Multimodal polyethylene tube

Abstract

The present invention relates to a multimodal polyethylene pipe, in particular to a reactor system for a multimodal polyethylene composition and the use of the multimodal polyethylene composition as a pipe, the system comprising: (a) a first reactor; (b) a hydrogen removal unit arranged between the first reactor and the second reactor, comprising at least one vessel connected to a decompression device, preferably selected from the group consisting of a vacuum pump, a compressor, a blower, an ejector or a combination thereof, the decompression device allowing the operating pressure to be adjusted to a pressure in the range of 100-200 kPa (abs); (c) a second reactor; and (d) a third reactor.

Description

[0001] This application is a divisional application of Chinese patent application No. 201780003702.7, with the application date of September 7, 2017 and the invention name of “Multimodal Polyethylene Pipe”. Technical Field

[0002] The present invention relates to a reactor system for a multimodal polyethylene polymerisation process, a method for producing a multimodal polyethylene composition using said reactor system, a multimodal polyethylene composition obtainable in this way and a pipe formed by using said multimodal polyethylene composition. Background Art

[0003] The demand for polyethylene resins is increasing for a variety of applications. Due to the high performance requirements of polyethylene for a relatively new plastic, polymerization process technologies have been developed to support the production of new polymer materials. In order to balance the processability and physical properties of ethylene copolymers, the development of multimodal polymerization processes has been studied.

[0004] In the prior art, multimodal polyethylene polymerization is used to produce polymers with different molecular weights by producing each resin fraction in a separate reactor. Excess hydrogen is used in the reactor to produce the low molecular weight fraction and control the molecular weight of the polymer suitable to provide good processability of the final polymer. The high molecular weight fraction, which has an impact on physical properties, is produced under polymerization conditions of low hydrogen concentration. It is well known in the art that low molecular weight polymers are preferably produced in the first reactor. In order to obtain multimodal polymers with good physical properties, all hydrogen from the first reactor should be removed before the polymerized slurry polymer is fed to the second reactor where the high molecular weight polymer production occurs.

[0005] US2010 / 0092709A1 describes a process for preparing bimodal polyethylene copolymers. The polymerization in the second reactor is run at high temperature with low comonomer-ethylene ratio and low hydrogen-ethylene ratio to obtain a resin with improved stress crack resistance and melt strength.

[0006] US 6,716,936 B1 describes a process for producing bimodal polyethylene copolymers. The second reactor is operated under hydrogen-deficient polyethylene polymerization by directing the polyethylene slurry stream from the first reactor to a hydrogen removal system. By using propane or isobutane as a light solvent, the polymerization in both the first and second reactors is carried out at the bubble point. The process is suitable for producing bimodal polyethylene of highly uniform high molecular weight resins.

[0007] US 6,291,601B1 describes a process for producing bimodal copolymers with relatively high molecular weight polyethylene. A hydrogenation catalyst is introduced into the second reactor to consume residual hydrogen from the first reactor by converting hydrogen into ethane, resulting in a low hydrogen concentration in the second reactor. Using this technology, the raw material consumption costs of both hydrogen and ethylene will increase due to the conversion of unreacted gas.

[0008] US2003 / 0191251A1 discloses a method for removing residual hydrogen from a polymer slurry by using two flash drums placed between a cascade reactor using a light solvent as a diluent. It is necessary to add a make-up solvent to the outlet of the first flash drum to prevent clogging of the slurry transfer pump. In addition, warm make-up solvent is necessary before transferring the slurry to the next flash drum.

[0009] EP 1 655 334 A1 discloses the multimodal production of ethylene polymers produced in a multistage process using a Ziegler-Natta catalyst based on MgCl2. The polymerization stages are carried out in the following order, firstly ultrahigh molecular weight polymers are obtained, then low molecular weight polymers are obtained and finally in the last step a high molecular weight polymer is obtained. The polymerization catalyst is charged to a prepolymerization step to prepare an ultrahigh molecular weight fraction.

[0010] WO 2013 / 144328 describes a composition of multimodal high density polyethylene for molding applications produced using a Ziegler-Natta catalyst. A small fraction of less than 15 wt% of ultra high density polyethylene is produced in a third reactor.

[0011] US2009 / 0105422 A1 describes a method for producing multimodal polyethylene using a process. The polymerization is carried out in three cascade reactors, wherein the molecular weight of the polymer in each reactor is controlled by the presence of hydrogen. The hydrogen concentration in each reactor is subsequently reduced by providing the highest hydrogen concentration in the first reactor and the lowest hydrogen concentration in the third reactor.

[0012] WO 2013 / 113797 describes a process for the preparation of polyethylene comprising three main subsequent steps of polymerizing ethylene and at least one other α-olefin in a first reactor, a second reactor and a third reactor in that order to obtain polyethylene having a lower molecular weight ethylene polymer, a first higher molecular weight ethylene polymer and a second higher molecular weight ethylene polymer, respectively.

[0013] Although many processes for preparing multimodal polyethylene are known and have been described, there is still a need to develop new processes for multimodal polymerisation, in particular for further improving the mechanical properties of polyethylene compositions. Summary of the invention

[0014] It is therefore an object of the present invention to provide a reactor system and a process for producing multimodal polyethylene which overcome the disadvantages of the prior art, in particular to enhance the performance of a hydrogen removal unit comprised in such a reactor.

[0015] A further object is to provide a multimodal polyethylene composition which overcomes the disadvantages of the prior art, in particular having improved mechanical properties like the Charpy index.

[0016] It is well known in the art that polyethylene pipes are widely used in various industrial applications, where the fluid may be pressurized or unpressurized. According to ISO 9080, in order to reach the PE125 requirement, a new pressure rating is made for PE. This means that the pipe has a minimum required strength (MRS) of 12.5 MPa for 50 years at 20°C without rupture. In order to improve the hydrostatic pressure resistance from the PE100 standard to the PE125 performance, an increased intercept or a decreased slope of the ductility is required.

[0017] The incorporation of ultra high molecular weight polyethylene (UHMWPE) polymers into polyethylene compositions as copolymers is also known and is disclosed in, for example, WO 2007 / 042216, WO 96 / 18677 and WO 2006 / 092378.

[0018] EP 1201713A1 discloses a material having a density of 0.928 g / cm 3 The high molecular weight polyethylene portion of the linear low density polyethylene and comprising a density of at least 0.969 g / cm 3 A blend of high-density polyethylene and low-molecular-weight polyethylene.

[0019] EP 2743305A1 reports the use of a co-rotating twin screw extruder to blend 10% UHMW polyethylene in 90% bimodal HDPE by extrusion. However, at least 3-4 extrusions are required to disperse the UHMW and HDPE matrices. This is also the main problem of UHMWPE containing HDPE in bimodal polymerization reported in EP1417260 B1. For these reasons, the content of UHMW polyethylene should be considered as a limited loading.

[0020] It is therefore a further object of the present invention to provide a pipe formed from a multimodal polyethylene composition which overcomes the disadvantages of the prior art, in particular having improved mechanical properties and allowing the pipe to withstand a wall stress of 12.5 MPa at 20°C without breaking. It would be desirable to make UHMWPE into a multimodal high density polyethylene matrix at commercially relevant loads without the need for multiple extrusions. Furthermore, an advantage of UHMW incorporation into a multimodal PE pipe is the resistance to sag against the gravity flow of the polymer melt during extrusion of the pipe while maintaining uniform dimensions around the pipe wall.

[0021] These objectives are achieved according to the technical solution of the present invention.

[0022] The object is firstly achieved by a reactor system for a multimodal polyethylene polymerization process, the reactor system comprising:

[0023] (a) a first reactor;

[0024] (b) a hydrogen removal unit arranged between the first reactor and the second reactor, comprising at least one vessel connected to a pressure reducing device preferably selected from a vacuum pump, a compressor, a blower, an ejector or a combination thereof, the pressure reducing device allowing adjustment of the operating pressure to a pressure in the range of 100-200 kPa (absolute pressure);

[0025] (d) a second reactor; and

[0026] (e) The third reactor.

[0027] Preferably, the pressure reduction device allows adjusting the operating pressure in the hydrogen removal unit to a pressure in the range of 103-145 kPa (absolute), preferably 104-130 kPa (absolute), most preferably 105-115 kPa (absolute).

[0028] Preferably, the hydrogen removal unit further comprises a stripping column for separating hydrogen and liquid diluent.

[0029] The object is further achieved by a process for producing a multimodal polyethylene composition in a reactor system according to the present invention, the process comprising (in this order):

[0030] (a) polymerizing ethylene in an inert hydrocarbon medium in a first reactor in the presence of a catalyst system selected from a Ziegler-Natta catalyst or a metallocene and 0.1-95 mol % of hydrogen relative to the total gas present in the gas phase of the first reactor to obtain a low molecular weight polyethylene having a weight average molecular weight (Mw) of 20,000 to 90,000 g / mol and a medium molecular weight polyethylene having a weight average molecular weight (Mw) of greater than 90,000 to 150,000 g / mol, wherein the low molecular weight polyethylene and the medium molecular weight polyethylene each have a density of 0.965 g / cm3, and the low molecular weight polyethylene has an MI2 in the range of 10 to 1,000 g / 10 min, and the medium-low molecular weight polyethylene has an MI2 in the range of 0.1 to 10 g / 10 min;

[0031] (b) removing 98.0 to 99.8 wt% of the hydrogen contained in the slurry mixture obtained from the first reactor in a hydrogen removal unit at a pressure in the range of 103-145 kPa (absolute pressure), and transferring the obtained residual mixture to a second reactor;

[0032] (c) in the presence of a catalyst system selected from Ziegler-Natta catalysts or metallocenes, in the presence of a certain amount of hydrogen obtained in step (b), reacting ethylene and optionally C4 to C 12 α-olefin comonomer is polymerized to obtain a first high molecular weight polyethylene having a weight average molecular weight (Mw) of greater than 150,000 to 1,000,000 g / mol or a first ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of greater than 1,000,000 to 5,000,000 g / mol in the form of a homopolymer or copolymer, and the obtained mixture is transferred to a third reactor; and

[0033] (d) in the presence of a catalyst system selected from a Ziegler-Natta catalyst or a metallocene and hydrogen (wherein the amount of hydrogen in the third reactor is in the range of 0.1-70 mol %, preferably 0.1-60 mol % relative to the total gas present in the gas phase of the third reactor) or alternatively in the substantial absence of hydrogen, ethylene and optionally C4 to C 12 The α-olefin comonomer is polymerized to obtain a second high molecular weight polyethylene having a weight average molecular weight (Mw) of greater than 150,000 to 1,000,000 g / mol or a second ultra-high molecular weight polyethylene homopolymer or copolymer having a weight average molecular weight (Mw) of greater than 1,000,000 to 5,000,000 g / mol;

[0034] "Substantially absent" in this context means that hydrogen is contained in the third reactor in an amount which cannot be avoided by technical means.

[0035] The slurry mixture obtained from the first reactor and subjected to the hydrogen removal step in the hydrogen removal unit contains all the solid and liquid components obtained in the first reactor, in particular the low molecular weight polyethylene or the medium molecular weight polyethylene. In addition, regardless of the amount of hydrogen used in the first reactor, the slurry mixture obtained from the first reactor is saturated with hydrogen.

[0036] Preferably, 98.0 to 99.8 wt%, more preferably 98.0 to 99.5 wt%, most preferably 98.0 to 99.1 wt% of the hydrogen is removed.

[0037] Preferably, the operating pressure of the hydrogen removal unit is in the range of 103-145 kPa (absolute pressure), more preferably 104-130 kPa (absolute pressure), and most preferably 105-115 kPa (absolute pressure).

[0038] Preferably, step (a) produces low molecular weight polyethylene or medium molecular weight polyethylene, step (c) produces high molecular weight polyethylene or ultra high molecular weight polyethylene, and step (d) produces high molecular weight polyethylene or ultra high molecular weight polyethylene.

[0039] The weight average molecular weight (Mw) of the low molecular weight polyethylene, medium molecular weight polyethylene, high molecular weight polyethylene and ultra-high molecular weight polyethylene described herein are respectively in the range of 20,000-90,000 g / mol (low), greater than 90,000-150,000 g / mol (medium), greater than 150,000-1,000,000 g / mol (high) and greater than 1,000,000-5,000,000 g / mol (ultra-high).

[0040] Furthermore, the object is achieved by a multimodal polyethylene composition obtainable by the process of the present invention, the multimodal polyethylene composition comprising:

[0041] (A) 30 to 65 parts by weight, preferably 40 to 65 parts by weight, more preferably 50 to 60 parts by weight of a low molecular weight polyethylene or a medium molecular weight polyethylene;

[0042] (B) 10 to 20 parts by weight, preferably 10 to 15 parts by weight, of a first high molecular weight polyethylene or a first ultra-high molecular weight polyethylene; and

[0043] (C) 25 to 40 parts by weight, preferably 30 to 38 parts by weight, of a second high molecular weight polyethylene or a second ultra high molecular weight polyethylene, wherein the multimodal polyethylene composition has a ratio Mw / Mn of weight average molecular weight Mw to number average molecular weight Mw of 10 to 60, preferably 10 to 40, preferably 20 to 30, wherein Mw and Mn are determined separately by gel permeation chromatography.

[0044] The object is further achieved by a multimodal polyethylene composition comprising

[0045] (A) 40 to 65 parts by weight, preferably 50 to 60 parts by weight of a low molecular weight polyethylene having a weight average molecular weight (Mw) of 20,000 to 90,000 g / mol or a medium molecular weight polyethylene having a weight average molecular weight (Mw) of greater than 90,000 to 150,000 g / mol;

[0046] (B) 10 to 20 parts by weight, preferably 10 to 15 parts by weight, of a first high molecular weight polyethylene having a weight average molecular weight (Mw) of greater than 150,000 to 1,000,000 g / mol or a first ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of greater than 1,000,000 to 5,000,000 g / mol; and

[0047] (C) 25 to 40 parts by weight, preferably 30 to 38 parts by weight, of a second high molecular weight polyethylene copolymer having a weight average molecular weight (Mw) of greater than 150,000 to 1,000,000 g / mol or a second ultra high molecular weight polyethylene having a weight average molecular weight (Mw) of greater than 1,000,000 to 5,000,000 g / mol, wherein the multimodal polyethylene composition has a ratio Mw / Mn of weight average molecular weight Mw to number average molecular weight Mw of 10 to 60, preferably 20 to 30, wherein Mw and Mn are determined separately by gel permeation chromatography, and has a degree of crystallinity of at least 57%, preferably at least 60%, determined by differential scanning calorimetry.

[0048] Preferably, the multimodal polyethylene composition has a degree of crystallinity as determined by differential scanning calorimetry of at least 60 %, preferably from 57 to 70 %, preferably from 60 to 70 %.

[0049] In a preferred embodiment the multimodal polyethylene composition has a weight average molecular weight of 80,000 to 1 ,300,000 g / mol, preferably 200,000 to 400,000 g / mol, preferably 250,000 to 350,000 g / mol, measured by gel permeation chromatography.

[0050] Furthermore, it is preferred that the multimodal polyethylene composition has a number average molecular weight measured by gel permeation chromatography of 5,000 to 30,000 g / mol, preferably 8,000 to 20,000 g / mol, more preferably 10,000 to 15,000 g / mol.

[0051] Preferably the multimodal polyethylene composition has a Z average molecular weight of 800,000 to 6,000,000 g / mol, preferably 800,000 to 3,000,000 g / mol, preferably 1,000,000 to 2,500,000 g / mol, measured by gel permeation chromatography.

[0052] Preferably, the multimodal polyethylene composition has a molecular weight according to ISO 1183 of 0.940 to 0.965 g / cm 3 , preferably 0.945 to 0.965 g / cm 3 , preferably 0.958 to 0.963 g / cm 3 The present invention relates to a polymer having a density of 0.1 to 60 g / 10 min, preferably 0.1 to 0.4 g / 10 min, preferably 0.15 to 0.3 g / 10 min, and / or a melt flow index MI5 of 0.1 to 60 g / 10 min, preferably 0.1 to 0.4 g / 10 min, preferably 0.15 to 0.3 g / 10 min.

[0053] It is further preferred that the multimodal polyethylene composition has a melt flow index MI5 of 0.1 to 0.5 g / 10 min, preferably 0.190 to 0.4 g / 10 min, preferably 0.195 to 0.275 g / 10 min.

[0054] More preferably the multimodal polyethylene composition has an average lamella repetition period comprising a crystalline part and an amorphous part, wherein the crystalline part has an average thickness of at least 7.2 nm and the amorphous part has an average thickness of at least 19 nm.

[0055] In a preferred embodiment, the crystalline portion has an average thickness of 7.2 nm to 9.0 nm, preferably 7.2 nm to 8.5 nm, preferably 7.2 nm to 8.3 nm, preferably 7.23 nm to 8.22 nm; and / or the amorphous portion has an average thickness of 19 nm to 30 nm, preferably 19 nm to 25 nm, preferably 19 nm to 23 nm, preferably 19 nm to 22 nm, preferably 19.55 nm to 21.38 nm.

[0056] Furthermore, the multimodal polyethylene composition preferably has an alpha temperature of at least 60°C, preferably at least 63°C, more preferably from 63 to 70°C, most preferably from 63 to 65°C.

[0057] Preferably, the alpha temperature is 63.46 to 64.46°C.

[0058] The object is further achieved by a layer consisting of the multimodal polyethylene composition of the invention, wherein the layer comprises at least one crystalline part and at least one amorphous part, wherein the thickness of the crystalline part perpendicular to the layer surface is at least 7.2 nm and the thickness of the amorphous part perpendicular to the layer surface is at least 19 nm.

[0059] Most preferably, the layer has an alpha temperature of at least 60°C, preferably at least 63°C, more preferably from 63 to 70°C, most preferably from 63 to 65°C.

[0060] The object is further achieved by a tube, wherein the wall of the tube is a layer according to the invention.

[0061] Finally, the object is achieved by using the pipe of the invention for applications at a pressure of at least 12.5 MPa.

[0062] In a preferred embodiment of the reactor system of the present invention, the process of the present invention and the multimodal polyethylene composition of the present invention "comprising" is "consisting of".

[0063] In a preferred embodiment, "parts by weight" means "weight percentage".

[0064] The embodiments mentioned above as being preferred result in even more improved mechanical properties of the obtained multimodal polyethylene composition. The best results are obtained by combining two or more of the above preferred embodiments. Likewise, the embodiments mentioned above as being more preferred or most preferred result in the best improvement of mechanical properties. DETAILED DESCRIPTION

[0065] Multimodal polyethylene polymers are well known in the art. Multimodal polyethylene systems are typically composed of a high molecular weight (HMW) component and a low molecular weight (LMW) component. The HMW component provides good mechanical properties to the system, while the LMW component provides good processability. By increasing the molecular weight of the HMW component, improved mechanical properties can be obtained. The combination of a specific high density multimodal polyethylene polymer with a specific copolymer ultra-high molecular weight polyethylene component and a copolymer high molecular weight polyethylene portion can produce a polymer that provides a balanced performance that meets the requirements of PE125 and good slow crack growth (SCG) and has good anti-sag properties. Moreover, these components can be uniformly blended with very high molecular weight admixtures.

[0066] The present invention relates to a reactor system for multimodal polyethylene polymerization, which comprises a first reactor, a second reactor, a third reactor and a hydrogen removal unit arranged between the first reactor and the second reactor.

[0067] The hydrogen-poor polyethylene from the first reactor affects the polymerization of high molecular weight in the subsequent reactor. Specifically, the high molecular weight leads to improved mechanical properties of polyethylene, which is advantageous for various product applications, including injection molding, blow molding and extrusion. The catalyst used to produce the multimodal polyethylene resin of the present invention is selected from Ziegler-Natta catalysts, and single-site catalysts including metallocene catalysts and non-metallocene catalysts or chromium-based catalysts can be used, preferably conventional Ziegler-Natta catalysts or single-site catalysts. The catalyst is usually used together with a cocatalyst known in the art.

[0068] Inert hydrocarbon preferably aliphatic hydrocarbon, including hexane, isohexane, heptane, isobutane. Preferably hexane (most preferably n-hexane) is used. Coordination catalyst, ethylene, hydrogen and optional alpha-olefin comonomer are polymerized in the first reactor. All products obtained from the first reactor are then transferred to a hydrogen removal unit to remove 98.0 to 99.8wt% of hydrogen, unreacted gas and some volatiles, which are then fed to a second reactor to continue polymerization. The polyethylene obtained from the second reactor is a bimodal polyethylene, which is a combination of the products obtained from the first reactor and the second reactor. The bimodal polyethylene is then fed to the third reactor to continue polymerization. The final multimodal (trimodal) polyethylene obtained from the third reactor is a mixture of polymers from the first, second and third reactors.

[0069] The polymerization in the first, second and third reactors is carried out under different process conditions. Therefore, the polyethylene obtained in each reactor has different molecular weights. Preferably, low molecular weight polyethylene or medium molecular weight polyethylene is produced in the first reactor, and high molecular weight polyethylene or ultra-high molecular weight polyethylene is produced in the second and third reactors respectively.

[0070] The term first reactor refers to the stage where low molecular weight polyethylene (LMW) or medium molecular weight polyethylene (MMW) is produced. The term second reactor refers to the stage where a first high or ultra-high molecular weight polyethylene (HMW1) is produced. The term third reactor refers to the stage where a second high molecular weight polyethylene or ultra-high molecular weight polyethylene (HMW2) is produced.

[0071] The term LMW refers to the low molecular weight polyethylene polymer polymerized in the first reactor having a weight average molecular weight (Mw) of 20,000-90,000 g / mol.

[0072] The term MMW refers to a medium molecular weight polyethylene polymer polymerized in the first reactor having a weight average molecular weight (Mw) of greater than 90,000 to 150,000 g / mol.

[0073] The term HMW1 refers to a high or ultra-high molecular weight polyethylene polymer polymerized in the second reactor having a weight average molecular weight (Mw) of greater than 150,000 to 5,000,000 g / mol.

[0074] The term HMW2 refers to a high or ultra-high molecular weight polyethylene polymer polymerized in the third reactor having a weight average molecular weight (Mw) of greater than 150,000 to 5,000,000 g / mol.

[0075] LMW or MMW is produced in the first reactor in the absence of comonomer to obtain a homopolymer.

[0076] In order to obtain the improved polyethylene properties of the present invention, ethylene is polymerized in the first reactor in the absence of comonomer to obtain a polyethylene having a molecular weight of ≥ 0.965 g / cm 3The high-density LMW polyethylene or MMW polyethylene with a density of 1000 g / 10min, the MI2 of LMW is in the range of 10-1000 g / 10min and that of MMW is 0.1-10 g / 10min. In order to obtain the target density and MI in the first reactor, the polymerization conditions are controlled and adjusted. The temperature in the first reactor is in the range of 70-90°C, preferably 80-85°C. Hydrogen is fed into the first reactor to control the molecular weight of the polyethylene. The molar ratio of hydrogen to ethylene in the gas phase may vary depending on the target MI. However, the preferred molar ratio is in the range of 0.01-8.0, more preferably 0.01-6.0. The first reactor is operated at a pressure of 250 to 900 kPa, preferably 400 to 850 kPa. The amount of hydrogen present in the gas phase of the first reactor is in the range of 0.1 to 95 mol%, preferably 0.1 to 90 mol%.

[0077] Prior to feeding to the second reactor, the slurry containing LMW or MMW polyethylene preferably in hexane obtained from the first reactor is transferred to a hydrogen removal unit which may have a flash tank connected to a pressure reducing device, the pressure reducing device preferably comprising one or a combination of a vacuum pump, a compressor, a blower and an ejector, wherein the pressure in the flash tank is reduced so that volatiles, unreacted gases and hydrogen are removed from the slurry stream. The operating pressure of the hydrogen removal unit is typically in the range of 103-145 kPa (absolute pressure), preferably 104-130 kPa (absolute pressure), wherein 98.0 to 99.8 wt%, preferably 98.0 to 99.5 wt% and most preferably 98.0 to 99.1 wt% of hydrogen can be removed.

[0078] In the present invention, when 98.0 to 99.8 wt% of hydrogen is removed and the polymerization is carried out under these conditions of hydrogen content, very high molecular weight polymers can be obtained in this way, and Charpy impact strength and flexural modulus are improved. Surprisingly, it was found that when working outside the range of 98.0 to 99.8 wt% of hydrogen removal, the effects of the invention of obtaining very high molecular weight polymers and improving Charpy impact strength and flexural modulus are not observed to the same extent. The effects are more pronounced in the preferred range mentioned.

[0079] The polymerization conditions of the second reactor are significantly different from those of the first reactor. The temperature range in the second reactor is 65-90°C, preferably 68-80°C. The molar ratio of hydrogen to ethylene is not controlled in this reactor because hydrogen is not fed into the second reactor. The hydrogen in the second reactor is the remaining hydrogen of the first reactor, which is retained in the slurry stream after flashing in the hydrogen removal unit. The polymerization pressure range in the second reactor is 100-3000kPa, preferably 150-900kPa, more preferably 150-400kPa.

[0080] Hydrogen removal is based on the comparison of the amount of hydrogen present in the slurry mixture before and after the hydrogen removal unit. The calculation of hydrogen removal is carried out according to the measurement results of the gas composition in the first and second reactors by gas chromatography.

[0081] After a large amount of hydrogen has been removed to reach the concentration of the present invention, the slurry from the hydrogen removal unit is transferred to the second reactor to continue the polymerization. In this reactor, ethylene can be polymerized with or without an α-olefin comonomer in the presence of LMW polyethylene or MMW polyethylene obtained from the first reactor to produce HMW1 polyethylene. The α-olefin comonomers that can be used for copolymerization include C 4-12 , preferably 1-butene and 1-hexene.

[0082] After the polymerization in the second reactor, the obtained slurry is transferred to the third reactor to continue the polymerization.

[0083] In the third reactor, HMW2 is produced by polymerizing ethylene with an optional α-olefin comonomer in the presence of LMW or MMW and HWM1 obtained from the first and second reactors. The α-olefin comonomers that can be used for copolymerization include C 4-12 , preferably 1-butene and 1-hexene.

[0084] In order to obtain the target density and target MI in the third reactor, the polymerization conditions are controlled and adjusted. However, the polymerization conditions in the third reactor are significantly different from those in the first and second reactors. The temperature range in the third reactor is 68 - 90 °C, preferably 68 - 80 °C. Hydrogen is optionally fed into the third reactor to control the molecular weight of the polyethylene. The molar ratio of hydrogen to ethylene can vary depending on the target MI. However, the preferred molar ratio is in the range of 0.01 - 2.0. The polymerization pressure range in the third reactor is 200 - 900 kPa, preferably 250 - 600 kPa, and is controlled by adding an inert gas such as nitrogen.

[0085] The amount of LMW or MMW present in the multimodal polyethylene composition of the present invention is 30 - 65 parts by weight. The amount of HMW1 present in the polyethylene of the present invention is 5 - 40 parts by weight, and the amount of HMW2 present in the polyethylene of the present invention is 10 - 60 parts by weight. Depending on the polymerization conditions employed, it is possible that HMW1 > HMW2 or HMW1 < HMW2.

[0086] The final (free-flowing) multimodal polyethylene composition is obtained by separating hexane from the slurry discharged from the third reactor.

[0087] Definitions and measurement methods

[0088] MI2, MI5, MI 21 a: The melt flow index (MI) of polyethylene is measured according to ASTM D1238 and is expressed in g / 10 min, which determines the fluidity of the polymer under the test conditions at 190° C. using loads of 2.16 kg, 5 kg and 21.6 kg, respectively.

[0089] MI5 b : Melt flow rate (MI) is determined at 190°C according to ISO 1133 and is expressed in g / 10 min. The composite at the melt flow rate is determined and indicated as a subscript, MI5 is measured under a load of 5 kg.

[0090] density a : The density of polyethylene is measured by observing the level at which a pellet sinks in a liquid gradient tube compared to a standard of known density. This method is based on ASTM D 1505 for solid plastics after annealing at 120°C.

[0091] density b : Density is measured according to ISO 1183 and is expressed in g / cm 3 Unit representation.

[0092] Intrinsic Viscosity (IV): The test method covers the determination of the dilute solution viscosity of HDPE at 135°C or ultra high molecular weight polyethylene (UHMWPE) at 150°C. The polymer solutions are prepared by dissolving the polymer in decalin with 0.2% wt / vol stabilizer (Irganox 1010 or equivalent). Details for the determination of IV are given according to ASTM D2515.

[0093] Flexural modulus: Samples were prepared and tested according to ISO 178. Flexural tests were performed using a universal testing machine equipped with a three-point bending fixture.

[0094] Comonomer content: Use 13 C-NMR determines the comonomer content in polyethylene copolymers. The characteristic signals corresponding to the incorporated 1-butene-1 and / or 1-hexene are determined and calculated to obtain the amount of comonomer in the polymer.

[0095] Crystallinity: Crystallinity is commonly used for characterization by differential scanning calorimetry (DSC) according to ASTM D 3418. Samples are identified by peak temperature and enthalpy, and the % crystallinity is calculated from the peak area. Molecular weight and polydispersity index (PDI) The weight average molecular weight (Mw), number average molecular weight (Mn) and Z average molecular weight (Mn) in g / mol were analyzed by gel permeation chromatography (GPC). Z ). Polydispersity Index (PDI) Calculated by Mw / Mn.

[0096] About 8 mg of sample was dissolved in 8 ml of 1,2,4-trichlorobenzene at 160°C for 90 minutes. Then 200 μl of the sample solution was injected into a high temperature GPC with an IR5 infrared detector (Polymer Char, Spain) at 145°C in the column zone and 160°C in the detection zone at a flow rate of 0.5 ml / min. Data were obtained by GPC from Polymer Char, Spain. Software processing.

[0097] Yield Stress: Tensile testing is performed by stretching a specimen and measuring a load frame that follows the specimen dimensions using a universal testing machine. Various tensile properties can be extracted from the stress-strain curve. The yield tensile stress is the point at which increasing strain does not result in an increase in stress.

[0098] Eta747: Viscosity at shear stress 747 Pa was performed on a DHR-3 controlled stress rotational rheometer from TA-instrument. A parallel plate geometry of 25 mm and a measuring gap of 1.8 mm were used. The samples were melted in compression molding at 190° C. for 5 minutes. A creep stress of 747 Pa was applied to the samples at 190° C. under nitrogen. The strain deformation (γ) was monitored as a function of time. Eta747 was calculated from the strain deformation at the equilibrium creep time.

[0099] Thin layer thickness (L): Structural parameters were analyzed by synchrotron X-ray analysis in terms of small angle X-ray scattering, SAXS and wide angle X-ray diffraction, WAXD measurements. The energy of the X-ray radiation was 9 KeV, while the wavelength was 0.1540 nm. All samples (polyethylene powder) were preheated at 170 °C for 5 min and subsequently heated at the same temperature under a pressure of 30 MPa in a 1.0 × 1.0 × 0.5 cm 3 The samples were compressed in an aluminum plate for 4 minutes, after which they were immediately removed at room temperature.

[0100] The layer repetition period is determined by using a plot of the Lorentz-corrected scattered intensity as a function of the scattering vector as follows:

[0101]

[0102] where L is the layer repetition period, and q m is the maximum peak position of the scattering vector.

[0103] Other structural parameters, including crystalline (Lc) and amorphous thickness (La), were calculated from the normalized one-dimensional (1D) correlation function using the following equations:

[0104]

[0105] where <> is designated as the ensemble average. η(z) and <η> are the electron density and the average electron density along the sheet normal, respectively. q is the scattering vector, and I(q) is the intensity at position q. Finally, Lc is estimated from the plot between K(z) and z. The amorphous thickness can be estimated by using the equation: L = Lc + La.

[0106] Dynamic Mechanical Analysis (DMA): DMA measurements were performed using a TA instrument Q800 in tension mode. The samples were preheated at 170°C for 5 minutes and then compressed at 170°C under pressure (30 bar) for 4 minutes. The samples were cut into 0.65×2×0.05 cm 3 The sample was loaded under a force track of 125% and the strain was modulated at a frequency of 40 Hz with an amplitude of 0.03%. The temperature was continuously varied from -70°C to 120°C at a heating rate of 1°C / min. The result obtained from DMA is the α-relaxation process temperature.

[0107] Strain hardening modulus <gp>:The strain hardening modulus can be determined according to ISO 18488:2015 (E). In brief, a pressed sheet is made from the compounded pellets by using compression molding at 180°C. The pressed sheet thickness should be in the range of 0.3±0.05 / -0.03mm. After pressing, the sample is annealed at 120±2°C for 1 hour. Then, the sample is slowly cooled to room temperature at a cooling rate of less than 2°C / min. According to ISO 1872-2, a specimen is punched from the pressed sheet. The measurement is in principle a standard tensile test using a 200N load cell. Prior to the test, the specimen is kept at 80°C for 30min to allow it to thermally equilibrate. The specimen is extended at a constant speed of 20±2mm / min until the true strain reaches 12.

[0108] Strain hardening modulus <gp>Calculated as the mean difference quotient:

[0109] All N difference quotients are averaged:

[0110]

[0111] <gp>The calculation of is usually carried out at a stretching ratio of 8 to 12 and is expressed in units of MPa.

[0112] Accelerated creep test (ACT): The test was performed by HESSEL Ingenieurtechnik GmbH according to PAS1075 standard and the result time is expressed in hours (h).

[0113] Charpy impact strength: Charpy impact strength is measured according to ISO179 at 23°C, 0°C and -30°C and is expressed in kJ / m 2 express.

[0114] Pressure resistance: Pressure resistance was evaluated on 32 mm SDR 11 pipes produced on a CINCINNATI extruder according to ISO 1167. The time to failure was determined in hours.

[0115] Experimental and Examples

[0116] Examples of multimodal polyethylene compositions

[0117] Medium density or high density polyethylene preparation is carried out in three reactors connected in series. Ethylene, hydrogen, hexane, catalyst and TEA (triethylaluminum) cocatalyst are added in the first reactor in the amount shown in Table 1. Commercially available Ziegler-Natta catalyst is used. Catalyst preparation is described in Hungarian patent application No. 0800771R, for example. Polymerization is carried out in the first reactor to prepare low molecular weight polyethylene or medium molecular weight polyethylene. Then the slurry polymer of all polymerizations from the first reactor is transferred to the hydrogen removal unit to remove unreacted gas and some hexanes from the polymer. The operating pressure in the hydrogen removal unit varies in the range of 100 to 115kPa (absolute pressure), wherein before being transferred to the second polymerization reactor, residual hydrogen is removed from hexane greater than 98wt% but not more than 99.8wt%. Some fresh hexanes, ethylene and / or comonomers are added to the second reactor to produce the first high molecular weight polyethylene (HMW1). All polymerized polymers from the second reactor are added to the third reactor, which produces the second high molecular weight polyethylene (HMW2). Ethylene, comonomer, hexane and / or hydrogen are fed to the third reactor.

[0118] Comparative Example 1 (CE1)

[0119] Before this polymer is transferred to the hydrogen removal unit, a homopolymer is generated in the first reactor to obtain a low molecular weight portion. The reaction mixture is introduced into the hydrogen removal unit and unreacted mixture is separated from the polymer. When the hydrogen removal unit is operated under the pressure of 150kPa (absolute pressure), residual hydrogen is removed by 97.6wt%. The low molecular weight polymer is then transferred to the second reactor and the first high molecular weight polymer is produced. Finally, the polymer generated from the second reactor is transferred to the third reactor and the second high molecular weight polymer is produced. In the third reactor, copolymerization is carried out as a comonomer by feeding 1-butene.

[0120] Example 1 (E1)

[0121] Example 1 was carried out in the same manner as Comparative Example 1, except that the hydrogen removal unit was operated at a pressure of 115 kPa (absolute). Residual hydrogen from the first reactor was removed by 98.0 wt%. Characteristic properties of these multimodal polymers are shown in Table 2. It can be seen that an improvement in the stiffness-impact balance is observed when the percentage of residual hydrogen removed is increased, compared to the properties of Comparative Example 1.

[0122] Example 2 (E2)

[0123] Example 2 was conducted in the same manner as Comparative Example 1, except that the hydrogen removal unit was operated at a pressure of 105 kPa (absolute pressure). Residual hydrogen from the first reactor was removed to an extent of 99.1 wt%. Operating the hydrogen removal unit at this pressure resulted in an expansion of the polymer property range. As shown in Table 2, the final melt flow rate of E2 was lower than that of CE1, resulting in an improvement in Charpy impact strength while still maintaining the flexural modulus.

[0124] Comparative Example 2 (CE2)

[0125] Comparative Example 2 was conducted in the same manner as Comparative Example 1, except that the hydrogen removal unit was operated at a pressure of 102 kPa (absolute). Residual hydrogen from the first reactor was removed to an extent of 99.9 wt%. Operating the hydrogen removal unit at this pressure resulted in an expansion of the polymer property range. As shown in Table 2, the final melt flow rate and density of CE2 were very similar to those of E2. CE2 showed a reduction in Charpy impact strength compared to E2.

[0126] Comparative Example 3 (CE3)

[0127] Before the polymer is transferred to the hydrogen removal unit, a homopolymer is produced in the first reactor to obtain a low molecular weight portion. The reaction mixture is introduced into the hydrogen removal unit to separate the unreacted mixture from the polymer. When the hydrogen removal unit is operated at 150kPa (absolute pressure), residual hydrogen is removed to a degree of 97.9wt%. The low molecular weight polymer is then transferred to the second reactor to prepare an ultra-high molecular weight polymer. In the second reactor, copolymerization is carried out by feeding 1-butene as a comonomer. Finally, the in-situ bimodal copolymer from the second reactor is transferred to the third reactor to produce a high molecular weight copolymer portion. The characteristic properties of this multimodal polymer are shown in Table 2. When copolymers are produced in both the second and third reactors, a significant improvement in Charpy impact strength at room temperature can be obtained by reducing the density of the final polymer.

[0128] Example 3 (E3)

[0129] Example 3 was carried out in the same manner as Comparative Example 3, except that the hydrogen removal unit was operated at a pressure of 105 kPa (absolute pressure). Residual hydrogen from the first reactor was removed to an extent of 98.8 wt%. The polymer obtained by this process had a melt flow rate of 0.195 g / 10 min (5 kg load), which was lower than that obtained from CE3. As shown in Table 2, this indicates an improvement in the balance of rigidity-impact when the percentage of residual hydrogen removed is increased, compared with the properties of Comparative Example 3.

[0130] Example 4 (E4)

[0131] Before this polymer is transferred to the hydrogen removal unit, a homopolymer is produced in the first reactor to obtain a medium molecular weight portion. The hydrogen removal device is operated at 105kPa (absolute pressure) to separate the unreacted mixture from the polymer. The residual hydrogen from the first reactor is removed to a degree of 98.9wt%. The medium molecular weight polymer is then transferred to the second reactor to produce a first ultra-high molecular weight polymer. Finally, the produced polymer from the second reactor is transferred to the third reactor to generate a second ultra-high molecular weight polymer. The second and third reactors are operated under hydrogen-poor polyethylene polymerization. The processable in-situ ultra-high molecular weight polyethylene produced by this process leads to an excellent improvement in Charpy impact strength while still maintaining the flexural modulus. It is known that conventional UHMWPE with very high IV cannot measure MI. 21 Inventive Example E4 has an IV of 9 dL / g, showing good melt flow properties exceeding the known art.

[0132] Comparative Example 4 (CE4)

[0133] Before this polymer is transferred to the hydrogen removal unit, a homopolymer is generated in the first reactor to obtain a low molecular weight portion. The reaction mixture is introduced into the hydrogen removal unit to separate the unreacted mixture from the polymer. When the hydrogen removal unit is operated under a pressure of 150kPa (absolute pressure), residual hydrogen is removed by 97.6wt%. The low molecular weight polymer is then transferred to the second reactor to produce the first high molecular weight polymer. Finally, the polymer generated from the second reactor is transferred to the third reactor to generate the second high molecular weight polymer. In the third reactor, copolymerization is carried out by feeding 1-butene as a comonomer. As shown in Tables 2 and 3, the final melt flow rate of CE4 is very similar to the final melt flow rate of E5. Compared with E5, CE4 shows a weakening of Charpy impact strength and flexural modulus, and even shows a lower density than E5.

[0134] Example 5 (E5)

[0135] Example 5 was conducted in the same manner as Comparative Example 4, except that the hydrogen removal unit was operated at a pressure of 115 kPa (absolute pressure). Residual hydrogen from the first reactor was removed to an extent of 98.5 wt%. The polymer obtained by this process had a melt flow rate of 48 g / 10 min (5 kg load), which is lower than that obtained by CE3. As shown in Table 2, the balance of stiffness-impact resistance improved as the percentage of residual hydrogen removed increased compared to the performance of Comparative Example 4.

[0136] Example 6 (E6)

[0137] Example 6 was conducted in the same manner as Example 4 except for the comonomer feed in the third ultra-high molecular weight polyethylene. The polymer produced by this process results in an excellent improvement in Charpy impact strength while still maintaining the flexural modulus. As shown in Table 2, Inventive Example 6 has an IV of 23 dl / g, showing high impact strength (no breakage in a single notched impact) and flexural modulus compared to the control sample, however, due to the high viscosity and high Mw, the melt flow index was not measurable.

[0138]

[0139]

[0140]

[0141] Tube-related embodiments

[0142] The multimodal PE product obtained from the third reactor was dried and the resulting powder was sent to a finishing operation where it was compounded with 2-2.5 wt% carbon black along with 2000 ppm Ca / Zn stearate and 3000 ppm hindered phenol / phosphate stabilizer in an extruder at 210°C under nitrogen atmosphere and subsequently pelletized. The pelletized resin was used to obtain density and MI.

[0143] Plastic pipes are produced by extruding molten polymer through an annular die. The pipe is formed by passing the molten extrudate through a calibrating sleeve and then to a cooling tank where water is sprayed on the outer surface. Solidification occurs radially inwards from the outer surface.

[0144] Inventive Example 7 (E7), Inventive Example 8 (E8) and Comparative Example 5 (CE5) were synthesized by multimodal technology using 1-hexene as comonomer according to the above method. Comparative Example 5 was made by bimodal technology with LMW and HMW and 1-hexene comonomers. Comparative Example CE6 was selected from Example 7 of EP 1417260B1.

[0145] The polymerization conditions and polymer properties are shown in Tables 3 to 4, respectively. Test results and analyses were performed on the compounds and recorded.

[0146] Table 3. Polymerization conditions for the inventive examples and comparative examples.

[0147]

[0148]

[0149] E7, E8 and E9 show higher pressure resistance at 20°C than the comparative examples at various hoop stresses of 12.0 to 13.2 MPa. Even though E7, E8, E9 and CE5 are produced by the process of the present invention, there is a specific multimodal polyethylene composition that enhances the excellent performance of pressure resistance. The thicker layer repetition period (L), amorphous thickness (La) and crystal thickness (Lc) of E7, E8 and E9 ensure the rigidity of the crystal structure, resulting in excellent pressure resistance. The obtained results support the influence of the ultra-high molecular weight part with appropriate 1-hexene comonomer content and distribution on the polymer properties. According to the DMA results, higher alpha temperatures are also observed in E7, E8 and E9 than in CE6. The higher alpha temperature means that the crystalline part of the polymer is stronger.

[0150] Table 4. Polymer properties of inventive and comparative examples.

[0151]

[0152]

[0153] The features disclosed in the preceding description and in the claims may, alone or in any combination, be material for realizing the invention in its various forms.< / gp> < / gp> < / gp>

Claims

1. A multimodal polyethylene composition obtainable by a process for producing a multimodal polyethylene composition in a reactor system for a multimodal polyethylene polymerization process, said reactor system include: (a) a first reactor; (b) a hydrogen removal unit arranged between the first reactor and the second reactor, comprising at least one vessel connected to a pressure reducing device preferably selected from a vacuum pump, a compressor, a blower, an ejector or a combination thereof, the pressure reducing device allowing the operating pressure to be adjusted to a pressure in the range of 100-200 kPa (absolute pressure); (c) the second reactor; and (d) a third reactor; The method comprises: (i) polymerizing ethylene in an inert hydrocarbon medium in the first reactor in the presence of a catalyst system selected from a Ziegler-Natta catalyst or a metallocene and hydrogen in an amount of 0.1 to 95 mol % relative to the total gas present in the gas phase of the first reactor to obtain a ethylene having a weight average molecular weight in the range of 20,000 to 90,000 g / mol, having a molecular weight of ≥ 0.965 g / cm 3 A low molecular weight polyethylene having a density of 10 to 1,000 g / 10 min and a MI2 in the range of 10 to 1,000 g / 10 min, or a polyethylene having a weight average molecular weight in the range of greater than 90,000 to 150,000 g / mol and having a molecular weight of ≥0.965 g / cm 3 A medium molecular weight polyethylene having a density of and a MI2 in the range of 0.1 to 10 g / 10 min; (ii) removing 98.0 to 99.8 wt% of the hydrogen contained in the slurry mixture obtained from the first reactor in the hydrogen removal unit at a pressure in the range of 103-145 kPa (absolute pressure), and transferring the obtained residual mixture to the second reactor; (iii) reacting ethylene and optionally C in the second reactor in the presence of a catalyst system selected from a Ziegler-Natta catalyst or a metallocene and in the presence of hydrogen in the amount obtained in step (b); 4 -C 12 α-olefin comonomer polymerization to obtain a first high molecular weight polyethylene having a weight average molecular weight in the range of greater than 150,000 to 1,000,000 g / mol or a first ultra-high molecular weight polyethylene in the form of a homopolymer or copolymer having a weight average molecular weight in the range of greater than 1,000,000 to 5,000,000 g / mol, and transferring the resulting mixture to the third reactor; and (iv) in the presence of a catalyst system selected from a Ziegler-Natta catalyst or a metallocene and hydrogen, wherein the amount of hydrogen in the third reactor is in the range of 0.1 to 70 mol%, preferably 0.1 to 60 mol%, relative to the total gas present in the gas phase in the third reactor, or alternatively in the substantial absence of hydrogen, ethylene and optionally C 4 -C 12 a-olefin comonomer is polymerized to obtain a second high molecular weight polyethylene having a weight average molecular weight in the range of greater than 150,000 to 1,000,000 g / mol or a second ultra-high molecular weight polyethylene homopolymer or copolymer having a weight average molecular weight in the range of greater than 1,000,000 to 5,000,000 g / mol, the multimodal polyethylene composition comprising: (A) 40 to 65 wt%, preferably 50 to 60 wt% of the low molecular weight polyethylene or the medium molecular weight polyethylene; (B) 10 to 20 wt%, preferably 10 to 15 wt% of the first high molecular weight polyethylene or the first ultra-high molecular weight polyethylene; and (C) 25 to 40 wt%, preferably 30 to 38 wt%, of the second high molecular weight polyethylene copolymer or the second ultra high molecular weight polyethylene, wherein the multimodal polyethylene composition has a ratio Mw / Mn of weight average molecular weight Mw to number average molecular weight Mn, wherein Mw and Mn are determined separately by gel permeation chromatography, of from 10 to 40, preferably 20 to 30, Wherein the multimodal polyethylene composition has a crystallinity of at least 60%, preferably from 60 to 70%, as determined by differential scanning calorimetry.

2. The multimodal polyethylene composition according to claim 1, in, The pressure reducing device allows adjusting the operating pressure in the hydrogen removal unit to a pressure in the range of 103-145 kPa (absolute pressure), preferably 104-130 kPa (absolute pressure), most preferably 105-115 kPa (absolute pressure).

3. The multimodal polyethylene composition according to claim 1 or 2, in, The hydrogen removal unit further comprises a stripping column for separating hydrogen and liquid diluent.

4. A multimodal polyethylene composition according to any one of the preceding claims, in, The removal is removal of 98.0 to 99.8 wt%, more preferably 98.0 to 99.5 wt%, and most preferably 98.0 to 99.1 wt% of the hydrogen.

5. A multimodal polyethylene composition according to any one of the preceding claims, in, The operating pressure in the hydrogen removal unit is in the range of 103-145 kPa (absolute pressure), more preferably 104-130 kPa (absolute pressure), and most preferably 105 to 115 kPa (absolute pressure).

6. A multimodal polyethylene composition according to any one of the preceding claims, in, The multimodal polyethylene composition has a weight average molecular weight of 200,000 to 400,000 g / mol, preferably 250,000 to 350,000 g / mol, determined by gel permeation chromatography.

7. A multimodal polyethylene composition according to any one of the preceding claims, in, The multimodal polyethylene composition has a number average molecular weight of 8,000 to 20,000 g / mol, preferably 10,000 to 15,000 g / mol, measured by gel permeation chromatography.

8. A multimodal polyethylene composition according to any one of the preceding claims, in, The multimodal polyethylene composition has a Z average molecular weight of 800,000 to 3,000,000 g / mol, preferably 1,000,000 to 2,500,000 g / mol, as measured by gel permeation chromatography.

9. A multimodal polyethylene composition according to any one of the preceding claims, in, The multimodal polyethylene composition has a relative humidity according to ISO 1183 of 0.945 to 0.965 g / cm 3 , preferably 0.958 to 0.963 g / cm 3 Density, and / or melt flow index MI of 0.1 to 0.4 g / 10 min 5 .

10. A multimodal polyethylene composition according to any one of the preceding claims, in, The multimodal polyethylene composition has an average lamellar repeat period comprising a crystalline part and an amorphous part, wherein the crystalline part has an average thickness of at least 7.2 nm and the amorphous part has an average thickness of at least 19 nm.

11. A multimodal polyethylene composition according to any one of the preceding claims, in, The multimodal polyethylene composition has an alpha temperature of at least 60°C, preferably at least 63°C, more preferably from 63 to 70°C, most preferably from 63 to 65°C.

12. Use of a pipe comprising a multimodal polyethylene composition according to any one of the preceding claims for applications at a pressure of at least 12.5 MPa.

Images