Studies On The New Technologies Of Bimodal Polyethylene Production

Bimodal polyethylene with bimodal molecular weight distribution and bimodal composition distribution include low molecular species for processability and high molecular weight species for properties have gained considerable popularity due to a better balance of processability and properties. Dual reactor systems and single reactor are main bimodal technology. There are many advantages in using single reactor bimodal technology vis-a-vis dual reactor systems. Lower investment costs, intimate mixing of high and low molecular weight components (improved product property).New technology and novel catalyst system were developed to produce bimodal polyethylene in single reactor based on the concept of "reactor granule technology". The thesis focuses on the four parts as follows:1. A new process to produce bimodal polyethylene through oscillating operatio in single reactor was presented.(a) Bimodal polyethylene was produced with the metallocene catalyst in a gas-phase reactor through oscillating operation of hydrogen periodically. Effects of hydrogen concentration, time distribution, oscillating operation period on molecular weight distributions with oscillating operation were studied. It was viable to control molecular weight distributions of bimodal polyethylene through hydrogen oscillating operation.(b) Bimodal polyethylene production in a continuous gas-phase fluidized bed through oscillating operation of hydrogen periodically was simulated. Flory distribution function was adopted to simulate the molecular weight distributions of single-site activity. Bimodal polyethylene was obtained through oscillating hydrogen concentration periodically. It shows that the feasibility of oscillating operation depended on the values of k_trH/kp (the ratio of termination rate constant by chain transfer to hydrogen to the propagation rate constant) and k_trH (termination rate constant by chain transfer to hydrogen). The bimodality of polyethylene will be easier to obtain by metallocene catalyst with the higher of k_trH/kp and k_trH. It also shows that the key factor to achieve oscillating operation is the changeover time from high to low hydrogen concentration in the fluidized bed reactor. The shorter the transition time is, the easier of oscillating operation will be.2. Blend of linear polyethylene (LPE) and branch polyethylene (BPE) by in-situ space-confined polymerization was studied.(a) Homogeneous polymerization using Cp₂TiCl₂ (Bis(cyclopentadienyl)titaniumDichloride)and nickel-diimine (ArN=C(CH₃)-C(CH₃)=NAr, Ar=2,6-(i-Pr)₂C₆H₃) (DMN) binary catalysts with methylaluminoxane (MAO) was studied. The effects of polymerization temperature and Cp₂TiCl₂ molar fraction (X_Ti) on binary catalysts performances were investigated. DSC and SEM have been used to study the polyethylene microblends, which both indicate that phase separation occurred in the polyethylene obtained at 50°C, while the linear and branched polyethylene obtained at 0°C are uniformly microblends. The GPC analysis shows that both Cp₂TiCl₂ and nickel-diimine have only one type of active site, while the binary catalysts have two, which are almost the same as that of Cp₂TiCl₂ and nickel-diimine.(b) Microblend of linear LPE/BPE by in-situ space-confined polymerization was studied. Cp₂TiCl₂ and a-nickel diimine catalysts (DMN) were supported on mesoporous particles having parallel hexagonal nanotube pore structure (MCM-41) for ethylene polymerization with methylaluminoxane (MAO) as cocatalyst. DSC, XRD and SEM have been used to investigate phase structures of LPE/BPE blends. Comparing with polyethylene obtained by homogeneous binary catalysts, it was able to blend LPE and BPE to microscale range through in-situ reaction without the need of a compatibilizer.3. A novel simple catalyst system, iron (III) acetylacetonate and bis(imino)pyridyl ligandmixture activated with methylaluminoxane (MAO) has been found to exhibit high activity for ethylene polymerization.(a) Effects of polymerization temperature and Al/Fe molar ratio have been systemically investigated on seven iron (III) acetylacetonate (Fe(acac)₃) and bis(imino)pyridyl ligand ( 2-R₁N=C(Me)-6-R₂N=C (Me)C₅H₃N ) ( L₁: R₁=R₂= 2,6-Me₂C₆H₃; L₂: R₁=R₂=2-Me-6-(i-Pr)C₆H₃; L₃: R,=R₂=2,6- (i-Pr)₂C₆H₃; L₄: R₁=R₂=2-MeC₆H₄; L₅: R₁=R₂=2-(i-Pr)C₆H4; L₆: R₁= 2-MeC₆H₄, R₂=2,6- (i-Pr)₂C₆H₃; L₇: R₁= cyclohexyl, R₂=2,6-(i-Pr)₂C₆H₃) catalyst systems. Fe(acac)₃ could not catalyze ethylene polymerization without bis(imino)pyridyl ligand. However, bimodal polyethylene was produced by active species were formed in situ and when Fe(acac)₃/ bis(imino)pyridyl ligand system was activated by MAO. Bis(imino)pyridyl ligand play a major role in the catalyst system. High molecular weight polyethylene was obtained using Fe(acac)₃/ L₁～L₃ catalyst systems, both liquid and solid products were obtained simultaneously using Fe(acac)₃/L₄, L₅ catalyst systems, and polyethylene with a few oligomers were produced with Fe(acac)₃/L₆, L₇ catalyst systems. As polymerization temperature increased, polyethylene with lower molecular weight was obtained. The effect of polymerization temperature on molecular weight distributions increased when decreasing the bulkiness ofligand. The chain transfer to MAO occurs more easily at higher equiv of Al/Fe, and the effect of Al/Fe molar ratio on molecular weight distributions increase when increasing the bulkiness of ligand when commercial MAO (including Al(Me)3) was used as cocatalyst.(b) Mechanisms of active species formation and bimodal polyethylene production were investigated. The results indicated that iron metal sites were coordinated with bis(imino)pyridyl ligand according one equiv molar ratio when MAO was added to Fe(acac)₃/ bis(imino)pyridyl ligand catalyst system by detailed study. Several kinds of active species were formed in situ and its action together with transfer to aluminum lead to the polyethylene product with broad/bimodal molecular weight distributions.4. Supporting of soluble single-site catalysts on preferably inorganic substrates is essential to provide "drop in" catalysts for use in existing technologies for slurry or gas-phase polymerization processes. Support method different with conventional support sequence was used because there was no activity when MAO was supported first. Activated Davision 955 silica gel was used to support compound by sequence of catalyst system and MAO. The supported catalyst has receivable activity when no more cocatalyst was added to the reactor. In all polymerization runs no reactor fouling was found with the supported catalyst. The polyethylene obtained showed high melting temperature and high molecular weight. The increase in activity is linear and demonstrates that the rate of propagation has a first-order rate dependence on ethylene, in accordance with the proposed Cossee-type mechanism. The fact that molecular weight remained essentially invariant with ethylene pressure indicated that the overall rate of chain transfer must also be first order in ethylene and β-H transfer was predominant chain transfer process. Experiments in which the temperature of the polymerization reaction was varied revealed that an increase in temperature results in large decreased in activity and molecular weight. As Al/Fe molar ratio increased from 32 to 59, the activity of supported catalysts increased obviously, while molecular weight remained constant indicated β-H transfer was predominant chain transfer process.