阅读说明：本技术 一种双峰高密度聚乙烯树脂及其制备方法 (Double-peak high-density polyethylene resin and preparation method thereof ) 是由 蒋斌波 马玉龙 陈湛旻 戴进成 叶健 杨遥 黄正梁 孙婧元 王靖岱 阳永荣 于 2019-09-04 设计创作，主要内容包括：本发明公开了一种双峰高密度聚乙烯树脂及其制备方法,所述树脂包含低分子量的组分A和高分子量的组分B,所述的组分A为乙烯均聚物或共聚物。其具有大于9的一级结构参数(PSP2),并且当根据ASTM F1473-18测量时,由该树脂制成的制品在80℃和3.2MPa下具有大于5000小时PENT耐慢速裂纹增长值。本发明的树脂的密度为0.940～0.960g/cm<Sup>3</Sup>,熔体流动速率MFR<Sub>5</Sub>为0.20～0.40g/10min,重均分子量为150000～300000g/mol,分子量分布为15～23,共聚单体含量为0.5～1.0mol％。(The invention discloses a bimodal high-density polyethylene resin and a preparation method thereof, wherein the resin comprises a component A with low molecular weight and a component B with high molecular weight, and the component A is an ethylene homopolymer or a copolymer. It has a primary structure parameter (PSP2) greater than 9 and an article made from the resin has a PENT Slow crack growth resistance value greater than 5000 hours at 80 ℃ and 3.2MPa when measured according to ASTM F1473-18. The density of the resin is 0.940-0.960 g/cm3, the melt flow rate MFR5 is 0.20-0.40 g/10min, the weight average molecular weight is 150000-300000 g/mol, the molecular weight distribution is 15-23, and the content of the comonomer is 0.5-1.0 mol%.)

1. A bimodal high density polyethylene resin comprising a low molecular weight component A and a high molecular weight component B, said component A being an ethylene homopolymer or copolymer and said component B being an ethylene copolymer;

The method is characterized in that: the density of the resin is 0.940-0.960 g/cm3, the melt flow rate MFR5 is 0.20-0.40 g/10min, the weight average molecular weight is 150000-300000 g/mol, the molecular weight distribution is 15-23, the comonomer content is 0.5-1.0 mol%, the value of primary structure parameter (PSP2) is equal to or more than 9, and the PENT slow crack growth resistance value of a compression-molded sample made of the polyethylene resin is equal to or more than 5000 hours at 80 ℃ and 3.2 MPa.

2. A method for preparing a bimodal high density polyethylene resin, the bimodal high density polyethylene resin comprises a low molecular weight component A and a high molecular weight component B, the component A is an ethylene homopolymer or copolymer, and the component B is an ethylene copolymer;

the preparation method of the high-density polyethylene resin comprises the following steps:

Introducing ethylene, hydrogen and a polymerization catalyst system into a first polymerization zone, and preparing a component A in the first polymerization zone, wherein the polymerization catalyst system consists of a solid catalyst containing a transition metal component and an aluminum alkyl cocatalyst, and the transition metal component comprises a metal of a group IVB or VB in a periodic table system;

Introducing the polymerization product of the first polymerization zone, ethylene, and optionally one or more alpha-olefin comonomers into a second polymerization zone, in which component B having a weight average molecular weight higher than that of component A is prepared, the proportion of said alpha-olefin comonomer to ethylene fed into the second polymerization zone being from 50 to 90 mol/kmol; the weight ratio of component a to component B is from 60: 40-40: 60.

3. The method of claim 2, wherein the low molecular weight component A is an ethylene homopolymer and the high molecular weight component B is a copolymer of ethylene and one or more alpha-olefin comonomers having 4 to 10 carbon atoms.

4. The method of claim 2, wherein the alpha-olefin comonomer comprises 1-butene, 1-hexene, 1-heptene, 1-octene, 1-decene, or combinations thereof.

5. The method of claim 2 or 3, wherein the ratio of hydrogen to ethylene in the first polymerization zone is 40 to 100 mol/kmol.

6. The method for preparing the bimodal high density polyethylene resin according to claim 2, wherein the polymerization product in the first polymerization zone is flashed to remove volatile substances before entering the second polymerization zone, and the ratio of hydrogen to ethylene in the second polymerization zone is 0-30 mol/kmol.

7. The high density polyethylene resin according to claim 1, wherein the resin has a density of 0.940-0.960 g/cm3, said density being measured on a compression molded specimen prepared according to ENISO1872-2-2007 according to ISO 1183-1: 2004 method A.

8. the bimodal high density polyethylene resin according to claim 1, wherein the resin has a melt flow rate MFR5 of 0.2 to 0.4g/10min, determined according to ISO 1133 at 190 ℃ under a load of 5 kg.

9. The bimodal high density polyethylene resin according to claim 1, wherein the resin has a primary structure parameter (PSP2) value equal to or greater than 9 and an article made from the polyethylene resin has a PENT slow crack growth resistance value at 80 ℃ and 3.2MPa of equal to or greater than 5000 hours when measured according to ASTM F1473-18, as determined according to ASTM F1473-18 on compression molded test specimens prepared according to ISO 293-2004.

10. Use of a bimodal high density polyethylene resin prepared according to the process of any one of claims 2 to 9 in a pipe.

Technical Field

The invention relates to a pipe product, in particular to a bimodal high-density polyethylene resin for a pipe and a preparation method thereof, wherein the resin has excellent slow crack growth resistance.

Background

polyethylene pipe material is an important direction of application for polyethylene resins. Because the polyethylene pipe has the characteristics of unique connection integrity, sealing property, firmness, flexibility, light weight and the like, the polyethylene pipe is simple and feasible in industrial application and wide in application due to the combination of the factors. Currently, polyethylene pipes have been industrially applied to temporary water supply pipelines, various bypass pipelines, oil and gas pipelines. Such pipes are exposed to numerous environmental stresses during their service life, which may lead to crack initiation or even fracture, especially if the pipe is buried in a structure or underground. For this reason, the tube must have sufficient resistance to Slow Crack Growth (SCG), which is the most common failure mode of the tube during use. SCG resistance is determined according to ASTM F1473-18 using the Pennsylvania notch tensile test (PENT). DesLaurisers and Rohlfing propose a method for estimating SCG resistance of polyethylene resin, and a model relation with the SCG resistance is established by calculating the tie molecule (tiemolecular) content in polyethylene, and the tie molecule content is called as primary structure parameter PSP 2.

bimodal molecular weight distribution polyethylene refers to a blend of linear polyethylene and branched polyethylene having a bimodal distribution of relative molecular mass, and can be produced using various series reactor polymerization processes. The bimodal polyethylene resin has wide molecular weight distribution (generally more than 20), a large number of lacing molecular structures exist in a branched polyethylene crystallization area of a high molecular weight part, so that the bimodal polyethylene resin has good SCG resistance, and linear polyethylene of a low molecular weight part has good fluidity, so that the processability of the bimodal polyethylene resin is improved. Such resins can therefore balance the processability and SCG resistance of tube materials under many extreme environmental conditions through a characteristic molecular weight distribution and comonomer distribution.

Compared with 1-butene copolymer resin, the high-density polyethylene (HDPE) resin produced by using 1-hexene as a comonomer has more branched chain carbon number, so that the capability of forming a lacing molecular structure in a crystallization region is stronger, the number of lacing molecules is more, the high-density polyethylene (HDPE) resin has better tensile strength, rheological property, rapid/slow crack growth resistance and impact resistance, the development and utilization prospect is very wide, the content of the comonomer is more, and the SCG resistance is more excellent under the condition of approximate molecular weight. The production of high performance HDPE resins using 1-hexene instead of l-butene has become a necessary trend for future development.

Chinese patent CN102046663A discloses a bimodal polyethylene pipe resin with improved SCG and RCP resistance by reducing long chain branching, which is manufactured by a slurry polymerization method with two reactors connected in series. Although the resin has high SCG resistance, the bimodal polyethylene resin is an ethylene/1-butene copolymer, and the PENT slow crack growth resistance value at 3.2MPa is 6677 hours. The bimodal polyethylene resin of the present invention is a mixture of ethylene homopolymer and ethylene/1-hexene copolymer, and produces pipe products having PENT values equal to or greater than 5000 hours, preferably 7000 hours or more.

chinese patent CN101415768B discloses a bimodal polyethylene composition of ethylene homopolymer and ethylene/1-hexene copolymer prepared by adopting metallocene catalyst, which is suitable for pipes, and the PENT slow crack growth resistance value is more than 3000 hours under 3.8 MPa. The resin of the invention adopts a Ziegler-Natta catalytic system.

Chinese patent CN105705572A discloses a bimodal polyethylene resin suitable for rotational moulding having a primary structure parameter (PSP2) of 4 to 7. CN102643465B discloses a bimodal polyethylene resin suitable for rotational molding having a PSP2 value equal to or greater than 8.9. Disclosed is a bimodal polyethylene resin suitable for pipe applications and having a PSP2 value of 9 or greater.

There is a continuing need in the industry for resins suitable for pipe applications having a balance of processability and SCG resistance, and in particular, there is a need to provide bimodal high density polyethylene pipe with excellent resistance to chronic crack growth.

The bimodal high-density polyethylene resin provided by the invention is a mixture of an ethylene homopolymer and an ethylene/1-hexene copolymer, and has good processability and excellent SCG resistance by matching molecular weight, molecular weight distribution and comonomer content.

Disclosure of Invention

One of the objects of the present invention is to provide a bimodal high density polyethylene resin for pipes. Another object of the present invention is to provide a method for preparing the bimodal high density polyethylene resin for pipes.

The bimodal high density polyethylene resin for pipes according to the present invention comprises a low molecular weight ethylene homo-or copolymer component a and a high molecular weight ethylene copolymer component B, characterized in that: the density of the resin is 0.940-0.960 g/cm3, the melt flow rate MFR5 is 0.20-0.40 g/10min, the weight average molecular weight is 150000-300000 g/mol, the molecular weight distribution is 15-23, the comonomer content is 0.5-1.0 mol%, the value of primary structure parameter (PSP2) is equal to or more than 9, and the PENT slow crack growth resistance value of a compression-molded sample made of the polyethylene resin is equal to or more than 5000 hours at 80 ℃ and 3.2 MPa.

The bimodal high density polyethylene resin for pipes according to the present invention is prepared comprising the steps of: introducing ethylene, hydrogen and a polymerization catalyst system into a first polymerization zone, and preparing a component A in the first polymerization zone, wherein the polymerization catalyst system consists of a solid catalyst containing a transition metal component and an aluminum alkyl cocatalyst, and the transition metal component comprises a metal of a group IVB or VB in a periodic table system; the polymerization product of the first polymerization zone, ethylene and optionally one or more alpha-olefin comonomers are introduced into a second polymerization zone where component B is prepared having a weight average molecular weight higher than that of component a. The weight ratio of component a to component B is from 60: 40-40: 60. the ratio of hydrogen to ethylene in the first polymerization zone is 40-100 mol/kmol. The ratio of hydrogen to ethylene in the second polymerization zone is 0-30 mol/kmol. The ratio of the comonomer to the ethylene fed to the second polymerization zone is 50-90 mol/kmol.

In another aspect, the invention provides a bimodal high density polyethylene resin for pipes as described above, the resin having a density of 0.940 to 0.960g/cm 3.

In another aspect, the present invention provides a bimodal high density polyethylene resin for pipes as described above, the resin having a weight average molecular weight of 150000 to 300000g/mol and a molecular weight distribution of 15 to 23.

In another aspect, the present invention provides a bimodal high density polyethylene resin as described above for pipes, the resin having a melt flow rate MFR5 of 0.2 to 0.4g/10 min.

In another aspect, the present invention provides a bimodal high density polyethylene resin for pipes as described above, the comonomer content of the resin being 0.5 to 1.0 mol%

In another aspect, the present invention provides a bimodal high density polyethylene resin as described above for pipes, the resin having a primary structure parameter (PSP2) value equal to or greater than 9, and resulting in compression molded specimens having a PENT Slow crack growth resistance value equal to or greater than 5000 hours at 80 ℃ and 3.2 MPa.

In another aspect, the present invention provides a bimodal high density polyethylene resin for pipes as described above, the resin low molecular weight component a being an ethylene homopolymer and the high molecular weight component B being a copolymer of ethylene and one or more alpha-olefin comonomers having from 4 to 10 carbon atoms.

In another embodiment, the invention provides a process for preparing the bimodal high density polyethylene resin for pipes and resins, the alpha-olefin comonomer comprising 1-butene, 1-hexene, 1-heptene, 1-octene, 1-decene or combinations thereof.

In another embodiment, the invention provides a use of the high density polyethylene resin in a pipe.

compared with the prior art, the invention has the following beneficial effects (advantages):

1) the bimodal high-density polyethylene resin provided by the invention has a wide molecular weight distribution (15-23), a large number of lacing molecular structures exist in a crystallization region of a copolymer component B of a high molecular weight part, so that the bimodal high-density polyethylene resin has good SCG resistance, and a homopolymer component A of a low molecular weight part has good fluidity, so that the processability of the bimodal high-density polyethylene resin can be improved.

2) According to the bimodal high-density polyethylene resin provided by the invention, 1-hexene is used as a comonomer of the high molecular weight component B, and compared with 1-butene copolymer resin, the carbon number of branched chains is more, so that the capability of forming a frenulum molecular structure in a crystallization region is stronger, the number of frenulum molecules is more, and the SCG resistance is more excellent under the condition that the molecular weights are close.

3) According to the preparation method of the bimodal high-density polyethylene resin, the proportion of hydrogen to ethylene in the first polymerization zone, the proportion of hydrogen to ethylene in the second polymerization zone, the proportion of comonomer to ethylene in the second polymerization zone and the weight ratio of the component A to the component B are optimized, and the prepared resin has bimodal molecular weight distribution and reverse comonomer distribution, and can balance the processing service performance and SCG resistance of a tube material under an extreme environment condition. The bimodal high density polyethylene resin provided by the invention has a primary structure parameter (PSP2) value equal to or more than 9, and the prepared compression-molded sample has a PENT slow crack growth resistance value equal to or more than 5000 hours at 80 ℃ and 3.2MPa

Drawings

in order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings briefly described below are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.

FIG. 1 is a plot of calculated PSP2 values versus logM for the molecular weight distribution curve of the resin of example 1.

Detailed Description

The invention discloses a polyethylene resin and a preparation method thereof. The bimodal high-density polyethylene resin has excellent SCG resistance and is very suitable for the application of pipes.

bimodal polyethylene resins can be deconvoluted into two different components, as is generally demonstrated by the molecular weight distribution curve obtained by Gel Permeation Chromatography (GPC). Bimodal polyethylene resins can exhibit two distinct peaks, which correspond to two individual components of two different molecular weights. The bimodal polyethylene resin comprises a low molecular weight ethylene homo-or copolymer component a and a high molecular weight ethylene copolymer component B.

In the present invention, the low molecular weight component A is an ethylene homopolymer and the high molecular weight component B is a copolymer of ethylene and one or more alpha-olefin comonomers having 4 to 10 carbon atoms. The alpha-olefin comonomer comprises 1-butene, 1-hexene, 1-heptene, 1-octene, 1-decene or combinations thereof, preferably 1-hexene.

In the present invention, the weight ratio of component a to component B is from 60: 40-40: 60. in other words, the lower molecular weight component a is present in an amount of from 40 to 60%, preferably from 45 to 55%, by weight of the total polymer. The higher molecular weight component B is present in an amount of from 60 to 40%, preferably from 55 to 45% by weight of the total polymer.

In the present invention, the bimodal high density polyethylene resin has the following overall characteristics:

On compression molded specimens prepared according to EN ISO1872-2-2007, a test specimen was prepared according to ISO 1183-1: the density of the resin is 0.940-0.960 g/cm3, preferably 0.945-0.955 g/cm3 measured by 2004 method A;

The melt flow rate MFR5 of the resin is 0.20-0.40 g/10min, preferably 0.25-0.33 g/10min, measured according to ISO 1133 at 190 ℃ under a temperature and load of 5 kg;

The weight average molecular weight of the resin is 150000-300000 g/mol, preferably 180000-250000 g/mol, and the molecular weight distribution is 15-23, preferably 16-20, determined by GPC according to ISO 16014-4-2012 and ASTM D6474-2012;

According to RandallRev. Macromol. chem. Chys., C29(2&3), page 285-297, the comonomer content of the resin is 0.5 to 1.0 mol%, preferably 0.65 to 0.85 mol%, as determined by NMR carbon spectroscopy;

Articles made from the polyethylene resin have a PENT Slow crack growth resistance value equal to or greater than 5000 hours, preferably greater than 7000 hours, at 80 ℃ and 3.2MPa, as determined according to ASTM F1473-18 on compression molded test specimens prepared according to ISO 293-2004;

The primary structure parameter (PSP2) value is equal to or greater than 9, preferably equal to or greater than 10.

PSP2 calculations as outlined by DesLaurisers and Rohlfing in the Macromolecular symposium (2009),282 (polyolefin characterization-ICPC 2008), pages 136 to 149 are hereby incorporated by reference. The PSP2 calculation can be generally described as a multi-step method. The first step involves estimating the homopolymer (or low comonomer polymer) density of the sample from the molecular weight distribution of the sample, as described in equation 1. The first step takes into account the effect of molecular weight on sample density.

Wherein ρ is 1.0748- (0.0241) LogM

according to this process, the density value at a molecular weight of less than 720g/mol is equal to 1.006g/cm 3. In a second step, to further account for the additional contribution to density inhibition by the presence of short chain branching per Molecular Weight (MW) segment, the difference between the measured copolymer bulk density and the calculated homopolymer density was divided by the total SCB level (as measured by size exclusion chromatography-fourier transform infrared spectroscopy or by C13-NMR) and then applied to the SCB level in each MW segment. The initial observed bulk density of the copolymer (down to 0.852g/cm3) was obtained by summing the MW fragments as described above. This calculation has been simplified by assuming that all SCB levels will have the same effect on density suppression. However, it should be understood that the effectiveness of a particular SCB level in suppressing density will vary (i.e., as the SCB level increases, the ability to disrupt crystallization in the SCB decreases). Alternatively, if the density of the copolymer is not known, then the effect of SCB on the sample density can be estimated in a second step by using equation 2 described by Deslauiers and Rohlfing in patent application publication 2007/0298508, where the density change Δ ρ refers to the value subtracted from the value given in equation 1 based on a fragment-by-fragment molecule.

wherein, C1 is 1.25E-0.2; c2 ═ 0.5; c3 ═ 7.51E-05; c4 ═ 0.62; n is 0.32

The third step in calculating the PSP2 is to calculate the amount of 2lc + la, where lc is the estimated crystalline platelet thickness (in nm) and la is the estimated thickness (in nm) of the amorphous material at the specific molecular weight given by the following equation:

In equation 3, assigned values of 20 ℃ and 142.5 ℃ give assigned density values of 40.852g/cm3 and l.01g/cm3, respectively. Equation 4 is in the form of the widely accepted Gibbs Thompson equation. The thickness of the amorphous layer (la) was calculated using equations 5a and 5 b:

la ═ ρ clc (1-wc)/ρ c wc (formula 5b)

Wherein wc is the crystallinity by weight fraction

Calculated density of Mw segments

ρ c-Density of 100% crystalline sample (1.006g/cm3)

ρ c-density of amorphous phase (0.852g/cm3)

The fourth step calculates the tie molecule (tie molecular) probability (P) per molecular weight and the respective 2lc + la values according to equations 6a and 6 b:

Wherein

The above symbols have the following meanings:

p is the probability of tie molecule formation;

l ═ critical distance (nm) ═ 2lc + la;

d ═ 6.8 (for polyethylene) chain extension factor in the melt;

n-number of links-Mw/14 (for polyethylene)

link length of 0.153nm (for polyethylene)

finally, the PSP2 value is calculated by equations 6a and 6b, which essentially serves as a weighting factor (Pi) for each MWD fragment, where arbitrary is multiplied by 100 and then defined as PSP2 i. As in all the above calculations, this value for each segment is multiplied by the corresponding weight fraction (wi) of the MWD curve to obtain the value for the bulk polymer. The calculated wiPSP2i value versus logM curve is shown in fig. 1, which is also very valuable when trying to understand and predict structural property relationships. The area under the obtained wiPSP2i vs logM curve is defined as the PSP2 value of the whole polymer sample. For the polymer sample depicted in fig. 1, PSP2 ═ 9.9.

preparation method

The bimodal high-density polyethylene pipe resin is produced by adopting a two-stage series polymerization method. In the first polymerization zone, a low molecular weight component A is produced, and in the second polymerization zone, a component B having a weight average molecular weight higher than that of component A is produced. The two-stage series process means that two polymerization reactors are connected in series, and the low molecular weight component a resin produced in the first reactor is fed to the second reactor and is present during the formation of the high molecular weight component B resin in the second reactor. The bimodal polyethylene resin product is thus an intimate mixture of a low molecular weight component a and a high molecular weight component B.

As used herein, the term first reactor, first polymerization zone or first reaction zone refers to the stage of producing a first relatively low molecular weight relatively high density polyethylene resin component a, and the term second reactor, second polymerization zone or second reaction zone refers to the stage of copolymerizing ethylene with a comonomer to form a second relatively higher molecular weight lower density polyethylene resin component B. Although the polyethylene formed in the first reactor is preferably a homopolymer, under certain operating conditions small amounts of comonomer may be present in the first reactor, for example in the case of industrial operations in which hydrocarbons recovered from the process, generally recovered at the end of the process, containing small amounts of unreacted/unrecovered comonomer are recycled to the first reactor.

any conventional ethylene homopolymerization or (co) polymerization reaction may be used to prepare the bimodal high density polyethylene resin of the present invention. Such conventional ethylene homopolymerization or (co) polymerization reactions include, but are not limited to, gas phase polymerization, slurry phase polymerization, liquid phase polymerization, and combinations thereof. They use conventional reactors such as gas phase reactors, loop reactors, stirred tank reactors and batch reactors in series, or in series and parallel. The polymerization system of the present invention is a two-stage polymerization system or a multistage polymerization system. Examples of two-stage polymerization systems include, but are not limited to, gas phase polymerization/gas phase polymerization; gas phase polymerization/liquid phase polymerization; liquid phase polymerization/gas phase polymerization; liquid phase polymerization/liquid phase polymerization; slurry phase polymerization/slurry phase polymerization; liquid phase polymerization/slurry phase polymerization; slurry phase polymerization/liquid phase polymerization; slurry phase polymerization/gas phase polymerization; gas phase polymerization/slurry phase polymerization. The multistage polymerization system comprises at least three polymerization reactions. The high-density polyethylene composition of the present invention is preferably produced by liquid phase polymerization/gas phase polymerization using a loop reactor/gas phase reactor cascade, in other words, the first polymerization zone uses a loop reactor for liquid phase polymerization to produce the low molecular weight component a and the second polymerization zone uses a gas phase reactor for gas phase polymerization to produce the high molecular weight component B. However, the present invention is not limited thereto, and any combination of the above may be employed.

The polymerization catalyst system used in the present invention is composed of a solid catalyst containing a transition metal component and a cocatalyst. The transition metal component includes a metal of group IVB or VB of the periodic Table system. Typical transition metal component-containing solid catalysts used for preparing high density polyethylene resins preferably use Ziegler-Natta catalysts. Particularly preferred transition metal components include titanium halides, alkoxyalkyl magnesium compounds, and alkyl aluminum dihalides supported on inorganic oxide supports. The cocatalyst is preferably an aluminum alkyl, especially a trialkylaluminum, for example trimethylaluminum, triethylaluminum and triisobutylaluminum.

Ethylene, hydrogen and the polymerization catalyst system are introduced into a first polymerization zone where component a is produced, and the density and melt flow rate of the resin produced in the first reactor are monitored during the polymerization process.

In the present invention, the reaction temperature in the first polymerization zone needs to be sufficiently high to achieve an acceptable activity of the catalyst. On the other hand, the temperature should not exceed the softening temperature of the polymer. Generally, the temperature in the first polymerization zone ranges from 60 to 100 ℃ and preferably from 70 to 90 ℃. If, as mentioned above, the first reactor is preferably a loop reactor, the pressure is in the range of 10 to 100bar, preferably 20 to 80 bar. Hydrogen is used to adjust the molecular weight, resulting in a low molecular weight component a. In the present invention, the ratio of hydrogen to ethylene in the first polymerization zone is from 40 to 100mol/kmol, preferably from 60 to 80 mol/kmol.

The low molecular weight component a produced in the first polymerization zone, the polymerization catalyst system, additional ethylene, additional hydrogen and optionally one or more alpha-olefin comonomers are then introduced into a second polymerization zone, where component B is produced having a weight average molecular weight higher than that of component a, resulting in the final bimodal polyethylene resin product. In the present invention, the weight ratio of component a to component B is from 60: 40-40: 60. in other words, the lower molecular weight component a is present in an amount of from 40 to 60%, preferably from 45 to 55%, by weight of the total polymer. The higher molecular weight component B is present in an amount of from 60 to 40%, preferably from 55 to 45% by weight of the total polymer.

a portion of the volatiles are removed prior to introducing the components from the first reactor to the second reactor, and substantially all of the hydrogen is removed at this stage. Since the hydrogen concentration required in the second reaction zone to form the high molecular weight component B is much lower than that used in the first reaction zone. In the present invention, the ratio of hydrogen to ethylene in the second polymerization zone is from 0 to 30mol/kmol, preferably from 10 to 20 mol/kmol.

the polymerization conditions in the second polymerization zone are different from those in the first polymerization zone. Since both polymerization zones use the same catalyst, the reaction temperatures in the first and second polymerization zones are in the same range, but the reaction pressures differ depending on the type of reactor selected. In the present invention, the temperature of the second polymerization zone is in the range of 60 to 100 ℃ and preferably 70 to 95 ℃. If, as mentioned above, the second reactor is preferably a gas phase reactor, the pressure is in the range of 1 to 50bar, preferably 1 to 20 bar.

In the second polymerization zone, it is also necessary to introduce optionally one or more alpha-olefin comonomers and also additional ethylene. The alpha-olefin comonomer comprises 1-butene, 1-hexene, 1-heptene, 1-octene, 1-decene or combinations thereof, preferably 1-hexene. The proportion of the comonomer to ethylene is 50-90 mol/kmol, preferably 60-80 mol/kmol.