BIMODAL ROTOMOLDING RESIN WITH ENHANCED DUCTILITY AND STRESS CRACK RESISTANCE (ESCR)

Abstract

The present invention relates to a polyethylene composition that has a high flow index and a bimodal composition, providing an outstanding combination of environmental stress cracking resistance (ESCR) and ductility in rotomolded articles. The composition is easy to process / mold.

Description

Field of Invention This description refers to polyethylene compositions for use in rotomolded articles. The compositions have exceptional resistance to environmental stress cracking (ESCR) and ductility. The compositions also have a high flow index, which facilitates molding, especially for longer parts. Background of the Invention There are a number of different considerations when manufacturing a resin suitable for use in rotational molding. The resin needs to be: capable of being produced at commercially acceptable production intervals; suitable for use in rotational molding processes (i.e., for example, it has a suitable sintering temperature and cooling rate for removal from the mold); and finally, it has properties suitable for the end-use application. One important desired property is resistance to environmental stress cracking. The resin must not develop cracking from exposure to chemicals, sunlight, etc., in applications such as tank sprayers for use Ref. 327196 agricultural, tanks and smaller rotomolded parts. U.S. Patents Nos. 5,382,630 and 5,382,631, filed on January 17, 1995, for Stehling and assigned to Exxon, disclose bimodal resins with superior physical properties. The patent requires that the mixture have two or more components, each with a polydispersity (Mw / Mn) of less than 3, and that the mixture have a polydispersity greater than 3. No single component in the mixture may have a relatively high molecular weight, and the comonomer content must be lower (i.e., the comonomer incorporation is reversed). The patent does not suggest an improved ESCR (Effective Carbon Rendering Ratio). U.S. Patent No. 6,969,741, filed November 29, 2005, by Lustiger et al. and assigned to ExxonMobil, discloses a polyethylene blend suitable for rotomolding. The patent states that the difference in density of each component is not less than 0.030 g / cm³. The difference in the densities of the polymer components in the present composition is less than 0.030 g / cm³. U.S. Patent No. 8,486,323, filed July 16, 2013, in the name of Davis and assigned to Dow Global Technologies Inc., discloses polymer blends used in rotationally molded articles that have high impact resistance. The blends have a residual unsaturation of less than 0.06 per 1000 carbon atoms. U.S. Patent No. 8,492,498 filed July 23, 2013 from an application filed February 21, 2011 in the name of Buck et al., assigned to Chevron Phillips describes a high-density polymer suitable for rotational molding having a folded web ESCR A condition greater than 1000 hours, as determined by ASTM D 1693 on 100% IGEPAL® CO-630. U.S. Patent No. 8,114,946, filed February 14, 2012, and U.S. Patent No. 8,475,899, filed July 2, 2013, both claiming a priority date of December 18, 2008, in the name of Yang et al., assigned to Chevron Phillips, disclose a polymer prepared using a bridged metallocene catalyst and having a long-chain branching (LCB) content of less than 0.008 per 1000 carbon atoms; by implication, LCBs are present in the polymer. The catalyst and processes used to make the compositions described herein do not produce detectable long-chain branches. Therefore, in summary, it has been difficult to prepare a rotomolding resin that has high flow ratios (to facilitate molding) and good ESCR. Brief Description of the Invention One modality provides: a bimodal polyethylene composition having a density of 0.934 to 0.940 g / cm2, a melt index 12 determined in accordance with ASTM D 1238 (2.16 kg 190°C2) of 4.0 to 7.0 g / 10 min, a 121 determined in accordance with ASTM D 1238 (21.6 kg 190°C-12i) of 140 to 170 g / 10 min, a 121 / 12 of 27 to 36, a folded-band ESCR in accordance with that determined by ASTM D 1693 on 100% octoxinol-9 for conditions A and B of more than 1000 hours, a folded-band ESCR in accordance with that determined by ASTM D1693 on 10% octoxin 1-9 for conditions B10 of more than 70 hours, a number-average molecular weight (Mn) from 11,000 to 35,000 as determined by GPC, a weight average molecular weight (Mw) of 55,000 to 82,000 as determined by GPC, a total Mw / Mn of 2.2 to 2.6, comprising 4 to 5% by weight (wt%) of one or more C4-8 alpha-olefin comonomers, as determined by FTIR, which when divided into two components consists of: (i) 20 to 45% by weight of a first component comprising 1 to 25% by weight of one or more C4-8 alpha-olefin comonomers and the remainder ethylene, such component having a density determined in accordance with ASTM D 792 of 0.915 to 0.925 g / cm3; a weight average molecular weight (Mw) of 180,000 to 220,000 g / mol, an Mw / Mn of 2 to 3; and (ii) 80 to 55% by weight of a second component comprising one or more of the C4-8 alpha olefin comonomers and the remainder ethylene, the component having a determined density in accordance with ASTM D 792 of 0.940 to 0.945 g / cm3, a weighted average molecular weight (Mw) of 30,000 to 50,000 and an Mw / Mn of 2 to 3. Another modality provides a bimodal polyethylene composition like the above in which component (i) is present in an amount of approximately 20 to approximately 35% by weight. Another modality provides a bimodal polyethylene composition like the one above, where one or more comonomers essentially consist of 1-octene. An additional embodiment provides a bimodal polyethylene composition as above wherein component (ii) is present in an amount of approximately 65 to approximately 80% by weight. Another modality provides a bimodal polyethylene composition as above where component (ii) has a weight average molecular weight (Mw) of approximately 30,000 to approximately 50,000 and a polydispersity of less than 2.5. Another embodiment provides a process for preparing a bimodal polyethylene composition as above, comprising feeding ethylene and one or more C4-8 comonomers to two sequential solution-phase reactors, in the presence of a single-site catalyst comprising a phosphinimine ligand together with one or more activators. In one embodiment, the catalyst is defined by the formula: (Pl). (L)n M (Y)p where M is selected from the group consisting of Ti, Zr and Hf; PI is a phosphinimine ligand of the formula: R21\ R2'_ p = N / R21 wherein each R21 is independently selected from the group consisting of a hydrogen atom; a halogen atom; hydrocarbyl radicals, typically C1-10, which are either unsubstituted or further substituted by a halogen atom; alkoxy radicals Ci-s; aryl or aryloxy radicals Cg-io; amido radicals; silyl radicals of the formula: --Si-- (R22) 3 wherein each R22 is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical and Ce-io aryl or aryloxy radicals; and a germanyl radical of formula: —Ge— (R22) 3 where R22 is as defined above; L is a monoanionic cyclopentadienyl-type ligand selected independently from the group consisting of cyclopentadienyl-type ligands, Y is selected independently from the group consisting of activatable ligands; m is 1 or 2; n is 0 or 1; p is an integer and the sum of m + n + p is equal to the valency state of M. In one embodiment, hydrogen is added to both reactors as follows: i) in an amount of 0.5 to 1.5 parts per million by weight (ppm) in the first reactor and ii) from 1.5 to 3.0 ppm in the second reactor. An additional embodiment provides a rotomolded part consisting essentially of the bimodal polyethylene composition described above. In another embodiment, rotomolded parts manufactured from the bimodal polyethylene composition described above exhibit ductile failure. Brief Description of the Figures Figure 1 is a graph of the molecular weight distribution obtained by gel permeation chromatograph (GPC), of a resin from example 1 and comparative examples. Figure 2 is a graph of the molecular weight distribution obtained by GPC and the short chain branching distribution determined from GPC-FTIR of a resin from example 1. Figure 3 is a graph of the molecular weight distribution obtained by GPC of the polymer of example 1 and the computer model predictions of the molecular weight distributions of the first and second ethylene polymers comprising the polymer of example 1. Figure 4 is a graph of temperature-increasing elution fractionation profiles (TREF) of the polymer from example 1 and comparative examples 3 and 4. Figure 5 is a TREF graph of the polymer from comparative examples 1, 3, and 4. Figure 6 is a TREF graph of the polymer from comparative example 1. Figure 7 presents the results of the cross-fraction chromatography obtained with the polymers of example 1 of the invention and comparative example 2, the graph of molecular weight distributions obtained from GPC on elution fractions obtained at 80°C. Figure 8 presents the results of the cross-fraction chromatography obtained with the polymers of example 1 of the invention and comparative example 2, the graph of molecular weight distributions obtained from GPC on elution fractions obtained at 89°C. Figure 9 presents the results of the cross-fraction chromatography obtained with the polymers of example 1 of the invention and comparative example 2, the graph of molecular weight distributions obtained from GPC on elution fractions obtained at 94°C. Detailed Description of the Invention Number intervals Apart from operational examples or where otherwise stated, all numbers or expressions referring to quantities of ingredients, reaction conditions, etc., used in the description and claims shall be understood to be modified in all cases by the term "approximately." Accordingly, unless otherwise stated, the numerical parameters stated in the following description and appended claims are approximations that may vary depending on the desired properties of the described embodiments. At a minimum, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter shall be interpreted at least in light of the number of significant digits reported and by applying ordinary rounding techniques. Although the numerical ranges and parameters that define the broad scope of this description are approximations, the numerical values ​​stated in the specific examples are reported as accurately as possible. However, any numerical value inherently contains certain errors that necessarily result from the standard deviation found in its respective test measurements. Furthermore, it should be understood that any numerical interval mentioned here is intended to include all subintervals within it. For example, an interval from 1 to 10 is intended to include all subintervals between and including the minimum value of 1 and the maximum value of 10; that is, it has a minimum value equal to or greater than 1 and a maximum value equal to or less than 10. Because the numerical ranges described are continuous, they include all values ​​between the minimum and maximum values. Unless expressly stated otherwise, the various numerical ranges specified in this application are approximations. All composition ranges expressed here are limited in total and do not exceed 100 percent (volume percent or weight percent) in practice. Where multiple components may be present in a composition, the sum of the maximum amounts of each component may exceed 100 percent, it being understood, and as will be readily understood by those skilled in the art, that the amounts of the components actually used will conform to the 100 percent maximum. The compositions described herein are bimodal polyethylene and can decompose into two distinct components. This is typically demonstrated by the presence of a shoulder on the right side of a gel permeation chromatography (GPC) curve (Figure 1). In this case, there is a small shoulder on the right side of the GPC curve, as shown in Figure 2, indicating a small amount of a higher molecular weight, low-density component. The overall composition of polyethylene is strictly defined with respect to the comonomer content and contains approximately 4 to 5% by weight of one or more alpha-Ce-e compounds, with the remainder being ethylene. In one embodiment, the comonomer is 1-octene or 1-hexene, particularly 1-octene. In one embodiment, the higher molecular weight component is present in an amount of approximately 20 to approximately 45% by weight of the total composition, specifically from approximately 20 to approximately 35% by weight, most especially from approximately 25 to approximately 30% by weight, based on the weight of the complete composition. The lower molecular weight component is present in corresponding amounts of approximately 80 to approximately 55% by weight of the total composition, specifically from approximately 80 to approximately 65% ​​by weight, most especially from approximately 75 to approximately 70% by weight, based on the weight of the complete composition. The highest molecular weight component has a weight-average molecular weight (Mw) of approximately 180,000 to approximately 220,000, as determined using gel permeation chromatography (GPC). This component has a polydispersity (Mw / Mn: weight-average molecular weight / numerical-average molecular weight) of less than 2.5. The melting index (I2) of the overall composition is approximately 4 to 7. It is unusual for a bimodal composition to have this relatively high I2 value while still having a first component with an Mw greater than 180,000. Without wishing to be bound to any theory, it is believed that this combination (i.e., I2 from 4 to 7 and Mw of the first component of the mixture greater than 180,000) is essential to this invention. The component with the higher molecular weight has a lower density than the component with the lower molecular weight. The density of the component with the higher molecular weight in the composition can range from approximately 0.915 to approximately 0.925 g / cm³. The density of the component, or that of any other component or the overall composition, is a function of the degree of comonomer incorporation. In one embodiment, the component with the highest molecular weight has no long-chain branches. The lower molecular weight component has a weight average molecular weight (Mw) of less than approximately 100,000, typically from approximately 30,000 to approximately 50,000, as determined using gel permeation chromatography (GPC). The lower molecular weight component has a polydispersity (Mw / Mn) of less than 2.5. The component with the lower molecular weight has a higher density than the component with the higher molecular weight. The density of the lower molecular weight component in the composition is greater than approximately 0.940 g / cm³, typically from approximately 0.940 to approximately 0.945 g / cm³. In one embodiment, the lower molecular weight component has no long-chain branching. In one embodiment, the catalysts used to produce the bimodal polyethylene compositions do not produce long-chain branching. The general properties of bimodal polyethylene compositions include the following: density of approximately 0.934 to approximately 0.940 g / cm3; melting index under a load of 2.16 kg (I2) at a temperature of 190°C, in accordance with ASTM 1238, is approximately 4 to approximately 7 and, in some cases, approximately 4.5 to approximately 6 g / 10 minutes; a melting index under a load of 21.6 kg (I21) at a temperature of 190°C, in accordance with ASTM 1238, of approximately 140 to approximately 170 and, in some cases, of approximately 140 to approximately 160 g / 10 minutes; a molten flow ratio (I21 / I2) of approximately 27 to approximately 36; an ESCR in Condition B, 10% of IGEPAL® CO-630 for more than 70 hours; an ESCR in condition A 100% IGEPAL CO-630 (octoxinol-9) greater than 1000 hours; and an ESCR in condition B 100% IGEPAL CO-630 greater than 1000 hours. In general, the composition includes approximately 4 to approximately 5% by weight of one or more C4-8 comonomers. The overall composition of bimodal polyethylene incorporates the following molecular characteristics: short chain branching frequency / 1000 carbon atoms by FTIR between approximately 5 and approximately 7; comonomer content (% by weight) by FTIR of approximately 4 to approximately 5; numerical average molecular weight (Mn) per GPC of approximately 15,000 to approximately 35,000 and, in some cases, of approximately 25,000 to approximately 30,000; average molecular weight (Mw) by GPC of approximately 55,000 to approximately 82,000 and, in some cases, from approximately 60,000 to approximately 75,000; and polydispersity (Mn / Mw) of approximately 2.0 to 2.6. The polymer can be prepared using a solution polymerization technique. In the solution polymerization of ethylene with one or more comonomers, non-limiting examples of comonomers include C3-8 α-olefins; in some cases, 1-hexene or 1-octene, especially 1-octene, is used. The monomers are typically dissolved in an inert hydrocarbon solvent, typically a C5-12 hydrocarbon, which may be unsubstituted or substituted with a Ci-β alkyl group, such as pentane, methylpentane, hexane, heptane, octane, cyclohexane, methylcyclohexane, and hydrogenated naphtha. An example of a suitable solvent that is commercially available is Isopar E (Cs-12 aliphatic solvent, Exxon Chemical Co.). The catalyst and activators are also dissolved in the solvent or suspended in a diluent miscible with the solvent under the reaction conditions. Catalyst In one embodiment, the catalyst is a compound of the formula: (Pl)r(L)n M (Y)p where M is selected from the group consisting of Ti, Zr and Hf; P1 is a phosphinimine ligand of formula: R21\ R?'_p=N / R21 where each R21 is independently selected from the group consisting of a hydrogen atom; a halogen atom; hydrocarbyl radicals, typically C1-10, which are either unsubstituted or further substituted by a halogen atom; alkoxy radicals Ci-s; aryl or aryloxy radicals Cg-io; amido radicals; silyl radicals of the formula: --SI— (R22) 3 wherein each R22 is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical and Ce-io aryl or aryloxy radicals; and a germanyl radical of formula: —Ge— (R22) 3 where R22 is as defined above; L is a monoanionic cyclopentadienyl-type ligand selected independently from the group consisting of cyclopentadienyl-type ligands, Y is selected independently from the group consisting of activatable ligands; m is 1 or 2; n is 0 or 1; p is an integer and the sum of m + n + p is equal to the valency state of M. Suitable phosphinimines are those in which each R21 is a hydrocarbyl radical, especially a Ci-6 hydrocarbyl radical, most especially a Ci-4 hydrocarbyl radical. The term cyclopentadienyl refers to a 5-membered carbon ring that has delocalized bonds within the ring and is typically attached to the active catalyst site, usually a group 4 (M) metal, through eta-5 bonds.The cyclopentadienyl ligand may be unsubstituted or up to fully substituted with one or more substituents selected from the group consisting of Ci-io hydrocarbyl radicals that are unsubstituted or further substituted by one or more substituents selected from the group consisting of a halogen atom and a C1-4 alkyl radical; a halogen atom; a Ci-s alkoxy radical; a Cg-io aryl or aryloxy radical; an amido radical that is unsubstituted or substituted with up to two Ci-8 alkyl radicals; a phosphide radical that is unsubstituted or substituted with up to two Ci-s alkyl radicals; silyl radicals of formula --Si--(R)3 wherein each R is independently selected from the group consisting of hydrogen, a Ci-8 alkyl or alkoxy radical, Cg-io aryl or aryloxy radical, and germanyl radicals of formula Ge--(R)s wherein R is as defined above. The cyclopentadienyl-type ligand can be selected from the group consisting of a cyclopentadienyl radical, an indenyl radical, and a fluorenyl radical, the radicals of which are either unsubstituted or fully substituted with one or more substituents selected from the group consisting of a fluorine atom, a chlorine atom, C1-4 alkyl radicals, and a phenyl or benzyl radical that is either unsubstituted or substituted with one or more fluorine atoms. Activatable ligands Y can be selected from the group consisting of a halogen atom, C1-4 alkyl radicals, Cg-20 aryl radicals, C7-12 arylalkyl radicals, Ce-ium phenoxy radicals, amido radicals that may be substituted with up to two C1-4 alkyl radicals, and C1-4 alkoxy radicals. In some cases, Y is selected from the group consisting of a chlorine atom, a methyl radical, an ethyl radical, and a benzyl radical. Suitable phosphinimine catalysts are Group 4 organometallic complexes containing a phosphinimine ligand (as described above), a cyclopentadienyl (L) type ligand, and two activatable ligands. The catalysts are not bridged. Activators The activators for the catalyst are typically selected from the group consisting of aluminoxanes (also known as alumoxanes by those skilled in the art) and ionic activators. Alumoxans The suitable alumoxane may have the formula: (R4) 2AIO (R4AIO)mAl (R4) 2 where each R4 is independently selected from the group consisting of C1-20 hydrocarbyl radicals and m is from 0 to 50. In one embodiment, R4 is a C1-4 alkyl radical and m is from 5 to 30. A non-limiting example of a suitable alumoxane is methylalumoxane (or MAO) where each R is methyl. Alumoxanes are well known as cocatalysts, particularly for metallocene-type catalysts. They are also readily available commercial products. The use of an alumoxane cocatalyst generally requires a molar ratio of aluminum to the transition metal in the catalyst of approximately 20:1 to approximately 1000:1; or in other cases, from approximately 50:1 to approximately 250:1. He Commercially available MAO typically contains free aluminum alkyl (e.g., trimethylaluminum or TMA) which can reduce catalyst activity and / or broaden the polymer's molecular weight distribution. If a polymer with a narrow molecular weight distribution is required, such commercially available MAO is known to be treated with an additive capable of reacting with the TMA; non-limiting examples of suitable additives include hindered alcohols or phenols. Ionic activators cocatalysts So-called ionic activators are also well known for metallocene catalysts. See, for example, U.S. Patent No. 5,198,401 (Hlatky and Turner) and U.S. Patent No. 5,132,380 (Stevens and Neithamer), both incorporated by reference. While not wishing to adhere to any one theory, those skilled in the art believe that ionic activators initially cause the abstraction of one or more of the activatable ligands in a way that ionizes the catalyst into a cation, then provide a bulky, labile, non-coordinating anion that stabilizes the catalyst in cationic form. The bulky, non-coordinating anion allows olefin polymerization to proceed at the center of the cationic catalyst (presumably because the non-coordinating anion is sufficiently labile to be displaced by the monomer that coordinates to the catalyst). Non-limiting examples of ionic activators are boron-containing ionic activators, such as: compounds of formula [R5] * [B (R7) 4] wherein B is a boron atom, R5 is an aromatic hydrocarbyl (e.g., triphenyl methyl cation) and each R7 is independently selected from the group consisting of phenyl radicals that are either unsubstituted or substituted with 3 to 5 substituents, selected from the group consisting of a fluorine atom, a C1-4 alkyl or alkoxy radical that is either unsubstituted or substituted with a fluorine atom; and a silyl radical of formula --Si--(R9)3,' wherein each R9 is independently selected from the group consisting of a hydrogen atom and a C1-4 alkyl radical;and compounds of formula [(R8)tZH] + [B(R7)4]* wherein B is a boron atom, H is a hydrogen atom, Z is a nitrogen atom or a phosphorus atom, t is 2 or 3, and R8 is selected from the group consisting of Ci-s alkyl radicals, a phenyl radical that is either unsubstituted or substituted with up to three C1-4 alkyl radicals, or an R8 taken together with the nitrogen atom can form an anilinium radical and R7 is as defined above; and compounds of formula B(R7)s wherein R7 is as defined above; In some of the above compounds, R7 is a pentafluorophenyl radical and R5 is a triphenylmethyl cation, Z is a nitrogen atom and R8 is a C1-4 alkyl radical or R3 together with the nitrogen atom forms an anilinium radical that is substituted by two C1-4 alkyl radicals. The ionic activator can abstract one or more activatable ligands to ionize the catalyst center into a cation but not to covalently bond with the catalyst and to provide sufficient distance between the catalyst and the ionizing activator to allow a polymerizable olefin to enter the resulting active site. Examples of ionic activators include: triethylamonium tetra(phenyl)boron; tripropylamonium tetra(phenyl)boron; tetra(phenyl)boron de tri(n-butyl)ammonium; tetra(p-tolyl)trimethylammonium boron; tetra(o-tolyl)trimethylammonium boron; tributylamoniotetra(pentafluorophenyl)boron; tripropylamonium tetra (o, p-dimethylphenyl)boron; tributylammonium tetra(m,m-dimethylphenyl)boron; tributylammonium tetra(ptrifluoromethylphenyl)boron; tributylammonium tetra(pentafluorophenyl)boron; tetra(o-tolyl)boron de tri(nbutyl)amonium; N,N-tetra(phenyl)boron de dimethylanilinium; N,Ndiethylanilinium tetra(phenyl)boron; N,N-tetra(phenyl)n-butylboron of diethylanilinium; N,N-2,4,6-pentamethylanilinium tetra(phenyl)boron; tetra(pentafluorophenyl)boron of di(isopropyl)amonium; tetra(phenyl)boron of dicyclohexylamonium; triphenylphosphonium tetra(phenyl)boron; tri(methylphenyl)phosphonium tetra(phenyl)boron; tetra(phenyl)boron of tri(dimethylphenyl)phosphonium; tetraquispentafluorophenyl borate of tropilum; triphenylmethyl tetraquispentafluorophenyl borate;benzene(diazonium)tetrakyspentafluorophenyl borate; tropillo phenyltrispentafluorophenyl borate; triphenylmethylium phenyltrispentafluorophenylborate; benzene(diazonium) phenyltrispentafluorophenyl borate; tropillo tetrakis(2,3,5,6tetrafluorophenyl)borate; triphenylmethyl tetrakis (2,3,5,6tetrafluorophenyl)borate; benzene(diazonium)tetrakis(3,4,5-trifluorophenyl)borate; tropillo tetrakis(3,4,5-trifluorophenyl) borate; benzene(diazonium)tetrakis(3,4,5-trifluorophenyl)borate; tropillo tetrakis(1,2,2-trifluoroethenyl)borate; triphenylmethyl tetrakis(1,2,2trifluoroethenyl)borate; benzene(diazonium) tetrakis(1,2,2-trifluoroethenyl)borate; tetrakis(2,3,4,5-tetrafluorophenyl)borate; triphenylmethyl tetrakis(2,3,4,5-tetrafluorophenyl)borate; and benzene(diazonium) tetrakis(2,3,4,5-tetrafluorophenyl)borate. Commercially available ionic activators include: N,N-dimethylanilinium tetrakispentafluorophenyl borate;triphenylmethyl tetraquispentafluorophenyl borate; and trispentafluorophenylborane. The ionic activator can be used in approximately molar equivalents of boron with respect to the group IV metal in the catalyst. Suitable molar ratios of the group IV metal from the catalyst to boron can range from approximately 1:1 to approximately 3:1, and in other cases, from approximately 1:1 to approximately 1:2. In some cases, the ionic activator can be used in combination with an alkylating activator (which can also serve as a scavenger). The ionic activator can be selected from the group consisting of (R3)pMgX2-P where X is a halide and each R3 is independently selected from the group consisting of Ci-io alkyl radicals and p is 1 or 2; R3Li where R3 is as defined above; (R3)qZnX2-q where R3 is as defined above, X is a halogen and q is 1 or 2;(R3)SA1X3-S where R3 is as defined above, X is a halogen, and is an integer from 1 to 3. In some of the above compounds, R3 is a C1-4 alkyl radical and X is chlorine. Commercially available compounds include triethylaluminum (TEAL), diethylaluminum chloride (DEAC), dibutyl magnesium (BuHMg), and butylethyl magnesium (BuEtMg or BuMgEt). If the phosphinimine catalyst is activated with a combination of ionic activators (e.g., boron compounds) and an alkylating agent, the molar ratio of the catalyst's group IV metal: the ionic activator's metalloid (boron): the alkylating agent's metal can vary from approximately 1:1:1 to approximately 1:3:10, and in other cases from approximately 1:1.3:5 to approximately 1:1.5:3. Polymerization process The temperature of the reactor(s) in a high-temperature solution polymerization process is approximately 80°C to approximately 300°C; in other cases, it ranges from approximately 120°C to 250°C. The upper temperature limit will be influenced by considerations well known to those skilled in the art, such as the desire to maximize the operating temperature (to reduce solution viscosity) while maintaining good polymer properties (since increasing polymerization temperatures generally reduces the polymer's molecular weight). In general, the upper polymerization temperature can be between approximately 200°C and approximately 300°C. A process using two reactors can be carried out at two different temperatures, with the temperature of the second reactor being higher than that of the first.A particularly suitable reaction process is a medium-pressure process, meaning that the pressure in the reactor(s) is normally less than approximately 42,000 kilopascals or kPa (approximately 6,000 psi). In some variations of the medium-pressure process, the pressures are from approximately 10,000 to approximately 40,000 kPa (1,450–5,800 psi), especially from approximately 14,000 to approximately 22,000 kPa (2,000 psi to 3,000 psi). In some reaction schemes, the pressure in the reactor system must be high enough to maintain the polymerization solution as a single-phase solution and to provide the upstream pressure necessary to feed the polymer solution from the reactor system through a heat exchanger and devolatilization system. Other systems allow the solvent to be separated into a polymer-rich stream and a polymer-lean stream to facilitate polymer separation. The solution polymerization process can be carried out in a stirred-tank reactor system comprising one or more stirred-tank reactors, in one or more circuit reactors, or in a mixed circuit reactor and stirred-tank system. The reactors can operate in tandem or in parallel. In a dual-tandem reactor system, the first polymerization reactor often operates at a lower temperature. The residence time in each reactor depends on the reactor design and capacity. Generally, the reactors must operate under conditions that ensure thorough mixing of the reactants. In one configuration, approximately 20 to approximately 60% by weight of the final polymer is polymerized in the first reactor, with the remainder polymerizing in the second reactor. A useful solution polymerization process uses at least two polymerization reactors in series (a multi-reactor process). The polymerization temperature in the first reactor is approximately 80°C to approximately 180°C (in other cases, approximately 120°C to 160°C), and the second reactor is typically operated at a higher temperature (up to approximately 220°C). In one embodiment, this multi-reactor process is a medium-pressure process, meaning that the pressure in each reactor is normally less than approximately 42,000 kilopascals or kPa (approximately 6,000 psi), specifically from approximately 14,000 to approximately 22,000 kPa (approximately 2,000 psi to approximately 3,000 psi). EXAMPLE Testing methods Mn, Mw, and Mz (g / mol) were determined by high-temperature gel permeation chromatography (GPC) with differential refractive index detection using universal calibration (e.g., ASTM-D646-99). The molecular weight distribution (MWD) is the ratio of the weight-average molecular weight (Mw) to the numerical average molecular weight (Mn). GPC-FTIR was used to determine comonomer content as a function of molecular weight. After polymer separation by GPC, an online FTIR measured the polymer concentration and the methyl end groups. The methyl end groups were used in branching frequency calculations. Conventional calibration allowed for the calculation of a molecular weight distribution. Mathematical deconvolutions were performed to determine the relative amount of polymer, molecular weight, and comonomer content of the component produced in each reactor, assuming that each polymer component follows a Flory molecular weight distribution function and has a homogeneous comonomer distribution across the entire molecular weight range. The uniform comonomer distribution of each resin component, resulting from the use of a single-site catalyst, allowed for the estimation of short-chain branching (SCB) content, in branches per 1000 carbon atoms, for the first and second ethylene polymers. This estimation was based on the deconvoluted relative amounts of the first and second ethylene polymer components in the polyethylene composition and their resin molecular weight parameters, estimated from the previous procedure. The short-chain branching frequency (SCB per 1000 carbon atoms) of copolymer samples was determined by Fourier transform infrared spectroscopy (FTIR) in accordance with ASTM D6645-01. A Thermo-Nicolet 750 Magna-IR spectrophotometer was used for the measurement. FTIR was also used to determine the levels of internal, side-chain, and terminal unsaturation. Comonomer content can also be measured using 13C NMR techniques as described in Randall Rev. Macromol. Chem. Phys., C29 (2 and 3), p. 285; Patent American No. 5,292,845 and WO 2005 / 121239. Information on the composition distribution was also obtained from temperature-raising elution fractionation (TREF). A polymer sample (80–100 mg) was introduced into the reactor vessel of the Polymer Char crystal-TREF unit. The reactor vessel was filled with 35 mL of 1,2,4-trichlorobenzene (TCP) and heated to the desired dissolution temperature (e.g., 150°C) for 2 hours. The solution (1.5 mL) was then loaded into the TREF column packed with stainless steel beads. After allowing it to equilibrate at a given stabilization temperature (e.g., 110°C) for 45 minutes, the polymer solution was allowed to crystallize with a temperature drop from the stabilization temperature to 30°C (0.09°C / minute). After equilibrating at 30°C for 30 minutes, the crystallized sample was eluted with TCB (0.75 ml / minute) with a temperature ramp from 30°C to the stabilization temperature (0.25°C / minute).The TREF column was cleaned at the end of the cycle for 30 minutes at the dissolution temperature. Data were processed using Polymer Char software, an Excel spreadsheet, and the internally developed TREF software. The CDBI is defined as the percentage of polymer whose composition is within 50% of the average comonomer composition. It is calculated from the composition distribution curing and the normalized cumulative integral of the composition distribution curve, as illustrated in U.S. Patent No. 5376439. We define the following quantities based on TREF TI profiles: 11: (Figure 6): High elution temperature. Maximum intensity of the elution peak at high temperature. T2: 12: Low elution temperature. Peak intensity of the elution peak at low temperature. T3: Temperature that marks the separation between the high and low temperature elution peaks. 13: Elution signal intensity in T3. Maximum intensity ratio: II / 12. Characterize the TREF profile with the slope between the two primary peaks: (11-12) / (T1-T2.) The weight fraction of the low elution peak is defined as the area under the curve for temperatures T3. Illustrated in the figure above as the dashed area. Cross-fractionated chromatography (CFG) was performed on selected samples. A polymer sample (100–150 mg) was introduced into the reactor vessel of the Polymer Char crystal-TREF unit. The reactor vessel was filled with 35 mL of 1,2,4-trichlorobenzene (TCB) and heated to the desired dissolution temperature (e.g., 150°C) for 2 hours. Then, 1.0 mL of the solution was loaded onto the TREF column packed with stainless steel beads. After allowing it to equilibrate at a given stabilization temperature (e.g., 110°C) for 45 minutes, the polymer solution was allowed to crystallize with a temperature drop from the stabilization temperature to 30°C (0.2°C / minute). After equilibrating at 30°C for 90 minutes, the crystallized sample was eluted with TCB from 30 to 110°C, which was divided into 15 to 20 fractions.For each fraction, the TREF column was heated to the specific dissolution temperature and held at that temperature for 55 minutes before the fraction solution was eluted and introduced into a GPC system via a heated transfer line. The polymer fractions were chromatographed at 140°C in a PL 220 high-temperature chromatography unit equipped with four SHODEX® columns (HT803, HT804, HT805, and HT806). The convolution of the individual ethylene polymer components was performed, in accordance with the results obtained from the deconvolution described above. The primary melting peak (°C), heat of fusion (J / g), and crystallinity (%) were determined using differential scanning calorimetry (DSC) as follows: the instrument was first calibrated with indium; after which a polymer sample was equilibrated at 0°C; the temperature was increased to 200°C at a heating rate of 10°C / min; the melt was then held at that temperature for five minutes; the melt was then cooled to 0°C at a cooling rate of 10°C / min and held at 0°C for five minutes; the sample was heated a second time to 200°C at a heating rate of 10°C / min. The reported melting peak (Tm), heat of fusion, and crystallinity are calculated based on the second heating cycle. The molded sheets from the polyethylene compositions were tested according to the following ASTM methods: Environmental Stress Cracking Resistance of Folded Web (ESCR), ASTM D1693; flexural properties, ASTM D 790; tensile properties, ASTM D 638. ESCR testing under ASTM D1693 Condition B was performed using a 100% octoxinol-9 solution (sold under the trade name IGEPAL® CO 360) and a 10% octoxinol-9 solution. Experts will recognize that the test using the 10% solution (Bio) is more severe than the test using the 100% solution; that is, Bio values ​​are typically lower than Bio values. The rotomolded parts were prepared on a rotational molding machine sold under the trade name ROTOSPEED® RS3-160 by Ferry Industries Inc. The machine has two arms that rotate around a central axis inside a closed oven. The arms are equipped with platens that rotate about an axis approximately perpendicular to the arm's axis of rotation. Each arm is equipped with six cast aluminum molds that produce plastic cubes with dimensions of 31.8 cm (12.5 in) x 12.5 in x 12.5 in. The arm rotation was set at approximately 8 revolutions per minute (rpm), and the platen rotation was set at approximately 2 rpm. These molds produce parts that have a nominal thickness of approximately 0.25 in (0.64 cm) when initially filled with a standard charge of approximately 3.7 kg of powdered polyethylene resin (US 35 mesh size).The temperature inside the closed oven was maintained at 293°C (560°F). The molds and their contents were heated for a specific period of time, until complete densification of the powder was achieved. The molds were then cooled in a controlled environment before the parts were removed. Samples of the molded parts were collected to measure density and color. The ARM impact test was performed according to ASTM D5628 at a test temperature of -40°C. The test specimens to be impacted must be from a rotationally molded part. The test specimens must be conditioned to achieve uniform cooling of the specimen's cross-section to no less than -40°C ± 2°C (-40°F ± 3.5°F). The impact testing technique for rotomolded parts is commonly called the Bruceton ladder method or the up-and-down method. The procedure establishes the height of a specific dart that will cause 50% of the specimens to fail. The percentage of ductility represents the percentage of failures that exhibited ductile characteristics. The specimens are subjected to impact testing using a drop-weight impact tester. If the specimen did not fail at a predetermined height / weight, the height or weight is gradually increased until failure occurs. Once failure has occurred, the height / weight is reduced in the same increment, and the process is repeated until all specimens have been used. The falling dart must impact the surface of the part that was in contact with the mold during molding. For polyethylene, ductile failure is the desired failure mode that generally occurs in properly processed specimens.A brittle failure or a failure by breakage generally indicates that the optimal properties have not been obtained through the processing parameters used. Ductile: This means the dart penetrates the specimen, leaving a hole with fibrous fibers at the point of failure, rather than cracking outward from the point of failure. The area beneath the dart is elongated and thinned at the point of failure. Brittle: This means that the piece physically disintegrates or cracks at the point of impact. The sample shows little or no elongation. The resin Bimodal polyethylene compositions were prepared in a dual-reactor pilot plant. In this dual-reactor process, the contents of the first reactor flow into the second reactor, both thoroughly mixed. The process operates using continuous feed flows. The catalyst (cyclopentadienyl tri(butyl tertiary)phosphinimine titanium dichloride) with activator was fed to both reactors. The total production rate was approximately 90 kg / h. The polymerization conditions are provided in Table 1. The polymer compositions prepared in the pilot plant were stabilized using a conventional additive package for rotational molding applications before carrying out plate tests. The properties of the resulting resins are compared with NOVA Chemicals' internal experimental resins, designated Comparative Examples 1 to 4, respectively. The results are presented in Table 2. The properties of the pressed plates and rotomolded parts manufactured from the polyethylene compositions are presented in Table 3. TABLE 1 Example Example Example Example Example Inventive Inventive Inventive Inventive Inventive 1 1 2 3 4 Division of ethylene between the first reactor (R1) and the second reactor (R2) 0.30 / 0.70 0.30 / 0.70 0.30 / 0.70 0.25 / 0.75 0.30 / 0.70 Octene is divided between the first reactor (R1) and the second reactor (R2), and the third reactor (R3) 1 / 0 1 / 0 1 / 0 1 / 0 1 / 0 Octene to ethylene ratio in fresh feed 0.144 0.110 0.080 0.115 0.190 Hydrogen in reactor 1 (ppm) 0.9 0.8 1.9 0.6 0.6 Hydrogen in reactor 2 (ppm) 2.9 7.8 2.9 0.5 9.0 Reactor 1 temperature (°C) 138 140 148 140 138 Reactor 2 temperature (°C) 210 212 208 210 208 Ethylene conversion in reactor 1 (%) 90 90 91 92 87 Ethylene conversion in reactor 2 (%) 88.0 89.9 85.3 88.7 85.0 Catalyst concentration in reactor 1 (ppm) 0.14 0.20 0.12 0.18 0.12 Catalyst concentration in reactor 2 (ppm) 0.69 0.47 0.40 0.90 0.46 TABLE 2 Example 1 Example 1 Example 2 Example 3 Example 4 Density (g / cm3) 0.9649 0.9398 0.9397 0.9361 0.9358 Melting index l2 (g / 10 min) 4.8 5.6 5.3 5.2 5.1 Melting index l6 (g / 10 min) 21.2 24.2 20.0 22.6 26.3 Melting index I21 (g / 10 min) 159 189 109 153 256 Melting flux ratio (I21 / I2) 33.1 33.9 20.3 29.6 51.0 Branching frequency / 1000C 6.2 4.6 3.8 6 6.7 (FTIR) Comonomer ID Octene Octene Octene Octene Octene Comonomer (mol %) 1.2 0.9 0.8 1.2 1.3 Comonomer (wt %) 4.8 3.6 2.9 4.6 5.2 Internal! Unsat / 1000C (FTIR) 0.027 0.019 0.02 0.034 0.018 Mn (GPC) 27,251 24,106 33,331 27,327 23,655 Mw (GPC) 68,845 67,459 69,334 74,040 71,156 Mz (GPC) 154,100 170,027 125,745 233,811 212,486 polydispersity index (Mw / Mn) 2.5 2.8 2.1 2.7 3.0 TABLE 3 Inventive Example Comparative Example Comparative Example Comparative Example Comparative Example Flex Secant Mod 1% (MPa) 784 898 891 792 694 Flex Secant Mod 1 % (MPa) Dev. 16 8 22 15 37 ESCRCond B10(hr) 10% CO-630 79 22 21 20 144 ESCRCond A100(hr) 100% CO-630 >1000 >1000 78 838 >1000 ESCRCond B100(hr) 100% CO-630 >1000 >1000 102 >1000 >1000 Low-temperature ARM impact performance Ductility (%) 100 100 100 90 27 Note: The comparative example of the Patent American Standard No. 9,540,505 also provides an ESCR of over 1000 hours for conditions A100 and B100 (however, the B10 value is only 22 hours as shown in Table 3). INDUSTRIAL APPLICABILITY Polyethylene compositions with a high flow index, desirable for ease of molding, are described. Rotomolded parts made from these compositions exhibit good resistance to environmental stress cracking and good ductility. The compositions can be used to prepare a wide variety of molded articles, such as kayaks, toys, and storage tanks. It is hereby stated that, with regard to this date, the best method known to the applicant to put the aforementioned invention into practice is the one that is clear from the present description of the invention.

Claims

1. A bimodal polyethylene composition, characterized in that it has a density of 0.934 to 0.940 g / cm3, a melting index I2 determined in accordance with ASTM D 1238 (2.16 kg 190°C—12) of 4.0 to 7.0 g / 10 min, and I2i determined in accordance with ASTM D 1238 (21.6 kg 190°C-I21) of 140 to 170 g / 10 min, an I21 / I2 of 27 to 36, a folded-band ESCR in accordance with that determined by ASTM D 1693 at 100% octoxinol-9 for conditions A and B for more than 1000 hours, a folded-band ESCR in accordance with that determined by ASTM D1693 at 10% octoxin 1-9 for conditions B10 for more than 70 hours, an average molecular weight in number (Mn) of 11,000 to 35,000 as determined by GPC, a weight average molecular weight (Mw) of 55,000 to 82,000 as determined by GPC, an overall Mw / Mn of 2.2 to 2.6, comprising 4 to 5% by weight of one or more C4-8 alpha-olefin comonomers as determined by FTIR, which when broken down into two components consists of: (i) 20 to 45% by weight of a first component consisting of 1 to 25% by weight of one or more C4-8 alpha-olefin comonomers and the remainder ethylene, the component having a density determined according to ASTM D 792 of 0.915 to 0.925 g / cm3; a weight average molecular weight (Mw) of 180,000 to 220,000 g / mol, an Mw / Mn of 2 to 3; and (ii) 80 to 55% by weight of a second component comprising one or more of the C48 alpha olefin comonomers and the remainder ethylene, the component having a density determined in accordance with ASTM D 792 of 0.940 to 0.945 g / cm3, a weight average molecular weight (Mw) of 30,000 to 50,000, and an Mw / Mn of 2 to 3.

2. The bimodal polyethylene composition according to claim 1, characterized in that component (i) is present in an amount of 20 to 35% by weight.

3. The bimodal polyethylene composition according to claim 2, characterized in that such one or more comonomers essentially consist of 1-octene.

4. The bimodal polyethylene composition according to claim 3, characterized in that component (ii) is present in an amount of 80 to 65% by weight.

5. The bimodal polyethylene composition according to claim 4, characterized in that component (ii) has a weighted average molecular weight (Mw) of 20,000 to 50,000 and a polydispersity of less than 3.

6. A process for preparing a bimodal polyethylene composition having a density of 0.934 to 0.940 g / cm³, a melt index I₂ determined in accordance with ASTM D 1238 (2.16 kg 190°C—12) of 4.0 to 7.0 g / 10 min, and I₂₁ determined in accordance with ASTM D 1238 (21.6 kg 190°C—I₂₁) of 140 to 170 g / 10 min, an I₂₁ / I₂ ratio of 27 to 36, a folded-band ESCR in accordance with ASTM D 1693 on 100% octoxinol-9 for conditions A and B for more than 1000 hours, a folded-band ESCR in accordance with ASTM D 1693 on 10% octoxinol-9 for condition B₁₀ for more than 70 hours, and an average molecular weight in number (Mn) of 11,000 to 35,000 in accordance with GPC determination, a weight average molecular weight (Mw) of 55,000 to 82,000 in accordance with GPC determination, an overall Mw / Mn of 2.2 to 2.6, comprising 4 to 5% by weight of one or more C4-8 alpha-olefin comonomers as determined by FTIR, which when broken down into two components consists of: (i) 20 to 45% by weight of a first component comprising 1 to 25% by weight of one or more C4-8 alpha-olefin comonomers and the remainder ethylene, such component having a density determined in accordance with ASTM D 792 of 0.915 to 0.925 g / cm3; a weight average molecular weight (Mw) of 180,000 to 220,000 g / mol, an Mw / Mn of 2 to 3; and (ii) 80 to 55% by weight of a second component comprising one or more of the C4-8 alpha olefin comonomers and the remainder ethylene, the component having a determined density in accordance with ASTM D 792 of 0.940 to 0.945 g / cm3, a weighted average molecular weight (Mw) of 30,000 to 50,000 and an Mw / Mn of 2 to 3; characterized in that it comprises feeding ethylene and one or more C4-8 comonomers to two sequential solution-phase polymerization reactors in the presence of a single-site catalyst comprising a phosphinimine ligand together with one or more activators.

7. The process according to claim 6, characterized in that such catalyst is defined by the formula: (Pl)n. (L)n M (Y)p wherein M is selected from the group consisting of Ti, Zr and Hf; PI is a phosphinimine ligand of formula: R21 \ R21 — P = N / R21 wherein each R21 is independently selected from the group consisting of a hydrogen atom; a halogen atom; hydrocarbyl radicals, typically C1-10, which are unsubstituted or further substituted by a halogen atom; Ci-s alkoxy radicals; Cg10 aryl or aryloxy radicals; amido radicals; silyl radicals of the formula: --Si-- (R22) 3 wherein each R22 is independently selected from the group consisting of hydrogen, a Ci-8 alkyl or alkoxy radical and Ce-io aryl or aryloxy radicals; and a germanyl radical of formula: --Ge— (R22) 3 where R22 is as defined above;L is a monoanionic cyclopentadienyl-type ligand selected independently from the group consisting of cyclopentadienyl-type ligands, Y is selected independently from the group consisting of activatable ligands; m is 1 or 2; n is 0 or 1; p is an integer and the sum of m + n + p is equal to the valency state of M.; 8. The process according to claim 6, characterized in that one or more activators comprise an alumoxane and an ionic activator.

9. The process according to claim 6, characterized in that it is carried out at temperatures between 120 and 250°C.

10. The process according to claim 6, characterized in that hydrogen is added to a first reactor in an amount of 0.05 to 1.5 parts per million by weight and to a second reactor in an amount of 1.5 to 3.0 parts per million by weight.