolymer that comprises at least 99.0 wt%, especially at least 99.5 wt% ethylene monomer units. Thus, a polyethylene homopolymer may comprise up to 1.0 wt% comonomer units, but preferably comprises only up to 0.5 wt%, like up to 0.2 wt% or even up to 0.1 wt% only. In an embodiment of the present invention, the amount of comonomer in the polyethylene homopolymer component is not detectable with13C-NMR.

[0018] Description

[0019] According to a first aspect of the present disclosure, there is provided a polyethylene composition comprising a metallocene catalysed multimodal medium density polyethylene (mMDPE) and 0.5 to 5 weight % carbon black; wherein the metallocene catalysed multimodal medium density polyethylene (mMDPE) comprises:

[0020] (i) 48 to 55 wt%, based on the total weight of the mMDPE, of a first polyethylene component (A), and

[0021] (ii) 45 to 52 wt%, based on the total weight of the mMDPE, of a second polyethylene component (B), wherein the first polyethylene component (A) has a density (ISO 1183) in the range of from 950 to 980 kg / m3, and a MFR2 (190°C, 2.16 kg, ISO 1133) in the range of from 20 to 500 g / 10 min; the second polyethylene component (B) has a density (ISO 1183) in the range of from 900 to 925 kg / m3, and wherein the polyethylene composition has a density of 945 to 960 kg / m3; and a MFRs (190°C, 5.0 kg, ISO 1133) in the range from 0.5 to 3.0 g / 10min.

[0022] It has been found that, by using a metallocene catalysed multimodal medium density polyethylene (mMDPE) defined above as a base resin of a polyethylene composition including 0.5 weight % to up to 5 weight % carbon black, it is possible to provide a polyethylene composition having an improved balance between tensile strength and resistance to slow crack growth. The polyethylene composition may also be used to produce pipes having desirable pressure resistance, for example, short term pressure resistance.

[0023] The strain hardening modulus (SH modulus) may be measured to provide an indication of the composition’s resistance to slow crack growth, while yield stress may be measured to provide an indication of the composition’s tensile strength. The inventors have found that yield stress is very well correlated with short term pressure resistance. Higher yield strength is indicative of longer failure time at 80°C, so indicating a longer life time of a pipe made from the composition.

[0024] The polyethylene composition may have a higher strain hardening modulus than expected for its density. In some examples, the polyethylene composition may have a high strain hardening modulus as well as a desirable yield strength. Higher SH modulus indicates a higher resistance to slow crack growth and so improved robustness of a pipe made from the composition. Improved robustness of PE pipes is desirable as the pipes can be used in a larger range of situations because the pipes can be used with a wider range of installation practices.

[0025] The polyethylene composition may have a strain hardening modulus of 65 MPa or higher; 70 MPa or higher; 75 MPa or higher; 80 MPa or higher, 85 MPa or higher, 90 MPa or higher; 95 MPa or higher, or 100 MPa or higher.

[0026] The polyethylene composition usually has a strain hardening modulus of not more than 150 MPa, not more than 145 MPa, not more than 140 MPa, not more than 135 MPa; not more than 130 MPa; not more than 125 MPa; not more than 120 MPa; or not more than 115 MPa.

[0027] For example, the polyethylene composition may have a strain hardening modulus of 65 to 150 MPa; 70 to 145 MPa; 75 to 140 MPa; 80 to 135 MPa; 85 to 130 MPa; 88 to 125 MPa; 90 to 120 MPa; or 100 to 115 MPa. The strain hardening modulus may be measured at 80°C and 20 mm / min on preconditioned (120°C / 1h) 0.3mm thick specimens according to ISO 18488.

[0028] Additionally or alternatively, the polyethylene composition may have a yield stress of 4.0 MPa or higher; 4.5 MPa or higher; 5.0 MPa or higher; 5.5 MPa or higher, or 6.0 MPa or higher The yield stress may be at most 20 MPa, at most 18 MPa, at most 15 MPa, at most 12 MPa or at most 10 MPa. For example, the yield stress may be 4.0 to 20 MPa, 4.5 to 18 MPa, 5.0 to 15 MPa, 5.5 to 12 MPa and 6.0 to 10 MPa.

[0029] Yield stress may be determined according to the tensile test according to ISO 527-1.

[0030] The pressure resistance may be determined by measuring the short term pressure resistance of a pipe formed using the polyethylene composition of the present disclosure. The short term pressure resistance of a pipe provides an indication of the pipe’s resistance to internal hydrostatic pressure at a given temperature. Short term pressure resistance may be measured according to ISO-1167-1. A hoop stress of 5.4 and a temperature of 80 °C may be applied. Pipes produced using the polyethylene composition of the present disclosure may have a short term pressure resistance (STPR) of at least 300 h, preferably at least 350 hours, for example, at least 400 hours or at least 450 hours (5.4 MPa / 80°C).

[0031] The polyethylene composition of the present invention is suitable for use in the manufacture of pipes and pipe fittings. According to a second aspect, the present disclosure also provides a pipe or pipe fitting comprising the polyethylene composition of the first aspect. As a third aspect, the present disclosure also provides the use of the polyethylene composition of the first aspect for producing a pipe or pipe fitting.

[0032] A fourth aspect of the present disclosure provides a process for producing the polyethylene composition of the first aspect, wherein the metallocene catalysed multimodal medium density polyethylene (mMDPE) is produced in the presence of metallocene complex of formula (I): wherein each X is independently a halogen atom, a Ci-6-alkyl group, Ci-6-alkoxy group, phenyl or benzyl group; each Het is independently a monocyclic heteroaromatic group containing at least one heteroatom selected from O or S;

[0033] L is -R'2Si-, wherein each R’ is independently Ci-20-hydrocarbyl or Ci- -alkyl substituted with alkoxy having 1 to 10 carbon atoms;

[0034] M is Ti, Zr or Hf; each Ri is selected from at least one of a Ci-6-alkyl group and Ci-6-alkoxy group; each n is 1 to 2; each R2 is selected from at least one of a Ci-6-alkyl group, Ci-6-alkoxy group or -Si(R)3 group; each R is selected from at least one of a C-i- -al kyl or phenyl group optionally substituted by 1 to 3 Ci-6-alkyl groups; and each p is 0 to 1.

[0035] Preferably, the metallocene catalysed multimodal medium density polyethylene (mMDPE) is produced in a multi-stage polymerisation process in the presence of the metallocene complex of formula (i).

[0036] The polyethylene composition may have a MFR21 (190°C, 21.6 kg, ISO 1133) in the range from 10 to 45 g / 10min.

[0037] Preferably, the second polyethylene component (B) has a MFR2 (190°C, 2.16 kg, ISO 1133) in the range of from 0.0001 to 1.0 g / 10 min.

[0038] The metallocene catalysed multimodal medium density polyethylene (mMDPE) (also referred to as the “base resin”) may have a density (ISO1183) of 932 to 955 kg / m3; and / or a MFR5 (190°C, 5.0 kg, ISO 1133) in the range from 0.5 to 3.0 g / 10min; and / or a MFR21 (190°C, 21.6 kg, ISO 1133) in the range from 10 to 45 g / 10min. The metallocene catalysed mMDPE may have a molecular weight distribution (MWD), Mw / Mn, in the range of 6.0 to 14.0, preferably 6.5 to 13.5, and more preferably 7.0 to 13.0.

[0039] The metallocene catalysed mMDPE may additionally have one or more or preferably all of the following properties: a weight average molecular weight, Mw, of at least 80000 g / mol, preferably in the range of 90000 to 150000 g / mol, more preferably 90000 to 130000 g / mol, still more preferably 100000 to 120000 g / mol, and / or a z average molecular weight, Mz, in the range of 200000 to 400000 g / mol, preferably 220000 to 350000 g / mol and more preferably from 250000 to 300000 g / mol, and / or a ratio of Mz / Mw in the range of 2.0 to 4.0, preferably 2.2 to 3.5, and more preferably 2.3 to 3.2.

[0040] Mw and Mz may be determined by Gel Permeation Chromatography (GPC) according to ASTM D 6474-99.

[0041] The first polyethylene component (A) may have a density in the range of 955 to 975 kg / m3, preferably 960 to 972 kg / m3and more preferably 962 to 970 kg / m3; and a MFR2 (190°C, 2.16 kg, ISO 1133) in the range of 30 to 400 g / 10min, preferably 50 to 300 g / 10min and more preferably 60 to 200 g / 10min. The second polyethylene component (B) can have a density in the range of 905 to 920 kg / m3, more preferably 908 to 918 kg / m3and even more preferably 910 to 916 kg / m3and an MFR2(190°C, 2.16 kg, ISO 1133) in the range of 0.0003 to 0.5 g / 10min, more preferably 0.0005 to 0.1 g / 10min and even more preferably 0.0008 to 0.01 g / 10min.

[0042] Detailed Description of Invention

[0043] A metallocene catalysed multimodal medium density polyethylene m(MDPE)

[0044] The metallocene catalysed multimodal mMDPE of the polyethylene composition of the present disclosure may have a density (ISO 1183) in the range of 932 to 955 kg / m3, preferably 935 to 950 kg / m3and more preferably 938 to 945 kg / m3.

[0045] The MFR2(190°C, 2.16 kg, ISO 1133) of the metallocene catalysed mMDPE may be in the range of 0.01 to 1.0 g / 10 min, preferably 0.05 to 0.90 g / 10 min, more preferably 0.10 to 0.80 g / 10 min and even more preferably 0.15 to 0.70 g / 10 min. The MFRs (190°C, 5.0 kg, ISO 1133) of the metallocene catalysed mMDPE may be in the range of 0.5 to 3.0 g / 10 min, preferably 0.6 to 2.8 g / 10 min, more preferably 0.7 to 2.5 g / 10 min and even more preferably 0.8 to 2.3 g / 10 min. In some examples, the MFR5 of the metallocene catalysed mMDPE may be in the range of 0.9 to 2.2 g / 10 min, for example, 1.0 to 1.8 g / 10 min.

[0046] The MFR21 (190°C, 21.6 kg, ISO 1133) of the metallocene catalysed mMDPE may be in the range of 10 to 45 g / 10 min, preferably in a range of 12 to 42 g / 10min, more preferably in the range of 14 to 40 g / 10 min, yet more preferably 15 to 35 g / 10 min or 16 to 32 g / 10min.

[0047] The metallocene catalysed mMDPE according to the present invention furthermore has a Flow Rate Ratio (FRR) of the MFR21 / MFR5 in the range of 10.0 to 30.0, preferably 12.0 to 25.0, and more preferably 15.0 to 20.0.

[0048] Additionally, the metallocene catalysed MDPE may have a molecular weight distribution (MWD), Mw / Mn, in the range of 6.0 to 14.0, preferably 6.5 to 13.5, and more preferably 7.0 to 13.0.

[0049] The metallocene catalysed mMDPE may have a weight average molecular weight, Mw, of at least 80000 g / mol, preferably in the range of from 90000 to 150000 g / mol, more preferably from 95000 to 140000 g / mol.

[0050] The z average molecular weight, Mz, may be in the range of 200000 to 420000 g / mol, preferably 250000 to 400000 g / mol and more preferably from 280000 to 380000 g / mol.

[0051] The ratio of Mz / Mw may be in the range of 2.5 to 4.0, preferably 2.7 to 3.5 and more preferably 2.8 to 3.2.

[0052] The metallocene catalysed mMDPE comprises or may consist of:

[0053] (i) 48 to 55 wt%, relative to the total weight of the mMDPE, of a first polyethylene component (A) with a density in the range of 950 to 980 kg / m3and a MFR2 (190°C, 2.16 kg, ISO 1133) of 20 to 500 g / 10 min; and

[0054] (ii) 45 to 52 wt%, relative to the total weight of the mMDPE, of a second polyethylene component (B) with a density in the range of 900 to 925 kg / m3and, optionally, a MFR2 (190°C, 2.16 kg, ISO 1133) of 0.0001 to 1.0 g / 10 min. The amounts of components (A) and (B) preferably sum up to 100 wt%.

[0055] The weight ratio of component (A) to component (B) in the metallocene catalysed mMDPE thus is in the range 48:52 to 55:45.

[0056] The first polyethylene component (A) and / or (B) can be a homopolymer or an ethylene copolymer.

[0057] Alternatively, the first polyethylene component (A) may be a homopolymer and second polyethylene component (B) may be a copolymer or vice versa (i.e., the first polyethylene component (A) being a copolymer and second polyethylene component (B) being a homopolymer).

[0058] Preferably, second polyethylene component (B) consists of a single ethylene copolymer or of a single ethylene homopolymer, more preferably of a single ethylene copolymer.

[0059] The first polyethylene component (A) may consist of a single ethylene homo- or copolymer. Alternatively, component (A) may be an ethylene polymer mixture comprising (e.g. consisting of) a first ethylene polymer fraction (A-1) and a second ethylene polymer fraction (A-2), whereby both fractions are either a homopolymer or a copolymer. Component (A) may be unimodal or multimodal. In case component (A) is an ethylene copolymer mixture, the comonomer(s) in the first and second ethylene copolymer fractions may be the same or different.

[0060] Preferred ethylene copolymers employ alpha-olefins (e.g. C3-C12 alpha-olefins) as comonomers. Examples of suitable alpha-olefins include 1-butene, 1-hexene and 1-octene. 1-butene and 1-hexene are especially preferred comonomers.

[0061] The first polyethylene component (A) may be a polyethylene homopolymer and the second polyethylene component (B) may be an ethylene-1 -hexene copolymer. In this embodiment, the polyethylene homopolymer of component (A) may be formed in the presence of a comonomer but the concentration of comonomer in the resulting polyethylene may be so low as to be below the detection limit of detection methods, such as13C-NMR. For example, the comonomer content may be less than 0.2 mol%, preferably less than 0.1 mol%.

[0062] Preferably, the first polyethylene component (A) may comprise or consist of a first ethylene polymer fraction (A-1) and a second ethylene polymer fraction (A-2), whereby both fractions are either a homopolymer, and the second polyethylene component (B) may be an ethylene copolymer, for example, an ethylene- 1 -hexene copolymer

[0063] The first polyethylene component (A) preferably has a MFR2 in the range of 20 to 500 g / 10min, more preferably 50 to 300 g / 10min, even more preferably 60 to 200 g / 10min and most preferably 70 to 150 g / 10 min. For example, the first polyethylene component (A) may have a MFR2 in the range of 80 to 145 g / 10 min.

[0064] The density of first polyethylene component (A) preferably is in the range of 955 to 975 kg / m3, more preferably 960 to 972 kg / m3and even more preferably 962 to 970 kg / m3.

[0065] It is further preferred that first polyethylene component (A) comprises at least two fractions, i.e. a first ethylene polymer fraction (A-1) and a second ethylene polymer fraction (A-2),. Component (A) may have a first ethylene homopolymer fraction (A-1) and a second ethylene homopolymer fraction (A-2); a first ethylene copolymer fraction (A-1) and a second ethylene copolymer fraction (A-2); a first ethylene copolymer fraction (A-1) and a second ethylene homopolymer fraction (A-2); or a first ethylene homopolymer fraction (A-1) and a second ethylene copolymer fraction (A-2). In one example, both ethylene polymer fractions (A-1) and (A-2) are ethylene homopolymer fractions.

[0066] It is possible that fraction (A-1) is produced first and then fraction (A-2) is produced in the presence of fraction (A-1) in a subsequent reactor or vice versa, i.e. fraction (A-2) is produced first and then fraction (A-1) is produced in the presence of fraction (A-2) in a subsequent reactor. Preferably, fraction (A-1) is produced first.

[0067] The MFR2 and / or the density of fractions (A-1) and (A-2) may be the same or may be different from each other.

[0068] Thus, the ethylene polymer fraction (A-1) preferably has a MFR2 (190°C, 2.16 kg, ISO 1133) in the range of 5.0 to 100.0 g / 10 min, preferably of 10.0 to 80.0 g / 10 min, more preferably of 15.0 to 70.0 g / 10 min and even more preferably of 20.0 to 60.0 g / 10 min, like 22.0 to 50.0 g / 10 min.

[0069] The ethylene polymer fraction (A-2) preferably has a MFR2 (190°C, 2.16 kg, ISO 1133) in the range of 100.0 to 1500.0 g / 10 min, preferably of 200.0 to 1200.0 g / 10 min, more preferably of 250.0 to 1000.0 g / 10 min and most preferably of 280.0 to 800.0 g / 10 min. Preferably, the MFR2 of fraction (A-2) may be higher than the MFR2 of fraction (A-1).

[0070] The density of the ethylene polymer fraction (A-1) preferably is in the range of 935 to 970 kg / m3, more preferably 940 to 965 kg / m3and even more preferably 945 to 962 kg / m3.

[0071] The ethylene polymer fraction (A-2) preferably has a density in the range of 960 to 982 kg / m3, more preferably 965 to 980 kg / m3.

[0072] Preferably, the density of fraction (A-2) is higher than the density of fraction (A-1).

[0073] The second polyethylene component (B) preferably has a MFR2 in the range of 0.0003 to 0.5 g / 10min, more preferably 0.0005 to 0.1 g / 10min, and even more preferably 0.0008 to 0.01 g / 10min.

[0074] The density of the second polyethylene component (B) preferably is in the range of 905 to 920 kg / m3, more preferably 908 to 918 kg / m3and even more preferably 910 to 916 kg / m3.

[0075] The metallocene catalysed mMDPE may be produced by polymerization using conditions which create a multimodal (e.g. bimodal) polymer product using a metallocene catalyst system.

[0076] Thus, the metallocene catalysed mMDPE of embodiment (I) can be produced in a 2-stage process, preferably comprising a slurry reactor (loop reactor), whereby the slurry (loop) reactor is connected in series to a gas phase reactor (GPR), whereby either first polyethylene component (A) or second polyethylene component (B) is produced in the loop reactor and the other ethylene polymer component is then produced in GPR in the presence of the first produced ethylene polymer component to produce the metallocene catalysed mMDPE, preferably the first polyethylene component (A) is produced in the loop reactor and the second polyethylene component (B) is produced in GPR in the presence of the first polyethylene component (A) to produce the metallocene catalysed mMDPE.

[0077] In case that the first polyethylene component (A) of the metallocene catalysed mMDPE consists of ethylene polymer fractions (A-1) and (A-2), the metallocene catalysed mMDPE can be produced with a 3-stage process, preferably comprising a first slurry reactor (loop reactor 1), whereby the first slurry loop reactor is connected in series with another slurry reactor (loop reactor 2), so that the first ethylene polymer fraction (A-1) produced in the loop reactor 1 is fed to the loop reactor 2, wherein the second ethylene polymer fraction (A-2) is produced in the presence of the first fraction (A-1). It is possible that fraction (A-1) is produced first and then fraction (A- 2) is produced in the presence of fraction (A-1) in a subsequent reactor or vice versa, i.e. fraction (A-2) is produced first and then fraction (A-1) is produced in the presence of fraction (A-2) in a subsequent reactor. Preferably, fraction (A-1) is produced first.

[0078] It is within the scope of the invention, that the first and the second ethylene polymer fraction (A-1 and A-2) of the first polyethylene component (A) are present in a weight ratio of 4:1 up to 1 :4, such as 3:1 to 1 :3, or 2:1 to 1 :2, or 1 :1 , based on the total weight of the first polyethylene component (A).

[0079] The first and the second polymerization stages are preferably slurry polymerization steps.

[0080] The slurry polymerization usually takes place in an inert diluent, typically a hydrocarbon diluent such as methane, ethane, propane, n-butane, isobutane, pentanes, hexanes, heptanes, octanes etc., or their mixtures. Preferably, the diluent is a low-boiling hydrocarbon having from 1 to 4 carbon atoms or a mixture of such hydrocarbons. An especially preferred diluent is propane, possibly containing minor amount of methane, ethane and / or butane.

[0081] The temperature in each of the first and second polymerization stages is typically from 60 to 100°C, preferably from 70 to 90°C. An excessively high temperature should be avoided to prevent partial dissolution of the polymer into the diluent and the fouling of the reactor. The pressure is from 1 to 150 bar, preferably from 40 to 80 bar.

[0082] The slurry polymerization may be conducted in any known reactor used for slurry polymerization. Such reactors include a continuous stirred tank reactor and a loop reactor. It is especially preferred to conduct the slurry polymerization in a loop reactor. In such reactors the slurry is circulated with a high velocity along a closed pipe by using a circulation pump. Loop reactors are generally known in the art and examples are given, for instance, in US-A- 4582816, US-A-3405109, US-A-3324093, EP-A-479186 and US-A-5391654. It is thus preferred to conduct the first and second polymerization stages as slurry polymerizations in two consecutive loop reactors.

[0083] The slurry may be withdrawn from each reactor either continuously or intermittently. A preferred way of intermittent withdrawal is the use of settling legs where slurry is allowed to concentrate before withdrawing a batch of the concentrated slurry from the reactor. The use of settling legs is disclosed, among others, in US-A-3374211 , US-A-3242150 and EP-A- 1310295. Continuous withdrawal is disclosed, among others, in EP-A-891990, EP-A-1415999, EP-A-1591460 and WO-A-2007 / 025640. The continuous withdrawal is advantageously combined with a suitable concentration method, as disclosed in EP-A-1310295 and EP-A- 1591460. It is preferred to withdraw the slurry from each of the first and second polymerization stages continuously.

[0084] Hydrogen is typically introduced into the first and second polymerization stages for controlling the MFR2 of the first and second ethylene polymers. The amount of hydrogen needed to reach the desired MFR depends on the catalyst used and the polymerization conditions.

[0085] The loop reactor 2 is thereby connected in series to a gas phase reactor (GPR), so that the first polyethylene component (A) leaving the second slurry reactor is fed to the GPR to produce a trimodal polyethylene copolymer. In this case, the reaction conditions in the two slurry reactors are chosen in a way that in the two slurry reactors different products in view of MFR and / or density are produced.

[0086] The third polymerization stage is a gas phase polymerization step, i.e. carried out in a gasphase reactor. Any suitable gas phase reactor known in the art may be used, such as a fluidised bed gas phase reactor.

[0087] For gas phase reactors, the reaction temperature used will generally be in the range 60 to 115°C (e.g. 70 to 110°C), the reactor pressure will generally be in the range 10 to 25 bar, and the residence time will generally be 1 to 8 hours. The gas used will commonly be a non- reactive gas such as nitrogen or low boiling point hydrocarbons such as propane together with monomer (e.g. ethylene).

[0088] A chain transfer agent (e.g. hydrogen) is typically added to the third polymerization stage.

[0089] Such a process is described inter alia in WO 2016 / 198273, WO 2021009189, WO 2021009190, WO 2021009191 and WO 2021009192. Full details of how to prepare suitable multimodal polymers can be found in these references.

[0090] A suitable process is the Borstar PE process or the Borstar PE 3G process.

[0091] The metallocene catalysed mMDPE according to the present invention is therefore preferably produced in a loop loop gas cascade.

[0092] Polymerisation may be preceded by a prepolymerization step.

[0093] The purpose of the prepolymerization is to polymerize a small amount of polymer onto the catalyst at a low temperature and / or a low monomer concentration. By prepolymerization it is possible to improve the performance of the catalyst in slurry and / or modify the properties of the final polymer. The prepolymerization step is preferably conducted in slurry. Thus, the prepolymerization step may be conducted in a loop reactor.

[0094] The prepolymerization may be preferably conducted in an inert diluent, typically a hydrocarbon diluent such as methane, ethane, propane, n-butane, isobutane, pentanes, hexanes, heptanes, octanes etc., or their mixtures. Preferably, the diluent is a low-boiling hydrocarbon having from 1 to 4 carbon atoms or a mixture of such hydrocarbons.

[0095] The temperature in the prepolymerization step is typically from 0 to 90°C, preferably from 20 to 80°C and more preferably from 45 to 75°C.

[0096] The pressure is not critical and is typically from 1 to 150 bar, preferably from 40 to 80 bar.

[0097] The amount of monomer is typically such that from 0.1 to 1000 grams of monomer per one gram of solid catalyst component is polymerized in the prepolymerization step. As the person skilled in the art knows, the catalyst particles recovered from a continuous prepolymerization reactor do not all contain the same amount of prepolymer. Instead, each particle has its own characteristic amount, which depends on the residence time of that particle in the prepolymerization reactor. As some particles remain in the reactor for a relatively long time and some for a relatively short time, then also the amount of prepolymer on different particles is different and some individual particles may contain an amount of prepolymer which is outside the above limits. However, the average amount of prepolymer on the catalyst typically is within the limits specified above.

[0098] The molecular weight of the prepolymer may be controlled by hydrogen as it is known in the art. Further, antistatic additives may be used to prevent the particles from adhering to each other or the walls of the reactor, as disclosed in WO-A-96 / 19503 and WO-A-96 / 32420.

[0099] The catalyst components are preferably all introduced to the prepolymerization step when a prepolymerization step is present. However, where the solid catalyst component and the cocatalyst can be fed separately it is possible that only a part of the cocatalyst is introduced into the prepolymerization stage and the remaining part into subsequent polymerization stages. Also in such cases it is necessary to introduce so much cocatalyst into the prepolymerization stage that a sufficient polymerization reaction is obtained therein. It is understood within the scope of the invention, that the amount or polymer produced in the prepolymerization lies within 1.0 to 5.0 wt% in respect to the final metallocene catalysed mMDPE. This can counted as part of the first first polyethylene component (A).

[0100] Catalyst

[0101] The metallocene catalysed mMDPE of the invention is one made using a metallocene catalyst. A metallocene catalyst comprises a metallocene complex and a cocatalyst. The metallocene compound or complex is referred herein also as organometallic compound (C).

[0102] The organometallic compound (C) comprises a transition metal (M) of Group 3 to 10 of the Periodic Table (IIIPAC 2007) or of an actinide or lanthanide.

[0103] The term "an organometallic compound (C)" in accordance with the present invention includes any metallocene or non-metallocene compound of a transition metal, which bears at least one organic (coordination) ligand and exhibits the catalytic activity alone or together with a cocatalyst. The transition metal compounds are well known in the art and the present invention covers compounds of metals from Group 3 to 10, e.g. Group 3 to 7, or 3 to 6, such as Group 4 to 6 of the Periodic Table, (IIIPAC 2007), as well as lanthanides or actinides.

[0104] In an embodiment, the organometallic compound (C) has the following formula (I): wherein each X is independently a halogen atom, a Ci-6-alkyl group, Ci-6-alkoxy group, phenyl or benzyl group; each Het is independently a monocyclic heteroaromatic group containing at least one heteroatom selected from O or S;

[0105] L is -R'2Si-, wherein each R’ is independently Ci-20-hydrocarbyl or Ci- -alkyl substituted with alkoxy having 1 to 10 carbon atoms;

[0106] M is Ti, Zr or Hf; each R1is the same or different and is a Ci-6-al kyl group or Ci-6-alkoxy group; each n is 1 to 2; each R2is the same or different and is a Ci-6-al kyl group, Ci-6-alkoxy group or -Si(R)3 group; each R is Ci- -alkyl or phenyl group optionally substituted by 1 to 3 Ci-6-alkyl groups; and each p is 0 to 1.

[0107] Preferably, the compound of formula (I) has the structure wherein each X is independently a halogen atom, a Ci-6-alkyl group, Ci-6-alkoxy group, phenyl or benzyl group;

[0108] L is a Me2Si-; each R1is the same or different and is a Ci-6-al kyl group, e.g. methyl or t-Bu; each n is 1 to 2;

[0109] R2is a -Si(R)3 alkyl group; each p is 1 ; each R is Ci-6-alkyl or phenyl group.

[0110] Highly preferred complexes of formula (I) are

[0111] Most preferably the complex dimethylsilanediylbis[2-(5-trimethylsilylfuran-2-yl)-4,5- dimethylcyclopentadien-1-yl] zirconium dichloride is used.

[0112] More preferably the polyethylene components (A) and (B) of the metallocene catalysed mMDPE are produced using, i.e. in the presence of, the same metallocene catalyst.

[0113] To form a catalyst, a cocatalyst, also known as an activator, is used, as is well known in the art. Cocatalysts comprising Al or B are well known and can be used here. The use of aluminoxanes (e.g. MAO) or boron based cocatalysts (such as borates) is preferred.

[0114] Polyethylene copolymers made using single site catalysis, as opposed to Ziegler Natta catalysis, have characteristic features that allow them to be distinguished from Ziegler Natta materials. In particular, the comonomer distribution is more homogeneous. This can be shown using TREF or Crystaf techniques. Catalyst residues may also indicate the catalyst used. Ziegler Natta catalysts would not contain a Zr or Hf group (IV) metal for example.

[0115] Polyethylene composition

[0116] The polyethylene composition of the present disclosure has desirable resistance to slow crack growth resistance. The slow crack growth resistance of a pipe material is imparted to the material by the metallocene catalysed multimodal medium density polyethylene (mMDPE) present in the polyethylene composition of the present disclosure. In particular, by combining components (A) and (B) and carbon black to provide a polyethylene composition with the density of 945 to 960 kg / m3; and a MFRs (190°C, 5.0 kg, ISO 1133) in the range from 0.5 to 3.0 g / 10min, it is possible to provide a polyethylene composition with a desirably high resistance to slow crack growth. At the same time, the polyethylene composition exhibits desirable strength e.g., tensile strength characteristics, as evidence by desirable yield stresses. The slow crack resistance can, for example, be tested by the strain hardening behaviour of the polyethylene composition.

[0117] In a preferred embodiment, the polyethylene composition has a strain hardening modulus of 65 MPa or higher; 70 MPa or higher; 75 MPa or higher; 80 MPa or higher; 85 MPa or higher; 88 MPa or higher; 90 MPa or higher or 100 MPa or higher. The polyethylene composition usually has a strain hardening modulus of not more than 150 MPa, not more than 145 MPa, not more than 140 MPa, not more than 135 MPa; not more than 130 MPa; not more than 125 MPa; not more than 120 MPa; or not more than 115 MPa.

[0118] For example, the polyethylene composition may have a strain hardening modulus of 65 to 150 MPa; 70 to 145 MPa; 75 to 140 MPa; 80 to 135 MPa; 85 to 130 MPa; 88 to 125 MPa; 90 to 120 MPa; or 100 to 115 MPa.

[0119] The yield stress of the composition is the highest stress the composition will experience before plastic deformation occurs. One way to measure the yield strength is with a tensile test. The tensile test may be carried out in accordance with ISO 527-1. The test may be carried out using a 5A (ISO 527-2) test specimen. The test specimen may be extended along its major longitudinal axis at constant speed. During this procedure, the load sustained by the specimen and the elongation, using the crosshead and following the ISO 527-1 (method A), are measured.

[0120] The polyethylene composition may have a yield stress of 4.0 MPa or higher; 4.5 MPa or higher; 5.0 MPa or higher; 5.5 MPa or higher, or 6.0 MPa or higher. The yield stress may be at most 20 MPa, at most 18 MPa, at most 15 MPa, at most 12 MPa or at most 10 MPa. For example, the yield stress may be 4.0 to 20 MPa, 4.5 to 18 MPa, 5.0 to 15 MPa, 5.5 to 12 MPa and 6.0 to 10 MPa.

[0121] To fulfil the requirements for a PE100 pipe material, the density of the multimodal medium density polyethylene (mMDPE) is preferably at least 945 kg / m3, more preferably of from 945 to 951 kg / m3.

[0122] The polyethylene composition of the present disclosure comprises carbon black. Carbon black imparts black colour to the polyethylene composition and, at the same time, protects the composition from UV radiation. Still further, the addition of carbon black increases the density of the polyethylene composition compared to that of the multimodal medium density polyethylene. Preferably, carbon black is present in the polyethylene composition in an amount of 1 to 4 wt.%, more preferably of 2.0 to 2.5 wt. % of the total composition.

[0123] Usually, the amount of carbon black added is selected so that the density of the carbon black-containing polyethylene composition is from 8 to 15 kg / m3, more preferably from 9 to 14 kg / m3higher than that of the multimodal medium density polyethylene.

[0124] Carbon black is usually added in the form of a master batch, i.e. as a mixture of e.g. an HDPE and carbon black, in which carbon black is present in an amount of e.g. 30 to 50 wt.%.

[0125] In addition to the multimodal medium density polyethylene and carbon black, usual additives for utilization with polyolefins, such as pigments, stabilizers (antioxidant agents), antiacids, antistatic agents and utilization agents (such as processing aid agents) may be present in the polyethylene composition. Preferably, the amount of these additives is 10 wt.% or below, further preferred 8 wt.% or below, still more preferred 5 wt.% or below, and still more preferred 4 wt.% or below of the total composition. The further additives more preferably are present in amount of 3 wt.% or less, more preferred of 2.5 wt.% or less, and most preferred of 2 wt.% or less.

[0126] Preferably, the multimodal medium density polyethylene makes up at least 92 wt.%, more preferably at least 93, and still more preferably at least 94 wt.% of the polyethylene composition.

[0127] The composition of the invention preferably is produced in a process comprising a compounding step, wherein the multimodal medium density polyethylene produced from the reactor is extruded in an extruder and then pelletised to polymer pellets in a manner known in the art. The extruder may be e.g. any conventionally used extruder. As an example of an extruder for the present compounding step may be those supplied by Japan Steel works, Kobe Steel, Coperion or Farrel-Pomini.

[0128] The resulting polyethylene composition may be extruded to form a pipe using known methods.

[0129] An exemplary embodiment of the present disclosure relates to a polyethylene composition comprising a metallocene catalysed multimodal medium density polyethylene (mMDPE) and 0.5 to 5 weight % carbon black; wherein the metallocene catalysed multimodal medium density polyethylene (mMDPE) comprises:

[0130] (i) 48 to 55 wt%, based on the total weight of the mMDPE, of a first polyethylene component (A), and

[0131] (ii) 45 to 52 wt%, based on the total weight of the mMDPE, of a second polyethylene component (B), wherein the first polyethylene component (A) has a density (ISO 1183) in the range of from 950 to 980 kg / m3, and a MFR2 (190°C, 2.16 kg, ISO 1133) in the range of from 20 to 500 g / 10 min; the second polyethylene component (B) has a density (ISO 1183) in the range of from 900 to 925 kg / m3, and a MFR2(190°C, 2.16 kg, ISO 1133) in the range of from 0.0001 to 1.0 g / 10 min. and wherein the polyethylene composition has a density of 945 to 960 kg / m3; a MFR5 (190°C, 5.0 kg, ISO 1133) in the range from 0.5 to 3.0 g / 10min; and a MFR21 (190°C, 21.6 kg, ISO 1133) in the range from 15 to 40 g / 10min.

[0132] In this embodiment, the metallocene catalysed multimodal medium density polyethylene (mMDPE) has a density (ISO1183) of 932 to 955 kg / m3; and / or a MFR5(190°C, 5.0 kg, ISO 1133) in the range from 0.5 to 3.0 g / 10min; and / or a MFR21 (190°C, 21.6 kg, ISO 1133) in the range from 10 to 45 g / 10min. Preferably, the metallocene catalysed multimodal mMDPE may have a density (ISO 1183) in the range preferably 935 to 950 kg / m3and more preferably 938 to 945 kg / m3.

[0133] As discussed above, the component (A) may comprise or consist of a blend of at least two components, for example, a blend of a component (A-1) and component (A-2). Components (A-1) and (A-2) may be homopolymers, while component (B) may be an ethylene copolymer.

[0134] As mentioned above, the ethylene polymer fraction (A-1) preferably has a MFR2(190°C, 2.16 kg, ISO 1133) in the range of 5.0 to 100.0 g / 10 min, preferably of 10.0 to 80.0 g / 10 min, more preferably of 15.0 to 70.0 g / 10 min and even more preferably of 20.0 to 60.0 g / 10 min, like 22.0 to 50.0 g / 10 min. The ethylene polymer fraction (A-2) preferably has a MFR2 (190°C, 2.16 kg, ISO 1133) in the range of 100.0 to 1500.0 g / 10 min, preferably of 200.0 to 1200.0 g / 10 min, more preferably of 250.0 to 1000.0 g / 10 min and most preferably of 280.0 to 800.0 g / 10 min.

[0135] Preferably, the MFR2 of fraction (A-2) may be higher than the MFR2 of fraction (A-1).

[0136] The density of the ethylene polymer fraction (A-1) preferably is in the range of 935 to 970 kg / m3, more preferably 940 to 965 kg / m3and even more preferably 945 to 962 kg / m3.

[0137] The ethylene polymer fraction (A-2) preferably has a density in the range of 960 to 982 kg / m3, more preferably 965 to 980 kg / m3.

[0138] Preferably, the density of fraction (A-2) is higher than the density of fraction (A-1).

[0139] The polyethylene composition preferably has a ratio of Mz / Mw in the range of 2.0 to 4.0, preferably 2.2 to 3.5, and more preferably 2.3 to 3.2.

[0140] The polyethylene composition preferably has a molecular weight distribution (MWD), Mw / Mn, in the range of 6.0 to 14.0, preferably 6.5 to 13.5, and more preferably 7.0 to 13.0

[0141] These exemplary embodiments may have strain hardening modulii of 65 MPa or higher; 70 MPa or higher; 75 MPa or higher; 80 MPa or higher, 85 MPa or higher, 90 MPa or higher; 95 MPa or higher, or 100 MPa or higher, and / or yield stresses of 4.0 MPa or higher; 4.5 MPa or higher; 5.0 MPa or higher; 5.5 MPa or higher, or 6.0 MPa or higher.

[0142] As discussed herein, strain hardening may be measured according to ISO 18488.

[0143] Yield stress may be measured using a tensile test according to ISO 527-1.

[0144] Unless explicitly described otherwise, the description of the present invention is to be understood so that one or more of any of the above described preferred embodiments of the invention can be combined with the invention described in its most general features.

[0145] Examples

[0146] Determination methods

[0147] Unless otherwise stated in the description or in the experimental part, the following methods were used for the property determinations of the polymers (including its fractions and components) and / or any sample preparations thereof as specified in the text or experimental part.

[0148] Melt Flow Rate

[0149] The melt flow rate (MFR) was determined according to ISO 1133 and is indicated in g / 10 min. The MFR is an indication of the flowability, and hence the processability, of the polymer. The MFR is determined at 190 °C for polyethylene. MFR may be determined at different loadings such as 2.16 kg (MFR2), 5 kg (MFR5) or 21.6 kg (MFR21).

[0150] Calculation of MFR2 of Component B and of Fraction (A-2) logA = ■ logB + { 1 — 1 1 ■ . •ogC

[0151] 1 logA—x.logB 1 C = 1061 J--.Y!

[0152] For Component B:

[0153] B = MFR2of Component (A)

[0154] C = MFR2of Component (B)

[0155] A = final MFR2(mixture) of multimodal mMDPE

[0156] X = weight fraction of Component (A).

[0157] For Fraction (A-2):

[0158] B = MFR2of 1stfraction (A-1)

[0159] C = MFR2of 2ndfraction (A-2)

[0160] A = final MFR2(mixture) of loop polymer (= Component (A))

[0161] X = weight fraction of the 1stfraction (A-1).

[0162] The quantity FRR (flow rate ratio) is an indication of molecular weight distribution and denotes the ratio of flow rate at different loadings. The FRR denotes the value of MFR2I / MFRS.

[0163] Density

[0164] Density of the polymer was measured according to ISO 1183-1- (method A) on compression moulded specimens prepared according to EN IS01872-2 and is given in kg / m3.

[0165] Strain Hardening Modulus

[0166] Strain hardening modulus of the compounds was obtained from a tensile stress- strain curve above the natural draw ratio and represents the slope of the increase in the stress-strain trend at very high strains (the strain hardening regime). It was measured at 80°C and 20 mm / min on preconditioned (120°C / 1h) 0.3mm thick specimens according to ISO 18488.

[0167] Yield Stress

[0168] Tensile test was conducted according to ISO 527-1 / -2 by using a 5A (ISO 527-2) test specimen. The test specimen is extended along its major longitudinal axis at constant speed. During this procedure, the load sustained by the specimen and the elongation, using the crosshead and following the ISO 527-1 (method A), are measured.

[0169] Standard conditions:

[0170] • Conditioning time: > 96 h at 23 ±2 °C ±10 %rh

[0171] • Test temperature: 80 °C

[0172] • Gripping distance: 50 mm

[0173] • Testspeed: 0,3mm / min

[0174] • Test results: o Stress at yield (MPa)

[0175] Short term pressure resistance

[0176] The pressure test on un-notched 32 mm SDR 11 pipes having a length of 450 mm is carried out in water-inside and water-outside environment according to ISO 1167-1 :2006. End caps type A were used. The time to failure is determined in hours. A hoop stress of 5.4 and a temperature of 80 °C were applied.

[0177] GPC

[0178] Molecular weight averages (Mz, Mw and Mn), Molecular weight distribution (MWD) and its broadness, described by polydispersity index, PDI= Mw / Mn (wherein Mn is the number average molecular weight and Mw is the weight average molecular weight) were determined by Gel Permeation Chromatography (GPC) according to ASTM D 6474-99 using the following formulas:

[0179] For a constant elution volume interval AVj, where Aj, and Mj are the chromatographic peak slice area and polyolefin molecular weight (MW), respectively associated with the elution volume, Vj, where N is equal to the number of data points obtained from the chromatogram between the integration limits. high temperature GPC instrument, equipped with either infrared (IR) detector (IR4 or IR5 from PolymerChar (Valencia, Spain), equipped with 3 x Agilent-PLgel Olexis and 1x Agilent- PLgel Olexis Guard columns was used. As the solvent and mobile phase 1 ,2,4- trichlorobenzene (TCB) stabilized with 250 mg / L 2,6-Di tert butyl-4-methyl-phenol) was used. The chromatographic system was operated at 160 °C and at a constant flow rate of 1 mL / min. 200 pL of sample solution was injected per analysis. Data collection was performed using either Agilent Cirrus software version 3.3 or PolymerChar GPC-IR control software.

[0180] The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with 19 narrow MWD polystyrene (PS) standards in the range of 0,5 kg / mol to 11 500 kg / mol. The PS standards were dissolved at room temperature over several hours. The conversion of the polystyrene peak molecular weight to polyolefin molecular weights is accomplished by using the Mark Houwink equation and the following Mark Houwink constants:

[0181] KPS = 19 x 10'3mL / g, OPS = 0.655

[0182] KPE= 39 x 10-3mL / g, aPE= 0.725

[0183] KPP = 19 x 10'3mL / g, OPP = 0.725

[0184] A third order polynomial fit was used to fit the calibration data.

[0185] All samples were prepared in the concentration range of 0,5 -1 mg / ml and dissolved at 160 °C for 2.5 hours for PP or 3 hours for PE under continuous gentle shaking.

[0186] To exclude the influence of additives such as antioxidants or some other low molecular weight oligomers, the low molecular weight integration limit was set in the valley between the antioxidant peak and the polymer peak (~ logM of 2.8 (PE equivalent) for the analysed samples).

[0187] Comonomer contents:

[0188] Quantification of microstructure by NMR spectroscopy

[0189] Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used to quantify the comonomer content of the polymers. Quantitative13C{1H} NMR spectra recorded in the molten-state using a Bruker Avance III 500 NMR spectrometer operating at 500.13 and 125.76 MHz for1H and13C respectively. All spectra were recorded using a13C optimized 7 mm magic-angle spinning (MAS) probe head at 150°C using nitrogen gas for all pneumatics. Approximately 200 mg of material was packed into a 7 mm outer diameter zirconia MAS rotor and spun at 4 kHz. This setup was chosen primarily for the high sensitivity needed for rapid identification and accurate quantification {klimke06, parkinson07, castignolles09}. Standard single-pulse excitation was employed utilizing the NOE at short recycle delays of 3 s {pollard04, klimke06} and the RS-HEPT decoupling scheme{fillip05,griffin07}. A total of 1024 (1k) transients were acquired per spectra.

[0190] Quantitative13C{1H} NMR spectra were processed, integrated and relevant quantitative properties determined from the integrals. All chemical shifts are internally referenced to the bulk methylene signal (8+) at 30.00 ppm.

[0191] The amount of ethylene was quantified using the integral of the methylene (8+) sites at 30.00 ppm accounting for the number of reporting sites per monomer:

[0192] E = l8+ / 2 the presence of isolated comonomer units is corrected for based on the number of isolated comonomer units present:

[0193] Etotal = E + (3*B + 2*H) 12 where B and H are defined for their respective comonomers. Correction for consecutive and non-consecutive commoner incorporation, when present, is undertaken in a similar way.

[0194] Characteristic signals corresponding to the incorporation of 1-butene were observed and the comonomer fraction calculated as the fraction of 1-butene in the polymer with respect to all monomer in the polymer: fBtotal = Btotal I (Etotal + Btotal + Htotal)

[0195] The amount isolated 1-butene incorporated in EEBEE sequences was quantified using the integral of the *B2 sites at 39.8 ppm accounting for the number of reporting sites per comonomer:

[0196] B = I.B2

[0197] If present the amount consecutively incorporated 1-butene in EEBBEE sequences was quantified using the integral of the aaB2B2 site at 39.4 ppm accounting for the number of reporting sites per comonomer: BB = 2 * laaB2B2

[0198] If present the amount non consecutively incorporated 1-butene in EEBEBEE sequences was quantified using the integral of the ppB2B2 site at 24.6 ppm accounting for the number of reporting sites per comonomer:

[0199] BEB = 2 * IPPB2B2

[0200] Due to the overlap of the *B2 and *pB2B2 sites of isolated (EEBEE) and non-consecutively incorporated (EEBEBEE) 1-butene respectively the total amount of isolated 1-butene incorporation is corrected based on the amount of non-consecutive 1-butene present:

[0201] B = I.B2 - 2 * I PPB2B2

[0202] Sequences of BBB were not observed. The total 1-butene content was calculated based on the sum of isolated, consecutive and non consecutively incorporated 1-butene:

[0203] Btotal = B + BB + BEB

[0204] The total mole fraction of 1-butene in the polymer was then calculated as: fB = Btotal I ( Etotal + Btotal + Htotal)

[0205] Characteristic signals corresponding to the incorporation of 1-hexene were observed and the comonomer fraction calculated as the fraction of 1-hexene in the polymer with respect to all monomer in the polymer: fHtotal = Htotal I (Etotal + Btotal + Htotal)

[0206] The amount isolated 1-hexene incorporated in EEHEE sequences was quantified using the integral of the *B4 sites at 38.3 ppm accounting for the number of reporting sites per comonomer:

[0207] H = I.B4

[0208] If present the amount consecutively incorporated 1-hexene in EEHHEE sequences was quantified using the integral of the aaB4B4 site at 40.5 ppm accounting for the number of reporting sites per comonomer:

[0209] HH = 2 * laaB4B4

[0210] If present the amount non consecutively incorporated 1-hexene in EEHEHEE sequences was quantified using the integral of the ppB4B4 site at 24.7 ppm accounting for the number of reporting sites per comonomer: 1

[0211] HEH = 2 * IPPB4B4

[0212] Sequences of HHH were not observed. The total 1-hexene content was calculated based on the sum of isolated, consecutive and non consecutively incorporated 1-hexene:

[0213] Htotal = H + HH + HEH

[0214] The total mole fraction of 1-hexene in the polymer was then calculated as: fH = Htotal I ( Etotal + Btotal + Htotal)

[0215] The mole percent comonomer incorporation is calculated from the mole fraction:

[0216] B [mol%] = 100 * fB

[0217] H [mol%] = 100 * fH

[0218] The weight percent comonomer incorporation is calculated from the mole fraction:

[0219] B [wt%] = 100 * ( fB * 56.11) / ( (fB * 56.11) + (fH * 84.16) + ((1-(fB + fH)) * 28.05) )

[0220] H [wt%] = 100 * ( fH * 84.16 ) / ( (fB * 56.11) + (fH * 84.16) + ((1-(fB + fH)) * 28.05) )

[0221] References:

[0222] Klimke, K., Parkinson, M., Piel, C., Kaminsky, W., Spiess, H.W., Wilhelm, M., Macromol. Chem. Phys. 2006;207:382.

[0223] Parkinson, M., Klimke, K., Spiess, H.W., Wilhelm, M., Macromol. Chem. Phys. 2007;208:2128.

[0224] Pollard, M., Klimke, K., Graf, R., Spiess, H.W., Wilhelm, M., Sperber, O., Piel, C., Kaminsky, W., Macromolecules 2004;37:813.

[0225] Filip, X., Tripon, C., Filip, C., J. Mag. Resn. 2005, 176, 239.

[0226] Griffin, J.M., Tripon, C., Samoson, A., Filip, C., and Brown, S.P., Mag. Res. in Chem. 200745, S1 , S198.

[0227] Castignolles, P., Graf, R., Parkinson, M., Wilhelm, M., Gaborieau, M., Polymer 50 (2009) 2373

[0228] Busico, V., Cipullo, R., Prog. Polym. Sci. 26 (2001) 443.

[0229] Busico, V., Cipullo, R., Monaco, G., Vacatello, M., Segre, A.L., Macromoleucles 30 (1997) 6251.

[0230] Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha, A., Baugh, D. Winniford, B., J. Mag. Reson. 187 (2007) 225. Busico, V., Carbonniere, P., Cipullo, R., Pellecchia, R., Severn, J., Talarico, G., Macromol.

[0231] Rapid Commun. 2007, 28, 1128.

[0232] Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev. 2000, 100, 1253.

[0233] Examples:

[0234] Cat.Example: Catalyst preparation for CAT1 for Inventive Examples IE1

[0235] Loading of SiO2:

[0236] 10 kg of silica (PQ Corporation ES757, calcined 600°C) was added from a feeding drum and inertized in the reactor until O2 level below 2 ppm was reached.

[0237] Preparation of MAO / tol / MC:

[0238] 30 wt% MAO in toluene (14.1 kg) was added into another reactor from a balance followed by toluene (4.0 kg) at 25°C (oil circulation temp) and stirring 95 rpm. Stirring speed was increased 95 rpm -> 200 rpm after toluene addition, stirring time 30 min. Metallocene Rac- dimethylsilanediylbis{2-(5-(trimethylsilyl)furan-2-yl)-4,5-dimethylcyclopentadien-1- yljzirconium dichloride 477 g was added from a metal cylinder followed by flushing with 4 kg toluene (total toluene amount 8.0 kg). Reactor stirring speed was changed to 95 rpm for MC feeding and returned back to 200 rpm for 3 h reaction time. After reaction time MAO / tol / MC solution was transferred into a feeding vessel.

[0239] Preparation of catalyst:

[0240] Reactor temperature was set to 10°C (oil circulation temp) and stirring was turned to 40 rpm during MAO / tol / MC addition. MAO / tol / MC solution (22.2 kg) was added within 205 min followed by 60 min stirring time (oil circulation temp was set to 25°C). After stirring “dry mixture” was stabilised for 12 h at 25°C (oil circulation temp), stirring 0 rpm. Reactor was turned 20° (back and forth) and stirring was turned on 5 rpm for few rounds once an hour.

[0241] After stabilisation the catalyst was dried at 60°C (oil circulation temp) for 2 h under nitrogen flow 2 kg / h, followed by 13 h under vacuum (same nitrogen flow with stirring 5 rpm). Dried catalyst was sampled and HC content was measured in the glove box with Sartorius Moisture Analyser, (Model MA45) using thermogravimetric method. Target HC level was < 2% (actual 1.3 %).

[0242] Cat.Example: Comparative Examples CE1 and CE2

[0243] In these comparative examples, a Lynx200, Ziegler-Natta catalyst supplied by Grace was used (CAT 2). Polymerization:

[0244] Borstar pilot plant with a 3-reactor set-up (loopl - Ioop2 - GPR 1) and a prepolymerization loop reactor.

[0245] Table 1 : Polymerization conditions for mMDPE

[0246] The polymer of IE1 was mixed with 0.027 wt% FX 5922 (3M Dynamar Polymer Processing Additive) and 0.24 wt% Irganox B 561 (BASF), where wt% are relative to total weight of composition (the sum of mMDPE powder + additive = 100%) compounded and extruded on a ZSK 57 twin screw extruder. The melt temperature was 224°C, production rate was 221 kg / h.

[0247] The resulting pellets were then re-pelletized with 5.5 weight % of a carbon black masterbatch, 0.26 weight % antioxidants and 0.04 weight % calcium stearate. The resulting carbon black content of the polyethylene composition was 2.2 weight %. Compounding and extrusion was done on a co-rotating twin screw extruder ZSK 18. The temperature in the beginning of the melting zone of the extruder was set to 150 °C and in all the remaining zones to 230 °C. The throughput was 1 kg / h and the screw speed was 120 RPM.

[0248] The polymers of CE1 and CE2 were pelletized with 5.5 weight % of a carbon black masterbatch and the same additive package used in IE1. The resulting carbon black content of the polyethylene composition was 2.2 weight %. Compounding and extrusion was done on a JSW CIMP90 twin screw extruder with the melt temperature of about 280 °C and SEI between 170-250 kwh / t.

[0249] Pipe extrusion

[0250] Pipe extrusion of 32x3 mm (outer diameter x wall thickness) pipes was performed on a Krauss-Maffei 45-36D (L / D) single screw extruder. The extruder has a modified PP-barrier screw installed with four heated cylinder zones and five tool zones. The downstream equipment is a 9m spray- cooling vacuum tank with two chambers and a defined water temperature of 20 °C.

[0251] The samples have been processed at an output rate of 50 kg / h and a screw speed of the extruder of ~ 57 rpm. The achieved melt temperature was 220-221 °C at a melt pressure of 213-216 bars. All samples have been produced at a constant meter weight of 280 g / m and a line speed of 2,97 m / minute.

[0252] Temperature profile for pipe extrusion:

[0253] Table 2: Material properties of polyethylene compositions of CE1 , CE2 and IE1

[0254] *below detection limit

[0255] **measurement on pipe

[0256] The polyethylene composition of IE1 has a lower density than the polyethylene compositions of CE1 and CE2. However, the yield stress of IE1 is comparable to that of CE1 and CE2, and the strain hardening modulus is significantly higher. Higher yield stress of compositions of the invention are indicative of a longer life time of a pipe made from the compositions. The lower tensile modulus of IE1 indicates improved flexibility of the pipe. It is advantageous that compositions of the invention have improved flexibility as this makes winding of the pipe easier (e.g. winding for storage and transport prior to installation).

[0257] Short term pressure resistance of pipe prepared from the polyethylene composition of IE1 is comparable to CE2, indicating the suitability of the composition of IE1 for the manufacture of pipes with desirable pressure resistance.

Claims

Claims1. A polyethylene composition comprising a metallocene catalysed multimodal medium density polyethylene (mMDPE) and 0.5 to 5 weight % carbon black; wherein the metallocene catalysed multimodal medium density polyethylene (mMDPE) comprises:(i) 48 to 55 wt%, based on the total weight of the mMDPE, of a first polyethylene component (A), and(ii) 45 to 52 wt%, based on the total weight of the mMDPE, of a second polyethylene component (B), wherein the first polyethylene component (A) has a density (ISO 1183) in the range of from 950 to 980 kg / m3, and a MFR2 (190°C, 2.16 kg, ISO 1133) in the range of from 20 to 500 g / 10 min; the second polyethylene component (B) has a density (ISO 1183) in the range of from 900 to 925 kg / m3, and wherein the polyethylene composition has a density of 945 to 960 kg / m3; and a MFR5 (190°C, 5.0 kg, ISO 1133) in the range from 0.5 to 3.0 g / 10min.

2. The polyethylene composition according to claim 1 , which has a MFR21 (190°C, 21.6 kg, ISO 1133) in the range from 10 to 45 g / 10min.

3. The polyethylene composition according to any one of the preceding claims, wherein the second polyethylene component (B) has a MFR2 (190°C, 2.16 kg, ISO 1133) in the range of from 0.0001 to 1.0 g / 10 min.

4. The polyethylene composition according to any one of the preceding claims, wherein the metallocene catalysed multimodal medium density polyethylene (mMDPE) has a density (ISO1183) of 932 to 955 kg / m3; and / or a MFR5 (190°C, 5.0 kg, ISO 1133) in the range from 0.5 to 3.0 g / 10min; and / or a MFR21 (190°C, 21.6 kg, ISO 1133) in the range from 10 to 45 g / 10min.

5. The polyethylene composition according to any one of the preceding claims, wherein the metallocene catalysed multimodal medium density polyethylene (mMDPE) has a molecularweight distribution (MWD), Mw / Mn, in the range of 6.0 to 14.0, preferably 6.5 to 13.5, and more preferably 7.0 to 13.0.

6. The polyethylene composition according to any one of the preceding claims, wherein the metallocene catalysed multimodal medium density polyethylene (mMDPE) has one or more or preferably all of the following properties: a weight average molecular weight, Mw, determined by Gel Permeation Chromatography (GPC) according to ASTM D 6474-99, of at least 80000 g / mol, preferably in the range of 90000 to 150000 g / mol, more preferably 100000 to 140000 g / mol, and / or a z average molecular weight, Mz, determined by Gel Permeation Chromatography (GPC) according to ASTM D 6474-99, in the range of 200000 to 420000 g / mol, preferably 250000 to 400000 g / mol and more preferably from 280000 to 380000 g / mol, and / or a ratio of Mz / Mw in the range of 2.5 to 4.0, preferably 2.7 to 3.5 and more preferably 2.8 to 3.2.

7. The polyethylene composition according to any one of the preceding claims, wherein the first polyethylene component (A) has a density in the range of 955 to 975 kg / m3, preferably 960 to 972 kg / m3and more preferably 962 to 970 kg / m3; and a MFR2 (190°C, 2.16 kg, ISO 1133) in the range of 30 to 400 g / 10min, preferably 50 to 300 g / 10min and more preferably 60 to 200 g / 10min, and the second polyethylene component (B) has a density in the range of 905 to 920 kg / m3, more preferably 908 to 918 kg / m3and even more preferably 910 to 916 kg / m3and an MFR2 (190°C, 2.16 kg, ISO 1133) in the range of 0.0003 to 0.5 g / 10min, more preferably 0.0005 to 0.1 g / 10min and even more preferably 0.0008 to 0.01 g / 10min.

8. The polyethylene composition according to any one of the preceding claims, which has a strain hardening modulus determined according to ISO 18488 of 65 MPa or higher; 70 MPa or higher; 75 MPa or higher; 80 MPa or higher, 85 MPa or higher, 90 MPa or higher; 95 MPa or higher, or 100 MPa or higher.

9. The polyethylene composition according to any one of the preceding claims, which has a yield stress of 4.0 MPa or higher; 4.5 MPa or higher; 5.0 MPa or higher; 5.5 MPa or higher, or 6.0 MPa or higher.

10. A process for producing a polyethylene composition according to any one of the preceding claims, wherein the metallocene catalysed multimodal medium density polyethylene (mMDPE) is produced in the presence of metallocene complex of formula (I):wherein each X is independently a halogen atom, a Ci-6-alkyl group, Ci-6-alkoxy group, phenyl or benzyl group; each Het is independently a monocyclic heteroaromatic group containing at least one heteroatom selected from O or S;L is -R'2Si-, wherein each R’ is independently Ci-20-hydrocarbyl or Ci- -alkyl substituted with alkoxy having 1 to 10 carbon atoms;M is Ti, Zr or Hf; each Ri is independently selected from a Ci-6-al kyl group and Ci-6-alkoxy group; each n is 1 to 2; each R2 is independently selected from a Ci-6-al kyl group, Ci-6-alkoxy group or -Si(R)3 group; each R is independently selected from a Ci-w-alkyl or phenyl group optionally substituted by 1 to 3 Ci-6-alkyl groups; and each p is 0 to 1.

11. A pipe or pipe fitting comprising the polyethylene composition according to any one of claims 1 to 9.

12. Use of a polyethylene composition according to any one of claims 1 to 9 for producing a Pipe.

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