Multimodal polyethylene tube

Abstract

The invention relates to a multimodal polyethylene tube. A reactor system for a multimodal polyethylene polymerization process, comprising: (a) a first reactor; (b) a hydrogen removal unit disposed between the first reactor and the second reactor, comprising at least one vessel connected to a reduced pressure device, preferably selected from the group consisting of a vacuum pump, a compressor, a blower, an ejector or a combination thereof, the reduced pressure 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 and its use as a tube.

Description

[0001] This application is a divisional application of the invention patent application with application date of September 7, 2017, application number 201780004083.3 and invention name “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, including a pipe comprising said multimodal polyethylene composition and the use thereof. Background Art

[0003] The demand for polyethylene resins is increasing for a variety of applications. Due to the high performance of polyethylene required for relatively new plastic products, 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, developments in multimodal polymerization processes have 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 to control the molecular weight of the polymer, suitable for providing good processability of the final polymer. The high molecular weight fraction has an impact on the physical properties and is produced under polymerization conditions with low hydrogen concentration. It is well known in the art that the low molecular weight polymer is preferably produced in the first reactor. In order to obtain a multimodal polymer with good physical properties, all of the hydrogen from the first reactor should be removed before the polymerized slurry polymer is delivered to the second reactor where the high molecular weight polymer is produced.

[0005] US 2010 / 0092709 A1 describes a process for preparing bimodal polyethylene copolymers. Polymerization is carried out in a second reactor at a low comonomer-to-ethylene ratio and a low hydrogen-to-ethylene ratio at high temperature to obtain a resin with improved stress crack resistance and melt strength.

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

[0007] US 6,291,601 B1 describes a process for producing bimodal copolymers with relatively high molecular weight polyethylene. A hydrogenation catalyst is introduced into the second reactor to consume the 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 cost of both hydrogen and ethylene raw material consumption is increased due to the conversion of unreacted gas.

[0008] US 2003 / 0191251 A1 discloses a method for removing residual hydrogen from a polymer slurry by using two flash drums placed between cascade reactors 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 the slurry transfer pump from clogging. In addition, the make-up solvent must be heated before the slurry is transferred to the next flash drum.

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

[0010] WO 2013 / 144328 describes a multimodal high density polyethylene composition for use in moulding applications produced using a Ziegler-Natta catalyst. A small fraction of very high polyethylene, less than 15% by weight, is produced in the third reactor.

[0011] US 2009 / 0105422 A1 describes a method for producing multimodal polyethylene. 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 concentration of hydrogen 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 the following three main steps: polymerizing ethylene and at least one other α-olefin to obtain a low molecular weight ethylene polymer, a first high molecular weight ethylene polymer and a second high molecular weight ethylene polymer, respectively, according to the order of a first reactor, a second reactor and a third reactor.

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

[0014] It is therefore an object of the present invention to provide a reactor system and a process for the preparation of multimodal polyethylene which overcomes the disadvantages of the prior art and, in particular, improves the performance of a hydrogen removal unit comprised in the 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 Charpy impact strength.

[0016] A variety of polyethylene-based pipes are well known in the art. The main end-use performance requirement of polyethylene pipes is long-term hydrostatic strength, which determines the pressure rating, slow crack growth (SCG) resistance and rapid crack growth (RCP) resistance. For polymer properties, the higher the polyethylene density, the higher the long-term hydrostatic strength. In order to improve the stress crack resistance of polyethylene pipes, attention should be paid to increasing the molecular weight of the polymer or reducing the density of the polymer. When the density is reduced, the rigidity of the polyethylene will be disadvantageously deteriorated.

[0017] Developing polyethylene for producing pipes having a balance of properties, such as improved SCG without adversely affecting RCP and pressure resistance and processability due to these opposing parameters is quite difficult.

[0018] WO 2013 / 113797 A1 discloses multimodal polyethylene polymers and a process for preparing the same.

[0019] EP 2 354 184 A1 discloses polyethylene molding compositions having a multimodal molecular weight distribution and addressing stress cracking / hardness, relationships and impact resistance.

[0020] WO 2009 / 147022 A1 relates to a high density polymer composition, a process for its preparation and a pipe prepared therefrom. The composition comprises a multimodal copolymer of ethylene and an alpha-olefin.

[0021] EP 2 017 302 A1 relates to moulded articles comprising a high density polyethylene copolymer comprising a multimodal high density ethylene-hexene copolymer and at least one low molecular weight polyethylene homopolymer component.

[0022] WO 2007 / 045415 A1 relates to a polyethylene composition comprising at least a first composition and a second composition, wherein the composition comprises a low molecular weight polyethylene and a high molecular weight polyethylene, both of which may be selected as homopolymers or copolymers.

[0023] WO 2008 / 131817 A1 relates to pipes or related articles comprising a multimodal polyethylene composition.

[0024] JP 2012-067914 A relates to polymer pipes formed by multimodal polymer compositions and to improved stress resistance. Summary of the invention

[0025] In light of the prior art, the object of the present invention is to provide a polyethylene pipe that overcomes the disadvantages of the prior art and, in particular, has an impact resistance sufficient to withstand the impacts during installation and long-term use characteristics, including excellent long-term durability under gas or water pressure as determined by slow crack growth (SCG) resistance.

[0026] This object is achieved according to the invention according to the subject matter of the independent claims. Preferred embodiments result from the dependent claims.

[0027] According to the invention, this aim of the art is achieved according to the subject matter of the independent claims. Preferred embodiments result from the dependent claims.

[0028] Firstly, the objectives are achieved by a reactor system for a multimodal polyethylene polymerization process, the reactor system comprising:

[0029] (a) a first reactor;

[0030] (b) a hydrogen removal unit disposed 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 and 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);

[0031] (c) a second reactor; and

[0032] (d) The third reactor.

[0033] 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 to 115 kPa (absolute).

[0034] Preferably, the hydrogen removal unit further comprises a stripping column for separating hydrogen and liquid diluent.

[0035] In the reactor system of the present invention, the object is also achieved by a process for producing a multimodal polyethylene composition comprising (in this order):

[0036] (a) in a first reactor, in the presence of a catalyst system selected from Ziegler-Natta catalysts or metallocenes and hydrogen in an amount of 0.1-95% by mole relative to the total gas present in the gas phase in the first reactor, polymerizing ethylene in an inert hydrocarbon medium to obtain 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 more 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 at least 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 molecular weight polyethylene has an MI2 in the range of 0.1 to 10 g / 10 min;

[0037] (b) removing 98.0 to 99.8% by weight of hydrogen from the slurry mixture obtained from the first reactor in a hydrogen removal unit at a pressure in the range of 103-145 kPa (absolute) and transferring the residual mixture obtained to a second reactor;

[0038] (c) in a second reactor, polymerizing ethylene and optionally a C4 to C12 alpha-olefin comonomer 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) 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 a copolymer, and transferring the resulting mixture to a third reactor; and

[0039] (d) in a third reactor, 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% by mole, preferably 0.1-60% by mole relative to the total gas present in the gas phase in the third reactor) or optionally in the substantial absence of hydrogen, polymerizing ethylene and optionally a C4 to C12 α-olefin comonomer 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.

[0040] In this context, "substantially absent" means that hydrogen is included in the third reactor only in an amount which is unavoidable by technical means.

[0041] 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, specifically, low molecular weight polyethylene or 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 hydrogen saturated.

[0042] Preferably, the removal is removal of 98.0 to 99.8% by weight of hydrogen, and more preferably 98.0 to 99.5% by weight, most preferably 98.0 to 99.1% by weight.

[0043] Preferably, the operating pressure of the hydrogen removal unit is in the range of 103-145 kPa (absolute), and more preferably 104-130 kPa (absolute), most preferably 105 to 115 kPa (absolute).

[0044] 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.

[0045] 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 in this article are 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), respectively.

[0046] Furthermore, the object is achieved by a multimodal polyethylene composition obtainable by a process according to any of claims 4 to 6, the composition comprising:

[0047] (A) 30 to 65, preferably 51 to 58 parts by weight of a low molecular weight polyethylene having a weight average molecular weight (Mw) of 20,000 to 90,000 g / mol;

[0048] (B) 5 to 40, preferably 12 to 21 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

[0049] (C) 10 to 60, preferably 27 to 33 parts by weight of 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 having a weight average molecular weight (Mw) of greater than 1,000,000 to 5,000,000 g / mol. The object is also achieved by a multimodal polyethylene composition comprising:

[0050] (A) 51 to 58 parts by weight of low molecular weight polyethylene or medium molecular weight polyethylene;

[0051] (B) 12 to 21 parts by weight of a first high molecular weight polyethylene or a first ultra-high molecular weight polyethylene; and

[0052] (C) 27 to 33 parts by weight of a second high molecular weight polyethylene or a second ultra-high molecular weight polyethylene copolymer.

[0053] Preferably, the ratio Mw / Mn of the weight average molecular weight Mw of the multimodal polyethylene composition to the number average molecular weight Mn of the multimodal polyethylene composition is less than 10-60, preferably 10-23, more preferably 15-22.

[0054] Preferably, the ratio is from 17 to 22, preferably from 18 to 22, preferably from 19 to 22, preferably from 19.59 to 21.24.

[0055] In a preferred embodiment the multimodal polyethylene composition has a weight average molecular weight as measured by gel permeation chromatography of 80,000 to 1,300,000 g / mol, preferably 200,000 to 500,000 g / mol, more preferably 200,000 to 300,000 g / mol.

[0056] Furthermore, preferably the multimodal polyethylene composition has a number average molecular weight as measured by gel permeation chromatography of 5,000 to 30,000 g / mol, preferably 10,000 to 25,000 g / mol, more preferably 10,000 to 15,000 g / mol.

[0057] Preferably, the multimodal polyethylene composition has a Z average molecular weight as measured by gel permeation chromatography of 1 ,000,000 to 6,000,000 g / mol, preferably 1 ,500,000 to 4,000,000 g / mol, more preferably 1 ,500,000 to 3,000,000 g / mol.

[0058] Preferably, the multimodal polyethylene composition has a molecular weight according to ISO 1183 of 0.945 to 0.965 g / cm 3, preferably a density of 0.958 to 0.963 g / cm3, 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.

[0059] Preferably, the melt flow index MI5 is from 0.15 to 0.3 g / 10 min, preferably from 0.20 to 0.25 g / 10 min, preferably from 0.20 to 0.23 g / 10 min, preferably from 0.21 to 0.22 g / 10 min.

[0060] Preferably, the total comonomer content is 0.5 to 5% by mol, preferably 1 to 3% by mol, relative to the total amount of monomer units included in the low molecular weight polyethylene, ultra high molecular weight polyethylene and high molecular weight polyethylene.

[0061] The object is also achieved by a pipe comprising the multimodal polyethylene composition of the invention.

[0062] Finally, the object is achieved by using the tube of the invention applied at a pressure of at least 10.0 MPa or at atmospheric pressure.

[0063] In a preferred embodiment of the inventive reactor system, the inventive process, the inventive multimodal polyethylene composition, the inventive pipe and the inventive use, "comprising" is "consisting of".

[0064] In a preferred embodiment, "parts by weight" means "percent by weight".

[0065] The above mentioned embodiments preferably result in more improved mechanical properties of the obtained multimodal polyethylene composition. The best results are achieved by combining two or more of the above preferred embodiments. Likewise, the above mentioned embodiments more preferably or most preferably result in the best improvement of mechanical properties.

[0066] Surprisingly, the inventors have found that the above process, the above multimodal polyethylene composition and the pipes produced therefrom solve the above objectives. The inventors have surprisingly found that controlling the comonomer distribution and the crystal structure of the polymer by controlling the content of hydrogen and comonomer in the second reactor improves the impact resistance and high slow crack growth (SCG) of the polyethylene pipe produced from the composition of the invention.

[0067] The present invention relates to a reactor system for multimodal polyethylene polymerization. The system comprises a first reactor, a second reactor, a third reactor and a hydrogen removal unit located between the first reactor and the second reactor.

[0068] The dehydrogenated 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 a variety of 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, single-site catalysts (including metallocene-base catalysts and non-metallocene-base catalysts) or chromium-based catalysts that can be used, preferably, conventional Ziegler-Natta catalysts or single-site catalysts. The catalyst is usually used with a cocatalyst well known in the art.

[0069] Inert hydrocarbon is preferably aliphatic hydrocarbon, which includes hexane, isohexane, heptane, isobutane. Preferably, hexane (most preferably, n-hexane) is used. In the first reactor, a coordination catalyst, ethylene, hydrogen and optionally alpha-olefin comonomer are polymerized. Then, before feeding to the second reactor to continue polymerization, all products from the first reactor are transferred to a hydrogen removal unit to remove 98.0 to 99.8% by weight of hydrogen, unreacted gas and some volatile substances. The polyethylene obtained from the second reactor is a bimodal polyethylene, which is a combination of the product obtained from the first reactor and the product obtained from the second reactor. Then, the bimodal polyethylene is 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.

[0070] Under different process conditions, polymerization is carried out in the first, second and third reactors. 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, while high molecular weight polyethylene or ultra-high molecular weight polyethylene is produced in the second and third reactors, respectively.

[0071] 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 first high or ultra-high molecular weight polyethylene (HMW1) is produced. The term third reactor refers to the stage where second high molecular weight polyethylene or ultra-high molecular weight polyethylene (HMW2) is produced.

[0072] The term LMW refers to a low molecular weight polyethylene polymer having a weight average molecular weight (Mw) of 20,000-90,000 g / mol polymerized in the first reactor.

[0073] 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.

[0074] The term HMW1 refers to a high or ultra-high molecular weight polyethylene polymer having a weight average molecular weight (Mw) of greater than 150,000 to 5,000,000 g / mol polymerized in the second reactor.

[0075] 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.

[0076] In the absence of comonomer, LMW or MMW is produced in the first reactor to obtain a homopolymer.

[0077] 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 density > 0.965 g / cm 3 And for LMW, MI2 is in the range of 5-1000 g / 10min, for MMW in the range of 1-10 g / 10min high density LMW polyethylene or MMW polyethylene. 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 70-90ºC, preferably 80-85ºC. Hydrogen is fed to the first reactor to control the molecular weight of the polyethylene. Depending on the target MI, the molar ratio of hydrogen to ethylene in the gas phase can be changed. However, the preferred molar ratio is in the range of 0.01-8.0, more preferably in the range of 0.01-6.0. The first reactor is operated at a pressure between 250 and 900 kPa, preferably 400-850 kPa. The amount of hydrogen present in the gas phase of the first reactor is in the range of 0.1-95% by mole, preferably in the range of 0.1-90% by mole.

[0078] 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 reduction 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, thereby removing volatile materials, unreacted gases and hydrogen from the slurry stream. The operating pressure of the hydrogen removal unit is generally in the range of 103-145 kPa (absolute pressure), preferably 104-130 kPa (absolute pressure), wherein 98.0 to 99.8% by weight of hydrogen, preferably 98.0 to 99.5% by weight of hydrogen, and most preferably 98.0 to 99.1% by weight of hydrogen can be removed.

[0079] In the present invention, when 98.0 to 99.8% by weight of hydrogen is removed and polymerization is performed under these hydrogen content conditions, ultra-high molecular weight polymers can be achieved by this method and the simple supported beam impact strength and flexural modulus are improved. It was unexpectedly found that when operating outside the hydrogen removal range of 98.0 to 99.8% by weight, the same degree of the inventive effect of obtaining ultra-high molecular weight polymers and improving the simple supported beam impact and flexural modulus cannot be observed. In the preferred range mentioned, this effect is more significant.

[0080] The polymerization conditions of the second reactor are significantly different from those of the first reactor. The temperature in the second reactor is in the range of 68-90°C, preferably 68-80°C. Since hydrogen is not fed to the second reactor, the molar ratio of hydrogen to ethylene is not controlled in this reactor. The hydrogen in the second reactor is the hydrogen left from the first reactor that is retained in the slurry stream after flashing in the hydrogen removal unit. The polymerization pressure in the second reactor is in the range of 100-3000 kPa, preferably 150-900 kPa, more preferably 150-400 kPa.

[0081] The removal of hydrogen is a comparison of the amount of hydrogen present in the slurry mixture before and after passing through the hydrogen removal unit.The calculation of hydrogen removal is performed based on the measurement of the gas composition of the first and second reactors by gas chromatography.

[0082] After removing a large amount of hydrogen to achieve the concentration of the present invention, the slurry from the hydrogen removal unit is transferred to a second reactor to continue polymerization. In this reactor, ethylene can be polymerized with or without α-olefin comonomers to form HMW1 polyethylene in the presence of LMW polyethylene or MMW polyethylene from the first reactor. The α-olefin comonomers used for copolymerization include C 4-12 , preferably 1-butene and 1-hexene.

[0083] After polymerization in the second reactor, the resulting slurry was transferred to a third reactor to continue polymerization.

[0084] HMW2 is produced in the third reactor by polymerizing ethylene and optionally an α-olefin comonomer in the presence of LMW or MMW and HWM1 from the first and second reactors. The α-olefin comonomers used for copolymerization include C 4-12 , preferably 1-butene and 1-hexene.

[0085] 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 of the third reactor are significantly different from those of the first and second reactors. The temperature in the third reactor is in the range of 68-90°C, preferably 68-80°C. Hydrogen is optionally fed to the third reactor to control the molecular weight of the polyethylene. Depending on the target MI, the molar ratio of hydrogen to ethylene can be changed. However, the preferred molar ratio is in the range of 0.01-2.0. The polymerization pressure in the third reactor is in the range of 250-900 kPa, preferably 250-600 kPa, and is controlled by adding an inert gas such as nitrogen.

[0086] 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 used, it may be that HMW1>HMW2 or HMW1> <HMW2。

[0087] By separating the hexane from the slurry released from the third reactor, a final (free-flowing) multimodal polyethylene composition is obtained.

[0088] The resulting polyethylene powder is then mixed with an antioxidant and optionally additives before being extruded and pelletized into pellets. DETAILED DESCRIPTION

[0089] Definition and measurement methods

[0090] MI2, MI5, MI 21 a : The melt flow index (MI) of polyethylene is measured according to ASTM D 1238 and expressed in g / 10min, which determines the flowability of the polymer under test conditions of 190ºC with loads of 2.16 kg, 5 kg and 21.6 kg.

[0091] MI5 b : Melt flow index (MI) is determined at 190°C according to ISO 1133 and is expressed in g / 10min. The load at which the melt flow rate is determined is expressed as a subscript, MI5 is measured at a load of 5 kg.

[0092] density a : The density of polyethylene is measured by observing the level at which a particle sinks into a liquid gradient tube compared to a standard of known density. This method is determined in accordance with ASTM D 1505 for solid plastic articles after annealing at 120ºC.

[0093] density b : Density is measured according to ISO 1183 and is expressed in g / cm 3 It is expressed as a unit.

[0094] Molecular weight and polydispersity index (PDI) : Weight average molecular weight (Mw), number average molecular weight (Mn) and Z average molecular weight (Mz) were analyzed by gel permeation chromatography (GPC) and expressed in g / mol. The polydispersity index was calculated by Mw / Mn. About 8 mg of sample was dissolved in 8 ml of 1,2,4-trichlorobenzene at 160ºC 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) using a flow rate of 0.5 ml / min, a column zone temperature of 145ºC and a detector zone temperature of 160ºC. The data were processed by GPC One® software, Polymer Char, Spain.

[0095] Intrinsic viscosity (IV)

[0096] This test method covers the determination of dilute solution viscosity at 135ºC for HDPE or at 150ºC for ultra-high molecular weight polyethylene (UHMWPE). The polymer solution is prepared by dissolving the polymer in decalin with 0.2% wt / vol stabilizer (Irganox 1010 or equivalent). Detailed information is provided for the determination of IV in accordance with ASTM D2515.

[0097] 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.

[0098] Comonomer content : Use quantitative 13 C-NMR determines the comonomer content of polyethylene copolymers. Characteristic signals corresponding to the incorporation of 1-butene and / or 1-hexene are identified and calculated to obtain the amount of comonomer in the polymer.

[0099] Simply supported beam impact strength :Charpy impact strength is determined at 23ºC, 0ºC and -30ºC according to ISO179 and is expressed in kJ / m 2 It is expressed as a unit.

[0100] 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.

[0101] Eta747: The viscosity at a shear stress of 747 Pa was measured 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 an extrusion mold at 190 ºC for 5 minutes. A creep stress of 747 Pa was applied to the sample at 190 ºC under nitrogen. The stress strain (γ) was monitored as a function of time. Eta747 was calculated from the stress strain at the equilibrium creep time.

[0102] Accelerated Creep Test (ACT) : Tested in accordance with PAS1075 by HESSEL Ingenieurtechnik GmbH and the result time is expressed in hours (h).

[0103] Experimental and Examples

[0104] The preparation of medium or high density polyethylene is carried out continuously in three reactors. Ethylene, hydrogen, hexane, catalyst and TEA (triethylaluminum) cocatalyst are fed to the first reactor in the amounts shown in Table 1. Commercially available Ziegler-Natta catalysts are used. The catalyst preparation is described, for example, in Hungarian patent application 08 00771r. Polymerization is carried out in the first reactor to prepare low molecular weight polyethylene or medium molecular weight polyethylene. Then, all polymerized slurry polymers are transferred from the first reactor to a hydrogen removal unit to remove unreacted gas and some hexane from the polymer. The operating pressure in the hydrogen removal unit is changed in the range of 100 to 115 kPa (absolute pressure), wherein before being transferred to the second polymerization reactor, residual hydrogen greater than 98% by weight but not greater than 99.8% by weight is removed from the hexane. Some fresh hexane, ethylene and / or comonomers are fed to the second reactor to produce the first high molecular weight polyethylene (HMW1). All polymerized polymers from the second reactor are fed 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.

[0105] Comparative Example 1 (CE1)

[0106] Before transferring the polymer 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 a pressure of 150 kPa (absolute pressure), 97.6% of the residual hydrogen by weight is removed. Then, the low molecular weight polymer is transferred to the second reactor to produce a first high molecular weight polymer. Finally, the produced polymer is transferred from the second reactor to the third reactor to produce a second high molecular weight polymer. In the third, copolymerization is carried out by feeding 1-butene as a comonomer.

[0107] Example 1 (E1)

[0108] 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). 98.0% by weight of the residual hydrogen from the first reactor was removed. Table 2 shows the characteristic properties of these multimodal polymers. It can be seen that an improvement in the hardness-impact balance is observed when the percentage of hydrogen residual removed is increased when compared to the properties of Comparative Example 1.

[0109] Example 2 (E2)

[0110] Example 2 was carried out in the same manner as Comparative Example 1, except that the hydrogen removal unit was operated at a pressure of 105 kPa (absolute). Residual hydrogen was removed from the first reactor to an extent of 99.1% by weight. Operation of the hydrogen removal unit at this pressure resulted in an expanded range of polymer properties. As shown in Table 2, the final melt flow rate of E2 was lower than that of CE1, which resulted in an improvement in Charpy impact strength while still maintaining flexural modulus.

[0111] Comparative Example 2 (CE2)

[0112] 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 was removed from the first reactor to an extent of 99.9% by weight. Operation of the hydrogen removal unit at this pressure resulted in an expanded range of polymer properties. As shown in Table 2, the final melt flow rate and density of CE2 were very similar to those of E2. A reduction in Charpy impact strength was shown in CE2 compared to E2.

[0113] Comparative Example 3 (CE3)

[0114] Before transferring the polymer 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 a pressure of 150 kPa (absolute pressure), the hydrogen is removed to a degree of 97.9% by weight. Then, the low molecular weight polymer is transferred to the second reactor to produce 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 is transferred from the second reactor to the third reactor to produce a high molecular weight copolymer portion. Table 2 shows the characteristic properties of the multimodal polymer. When the copolymer is produced in both the second and third reactors, a significant improvement in the simply supported beam impact strength can be obtained at room temperature by reducing the density of the final polymer.

[0115] Example 3 (E3)

[0116] 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 was removed from the first reactor to an extent of 98.8% by weight. The polymer obtained by operating this method had a melt flow rate (5 kg load) of 0.195 g / 10 min, which was less than that obtained from CE3. As shown in Table 2, it is shown that when the percentage of hydrogen residue removed is increased compared to the properties of Comparative Example 3, the hardness-impact balance improves.

[0117] Example 4 (E4)

[0118] A homopolymer is produced in the first reactor to obtain a medium molecular weight portion before the polymer is transferred to a hydrogen removal unit. The hydrogen removal unit is operated at a pressure of 105 kPa (absolute) to separate the unreacted mixture from the polymer. Residual hydrogen is removed from the first reactor to an extent of 98.9% by weight. 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 is transferred from the second reactor to the third reactor to produce a second ultra-high molecular weight polymer. The second and third reactors are operated under a polyethylene polymerization reaction with hydrogen removed. The processable in-situ ultra-high molecular weight polyethylene produced by operating this method results in an excellent improvement in the simply supported beam impact strength while still maintaining the flexural modulus. It is known that conventional UHMWPE with extremely high IV cannot measure MI 21 Inventive Example E4 with an IV of 9 dl / g showed excellent melt flow capability beyond the known art.

[0119] Comparative Example 4 (CE4)

[0120] 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 a pressure of 150 kPa (absolute pressure), 97.6% of the residual hydrogen by weight is removed. Then, the low molecular weight polymer is transferred to the second reactor to produce the first high molecular weight polymer. Finally, the produced polymer is transferred from the second reactor to the third reactor to produce the second high molecular weight polymer. In the third, 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 that of E5. Compared with E5, a reduction in simply supported beam impact and flexural modulus is shown in CE4, and even a lower density of E5 is shown.

[0121] Example 5 (E5)

[0122] Example 5 was carried out 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 was removed from the first reactor to an extent of 98.5% by weight. The polymer obtained by operating this method had a melt flow rate of 48 g / 10 min (5 kg load), which was less than that obtained from CE3. As shown in Table 2, it has been shown that when the percentage of hydrogen residue removed is increased compared to the properties of Comparative Example 4, the hardness-impact balance improves.

[0123] Example 6 (E6)

[0124] Example 6 was carried out in the same manner as Example 4, except that the comonomer fed in the third reactor was the third ultra-high molecular weight polyethylene. The polymer produced by this method resulted in an excellent improvement in the Charpy impact strength while still maintaining the flexural modulus. As shown in Table 2, Inventive Example 6 having an IV of 23 dl / g showed high impact strength (1 notch impact without breaking) and flexural modulus compared to the comparative sample, but the melt flow index was not measurable due to the high viscosity and high Mw.

[0125] Table 1

[0126]

[0127] W A Indicates the percentage by weight of polymer in the first reactor

[0128] W B Indicates the percentage by weight of polymer in the second reactor

[0129] W C Indicates the percentage by weight of polymer in the third reactor

[0130] Table 2

[0131]

[0132] Tube-related embodiments

[0133] The multimodal PE product obtained from the third reactor was dried and the resulting powder was sent to the final process where it was mixed with 2-2.5 wt% carbon black, 2000 ppm Ca / Zn stearate and 3000 ppm hindered phenol / phosphate stabilizer in an extruder at 200ºC under nitrogen atmosphere and then pelletized. The pelletized resin was used to obtain density and MI.

[0134] Plastic pipe is produced by extruding molten polymer through an annular die. The molten extrudate is passed through a sizing sleeve and then to a cooling trough where water is sprayed on the outer surface to form the pipe. Solidification proceeds radially inward from the outer surface.

[0135] Tables 3-4 show the polymerization conditions and polymer properties respectively. Test results and analysis were applied to the compounds and recorded.

[0136] Table 3. Polymerization conditions for inventive and comparative examples.

[0137] All inventive examples (present invention) 7, 8, 9, 10 and comparative examples (comparative) 5, 6, 7 were synthesized by multimodal technology according to the process patent described above. The polymer compositions are described in Table 4.

[0138] Once the polymerization conditions are changed, the composition and properties of the polymer are adjusted by split ratio, MI, density and comonomer content. For samples Comparative Examples 4 and 5, polymers were produced without comonomer feed in the second reactor to obtain ultra-high molecular weight homopolymers. Only 1-butene comonomer was added in the third reactor.

[0139] For samples Inventive Examples 7, 8, 9, 11 and Comparative Example 7, 1-butene comonomer was added in the second and third reactors to obtain copolymers in the ultra-high molecular weight fraction. The GPC results of samples Comparative Examples 5 and Comparative Examples 6 containing homopolymers in the second reactor prototype showed a large range of Mw greater than 300,000 g / mol and Mz up to 3,500,000 g / mol, which confirmed that the ultra-high molecular weight fraction was increased by controlling the comonomer content in the second reactor. In addition, the excellent anti-sag properties observed by Eta 747 of Comparative Example 6 also support the role of UHMW.

[0140] Inventive Examples 5-8 show high Charpy impact resistance at 23°C, 0°C and -30°C according to the process for multimodal polymerization of the present invention. Moreover, according to the ACT results, Inventive Examples 9 and 11 also provide higher SCG than Comparative Example 7 by specific multimodal polyethylene compositions. An improved SCG balance with excellent RCP from a multimodal polyethylene composition is proposed in the present invention.

[0141] Table 3. Polymerization conditions for inventive examples and comparative examples.

[0142]

[0143] Table 4. Polymer properties of all inventive examples and comparative examples

[0144]

[0145]

[0146] The features disclosed in the above description and in the claims can be material for realizing the invention both individually and in any combination in its different forms.

Claims

1. A multimodal polyethylene composition obtained by a process for producing a multimodal polyethylene composition in a reactor system for a multimodal polyethylene polymerization process, said reactor system comprising: (a) a first reactor; (b) a hydrogen removal unit disposed 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); (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 Ziegler-Natta catalysts or metallocenes and an amount of hydrogen in the range of 0.1 to 95% by mole relative to the total gas present in the gas phase in the first reactor to obtain a catalyst having a weight average molecular weight in the range of 20,000 to 90,000 g / mol, ≥ 0.965 g / cm 3 and a low molecular weight polyethylene having a density of 5 to 1,000 g / 10min and a MI2 in the range of 5 to 1,000 g / 10min, or a weight average molecular weight in the range of greater than 90,000 to 150,000 g / mol, a density of ≥0.965 g / cm 3 A medium molecular weight polyethylene having a density in the range of 1 to 10 g / 10min; (ii) removing 98.0 to 99.8% by weight of hydrogen included 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) and transferring the obtained residual mixture to the second reactor; (iii) reacting ethylene and optionally C4 to C6 hydrocarbons in the second reactor in the presence of a catalyst system selected from Ziegler-Natta catalysts or metallocenes and in the presence of hydrogen in the amount obtained in step (b); 12 α-olefin comonomer polymerization to obtain a first high molecular weight polyethylene having a weight average molecular weight in the range of 150,000 to 1,000,000 g / mol or a first ultra-high molecular weight polyethylene having a weight average molecular weight in the range of 1,000,000 to 5,000,000 g / mol in the form of a homopolymer or a copolymer, and transferring the product mixture to the third reactor; and (d) in the presence of a catalyst system selected from Ziegler-Natta catalysts or metallocenes and wherein the amount of hydrogen in the third reactor is in the range of 0.1 to 70% by mole, preferably 0.1 to 60% by mole, of hydrogen relative to the total gas present in the gas phase in the third reactor, or optionally in the substantial absence of hydrogen, ethylene and optionally C4 to C6 hydrocarbons are reacted in the third reactor. 12 an α-olefin comonomer to obtain a second high molecular weight polyethylene having a weight average molecular weight in the range of 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 1,000,000 to 5,000,000 g / mol, The multimodal polyethylene composition comprises: (A) 51 to 58 parts by weight of the low molecular weight polyethylene; (B) 12 to 21 parts by weight of the first high molecular weight polyethylene or the first ultra-high molecular weight polyethylene; and (C) 27 to 33 parts by weight of the second high molecular weight polyethylene or the second ultra-high molecular weight polyethylene copolymer; wherein the ratio Mw / Mn of the weight average molecular weight Mw of the multimodal polyethylene composition to the number average molecular weight Mn of the multimodal polyethylene composition is less than 23, preferably from 15 to 22.

2. The multimodal polyethylene composition according to claim 1, wherein 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 to 115 kPa (absolute).

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

4. The multimodal polyethylene composition according to any one of the preceding claims, wherein the removal is a removal of 98.0-99.8% by weight, more preferably 98.0-99.5% by weight, and most preferably 98.0 to 99.1% by weight.

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

6. A multimodal polyethylene composition according to any one of the preceding claims, wherein the multimodal polyethylene composition has a weight average molecular weight as measured by gel permeation chromatography of 200,000 to 500,000 g / mol, preferably 200,000 to 300,000 g / mol.

7. A multimodal polyethylene composition according to any one of the preceding claims, wherein the multimodal polyethylene composition has a number average molecular weight as measured by gel permeation chromatography of from 10,000 to 25,000 g / mol, preferably from 10,000 to 15,000 g / mol.

8. A multimodal polyethylene composition according to any one of the preceding claims, wherein the multimodal polyethylene composition has a Z average molecular weight as measured by gel permeation chromatography of from 1,500,000 to 4,000,000 g / mol, preferably from 1,500,000 to 3,000,000 g / mol.

9. The polyethylene composition according to any one of the preceding claims, wherein the multimodal polyethylene composition has a molecular weight according to ISO 1183 of 0.945 to 0.965 g / cm 3 , preferably 0.958 to 0.963 g / cm 3 and / or a melt flow index MI5 of 0.1 to 0.4 g / 10 min, preferably 0.15 to 0.3 g / 10 min.

10. The polyethylene composition according to any one of the preceding claims, wherein the total comonomer content is 0.5 to 5 mol%, preferably 1 to 3 mol%, relative to the total amount of monomer units contained in the low molecular weight polyethylene, ultra high molecular weight polyethylene and high molecular weight polyethylene.

11. A pipe comprising a multimodal polyethylene composition according to any one of the preceding claims.

12. Use of a tube according to claim 11 for applications at a pressure of at least 10.0 MPa or at atmospheric pressure.

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