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  Polyethylene resinsUSPTO Application #: 20080039606Title: Polyethylene resins Abstract: The invention provides ethylene/α-olefin copolymers exhibiting improved environmental stress cracking resistance properties, and methods for the production of the copolymers in a single reactor by means of a bimetallic catalyst including a Ziegler component and a metallocene component. (end of abstract) Agent: Exxonmobil Chemical Company - Baytown, TX, US Inventors: David Bruce Barry, Gary M. Diamond, Hitesh A. Fruitwala, Shih-May Christine Ong, Chunming Wang USPTO Applicaton #: 20080039606 - Class: 526348200 (USPTO) The Patent Description & Claims data below is from USPTO Patent Application 20080039606. Brief Patent Description - Full Patent Description - Patent Application Claims
1. FIELD OF THE INVENTION [0001] The invention relates generally to polyethylene resins. In particular, the invention provides ethylene/.alpha.-olefin copolymers exhibiting improved environmental stress cracking resistance properties, and methods for the production of the copolymers in a single reactor by means of a bimetallic catalyst including a Ziegler component and a metallocene component. 2. BACKGROUND [0002] Environmental stress cracking is the phenomenon whereby a stressed resin develops brittle cracks when exposed to a fluid such as a detergent or an organic liquid. This phenomenon can cause premature failure of articles manufactured from the resin. Environmental stress cracking resistance ("ESCR") tests have been developed to measure the resistance of resins to their environment. One such test is described in ASTM D1693. ESCR is commercially important particularly when resins come into contact with detergents and organic chemicals, such as household chemical containers and organic chemical containers. [0003] ESCR testing can also be used as a measure of a resin's resistance to slow crack propagation. Slow crack propagation occurs in resins that are at low stress levels, over extended periods of time. In this case a brittle crack propagates through the materials. This type of failure mechanism is seen in commercial applications of polyethylene in pressure pipe, containers, and vessels. Commercial polyethylene pressure pipe systems are designed to have a lifetime in excess of fifty years. Improved ESCR at high stiffness would be particularly desirable for such applications. [0004] It is known in the art that lowering resin density of linear polyethylene resins, such as linear low density polyethylene ("LLDPE"), medium density polyethylene ("MDPE") and high density polyethylene ("HDPE"), greatly improves the ESCR of the resins. However, this improvement in ESCR is at the expense of resin stiffness. As a result, conventional single reactor resins have a poor balance of ESCR and resin stiffness. [0005] Resins with a bimodal molecular weight distribution, also termed "bimodal resins," are resins having at least two polymer components with different average molecular weights. In this description, the resin with the higher average molecular weight is referred to as the "HMW polymer component", and the resin with the lower average molecular weight is referred to as the "LMW polymer component". Resins with a bimodal molecular weight distribution ("MWD") can be produced in a single reactor using the technology disclosed in, for example, U.S. Pat. No. 5,539,076, or by the use of a series of reactors or reaction steps. For example, bimodal MWD polyethylene resins can be produced in a tandem slurry processes. Bimodal resins such as those produced in series reactors are known to have a good combination of high ESCR and stiffness, believed to be because the polymerization process is controlled to ensure that the comonomer is incorporated in the HMW polymer component. U.S. Pat. No. 4,461,783 to Baily et al. discloses that high ESCR, high density resins may be obtained with independently prepared, mechanically blended polyethylene resins of different MWD where the HMW polymer component contains the majority of the comonomer, and the LMW polymer component is essentially a homopolymer. [0006] U.S. Pat. No. 5,539,076 to Nowlin et al. discloses the production of polyethylene resins with bimodal MWD in a single reactor using Ti/Zr bimetallic catalyst systems. However, in these resins, the comonomer is predominantly in the LMW polymer component of the bimodal resin. This type of comonomer distribution does not meet the requirements as disclosed in U.S. Pat. No. 4,461,783 for high ESCR at high resin density. Other background references include WO 00/50466, WO 98/57998, WO 99/31146, U.S. Pat. No. 5,624,877 to Bergmeister et al., EP 0 619 325 A1, and EP 0 882 744 A1. 3. SUMMARY OF THE INVENTION [0007] It has now surprisingly been found that despite the expected unfavorable branching distribution in ethylene/.alpha.-olefin copolymers with bimodal MWD produced by bimetallic (e.g., Ti/Zr) catalysts in a single reactor, it is possible to produce such resins which exhibit very high ESCR at high resin density. This unexpected result makes it possible to produce polyethylene resins with a superior balance of density, stiffness, ESCR and fracture toughness, in a single reactor. The ESCR of these resins is better than that of comparable commercial materials with similar resin density, and better than that of resins previously made with bimetallic catalysts, including those described in U.S. Pat. No. 5,539,076. [0008] Accordingly, the present invention generally relates to an ethylene/.alpha.-olefin copolymer having a density of at least 0.953 g/cm.sup.3 and a Bent Strip ESCR, T.sub.50, of at least 175 hours, the copolymer prepared in a single reactor. The copolymer generally will have a density of at least 0.955 g/cm.sup.3, at least 0.957 g/cm.sup.3, at least 0.959 g/cm.sup.3 or at least 0.960 g/cm.sup.3, with the T.sub.50 being generally at least 200 hours, at least 250 hours, least 300 hours, at least 350 hours, or at least 400 hours. [0009] The copolymer may have a Melt Flow Rate I.sub.21, determined according to ASTM D-1238, at 190.degree. C. and 21.6 kg, of at least 20 g/10 min., at least about 22 g/10 min., or at least 24 g/10 min. [0010] According to a further embodiment the copolymer has a Melt Flow Ratio, I.sub.21/I.sub.2 of at least 100 or at least 120, with the Melt Index, I.sub.2, being determined according to ASTM D-1238, at 190.degree. C. and 2.16 kg. [0011] In another aspect, the ethylene/.alpha.-olefin copolymer of the present invention has a bimodal molecular weight distribution and includes a HMW polymer component and a LMW polymer component which has a lower average molecular weight (weight average molecular weight, M.sub.w, determined by Gel Permeation Chromatography) and a higher density than the HMW polymer component, the copolymer prepared in a single reactor with a polymerization catalyst including a Ziegler component and a metallocene component. [0012] The density of the HMW polymer component generally will range from 0.930 g/cm.sup.3 to 0.950 g/cm.sup.3. The density of the LMW polymer component can be at least 0.955 g/cm.sup.3. [0013] The total density of the copolymer can be at least 0.954 g/cm.sup.3. [0014] In another embodiment, the HMW polymer component has a molecular weight distribution, weight average molecular weight/number average molecular weight M.sub.w/M.sub.n, of from 3 to 6 and the LMW polymer component has a M.sub.w/M.sub.n of not higher than 6. [0015] It may be particularly advantageous for the weight ratio of HMW polymer component to LMW polymer component to range from 65:35 to 35:65, or from 60:40 to 40:60. [0016] In a further embodiment, the copolymer includes units derived from one or more .alpha.-olefins containing 3 to 10 carbon atoms, or 4 to 8 carbon atoms, such as 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene or 1-octene. [0017] The copolymer will generally include 0.5 to 20 mol %, or 1 to 10 mol %, of units derived from one or more .alpha.-olefins. [0018] A further aspect of the present invention is an ethylene/.alpha.-olefin copolymer prepared in a single reactor and having a bimodal MWD and a density of at least 0.953 g/cm.sup.3, the copolymer including a HMW polymer component and a LMW polymer component having a lower average molecular weight than the HMW polymer component, the HMW polymer component including at least 30 mol % or at least 35 mol %, of the total .alpha.-olefin present in the copolymer. [0019] The present invention also generally relates to a process for making an ethylene/.alpha.-olefin copolymer as described above, in a single reactor. The process includes contacting, under polymerization conditions, ethylene, one or more .alpha.-olefins, hydrogen and a polymerization catalyst having a Ziegler component and a metallocene component, the combination of Ziegler component and metallocene component being selected to form a copolymer which has a HMW polymer component and a LMW polymer component, the HMW polymer component including at least 30 mol % of the total .alpha.-olefin incorporated into the copolymer. In a particular aspect, the copolymer is a copolymer of ethylene and 1-hexene. [0020] The process may advantageously be carried out in a gas phase reactor or in a slurry reactor, although other reactors are also suitable. [0021] In a still further aspect, the present invention relates to a method of improving the ESCR of an ethylene/.alpha.-olefin copolymer produced in a single reactor and having a bimodal MWD. According to this method, the comonomers are polymerized in the presence of a polymerization catalyst including a Ziegler component and a metallocene component, which affords a copolymer having a HMW component and a LMW component, the HMW component including at least 30 mol % of the total .alpha.-olefin incorporated into the copolymer. Continue reading... Full patent description for Polyethylene resins Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Polyethylene resins patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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