Polyethylene compositions having ethylene copolymer prepolymer fractions

Abstract

A polyethylene composition (PE) comprising: a) from 0.1 to 5.0 wt.-% of a first polyethylene fraction (PE1) being a copolymer of ethylene and one or more alpha olefins selected from C3-8 alpha olefins; b) from 30.0 to 69.0 wt.-% of a second polyethylene fraction (PE2) being an ethylene polymer consisting of ethylene monomer units and optionally the same comonomer(s) as the first polyethylene fraction (PE1) wherein the content of said optional comonomer(s) is not more than 0.20 wt.-%; and c) from 30.0 to 69.0 wt.-% of a third polyethylene fraction (PE3) being a copolymer of ethylene and 1-hexene; wherein the combined weight of the first polyethylene fraction (PE1), the second polyethylene fraction (PE2), and the third polyethylene fraction (PE3) is at least 90 wt.-%, relative to the total weight of the polyethylene composition (PE).

Description

[0001] Polyethylene compositions having ethylene copolymer prepolymer fractions

[0002] Field of the Invention

[0003] The present invention relates to a polyethylene composition comprising a prepolymer being a copolymer, an ethylene copolymer fraction having negligible comonomer content, and an ethylene-hexene copolymer fraction, to a method for preparing said polyethylene composition, and to pipes comprising said polyethylene composition.

[0004] Background to the Invention

[0005] Polyethylene pipes are widely used for the transportation of water and gas due to various advantages compared to other materials like iron, concrete or fiber reinforced plastic. The main advantages of plastic pipes are that they are corrosion free and flexible, along with good mechanical strength. Polyethylene pipes are normally used in the temperature range of 0 °C to 50 °C. The use of these plastic pipes in hot climatic conditions above 20 °C requires derating of pipes in accordance with ISO 13761. Derating also leads to thicker pipes in order to withstand said operating conditions. Therefore, in order to reduce the thickness and the material consumption, polyethylene with higher Minimum Required Strength (MRS) at 20 °C is desired.

[0006] The properties of polyethylene resins, namely mechanical strength and good long term strength (stress crack resistance), are opposing properties and therefore the preparation of products having high MRS and good mechanical strength poses a challenge.

[0007] For many years, such pipes have had to fulfil the requirements of the PE 100 classification, wherein a balance of numerous pipe properties must be obtained.

[0008] In recent years, the new PE 100 RC pipe standard has been developed, which has yet higher requirements in terms of long term stability and crack resistance. In order to fulfil the PE100RC classification according to DIN EN 1555, the polyethylene must have, inter alia, a strain hardening modulus of greater than 53 MPa, aFNCT of greater than 550 hours, and aNPT of greater than 300 hours. The aFNCT and aNPT test are newer methods used in the DIN EN 1555 standard to assess the performance of polymeric pipe materials without waiting for more than a year, as would often be required for the more traditional HPT and NPT tests. Given the continuing drive for ever stricter classifications in polyethylene pipes, especially in view of long-term crack resistance, there is continuing need to develop new polyethylene grades capable of fulfilling said classifications.

[0009] In conventional polymerization processes producing polyethylene meeting the PE 100 standards, a two-stage polymerization process is typically employed, with the first reactor producing an ethylene homopolymer and the second an ethylene / alpha olefin copolymer. Any prepolymerization step will typically use similar conditions to the first polymerization reactor, i.e. only using ethylene monomer for such pipe grades.

[0010] Summary of the Invention

[0011] The finding of the present invention is based upon the finding that adding alpha olefin comonomers to the prepolymerization reactor in a typical bimodal C2 / C2C6 polymerization process results in polyethylene compositions having considerably improved pipe properties.

[0012] As such, in a first aspect, the present invention is directed to a polyethylene composition (PE) comprising: a) from 0.1 to 5.0 wt.-%, relative to the total weight of the polyethylene composition (PE), of a first polyethylene fraction (PEI) being a copolymer of ethylene and one or more alpha olefins selected from C3-8 alpha olefins; b) from 30.0 to 69.0 wt.-%, relative to the total weight of the polyethylene composition (PE), of a second polyethylene fraction (PE2) being an ethylene polymer consisting of ethylene monomer units and optionally the same comonomer(s) as the first polyethylene fraction (PEI) wherein the content of said optional comonomer(s) is not more than 0.20 wt.-%; and c) from 30.0 to 69.0 wt.-%, relative to the total weight of the polyethylene composition (PE), of a third polyethylene fraction (PE3) being a copolymer of ethylene and 1 -hexene; wherein the combined weight of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2), and the third polyethylene fraction (PE3) is at least 90 wt.-%, relative to the total weight of the polyethylene composition (PE). In a second aspect, the present invention is directed to a method for producing the polyethylene composition (PE) according to the first aspect, comprising the following steps in the given order: a) providing a Ziegler-Natta catalyst composition (ZNC); b) in the presence of the Ziegler-Natta catalyst composition (ZNC), in a first polymerization reactor (Rl), polymerizing ethylene and one or more alpha olefins selected from C3-8 alpha olefins, thereby obtaining the first polyethylene fraction (PEI); c) transferring the mixture of the Ziegler-Natta catalyst composition (ZNC) and the first polyethylene fraction (PEI) into a second polymerization reactor (R2) and subsequently polymerizing ethylene, thereby obtaining the second polyethylene fraction (PE2); d) transferring the mixture of the Ziegler-Natta catalyst composition (ZNC), the first polyethylene fraction (PEI) and the second polyethylene fraction (PE2) into a third polymerization reactor (R3) and subsequently polymerizing ethylene and 1 -hexene, thereby obtaining the third polyethylene fraction (PE3); e) removing the mixture of the Ziegler-Natta catalyst composition (ZNC), the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) from the third polymerization reactor, removing unreacted monomers and comonomers; and f) melt extruding, more preferably pelletizing, the mixture obtained in step e), optionally with one or more further additives (A), thereby obtaining the polyethylene composition (PE).

[0013] In a final aspect, the present invention is directed to a pipe comprising at least 90 wt.-% of, more preferably at least 95 wt.-% of, most preferably consisting of the polyethylene composition (PE) of the first aspect.

[0014] Definitions

[0015] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although, any methods and materials similar or equivalent to those described herein can be used in practice fortesting of the present invention, the preferred materials and methods are described herein. In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below.

[0016] Unless clearly indicated otherwise, use of the terms “a,” “an,” and the like refers to one or more.

[0017] In the following, amounts are given in % by weight (wt.-%) unless it is stated otherwise.

[0018] An ethylene homopolymer is a polymer that essentially consists of ethylene monomer units. Due to impurities especially during commercial polymerization processes, an ethylene homopolymer can comprise up to 0.1 mol-% comonomer units, preferably up to 0.05 mol-% comonomer units and most preferably up to 0.01 mol-% comonomer units.

[0019] An ethylene copolymer is a copolymer of ethylene monomer units and comonomer units in an amount of at least 0.1 mol-%, preferably selected from Cs-Cs alpha-olefins. Ethylene copolymers can comprise comonomer units from one or more comonomers different in their amounts of carbon atoms.

[0020] In the context of the present invention, a “copolymer of ethylene and 1 -hexene” (for example) is a copolymer that consists essentially of only ethylene monomers and hexene comonomers.

[0021] The present invention will now be described in more detail.

[0022] Detailed Description

[0023] In a first aspect, the present invention is directed to a polyethylene composition (PE) comprising: a) from 0.1 to 5.0 wt.-%, relative to the total weight of the polyethylene composition (PE), of a first polyethylene fraction (PEI) being a copolymer of ethylene and one or more alpha olefins selected from C3-8 alpha olefins; b) from 30.0 to 69.0 wt.-%, relative to the total weight of the polyethylene composition (PE), of a second polyethylene fraction (PE2) being an ethylene polymer consisting of ethylene monomer units and optionally the same comonomer(s) as the first polyethylene fraction (PEI) wherein the content of said optional comonomer(s) is not more than 0.20 wt.-%; and c) from 30.0 to 69.0 wt.-%, relative to the total weight of the polyethylene composition (PE), of a third polyethylene fraction (PE3) being a copolymer of ethylene and 1 -hexene; wherein the combined weight of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2), and the third polyethylene fraction (PE3) is at least 90 wt.-%, relative to the total weight of the polyethylene composition (PE).

[0024] More preferably, the combined weight of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2), and the third polyethylene fraction (PE3) is at least 95 wt.-%, yet more preferably at least 98 wt.-%, relative to the total weight of the polyethylene composition (PE)

[0025] The polyethylene composition (PE) preferably comprises, more preferably consists of: a) from 0.1 to 5.0 wt.-%, more preferably in the range from 0.3 to 3.0 wt.-%, most preferably in the range from 0.5 to 2.0 wt.-% relative to the total weight of the polyethylene composition, of the first polyethylene fraction (PEI); b) from 30.0 to 69.0 wt.-%, more preferably in the range from 39.0 to 59.0 wt.-%, most preferably in the range from 43.0 to 53.0 wt.-%, relative to the total weight of the polyethylene composition, of the second polyethylene fraction (PE2); c) from 30.0 to 69.0 wt.-%, more preferably in the range from 40.0 to 60.0 wt.-%, most preferably in the range from 46.0 to 56.0 wt.-%, relative to the total weight of the polyethylene composition, of the third polyethylene fraction (PE3); d) optionally, from 0.0 to 10.0 wt.-% of one or more further additives (A).

[0026] In one embodiment, the polyethylene composition (PE) comprises, more preferably consists of: a) from 0.1 to 5.0 wt.-%, relative to the total weight of the polyethylene composition, of the first polyethylene fraction (PEI); b) from 30.0 to 69.0 wt.-%, relative to the total weight of the polyethylene composition, of the second polyethylene fraction (PE2); c) from 30.0 to 69.0 wt.-%, relative to the total weight of the polyethylene composition, of the third polyethylene fraction (PE3); d) optionally, from 0.0 to 10.0 wt.-% of one or more further additives (A).

[0027] In a further embodiment, the polyethylene composition (PE) comprises, more preferably consists of: a) from 0.3 to 3.0 wt.-%, relative to the total weight of the polyethylene composition, of the first polyethylene fraction (PEI); b) from 39.0 to 59.0 wt.-%, relative to the total weight of the polyethylene composition, of the second polyethylene fraction (PE2); c) from 40.0 to 60.0 wt.-%, relative to the total weight of the polyethylene composition, of the third polyethylene fraction (PE3); d) optionally, from 0.0 to 10.0 wt.-% of one or more further additives (A).

[0028] In yet a further embodiment, the polyethylene composition (PE) comprises, more preferably consists of: a) from 0.5 to 2.0 wt.-% relative to the total weight of the polyethylene composition, of the first polyethylene fraction (PEI); b) from 43.0 to 53.0 wt.-%, relative to the total weight of the polyethylene composition, of the second polyethylene fraction (PE2); c) from 46.0 to 56.0 wt.-%, relative to the total weight of the polyethylene composition, of the third polyethylene fraction (PE3); d) optionally, from 0.0 to 10.0 wt.-% of one or more further additives (A).

[0029] It is preferred that the polyethylene composition (PE) is a reactor blend of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2), and the third polyethylene fraction (PE3).

[0030] It is further preferred that each of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2), and the third polyethylene fraction (PE3) are polymerized in the presence of a Ziegler-Nata catalyst. It is also preferred that the polymeric part of the polyethylene composition (PE) is polymerized in the presence of a Ziegler Natta catalyst.

[0031] The individual components of the polyethylene composition (PE) will be described in more detail.

[0032] The first polyethylene fraction (PEI)

[0033] One essential component of the polyethylene composition (PE) is the first polyethylene fraction (PEI).

[0034] The first polyethylene fraction is a copolymer of ethylene and one or more alpha olefins selected from C3-8 alpha olefins.

[0035] It is preferred that the first polyethylene fraction (PEI) is a copolymer of ethylene and one alpha olefin selected from C3-8 alpha olefins.

[0036] More preferably the first polyethylene fraction (PEI) is a copolymer of ethylene and one alpha olefin selected from C4-6 alpha olefins

[0037] It is particularly preferred that the first polyethylene fraction (PEI) is free from 1 -hexene.

[0038] Most preferably the first polyethylene fraction (PEI) is a copolymer of ethylene and 1- butene.

[0039] When the polyethylene composition (PE) is produced in a multistage polymerization process, the first polyethylene fraction (PEI) corresponds to the prepolymer.

[0040] The present invention is based on the finding that, in contrast to the general understanding in the art that prepolymerization should be carried out using the same monomer / comonomer combination as the first polymerization reactor proper (i.e. only ethylene in the context of a C2 / C2C6 copolymer, or C2C4 in the case of a C2C4 / C2C6 polymer), that a comonomer that is not fed into the first polymerization reactor proper should be added to the prepolymerization reactor.

[0041] The second polyethylene fraction may contain trace amounts of this extra comonomer, since unreacted comonomer may be transferred into the first polymerization reactor proper from the prepolymerization reactor; however, given the reactor splits and the fact that no further comonomer is added to the first polymerization reactor proper, the amounts of the additional comonomer in the product of the first polymerization reactor (i.e. the second polyethylene fraction as defined above and below) will be very low, much lower than the comonomer content in the prepolymerization product. Thus, despite the definition of the second polyethylene fraction (PE2) allowing for the presence of trace amounts of the extra comonomer, this comonomer may not be added to the reactor in which the second polyethylene fraction is produced.

[0042] The first polyethylene fraction (PEI) is present in an amount in the range from 0.1 to 5.0 wt.- %, more preferably in the range from 0.3 to 3.0 wt.-%, most preferably in the range from 0.5 to 2.0 wt.-% relative to the total weight of the polyethylene composition (PE).

[0043] It is preferred that the first polyethylene fraction (PEI) has a density, determined according to ISO 1183, in the range from 960 to 968 kg / m3, more preferably in the range from 961 to 968 kg / m3, most preferably in the range from 962 to 967 kg / m3.

[0044] The second polyethylene fraction (PE2)

[0045] Another essential component of the polyethylene composition (PE) is the second polyethylene fraction (PE2).

[0046] The second polyethylene fraction (PE2) is an ethylene polymer consisting of ethylene monomer units and optionally the same comonomer(s) as the first polyethylene fraction (PEI) wherein the content of said optional comonomer(s) is not more than 0.20 wt.-%. As explained above, the second polyethylene fraction (PE2) is essentially an ethylene homopolymer, with the potential for small amounts of the comonomer(s) of the first polyethylene fraction (PEI) to be incorporated if this comonomer(s) is / are transferred with the first polyethylene fraction (PEI) from the prepolymerization reactor into the first polymerization reactor proper in a multistage polymerization process.

[0047] As any trace amounts of the optional comonomer(s) present in the second polyethylene fraction (PE2) must be the same comonomer(s) as present in the first polyethylene fraction (PEI), the fallback positions and preferred embodiments of the comonomer(s) of the first polyethylene fraction (PEI) apply mutatis mutandis to the optional comonomer(s) of the second polyethylene fraction (PE2). It is particularly preferred that this comonomer is not 1- hexene, most preferably it is 1 -butene.

[0048] If present, the total content of the optional comonomer(s) in the second polyethylene fraction (PE2) is not more than 0.20 wt.-%, more preferably not more than 0.15 wt.-%, most preferably not more than 0. 10 wt.-%.

[0049] It is preferred that the combination of the first polyethylene fraction (PEI) and the second polyethylene fraction (PE2) have a melt flow rate (MFR2), determined according to ISO 1133 at 190 °C and 2.16 kg, in the range from 100 to 800 g / 10 min, more preferably in the range from 150 to 600 g / 10 min, most preferably in the range from 150 to 500 g / 10 min.

[0050] It is preferred that the combination of the first polyethylene fraction (PEI) and the second polyethylene fraction (PE2) have a density, determined according to ISO 1183, in the range from 965 to 975 kg / m3, more preferably in the range from 968 to 975 kg / m3, most preferably in the range from 970 to 974 kg / m3.

[0051] The second polyethylene fraction (PE2) is present in an amount in the range from 30.0 to 69.0 wt.-%, more preferably in the range from 39.0 to 59.0 wt.-%, most preferably in the range from 43.0 to 53.0 wt.-%, relative to the total weight of the polyethylene composition (PE). The third polyethylene fraction (PE3)

[0052] The final essential component of the polyethylene composition (PE) is the third polyethylene fraction (PE3).

[0053] The third polyethylene fraction (PE3) is a copolymer of ethylene and 1 -hexene. In other words, the third polyethylene fraction (PE3) consists of ethylene monomer units and 1- hexene comonomer units.

[0054] It is particularly preferred that the third polyethylene fraction (PE3) has a density, determined according to ISO 1183, in the range from 915 to 935 kg / m3, more preferably in the range from 918 to 932 kg / m3, most preferably in the range from 920 to 930 kg / m3.

[0055] It is preferred that the third polyethylene fraction (PE3) has a 1 -hexene content, determined according to quantitative13C-NMR spectroscopic analysis, in the range from 2.4 to 4.4 wt.- %, more preferably in the range from 2.8 to 4.0 wt.-%, most preferably in the range from 2.8 to 3.8 wt.-%.

[0056] The third polyethylene fraction (PE3) is present in an amount in the range from 30.0 to 69.0 wt.-%, more preferably in the range from 39.0 to 59.0 wt.-%, most preferably in the range from 43.0 to 53.0 wt.-%, relative to the total weight of the polyethylene composition (PE).

[0057] One or more further additives (A)

[0058] An optional component of the polyethylene composition (PE) is one or more further additives.

[0059] The skilled practitioner would be able to select suitable additives that are well known in the art. The additives are preferably selected from pigments, nucleating agents, antioxidants, UV- stabilizers, anti-scratch agents, mold release agents, acid scavengers, lubricants, anti-static agents, and mixtures thereof.

[0060] It is understood that the content of additives includes any carrier polymers used to introduce the additives to the precursor polyethylene composition (PPC), i.e. masterbatch carrier polymers. An example of such a carrier polymer would be a high-density polyethylene in the form of powder or pellets.

[0061] If present, the one or more further additives (A) are present in an amount in the range from 0.0 to 10.0 wt.-%, relative to the total weight of the polyethylene composition (PE).

[0062] For embodiments wherein the combined weight of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2), and the third polyethylene fraction (PE3) is at least 95 wt.-%, relative to the total weight of the polyethylene composition (PE), the one or more further additives (A) are present in an amount in the range from 0.0 to 5.0 wt.-%, relative to the total weight of the polyethylene composition (PE).

[0063] For embodiments wherein the combined weight of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2), and the third polyethylene fraction (PE3) is at least 98 wt.-%, relative to the total weight of the polyethylene composition (PE), the one or more further additives (A) are present in an amount in the range from 0.0 to 2.0 wt.-%, relative to the total weight of the polyethylene composition (PE).

[0064] The polyethylene composition (PE)

[0065] The polyethylene composition (PE) preferably has a melt flow rate (MFR5), determined according to ISO 1133 at 190 °C and 5.0 kg, in the range from 0.05 to 1.0 g / 10 min, more preferably in the range from 0. 10 to 0.70 g / 10 min, most preferably in the range from 0.20 to 0.40 g / 10 min. The combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) preferably has a melt flow rate (MFR5), determined according to ISO 1133 at 190 °C and 5.0 kg, in the range from 0.05 to 1.0 g / 10 min, more preferably in the range from 0.10 to 0.70 g / 10 min, most preferably in the range from 0.20 to 0.40 g / 10 min.

[0066] The polyethylene composition (PE) preferably has a density, determined according to ISO 1183, in the range from 940 to 965 kg / m3, more preferably in the range from 943 to 963 kg / m3, most preferably in the range from 945 to 962 kg / m3.

[0067] The combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) preferably has a density, determined according to ISO 1183, in the range from 942 to 955 kg / m3, more preferably in the range from 944 to 953 kg / m3, most preferably in the range from 946 to 950 kg / m3.

[0068] The polyethylene composition (PE) preferably has a melting temperature (Tm), determined by differential scanning calorimetry (DSC) according to ISO 11357 / part 3 / method C2, in the range from 125.0 to 135.0 °C, more preferably in the range from 127.0 to 133.0 °C, most preferably in the range from 128.0 to 131.0 °C.

[0069] The combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) preferably has a melting temperature (Tm), determined by differential scanning calorimetry (DSC) according to ISO 11357 / part 3 / method C2, in the range from 125.0 to 135.0 °C, more preferably in the range from 127.0 to 133.0 °C, most preferably in the range from 128.0 to 131.0 °C.

[0070] The polyethylene composition (PE) preferably has a 1 -hexene content, determined according to quantitative13C-NMR spectroscopic analysis, in the range from 1.2 to 2.2 wt.-%, more preferably in the range from 1.4 to 2.0 wt.-%, most preferably in the range from 1.4 to 1.9 wt.-%. The combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) preferably has a 1 -hexene content, determined according to quantitative13C-NMR spectroscopic analysis, in the range from 1.2 to 2.2 wt.-%, more preferably in the range from 1.4 to 2.0 wt.-%, most preferably in the range from 1.4 to 1.9 wt.-%.

[0071] The polyethylene composition (PE) preferably has a total comonomer content, determined according to quantitative13C-NMR spectroscopic analysis, in the range from 1.2 to 2.2 wt.- %, more preferably in the range from 1.4 to 2.0 wt.-%, most preferably in the range from 1.4 to 1.9 wt.-%.

[0072] The combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) preferably has a total comonomer content, determined according to quantitative13C-NMR spectroscopic analysis, in the range from 1.2 to 2.2 wt.-%, more preferably in the range from 1.4 to 2.0 wt.-%, most preferably in the range from 1.4 to 1.9 wt.-%.

[0073] The polyethylene composition (PE) preferably has a number average molecular weight (Mn), determined according to ASTM D 6474-12, in the range from 7,500 to 11,000 g / mol, more preferably in the range from 7,700 to 10,000 g / mol, most preferably, in the range from 7,800 to 9,000 g / mol.

[0074] The combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) preferably has a number average molecular weight (Mn), determined according to ASTM D 6474-12, in the range from 7,500 to 11,000 g / mol, more preferably in the range from 7,700 to 10,000 g / mol, most preferably, in the range from 7,800 to 9,000 g / mol.

[0075] The polyethylene composition (PE) preferably has a weight average molecular weight (Mw), determined according to ASTM D 6474-12, in the range from 220,000 to 260,000 g / mol, more preferably in the range from 230,000 to 258,000 g / mol, most preferably, in the range from 240,000 to 255,000 g / mol. The combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) preferably has a weight average molecular weight (Mw), determined according to ASTM D 6474-12, in the range from 220,000 to 260,000 g / mol, more preferably in the range from 230,000 to 258,000 g / mol, most preferably, in the range from 240,000 to 255,000 g / mol.

[0076] The polyethylene composition (PE) preferably has a z-average molecular weight (Mz), determined according to ASTM D 6474-12, in the range from 1,300,000 to 1,600,000 g / mol, more preferably in the range from 1,350,000 to 1,540,000 g / mol, most preferably, in the range from 1,400,000 to 1,480,000 g / mol.

[0077] The combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) preferably has a z-average molecular weight (Mz), determined according to ASTM D 6474-12, in the range from 1,300,000 to 1,600,000 g / mol, more preferably in the range from 1,350,000 to 1,540,000 g / mol, most preferably, in the range from 1,400,000 to 1,480,000 g / mol.

[0078] The polyethylene composition (PE) preferably has a z+l-average molecular weight (Mz+1), determined according to ASTM D 6474-12, in the range from 2,000,000 to 3,500,000 g / mol, more preferably in the range from 2,500,000 to 3,200,000 g / mol, most preferably, in the range from 2,770,000 to 2,900,000 g / mol.

[0079] The combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) preferably has a z+l-average molecular weight (Mz+1), determined according to ASTM D 6474-12, in the range from 2,000,000 to 3,500,000 g / mol, more preferably in the range from 2,500,000 to 3,200,000 g / mol, most preferably, in the range from 2,770,000 to 2,900,000 g / mol.

[0080] The polyethylene composition (PE) preferably has a volume average molecular weight (Mv), determined according to ASTM D 6474-12, in the range from 100,000 to 200,000 g / mol, more preferably in the range from 130,000 to 190,000 g / mol, most preferably, in the range from 160,000 to 180,000 g / mol.

[0081] The combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) preferably has a volume average molecular weight (Mv), determined according to ASTM D 6474-12, in the range from 100,000 to 200,000 g / mol, more preferably in the range from 130,000 to 190,000 g / mol, most preferably, in the range from 160,000 to 180,000 g / mol.

[0082] The polyethylene composition (PE) preferably has a molecular weight distribution (Mw / Mn), determined according to ASTM D 6474-12, in the range from 20.0 to 35.0, more preferably in the range from 23.0 to 33.5, most preferably, in the range from 26.0 to 32.0.

[0083] The combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) preferably has a molecular weight distribution (Mw / Mn), determined according to ASTM D 6474-12, in the range from 20.0 to 35.0, more preferably in the range from 23.0 to 33.5, most preferably, in the range from 26.0 to 32.0.

[0084] The polyethylene composition (PE) preferably has a hexane soluble fraction according to the FDA method (C6FDA) in the range from 0.00 to 0.20 wt.-%, more preferably in the range from 0.00 to 0. 18 wt.-%, most preferably in the range from 0.00 to 0. 17 wt.-%.

[0085] The combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) preferably has a hexane soluble fraction according to the FDA method (C6FDA) in the range from 0.00 to 0.20 wt.-%, more preferably in the range from 0.00 to 0.18 wt.-%, most preferably in the range from 0.00 to 0.17 wt.-%.

[0086] The polyethylene composition (PE) preferably has a chain length, determined according to quantitative13C-NMR spectroscopic analysis, in the range from 740 to 1000, more preferably in the range from 750 to 950, most preferably in the range from 760 to 900. The combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) preferably has a chain length, determined according to quantitative13C-NMR spectroscopic analysis, in the range from 740 to 1000, more preferably in the range from 750 to 950, most preferably in the range from 760 to 900.

[0087] The polyethylene composition (PE) preferably has a degree of polymerization, determined according to quantitative13C-NMR spectroscopic analysis, in the range from 370 to 500, more preferably in the range from 380 to 450, most preferably in the range from 385 to 430.

[0088] The combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) preferably has a degree of polymerization, determined according to quantitative13C-NMR spectroscopic analysis, in the range from 370 to 500, more preferably in the range from 380 to 450, most preferably in the range from 385 to 430.

[0089] The polyethylene composition (PE) preferably has a molecular structure comprising fractions of varying lamellar thickness. The thickness the different lamellar fractions can be measured by Successive Self-nucleation and Annealing (SSA) thermal fractionation technique that can be run on a Differential Scanning Calorimeter (DSC) as described below in the measurement method section.

[0090] The SSA technique consists of sequential application of self-nucleation and annealing steps to a polymer sample. After thermal conditioning (multiple heating and cooling cycles), a final DSC heating run reveals the distribution of melting temperatures induced by the SSA thermal treatment as a result of the heterogeneous nature of the chain structure of the polymer under analysis. The SSA technique is particularly useful to study the degree and distribution of short chain branches produced by the copolymerization of ethylene with alpha-olefins.

[0091] It is preferred that the amount of the fraction having a lamellar thickness of more than 21.7 nm is from 18.0 to 35.0%, preferably from 20.0 to 32.5%, more preferably from 22.0 to 31.0%, based on the total amount of lamellar fractions. It is preferred that the amount of the fraction having a lamellar thickness of more than 11.8 nm to 21.7 nm is from 45.0 to 58.0%, preferably from 47.0 to 57.0%, more preferably from 48.0 to 56.0%, based on the total amount of lamellar fractions.

[0092] It is preferred that the amount of the fraction having a lamellar thickness of more than 8.1 nm to 11.8 nm is from 8.0 to 15.0%, preferably from 9.0 to 14.0%, more preferably from 10.0 to 13.0%, based on the total amount of lamellar fractions.

[0093] It is preferred that the amount of the fraction having a lamellar thickness of more than 6. 1 nm to 8.1 nm is from 4.0 to 10.0%, preferably from 5.0 to 9.0%, more preferably from 6.0 to 8.0%, based on the total amount of lamellar fractions.

[0094] It is preferred that the amount of the fraction having a lamellar thickness of 0 nm to 6.1 nm is from 2.0 to 6.0%, preferably from 2.5 to 5.5%, more preferably from 3.0 to 5.0%, based on the total amount of lamellar fractions.

[0095] Additionally or alternatively, the ranges given above for the amounts of each lamellar thickness may apply mutatis mutandis to the combination of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3).

[0096] The polyethylene composition (PE) preferably has a strain hardening modulus (SH), determined according to the method given in the determination methods, in the range from 50 to 90 MPa, more preferably in the range from 60 to 80 MPa, most preferably in the range from 65 to 72 MPa.

[0097] The polyethylene composition (PE) preferably has an aFNCT, determined according to ISO 16770:2019 on a Type A specimen, in the range from 500 to 2000 hours, more preferably in the range from 600 to 1500 hours, most preferably in the range from 700 to 1000 hours.

[0098] The polyethylene composition (PE) preferably has an aNPT, determined according to ISO 13479:2020 on a pipe specimen having an outer diameter of 110 mm and an average wall thickness of 10 mm, in the range from 300 to 1000 MPa, more preferably in the range from 320 to 700 MPa, most preferably in the range from 340 to 500 MPa.

[0099] The polyethylene composition (PE) preferably has a hydrostatic pressure resistance (HPT), determined according to ISO 1167-1 on a pipe specimen having an outer diameter of 32 mm and an average wall thickness of 3.2 mm at a temperature of 20 °C and a hoop stress of 12.4 MPa, in the range from 80 to 400 hours, more preferably in the range from 95 to 300 hours, most preferably in the range from 140 to 250 hours.

[0100] The polyethylene composition (PE) preferably has a hydrostatic pressure resistance (HPT), determined according to ISO 1167-1 on a pipe specimen having an outer diameter of 32 mm and an average wall thickness of 3.2 mm at a temperature of 80 °C and a hoop stress of 5.5 MPa, in the range from 400 to 1300 hours, more preferably in the range from 480 to 1100 hours, most preferably in the range from 600 to 900 hours.

[0101] It is particularly preferred that the polyethylene composition (PE) is obtainable by, more preferably obtained by, the method of the second aspect.

[0102] The method

[0103] In a second aspect, the present invention is directed to a method for producing the polyethylene composition (PE) according to the first aspect, comprising the following steps in the given order: a) providing a Ziegler-Natta catalyst composition (ZNC); b) in the presence of the Ziegler-Natta catalyst composition (ZNC), in a first polymerization reactor (Rl), polymerizing ethylene and one or more alpha olefins selected from C3-8 alpha olefins, thereby obtaining the first polyethylene fraction (PEI); c) transferring the mixture of the Ziegler-Natta catalyst composition (ZNC) and the first polyethylene fraction (PEI) into a second polymerization reactor (R2) and subsequently polymerizing ethylene, thereby obtaining the second polyethylene fraction (PE2); d) transferring the mixture of the Ziegler-Natta catalyst composition (ZNC), the first polyethylene fraction (PEI) and the second polyethylene fraction (PE2) into a third polymerization reactor (R3) and subsequently polymerizing ethylene and 1 -hexene, thereby obtaining the third polyethylene fraction (PE3); e) removing the mixture of the Ziegler-Natta catalyst composition (ZNC), the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) from the third polymerization reactor, removing unreacted monomers and comonomers; and f) melt extruding, more preferably pelletizing, the mixture obtained in step e), optionally with one or more further additives (A), thereby obtaining the polyethylene composition (PE).

[0104] The present invention is based on the finding that, in contrast to the general understanding in the art that prepolymerization should be carried out using the same monomer / comonomer combination as the first polymerization reactor proper (i.e. only ethylene in the context of a C2 / C2C6 copolymer, or C2C4 in the case of a C2C4 / C2C6 polymer), that a comonomer that is not fed into the first polymerization reactor proper (i.e. the second polymerization reactor (R2)) should be added to the prepolymerization reactor.

[0105] As explained above, although the second polyethylene fraction (PE2) may contain trace amounts of comonomer, since unreacted comonomer may be transferred into the first polymerization reactor proper from the prepolymerization reactor; however, given the reactor splits and the fact that no further comonomer is added to the first polymerization reactor proper, the amounts of the additional comonomer in the product of the first polymerization reactor (i.e. the second polyethylene fraction as defined above and below) will be very low, much lower than the comonomer content in the prepolymerization product. Thus despite the definition of the second polyethylene fraction (PE2) allowing for the presence of trace amounts of the extra comonomer, this comonomer may not be added to the reactor in which the second polyethylene fraction is produced.

[0106] In some embodiments, the transferring of the mixture of the Ziegler-Natta catalyst composition (ZNC) and the first polyethylene fraction (PEI) into a second polymerization reactor (R2) of step c) is carried out directly without any alteration to the mixture, meaning that unreacted monomers / comonomers from the first polymerization reactor are also transferred into the second polymerization reactor (R2).

[0107] In other embodiments, unreacted monomers / comonomers from the prepolymerization reactor (i.e. the first polymerization reactor (Rl)) may be removed from the mixture of the Ziegler- Natta catalyst composition (ZNC) and the first polyethylene fraction (PEI) before addition into the second polymerization reactor (R2), using flash tanks, hydrocyclones and / or other technologies well-known to the person skilled in the art.

[0108] Additionally or alternatively, unreacted monomers / comonomers from the second polymerization reactor (R2) may be removed from the mixture of the Ziegler-Natta catalyst composition (ZNC), the first polyethylene fraction (PEI) and the second polyethylene fraction (PE2) before addition into the third polymerization reactor (R3), using flash tanks, hydrocyclones and / or other technologies well-known to the person skilled in the art.

[0109] It is particularly preferred that the first polymerization reactor (Rl) is a prepolymerization reactor.

[0110] It is also particularly preferred that the second polymerization reactor (R2) is a slurry reactor, more preferably a loop reactor.

[0111] It is also particularly preferred that the third polymerization reactor (R3) is a gas phase reactor.

[0112] In the broadest sense, the Ziegler-Natta catalyst composition (ZNC) may be any Ziegler- Natta catalyst composition (ZNC) suitable for ethylene polymerization.

[0113] A Ziegler-Natta type catalyst typically used for ethylene polymerization and / or copolymerization will be a stereospecific, solid, high yield Ziegler-Natta catalyst component comprising as main components Mg, Ti and Cl. In addition to the solid catalyst component, a cocatalyst(s) as well as external donor(s) will generally be used in the polymerization process. The components of the catalyst may be supported on a particulate support, such as inorganic oxide, like silica or alumina, or, usually, a magnesium halide may form the solid support. It is also possible that the catalyst components are not supported on an external support, but the catalyst is prepared by an emulsion-solidification method or by a precipitation method, as is well-known by the man skilled in the art of catalyst preparation.

[0114] The solid catalyst usually also comprises at least one electron donor (internal electron donor) and optionally aluminium.

[0115] Suitable external electron donors used in the polymerization are well known in the art and include ethers, ketones, amines, alcohols, phenols, phosphines and silanes.

[0116] Examples of suitable Ziegler-Natta catalysts and components in the catalysts are described among others in W087 / 07620, WO92 / 21705, WO93 / 11165, WO93 / 11166, W093 / 19100, WO97 / 36939, WO98 / 12234, WO99 / 33842, W003 / 000756, W003 / 000757, W003 / 000754, W003 / 000755, W02004 / 029112, EP2610271, WO2012 / 007430, WO92 / 19659, WO92 / 19653, WO92 / 19658, US4382019, US4435550, US4465782, US4473660, US4560671, US5539067, US5618771, EP45975, EP45976, EP45977, WO95 / 32994, US4107414, US4186107, US4226963, US4347160, US4472524, US4522930, US4530912, US4532313, US4657882, US4581342, US4657882.

[0117] The catalyst may be commercially available or be produced in accordance or analogously to the literature. For the preparation of the preferable catalyst usable in the invention, reference is made to WO 2004 / 055068 Al, WO 2004 / 055069 Al of Borealis AG and WO 2015 / 078924 of Abu Dhabi Polymers Co., Ltd. (Borouge) / Borealis AG and EP 0 810 235 Al. The content of these documents in its entirety is incorporated herein by reference, in particular concerning the general and all preferred embodiments of the catalysts described therein as well as the methods for the production of the catalysts. Particularly preferred Ziegler-Natta catalysts are described in EP 0 810 235 Al. All fallback positions given above for the polyethylene composition (PE) of the first aspect apply mutatis mutandis to the polyethylene composition produced by the method of the second aspect.

[0118] The pipe

[0119] In a final aspect, the present invention is directed to a pipe comprising at least 90 wt.-% of, more preferably at least 95 wt.-% of, most preferably consisting of the polyethylene composition (PE) of the first aspect.

[0120] The pipe preferably has an aNPT, determined according to ISO 13479:2020, in the range from 300 to 1000 MPa, more preferably in the range from 320 to 700 MPa, most preferably in the range from 340 to 500 MPa.

[0121] The pipe preferably has a hydrostatic pressure resistance (HPT), determined according to ISO 1167-1 at a temperature of 20 °C and a hoop stress of 12.4 MPa, in the range from 80 to 400 hours, more preferably in the range from 95 to 300 hours, most preferably in the range from 140 to 250 hours.

[0122] The pipe preferably has a hydrostatic pressure resistance (HPT), determined according to ISO 1167-1 at a temperature of 80 °C and a hoop stress of 5.5 MPa, in the range from 400 to 1300 hours, more preferably in the range from 480 to 1100 hours, most preferably in the range from 600 to 900 hours.

[0123] All fallback positions given above for the polyethylene composition (PE) of the first aspect apply mutatis mutandis to the polyethylene composition in the pipe of the final aspect. E X A M P L E S

[0124] 1. Determination Methods

[0125] 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.

[0126] Melt flow rate

[0127] 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 higher the melt flow rate, the lower the viscosity of the polymer. The MFR5 of polyethylene was measured at a temperature of 190 °C and a load of 5 kg and the MFR2 of polyethylene at a temperature of 190 °C and a load of 2. 16 kg.

[0128] Density

[0129] Density of the polymer was 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.

[0130] Comonomer content

[0131] Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used to quantify the comonomer content of the polymers.

[0132] Quantitative13C {1H} NMR spectra recorded in the molten-state using a Bruker Advance III 500 NMR spectrometer operating at 500. 13 and 125.76 MHz for 'H and13C respectively. All spectra were recorded using a13C optimised 7 mm magic-angle spinning (MAS) probehead 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 utilising the NOE at short recycle delays {pollard04, klimke06} and the RS-HEPT decoupling scheme {fillipO5,griffmO7}. A total of 1024 (Ik) transients were acquired per spectra. Quantitative13C{’H} NMR spectra were processed, integrated and relevant quantitative properties determined from the integrals. All chemical shifts are internally referenced to the bulk methylene signal (5+) at 30.00 ppm.

[0133] The amount of ethylene was quantified using the integral of the methylene (5+) sites at 30.00 ppm accounting for the number of reporting sites per monomer:

[0134] E= I5+ / 2 the presence of isolated comonomer units is corrected for based on the number of isolated comonomer units present:

[0135] Etotal = E + (3*B + 2*H) / 2 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. 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 / (Etotal + Btotal + Htotal))

[0136] The amount isolated 1 -butene incorporated in EEBEE sequences was quantified using the integral of the *B2 sites at 38.3 ppm accounting for the number of reporting sites per comonomer:

[0137] B = I„B2

[0138] 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:

[0139] BB = 2 * IaaB2B2

[0140] The amount non consecutively incorporated 1 -butene in EEBEBEE sequences was quantified using the integral of the PPB2B2 site at 24.7 ppm accounting for the number of reporting sites per comonomer:

[0141] BEB = 2 * IPPB2B2

[0142] Due to the overlap of the *B2 and *PB2B2 sites of isolated (EEBEE) and non-consecutivly 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: B = I„B2 - 2 * IppB2B2 The total 1 -butene content was calculated based on the sum of isolated, consecutive and non consecutively incorporated 1 -butene:

[0143] Btotal = B + BB + BEB

[0144] The total mole fraction of 1 -butene in the polymer was then calculated as: fB = (Btotal / ( Etotal + Btotal + Htotal)

[0145] 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 / (Etotal + Btotal + Htotal)

[0146] The amount isolated 1 -hexene incorporated in EEHEE sequences was quantified using the integral of the *B4 sites at 39.9 ppm accounting for the number of reporting sites per comonomer:

[0147] H = I.B4

[0148] 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:

[0149] HH = 2 * IaaB4B4

[0150] 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:

[0151] HEH = 2 * IPPB4B4

[0152] The total mole fraction of 1 -hexene in the polymer was then calculated as: fH = (Htotal / (Etotal + Btotal + Htotal)

[0153] The mole percent comonomer incorporation is calculated from the mole fraction:

[0154] B [mol%] = 100 * fB

[0155] H [mol%] = 100 * fH

[0156] The weight percent comonomer incorporation is calculated from the mole fraction:

[0157] B [wt%] = 100 * (fB * 56.11) / ((fB * 56.11) + (fH * 84.16) + ((l-(fB + fH)) * 28.05))

[0158] H [wt%] = 100 * (fH * 84.16) / ((fB * 56.11) + (fH * 84.16) + ((l-(fB + fH)) * 28.05)) Quantitative estimation of each type of side chain branch is possible using the ’3C NMR spectrum plot as mentioned previously. Such quantification is possible using following equationarea under branch peak

[0159] Branch type / lOOCI carbons = - * 1000 area under back bone peak

[0160] Using above equation, by dividing the area under a spectrum peak, specific for one type of branch, by the area under the backbone peak, the number of branches per thousand carbon atoms can be estimated.

[0161] Similarly, the Mn of polyethylene by comparing the integrals (I) of the backbone methylene carbons to those of the end-groups or isolated branch carbons. The integral corresponding to the total C (Total) is given by the sum of integrals, the integrals of the four aliphatic carbons out of the main peak, and the integrals of the two olefinic carbon atoms. The integral corresponding to one carbon (1C) can be obtained using the isolated signals. The equation E and E otai as previously mentioned supports such calculation.

[0162] With the determination of Mn, average chain length can be understood by the following formula-

[0163] Mn Avg. chain length = — 14

[0164] The degree of polymerization (DP) can be calculated using13C-NMR spectroscopy by integrating specific signals in the13C-NMR spectrum that correspond to the end groups and the repeating units of the polymer chain. It is calculated using the following formula- p integral of back bone peak integral of end group peak References: klimke06

[0165] Klimke, K., Parkinson, M., Piel, C., Kaminsky, W., Spiess, H.W., Wilhelm, M., Macromol.

[0166] Chem. Phys. 2006;207:382. parkinson07

[0167] Parkinson, M., Klimke, K., Spiess, H.W., Wilhelm, M., Macromol. Chem. Phys.

[0168] 2007;208:2128. pollard04

[0169] Pollard, M., Klimke, K., Graf, R., Spiess, H.W., Wilhelm, M., Sperber, O., Piel, C., Kaminsky, W., Macromolecules 2004;37:813. filipO5

[0170] Filip, X., Tripon, C„ Filip, C„ J. Mag. Resn. 2005, 176, 239 griffin07

[0171] Griffin, J.M., Tripon, C., Samoson, A., Filip, C., and Brown, S.P., Mag. Res. in Chem. 2007

[0172] 45, SI, S198 castignolles09

[0173] Castignolles, P., Graf, R., Parkinson, M., Wilhelm, M., Gaborieau, M., Polymer 50 (2009) 2373 busicoOl

[0174] Busico, V., Cipullo, R., Prog. Polym. Sci. 26 (2001) 443 busico97

[0175] Busico, V., Cipullo, R., Monaco, G., Vacatello, M., Segre, A.L., Macromoleucles 30 (1997) 6251 zhou07

[0176] Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha, A., Baugh, D. Winniford,

[0177] B„ J. Mag. Reson. 187 (2007) 225 busico07

[0178] Busico, V., Carbonniere, P., Cipullo, R., Pellecchia, R., Severn, J., Talarico, G., Macromol.

[0179] Rapid Commun. 2007, 28, 1128 resconiOO

[0180] Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev. 2000, 100, 1253 DSC analysis, melting temperature (Tm) and heat of fusion (Hf), crystallization temperature (Tc) and melt enthalpy (Hm)

[0181] Measured with a TA Instrument Q2000 differential scanning calorimetry (DSC) on 5 to 7 mg samples. DSC was run according to ISO 11357 / part 3 / method C2 in a heat / cool / heat cycle with a scan rate of 10 °C / min in the temperature range of 0 to +200 °C. The crystallization temperature (Tc) was determined from the cooling step, while melting temperature (Tm) and melting enthalpy (AHm) were determined from the second heating step. The crystallinity was calculated from the melting enthalpy by assuming an AHm -value of 290 J / g for a fully crystalline polyethylene (see L. Mandelkem, R. G. Alamo, In Physical properties of polymers handbook, 2nd ed., Mark, J.E., Eds. Springer: New York, 2007)

[0182] Molecular Weight

[0183] Molecular weight averages (Mn, Mw, Mz, Mz+1, and Mv), Molecular weight distribution

[0184] (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 ISO 16014-4:2003 and

[0185] ASTM D 6474-99 using the following formulas:

[0186] For a constant elution volume interval AVi, where Ai and Mi are the chromatographic peak slice area and polyolefin molecular weight (MW). A GPC instrument, equipped with infrared (IR) detector was used with 3 x Olexis and lx Olexis Guard columns and 1,2,4- trichlorobenzene (TCB, stabilized with 250 mg / L 2,6-Di tert butyl-4-methyl-phenol) as solvent at 160 °C and at a constant flow rate of 1 mL / min. 200 pL of sample solution were injected per analysis. The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with at least 15 narrow MWD polystyrene (PS) standards in the range of 0,5 kg / mol to 11 500 kg / mol. Mark Houwink constants for PS, PE and PP used are as described per ASTM D 6474-99. All samples were prepared by dissolving 5.0 - 9.0 mg of polymer in 8 m (at 160 °C) of stabilized TCB (same as mobile phase) for 2.5 hours for PP or 3 hours for PE at 160°C under continuous gentle shaking in the autosampler of the GPC instrument.

[0187] The amount of hexane solubles (C6) [in wt.-%] was determined via Soxhlet extraction. Around 1 g of a powdered test portion is carefully dried and weighed out exactly. The powdered test portion is extracted in a Soxhlet-apparatus with 150 ml n-hexane (p.a. quality) during 24 h. As thimble a standard 603 cellulose extraction thimble was used. To avoid that polymer powder will leave the extraction thimble it was closed on the top with another half cut thimble. The weight of the glass extractor including the polymer after extraction is weighed out after reaching constant weight. The mass of residue of the test portion in the glass extractor is determined in the following way:

[0188] C6 solubles (%))=((m2 - mt )) / ml x 100 with: ml= original sample weight m2 = weight of the glass extractor with polymer after extraction mt = weight of the glass extractor.

[0189] Determination of the chemical heterogeneity by Cross Fractionation Chromatography (CFC)

[0190] The chemical composition distribution as well as the determination of the molecular weight distribution and the corresponded molecular weight averages (Mn, Mw and Mv) at a certain elution temperature (polymer crystallinity in solution) were determined by a full automated Cross Fractionation Chromatography (CFC) as described by Ortin A., Monrabal B., Sancho- Tello J., Macromol. Symp., 2007, 257, 13-28.

[0191] A CFC instrument (PolymerChar, Valencia, Spain) was used to perform the crossfractionation chromatography (TREF x SEC). A four-band IR5 infrared detector (PolymerChar, Valencia, Spain) was used to monitor the concentration. Around 40 mg of the polymer sample was dissolved in 25 ml TCB in the stainless steel vessel for 150 min at 150 °C. Once the sample was completely dissolved an aliquot of 0,5 ml was loaded into the TREF column and stabilized for 60 minutes at 110 °C. The polymer was crystallized and precipitate to a temperature of 30°C by applying a constant cooling rate of 0.1 °C / min. A discontinuous elution process is performed using the following temperature steps: (35, 40, 45, 50, 53, 56, 59, 62, 64, 66, 69, 72, 76, 79, 82, 85, 89, 91 , 93, 95, 97, 100, 110, and 120) In the second dimension, the GPC analysis, 3 PL Olexis columns and lx Olexis Guard columns from Agilent (Church Stretton, UK) were used as stationary phase. As eluent 1 ,2,4- trichlorobenzene (TCB, stabilized with 250 mg / L 2,6-Di tert butyl-4-methyl-phenol) at 150 °C and a constant flow rate of 1 mL / min were applied. The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with at least 15 narrow MWD polystyrene (PS) standards in the range of 0.5 kg / mol to 11,500 kg / mol. The following Mark Houwink constants were used to convert PS molecular weights into the PE molecular weight equivalents.

[0192] KPS = 19 x 10'3mL / g, aps = 0.655

[0193] KPE = 39 x W3mL / g, aPS= 0.725

[0194] A third order polynomial fit was used to fit the calibration data. Data processing was performed using the software provided from PolymerChar with the CFC instrument.

[0195] Soluble fraction SF@60°C(CFC)

[0196] The soluble fraction SF@60°C (CFC) is simply the polymer fraction eluting at 60°C in the Cross Fractionation Chromatography analysis described above.

[0197] Lamella Thickness

[0198] Successive Self-Nucleation / Annealing (SSA) technique enhances the potential molecular fractionation that can occur during crystallization by magnifying the small effects through annealing of the unmelted crystals at each stage of the process. General references and additional information can be found in a review article by Muller and Amal [A. J. Muller, M.L. Amal, Thermal fractionation of polymers, Prog. Polym. Sci. 30 (2005) 559-603], SSA was performed on an indium and tin calibrated TA Instruments Q2000 differential scanning calorimeter equipped with RCS90 cooling system. A sample mass of 5-10 mg was used from compression moulded sheets. All samples were encapsulated in Tzero aluminium pans. Ultra-high purity dry nitrogen was used as an inert atmosphere. The experimental protocol is as follows:

[0199] (a) Erasure of previous thermal history by heating the sample from room temperature to 180°C at 20°C / min and by holding isothermally for 5 minutes.

[0200] (b) In this method, 8 subsequent cooling / heating cycles at 10°C / min were utilized and the first isothermal crystallization temperature was selected to be 126°C. The duration of each isothermal crystallization step is 5 minutes. After each isothermal crystallization step, the sample is cooled to 30°C and held isothermally for 5 minutes.

[0201] (c) The subsequent isothermal temperatures were always maintained at 5°C lower than the previous step, i.e., the sample is heated from 30°C to the second isothermal crystallization temperature of 121 °C.

[0202] (d) After the last isothermal crystallization step at 91 °C, the sample is cooled down to 0°C, and the melting curve is obtained by heating the cooled sample at a heating rate of 10°C / min up to 180°C.

[0203] The final heating run after an SSA treatment exhibits a series of melting peaks that correspond to the number of SSA cycles, where annealing was promoted. Gibbs-Thompson equation can then be used to establish a correlation between melting temperature and lamellae thickness:

[0204] Tm= T° {1 - (2e / A / / ° Lc)}, (4) is equilibrium melting point of infinitely thick crystal, <Jeis the specific surface energy, AHnis enthalpy of melting per mass unit, Lcis lamellae thickness and Tmis melting temperature of the lamellae. . ae. and 4 H,°nhave been taken to be 415 K, 93 x 10'3J / m3, and 3 x 108J / m3, respectively [L. Mandelkem, R.G. Alamo, Thermodynamic quantities governing melting, in: Phys. Prop. Polym. Handb., Springer, 2007: pp. 165-186.].

[0205] The melt enthalpy is calculated from the quotient of the heat flow volume and initial weight of the sample between the extrapolated onset and end of melting, as described in ISO 11357- 3. A fixed integration approach, in steps of 10°C, is used for the melting curve quantification to determine the lamellae thickness distribution. Strain Hardening Modulus

[0206] 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 (strain hardening regime). It was measured at 80°C and 20mm / min on pre-conditioned (120°C / hour) 300 pm thick specimens according to ISO 18488 method.

[0207] Hydrostatic Pressure Testing

[0208] HPT testing was conducted according to; ISO 1167-1:2006 Thermoplastics pipes, fittings and assemblies for the conveyance of fluids — Determination of the resistance to internal pressure — Part 1: General method. Tests were carried out at specified temperatures and stress levels. The pipes specimens used had an outer diameter of 32 mm and an average wall thickness of 3.2 mm. aFNCT aFNCT testing was conducted according to ISO 16770:2019: Determination of environmental stress cracking (ESC) of polyethylene - full - notch creep test (FNCT) Compression moulding and annealing was performed in accordance with ISO 16770. The plaque was compression moulded at 200°C. Afterwards, the 10 mm thick plaque was annealed for 3 h at 100°C and then slowly cooled down to room temperature. The test specimens (type A) were machined from the compression moulded plaque and notched on all four sides using a razor blade. The test specimens were submerged and conditioned in the aged detergent solution. The pH of the detergent was monitored during preparation and the test. Subsequently, the test pieces were loaded according to ISO 16770.

[0209] Specimen preparation

[0210] Moulding temperature : 200 °C

[0211] Average cooling rate : (3,5 ± 1,5) °C

[0212] Preheating time : 25 min

[0213] Full pressure : 10 MPa

[0214] Full pressure time : 40 min

[0215] Annealing temperature : 100 °C

[0216] Annealing time : 3 h Apparatus

[0217] Fontijne Presses, LabEcon 300.

[0218] Mitutoyo micrometer, Type ABS IDC 543-300B.

[0219] IPT CNC milling machine, model 1643.

[0220] IPT notching device, model 1719-005 with a notching speed > 0.5 mm / min.

[0221] Test parameters

[0222] Conditioning period (machining) : >24 h at (23 ± 2) °C

[0223] Conditioning period (notching) : >24 h at (23 ± 2) °C

[0224] Specimen type : Type A (100x10x10 mm (LxWxT) and 1,6 mm notch depth)

[0225] Test temperature : 90 °C

[0226] Environment : Test environment no. 2

[0227] Water type : Demineralized water

[0228] Detergent type : Lauramine oxide (Dehyton PL, CAS 85408-49-7)

[0229] Concentration : 2,0 % (6,67 % Dehyton PL)

[0230] Number of test pieces : 8

[0231] Conditioning period (environment) : 16 - 24 h at 90 °C

[0232] Evaluation method : log-log aNPT aNPT was measured according to ISO 13479:2020, Determination of resistance to crack propagation - test method for slow crack growth on notched pipes (ISO 13479:2020). For the data presented in this application, pipes having pipe specimens having an outer diameter of 110 mm and an average wall thickness of 10 mm were used (i.e. nominal diameter 110 mm and SDR of 11) Sample preparation, conditioning

[0233] After the determination of the dimensions, 4 notches were placed at the outside diameter of the test pieces, according to ISO 13479. Each notch was machined by climb milling.

[0234] Then the test pieces were assembled with end caps and filled with water. Subsequently the test pieces were conditioned according to ISO 13479. Milling description

[0235] Cutter size : 0 168mm

[0236] Amount of teeth : 22

[0237] Cutter speed : 650 r / min Traverse speed : 150 mm / min

[0238] Apparatus

[0239] Tank with an average temperature of ± 0.5°C of the required value.

[0240] Pressure unit with an average pressure of +2 en -1% of the required value.

[0241] Measuring equipment according to ISO 3126. Test parameters

[0242] Test temperature : (80 ± 1) °C

[0243] Type of end caps : A

[0244] Environment : water in water

[0245] Orientation of the test pieces : Vertical Conditioning period : 6 h (8mm < emin< 16mm)

[0246] Time to achieve test pressure : 1 min

[0247] Accuracy pressure equipment : +2 / -1 %

[0248] Number of test pieces : 3

[0249] Free length of the samples : 580mm

[0250] 2. Examples

[0251] 2.1 Catalyst preparation

[0252] For the polymerization of CE1, IE1, and IE2, Lynx 200 catalyst, commercially available from W.R. Grace (US), was employed.

[0253] 2.2 Polymerization of CE1, IE1, and IE2

[0254] CE1, IE1, and IE2 were polymerized according to the following general procedure, according to the precise conditions given in Table 1.

[0255] Into a first loop reactor having a volume of 50 dm3and operating at a temperature and pressure as indicated in Table 1, propane, ethylene, hydrogen, and, for IE1 and IE2, 1-butene (see Table 1) were introduced for conducting a pre-polymerization step. In addition, the Ziegler-Natta catalyst as described above was introduced into the reactor together with triethylaluminium cocatalyst so that the ratio of aluminium to titanium was 15 mol / mol.

[0256] The slurry was withdrawn intermittently from the prepolymerization reactor and directed to a second loop reactor having a volume of 500 dm3and operating at a temperature and pressure of as indicated in Table 1. Additionally, propane, ethylene and hydrogen were fed to the second loop reactor whereby the ethylene concentration and the hydrogen to ethylene ratio for example CE 1 , IE 1 , and IE2 are listed in Table 1. The split, the density and melt index of the polymer fractions produced in the second loop reactor are listed in Table 1.

[0257] The slurry was withdrawn intermittently from the second loop reactor by using settling legs and directed to a gas phase reactor. The gas phase reactor was operated at a temperature and pressure as indicated in Table 1. Additional ethylene, 1-hexene comonomer (Ce), and hydrogen were fed whereby the ethylene concentration, the 1-hexene to ethylene ratio and the hydrogen to ethylene ratio as well as the production split and the density of the polymers of example Ex 1 withdrawn from the gas phase reactor are listed in Table 1. Table 1 Polymerization conditions of CE1, IE1, and IE2

[0258] The resultant polymer powder was purged with nitrogen (about 50 kg / h) for one hour, stabilized with commercial stabilizers (1200 ppm of Irganox 1010, 1200 ppm of Irgafos 168, and 1100 ppm of calcium stearate) and mixed with 1 wt.-% of a pigment masterbatch and then extruded to pellets in a Mixtron LCM 80 counter-rotating twin screw extruder (manufactured by KOBELCO, Kobe Steel, Japan). The properties of compositions CE1, IE1 and IE2 are given in Table 2.

[0259] Table 2 Polymerization conditions of CE1, IE1, and IE2 As can be seen from the data presented in Table 2, IE1 and IE2, which involve a C2C4 prepolymerization step, have a significantly improved balance of pipe properties. In particular the aFNCT and aNPT properties are noticeably better than those of CE1, with both IE1 and IE2 fulfilling the requirements of the PE100RC classification.

[0260] Furthermore, IE1 and IE2 appears to have fewer oligomeric chains, with lower hexane soluble values and a narrower molecular weight distribution. This is also reflected in the NMR data, with higher chain lengths seemingly helping to contribute to the improved pipe properties.

[0261] The SSA data also backs up this trend, with much higher amounts of the >21.7 nm lamellar fraction.

[0262] Interestingly, despite IE2 having a higher MFR5, the pipe properties are maintained, which is in defiance of the usual understanding that higher MFRs tend to lead to inferior HPT and

[0263] NPT properties.

Claims

C L A I M S1. A polyethylene composition (PE) comprising: a) from 0.1 to 5.0 wt.-%, relative to the total weight of the polyethylene composition (PE), of a first polyethylene fraction (PEI) being a copolymer of ethylene and one or more alpha olefins selected from C3-8 alpha olefins; b) from 30.0 to 69.0 wt.-%, relative to the total weight of the polyethylene composition (PE), of a second polyethylene fraction (PE2) being an ethylene polymer consisting of ethylene monomer units and optionally the same comonomer(s) as the first polyethylene fraction (PEI) wherein the content of said optional comonomer(s) is not more than 0.20 wt.-%; and c) from 30.0 to 69.0 wt.-%, relative to the total weight of the polyethylene composition (PE), of a third polyethylene fraction (PE3) being a copolymer of ethylene and 1 -hexene; wherein the combined weight of the first polyethylene fraction (PEI), the second polyethylene fraction (PE2), and the third polyethylene fraction (PE3) is at least 90 wt.-%, relative to the total weight of the polyethylene composition (PE).

2. The polyethylene composition (PE) according to claim 1, wherein the first polyethylene fraction (PEI) is a copolymer of ethylene and one alpha olefin selected from C3-8 alpha olefins, more preferably from C4-6 alpha olefins, most preferably the first polyethylene fraction (PEI) is a copolymer of ethylene and 1 -butene.

3. The polyethylene composition (PE) according to claim 1 or claim 2, wherein the first polyethylene fraction (PEI) is free from 1 -hexene.

4. The polyethylene composition (PE) according to any one of the preceding claims, having one or more, preferably all, of the following properties: a) a melt flow rate (MFR5), determined according to ISO 1133 at 190 °C and 5.0 kg, in the range from 0.05 to 1.0 g / 10 min, more preferably in the range from 0.10 to 0.70 g / 10 min, most preferably in the range from 0.20 to 0.40 g / 10 min;b) a density, determined according to ISO 1183, in the range from 940 to 965 kg / m3, more preferably in the range from 943 to 963 kg / m3, most preferably in the range from 945 to 962 kg / m3; and / or c) a melting temperature (Tm), determined by differential scanning calorimetry (DSC) according to ISO 11357 / part 3 / method C2, in the range from 125.0 to 135.0 °C, more preferably in the range from 127.0 to 133.0 °C, most preferably in the range from 128.0 to 131.0 °C.

5. The polyethylene composition (PE) according to any one of the preceding claims, having one or more, preferably, all, of the following properties: a) a number average molecular weight (Mn), determined according to ASTM D 6474-12, in the range from 7,500 to 11,000 g / mol, more preferably in the range from 7,700 to 10,000 g / mol, most preferably, in the range from 7,800 to 9,000 g / mol; b) a weight average molecular weight (Mw), determined according to ASTM D 6474-12, in the range from 220,000 to 260,000 g / mol, more preferably in the range from 230,000 to 258,000 g / mol, most preferably, in the range from 240,000 to 255,000 g / mol; and / or c) a molecular weight distribution (Mw / Mn), determined according to ASTM D 6474-12, in the range from 20.0 to 35.0, more preferably in the range from 23.0 to 33.5, most preferably, in the range from 26.0 to 32.0.

6. The polyethylene composition (PE) according to any one of the preceding claims, having a hexane soluble fraction according to the FDA method (C6FDA) in the range from 0.00 to 0.20 wt.-%, more preferably in the range from 0.00 to 0.18 wt.-%, most preferably in the range from 0.00 to 0.17 wt.-%.

7. The polyethylene composition (PE) according to any one of the preceding claims, having a chain length, determined according to quantitative13C-NMR spectroscopic analysis, in the range from 740 to 1000, more preferably in the range from 750 to 950, most preferably in the range from 760 to 900.

8. The polyethylene composition (PE) according to any one of the preceding claims, wherein the polyethylene composition has a molecular structure comprising fractions of varying lamellar thickness, having one or more, preferably both, of the following properties: a) the amount of the fraction having a lamellar thickness of more than 21.7 nm is from 18.0 to 35.0%, preferably from 20.0 to 32.5%, more preferably from 22.0 to 31.0%, based on the total amount of lamellar fractions; b) the amount of the fraction having a lamellar thickness of more than 11.8 nm to 21.7 nm is from 45.0 to 58.0%, preferably from 47.0 to 57.0%, more preferably from 48.0 to 56.0%, based on the total amount of lamellar fractions; wherein the amount of the fractions having varying lamellar thicknesses is determined by Successive Self-nucleation and Annealing (SSA) thermal fractionation.

9. The polyethylene composition (PE) according to any one of the preceding claims, consisting of: a) from 0.1 to 5.0 wt.-%, more preferably in the range from 0.3 to 3.0 wt.-%, most preferably in the range from 0.5 to 2.0 wt.-%, relative to the total weight of the polyethylene composition (PE), of the first polyethylene fraction (PEI); b) from 30.0 to 69.0 wt.-%, more preferably in the range from 39.0 to 59.0 wt.-%, most preferably in the range from 43.0 to 53.0 wt.-%, relative to the total weight of the polyethylene composition (PE), of the second polyethylene fraction (PE2); c) from 30.0 to 69.0 wt.-%, more preferably in the range from 40.0 to 60.0 wt.-%, most preferably in the range from 46.0 to 56.0 wt.-%, relative to the total weight of the polyethylene composition (PE), of the third polyethylene fraction (PE3); d) optionally, from 0.0 to 10.0 wt.-% of one or more further additives (A).

10. The polyethylene composition (PE) according to any one of the preceding claims, having one or more, preferably all, of the following properties: a) a strain hardening modulus (SH), determined according to the method given in the determination methods, in the range from 50 to 90 MPa, more preferably in the range from 60 to 80 MPa, most preferably in the range from 65 to 72 MPa; b) an aFNCT, determined according to ISO 16770:2019 on a Type A specimen, in the range from 500 to 2000 hours, more preferably in the range from 600 to 1500 hours, most preferably in the range from 700 to 1000 hours; c) an aNPT, determined according to ISO 13479:2020 on a pipe specimen having an outer diameter of 110 mm and an average wall thickness of 10 mm, in the range from 300 to 1000 MPa, more preferably in the range from 320 to 700 MPa, most preferably in the range from 340 to 500 MPa; d) a hydrostatic pressure resistance (HPT), determined according to ISO 1167-1 on a pipe specimen having an outer diameter of 32 mm and an average wall thickness of 3.2 mm at a temperature of 20 °C and a hoop stress of 12.4 MPa, in the range from 80 to 400 hours, more preferably in the range from 95 to 300 hours, most preferably in the range from 140 to 250 hours; e) a hydrostatic pressure resistance (HPT), determined according to ISO 1167-1 on a pipe specimen having an outer diameter of 32 mm and an average wall thickness of 3.2 mm at a temperature of 80 °C and a hoop stress of 5.5 MPa, in the range from 400 to 1300 hours, more preferably in the range from 480 to 1100 hours, most preferably in the range from 600 to 900 hours.

11. A method for producing the polyethylene composition (PE) according to any one of the preceding claims, comprising the following steps in the given order: a) providing a Ziegler-Natta catalyst composition (ZNC); b) in the presence of the Ziegler-Natta catalyst composition (ZNC), in a first polymerization reactor (Rl), polymerizing ethylene and one or more alpha olefins selected from C3-8 alpha olefins, thereby obtaining the first polyethylene fraction (PEI); c) transferring the mixture of the Ziegler-Natta catalyst composition (ZNC) and the first polyethylene fraction (PEI) into a second polymerization reactor (R2) andsubsequently polymerizing ethylene, thereby obtaining the second polyethylene fraction (PE2); d) transferring the mixture of the Ziegler-Natta catalyst composition (ZNC), the first polyethylene fraction (PEI) and the second polyethylene fraction (PE2) into a third polymerization reactor (R3) and subsequently polymerizing ethylene and 1 -hexene, thereby obtaining the third polyethylene fraction (PE3); e) removing the mixture of the Ziegler-Natta catalyst composition (ZNC), the first polyethylene fraction (PEI), the second polyethylene fraction (PE2) and the third polyethylene fraction (PE3) from the third polymerization reactor, removing unreacted monomers and comonomers; and f) melt extruding, more preferably pelletizing, the mixture obtained in step e), optionally with one or more further additives (A), thereby obtaining the polyethylene composition (PE).

12. The method according to claim 11, wherein: a) the first polymerization reactor (Rl) is a prepolymerization reactor; b) the second polymerization reactor (R2) is a slurry reactor, more preferably a loop reactor; and / or c) the third polymerization reactor (R3) is a gas phase reactor.

13. The polyethylene composition (PE) according to any one of claims 1 to 11, being obtainable by the method according to claim 11 or claim 12.

14. A pipe comprising at least 90 wt.-% of, more preferably at least 95 wt.-% of, most preferably consisting of the polyethylene composition (PE) according to any one of claims 1 to 10 or 13.

15. The pipe according to claim 14, having one or more, preferably all, of the following properties: a) an aNPT, determined according to ISO 13479:2009, in the range from 300 to 1000 MPa, more preferably in the range from 320 to 700 MPa, most preferably in the range from 340 to 500 MPa;b) a hydrostatic pressure resistance (HPT), determined according to ISO 1167-1 at a temperature of 20 °C and a hoop stress of 12.4 MPa, in the range from 80 to 400 hours, more preferably in the range from 95 to 300 hours, most preferably in the range from 140 to 250 hours; c) a hydrostatic pressure resistance (HPT), determined according to ISO 1167-1 at a temperature of 80 °C and a hoop stress of 5.5 MPa, in the range from 400 to 1300 hours, more preferably in the range from 480 to 1100 hours, most preferably in the range from 600 to 900 hours.

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