Bimodal polyethylene resin for pipe and its preparation method and application

Abstract

This invention provides a bimodal polyethylene resin for pipe production, its preparation method, and its application. The polyethylene resin has a bimodal molecular weight distribution, with a weight-average molecular weight greater than or equal to 250,000 g / mol; its molecular weight distribution M... w / M n The resin content is 18–30 g / cm³. The resin density is 0.946–0.949 g / cm³. 3 The melt flow index is 0.20–0.35 g / 10 min. The bimodal polyethylene resin is obtained by polymerizing ethylene homopolymerization and copolymerization reactions in two or more tandem reaction stages in the presence of a Ziegler-Natta catalyst system. The bimodal polyethylene resin exhibits excellent impact resistance and excellent resistance to slow crack growth, with pipe notch test results exceeding 8760 hours. This bimodal polyethylene resin is suitable for preparing extruded pipe products and is applicable to non-traditional pipe installation technologies.

Description

Technical Field

[0001] This invention relates to the field of polyethylene, and more specifically, to a bimodal polyethylene resin prepared using a Ziegler-Natta catalyst for the production of pipes with excellent resistance to slow crack growth, as well as its preparation method and applications. Background Technology

[0002] PE100 pipe-specific materials originated in the late 1980s, achieving a perfect combination of creep resistance, slow crack growth resistance, and good processing performance. This significantly improved transportation safety and capacity, leading to its widespread use in municipal and building water supply and drainage, as well as gas applications. Non-traditional pipeline installation methods, such as sandless backfilling or trenchless installation, place higher demands on PE pipe materials. Starting in 2001, through molecular structure design, new catalyst development, and advancements in polymerization technology, fourth-generation polyethylene pipe-specific materials, while maintaining the material's pipe processing, pressure resistance, and welding performance, exhibit excellent resistance to slow crack growth. Even under scratches and point loads on the pipe wall, it can still achieve a design service life of 100 years, hence the name "ultra-tough PE100." To adapt to new installation methods, there is an urgent need to develop polyethylene resins and products that meet PE100 grade standards while possessing enhanced slow crack growth resistance.

[0003] Currently, the more mature bimodal pipe materials on the market use Ziegler-Natta catalysts to produce PE100 pipe materials. However, this process suffers from problems such as high polymer fine powder content, high oligomer content, and short equipment operating cycles. Furthermore, the high-density polyethylene pipe resin produced cannot meet the requirement of at least 8760 hours of slow-crack resistance testing for trenchless installation technology. Summary of the Invention

[0004] Therefore, addressing the problems existing in the prior art, this invention provides a method for directly catalytically polymerizing bimodal polyethylene resin with a titanium-containing Ziegler-Natta catalyst in multiple reactors connected in series. Further, by varying the amounts of hydrogen (a chain transfer agent) and comonomers used in the reaction, a bimodal polyethylene resin with a specific melt index, molecular weight, and molecular weight distribution can be prepared. The bimodal polyethylene resin of this invention exhibits excellent resistance to slow crack growth and high impact strength, making it suitable for pipe manufacturing. It offers longer service life and higher safety, and its application in non-traditional pipe installation technologies significantly reduces installation costs.

[0005] One object of the present invention is to provide a bimodal polyethylene resin for pipes.

[0006] The bimodal polyethylene resin for pipes described in this invention exhibits excellent resistance to slow crack growth. This bimodal polyethylene resin comprises a low molecular weight fraction (homopolymer unit fraction) and a high molecular weight fraction (copolymer unit fraction), thus possessing a bimodal molecular weight distribution.

[0007] The weight-average molecular weight M of the bimodal polyethylene resin described in this invention w ≥250,000 g / mol, preferably ≥280,000 g / mol; number-average molecular weight M n Greater than or equal to 8000 g / mol, preferably greater than or equal to 9000 g / mol; its molecular weight distribution M w / M n The value is 23 to 35, preferably 26 to 32.

[0008] Furthermore,

[0009] The melt index of the bimodal polyethylene resin of the present invention is 0.20-0.35 g / 10 min under a load of 5.0 kg, preferably 0.22-0.30 g / 10 min.

[0010] The density range of the bimodal polyethylene resin described in this invention is 0.945–0.950 g / cm³. 3 The preferred value is 0.946–0.949 g / cm³. 3 .

[0011] The bimodal polyethylene resin of the present invention has a particle size of 150μm to 250μm for 80.00wt% or more of the bimodal polyethylene resin, preferably 150μm to 250μm for 82.00wt% or more of the bimodal polyethylene resin; and a particle size of less than 0.20wt% of the bimodal polyethylene resin is less than or equal to 45μm.

[0012] According to one aspect of the technical solution of the present invention, the weight-average molecular weight of the low molecular weight fraction (i.e., the ethylene homopolymer unit fraction) of the bimodal polyethylene resin is greater than or equal to 40,000 g / mol, preferably greater than or equal to 41,000 g / mol; its molecular weight distribution M w / M n The concentration is 6–15, preferably 7–14; its density range is ≥0.968 g / cm³. 3 The preferred concentration is 0.969–0.975 g / cm³. 3 A more preferred range is 0.969–0.973 g / cm³. 3 The melt flow index is 8–25 g / 10 min under a load of 2.16 kg, preferably 10–20 g / 10 min, and more preferably 11–16 g / 10 min.

[0013] According to one aspect of the present invention, the bimodal polyethylene resin contains copolymer units derived from comonomers copolymerized with ethylene. The comonomers of the copolymer units include α-olefin monomers.

[0014] Further, the comonomer is preferably CH2=CHR; wherein R is preferably a linear or branched alkane having 1 to 10 carbon atoms; the comonomer is more preferably at least one of propylene, butene-1, pentene-1, hexene-1, octene-1, and decene-1, and most preferably at least one of hexene-1, butene-1, and octene-1.

[0015] The comonomer of the copolymer unit has a weight percentage content of greater than 1.4 wt% in the bimodal polyethylene resin, preferably greater than or equal to 1.5 wt%, and more preferably greater than or equal to 1.8 wt%. Preferably, the weight percentage content of the comonomer of the copolymer unit in the bimodal polyethylene resin is less than 2.3 wt%.

[0016] The comonomer of the copolymer unit has a molar percentage content of greater than or equal to 0.6x in the bimodal polyethylene resin, preferably greater than or equal to 0.7x. Preferably, the molar percentage content of the comonomer of the copolymer unit in the bimodal polyethylene resin is less than 1.1x.

[0017] According to one aspect of the technical solution of the present invention, the bimodal polyethylene of the present invention exhibits good impact resistance in simply supported beams and excellent resistance to slow crack growth. The bimodal polyethylene resin of the present invention demonstrates a slow crack growth notch test resistance of greater than or equal to 5000 hours, preferably greater than or equal to 8000 hours, and most preferably greater than or equal to 8760 hours. The notched impact strength of a simply supported beam (measured at 23°C) is greater than or equal to 32 kJ / m. 2 Preferably greater than or equal to 34 kJ / m 2 .

[0018] The yellow index of the bimodal polyethylene resin is less than -1.0, preferably less than -1.5.

[0019] The tensile yield stress of the bimodal polyethylene resin is greater than or equal to 20 MPa, preferably greater than or equal to 21 MPa. The nominal tensile strain at break of the bimodal polyethylene resin is greater than or equal to 500%, preferably greater than or equal to 600%.

[0020] Another object of the present invention is to provide a method for preparing the bimodal polyethylene resin for the pipe.

[0021] The method for preparing bimodal polyethylene resin according to the present invention comprises a series of ethylene homopolymerization and ethylene copolymerization reactions in the presence of a titanium-containing Ziegler-Natta catalyst system. Preferably, the method of the present invention is carried out in two or more reactors operating in series.

[0022] The tandem ethylene homopolymerization and ethylene copolymerization reactions may include two or more reaction stages; wherein both the ethylene homopolymerization and ethylene copolymerization reactions may be carried out in one or more stages; preferably, the ethylene homopolymerization reaction is carried out in one stage, and the ethylene copolymerization reaction is carried out in one or two stages; more preferably, the tandem ethylene homopolymerization and ethylene copolymerization reactions include one ethylene homopolymerization reaction stage and a subsequent ethylene copolymerization reaction stage.

[0023] According to one aspect of the technical solution of the present invention, the method for preparing bimodal polyethylene resin of the present invention includes the following steps:

[0024] The first stage of ethylene homopolymerization involves homopolymerizing ethylene monomers with optional hydrogen in the presence of a titanium-containing Ziegler-Natta catalyst system to obtain a stream containing ethylene homopolymer.

[0025] The resulting ethylene homopolymer has a weight-average molecular weight greater than or equal to 40,000 g / mol, preferably greater than or equal to 41,000 g / mol, and a molecular weight distribution of 6–15, preferably 7–14; a melt index of 8–25 g / 10 min under a 2.16 kg load, preferably 10–20 g / 10 min, more preferably 11–16 g / 10 min; and a density range of ≥0.968 g / cm³. 3 The preferred value is 0.969–0.975 g / cm³. 3 More preferably, it is 0.969–0.973 g / cm³. 3 .

[0026] In the second stage of ethylene copolymerization, the stream containing ethylene homopolymer obtained in the previous stage is copolymerized together with ethylene monomers, comonomers, and optional hydrogen to produce an ethylene copolymer component, thereby obtaining the bimodal polyethylene resin.

[0027] The preparation method described above includes a series of ethylene homopolymerization and ethylene copolymerization reactions. By controlling the hydrogen-to-ethylene ratio and the ethylene copolymerization conditions, two molecular weight distribution structures containing a low molecular weight portion (homogeneous portion) and a high molecular weight portion (copolymerized portion) are obtained, thereby obtaining the bimodal polyethylene resin.

[0028] More specifically, the preparation method described in this invention:

[0029] The reaction temperature of the first-stage ethylene homopolymerization reaction is 80–110°C, preferably 90–100°C; the reaction pressure is approximately 1.0–5.0 MPa, preferably 4.0–4.5 MPa; and / or,

[0030] The reaction temperature of the second-stage ethylene copolymerization reaction is 60–110°C, preferably 80–90°C; the reaction pressure is 1.0–5.0 MPa, preferably 2.5–3.0 MPa.

[0031] Preferably, the first stage of ethylene homopolymerization is carried out in the presence of hydrogen, and the molar ratio (% / %) of hydrogen to ethylene in the ethylene homopolymerization stage is 0.25-0.55, preferably 0.30-0.50. The hydrogen-ethylene molar ratio (% / %) described in this invention is the ratio of the molar percentage concentration of hydrogen (mol%) to the molar percentage concentration of ethylene (mol%) in actual production.

[0032] Preferably, the second-stage ethylene copolymerization reaction is carried out in the presence of hydrogen, and the hydrogen-to-ethylene ratio (%) in the ethylene copolymerization reaction stage is 0.0051 to 0.0100, preferably 0.0055 to 0.0095, and most preferably 0.0057 to 0.0090.

[0033] Preferably, the molar ratio (% / %) of the comonomer to ethylene in the second-stage ethylene copolymerization reaction is 0.5–1.5, more preferably 0.6–1.2, and even more preferably 0.7–1.0. The molar ratio (% / %) of the comonomer to ethylene described in this invention is the ratio of the actual molar percentage concentration (mol%) of the comonomer to the molar percentage concentration (mol%) of ethylene in production.

[0034] The packing material of the catalyst, the flow rate of each stage, or the range of reaction time are routinely adjusted and selected according to the actual reactor load.

[0035] In the method for preparing bimodal polyethylene resin according to the present invention, the same catalyst system is used for the first-stage homopolymerization reaction and the second-stage copolymerization reaction. Specifically, the reactor for the two-stage reaction only needs to inject the catalyst during the first-stage homopolymerization reaction, and it is not necessary to re-inject the catalyst during the second-stage copolymerization reaction.

[0036] The titanium-containing Ziegler-Natta catalyst system used in the preparation method of bimodal polyethylene resin of this invention can be any existing titanium-containing Ziegler-Natta catalyst system. Specific examples of the titanium-containing Ziegler-Natta catalyst system of this invention can be found in patent documents CN1958620A, CN1958622A, CN102344514A, CN102344515A, CN102875708A, CN103772536A, CN102875709A, CN102993344A, and CN102875707A. The entire contents of these patent documents are incorporated herein by reference.

[0037] Preferably, in the method for preparing bimodal polyethylene resin of the present invention, the titanium-containing Ziegler-Natta catalyst system comprises the following components: (1) a magnesium-containing compound; (2) an organophosphorus compound; (3) an organoalcohol compound; (4) an organoepoxide compound; (5) a silicon-containing compound; (6) a titanium-containing compound; and optionally (7) an aluminum-containing compound.

[0038] The titanium-containing Ziegler-Natta catalyst system of this invention is formed by forming a magnesium complex from a magnesium-containing compound in a solvent system containing an organophosphorus compound, an organoepoxide compound, and an organoalcohol compound. Typically, this magnesium complex is a homogeneous and transparent solution. The magnesium complex is then reacted with a silicon-containing compound, a titanium-containing compound, and optionally an aluminum-containing compound to form the catalyst system.

[0039] The components of the titanium-containing Ziegler-Natta catalyst system include:

[0040] The magnesium-containing compound preferably has the general formula Mg(OR) 6 ) p X 1 2-p Magnesium-containing compounds. In the formula R 6 For C1-C 20 Saturated or unsaturated straight-chain or branched hydrocarbon groups or C3-C 20 Cyclic hydrocarbon group; X 1 It is a halogen, preferably chlorine, where p is an integer and 0 ≤ p ≤ 2.

[0041] Further, the magnesium-containing compound is preferably selected from at least one of magnesium chloride, magnesium bromide, magnesium chloromethoxy, magnesium chloroethoxy, magnesium chloroisopropoxy, magnesium chlorobutoxy, magnesium chlorooctoxy, magnesium diethoxy, magnesium dipropoxy, magnesium dibutoxy, magnesium dioctoxy, magnesium isopropoxy, magnesium butoxy, magnesium n-octoxy, and magnesium 2-ethylhexyloxy; more preferably selected from at least one of magnesium chloride, magnesium diethoxy, magnesium dipropoxy, magnesium dibutoxy, and magnesium dioctoxy; and most preferably selected from magnesium chloride and / or magnesium diethoxy.

[0042] The organophosphorus compound is preferably selected from at least one of the following: a hydrocarbon ester of orthophosphoric acid, a hydrocarbon ester of phosphorous acid, a halohydrocarbon ester of orthophosphoric acid, and a halohydrocarbon ester of phosphorous acid; more preferably selected from at least one of the following: triethyl phosphate, tributyl phosphate, triisooctyl phosphate, triphenyl phosphate, triethyl phosphite, tributyl phosphite, and di-n-butyl phosphite.

[0043] The organic epoxy compound is preferably selected from at least one of C2-C8 aliphatic olefins, C2-C8 aliphatic dienes, C2-C8 halogenated aliphatic olefins or oxides of C2-C8 halogenated aliphatic dienes, glycidyl ethers and internal ethers; preferably selected from at least one of ethylene oxide, propylene oxide, butane oxide, butadiene oxide, butadiene dioxide, epichlorohydrin, tetrahydrofuran, methyl glycidyl ether and diglycidyl ether; most preferably epichlorohydrin and / or tetrahydrofuran.

[0044] The organic alcohol compound is preferably a straight-chain, branched, or cycloalkyl alcohol with 1 to 10 carbon atoms or an aryl alcohol with 6 to 20 carbon atoms, wherein the hydrogen atoms in the organic alcohol compound may optionally be replaced by halogen atoms; more preferably, the organic alcohol compound is selected from at least one of ethanol, propanol, butanol, 2-ethylhexanol, and glycerol; most preferably, it is selected from at least one of ethanol and 2-ethylhexanol.

[0045] The solvent system for the magnesium complex may optionally contain an inert diluent, typically an aromatic or alkane compound. Aromatic compounds include benzene, toluene, xylene, monochlorobenzene, dichlorobenzene, trichlorobenzene, monochlorotoluene, and their derivatives. Alkane compounds include one or a mixture of straight-chain alkanes, branched alkanes, or cycloalkanes with 3 to 20 carbon atoms, such as butane, pentane, hexane, cyclohexane, heptane, etc., as long as they facilitate the dissolution of magnesium halides. The aforementioned inert diluents may be used alone or in combination.

[0046] The silicon-containing compound is preferably an organosilicon compound without active hydrogen atoms, and its general formula is R. 1 x R 2 y Si(OR 3 ) Z , where R 1 R 2 R 3 Same or different, R 1 and R 2 These are hydrocarbon groups or halogens with 1 to 10 carbon atoms, respectively, R 3 It is a hydrocarbon group with 1 to 10 carbon atoms, where x, y, and z are positive integers, 0≤x≤2, 0≤y≤2, 0≤z≤4, and x+y+z=4.

[0047] The silicon-containing compound is more preferably selected from silicon tetrachloride, silicon tetrabromide, tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, tetra(2-ethylhexyloxy)silane, ethyltrimethoxysilane, ethyltriethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, n-propyltriethoxysilane, n-propyltrimethoxysilane, decyltrimethoxysilane, decyltriethoxysilane, cyclopentyltrimethoxysilane, cyclopentyltriethoxysilane, and 2-methylcyclopentyl 2,3-Dimethylcyclopentyltrimethoxysilane, cyclohexyltrimethoxysilane, cyclohexyltriethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, ethyltriethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, tert-butyltriethoxysilane, n-butyltrimethoxysilane, n-butyltriethoxysilane, isobutyltrimethoxysilane, isobutyltriethoxysilane, cyclohexyltriethoxysilane, cyclohexyltrimethoxysilane, benzene Trimethyltrimethoxysilane, phenyltriethoxysilane, monochlorotrimethoxysilane, monochlorotriethoxysilane, ethyltriisopropoxysilane, vinyltributoxysilane, trimethylphenoxysilane, methyltrienylpropoxysilane, vinyltriacetylsilane, dimethyldimethoxysilane, dimethyldiethoxysilane, diisopropyldimethoxysilane, diisopropyldiethoxysilane, tert-butylmethyldimethoxysilane, tert-butylmethyldiethoxysilane, tert-pentylmethyldiethoxysilane, di At least one of cyclopentyldimethoxysilane, dicyclopentyldiethoxysilane, methylcyclopentyldiethoxysilane, methylcyclopentyldimethoxysilane, trimethylmethoxysilane, trimethylethoxysilane, tricyclopentylmethoxysilane, tricyclopentylethoxysilane, dicyclopentylmethylmethoxysilane, and cyclopentyldimethylmethoxysilane; most preferably at least one of tetraethoxysilane, tetramethoxysilane, and tetrabutoxysilane; most preferably selected from tetraethoxysilane and / or silicon tetrachloride.

[0048] The titanium-containing compound preferably has the general formula Ti(OR) 5 ) a X 2 b Titanium-containing compounds, where R 5 X is an aliphatic or aromatic hydrocarbon group with 1 to 14 carbon atoms. 2 The compound is a halogen, a is 0, 1 or 2, b is an integer from 0 to 4, and a+b=3 or 4; the titanium-containing compound is more preferably selected from at least one of titanium tetrachloride, titanium tetrabromide, titanium tetraiodide, titanium tetrabutoxy, titanium tetraethoxy, titanium monochlorotriethoxy, titanium trichloride, titanium dichlorodiethoxy, and titanium trichloromonoethoxy, most preferably selected from at least one of titanium tetrachloride, titanium tetraethoxy, and titanium tetrabutoxy; and most preferably titanium tetrachloride.

[0049] The aluminum-containing compound is preferably of the general formula AlR. 4 n X 3 3-n Organoaluminum compounds, where R 4 X is a hydrogen or hydrocarbon group with 1 to 20 carbon atoms. 3 The halogen is n, where n is an integer from 0 to n ≤ 3; the organoaluminum compound is more preferably selected from at least one of triethylaluminum, diethylaluminum monochloro, diethylaluminum monochloro, sesquiethylaluminum, diisobutylaluminum dichloro, triisobutylaluminum, diisopropylaluminum monochloro, methylpropylaluminum monochloro, and diphenylaluminum monochloro; most preferably, at least one of diethylaluminum monochloro, diethylaluminum monochloro, and triethylaluminum.

[0050] The components of the titanium-containing Ziegler-Natta catalyst system of the present invention, based on each mole of magnesium compound in the magnesium complex, are as follows: silicon compound: 0.05-1 mole, preferably 0.1-0.5 mole, for example, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 mole; aluminum compound: 0-5 mole, preferably 0.01-3 mole, for example, 0, 0.01, 0.05, 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 mole; titanium compound: 1-15 mole, preferably 2-10 mole, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 mole.

[0051] In the solvent system for forming the magnesium complex in the titanium-containing Ziegler-Natta catalyst system of the present invention, the organic epoxy compound is 0.2 to 10 moles, preferably 0.3 to 4 moles, for example, 0.2, 0.3, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 moles per mole of magnesium compound; the organic phosphorus compound is 0.1 to 10 moles, preferably 0.2 to 4 moles, for example, 0.1, 0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 moles per mole of magnesium compound; and the organic alcohol compound is 0.1 to 10 moles, preferably 1 to 4 moles, for example, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 moles per mole of magnesium compound.

[0052] Preferably, the titanium-containing Ziegler-Natta catalyst system of the present invention is prepared by a method comprising the following steps:

[0053] (1) Under inert gas protection, the magnesium-containing compound is dissolved in a solvent system containing the organic epoxy compound and the organic phosphorus compound to form a homogeneous solution of the magnesium complex at a dissolution temperature of 50–90°C; the organic alcohol compound is added during or after the solution is formed to obtain a magnesium complex reaction solution. The reaction should be allowed to proceed until it is complete, generally for 0.5–6 hours, preferably 1–6 hours.

[0054] (2) The above reaction solution is brought into contact with the titanium-containing compound at a temperature of -30℃ to 20℃. The silicon-containing compound and optionally an aluminum-containing compound are introduced before, after, or during the reaction. The mixture is then slowly heated to 60 to 110℃, and the solid gradually precipitates and forms particles. The reaction is allowed to proceed until it is complete, generally for 0.5 to 10 hours, preferably 0.5 to 6 hours.

[0055] The process may then include: (3) removing unreacted material and solvent from the mixture (i.e., the product after the reaction in step (2)) to obtain the titanium-containing Ziegler-Natta catalyst system.

[0056] Unreacted materials and solvents can be removed using conventional methods in the prior art, such as filtration. Furthermore, it is preferable to wash the reaction products with an inert diluent, such as hexane, to obtain the titanium-containing Ziegler-Natta catalyst system.

[0057] The bimodal polyethylene resin of the present invention can also be used with titanium-containing Ziegler-Natta catalyst system during polymerization reaction, and a co-catalyst commonly used in ethylene polymerization reaction, such as an organoaluminum co-catalyst, can be added in the usual amount.

[0058] The method for preparing bimodal polyethylene resin of the present invention employs slurry polymerization, wherein ethylene polymerization is carried out in at least two slurry reactors connected in series. The series-connected reactors yield high-molecular-weight polyethylene (i.e., the ethylene copolymer portion) and low-molecular-weight polyethylene (i.e., the ethylene homopolymer portion) with different molecular weights, and the two types of polyethylene can be well reacted and mixed in the reactors.

[0059] The slurry polymerization medium can be any common slurry polymerization medium, including at least one of the following: isobutane, hexane, heptane, cyclohexane, naphtha, raffinate, hydrogenated gasoline, kerosene, benzene, toluene, xylene, and other saturated aliphatic hydrocarbons or aromatic hydrocarbons, etc.

[0060] Hydrogen is typically used as a molecular weight regulator to adjust the molecular weight of the final polymer.

[0061] Another object of the present invention is to provide a bimodal polyethylene resin for pipes prepared by the said preparation method.

[0062] Another object of the present invention is to provide a pipe made of bimodal polyethylene resin according to the present invention, which has excellent resistance to slow crack growth and high impact strength, and is applicable to non-traditional pipe installation technologies.

[0063] The bimodal polyethylene resin for pipe production described in this invention is prepared using a titanium-containing Ziegler-Natta catalyst system in multiple reactors connected in series under specific process conditions. This catalyst system has advantages such as controllable particle size distribution, narrow particle size distribution, high catalytic activity, and good copolymerization performance. The bimodal polyethylene resin prepared by the titanium-based catalyst of this invention has adjustable molecular weight, good mechanical properties, and excellent resistance to slow crack propagation.

[0064] The bimodal polyethylene resin of this invention, prepared using the titanium-containing Ziegler-Natta catalyst system described in this invention and combined with the specific conditions of the bimodal polyethylene resin preparation method described in this invention, exhibits good particle morphology, concentrated particle size distribution, mainly concentrated in the range of 150 μm to 250 μm (more than 80 wt%, preferably more than 82 wt%, of which have a particle size of 150 μm to 250 μm), low fine powder content, and particles smaller than 45 μm having a particle size smaller than 0.20 wt% of the bimodal polyethylene resin; it also has good flowability, which is beneficial for processing. Furthermore, the bimodal polyethylene resin of this invention has low oligomer content, and its molecular weight and molecular weight distribution are controllable, resulting in excellent processing performance for extruded pipes. The unique resin structure of the bimodal polyethylene resin of this invention gives the pipe products prepared from it excellent resistance to slow crack growth and excellent mechanical properties, making it suitable for non-traditional pipe installation technologies. The bimodal polyethylene resin of this invention has a melt index of 0.20 to 0.35 g / 10 min, preferably 0.22 to 0.30 g / 10 min, and a density of 0.945 to 0.950 g / cm³. 3 The preferred value is 0.946–0.949 g / cm³. 3 At that time, it was very suitable for extruding pipe products. Detailed Implementation

[0065] The present invention will be further described below through specific embodiments, but these do not constitute any limitation on the present invention.

[0066] The polymer-related data in the examples were obtained using the following test methods:

[0067] (1) Resin tensile properties: determined according to the method described in GB / T 1040.2-2006, with a test speed of 50 mm / min.

[0068] (2) Melt mass flow rate (also known as melt index, MI): Measured using a CEAST 7026 melt indexer at 190°C and under a load of 2.16 kg or 5.0 kg, according to the method described in ASTM D1238-2038.

[0069] The melt index of the resin after ethylene homopolymerization was determined using a 1.00 mm die under a load of 2.16 kg; the melt index of the resin after ethylene copolymerization was determined using a normal die under a load of 5.0 kg.

[0070] (3) Notched impact strength of simply supported beam: determined according to the method described in GB / T 1043.1.

[0071] (4) Resistance to slow crack growth: The notched tube test was conducted in accordance with GB / T 18476-2019.

[0072] (5) Resin density: determined according to the method described in GB / T 15558.1-2015.

[0073] (6) Content of comonomers in bimodal polyethylene resin: determined by nuclear magnetic resonance (NMR). A Bruker Avance III 400MHz NMR spectrometer with a 10mm probe was used. The solvent was deuterated o-dichlorobenzene; approximately 250 mg of sample was placed in 2.5 mL of deuterated solvent and heated in an oil bath at 140°C to dissolve the sample and form a homogeneous solution. 13C-NMR was collected at a probe temperature of 125°C, using a 90° pulse, a sampling time AQ of 5 seconds, a delay time D1 of 10 seconds, and more than 5000 scans.

[0074] (7) Molecular weight (M) w M n M z ) and molecular weight distribution M w / M n All results were obtained by gel permeation chromatography (GPC). Specifically, a Polymer Laboratories PL-GPC 220 gel permeation chromatograph, connected to a Polymerchar SA IR5 infrared detector, was used. The chromatographic column consisted of three tandem PLgel 13μm Olexis columns. The solvent and mobile phase were 1,2,4-trichlorobenzene (containing 250 ppm of the antioxidant 2,6-dibutyl-p-cresol). The column temperature was 150℃, and the flow rate was 1.0 mL / min. Universal standardization was performed using PL's EasiCal PS-1 narrow-distribution polystyrene standard.

[0075] (8) Particle size distribution of resin powder: The particle size distribution of resin powder was investigated by using a German Retsch sieve analyzer.

[0076] (9) The yellow index of plastics shall be tested according to the method of GB2409-80.

[0077] The raw materials used in the embodiments and comparative examples of this invention are all commercially available.

[0078] Example 1

[0079] (1) Preparation of the titanium-containing Ziegler-Natta catalyst system:

[0080] The preparation method of the solid catalyst is the same as that in patent ZL200510117428.5 (CN1958620A). In a reactor thoroughly purged with high-purity nitrogen, 4.4 g of magnesium dichloride, 80 mL of toluene, 4.0 mL of epichlorohydrin, 4.0 mL of tributyl phosphate, and 3.5 mL of ethanol were added sequentially. The mixture was heated to 70°C with stirring, and the reaction was continued for 2 hours after the solid had completely dissolved to form a homogeneous solution. The system was then cooled to -15°C, and 50 mL of titanium tetrachloride was slowly added dropwise, with stirring for 0.5 hours. Then, 3.6 mL of tetraethoxysilane was added, and the reaction was continued for 1 hour. The temperature was slowly increased to 85°C, and the reaction was continued for 2 hours. Stirring was stopped, and the mixture was allowed to stand. The suspension quickly separated into layers. The supernatant was removed, and the mixture was washed four times with hexane and dried with high-purity nitrogen to obtain a titanium-containing Ziegler-Natta catalyst system with good flowability and a narrow particle size distribution.

[0081] (2) Polymerization reaction:

[0082] The polymerization reaction is carried out in two slurry reactors connected in series.

[0083] The titanium-containing Ziegler-Natta catalyst system and co-catalyst (triethylaluminum) obtained above were continuously fed into the first reactor through a catalyst storage tank to complete the first stage of ethylene homopolymerization. The polymerization temperature in the first reactor was 95.00℃, and the reaction pressure was 4.002MPa. Hydrogen was added to the feed of the first reactor as a molecular weight regulator, and isobutane was used as a diluent. The ethylene addition rate was 15.51 t / h, and the hydrogen addition rate was 16.0 kg / h, with a hydrogen / ethylene molar ratio (%) of 0.418, yielding ethylene homopolymer. The ethylene homopolymer prepared in the first reactor had a melt index of 14.48 g / 10 min and a density of 0.9693 g / cm³. 3 M w It is 4.11×10 4 g / mol.

[0084] The ethylene-containing homopolymer stream from the first reactor enters the second reactor. The polymerization temperature in the second reactor is 85.01℃, and the reaction pressure is 2.808 MPa. The feed to the second reactor includes 17.49 t / h of ethylene, 364 g / h of hydrogen, and 950 kg / h of hexene-1 comonomer. The hydrogen / ethylene molar ratio (%) is 0.0065, and the hexene-1 / ethylene molar ratio (%) is 0.80. The bimodal polyethylene resin obtained after the reaction in the second reactor has a molecular weight (M). w 28.98×10 4 M n 0.98×10 4 M w / M n The melting point is 29.57; the melt index is 0.22 g / 10 min; and the density is 0.9478 g / cm³. 3 The comonomer content of the bimodal polyethylene resin is 2.2 wt%. Specific test data for the basic and mechanical properties of the resin are shown in Tables 1-3.

[0085] The unique resin structure of the bimodal polyethylene resin of this invention enables the prepared polyethylene pipe to have excellent mechanical properties, high simply supported beam impact strength and excellent resistance to slow crack propagation.

[0086] Example 2

[0087] (1) Preparation of the titanium-containing Ziegler-Natta catalyst system:

[0088] The preparation method of the solid catalyst is the same as that in patent ZL200510117428.5 (CN1958620A). In a reactor thoroughly purged with high-purity nitrogen, 4.4 g of magnesium dichloride, 80 mL of toluene, 4.0 mL of epichlorohydrin, 6.0 mL of tributyl phosphate, and 3.4 mL of ethanol were added sequentially. The mixture was heated to 65°C with stirring, and the reaction was allowed to proceed for 2 hours after the solid had completely dissolved to form a homogeneous solution. The system was then cooled to -15°C, and 65 mL of titanium tetrachloride was slowly added dropwise, with stirring for 0.5 hours. Then, 4.0 mL of tetraethoxysilane was added, and the reaction was allowed to proceed for 1 hour. The temperature was then slowly raised to 85°C, and the reaction was allowed to proceed for 2 hours. Stirring was stopped, and the mixture was allowed to stand. The suspension quickly separated into layers. The supernatant was removed, and the mixture was washed four times with hexane and dried with high-purity nitrogen to obtain a titanium-containing Ziegler-Natta catalyst system with good flowability and a narrow particle size distribution.

[0089] (2) Polymerization reaction:

[0090] The polymerization reaction is carried out in two slurry reactors connected in series.

[0091] The titanium-containing Ziegler-Natta catalyst system and co-catalyst (triethylaluminum) obtained above were continuously fed into the first reactor through a catalyst storage tank to complete the first stage of ethylene homopolymerization. The polymerization temperature in the first reactor was 94.98℃, and the reaction pressure was 4.001MPa. Hydrogen was added to the feed of the first reactor as a molecular weight regulator, and isobutane was added as a diluent. The ethylene addition rate was 16.0 t / h, and the hydrogen addition rate was 16.8 kg / h, with a hydrogen / ethylene molar ratio (%) of 0.454, yielding ethylene homopolymer. The ethylene homopolymer prepared in the first reactor had a melt index of 15.61 g / 10 min and a density of 0.9701 g / cm³. 3 M w 4.02×10 4 g / mol.

[0092] The ethylene-containing homopolymer stream from the first reactor enters the second reactor. The polymerization temperature in the second reactor is 85.03℃, and the reaction pressure is 2.801 MPa. The feed to the second reactor includes 17.0 t / h of ethylene, 352 g / h of hydrogen, and 900 kg / h of hexene-1 comonomer. The hydrogen / ethylene molar ratio (%) is 0.0059, and the hexene-1 / ethylene molar ratio (%) is 0.76. The bimodal polyethylene resin obtained after the reaction in the second reactor has a molecular weight (M). w 28.19×10 4 M n It is 0.97×10 4 M w / M n The melting point was 29.06; the melt index was 0.23 g / 10 min; and the density was 0.9482 g / cm³. 3 The comonomer content of the bimodal polyethylene resin is 1.8 wt%. Specific test data for the basic and mechanical properties of the resin are shown in Tables 1-3.

[0093] The unique resin structure of the bimodal polyethylene resin of this invention enables the prepared polyethylene pipe to have excellent mechanical properties, high simply supported beam impact strength and excellent resistance to slow crack propagation.

[0094] Example 3

[0095] (1) Preparation of the titanium-containing Ziegler-Natta catalyst system:

[0096] The preparation method of the solid catalyst is the same as that in patent ZL200510117428.5 (CN1958620A). In a reactor thoroughly purged with high-purity nitrogen, 4.4 g of magnesium dichloride, 70 mL of toluene, 4.0 mL of epichlorohydrin, 6.0 mL of tributyl phosphate, and 4.6 mL of ethanol were added sequentially. The mixture was heated to 65°C with stirring, and the reaction was continued for 2 hours after the solid had completely dissolved to form a homogeneous solution. The system was then cooled to -15°C, and 55 mL of titanium tetrachloride was slowly added dropwise, with stirring for 0.5 hours. Then, 4.2 mL of tetraethoxysilane was added, and the reaction was continued for 1 hour. The temperature was then slowly increased to 85°C, and the reaction was continued for 2 hours. Stirring was stopped, and the mixture was allowed to stand. The suspension quickly separated into layers. The supernatant was removed, and the mixture was washed four times with hexane and dried with high-purity nitrogen to obtain a titanium-containing Ziegler-Natta catalyst system with good flowability and a narrow particle size distribution.

[0097] (2) Polymerization reaction:

[0098] The polymerization reaction is carried out in two slurry reactors connected in series.

[0099] The titanium-containing Ziegler-Natta catalyst system and co-catalyst (triethylaluminum) obtained above were continuously fed into the first reactor through a catalyst storage tank to complete the first stage of ethylene homopolymerization. The polymerization temperature in the first reactor was 95.01℃, and the reaction pressure was 3.998MPa. Hydrogen was added to the feed of the first reactor as a molecular weight regulator, and isobutane was added as a diluent. The ethylene addition rate was 16.12 t / h, and the hydrogen addition rate was 15.72 kg / h, with a hydrogen / ethylene molar ratio (%) of 0.392, yielding ethylene homopolymer. The ethylene homopolymer prepared in the first reactor had a melt index of 13.65 g / 10 min and a density of 0.9701 g / cm³. 3 M w 4.24×10 4 g / mol.

[0100] The ethylene-containing homopolymer stream from the first reactor enters the second reactor. The polymerization temperature in the second reactor is 84.96℃, and the reaction pressure is 2.790 MPa. The feed to the second reactor includes 16.88 t / h of ethylene, 348 g / h of hydrogen, and 880 kg / h of hexene-1 comonomer. The hydrogen / ethylene molar ratio (%) is 0.0055, and the hexene-1 / ethylene molar ratio (%) is 0.69. The bimodal polyethylene resin obtained after the reaction in the second reactor has a molecular weight (M). w 27.25×10 4 M n 0.98×10 4 M w / M nThe melting point was 27.81; the melt index was 0.22 g / 10 min; and the density was 0.9490 g / cm³. 3 The comonomer content of the bimodal polyethylene resin is 1.6 wt%. Specific test data for the basic and mechanical properties of the resin are shown in Tables 1-3.

[0101] The unique resin structure of the bimodal polyethylene resin of this invention enables the prepared polyethylene pipe to have excellent mechanical properties, high simply supported beam impact strength and excellent resistance to slow crack propagation.

[0102] Example 4

[0103] (1) Preparation of the titanium-containing Ziegler-Natta catalyst system:

[0104] The preparation method of the solid catalyst is the same as that in patent ZL200510117428.5 (CN1958620A). In a reactor thoroughly purged with high-purity nitrogen, 4.4 g of magnesium dichloride, 70 mL of toluene, 4.2 mL of epichlorohydrin, 6.0 mL of tributyl phosphate, and 6.4 mL of ethanol were added sequentially. The mixture was heated to 65°C with stirring, and the reaction was continued for 2 hours after the solid had completely dissolved to form a homogeneous solution. The system was then cooled to -20°C, and 65 mL of titanium tetrachloride was slowly added dropwise, with stirring for 0.5 hours. Then, 4.0 mL of tetraethoxysilane was added, and the reaction was continued for 1 hour. The temperature was slowly increased to 85°C, and the reaction was continued for 2 hours. Stirring was stopped, and the mixture was allowed to stand. The suspension quickly separated into layers. The supernatant was removed, and the mixture was washed four times with hexane and dried with high-purity nitrogen to obtain a titanium-containing Ziegler-Natta catalyst system with good flowability and a narrow particle size distribution.

[0105] (2) Polymerization reaction:

[0106] The polymerization reaction is carried out in two slurry reactors connected in series.

[0107] The titanium-containing Ziegler-Natta catalyst system and co-catalyst (triethylaluminum) obtained above were continuously fed into the first reactor through a catalyst storage tank to complete the first stage of ethylene homopolymerization. The polymerization temperature in the first reactor was 94.97℃, and the reaction pressure was 4.001MPa. Hydrogen was added to the feed of the first reactor as a molecular weight regulator, and isobutane was added as a diluent. The ethylene addition rate was 15.88 t / h, and the hydrogen addition rate was 15.02 kg / h, with a hydrogen / ethylene molar ratio (%) of 0.354, yielding ethylene homopolymer. The ethylene homopolymer prepared in the first reactor had a melt index of 12.34 g / 10 min and a density of 0.9703 g / cm³. 3 M w 4.42×10 4 g / mol.

[0108] The ethylene-containing homopolymer stream from the first reactor enters the second reactor. The polymerization temperature in the second reactor is 84.97℃, and the reaction pressure is 2.810 MPa. The feed to the second reactor includes 17.12 t / h of ethylene, 335 g / h of hydrogen, and 800 kg / h of hexene-1 comonomer. The hydrogen / ethylene molar ratio (%) is 0.0051, and the hexene-1 / ethylene molar ratio (%) is 0.50. The bimodal polyethylene resin obtained after the reaction in the second reactor has a molecular weight (M). w It is 27.14×10 4 M n 1.01×10 4 M w / M n The melting point is 26.87; the melt index is 0.23 g / 10 min; and the density is 0.9500 g / cm³. 3 The comonomer content of the bimodal polyethylene resin is 1.5 wt%. Specific test data for the basic and mechanical properties of the resin are shown in Tables 1-3.

[0109] The unique resin structure of the bimodal polyethylene resin of this invention enables the prepared polyethylene pipe to have excellent mechanical properties, high simply supported beam impact strength and excellent resistance to slow crack propagation.

[0110] Comparative Example 1

[0111] (1) The titanium-containing Ziegler-Natta catalyst system is the same as the titanium-containing Ziegler-Natta catalyst system obtained in Example 4;

[0112] (2) Polymerization reaction:

[0113] The polymerization reaction is carried out in two slurry reactors connected in series.

[0114] The titanium-containing Ziegler-Natta catalyst system and co-catalyst (triethylaluminum) obtained above were continuously fed into the first reactor through a catalyst storage tank to complete the first stage of ethylene homopolymerization. The polymerization temperature in the first reactor was 95.00℃, and the reaction pressure was 4.002MPa. Hydrogen was added to the feed of the first reactor as a molecular weight regulator, and isobutane was used as a diluent. The ethylene addition rate was 15.90 t / h, and the hydrogen addition rate was 14.92 kg / h, with a hydrogen / ethylene molar ratio (%) of 0.348, yielding ethylene homopolymer. The ethylene homopolymer prepared in the first reactor had a melt index of 12.06 g / 10 min and a density of 0.9700 g / cm³. 3 M w 4.38×10 4 g / mol.

[0115] The ethylene-containing homopolymer stream from the first reactor enters the second reactor. The polymerization temperature in the second reactor is 85.01℃, and the reaction pressure is 2.799 MPa. The feed to the second reactor includes 17.11 t / h of ethylene, 321 g / h of hydrogen, and 760 kg / h of hexene-1 comonomer. The hydrogen / ethylene molar ratio (%) is 0.0040, and the hexene-1 / ethylene molar ratio (%) is 0.45. The bimodal polyethylene resin obtained after the reaction in the second reactor has a molecular weight (M). w 26.35×10 4 M n 1.01×10 4 M w / M n The value is 26.09; the melt index is 0.23 g / 10 min; and the density is 0.9501 g / cm³. 3 The comonomer content of the bimodal polyethylene resin is 1.3 wt%. Specific test data for the basic and mechanical properties of the resin are shown in Tables 1-3.

[0116] Comparative Example 2

[0117] Bimodal polyethylene resin was prepared using an imported industrial catalyst (MT2110).

[0118] The aforementioned industrial-grade imported titanium-containing Ziegler-Natta catalyst system and co-catalyst (triethylaluminum) were continuously fed into the first reactor through a catalyst storage tank to complete the first stage of ethylene homopolymerization. The polymerization temperature in the first reactor was 95.07℃, and the reaction pressure was 4.001 MPa. Hydrogen was added to the feed of the first reactor as a molecular weight regulator, and isobutane was used as a diluent. The ethylene addition rate was 16.0 t / h, and the hydrogen addition rate was 13.98 kg / h, with a hydrogen / ethylene molar ratio (%) of 0.24, yielding an ethylene homopolymer. The ethylene homopolymer prepared in the first reactor had a melt index of 11.89 g / 10 min and a density of 0.9694 g / cm³. 3 M w 4.62×10 4 g / mol.

[0119] The ethylene-containing homopolymer stream from the first reactor enters the second reactor. The polymerization temperature in the second reactor is 85.18℃, and the reaction pressure is 2.80 MPa. The feed to the second reactor includes 16.5 t / h of ethylene, 316 g / h of hydrogen, and 918 kg / h of hexene-1 comonomer. The hydrogen / ethylene molar ratio (%) is 0.0045, and the hexene-1 / ethylene molar ratio (%) is 1.21. The bimodal polyethylene resin obtained after the reaction in the second reactor has a molecular weight (M). w 24.95×104 M n 1.02×10 4 M w / M n The melting point is 24.46; the melt index is 0.23 g / 10 min; and the density is 0.9502 g / cm³. 3 The comonomer content of the bimodal polyethylene resin is 1.3 wt%. Specific test data for the basic and mechanical properties of the resin are shown in Tables 1-3.

[0120] Table 1. Particle size distribution of resin powder and catalyst activity

[0121]

[0122] Table 2. Basic Properties of Resins

[0123]

[0124] Table 3. Mechanical properties of resin

[0125]

[0126] As can be seen from the data in Tables 1-3, the titanium-containing Ziegler-Natta catalyst prepared by the method provided in this invention exhibits high catalytic activity and good copolymerization performance in ethylene polymerization of two or more reactions connected in series. It can produce bimodal polyethylene resin with good particle morphology, fewer small particles, a concentrated polymer resin particle size distribution mainly between 150 μm and 250 μm, and good flowability. The bimodal polyethylene resin prepared by the method provided in this invention has higher impact strength and excellent resistance to slow crack growth compared to the comparative polyethylene resin, making it suitable for extruded polyethylene pipe products with longer service life and higher safety, and applicable to non-traditional pipeline installation technologies.

Claims

1. A bimodal polyethylene resin for pipes, having a bimodal molecular weight distribution; the weight-average molecular weight of the bimodal polyethylene resin is greater than or equal to 250,000 g / mol; its molecular weight distribution is 23-35; the bimodal polyethylene resin contains copolymer units, wherein the weight percentage of the comonomer of the copolymer unit in the bimodal polyethylene resin is greater than or equal to 1.4 wt%.

2. The bimodal polyethylene resin according to claim 1, characterized in that: The weight-average molecular weight of the bimodal polyethylene resin is greater than or equal to 280,000 g / mol; its molecular weight distribution is 26~32.

3. The bimodal polyethylene resin according to claim 1, characterized in that: The bimodal polyethylene resin has a density of 0.945 to 0.950 g / cm 3 ; and / or, The melt index of the bimodal polyethylene resin is 0.20~0.35 g / 10min at a load of 5.0 kg; and / or, The number-average molecular weight of the bimodal polyethylene resin is greater than or equal to 8000 g / mol; The bimodal polyethylene resin has a particle size of 150μm to 250μm for 80.00wt% or more of the particles, and a particle size of less than 45μm for 0.20wt% or less of the particles.

4. The bimodal polyethylene resin according to claim 3, characterized in that: The bimodal polyethylene resin has a density of 0.946 to 0.949 g / cm 3 ; and / or, The melt index of the bimodal polyethylene resin is 0.22~0.30 g / 10 min at a load of 5.0 kg; and / or, The bimodal polyethylene resin has a number-average molecular weight greater than or equal to 9000 g / mol.

5. The bimodal polyethylene resin according to claim 1, characterized in that: The ethylene homopolymer fraction of the bimodal polyethylene resin has a density greater than or equal to 0.968 g / cm 3 ; and / or, The melt index of the homopolymer portion of the bimodal polyethylene resin is 8-25 g / 10min at a load of 2.16 kg; and / or, The weight-average molecular weight of the homopolymer portion of the bimodal polyethylene resin is greater than or equal to 40,000 g / mol; and / or, The molecular weight distribution of the ethylene homopolymer portion of the bimodal polyethylene resin is 6-15.

6. The bimodal polyethylene resin according to claim 5, characterized in that: The ethylene homopolymer fraction of the bimodal polyethylene resin has a density of 0.969 to 0.975 g / cm 3 ; and / or, The melt index of the homopolymer portion of the bimodal polyethylene resin is 10-20 g / 10 min at a load of 2.16 kg; and / or, The weight-average molecular weight of the homopolymer portion of the bimodal polyethylene resin is greater than or equal to 41000 g / mol; and / or, The molecular weight distribution of the ethylene homopolymer portion of the bimodal polyethylene resin is 7-14.

7. The bimodal polyethylene resin according to claim 6, characterized in that: The ethylene homopolymer fraction of the bimodal polyethylene resin has a density of 0.969 to 0.973 g / cm 3 ; and / or, The melt index of the ethylene homopolymer portion of the bimodal polyethylene resin is 11~16 g / 10 min under a load of 2.16 kg.

8. The bimodal polyethylene resin according to claim 1, characterized in that: The comonomers of the copolymer unit include α-olefin monomers.

9. The bimodal polyethylene resin according to claim 8, characterized in that: The comonomer has the following structural formula: CH2=CHR, where R is a linear or branched alkane with 1 to 10 carbon atoms.

10. The bimodal polyethylene resin according to claim 9, characterized in that: The comonomer is at least one of propylene, butene-1, pentene-1, hexene-1, octene-1, and decene-1.

11. The bimodal polyethylene resin according to claim 1, characterized in that: The comonomer of the copolymer unit has a weight percentage of greater than or equal to 1.5 wt% and less than 2.3 wt% in the bimodal polyethylene resin.

12. The polyethylene resin according to any one of claims 1 to 11, characterized in that: The tensile yield stress of the bimodal polyethylene resin is greater than or equal to 20 MPa; and / or, The nominal tensile strain at break of the bimodal polyethylene resin is greater than or equal to 500%; and / or The notched impact strength of the bimodal polyethylene resin in a simply supported beam, measured at 23°C, is greater than or equal to 32 kJ / m. 2 ; and / or, The bimodal polyethylene resin undergoes a slow crack growth notch test of 5000 hours or more; and / or, The yellow index of the bimodal polyethylene resin is less than -1.

0.

13. The polyethylene resin according to claim 12, characterized in that: The tensile yield stress of the bimodal polyethylene resin is greater than or equal to 21 MPa; and / or, The nominal tensile strain at break of the bimodal polyethylene resin is 600%; and / or The bimodal polyethylene resin has a Charpy notched impact strength at 23 °C of greater than or equal to 34 kJ / m 2 ; and / or, The bimodal polyethylene resin undergoes a slow crack growth notch test of ≥8000 hours; and / or, The yellow index of the bimodal polyethylene resin is less than -1.

5.

14. The method for preparing bimodal polyethylene resin for pipes according to any one of claims 1 to 13, comprising a series of ethylene homopolymerization and ethylene copolymerization reactions, wherein the bimodal polyethylene resin is prepared in the presence of a titanium-containing Ziegler-Natta catalyst system.

15. The preparation method according to claim 14, characterized in that: The first stage of homopolymerization involves homopolymerizing ethylene monomers with optional hydrogen in the presence of a titanium-containing Ziegler-Natta catalyst system to obtain a stream containing ethylene homopolymer. Second-stage copolymerization reaction: The stream containing ethylene homopolymer obtained in the previous stage is subjected to ethylene copolymerization reaction with ethylene monomers, comonomers and optional hydrogen to obtain the bimodal polyethylene resin.

16. The preparation method according to claim 15, characterized in that: The reaction temperature for the first stage of ethylene homopolymerization is 80~110℃; the reaction pressure is 1.0~5.0MPa; and / or, The reaction temperature for the second-stage ethylene copolymerization reaction is 60~110℃; the reaction pressure is 1.0~5.0MPa.

17. The preparation method according to claim 16, characterized in that: The reaction temperature for the first stage of ethylene homopolymerization is 90~100℃; the reaction pressure is 4.0~4.5MPa; and / or, The reaction temperature for the second-stage ethylene copolymerization reaction is 80~90℃; the reaction pressure is 2.5~3.0MPa.

18. The preparation method according to claim 15, characterized in that: The first stage of ethylene homopolymerization is carried out in the presence of hydrogen; wherein the molar ratio of hydrogen to ethylene is 0.25~0.55; and / or, The second-stage ethylene copolymerization reaction is carried out in the presence of hydrogen; wherein the molar ratio of hydrogen to ethylene is 0.0051~0.0100; and / or, The molar ratio of the comonomer to ethylene in the second-stage ethylene copolymerization reaction is 0.5 to 1.

5.

19. The preparation method according to claim 18, characterized in that: In the first stage, the ethylene homopolymerization reaction, the molar ratio of hydrogen to ethylene is 0.30~0.50; and / or, In the second stage of ethylene copolymerization, the molar ratio of hydrogen to ethylene is 0.0055~0.0095; and / or, The molar ratio of the comonomer to ethylene in the second-stage ethylene copolymerization reaction is 0.6 to 1.

2.

20. The preparation method according to claim 15, characterized in that: The density of the ethylene homopolymer obtained in the first stage ethylene homopolymerization is greater than or equal to 0.968 g / cm 3 ; and / or, The melt index of the ethylene homopolymer obtained from the first-stage ethylene homopolymerization reaction is 8~25 g / 10 min at a load of 2.16 kg; and / or, The weight-average molecular weight of the ethylene homopolymer obtained from the first-stage ethylene homopolymerization reaction is greater than or equal to 40,000 g / mol; and / or, The molecular weight distribution of the ethylene homopolymer obtained from the first stage of ethylene homopolymerization reaction is 6~15.

21. The preparation method according to claim 20, characterized in that: The density of the ethylene homopolymer obtained in the first stage ethylene homopolymerization is 0.969 to 0.975 g / cm 3 ; and / or, The melt index of the ethylene homopolymer obtained from the first-stage ethylene homopolymerization reaction is 10~20 g / 10 min at a load of 2.16 kg; and / or, The weight-average molecular weight of the ethylene homopolymer obtained from the first-stage ethylene homopolymerization reaction is greater than or equal to 41000 g / mol; and / or, The molecular weight distribution of the ethylene homopolymer obtained from the first stage of ethylene homopolymerization reaction is 7~14.

22. The preparation method according to claim 21, characterized in that: The density of the ethylene homopolymer obtained in the first stage ethylene homopolymerization is 0.969 to 0.973 g / cm 3 ; and / or, The melt index of the ethylene homopolymer obtained from the first stage of ethylene homopolymerization reaction was 11~16 g / 10 min under a load of 2.16 kg.

23. The preparation method according to claim 14 or 15, characterized in that: The titanium-containing Ziegler-Natta catalyst system comprises the following components: (1) a magnesium-containing compound; (2) an organophosphorus compound; (3) an organoalcohol compound; (4) an organoepoxide compound; (5) a silicon-containing compound; (6) a titanium-containing compound; optionally (7) an aluminum-containing compound; the catalyst system is formed by forming a magnesium complex from the magnesium-containing compound in a solvent system containing the organophosphorus compound, the organoepoxide compound and the organoalcohol compound; and by reacting the magnesium complex with the silicon-containing compound, the titanium-containing compound and optionally the aluminum-containing compound.

24. The preparation method according to claim 23, characterized in that: The magnesium-containing compound is selected from the general formula Mg(OR) 6 ) p X 1 2-p Magnesium-containing compounds; where R 6 For C1-C 20 Saturated or unsaturated straight-chain or branched hydrocarbon groups or C3-C 20 Cyclic hydrocarbon group; X 1 For halogens, p is an integer and 0 ≤ p ≤ 2; and / or, The organophosphorus compound is selected from at least one of the following: alkyl esters of orthophosphoric acid, alkyl esters of phosphorous acid, haloalkyl esters of orthophosphoric acid, and haloalkyl esters of phosphorous acid; and / or, The organic epoxy compound is selected from at least one of C2-C8 aliphatic olefins, C2-C8 aliphatic dienes, C2-C8 haloaliphatic olefins, or oxides of C2-C8 haloaliphatic dienes, glycidyl ethers, and internal ethers; and / or, The organic alcohol compound is a straight-chain, branched, or cycloalkyl alcohol with 1 to 10 carbon atoms, or an aryl alcohol with 6 to 20 carbon atoms, wherein the hydrogen atoms in the organic alcohol compound may optionally be substituted with halogen atoms; and / or, The silicon-containing compound is an organosilicon compound without active hydrogen atoms, and its general formula is R. 1 x R 2 y Si(OR 3 ) Z , where R 1 R 2 R 3 Same or different, R 1 and R 2 These are hydrocarbon groups or halogens with 1 to 10 carbon atoms, respectively, R 3 For hydrocarbon groups with 1 to 10 carbon atoms, where x, y, and z are positive integers, 0 ≤ x ≤ 2, 0 ≤ y ≤ 2, 0 ≤ z ≤ 4, and x + y + z = 4; and / or, The general formula of the titanium-containing compound is Ti(OR). 5 ) a X 2 b In the formula R 5 X is an aliphatic or aromatic hydrocarbon group with 1 to 14 carbon atoms. 2 For halogens, a is 0, 1, or 2, b is an integer from 0 to 4, a + b = 3 or 4; and / or, The aluminum-containing compound has the general formula AlR. 4 n X 3 3-n Organoaluminum compounds, where R 4 X is a hydrogen or hydrocarbon group with 1 to 20 carbon atoms. 3 For halogens, n is an integer where 0 < n ≤ 3.

25. The preparation method according to claim 24, characterized in that: The magnesium-containing compound is selected from the general formula Mg(OR) 6 ) p X 1 2-p Magnesium-containing compounds; where X 1 The magnesium-containing compound is chlorine; the magnesium compound is selected from at least one of magnesium chloride, magnesium bromide, magnesium chloromethoxy, magnesium chloroethoxy, magnesium chloroisopropoxy, magnesium chlorobutoxy, magnesium chlorooctoxy, magnesium diethoxy, magnesium dipropoxy, magnesium dibutoxy, magnesium dioctoxy, magnesium isopropoxy, magnesium butoxy, magnesium n-octoxy, and magnesium 2-ethylhexyloxy; and / or, The organophosphorus compound is selected from at least one of triethyl phosphate, tributyl phosphate, triisooctyl phosphate, triphenyl phosphate, triethyl phosphite, tributyl phosphite, and di-n-butyl phosphite; and / or, The organic epoxy compound is selected from at least one of ethylene oxide, propylene oxide, butane oxide, butadiene oxide, butadiene dioxide, epichlorohydrin, tetrahydrofuran, methyl glycidyl ether, and diglycidyl ether; and / or, The organic alcohol compound is selected from at least one of ethanol, propanol, butanol, 2-ethylhexanol, and glycerol; and / or, The silicon-containing compound is selected from silicon tetrachloride and / or tetraethoxysilane; and / or, The titanium-containing compound is selected from at least one of titanium tetrachloride, titanium tetrabromide, titanium tetraiodide, titanium trichloride, titanium dichloroethoxy, and titanium trichloromonoethoxy; and / or, The aluminum-containing compound is selected from at least one of triethylaluminum, diethylaluminum chloride, diethylaluminum chloride, sesquiethylaluminum, diisobutylaluminum chloride, triisobutylaluminum chloride, diisopropylaluminum chloride, methylpropylaluminum chloride, and diphenylaluminum chloride.

26. The preparation method according to claim 23, characterized in that: The components of the titanium-containing Ziegler-Natta catalyst system, based on the amount of magnesium compound per mole in the magnesium complex, are: silicon compound 0.05-1 mole; aluminum compound 0-5 moles; titanium compound 1-15 moles. In the solvent system in which the magnesium complex is formed in the titanium-containing Ziegler-Natta catalyst system, the organic alcohol compound is 0.1 to 10 moles per mole of magnesium compound. The amount of organic epoxy compounds is 0.2 to 10 moles; the amount of organic phosphorus compounds is 0.1 to 10 moles.

27. The preparation method according to claim 26, characterized in that: The components of the titanium-containing Ziegler-Natta catalyst system, based on the amount of magnesium compound per mole in the magnesium complex, are: silicon compound 0.1-0.5 moles; aluminum compound 0.01-3 moles; titanium compound 2-10 moles; In the solvent system for forming the magnesium complex in the titanium-containing Ziegler-Natta catalyst system, the organic alcohol compound is 1-4 moles, the organic epoxy compound is 0.3-4 moles, and the organic phosphorus compound is 0.2-4 moles per mole of magnesium compound.

28. Bimodal polyethylene resin for pipes prepared by the preparation method according to any one of claims 14 to 27.

29. The pipe made of bimodal polyethylene resin according to any one of claims 1 to 13 and 28.

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