Multimodal polyethylene compositions

Abstract

Provided are multimodal polyethylene compositions, and geomembranes including the same. The multimodal polyethylene compositions according to embodiments disclosed herein include a first polyethylene component and a second polyethylene component, wherein the composition has the following: a density of from 0.933 to 0.945 g / cm3; a high flow melt index (I21) of from 7.5 to 15.0 g / 10 min; a I21 / I5 of greater than 20.0; a strain hardening modulus of at least 45 MPa; and wherein the first polyethylene component has a density of less than 0.930 g / cm3. The multimodal polyethylene composition can be formed into geomembrane and can exhibit a desirable balance of processability, slow crack growth resistance, and other mechanical properties.

Description

MULTIMODAL POLYETHYLENE COMPOSITIONSTECHNICAL FIELD

[0001] Embodiments of the present disclosure generally relate to multimodal polyethylene compositions, and geomembranes including the same.INTRODUCTION

[0002] Polyethylene compositions can be formed into useful articles such as films, liners and geomembranes. Polymeric sheets such as geomembranes can be used as part of containment structures to provide barrier protection to the migration of materials into the environment. When extruding polyethylene compositions to form articles, it is generally desirable for the polyethylene compositions to have a lower molecular weight and lower viscosity, particularly under shear conditions for articles such as geomembranes, so that the polyethylene compositions can be more easily processed. However, lower molecular weight and viscosity can compromise slow growth crack resistance properties such as environmental stress crack resistance (ESCR), PENT, and a notched constant tensile load failure time (NCTL). These properties are of particular importance in polyethylene compositions used to form articles like geomembranes because longevity and anti-leak performance is required to address potential environmental protection concerns. In addition to processability and slow growth crack performance, polyethylene compositions formed into articles such as geomembranes must balance a number of other properties, including, for example, melt strength, tensile elongation, puncture resistance, low temperature flexibility, strong corrosion resistance, and good weldability.

[0003] Attempts to achieve a desirable balance of properties from polyethylene compositions include the introduction of narrow molecular weight distribution catalysts in dual reactor systems to produce multimodal polyethylene compositions. With multimodal compositions in dual reactor systems, it is possible, for example, to increase stress crack resistance by increasing the molecular weight or increasing the comonomer content of the high molecular weight fraction, which in turn decreases density. Altering properties of the higher molecular fraction, however, can lead to undesirable behavior such as an increase in viscosity. Accordingly, there remains a need for polyethylene compositions that can exhibit a desirable balance of properties such as superior slow growth crack resistance, good processability, and maintained tensile elongation properties.SUMMARY

[0004] Embodiments of the present disclosure meet one or more of the foregoing needs by providing a polyethylene composition that can achieve desirable processability and slow growth crack resistance for applications such as geomembranes. The multimodal composition according to embodiments disclosed herein are particularly suitable for blown extrusion for forming geomembranes.

[0005] Disclosed herein is a multimodal polyethylene composition. In a first aspect, the multimodal polyethylene composition comprises from 40 to 65 wt.% of a first polyethylene component, based on the total weight of the multimodal polyethylene composition, and a second polyethylene component, wherein the first polyethylene component has a higher molecular weight than the second polyethylene component, and wherein the multimodal polyethylene composition has the following: a density of from 0.933 to 0.945 g / cm3; a high flow melt index (I21) of from 7.5 to 15.0 g / 10 min; a I21 / I5 of greater than 20.0; a strain hardening modulus of at least 45 MPa; and wherein the first polyethylene component has a density of less than 0.930 g / cm3.

[0006] Disclosed herein are geomembranes. In a second aspect, the geomembrane comprises the multimodal polyethylene composition according to the first aspect of the invention.

[0007] These and other embodiments are described in more detail in the Detailed Description.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 is a GPC chromatogram of an inventive example according to an embodiment of the present invention.DETAILED DESCRIPTION

[0009] Aspects of the disclosed multimodal polyethylene compositions are described in more detail below. The multimodal polyethylene compositions are suitable for use in forming geomembranes and can have a wide variety of applications, including, for example, liners, hoses, or the like.

[0010] As used herein, the term “polymer” means a polymeric compound prepared by polymerizing monomers, whether of the same or a different type. The generic term polymer thus embraces the term homopolymer (employed to refer to polymers prepared from only one type of monomer), and the term copolymer or interpolymer. Trace amounts of impurities (for example, catalyst residues) may be incorporated into and / or within the polymer. A polymer may be a single polymer, a polymer blend, or a polymer mixture, including mixtures of polymers that are formed in situ during polymerization.

[0011] As used herein, the term “copolymer” means a polymer formed by the polymerization reaction of at least two structurally different monomers. The term “copolymer” is inclusive of terpolymers.

[0012] As used herein, the terms “polyethylene” or “ethylene-based polymer” shall mean polymers comprising a majority amount (>50 mol %) of units which have been derived from ethylene monomer. This includes polyethylene homopolymers and copolymers (meaning units derived from two or more comonomers). The terms “ethylene-based polymer” and “polyethylene” may be used interchangeably. Generally, polyethylene may be produced in gas-phase, fluidized bed reactors, liquid phase slurry process reactors, or liquid phase solution process reactors, using a heterogeneous catalyst system, such as Ziegler-Natta catalyst, a homogeneous catalyst system, comprising Group 4 transition metals and ligand structures such as metallocene, non-metallocene metal-centered, heteroaryl, heterovalent aryloxyether, phosphinimine, and others. Combinations of heterogeneous and / or homogeneous catalysts also may be used in either single reactor or dual reactor configurations.

[0013] As used herein, the term “composition” refers to a mixture of materials which comprise the composition, as well as reaction products and decomposition products formed from the materials of the composition.

[0014] The term “multimodal” means compositions that can be characterized by having at least two (2) polymer components or subcomponents with different molecular weights and / or different comonomer contents. In one embodiment, multimodal may be defined by having at least two distinct peaks in an Absolute Gel Permeation Chromatography (GPC) chromatogram showing the molecular weight distribution of the composition. The term “bimodal” means compositions that can be characterized by having two (2) polymer components or subcomponents with different molecular weights and / or different comonomer contents. In oneembodiment, bimodal may be defined by having two distinct peaks in an Absolute Gel Permeation Chromatography (GPC) chromatogram showing the molecular weight distribution of the composition. All GPC measurement values (e.g., Mw, Mn, Mz) recited herein are Absolute GPC measurements provided in accordance with the test methods described below.

[0015] The terms “comprising,” “including,” “having,” and their derivatives, are not intended to exclude the presence of any additional component, step or procedure, whether or not the same is specifically disclosed. In order to avoid any doubt, all compositions claimed through use of the term “comprising” may include any additional additive, adjuvant, or compound, whether polymeric or otherwise, unless stated to the contrary. In contrast, the term, “consisting essentially of’ excludes from the scope of any succeeding recitation any other component, step or procedure, excepting those that are not essential to operability. The term “consisting of’ excludes any component, step or procedure not specifically delineated or listed.

[0016] Disclosed herein are multimodal polyethylene compositions. The multimodal polyethylene composition according to embodiments disclosed herein comprises a first polyethylene component and a second polyethylene component. The first polyethylene component has a higher molecular weight (Mw) than the second polyethylene component. In some embodiments, the multimodal composition is a bimodal polyethylene composition consisting essentially of a first polyethylene component and a second polyethylene component.

[0017] The first polyethylene component is a copolymer of ethylene and one or more alpha- olefin comonomers. The second polyethylene component is also a copolymer of ethylene and one or more alpha-olefin comonomers. The alpha-olefin comonomers can have 3 to 10 carbon atoms or 3 to 8 carbon atoms. Exemplary alpha-olefin comonomers include, but are not limited to, propylene, 1 -butene, 1 -pentene, 1 -hexene, 1 -heptene, 1 -octene, 1 -nonene, 1 -decene, and 4- methyl- 1 -pentene. In some embodiments, the alpha-olefin comonomers may be selected from the group consisting of 1 -butene, 1 -hexene, and 1 -octene, or from the group consisting of 1- butene and 1 -hexene, or from the group consisting of 1 -hexene or 1 -octene. In some embodiments, the first polyethylene component and the second polyethylene component comprise 1 -hexene. In some embodiments, the first polyethylene component and the second polyethylene component are void of comonomers other than 1 -hexene, or 1 -hexene and 1- octene.

[0018] The multimodal polyethylene composition comprises from 40 to 65 wt.% of a first polyethylene component, based on the total weight of the multimodal polyethylene composition. All individual values and subranges from 40 to 65 wt.% are disclosed and included herein. For example, the multimodal polyethylene composition can comprise from a lower limit of 40, 45, 50, 55, or 60 wt.% to an upper limit of 65, 60, 55, 50 or 45 wt.% of the first polyethylene component, based on the total weight of the multimodal polyethylene composition. In some embodiments, the multimodal polyethylene composition comprises from 35 to 60 wt.% of the second polyethylene component, based on the total weight of the multimodal polyethylene composition. For example, in some embodiments, the multimodal polyethylene composition can comprise from 35, 40, 45, 50, or 55 wt.% to an upper limit of 60, 55, 50 or 45 wt.% of the second polyethylene component, based on the total weight of the multimodal polyethylene composition.

[0019] The first polyethylene component has a density less than 0.930 g / cm3. In some embodiments, the first polyethylene component can have a density of less than 0.928 g / cm3, less than 0.925 g / cm3, less than 0.922 g / cm3, less than 0.919 g / cm3or less than 0.918 g / cm3. In some embodiments, the first polyethylene component can have a high flow melt index (I21) of less than 1.00 g / 10 min, or less than 0.80 g / 10 min or less than 0.60 g / 10 min, or less than 0.55 g / 10 min, or from a range of 0.10 to 1.00 g / 10 min, or 0.20 to 1.00 g / 10 min.

[0020] The multimodal polyethylene composition has a density of from 0.933 to 0.945 g / cm3. In some embodiments, the multimodal polyethylene composition can have a density from a lower limit of 0.934, 0.935, 0.936, 0.937, 0.938, 0.939, 0.940, 0.941, 0.942, 0.943, or 0.944 g / cm3to an upper limit of 0.935, 0.936, 0.937, 0.938, 0.939, 0.940, 0.941, 0.942, 0.943, 0.944, or 0.945 g / cm3.

[0021] The multimodal polyethylene composition has a high flow melt index (I21) of from 7.5 to 15.0 g / 10 min. In some embodiments, multimodal polyethylene composition can have a high flow melt index (I21) of from a lower limit of 7.5, 8.0. 8.5, 9.0, 9.5, 10.0, 11.0, 12.0, 13.0, or 14.0 g / 10 min to an upper limit of 8.0. 8.5, 9.0, 9.5, 10.0, 11.0, 12.0, 13.0, 14.0, or 15.0 g / 10 min.

[0022] The multimodal polyethylene composition has a I21 / I5 of at least 20.0. In some embodiments, the multimodal polyethylene can have a I21 / I5 of at least 21.0, or 22.0, or 23.0.or 24.0. In some embodiments, the multimodal polyethylene composition can have a I21 / I5 of less than 50.0, or less than 45.0, or less than 40.0 or less than 36.0.

[0023] The multimodal polyethylene composition has a strain hardening modulus of at least 45 MPa. In some embodiments, the multimodal polyethylene composition can have a strain hardening modulus of at least 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, or 57 MPa. In some embodiments, the multimodal polyethylene composition can have a strain hardening modulus of no more than 75, 74, 73, 72, 71, or 70 MPa.

[0024] In some embodiments, the multimodal polyethylene composition has a PENT value as measured according to ASTM F1472 of at least 4,000 hours. In some embodiments, the multimodal polyethylene composition can have a PENT value as measured according to ASTM F1472 of at least 4,500 hours, at least 5,000 hours, or at least 5,500 hours.

[0025] In some embodiments, the multimodal polyethylene composition has a molecular weight distribution (Mw / Mn), measured by absolute GPC, between 10.0 and 30.0. In some embodiments, the multimodal polyethylene composition has a molecular weight distribution of between 12.0 and 28.0, or 13.0 and 26.0.

[0026] In some embodiments, the multimodal polyethylene composition has a weight average molecular weight (Mw), as measured by absolute GPC, of greater than 200,000 g / mol, or greater than 210,000 g / mol, or greater than 220,000 g / mol, or greater than 230,000 g / mol, or greater than 240,000 g / mol, or greater than 250,000 g / mol.

[0027] In some embodiments, the multimodal polyethylene composition can have an ESCR of at least 1,000 hours. In some embodiments, the multimodal polyethylene composition can have an ESCR of at least 1,200 hours, 1,300 hours, 1,400 hours, or 1,500 hours.

[0028] In some embodiments, the multimodal polyethylene composition can have a I21 / I2 of greater than of greater than 70, or greater 80, or greater than 90, or greater than 95. In some embodiments the multimodal polyethylene composition can have a I21 / I2 of no more than 200, or no more than 180 or no more than 150.

[0029] In some embodiments, the multimodal polyethylene composition can have a Mz of greater than 1,300,000 g / mol or greater than 1,400,000 g / mol. In some embodiments, the multimodal polyethylene composition can have a Mz of no more than 3,000,000 g / mol. Insome embodiments, the multimodal polyethylene composition can have a Mz / Mw of less than 15, or less than 14, or less than 13, or less than 12, or less than 11, or greater than 3, or greater than 4, or greater than 5.

[0030] In some embodiments, the multimodal polyethylene composition can have a melt strength of greater than 10.0 cN, or greater than 12.0 cN, or greater than 14.0 cN, or greater than 15.0 cN. In some embodiments, the multimodal polyethylene composition can have a melt strength of no more than 25.0 cN.

[0031] In some embodiments, the multimodal polyethylene composition can have a molecular weight distribution from Absolute GPC, where the Absolute GPC molecular weight distribution has a first peak, a local minimum, and a second peak in a range of Log(molecular weight) of 3.5 to 6.0, wherein the local minimum is an inflection point between the first peak and the second peak, and the first peak corresponds to the low molecular weight component and the second peak corresponds to the high molecular weight component. Within this range of Log(molecular weight) of 3.5 to 6.0, a first and then second derivative of the equally spaced data produces three inflexion points for a molecular weight distribution. Two positive inflexion points, derivative values going from positive to negative values as Log(molecular weight) increases, and one negative inflexion point, derivative values going from negative to positive as Log(molecular weight) increases. The local minimum is located between the first peak and the second peak. The first peak, which can be designated as the local maximum (Mmaxl), is the molecular weight at the inflexion point that corresponds to the low molecular weight component and the second peak, which can be designated as the local maximum (Mmax2), is the molecular weight at the inflexion point that corresponds to the high molecular weight component. The local minimum is the lowest molecular weight value between the first peak and the second peak and is the negative inflection point between the first peak and the second peak. A person of ordinary skill in the art understands that the GPC chromatogram relates to the molecular architecture of the multimodal polyethylene composition and is in part a result of the particular catalyst system used to form the composition. It has been found that, according to embodiments disclosed herein, a particular type of catalyst is suitable for producing the multimodal polyethylene composition in a single reactor and, relatedly, delivering a specific GPC chromatogram, whereas prior art compositions with similar features or different catalyst systems cannot be made in a single reactor system, deliver the specific GPC chromatogram, and / or deliver the desirable properties disclosed herein. In some embodiments, the AbsoluteGPC chromatogram has a first peak, a local minimum, and a second peak in a range of Log(molecular weight) of from 3.5 to 5.8 or from 3.5 to 5.5.

[0032] In some embodiments, the multimodal polyethylene composition has a complex viscosity at 100 rad / s of less than 2,800 Pa.s, or less than 2,700 Pa.s, or less than 2,500 Pa.s, or less than 2,400 Pa.s.

[0033] In some embodiments, when the multimodal polyethylene composition is formed into a geomembrane, the geomembrane has a notched constant tensile load failure time at 30% yield stress, as measured according to ASTMD5397, of greater than 2,500 hours.

[0034] In some embodiments, in addition to the properties above, when the multimodal polyethylene composition is formed into a geomembrane, the geomembrane can exhibit at least one of the following properties: a yield strength of at least 2,300 psi (or at least 2,400 psi); a break strength of at least 4,000 psi (or at least 4,500 psi, or at least 4,900 psi); a yield elongation of at least 13% (or at least 14% or at least 14.5%); a break elongation of at least 700% (or at least 720%); a tear strength of at least 700 Ib / inch; a puncture strength of at least 2,000 Ib / inch (or at least 2,100 Ib / inch).

[0035] The multimodal polyethylene composition of the present invention is suitable for fabrication of geomembranes. A geomembrane is a low permeability barrier polymeric sheet used in any geotechnical applications to regulate the migration of a liquid or gas in industrial systems. Geomembranes can be used to receive or transmit fluids, gas, or solids and used to protect water or the environment from impurities or pollution. Geomembranes can be used as a hydraulic barrier in purification processes and as a gas barrier. Geomembranes generally must meet certain regulatory or industrial thresholds for performance such as Geosynthetic Institute’s GM- 13 standard.

[0036] In some embodiments, the multimodal polyethylene composition is formed into a geomembrane meeting or exceeding Geosynthetic Institute’s GM- 13 standard. The features of the multimodal high density polyethylene composition, including its density, melt flow properties, elongation properties, and modulus properties, contribute to making it particularly suitable for geomembranes. The geomembrane may be extruded by methods known to those skilled in the art and may be formed by methods known to those skilled in the art. For example, a geomembrane may formed by sealing, via heat or other means, polymeric sheets formed from polyethylene compositions, along one or more overlapping seams, to create a long, wide sheetwith fused overlaps. A geomembrane may also be formed from polymeric sheets that are welded together. In some embodiments, a blown-extruded geomembrane comprising the multimodal polyethylene composition disclosed herein can be formed. Cast extrusion and blown extrusion are methods known to produce geomembranes, each with unique characteristics and applications. In cast extrusion, the molten polymer is extruded through a flat die to form a thin, continuous sheet, which is then rapidly cooled on a chill roll. In contrast, blown extrusion involves extruding the molten polymer through a circular die to create a tube, which is subsequently inflated with air to form a bubble. The bubble is then collapsed and wound into rolls. The multimodal compositions according to embodiments can be made via blown extrusion process and deliver or maintain desirable properties for geomembrane applications

[0037] The geomembrane according to embodiments disclosed herein may be a monolayer geomembrane and may comprise suitable additives used for extrusion or geomembrane applications. Such additives include colorants and materials suitable to protect the composition from adverse environmental effect, for example, oxidation during extrusion or degradation under service conditions. Suitable additives include process stabilizers, antioxidants, and pigments. In some embodiments, the geomembrane may comprise multiple layers wherein at least one layer comprises the composition according to the present invention. Additional polyolefins may be coextruded with other polymers such as polyamides, ethylene vinyl alcohol copolymers and polyesters.

[0038] The multimodal polyethylene composition can be made by a variety of methods. For example, such methods may include, but are not limited to, gas phase polymerization process, slurry phase polymerization process, liquid phase polymerization process, and combinations thereof using one or more conventional reactors, e.g., fluidized bed gas phase reactors, loop reactors, stirred tank reactors, batch reactors in parallel, series, and / or any combinations thereof. In the alternative, the multimodal polyethylene composition may be produced in a high pressure reactor via a coordination catalyst system. For example, the multimodal polyethylene composition may be produced via gas phase polymerization process in a gas phase reactor; however, any of the above polymerization processes may be employed. In an embodiment, the polymerization reactor may comprise of two or more reactors in series, parallel, or combinations thereof, and wherein each polymerization takes place in solution, in slurry, or in the gas phase. In another embodiment, a dual reactor configuration is used wherethe polymer made in the first reactor can be either the first polyethylene component or the second polyethylene component. The polymer made in the second reactor may have a density and melt flow rate such that the overall density and melt flow rate of the multimodal polyethylene composition are met. Similar polymerization processes are described in, for example, USP 7,714,072, which is incorporated herein by reference.

[0039] In an embodiment, the method of manufacturing the multimodal polyethylene composition includes polymerizing a first polyethylene component, as previously described herein, in a reactor, and polymerizing a second polyethylene component, as previously described herein, in a different reactor, thereby producing a multimodal polyethylene composition. The two reactors may be operated in series. In some embodiments, the first polyethylene component is polymerized in a first reactor, and the second polyethylene component is polymerized in a second reactor. In other embodiments, the second polyethylene component is polymerized in a first reactor, and the first polyethylene component is polymerized in a second reactor.

[0040] In an embodiment, the weight ratio of copolymer prepared in the first polyethylene component reactor to copolymer prepared in second polyethylene component reactor is in the range of from 30:70 to 70:30, or in the range of from 40:60 to 60:40. This is also known as the polymer split.

[0041] In an embodiment, the multimodal polyethylene composition is manufactured using at least one Ziegler-Natta (Z-N) catalyst system. In other embodiments, the multimodal polyethylene composition is manufactured using multiple reactors in series with a Z-N catalyst being fed to either each reactor or to just the first reactor. In further embodiments, the Z-N catalyst system may be fed into one or two independently-controlled reactors configured sequentially, and operated in solution, slurry or gas phase. In even further embodiments, the Z-N catalyst system may be fed into one or two independently-controlled reactors configured sequentially, and operated in gas phase. Sequential polymerization may be conducted such that fresh catalyst is injected into one reactor, and active catalyst is carried over from the first reactor into the second reactor.

[0042] The term “procatalysf ’ or “precursor”, are used interchangeably herein, and denote a compound including a ligand, a transition metal, and optionally, an electron donor. The procatalyst may further undergo halogenation by contacting with one or more halogenatingagents. A procatalyst can be converted into a catalyst upon activation. Such catalysts are commonly referred to as Ziegler-Natta catalysts. Suitable Zeigler-Natta catalysts are known in the art and include, for example, the catalysts taught in U.S. Patent Nos. 4,302,565; 4,482,687; 4,508,842; 4,990,479; 5,122,494; 5,290,745; and, 6,187,866 Bl, the disclosures of which are hereby incorporated by reference. The collection of catalyst components, such as procatalyst(s), cocatalyst(s), is referred to as a catalyst system.

[0043] The transition metal compound of the procatalyst composition can include compounds of different kinds. The most usual are titanium compounds — organic or inorganic — having an oxidation degree of 3 or 4. Other transition metals such as, vanadium, zirconium, hafnium, chromium, molybdenum, cobalt, nickel, tungsten and many rare earth metals are also suitable for use in Ziegler-Natta catalysts. The transition metal compound is usually a halide or oxyhalide, an organic metal halide or purely a metal organic compound. In the last-mentioned compounds, there are only organic ligands attached to the transition metal.

[0044] In an embodiment, the procatalyst has the formula Mga Me(OR)eXf (ED)gwherein R is an aliphatic or aromatic hydrocarbon radical having 1 to 14 carbon atoms or COR' wherein R' is a aliphatic or aromatic hydrocarbon radical having 1 to 14 carbon atoms; each OR group is the same or different; X is independently chlorine, bromine or iodine; ED is an electron donor; d is 0.5 to 56; e is 0, 1, or 2; f is 2 to 116; and g is > 1 to 1.5(d). Me is a transition metal selected from the group of titanium, zirconium, hafnium and vanadium. Some specific examples of suitable titanium compounds are: TiCl3, TiCl4, Ti(OC2H5)2Br2, Ti(OC6H5)Cl3, Ti(OCOCH3)Cl3, Ti(acetylacetonate)2C12, TiCl3(acetylacetonate), and TiBr4.

[0045] The magnesium compounds include magnesium halides such as MgCh (including anhydrous MgCh), MgBr2, and Mgl2. Nonlimiting examples of other suitable compounds are Mg(OR)2, Mg(OCO2Et) and MgRCl where R is defined above. From 0.5 to 56 moles, or from 1 to 20 moles of the magnesium compounds are used per mole of transition metal compound. Mixtures of these compounds may also be used.

[0046] The procatalyst compound can be recovered as a solid using techniques known in the art, such as precipitation of the procatalyst or by spray drying, with or without fillers. Spray drying is a particularly preferred method for recovery of the procatalyst compound. Spray drying is taught in U.S. Pat. 5,290,745 and is hereby incorporated by reference. A further procatalyst including magnesium halide or alkoxide, a transition metal halide, alkoxide ormixed ligand transition metal compound, an electron donor and optionally, a filler can be prepared by spray drying a solution of said compounds from an electron donor solvent.

[0047] The electron donor is typically an organic Lewis base, liquid at temperatures in the range of from 0°C to 200°C, in which the magnesium and transition metal compounds are soluble. The electron donor can be an alkyl ester of an aliphatic or aromatic carboxylic acid, an aliphatic ketone, an aliphatic amine, an aliphatic alcohol, an alkyl or cycloalkyl ether, or mixtures thereof, each electron donor having 2 to 20 carbon atoms. Among these electron donors, the preferred are alkyl and cycloalkyl mono-ethers having 2 to 20 carbon atoms; dialkyl, diaryl, and alkylaryl ketones having 3 to 20 carbon atoms; and alkyl, alkoxy, and alkylalkoxy esters of alkyl and aryl carboxylic acids having 2 to 20 carbon atoms. Mono-ether is defined herein as a compound that contains only one ether functional group in the molecule. For ethylene homo and copolymerization, the most preferred electron donor is tetrahydrofuran. Other examples of suitable electron donors are methyl formate, ethyl acetate, butyl acetate, ethyl ether, dioxane, di-n-propyl ether, dibutyl ether, ethanol, 1 -butanol, ethyl formate, methyl acetate, ethyl anisate, ethylene carbonate, tetrahydropyran, and ethyl propionate.

[0048] While an excess of electron donor may be used initially to provide the reaction product of transition metal compound and electron donor, the reaction product finally contains from 1 to 20 moles of electron donor per mole of transition metal compound, or from 1 to 10 moles of electron donor per mole of transition metal compound. The ligands include halogen, alkoxide, aryloxide, acetyl acetonate, and amide anions.

[0049] Partial activation of the procatalyst can be carried out prior to the introduction of the procatalyst into the reactor. The partially activated catalyst alone can function as a polymerization catalyst but at greatly reduced and commercially unsuitable catalyst productivity. Complete activation by additional cocatalyst is required to achieve full activity. The complete activation occurs in the polymerization reactor via addition of cocatalyst.

[0050] The catalyst procatalyst can be used as dry powder or slurry in an inert liquid. The inert liquid is typically a mineral oil. The slurry prepared from the catalyst and the inert liquid has a viscosity measured at 1 sec'1of at least 500 cp (500 mPa»s) at 20°C. Nonlimiting examples of suitable mineral oils are the Kaydol™ and Hydrobrite™ mineral oils from Crompton.

[0051] In an embodiment of the polymerization process, the procatalyst undergoes in-line reduction using reducing agent(s). The procatalyst is introduced into a slurry feed tank; theslurry then passes via a pump to a first reaction zone immediately downstream of a reagent injection port where the slurry is mixed with the first reagent, as described below. Optionally, the mixture then passes to a second reaction zone immediately downstream of a second reagent injection port where it is mixed with the second reagent (as described below) in a second reaction zone. While only two reagent injection and reaction zones are described above, additional reagent injection zones and reaction zones may be included, depending on the number of steps required to fully activate and modify the catalyst to allow control of the specified fractions of the polymer molecular weight distribution. Methods to control the temperature of the catalyst procatalyst feed tank and the individual mixing and reaction zones are provided.

[0052] Depending on the activator compound used, some reaction time may be required for the reaction of the activator compound with the catalyst procatalyst. This is conveniently done using a residence time zone, which can consist either of an additional length of slurry feed pipe or an essentially plug flow holding vessel. A residence time zone can be used for both activator compounds, for only one or for neither, depending entirely on the rate of reaction between activator compound and catalyst procatalyst.

[0053] Exemplary in-line reducing agents are aluminum alkyls and aluminum alkyl chlorides of the formula AlRxClywhere X+Y=3 and y is 0 to 2 and R is a Cl to C14 alkyl or aryl radical. Nonlimiting examples of in-line reducing agents include di ethyl aluminum chloride, ethylaluminum dichloride, di-isobutyaluminum chloride, dimethylaluminum chloride, methylaluminum sesquichloride, ethylaluminum sesquichloride, triethylaluminum, trimethylaluminum, triisobutylaluminum, tri-n-hexylaluminum, tri-n-octylaluminum, and dimethylaluminum chloride.

[0054] The entire mixture is then introduced into the reactor where the activation is completed by the cocatalyst. Additional reactors may be sequenced with the first reactor, however, catalyst is typically only injected into the first of these linked, sequenced reactors with active catalyst transferred from a first reactor into subsequent reactors as part of the polymer thus produced.

[0055] The cocatalysts, which are reducing agents, conventionally used are comprised of aluminum compounds, but compounds of lithium, sodium and potassium, alkaline earth metals as well as compounds of other earth metals than aluminum are possible. The compounds are usually hydrides, organometal or halide compounds. Conventionally, the cocatalysts are selected from thegroup comprising Al-trialkyls, Al-alkyl halides, Al-alkyl alkoxides and Al-alkyl alkoxy halides. In particular, Al-alkyls and Al-alkyl chlorides are used. These compounds are exemplified by trimethylaluminum, triethylaluminum, tri-isobutylaluminum, tri-n-hexylaluminum, dimethylaluminum chloride, diethylaluminum chloride, ethylaluminum dichloride and diisobutylaluminum chloride, isobutyl aluminum dichloride and the like. Butyllithium and dibutylmagnesium are examples of useful compounds of other metals.

[0056] TEST METHODS

[0057] Density

[0058] Density is measured in accordance with ASTM D792, and expressed in grams / cm3(g / cm3or g / cc).

[0059] Melt Flow Rate (12, 15 and 121)

[0060] The procedure described in ASTM D1238 is followed to determine the melt flow rate. Method B of ASTM D1238 is used. Samples are ran with loads of 21.6 kg, 5.0 kg or 2.16 kg (i.e., 121, 15 or 12, respectively).

[0061] Absolute GPC (Molecular Weight Distribution)

[0062] The chromatographic system consists of a PolymerChar GPC-IR (Valencia, Spain) high temperature GPC chromatograph equipped with an internal IR5 infra-red detector (IR5) and 4-capillary viscometer (DV) coupled to a Precision Detectors (Now Agilent Technologies) 2-angle laser light scattering (LS) detector Model 2040. For all absolute Light scattering measurements, the 15 degree angle is used for measurement. The autosampler oven compartment was set at 160° Celsius and the column and detector compartment were set at 150° Celsius. The columns used were 4 Agilent “Mixed A” 30cm 20-micron linear mixed-bed columns. The chromatographic solvent used was 1,2,4 tri chlorobenzene and contained 200 ppm of butylated hydroxytoluene (BHT). The solvent source was nitrogen sparged. The injection volume used was 200 microliters and the flow rate was 1.0 milliliters / minute.

[0063] The total plate count of the GPC column set was performed with decane which was introduced into blank sample via a micropump controlled with the PolymerChar GPC-IR system. The plate count for the chromatographic system should be greater than 18,000 for the 4 Agilent “Mixed A” 30cm 20-micron linear mixed-bed columns.

[0064] Samples were prepared in a semi-automatic manner with the PolymerChar “Instrument Control” Software, wherein the samples were weight-targeted at 2 mg / ml, and the solvent (contained 200ppm BHT) was added to a pre nitrogen-sparged septa-capped vial, via the PolymerChar high temperature autosampler. The samples were dissolved for 2 hours at 160° Celsius under “low speed” shaking.

[0065] In order to monitor the deviations over time, a flowrate marker (decane) was introduced into each sample via a micropump controlled with the PolymerChar GPC-IR system. This flowrate marker (FM) was used to linearly correct the pump flowrate (Flowrate(nominal)) for each sample by RV alignment of the respective decane peak within the sample (RV(FM Sample)) to that of the decane peak within the narrow standards calibration (RV(FM Calibrated)). Any changes in the time of the decane marker peak are then assumed to be related to a linear-shift in flowrate (Flowrate(effective)) for the entire run. After calibrating the system based on a flow marker peak, the effective flowrate (with respect to the narrow standards calibration) is calculated as Equation 1. Processing of the flow marker peak was done via the PolymerChar GPCOne™ Software. Acceptable flowrate correction is such that the effective flowrate should be within + / -0.5% of the nominal flowrate.

[0066] Flowrate(effective) = Flowrate(nominal) * (RV(FM Calibrated) / RV(FM Sample)) (EQI)

[0067] For the determination of the viscometer and light scattering detector offsets from the IR5 detector, the Systematic Approach for the determination of multi -detector offsets is done in a manner consistent with that published by Balke, Mourey, et. al. (Mourey and Balke, Chromatography Polym. Chpt 12, (1992)) (Balke, Thitiratsakul, Lew, Cheung, Mourey, Chromatography Polym. Chpt 13, (1992)), optimizing triple detector log (MW and IV) results from a linear homopolymer polyethylene standard (3.5 > Mw / Mn > 2.2) with a molecular weight in the range of 115,000 to 125,000 g / mol to the narrow standard column calibration results from the narrow standards calibration curve using PolymerChar GPCOne™ Software.

[0068] The absolute molecular weight data was obtained in a manner consistent with that published by Zimm (Zimm, B.H., J. Chem. Phys., 16, 1099 (1948)) and Kratochvil (Kratochvil, P., Classical Light Scattering from Polymer Solutions, Elsevier, Oxford, NY (1987)) using PolymerChar GPCOne™ software. The overall injected concentration, used in the determination of the molecular weight, was obtained from the mass detector area and the massdetector constant, derived from a suitable linear polyethylene homopolymer, or one of the polyethylene standards of known weight-average molecular weight. The calculated molecular weights (using GPCOne™) were obtained using a light scattering constant, derived from one or more of the polyethylene standards mentioned below, and a refractive index concentration coefficient, dn / dc, of -0.104. Generally, the mass detector response (IR5) and the light scattering constant (determined using GPCOne™) should be determined from a linear standard with a molecular weight in excess of about 50,000 g / mole. The viscometer calibration (determined using GPCOne™) can be accomplished using the methods described by the manufacturer, or, alternatively, by using the published values of suitable linear standards, such as Standard Reference Materials (SRM) 1475 (available from National Institute of Standards and Technology (NIST)). A viscometer constant (obtained using GPCOne™) is calculated which relates specific viscosity area (DV) and injected mass for the calibration standard to its intrinsic viscosity. The chromatographic concentrations are assumed low enough to eliminate addressing 2nd viral coefficient effects (concentration effects on molecular weight).

[0069] The absolute weight average molecular weight (MW(Abs)) is obtained (using GPCOne™) from the Area of the Light Scattering (LS) integrated chromatogram (factored by the light scattering constant) divided by the mass recovered from the mass constant and the mass detector (IR5) area. The molecular weight and intrinsic viscosity responses are linearly extrapolated at chromatographic ends where signal to noise becomes low (using GPCOne™). Other respective moments, Mn(Abs) and Mz(Abs) are be calculated according to the following equations:

[0070] Pennsylvania Notch Test (PENT)

[0071] PENT is measured in accordance with ASTM F1473. Results are reported in hours.

[0072] Strain Hardening Modulus

[0073] The ISO 18488 standard is followed to determine the strain hardening modulus. The samples are compression molded at 180°C with a preheating time of 5 to 15 minutes followed by 5 MPa full-pressure application for 5 minutes. A controlled 15°C / min cooling rate is used in the last step. The compression molded sheet is conditioned at 120°C for one hour followed by controlled cooling at a rate of 2°C / min to RT. Tensile bars are punched out of the compression molded sheets. The tensile test is conducted at 80°C. A non-contact extensometer is used to record the strain. As specified in ISO 18488, Neo-Hookean Strain Measure (NHSM) and the true stress plot is used to calculate the slope between a draw ratio of 8 and 12. If the failure occurred before a draw ratio of 12, then the draw ratio corresponding to the failure strain is considered as the upper limit of the slope. If the failure occurred before a draw ratio of 8.5, then the test is considered invalid.

[0074] Melt Strength

[0075] Melt strength is determined with a Gbttfert Rheotens unit model 71.9 in combination with a capillary rheometer (such as Rheotester 2000 from Gbttfert, e.g.). A polymer melt (about 20-30 grams, pellets) is extruded through a capillary die with a flat entrance angle (180 degrees) with a capillary diameter of 2.0 mm and an aspect ratio (capillary length / capillary diameter) of 15. After equilibrating the samples at 190° C. for 10 minutes, molten polymer is extruded out of the die at a constant volume flow rate corresponding to a theoretical average exit velocity of 9.5 mm / s and an apparent wall shear rate of 38.2 s'1. The wheels of the Rheotens were at standard laboratory temperature. The distance between the die exit and the wheels was 100 mm. The extruded strand was drawn by a set of standard smooth wheels with a 0.4 mm gap. The wheels were accelerated at a rate of 2.4 mm / s2and the tensile force recorded as a function of take-up speed until the filament broke. The velocity at break is a measure for the drawability of the polymer melt. Melt strength is defined as the plateau value of the force-velocity curve just before the strand broke and is reported in Newtons (cN).

[0076] Complex Viscosity

[0077] Complex viscosities (q*) are calculated using Dynamic Mechanical Spectroscopy and are reported in pascal-seconds (Pa-s). Samples are compression-molded into a 9.75 inch x10.25 inch x 1.85 mm thick rectangular plaque at 190 °C, for 6.5 minutes, under 25,000 psi pressure, in air. The sample is then taken out of the press, and allowed to cool. The resulting plaque is subjected to a 25mm diameter die cutter to extract disk-shaped samples for rheological testing. A constant temperature frequency sweep is performed using a TA Instruments “Advanced Rheometric Expansion System (ARES),” equipped with 25 mm (diameter) parallel plates, under a nitrogen purge. Samples are placed on the plate and allowed to melt for five minutes at 190 °C. The plates are then closed to a gap of “1.8 mm,” the samples trimmed (extra sample that extends beyond the circumference of the “25 mm diameter” plate was removed), and then the tests are started. The method had an additional five minute delay built in to allow for temperature equilibrium. The tests are performed at 190 °C over a frequency range of from 0.1 radians per second (rad / s) to 100 rad / s at a constant strain amplitude of 10%.

[0078] ESCR

[0079] All ESCR values disclosed herein are F50 failure times reported in hours and are measured according to ASTM DI 693, Method B, on compression molded samples having a thickness of 1.90 mm in a 10% Igepal solution at 50°C.

[0080] Tensile / Tear / Puncture / NCTL on Geomembrane

[0081] Tensile, tear, and puncture tests are performed on geomembranes formed from the inventive and comparative multimodal compositions according to GM-13 standards for MDPE membranes. Tear strength is measured in accordance with ASTM D1004; puncture resistance is measured in accordance with ASTM D4833; single-point NCTL is measured in accordance with ASTM D5397; tensile properties (i.e., tensile yield strength, elongation at break, yield stress, and tensile strength) are measured in accordance with ASTM D6693.

[0082] Geomembrane Fabrication

[0083] Geomembranes are fabricated on Collin cast line. Melt temperature is set at 240 °C and the output rate is about 20 Ib / hr. Approximately 60 mil thick and 8-inch-wide membranes were produced. Very low winder speed was used to minimize orientation in machine direction.

[0084] EXAMPLES

[0085] Materials Used

[0086] The following materials were included in the examples discussed below.

[0087] Comparative Example 1 is a unimodal ethylene copolymer medium density polyethylene.

[0088] Comparative Example 2 is a bimodal ethylene copolymer medium density polyethylene.

[0089] Inventive Examples 1-6

[0090] Example bimodal polyethylene compositions designated as Inventive Examples 1 through 6 are produced using a catalyst system including a procatalyst, UCAT™ J (commercially available from Univation Technologies, LLC, Houston, TX), and a cocatalyst, triethylaluminum (TEAL), in a gas phase polymerization process. The UCAT™ J catalyst is partially activated by contact at room temperature with an appropriate amount of a 40 percent mineral oil solution of tri-n-hexyl aluminum (TNHA). The catalyst slurry is added to a mixing vessel. While stirring, a 40 percent mineral oil solution of tri-n-hexyl aluminum (TNHA) is added at ratio of 0.17 moles of TNHA to mole of residual THF in the catalyst and stirred for at least 1 hour prior to use. Ethylene (C2) and 1 -hexene (C6) are polymerized in two fluidized bed reactors. Each polymerization is continuously conducted, after equilibrium is reached, under the respective conditions, as shown below in Tables 1. Polymerization is initiated in the first reactor by continuously feeding the catalyst and cocatalyst (trialkyl aluminum, specifically tri ethyl aluminum or TEAL) into a fluidized bed of polyethylene granules, together with ethylene, hydrogen, and 1 -hexene. The resulting polymer, mixed with active catalyst, is withdrawn from the first reactor, and transferred to the second reactor, using second reactor gas as a transfer medium. The second reactor also contains a fluidized bed of polyethylene granules. Ethylene, hydrogen and hexene are introduced into the second reactor, where the gases come into contact with the polymer and catalyst from the first reactor. In the second reactor, the cocatalyst (TEAL) is again introduced. Inert gases, nitrogen and isopentane, make up the remaining pressure in both the first and second reactors. The final product blend is continuously removed and combined with additives (i.e., antioxidants and acid neutralizers) and fed to a continuous mixer (Kobe Steel, Ltd. LCM-100), which is coupled to a gear pump, and equipped with a melt filtration device and an underwater pelletizing system.

[0091] Table 1 - Inventive Examples 1-6 Polymerization Conditions

[0092] Polyethylene compositional properties are measured in accordance with the test methods described above. Geomembranes are formed from certain of the polyethylene compositions in accordance with the geomembrane fabrication process described above.

[0093] Table 2A - Properties of Comparative Examples 1-2

[0094] Table 2B - Properties of Inventive Examples 1-6

Claims

We Claim:

1. A multimodal polyethylene composition comprising from 40 to 65 wt.% of a first polyethylene component, based on the total weight of the multimodal polyethylene composition, and a second polyethylene component, wherein the first polyethylene component has a higher molecular weight (Mw) than the second polyethylene component, and wherein the multimodal polyethylene composition has the following:(a) a density of from 0.933 to 0.945 g / cm3;(b) a high flow melt index (I21) of from 7.5 to 15.0 g / 10 min;(c) a I21 / I5 of at least 20.0;(d) a strain hardening modulus of at least 45 MPa; and wherein the first polyethylene component has a density of less than 0.930 g / cm3.

2. The multimodal polyethylene composition of any preceding claims, wherein the composition has a PENT value as measured according to ASTM F1472 of at least 4,000 hours.

3. The multimodal polyethylene composition of any preceding claim, wherein the first polyethylene component has a high load melt index (I21) of less than 1.00 g / 10 min.

4. The multimodal polyethylene composition of any preceding claim, wherein the composition has a I21 / I2 of greater than 70.

5. The multimodal polyethylene composition of any preceding claim, wherein the composition has an I2 of less than 0.15 g / 10 min.

6. The multimodal polyethylene composition of any preceding claim, wherein the first polyethylene component and the second polyethylene component comprise 1 -hexene.

7. The multimodal polyethylene composition of any preceding claim, wherein the composition has a complex viscosity at 100 rad / s of less than 3,000 Pa.s.

8. The multimodal polyethylene composition of any preceding claim, wherein the composition has a molecular weight distribution from Absolute GPC, where the Absolute GPCmolecular weight distribution has a first peak, a local minimum, and a second peak in a range of Log(molecular weight) of 3.5 to 6.0, wherein the local minimum is an inflection point between the first peak and the second peak, and the first peak corresponds to the low molecular weight component and the second peak corresponds to the high molecular weight component.

9. The multimodal polyethylene composition of any preceding claim, wherein the composition has a molecular weight distribution (Mw / Mn), as measured by absolute GPC, between 10.0 and 30.0.

10. The multimodal polyethylene composition of any preceding claim, wherein the composition wherein the composition has a weight average molecular weight (Mw), as measured by absolute GPC, of greater than 200,000 g / mol.

11. The multimodal polyethylene composition of any preceding claim, wherein the composition comprises from 50 to 60 wt.% of the first polyethylene component, based on the total weight of the multimodal polyethylene composition.

12. The multimodal polyethylene composition of any preceding claim, wherein the composition has a Mz of greater than 1,3000,000 g / mol.

13. The multimodal polyethylene composition of any preceding claim, wherein the composition is a bimodal polyethylene composition.

14. The multimodal polyethylene composition of any preceding claim, wherein when the multimodal polyethylene composition is formed into a geomembrane, the geomembrane has a notched constant tensile load failure time at 30% yield stress, as measured according to ASTMD5397, of greater than 2,500 hours.

15. A geomembrane comprising the multimodal polyethylene composition of any preceding claim.

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