Broad-peak distribution polyethylene, and preparation method and application thereof

Abstract

The invention relates to a preparation method and application of broad-peak distribution polyethylene. More specifically, the molecular weight distribution (MWD) of the polyethylene is between 10 and 30, the melt index is between 0.1 and 1.6, the weight-average molecular weight is between 10000 and 40000, and the short-chain branch content is between 0.40 wt% and 2.0 wt%. The preparation method comprises the following steps: a first-stage slurry polymerization step: carrying out slurry polymerization on ethylene at the reaction temperature of 70-100 DEG C and the reaction pressure of 0.2-4.0 Mpa in the presence of hydrogen by using a polyethylene catalytic system; a second-stage slurry polymerization step in which the product from the first-stage slurry polymerization step is subjected to slurry polymerization with ethylene and a comonomer at a reaction temperature of 70-100 DEG C and a reaction pressure of 0.1-3.0 Mpa, optionally in the presence of hydrogen, to obtain polyethylene, the amount ratio of the product from the first-stage slurry polymerization step to the product from the second-stage slurry polymerization step being 1 / 3 to 2.5, and the amount ratio of the product from the second-stage slurry polymerization step being 1 / 3 to 2 / 3. The amount of the comonomer in the second-stage slurry polymerization step is 10 <-5 >-10 <-1 > parts by mass relative to the total amount of ethylene in the first-stage slurry polymerization step and the second-stage slurry polymerization step.

Description

Technical Field

[0001] The present invention relates to a broad distribution polyethylene, a preparation method of the polyethylene and applications thereof. More specifically, it relates to a broad peak distribution medium density polyethylene with a short branched chain structure, a preparation method of the polyethylene and applications thereof. Background Art

[0002] Polyethylene can be generally divided into high-density polyethylene, medium-density polyethylene and low-density polyethylene according to its density. Polyethylene materials have good chemical corrosion resistance, wear resistance, heat resistance and welding performance. Pipes made of polyethylene materials are widely used in gas transportation, water supply and other fields. Among them, high-density polyethylene is widely used in plastic pipes due to its high strength and modulus.

[0003] High-grade pressure-resistant high-density polyethylene pipes are required to have high mechanical strength, excellent toughness, environmental cracking resistance, slow crack growth resistance and rapid crack growth resistance. However, ordinary high-density polyethylene has few branches, low toughness, low hardness and poor crack resistance. The relative molecular weight and crystallinity of the polymer are the key factors affecting its rigidity and toughness. The higher the relative molecular weight and crystallinity of the polymer, the stronger its rigidity, the lower the crystallinity and the better its toughness. Therefore, with the increase of the relative molecular mass of the polymer, the stiffness is improved and the toughness is also higher, but the processing performance of the polymer becomes worse. On the other hand, polyethylene with a wide molecular weight distribution is conducive to the improvement of processing performance. Low relative molecular weight forms a regular crystal region of the molecular chain, and the mechanical properties of the polymer are improved. The high relative molecular weight part is not easy to be regular and only participates in partial crystallization due to its long molecular weight and relatively high branching. The molecular chain part that is difficult to fold can be interspersed in the crystalline and amorphous regions of the polymer to become a tie molecule, which is beneficial to the long-term mechanical properties of the polymer. Therefore, high-pressure grade polyethylene pipe special resins often have multi-peak or broad-peak molecular weight distribution.

[0004] Tie molecules are key factors affecting the environmental cracking resistance, slow crack growth resistance and rapid crack growth resistance of polymers. Related studies have shown that only molecules with very high relative molecular weight can form tie molecules. In addition, the presence and content of short branches also affect the mechanical properties of polymers. When the molecular chains of the high molecular weight part contain an appropriate amount of short branches that are evenly distributed in the high molecular weight part, it is not only conducive to the disentanglement effect, but also conducive to the miniaturization of the lamellar region. When the length of the short branches is increased, the resistance to slippage between the polymer molecular chains will increase. This is more conducive to the formation of tie molecules. Therefore, the distribution of comonomers in the higher molecular weight part is an extremely critical factor.

[0005] Philips Chemical Company (CN201410086775.5) uses a dual-active center catalyst, and the reactor system is a plurality of loop reactors, or includes at least one loop slurry reactor including a vertical and / or horizontal loop, to prepare metallocene medium-density polyethylene for film blowing process. Iquista Chemical Co., Ltd. (CN01814692.9) uses a loop slurry method plus a gas phase reaction method to prepare polyethylene, wherein the loop slurry is used to prepare high melt index polyethylene, the gas phase method is used to prepare low melt index polyethylene, and the ZN catalyst is used to prepare high molecular weight medium-density polyethylene. In the technical proposal of Total Petrochemical Product Research Flue (CN200780013361.8), the catalyst needs to be titanated at 500°C and gas phase polymerization is used; in the technical proposal of Borelix Technologies (CN200880002742.0), the polymerization method is loop liquid phase polymerization + gas phase polymerization, the liquid phase reaction pressure is about 4.0Mpa, the reaction temperature is 95°C, and high melt index polyethylene is prepared; the gas phase polymerization pressure is 2.0Mpa, copolymerization occurs, and the lacing molecule is prepared. The obtained multimodal medium density polyethylene patent is used for film materials; in the technical proposal of PetroChina (CN201611258743.4), a single reactor is used, the reaction pressure reaches 2.5Mpa, the polymerization temperature is 95°C, and non-multimodal polyethylene is produced, and the product is used for film materials. The DGDA-2420NT medium density polyethylene developed by Dow Chemical's North American Plastics Business Department using Unipol II process technology has excellent performance and is used in the pipe field.

[0006] However, the polyethylene materials provided in the prior art still have the problems of insufficient toughness, insufficient environmental cracking resistance, insufficient resistance to slow crack growth and insufficient resistance to rapid crack growth when used as pipe materials. In addition, in the prior art, the process for preparing such polyethylene materials is complicated and costly. Summary of the invention

[0007] After in-depth research, the inventors of the present invention found that by adopting the method of the present invention, polyethylene with excellent performance can be prepared. The polyethylene has a broad-peak molecular weight distribution and a certain amount of short branching distribution in the relatively high molecular weight part. It is a medium-density polyethylene. When used as a raw material for pipes, it exhibits excellent mechanical properties and toughness, resistance to environmental cracking, resistance to slow crack growth and resistance to rapid crack growth.

[0008] Specifically, the present invention provides the following technical solutions.

[0009] A method for preparing polyethylene, comprising:

[0010] The first stage slurry polymerization step, wherein the polyethylene catalyst system is used to carry out slurry polymerization of ethylene at a reaction temperature of 70-100°C and a reaction pressure of 0.2-4.0 MPa in the presence of hydrogen,

[0011] The second stage slurry polymerization step, wherein the product from the first stage slurry polymerization step is subjected to slurry polymerization with ethylene and a comonomer at a reaction temperature of 70-100° C. and a reaction pressure of 0.1-3.0 MPa, optionally in the presence of hydrogen, to obtain polyethylene.

[0012] The ratio of the amount of the product of the first stage slurry polymerization step to the amount of the product of the second stage slurry polymerization step is 1 / 3 to 2.5, and the mass of the comonomer is 10 relative to the total amount of ethylene in the first stage slurry polymerization step and the second stage slurry polymerization step. -5 -10 -1 Quality parts.

[0013] Effects of the Invention

[0014] The preparation method of the present invention can obtain medium-density polyethylene with excellent mechanical properties and toughness, excellent environmental crack resistance, slow crack growth resistance and rapid crack growth resistance. Therefore, the polyethylene of the present invention is suitable for use as a pipe raw material.

[0015] Moreover, the polyethylene preparation method of the present invention is simple, and the performance of the polyethylene finally obtained can be conveniently adjusted by simply adjusting the raw material ratio between the first stage slurry polymerization step and the second stage slurry polymerization step, the yield ratio between the first stage slurry polymerization step and the second stage slurry polymerization step, the amount of comonomer added, etc.

[0016] Furthermore, the polyethylene of the present invention has a broad-peak molecular weight distribution and therefore has excellent processability. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a schematic diagram of one embodiment of the present invention.

[0018] Figure 2 is a schematic diagram of another embodiment of the present invention. Summary of the invention

[0020] The specific embodiments of the present invention are described in detail below, but it should be pointed out that the protection scope of the present invention is not limited by these specific embodiments, but is determined by the claims in the appendix.

[0021] In the context of this specification, except for the contents explicitly described, any matters or items not mentioned are directly applicable to those known in the art without any changes. Moreover, any embodiment described herein can be freely combined with one or more other embodiments described herein, and the technical solutions or technical ideas formed thereby are regarded as part of the original disclosure or original record of the present invention, and should not be regarded as new contents not disclosed or anticipated herein, unless a person skilled in the art considers that the combination is obviously unreasonable.

[0022] In the context of the present invention, unless otherwise expressly defined or the meaning is beyond the comprehension of a person skilled in the art, hydrocarbon or hydrocarbon derivative groups having more than 3 carbon atoms (such as propyl, propoxy, butyl, butane, butene, butenyl, hexane, etc.) when not preceded by the prefix "n" have the same meaning as when preceded by the prefix "n". For example, propyl is generally understood to be n-propyl, and butyl is generally understood to be n-butyl.

[0023] In the context of the present invention, unless otherwise specified, the physical properties of a substance (such as boiling point) are measured at normal temperature (25° C.) and normal pressure (101325 Pa).

[0024] The method for preparing the broad peak distribution polyethylene of the present invention is carried out in a two-stage polymerization reaction mode, wherein the first-stage and the second-stage polymerization modes are both slurry polymerization methods, preferably kettle slurry polymerization.

[0025] The method for preparing the broad peak distribution polyethylene of the present invention can be carried out by continuous polymerization or intermittent polymerization, preferably by continuous polymerization.

[0026] The preparation method of the broad peak distribution polyethylene of the present invention can be carried out in one polymerization reactor or in two reactors. When carried out in one polymerization reactor, after the first stage slurry polymerization step is completed, the second stage slurry polymerization step is carried out. When carried out in two independent reactors, the first stage slurry polymerization step and the second stage slurry polymerization step are respectively and continuously carried out in two independent polymerization reactors connected in series. Preferably, polymerization is carried out in two independent reactors connected in series. More preferably, polymerization is carried out in two independent reactors connected in series in a continuous polymerization manner.

[0027] In the present invention Figure 1 In the embodiment of the present invention, it is shown that in two independent polymerization reactors connected in series, the first stage slurry polymerization step and the second stage slurry polymerization step are respectively and continuously carried out. Figure 2 In the figure, it is shown that the first stage slurry polymerization step and the second stage slurry polymerization step are respectively and continuously carried out in a single polymerization reactor.

[0028] In the present invention, the first stage slurry polymerization step and the second stage slurry polymerization step are carried out in a conventional polymerization solvent in the art. The conventional polymerization solvent includes, but is not limited to, C 4-10 Alkanes (such as butane, pentane, hexane, heptane, octane, nonane or decane, etc.), halogenated C 1-10 Alkanes (such as dichloromethane), aromatic hydrocarbon solvents (such as toluene and xylene), ether solvents (such as diethyl ether or tetrahydrofuran), ester solvents (such as ethyl acetate) and ketone solvents (such as acetone), etc. Among them, hexane, isopropane, etc. are preferably used as the polymerization solvent.

[0029] The method for preparing polyethylene of the present invention comprises:

[0030] The first stage slurry polymerization step, wherein ethylene is subjected to slurry polymerization using a polyethylene catalyst system at a reaction temperature of 70-100°C and a reaction pressure of 0.2-4.0 MPa in the presence of hydrogen, and

[0031] The second stage slurry polymerization step, wherein the product from the first stage slurry polymerization step is subjected to slurry polymerization with ethylene and a comonomer at a reaction temperature of 70-100° C. and a reaction pressure of 0.1-3.0 MPa, optionally in the presence of hydrogen, to obtain polyethylene.

[0032] The ratio of the amount of the product of the first stage slurry polymerization step to the amount of the product of the second stage slurry polymerization step is 1 / 3 to 2.5, and the mass of the comonomer is 10 relative to the total amount of ethylene in the first stage slurry polymerization step and the second stage slurry polymerization step. -5 -10 -1 Mass parts.

[0033] In the present invention, the first stage slurry polymerization step is sometimes also referred to as a first reaction step, and the second stage slurry polymerization step is sometimes also referred to as a second reaction step.

[0034] In the present invention, the polyethylene catalyst system is a conventional catalyst system in the art that can be used for ethylene polymerization, which generally includes a main catalyst, a co-catalyst and an optional carrier.

[0035] In the present invention, as the main catalyst of the polyethylene catalyst system, it can be selected from non-metallocene catalysts, metallocene catalysts, Ziegler-Natta catalysts, or the composites between them. Specifically, there can be listed a supported single-center metal catalyst with catalytic ethylene polymerization activity, a supported multi-active center metal catalyst, a Ziegler-Natta catalyst, a supported Ziegler-Natta and metallocene composite catalyst, a supported double or multiple metallocene catalysts, a supported Ziegler-Natta and non-metallocene composite catalyst, a supported double or multiple non-metallocene catalysts, a supported metallocene and non-metallocene composite catalyst, a supported organic chromium catalyst, a supported inorganic chromium catalyst, a mixed organic chromium / chromium oxide catalyst, a supported late transition metal catalyst, a supported Ziegler-Natta catalyst, etc.

[0036] Specifically, as a non-metallocene catalyst, such as invention patents CN200310106156.X, CN200310106157.4, CN200410066068.6, CN200510119401.X, CN200610107651.6, CN200710162677.5, CN 200710162667.1, CN200710162672.2, CN 200710162675.6, CN200710162676.0, CN200710162666.7, CN200910180100.6, CN 200910180607.1, CN200910180601.4, CN200910180606.7, CN200910180602.9, CN200910180605.2, CN2009101806 03.3, CN200910180604.8, CN200910210988.3, ​​CN200910210984.5, CN200910210989.8, CN200910210986.4, CN200 910210985. 7. CN201010284870.8, CN201010285982.5, CN201010284856.8, CN201010285970.2, CN201010285956.2, CN201010 285969.X, CN201010285958.1, CN201010285967.0, CN201010285994.8, CN201110259336.6, CN201110259219.X, C N201110259330.9, CN201110259327.7, CN201110259367.1, CN201110259289.5, CN201110259359.7, CN201110259 282.3, CN201110259318.8, CN201110259258.X, CN201110259300.8, CN201110259254.1, CN001110259299.9, CN20 1110259245.2, CN201110259296.5, CN201110259338.5, CN201110259370.3, CN201110259339.X, CN201110259293.1. CN201110259356.3, CN201210063756.1, CN201210063777.3, CN201210063788.1, CN201210 063818.9, CN201210063824.4, CN201210063843.7, CN201210063854.5, CN201210063876.1, CN 201210063878.0, CN201210063891.6, CN201210063894.X, CN201210063907.3, CN2012100639 09.2, CN201210063935.5, CN201210063941.0, CN201210063945.9, CN201310189677.X, CN2013 10227368.7, CN201310227370.4, CN201310227830.3, CN201310227393.5, CN201310452714.1 , CN201710814678.7, CN201710814595.8, CN201710814594.3, CN201710814593.9, CN20171081 Supported non-metallocene catalysts involved in CN201710814591.X, CN201811144599.0, CN201811144768.0, CN201811139936.7, CN201811140811.6, CN201811139946.0, CN201811139741.2, CN201310091208.4, etc.

[0037] In the present invention, the non-metallocene catalyst is preferably a supported non-metallocene catalyst, and more preferably a supported single-component non-metallocene catalyst.

[0038] The metallocene catalyst may be any catalyst containing a metallocene catalyst. It may be a single-component metallocene catalyst or a composite catalyst of a metallocene catalyst and other catalysts. Specifically, a composite catalyst of a supported Ziegler-Natta and a metallocene, a supported dual or multi-metallocene catalyst, etc. may be mentioned.

[0039] More specifically, as a metallocene catalyst, such as invention patents CN201110247347.2, CN201110080343.X, CN201010518904.5, CN201010519660.2, CN201210289014.0, CN200910078596.6, CN201310090758.4, CN201310090736.8, 201310521768.9, CN201410589467.4, CN201410590067.5, CN201610835700.1, 201610944191.6, CN201710959 423.X, CN201110247349.1, CN201110080294.X, CN201110080395.7, CN201 210289017.4, CN201210289031.4, CN201310091192.7, CN201310540973.X , CN201510724626.1, CN200410086283.2, CN200610137777.8, CN20161094 4182.7, CN201710312720.5, CN201110080422.0, CN201110080422.0, CN20 1110080394.2, CN201010519406.2, CN201010519715.X, CN201010519174. 0. CN201010519429.3, CN201210289004.7, CN201310090847.9, CN2013100 91209.9, CN201310540975.9, CN201410554709.6, CN201410513506.2, CN0 0130388.0, CN200710176589.0, CN201610944083.9, CN201110246705.8, C N201110247085.X, CN2011102914899, CN201010521674.8, CN20131009075 2.7, CN201310090848.3, CN2013100908483, CN201510624502.6, CN201710 166709.2, CN20171031225.2, CN201110246710.9, CN201110080374.5, CN2 01010519797.8, CN201210289012.1, CN201210418645.8, CN201310090998.4. Supported metallocene catalysts involved in CN201410252254.2, CN201610393399.3, CN201610956141.X, CN201710958837.0, etc., as well as Grace Davison's 101 and 201 supported metallocene catalysts and Univation's XCAT metallocene catalysts.

[0040] Among them, the metallocene ligand and metallocene complex structure in the metallocene catalyst can be selected from a homoleptic bis-metallocene structure, a mixed bis-metallocene structure, a chiral metallocene structure, a single / double / multi-bridged, a monometallocene, a constrained geometry monometallocene or bis-metallocene, a bridged mixed metallocene, a cationic type, etc.

[0041] In the present invention, the metallocene catalyst is preferably a supported metallocene catalyst, and more preferably a supported single-component metallocene catalyst.

[0042] As the Ziegler-Natta catalyst, any catalyst containing a Ziegler-Natta catalyst may be used. It may be a single-component Ziegler-Natta catalyst or a composite catalyst of a Ziegler-Natta catalyst and other catalysts. Specifically, there may be listed Ziegler-Natta catalysts, composite catalysts of supported Ziegler-Natta and metallocene, composite catalysts of supported Ziegler-Natta and non-metallocene, supported Ziegler-Natta catalysts, and the like.

[0043] More specifically, as a Ziegler-Natta catalyst, such as invention patents CN201010522112.5, CN201010240555.X, CN201010502803.9, CN201010511310.1, CN200710121105.2, CN201010502778.4, CN201010502717.8, CN201010240379.5, CN201110148492.5, CN201110148493.X, CN201110148527.5, CN201110148545.3, CN201 Ziegler-Natta type catalysts involved in 110306102.2, CN201010240378.0, CN200410086382.0, CN98101108.X, CN200410078280.4, CN200910088546.6, etc., such as the BCH, BCE, BCG, BCS and other series catalysts of Beijing Aoda Branch of Sinopec Catalyst Co., Ltd., SCG series catalysts of Shanghai Lide Catalyst Co., Ltd., TH series catalysts of Zibo Xinsu Chemical Co., Ltd., M and J type catalysts of Univation, XY-H and XY-S type catalysts of Yingkou, Liaoning, etc.

[0044] In the present invention, as the Ziegler-Natta catalyst, a supported Ziegler-Natta catalyst is preferred, and a supported single-component Ziegler-Natta catalyst is more preferred. For example, the BCH, BCE, BCG, BCS and other series of catalysts sold on the market by Sinopec Aoda Catalyst Company, the SCG series of catalysts by Shanghai Lide Catalyst Co., Ltd., the TH series of catalysts by Zibo Xinsu Chemical Co., Ltd., and the XY-H type catalysts by Yingkou Xiangyang Catalyst Co., Ltd.

[0045] It should be noted that, in the present invention, unless otherwise specified, the molar amount of the main catalyst is based on the active metal element in the main catalyst.

[0046] As the main catalyst of the present invention, a metallocene catalyst or a Ziegler-Natta catalyst is preferred, and a metallocene catalyst is more preferred.

[0047] In the present invention, the cocatalyst of the polyethylene catalyst system can be selected from aluminoxane, alkyl aluminum, halogenated alkyl aluminum, borofluorane, alkyl boron or alkyl boron ammonium salt or a mixture thereof. Preferably, it is selected from aluminoxane, alkyl aluminum, halogenated alkyl aluminum or a mixture thereof, and more preferably, it is selected from aluminoxane, alkyl aluminum or a mixture thereof.

[0048] The aluminoxane used as a co-catalyst may be, for example, a linear aluminoxane represented by the following general formula (I): (R)(R)Al-(Al(R)-O)n -O-Al(R)(R), and a cyclic aluminoxane represented by the following general formula (II): -(Al(R)-O-) n+2 -.

[0049]

[0050] In the aforementioned general formulae (I) and (II), the groups R are identical or different (preferably identical) to each other, and are independently selected from C1-C8 alkyl, preferably methyl, ethyl, propyl, butyl and isobutyl, most preferably methyl and isobutyl; n is any integer in the range of 1-50, preferably any integer in the range of 10-30.

[0051] As the aluminoxane, methylaluminoxane, ethylaluminoxane, isobutylaluminoxane and n-butylaluminoxane are preferred, and methylaluminoxane and isobutylaluminoxane are more preferred.

[0052] These aluminoxanes may be used alone or in combination of two or more in any ratio.

[0053] As the alkyl aluminum, for example, there can be mentioned compounds represented by the following general formula:

[0054] Al(R)3

[0055] The groups R are identical or different (preferably identical) to each other, and are each independently selected from C1-C8 alkyl, preferably methyl, ethyl, propyl, butyl and isobutyl, and most preferably methyl and isobutyl.

[0056] Specifically, examples of the alkyl aluminum include trimethyl aluminum (Al(CH3)3), triethyl aluminum (Al(CH3CH2)3), tri-n-propyl aluminum (Al(C3H7)3), tri-isopropyl aluminum (Al(i-C3H7)3), tri-isobutyl aluminum (Al(i-C4H9)3), tri-n-butyl aluminum (Al(C4H9)3), tri-isopentyl aluminum (Al(i-C5H 11 )3), tri-n-pentyl aluminum (Al(C5H 11 )3) Tri-n-hexyl aluminum (Al(C6H 13 )3) Triisohexyl aluminum (Al(i-C6H 13 )3), diethylmethylaluminum (Al(CH3)(CH3CH2)2) and dimethylethylaluminum ((Al(CH3CH2)(CH3)2), etc., among which trimethylaluminum, triethylaluminum, tripropylaluminum and triisobutylaluminum are preferred, and triethylaluminum and triisobutylaluminum are most preferred.

[0057] These aluminum alkyls may be used alone or in combination of two or more in any ratio.

[0058] As the halogenated alkyl aluminum, for example, there can be mentioned compounds represented by the following general formula:

[0059] Al(R) n X 3-n

[0060] Wherein, the groups R are identical or different (preferably identical) to each other, and are independently selected from C1-C8 alkyl, preferably methyl, ethyl, propyl, butyl and isobutyl, most preferably methyl and isobutyl; X represents fluorine, chlorine, bromine or iodine; and n represents 1 or 2.

[0061] Specifically, as the halogenated alkyl aluminum, for example, there can be mentioned dimethylaluminum monochloride (Al(CH3)2Cl), dimethylaluminum dichloride (Al(CH3)Cl2)), diethylaluminum monochloride (Al(CH3CH2)2Cl), diethylaluminum dichloride (Al(CH3CH2)Cl2), dipropylaluminum monochloride (Al(C3H7)2Cl), dipropylaluminum dichloride (Al(C3H7)Cl2), di-n-butylaluminum monochloride (Al(C4H9)2Cl), di-n-butylaluminum dichloride (Al(C4H9)Cl2), diisobutylaluminum monochloride (Al(i-C4H9)2Cl), diisobutylaluminum dichloride (Al(i-C4H9)Cl2), di-n-pentylaluminum monochloride (Al(C5H 11 )2Cl), dichloropentylaluminum (Al(C5H 11 )Cl2), diisopentylaluminum monochloride (Al(i-C5H 11 )2Cl), dichloroisopentylaluminum (Al(i-C5H 11 )Cl2), di-n-hexylaluminum chloride (Al(C6H 13 )2Cl), dichlorohexyl aluminum (Al(C6H 13 )Cl2), diisohexylaluminum monochloride (Al(i-C6H 13 )2Cl), dichloroisohexylaluminum (Al(i-C6H 13)Cl2), chloromethyl ethylaluminum (Al(CH3)(CH3CH2)Cl), chloromethyl propylaluminum (Al(CH3)(C3H7)Cl), chloromethyl n-butylaluminum (Al(CH3)(C4H9)Cl), chloromethyl isobutylaluminum (Al(CH3)(i-C4H9)Cl), chloroethyl propylaluminum (Al(CH2CH3)(C3H7)Cl), chloroethyl n-butylaluminum (AlCH2CH3)(C4H9)Cl), chloromethyl isobutylaluminum (Al(CH2CH3)(i-C4H9)Cl), etc., among which diethylaluminum chloride, ethylaluminum dichloride, di-n-butylaluminum chloride, n-butylaluminum dichloride, diisobutylaluminum chloride, isobutylaluminum dichloride, di-n-hexylaluminum chloride, and n-hexylaluminum dichloride are preferred, and diethylaluminum chloride, ethylaluminum dichloride and di-n-hexylaluminum chloride are further preferred, and diethylaluminum chloride is most preferred.

[0062] These alkylaluminum halides may be used alone or in combination of two or more in any ratio.

[0063] As the borofluorane, alkyl boron and alkyl boron ammonium salt, those conventionally used in the art can be directly used without particular limitation, for example, trimethyl boron, triethyl boron, triphenyl boron, tris(pentafluorophenyl) boron, tris[3,5-bis(trifluoromethyl)phenyl] boron, hexafluorophenyl boron, trityl tetra(pentafluorophenyl) borate, N,N-dimethylanilinium tetra(pentafluorophenyl) borate, 1-butyl-3-methylimidazole tetrafluoroborate, ferrocene tetrafluoroborate, trimethylammonium tetraphenyl borate, triethylammonium tetraphenyl borate, tripropylammonium tetraphenylborate, tributylammonium tetraphenylborate, trimethylammonium tetra(p-tolyl)borate, tripropylammonium tetra(p-tolyl)borate, trimethylammonium tetra(o,p-dimethylphenyl)borate, triethylammonium tetra(o,p-dimethylphenyl)borate, trimethylammonium tetra(p-trifluoromethylphenyl)borate, tributylammonium tetra(p-trifluoromethylphenyl)borate, tributylammonium tetra(pentafluorophenyl)borate, N,N-diethylanilinium tetraphenylborate, N,N-diethylanilinium tetrapentafluorophenylborate, diethylammonium tetrapentafluorophenylborate, and the like.

[0064] In addition, according to the present invention, the co-catalyst may be used alone or in combination of multiple co-catalysts in any proportion as required. Moreover, there is no particular limitation on the proportion of each component in the mixture and it can be selected as required.

[0065] According to the present invention, the cocatalyst is generally used in the form of a solution. When preparing the solution of the cocatalyst, there is no particular limitation on the solvent used at this time, as long as it can dissolve the cocatalyst, and generally an alkane solvent such as n-pentane, isopentane, cyclopentane, neopentane, etc., or an aromatic solvent such as toluene, ethylbenzene, xylene, etc. is selected. According to the present invention, in order to facilitate subsequent separation, the same solvent as the polymerization solvent is preferred; or the same solvent as one of the solvents in the mixed solvent for polymerization.

[0066] It should be noted that, unless otherwise specified, in the present invention, when the co-catalyst is aluminoxane, alkyl aluminum or halogenated alkyl aluminum, the molar amount of the co-catalyst is based on the molar amount of the Al element; when the co-catalyst is borofluoroalkane, alkyl boron or alkyl boron ammonium salt, the molar amount of the co-catalyst is based on the molar amount of the B element.

[0067] In the present invention, the main catalyst of the polyethylene catalytic system can be optionally supported on a carrier, and the carrier can be selected from porous silica gel carriers, layered porous carriers, organic polymer carriers, magnesium compound carriers, oxide carriers, and the like.

[0068] The magnesium compound carrier may be selected from magnesium compounds such as magnesium halide, alkoxymagnesium halide, alkoxymagnesium, alkylmagnesium, alkylmagnesium halide and alkylalkoxymagnesium.

[0069] Specifically, examples of the magnesium halide include magnesium chloride (MgCl 2 ), magnesium bromide (MgBr 2 ), magnesium iodide (MgI 2 ) and magnesium fluoride (MgF 2 ), among which magnesium chloride is preferred.

[0070] Examples of the alkoxy magnesium halide include methoxymagnesium chloride (Mg(OCH3)Cl), ethoxymagnesium chloride (Mg(OC2H5)Cl), propoxymagnesium chloride (Mg(OC3H7)Cl), n-butoxymagnesium chloride (Mg(OC4H9)Cl), isobutoxymagnesium chloride (Mg(i-OC4H9)Cl), methoxymagnesium bromide (Mg(OCH3)Br), ethoxymagnesium bromide (Mg(OC2H5)Br), propoxymagnesium bromide (Mg(OC3H7) Br), n-butoxymagnesium bromide (Mg(OC4H9)Br), isobutoxymagnesium bromide (Mg(i-OC4H9)Br), methoxymagnesium iodide (Mg(OCH3)I), ethoxymagnesium iodide (Mg(OC2H5)I), propoxymagnesium iodide (Mg(OC3H7)I), n-butoxymagnesium iodide (Mg(OC4H9)I) and isobutoxymagnesium iodide (Mg(i-OC4H9)I), etc., among which methoxymagnesium chloride, ethoxymagnesium chloride and isobutoxymagnesium chloride are preferred.

[0071] As alkoxymagnesium, for example, methoxymagnesium (Mg(OCH3)2), ethoxymagnesium (Mg(OC2H5)2), propoxymagnesium (Mg(OC3H7)2), butoxymagnesium (Mg(OC4H9)2), isobutoxymagnesium (Mg(i-OC4H9)2) and 2-ethylhexyloxymagnesium (Mg(OCH2CH(C2H5)C4H8)2) can be mentioned, among which ethoxymagnesium and isobutoxymagnesium are preferred.

[0072] As alkyl magnesium, for example, methyl magnesium (Mg(CH3)2), ethyl magnesium (Mg(C2H5)2), propyl magnesium (Mg(C3H7)2), n-butyl magnesium (Mg(C4H9)2) and isobutyl magnesium (Mg(i-C4H9)2) can be mentioned, among which ethyl magnesium and n-butyl magnesium are preferred.

[0073] Examples of the alkyl magnesium halide include methyl magnesium chloride (Mg(CH3)Cl), ethyl magnesium chloride (Mg(C2H5)Cl), propyl magnesium chloride (Mg(C3H7)Cl), n-butyl magnesium chloride (Mg(C4H9)Cl), isobutyl magnesium chloride (Mg(i-C4H9)Cl), methyl magnesium bromide (Mg(CH3)Br), ethyl magnesium bromide (Mg(C2H5)Br), propyl magnesium bromide (Mg(C3H7)Cl), Br), n-butylmagnesium bromide (Mg(C4H9)Br), isobutylmagnesium bromide (Mg(i-C4H9)Br), methylmagnesium iodide (Mg(CH3)I), ethylmagnesium iodide (Mg(C2H5)I), propylmagnesium iodide (Mg(C3H7)I), n-butylmagnesium iodide (Mg(C4H9)I) and isobutylmagnesium iodide (Mg(i-C4H9)I), etc., among which methylmagnesium chloride, ethylmagnesium chloride and isobutylmagnesium chloride are preferred.

[0074] Examples of the alkyl magnesium alkoxide include methyl methoxy magnesium (Mg(OCH3)(CH3)), methyl ethoxy magnesium (Mg(OC2H5)(CH3)), methyl propoxy magnesium (Mg(OC3H7)(CH3)), methyl n-butoxy magnesium (Mg(OC4H9)(CH3)), methyl isobutoxy magnesium (Mg(i-OC4H9)(CH3)), ethyl methoxy magnesium (Mg(OCH3)(C2H5)), ethyl ethoxy magnesium (Mg(OC2H5)(C2H5)), ethyl propoxy magnesium (Mg(OC3H7)(C2H5)), ethyl n-butoxy magnesium (Mg(OC4H9)(C2H5)), ethyl isobutoxy magnesium (Mg(i-OC4H9)(C2H5)), propyl methoxy magnesium (Mg(OCH3)(C3H7)), propyl ethoxy magnesium (Mg(OC2H5)(C3H7)), propyl propoxy magnesium (Mg(OC3H7)(C3H7)), propyl Magnesium methoxide (Mg(OCH3)(C4H9)), magnesium ethoxide (Mg(OC2H5)(C4H9)), magnesium propoxide (Mg(OC3H7)(C4H9)), magnesium n-butyl-n-butoxide (Mg(OC4H9)(C4H9)), magnesium n-butyl-isobutoxide (Mg(i-OC4H9)(C3H7)), magnesium n-butyl-methoxide (Mg(OCH3)(C4H9)), magnesium n-butyl-ethoxide (Mg(OC2H5)(C4H9)), magnesium n-butyl-prop ... -OC4H9)(C4H9)), isobutyl methoxymagnesium (Mg(OCH3)(i-C4H9)), isobutyl ethoxymagnesium (Mg(OC2H5)(i-C4H9)), isobutyl propoxymagnesium (Mg(OC3H7)(i-C4H9)), isobutyl n-butoxymagnesium (Mg(OC4H9)(i-C4H9)) and isobutyl isobutoxymagnesium (Mg(i-OC4H9)(i-C4H9)), etc., among which butyl ethoxymagnesium is preferred.

[0075] These magnesium compounds may be used alone or in combination of two or more.

[0076] Examples of the porous carrier include organic or inorganic porous solids conventionally used as carriers in the preparation of supported olefin polymerization catalysts in the art.

[0077] Specifically, as organic porous solids, for example, there can be mentioned olefin homopolymers or copolymers, polyvinyl alcohol or its copolymers, cyclodextrin, (co)polyester, (co)polyamide, vinyl chloride homopolymers or copolymers, acrylate homopolymers or copolymers, methacrylate homopolymers or copolymers, and styrene homopolymers or copolymers, as well as partially cross-linked forms of these homopolymers or copolymers, among which partially cross-linked (for example, a cross-linking degree of at least 2% but less than 100%) styrene polymers are preferred.

[0078] According to the present invention, when an organic porous solid is used as a carrier, the organic porous solid can also be subjected to a heat activation treatment before use. The heat activation treatment can be carried out in a usual manner, such as heating the organic porous solid under reduced pressure or in an inert atmosphere. The inert atmosphere mentioned here refers to a gas containing only extremely trace amounts or no components that can react with the organic porous solid. As the inert atmosphere, for example, nitrogen or a rare gas atmosphere can be cited, preferably a nitrogen atmosphere. Since the organic porous solid has poor heat resistance, the heat activation process is based on the premise of not destroying the structure and basic composition of the organic porous solid itself. Generally, the heat activation temperature is 50-400°C, preferably 100-250°C, and the heat activation time is 1-24h, preferably 2-12h. After the heat activation treatment, the organic porous solid needs to be stored under positive pressure in an inert atmosphere for standby use.

[0079] As inorganic porous solids, for example, refractory oxides of metals of Groups IIA, IIIA, IVA or IVB of the periodic table (such as silicon dioxide (also known as silicon oxide or silica gel), aluminum oxide, magnesium oxide, titanium oxide, zirconium oxide or thorium oxide, etc.), or any refractory composite oxides of these metals (such as silicon aluminum oxide, magnesium aluminum oxide, titanium silicon oxide, titanium magnesium oxide and titanium aluminum oxide, etc.), as well as clay, molecular sieves (such as ZSM-5 and MCM-41), mica, montmorillonite, bentonite and diatomaceous earth, etc. As the inorganic porous solids, oxides generated by high-temperature hydrolysis of gaseous metal halides or gaseous silicon compounds, such as silica gel obtained by high-temperature hydrolysis of silicon tetrachloride, or aluminum oxide obtained by high-temperature hydrolysis of aluminum trichloride, etc. are also mentioned. Silicon dioxide, aluminum oxide, magnesium oxide, silicon aluminum oxide, magnesium aluminum oxide, titanium silicon oxide, titanium dioxide, molecular sieves and montmorillonite, etc. are preferred, and silicon dioxide is particularly preferred. Suitable silica can be prepared by conventional methods or can be any commercially available product, such as Grace 955, Grace 948, Grace SP9-351, Grace SP9-485, Grace SP9-10046, Davsion Syloid 245 and Aerosil812 from Grace, ES70, ES70X, ES70Y, ES70W, ES757, EP10X and EP11 from Ineos, and CS-2133 and MS-3040 from PQ.

[0080] In one embodiment of the present invention, the polymerization reaction temperature of the first stage slurry polymerization step and the second stage slurry polymerization step is 70-100°C, preferably 75-90°C, and more preferably 80-90°C. Wherein, if the ethylene slurry polymerization is carried out at a higher polymerization temperature, a solvent with a higher boiling point can be selected, and vice versa, if the ethylene slurry polymerization is carried out at a lower polymerization temperature, a solvent with a lower boiling point can be selected. It is known that under the conditions of ethylene slurry polymerization, under similar and comparable conditions such as polymerization pressure, main catalyst, co-catalyst and solvent, within the polymerization temperature range described in the present invention, as the polymerization temperature increases, the molecular weight of the polyethylene obtained thereby first increases and then decreases. Therefore, according to the present invention, the molecular weight and molecular weight distribution of the polyethylene obtained by ethylene slurry polymerization can be adjusted and controlled by the polymerization temperature.

[0081] In one embodiment of the present invention, the polymerization reaction pressure of the first stage slurry polymerization step is 0.2-4.0MPa, preferably 0.3-3.0MPa, and more preferably 0.5-2.0MPa. Wherein, if the ethylene slurry polymerization is carried out at a higher polymerization temperature, a lower polymerization pressure can be selected, and vice versa, if the ethylene slurry polymerization is carried out at a lower polymerization temperature, a higher polymerization pressure can be selected. Under the conditions of ethylene slurry polymerization, under similar and comparable conditions such as polymerization temperature, main catalyst, co-catalyst and solvent, within the polymerization pressure range described in the present invention, as the polymerization pressure increases, the molecular weight of the polyethylene obtained thereby also increases first and then decreases. Therefore, according to the present invention, the molecular weight and molecular weight distribution of the polyethylene obtained by ethylene slurry polymerization can also be adjusted and controlled by the polymerization pressure.

[0082] In one embodiment of the present invention, the polymerization reaction pressure of the second stage slurry polymerization step is 0.1-3.0MPa, preferably 0.2-2.0MPa, and more preferably 0.3-2.0MPa. Wherein, if the ethylene slurry polymerization is carried out at a higher polymerization temperature, a lower polymerization pressure can be selected, and vice versa, if the ethylene slurry polymerization is carried out at a lower polymerization temperature, a higher polymerization pressure can be selected. Under the conditions of ethylene slurry polymerization, under similar and comparable conditions such as polymerization temperature, primary catalyst, co-catalyst and solvent, within the polymerization pressure range described in the present invention, as the polymerization pressure increases, the molecular weight of the polyethylene obtained thereby also increases first and then decreases. Therefore, according to the present invention, the molecular weight and molecular weight distribution of the polyethylene obtained by ethylene slurry polymerization can also be adjusted and controlled by the polymerization pressure.

[0083] In one embodiment of the present invention, the polymerization reaction pressure of the second stage slurry polymerization step is lower than or equal to the polymerization reaction pressure of the first stage slurry polymerization step.

[0084] In one embodiment of the present invention, the polymerization temperature and polymerization pressure of the first stage slurry polymerization step and the second stage slurry polymerization step can be adjusted as needed. Specifically, the appropriate reaction temperature and reaction pressure can be set according to the molecular weight and molecular weight distribution of the polyethylene.

[0085] In addition, it is well known in the art that in order to adjust the polymerization pressure, an inert gas that does not participate in the reaction (such as nitrogen) can be introduced into the polymerization reactor.

[0086] In one embodiment of the present invention, in the first stage slurry polymerization step, the amount of hydrogen can be 10 -7 -10 -1 parts by mass, preferably 10 -6 -10 -2 parts by mass, more preferably 10 -5 -10 -2 In the case of a continuous polymerization method, the mass ratio of hydrogen to ethylene can be expressed as their flow ratio.

[0087] In one embodiment of the present invention, in the first stage slurry polymerization step, the polymerization solvent is the polymerization solvent of the present invention described above. The amount of the polymerization solvent can be arbitrarily adjusted as needed, and the amount of the polymerization solvent can be 0.1-20 parts by mass, preferably 0.5-10 parts by mass, and more preferably 0.8-6 parts by mass relative to the amount of ethylene in the first stage slurry polymerization step.

[0088] In one embodiment of the present invention, the reaction time of the first stage slurry polymerization step is not particularly limited and can be 20 minutes to 6 hours, preferably 40 minutes to 2 hours. Specifically, the appropriate reaction time can be set according to the desired molecular weight and molecular weight distribution of the polyethylene. In the continuous polymerization method, the weight hourly space velocity of ethylene can be 0.2-1h -1 , preferably 0.3-0.5h -1 .

[0089] In one embodiment of the present invention, the first stage slurry polymerization step may be carried out under stirring.

[0090] In one embodiment of the present invention, the melt index of the output material (ethylene homopolymer) of the first stage slurry polymerization step is in the range of 1-900 g / 10 min, preferably 20-900 g / 10 min, more preferably 100-900 g / 10 min.

[0091] In one embodiment of the present invention, the discharge of the first stage slurry polymerization step is not treated, that is, fed to the second stage slurry polymerization step. In one embodiment of the present invention, the discharge of the first stage slurry polymerization step is dehydrogenated and then fed to the second stage slurry polymerization step. Therefore, in one embodiment of the present invention, a dehydrogenation step is further provided between the first stage slurry polymerization step and the second stage slurry polymerization step.

[0092] In one embodiment of the present invention, the second stage slurry polymerization step can be carried out in the presence of hydrogen or in the absence of hydrogen. When hydrogen is used in the second stage slurry polymerization step, the amount of hydrogen can be 10:1 relative to the amount of ethylene in the second stage slurry polymerization step. -8 -10 -3 parts by mass, preferably 10 -6 -10 -4 Mass parts.

[0093] In the present invention, in the second stage slurry polymerization step, there is no need to supplement the catalyst.

[0094] In one embodiment of the present invention, in the second stage slurry polymerization step, the comonomer is selected from α-olefins, diolefins, cyclic olefins and other ethylenically unsaturated compounds. The α-olefin may be C3-C 10 As the α-olefins, for example, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 4-methyl-1-pentene, 4-methyl-1-hexene, 1-octene, 1-decene, 1-undecene, 1-dodecene and styrene can be mentioned. As the cyclic olefins, for example, 1-cyclopentene, ethylidene norbornene and norbornene can be mentioned. As the dienes, for example, 1,4-butadiene, 2,5-pentadiene, 1,5-hexadiene, vinyl norbornene, norbornene and 1,7-octadiene can be mentioned. As the other ethylenically unsaturated compounds, for example, vinyl acetate and (meth)acrylate can be mentioned. Among them, the comonomer is preferably C3-C 10 The α-olefin is more preferably at least one selected from 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene, and further preferably at least one selected from 1-butene, 1-hexene, and 1-octene.

[0095] The short chain branches of the polyethylene of the present invention come from the comonomer, so the amount of the comonomer used affects the content of the short chain branches in the final polyethylene product. Therefore, the number of carbon atoms of the short chain branches of the polyethylene of the present invention is different according to the different comonomers used. For example, when 1-hexene is used as a comonomer, the short chain branches are C4 branches.

[0096] In one embodiment of the present invention, the amount of the comonomer in the second stage slurry polymerization step is 10 -5 -10 -1 parts by mass, preferably 10 -4 -5×10 -2 parts by mass, more preferably 10 -3 -5×10 -2 In the case of continuous polymerization, the mass ratio of the comonomer to ethylene can be expressed as its flow ratio.

[0097] In one embodiment of the present invention, when hydrogen is used in the second stage slurry polymerization step, the amount of hydrogen is 10 -8 -10 -3 parts by mass, preferably 10 -6 -10 -4 In the case of continuous polymerization, the mass ratio of hydrogen to comonomer can be expressed as its flow ratio.

[0098] In one embodiment of the present invention, the distribution ratio of ethylene in the first stage slurry polymerization step and the second stage slurry polymerization step, that is, the ratio of the amount of ethylene participating in the first stage slurry polymerization step to the amount of ethylene participating in the second stage slurry polymerization step (first stage slurry polymerization step: second stage slurry polymerization step), is 0.1-3, preferably 0.2-2.5, and more preferably 0.25-2.2 in terms of mass ratio.

[0099] In one embodiment of the present invention, the reaction time of the second stage slurry polymerization step is not particularly limited and can be 10 minutes to 6 hours, preferably 20 minutes to 2 hours. Specifically, the appropriate reaction time can be set according to the desired molecular weight and molecular weight distribution of the polyethylene. In the continuous polymerization method, the weight hourly space velocity of ethylene can be 0.2-1h -1 , preferably 0.5-1h -1 .

[0100] In one embodiment of the present invention, in the second stage slurry polymerization step, a polymerization solvent may be added as needed to adjust the slurry concentration.

[0101] In one embodiment of the present invention, the second stage slurry polymerization step may be carried out under stirring.

[0102] In the present invention, it is important that the amount ratio of the products of the first stage slurry polymerization step and the second stage slurry polymerization step is 1 / 3 to 2.5, preferably 0.0.5-2, more preferably 0.75-1.5 in terms of mass ratio.

[0103] In the present invention, the ratio of the amount of the product of the first stage slurry polymerization step to the amount of the product of the second stage slurry polymerization step refers to the ratio of the amount of the product of the first stage slurry polymerization step to the amount of the product of the second stage slurry polymerization step relative to the feed from the first stage slurry polymerization step. More specifically, the product of the first stage slurry polymerization step is the homopolymer of ethylene discharged from the first stage slurry polymerization step; in the second stage slurry polymerization step, the raw materials participating in the reaction are the product of the first stage slurry polymerization step, ethylene and comonomers, and the product of the second stage slurry polymerization step is: the amount of the product obtained by subtracting the feed from the first stage slurry polymerization step (i.e., the product of the first slurry polymerization step) from the final product of the second stage slurry polymerization step is the amount of the product of the second stage slurry polymerization step relative to the feed from the first stage slurry polymerization step, i.e., the product of the second stage slurry polymerization step.

[0104] In the present invention, by controlling the amount ratio of the products of the first stage slurry polymerization step and the second stage slurry polymerization step to be within the specific range of the present invention, the molecular weight of the polyethylene can be made to have a broad peak distribution, so that the processability of the polyethylene is good. In addition, the short branches derived from the comonomer are distributed in a relatively high molecular weight portion of the polyethylene chain at a specific content, so that the pipe prepared from the polyethylene has excellent toughness.

[0105] In the present invention, the ratio of the amount of the product of the first stage slurry polymerization step and the second stage slurry polymerization step can be adjusted by adjusting the reaction raw materials, catalyst type, catalyst amount, reaction conditions, polymerization pressure, polymerization temperature, polymerization time, etc. in the first stage slurry polymerization step and the second stage slurry polymerization step. In the present invention, there is no limitation on the adjustment method of the reaction raw materials, catalyst type, catalyst amount, reaction conditions, polymerization pressure, polymerization temperature, and polymerization time, as long as it is a conventional method in the art.

[0106] In one embodiment of the present invention, the method for preparing polyethylene of the present invention is carried out as follows:

[0107] In the first slurry polymerization reactor, in the presence of a polyethylene catalyst system, at a reaction temperature of 70-100° C., a reaction pressure of 0.2-4.0 MPa, and in the presence of hydrogen, ethylene is subjected to slurry polymerization;

[0108] In the second slurry polymerization reactor, at a reaction temperature of 70-100°C and a reaction pressure of 0.1-3.0 MPa, in the presence of hydrogen, the product from the first stage slurry polymerization reactor is subjected to slurry polymerization with ethylene and a comonomer to obtain polyethylene.

[0109] The ratio of the amount of the product in the first slurry polymerization reactor to that in the second slurry polymerization reactor is 1 / 3 to 2.5, and the mass of the comonomer is 10 relative to the total amount of ethylene in the first slurry polymerization reactor and the second slurry polymerization reactor. -5 -10 -1 Mass parts.

[0110] In one embodiment of the present invention, the method for preparing polyethylene of the present invention is carried out as follows:

[0111] In the first slurry polymerization reactor, in the presence of a polyethylene catalyst system, at a reaction temperature of 70-100° C., a reaction pressure of 0.2-4.0 MPa, and in the presence of hydrogen, ethylene is subjected to slurry polymerization;

[0112] Dehydrogenating the material obtained from the first slurry polymerization reactor;

[0113] In the second slurry polymerization reactor, at a reaction temperature of 70-100° C. and a reaction pressure of 0.1-3.0 MPa, in the absence of hydrogen, the dehydrogenated material is subjected to slurry polymerization with ethylene and a comonomer to obtain polyethylene.

[0114] The ratio of the amount of the product in the first slurry polymerization reactor to that in the second slurry polymerization reactor is 1 / 3 to 2.5, and the mass of the comonomer is 10 relative to the total amount of ethylene in the first slurry polymerization reactor and the second slurry polymerization reactor. -5 -10 -1 Mass parts.

[0115] The method for preparing polyethylene of the present invention further comprises a separation step after the second slurry polymerization step, wherein a polyethylene product is separated.

[0116] The polyethylene obtained by the preparation method of the present invention is a medium-density polyethylene with a wide molecular weight distribution.

[0117] In one embodiment of the present invention, the molecular weight distribution (MWD) of the polyethylene of the present invention is 10-30, more preferably 12-25. The melt index of the polyethylene of the present invention is between 0.1 and 1.6, preferably 0.15-1.6, more preferably 0.2-1.6.

[0118] The weight average molecular weight of the polyethylene of the present invention is 10,000 to 40,000, preferably 15,000-35,000, more preferably 18,000-30,000.

[0119] The number average molecular weight of the polyethylene of the present invention is 5,000 to 35,000, preferably 8,000-30,000, more preferably 10,000-20,000.

[0120] The density of the polyethylene of the present invention is 0.930 g / cm 3 Up to 0.940g / cm 3 , preferably 0.931 g / cm 3 -0.940g / cm 3 , more preferably 0.932 g / cm 3 -0.938g / cm 3 .

[0121] In the polyethylene of the present invention, the mass content of short chain branches is between 0.40wt% and 2.0wt%, preferably 0.5wt%-1.5wt%, and more preferably 0.55wt%-1.25wt%, relative to the total amount of polyethylene.

[0122] The polyethylene material obtained from the polyethylene of the present invention has a tensile strength greater than 10 MPa, preferably greater than 13 MPa, and more preferably greater than 20 MPa.

[0123] The thermal stability time (210° C.) of the polyethylene material obtained from the polyethylene of the present invention is greater than 18 min, preferably greater than 25 min, and more preferably greater than 28 min.

[0124] The impact strength of the polyethylene material obtained from the polyethylene of the present invention is greater than or equal to 13KJ / cm 2 , preferably greater than 15KJ / cm 2 , more preferably greater than 18KJ / cm 2 .

[0125] The polyethylene material obtained from the polyethylene of the present invention has a slow crack growth resistance of more than 800 hours, preferably more than 1000 hours, and more preferably more than 1200 hours.

[0126] The present invention also provides use of the polyethylene prepared by the method of the present invention in the preparation of polyethylene pipes.

[0127] When the polyethylene pipe is prepared from the polyethylene of the present invention, it can be manufactured by methods known in the art. For example, the polyethylene material of the present invention can be extruded to prepare the pipe. Example

[0128] Hereinafter, the present invention will be further described by way of examples, but the present invention is not limited to the following examples.

[0129] In the present invention, the relative molecular weight and molecular weight distribution and short chain branching and distribution are determined as follows: the solvent is 1,2,4-trichlorobenzene, the test temperature is 150°C, the flow rate is 1.0 mL / min, the narrow distribution polystyrene is used as the calibration sample, and the infrared data collection range is 300-2700 cm -1 .

[0130] Melt index (g / 10min): tested in accordance with GB / T 3682-2000.

[0131] Density: Tested in accordance with GB / T 1033.2-2010.

[0132] Tensile properties: Tested according to GB / T 1040.2-2006

[0133] Impact performance: Tested according to GB / T1043-2008

[0134] Oxidation induction period: Tested according to GB / T19466.6-2009

[0135] Static pressure test: Tested in accordance with GB / T28799-2012

[0136] Slow crack growth resistance test: Tested according to ISO4437-1997

[0137] Source of BCS catalyst (model: BCS): Beijing Research Institute of Chemical Industry

[0138] Detailed information about metallocene catalysts, from: GraceAT101, Beijing Research Institute of Chemical Industry 38#

[0139] Example 1

[0140] Preparation of medium density polyethylene powder

[0141] The BCS catalyst is used and two slurry polymerization reactors are connected in series for production. Among them, the volume of the first kettle is 100L, under the conditions of ethylene flow rate of 10Kg / h and hydrogen flow rate of 5g / h in the kettle, hexane is used as the polymerization solvent, the hexane flow rate is 30kg / h, the pressure in the kettle is maintained at 0.6MPa, and after reacting at 82°C for 60 minutes in a slurry state, the reaction slurry is fed into the second reactor, the volume of the second reactor is 300L, and the reaction is carried out under the conditions of ethylene flow rate of 17kg / h, hydrogen flow rate of 1g / h, and 1-hexene flow rate of 180g / h in the kettle, the hexane flow rate is 30kg / h, the pressure in the kettle is maintained at 0.3MPa, and the reaction is carried out at 82°C for 35 minutes in a slurry state. The ratio of the amount of the product of the first reactor and the second reactor is 1:1, the material is fed into a centrifugal concentration flash evaporation system to recover the solvent hexane, and the obtained polyethylene masterbatch enters a drying system to obtain polyethylene powder. Its molecular weight, molecular weight distribution, short chain branch content and other physical properties are shown in Table 1.

[0142] Preparation of medium density polyethylene pellets

[0143] 1000 ppm by weight of antioxidant 1010, 1000 ppm of zinc stearate and 5 ppm of PPA were mixed evenly with the polyethylene powder obtained above in a high-speed mixer, and then granulated at a screw temperature range of 185°C to 210°C and an extrusion rate of 50 m / min. The mechanical properties of the granules are shown in Table 1.

[0144] Preparation of medium density polyethylene pipe products

[0145] In a tube extruder with a screw temperature range of 190±10°C, a pulling rate of 80m / min, and a die diameter that meets the tube diameter standard of 32mm, the medium density polyethylene pellets are extruded into tubes. The static pressure test data of the tubes are shown in Table 1.

[0146] Example 2

[0147] Preparation of medium density polyethylene powder

[0148] The product is produced by using a BCS catalyst and two slurry polymerization reactors in series. Among them, the volume of the first kettle is 100L, under the conditions of ethylene flow rate of 5Kg / h and hydrogen flow rate of 8g / h in the kettle, hexane is used as the polymerization solvent, the hexane flow rate is 20kg / h, the pressure in the kettle is maintained at 0.7MPa, and after reacting at 82°C for 60 minutes in a slurry state, the reaction slurry is fed into the second reactor, the volume of the second reactor is 300L, and the reaction is carried out under the conditions of ethylene flow rate of 17kg / h, hydrogen flow rate of 1g / h, and 1-hexene flow rate of 700g / h in the kettle. The reaction is carried out at 82°C for 20 minutes under the conditions of hexane flow rate of 30kg / h, the pressure in the kettle is maintained at 0.3MPa, and the reaction is carried out at 82°C in a slurry state for 20 minutes. The ratio of the amount of the products of the first reactor and the second reactor is 100:85, the material enters the centrifugal concentration flash evaporation system to recover the solvent hexane, and the obtained polyethylene masterbatch enters the drying system to obtain polyethylene powder, and its molecular weight, molecular weight distribution, short chain branch content and other physical properties are shown in Table 1.

[0149] Preparation of medium density polyethylene pellets

[0150] 800 ppm by weight of antioxidant 1010, 1000 ppm of zinc stearate and 10 ppm of PPA were mixed evenly with the above polyethylene powder in a high-speed mixer, and then granulated at a screw temperature range of 185°C to 210°C and an extrusion rate of 80 m / min. The mechanical properties of the granules are shown in Table 1.

[0151] Preparation of medium density polyethylene pipe products

[0152] In a tube extruder with a screw temperature range of 190±10°C, a pulling rate of 100m / min, and a die diameter that meets the tube diameter standard of 32mm, the medium density polyethylene pellets are extruded into tubes. The static pressure test data of the tubes are shown in Table 1.

[0153] Example 3

[0154] Preparation of medium density polyethylene powder

[0155] Metallocene catalyst was used and single-pot slurry polymerization was adopted for production. The reactor volume was 300L. In the first polymerization step, under the conditions of ethylene flow rate of 17kg / h and hydrogen flow rate of 0.5g / h in the reactor, hexane was used as the polymerization solvent, the hexane flow rate was 20kg / h, and the pressure in the reactor was maintained at 1.8MPa. After reacting at 82°C for 1 hour in the slurry state, the second polymerization step was entered, wherein the ethylene flow rate in the reactor was adjusted to 17kg / h, the hydrogen flow rate was 0.02g / h, the flow rate of 1-hexene was 240g / h, the hexane flow rate was maintained at 20kg / h, and the pressure in the reactor was maintained at 1.8MPa. After reacting at 82°C for 1 hour in the slurry state, the ratio of the amount of the product in the first reaction step to the second reaction step was 1:1, the material was introduced into the centrifugal concentration flash evaporation system to recover the solvent hexane, and the obtained polyethylene masterbatch was introduced into the drying system to obtain polyethylene powder. The molecular weight, molecular weight distribution, short chain branch content and other physical properties thereof are shown in Table 1.

[0156] Preparation of medium density polyethylene pellets

[0157] 800 ppm by weight of antioxidant 1010, 1000 ppm of zinc stearate and 5 ppm of PPA were mixed evenly with the above polyethylene powder in a high-speed mixer, and then granulated at a screw temperature range of 185°C to 210°C and an extrusion rate of 80 m / min. The mechanical properties of the granules are shown in Table 1.

[0158] Preparation of medium density polyethylene pipe products

[0159] In a tube extruder with a screw temperature range of 190±10℃, a pulling speed of 100m / min, and a die diameter of 32mm, the above medium-density polyethylene pellets were extruded into tubes. The static pressure test data of the tubes are shown in Table 1.

[0160] Example 4

[0161] Preparation of medium density polyethylene powder

[0162] Metallocene catalysts are used and two slurry polymerization reactors are connected in series for production. Among them, the volume of the first kettle is 100L, under the conditions of ethylene flow rate of 5Kg / h and hydrogen flow rate of 1g / h in the kettle, hexane is used as the polymerization solvent, the hexane flow rate is 30kg / h, the pressure in the kettle is maintained at 0.6MPa, and after reacting at 82°C for 60 minutes in a slurry state, the reaction slurry is fed into the second reactor, the volume of the second reactor is 300L, and the reaction is carried out under the conditions of ethylene flow rate of 10kg / h, hydrogen flow rate of 1g / h, and 1-hexene flow rate of 90g / h in the kettle, the hexane flow rate is 30kg / h, the pressure in the kettle is maintained at 0.3MPa, and the reaction is carried out at 82°C for 60 minutes in a slurry state. The amount ratio of the products of the first reactor and the second reactor is 5:10, the material is fed into a centrifugal concentration flash evaporation system to recover the solvent hexane, and the obtained polyethylene masterbatch enters a drying system to obtain polyethylene powder, and its molecular weight, molecular weight distribution, short chain branch content and other physical properties are shown in Table 1.

[0163] Preparation of medium density polyethylene pellets

[0164] 1000 ppm by weight of antioxidant 1010, 1000 ppm of zinc stearate and 5 ppm of PPA were mixed evenly with the polyethylene powder obtained above in a high-speed mixer, and then granulated at a screw temperature range of 185°C to 210°C and an extrusion rate of 50 m / min. The mechanical properties of the granules are shown in Table 1.

[0165] Preparation of medium density polyethylene pipe products

[0166] In a tube extruder with a screw temperature range of 190±10°C, a pulling rate of 80m / min, and a die diameter that meets the tube diameter standard of 32mm, the medium density polyethylene pellets are extruded into tubes. The static pressure test data of the tubes are shown in Table 1.

[0167] Example 5

[0168] Preparation of medium density polyethylene powder

[0169] Metallocene catalysts are used and two slurry polymerization reactors are connected in series for production. Among them, the volume of the first kettle is 100L, under the conditions of ethylene flow rate of 12Kg / h and hydrogen flow rate of 3g / h in the kettle, hexane is used as the polymerization solvent, the hexane flow rate is 30kg / h, the pressure in the kettle is maintained at 0.6MPa, and after reacting at 82°C for 60 minutes in a slurry state, the reaction slurry is fed into the second reactor, the volume of the second reactor is 300L, and the reaction is carried out under the conditions of ethylene flow rate of 8kg / h, hydrogen flow rate of 0.5g / h, and 1-hexene flow rate of 120g / h in the kettle, the hexane flow rate is 25kg / h, the pressure in the kettle is maintained at 0.3MPa, and the reaction is carried out at 82°C for 60 minutes in a slurry state. The ratio of the amount of the product of the first reactor and the second reactor is 1.5:1, the material is fed into a centrifugal concentration flash evaporation system to recover the solvent hexane, and the obtained polyethylene masterbatch enters a drying system to obtain polyethylene powder, and its molecular weight, molecular weight distribution, short chain branch content and other physical properties are shown in Table 1.

[0170] Preparation of medium density polyethylene pellets

[0171] 1000 ppm by weight of antioxidant 1010, 1000 ppm of zinc stearate and 5 ppm of PPA were mixed evenly with the polyethylene powder obtained above in a high-speed mixer, and then granulated at a screw temperature range of 185°C to 210°C and an extrusion rate of 50 m / min. The mechanical properties of the granules are shown in Table 1.

[0172] Preparation of medium density polyethylene pipe products

[0173] In a tube extruder with a screw temperature range of 190±10°C, a pulling rate of 80m / min, and a die diameter that meets the tube diameter standard of 32mm, the medium density polyethylene pellets are extruded into tubes. The static pressure test data of the tubes are shown in Table 1.

[0174] Example 6

[0175] Preparation of medium density polyethylene powder

[0176] Metallocene catalysts are used and two slurry polymerization reactors are connected in series for production. Among them, the volume of the first kettle is 100L, under the conditions of ethylene flow rate of 10Kg / h and hydrogen flow rate of 2g / h in the kettle, hexane is used as the polymerization solvent, the hexane flow rate is 30kg / h, the pressure in the kettle is maintained at 0.6MPa, and after reacting at 82°C for 60 minutes in a slurry state, the reaction slurry is fed for dehydrogenation and then fed into the second reactor. The volume of the second reactor is 300L, and the reaction is carried out under the conditions of ethylene flow rate of 5kg / h and 1-hexene flow rate of 130g / h in the kettle. The hexane flow rate is 30kg / h, the pressure in the kettle is maintained at 0.3MPa, and the reaction is carried out at 82°C for 60 minutes in a slurry state. The amount ratio of the products of the first reactor and the second reactor is 2:1, and the material is fed into a centrifugal concentration flash evaporation system to recover the solvent hexane, and the obtained polyethylene masterbatch enters a drying system to obtain polyethylene powder. Its molecular weight, molecular weight distribution, short chain branch content and other physical properties are shown in Table 1.

[0177] Preparation of medium density polyethylene pellets

[0178] 1000 ppm by weight of antioxidant 1010, 1000 ppm of zinc stearate and 5 ppm of PPA were mixed evenly with the polyethylene powder obtained above in a high-speed mixer, and then granulated at a screw temperature range of 185°C to 210°C and an extrusion rate of 50 m / min. The mechanical properties of the granules are shown in Table 1.

[0179] Preparation of medium density polyethylene pipe products

[0180] In a tube extruder with a screw temperature range of 190±10°C, a pulling rate of 80m / min, and a die diameter that meets the tube diameter standard of 32mm, the medium density polyethylene pellets are extruded into tubes. The static pressure test data of the tubes are shown in Table 1.

[0181]

[0182] As can be seen from Table 1, the medium density polyethylene of the present invention has a wide relative molecular weight distribution, a high weight average molecular weight, and a certain short chain branch content. Therefore, more effective tie molecules can be formed in the polyethylene of the present invention.

[0183] This can be confirmed by the polymer properties in Table 1, that the polyethylene material of the present invention has excellent mechanical properties and toughness, and exhibits excellent oxidation stability, resistance to slow crack growth and resistance to rapid crack growth.

[0184] In addition, the medium density polyethylene pipe product prepared according to the present invention has excellent static pressure performance.

[0185] Industrial Applicability

[0186] By the method of the present invention, medium density polyethylene with excellent mechanical properties and toughness, excellent environmental crack resistance, slow crack growth resistance and rapid crack growth resistance can be obtained. Therefore, the polyethylene of the present invention is suitable for use as a pipe raw material.

Claims

1. A method for preparing polyethylene, comprising: A first slurry polymerization step, wherein Using a polyethylene catalyst system, ethylene is polymerized in slurry at a reaction temperature of 70-100°C and a reaction pressure of 0.2-4.0 MPa in the presence of hydrogen, and The second stage slurry polymerization step, wherein the product from the first stage slurry polymerization step is subjected to slurry polymerization with ethylene and a comonomer at a reaction temperature of 70-100° C. and a reaction pressure of 0.1-3.0 MPa, optionally in the presence of hydrogen, to obtain polyethylene. The ratio of the amount of the product of the first stage slurry polymerization step to that of the second stage slurry polymerization step is 1 / 3 to 2.5, and the amount of the comonomer in the second stage slurry polymerization step is 10 to 20%, relative to the total amount of ethylene in the first stage slurry polymerization step and the second stage slurry polymerization step. -5 -10 -1 Quality parts.

2. The method according to claim 1, wherein: The polyethylene catalytic system comprises a main catalyst and a co-catalyst. The main catalyst is selected from non-metallocene catalysts, metallocene catalysts, Ziegler-Natta catalysts, or a composite of at least two thereof, The cocatalyst is selected from aluminoxane, alkylaluminum, halogenated alkylaluminum, borofluorane, alkyl boron or alkyl boron ammonium salt or a mixture thereof, preferably selected from aluminoxane, alkylaluminum, halogenated alkylaluminum or a mixture thereof, more preferably selected from aluminoxane, alkylaluminum, or a mixture thereof.

3. The method according to claim 1 or 2, wherein: In the first stage slurry polymerization step, the amount of hydrogen is 10:1 relative to the amount of ethylene in the first stage slurry polymerization step. -7 -10 -1 parts by mass, preferably 10 -6 -10 -2 parts by mass, more preferably 10 -5 -10 -2 Quality parts.

4. The method according to any one of claims 1 to 3, wherein: In the second slurry polymerization step, the comonomer is selected from α-olefins, diolefins, cyclic olefins and other ethylenically unsaturated compounds, more preferably C3-C 10 The α-olefin is more preferably at least one selected from propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene, and further preferably at least one selected from 1-butene, 1-hexene, and 1-octene.

5. The method according to any one of claims 1 to 4, wherein: The ratio of the amount of the product of the first stage slurry polymerization step to the amount of the product of the second stage slurry polymerization step is 0.0.5-2, preferably 0.75-1.5, The amount of the comonomer in the second stage slurry polymerization step is 10 -4 -5×10 -2 parts by mass, preferably 10 -3 -5×10 -2 Quality parts.

6. The method according to any one of claims 1 to 5, wherein: In the second stage slurry polymerization step, when hydrogen is used, the amount of hydrogen is 10:1 relative to the amount of ethylene used in the second stage slurry polymerization step. -8 -10 -3 parts by mass, preferably 10 -6 -10 -4 Quality parts.

7. The method according to any one of claims 1 to 5, wherein: In the second slurry polymerization step, the reaction is carried out in the absence of hydrogen.

8. The method according to any one of claims 1 to 7, wherein: The ratio of the amount of ethylene participating in the first stage slurry polymerization step to the amount of ethylene participating in the second stage slurry polymerization step (first stage slurry polymerization step: second stage slurry polymerization step) is 0.1-3, preferably 0.2-2.5, and more preferably 0.25-2.2, in terms of mass ratio.

9. The method according to any one of claims 1 to 8, wherein: Between the first stage slurry polymerisation step and the second stage slurry polymerisation step, there is a dehydrogenation step.

10. The method according to any one of claims 1 to 9, wherein: The first stage slurry polymerization step and the second stage slurry polymerization step are each a tank type slurry polymerization.

11. The method according to any one of claims 1 to 10, wherein: The first stage slurry polymerization step and the second stage slurry polymerization step are carried out in two separate reactors connected in series.

12. The method according to any one of claims 1 to 10, wherein: The first stage slurry polymerization step and the second stage slurry polymerization step are carried out in the same reactor.

13. Polyethylene having a molecular weight distribution (MWD) between 10 and 30, a melt index between 0.1 and 1.6, a weight average molecular weight between 10,000 and 40,000, and a short chain branch content between 0.40 wt% and 2.0 wt%.

14. Use of the polyethylene prepared by the method for preparing polyethylene according to any one of claims 1 to 12 or the polyethylene according to claim 13 in preparing polyethylene pipes.

Images