| Broad/bimodal polyethylene, which can well balance the processability and mechanical properties for application, is one typical tailor-made polyethylene. At present, the cascade-reactor technology is the normal used method for production of broad/bimodal polyethylene. Large capital and energy consumption are needed to build up such multiple reactors, with complicated operation procedure. As a result, researchers in various institutes and companies are making efforts to develop effective hybrid catalysts to produce broad/bimodal polyethylene in a single reactor, instead of the existed cascade-reactor technology.This thesis focuses on polymer-coated particles used as supports of hybrid catalysts for production of broad/bimodal polyethylene, the main contents are following:Firstly, a heat-driven polymer coating technology, based on solution-phase-inversion principle was established for synthesizing polymer-coated particles and the diffusion behavior of small molecules in such particles were also studied. Spherical and well-dispersed polymer-coated particles (SiO2/PSA) were successfully prepared by using silica (SiO2) and poly[styrene-co-(acrylic acid)](PSA). Through the vapor phase, instead of the liquid phase in the traditional process, a non-solvent was introduced into the mixture of micrometer-sized SiO2and PSA solution. The physical structure of polymer-coated particles can be manipulated by optimizing the weight ratio between PSA and SiO2, the concentration of PSA solution and the particle size of SiO2. Constant gradient field NMR and gravimetric adsorption method were used to study the diffusion behavior of small molecules in polymer-coated particles respectively. Both results indicated that the polymer layer has a barrier effect on the diffusion of n-hexane,1-hexene, etc. in SiO2/PSA particles while no barrier effect was shown during the diffusion of ethylene compared to the diffusion in SiO2.Secondly, poly[styrene-co-(acrylic acid)](PSA) and SiO2/MgCl2·xBD used as support of titanium-based Ziegler-Natta catalyst were carefully studied to support the continued study. PSA was modified by different magnesium compounds, containing MgCl2,(n-Bu)MgCl and (n-Bu)2Mg, and then supported TiCl4.Four catalysts PSA/TiCl4, PSA/MgCl2/TiCl4, PSA/(n-Bu)MgCl/TiCl4and PSA/(n-Bu)2Mg/TiCl4were synthesized. A series of characterization results indicated how magnesium compounds affected the formation of polymer-supported catalysts. The chemical reactions between magnesium compounds, PSA and TiCl4were provided finally. Ethylene polymerization results showed that each catalyst has its own characteristics. In the other aspect, MgCl2was recrystallized from THF solution by1,4-butanediol (BD) and dispersed on the surface of SiO2, forming SiO2/MgCl2·xBD. The influences of the1,4-butanediol on the hybrid inorganic support SiO2/MgCl2·xBD was investigated.Thirdly,"core-mantle-shell" polymer-coated particles supported titanium-based Ziegler-Natta catalysts (SiO2/MgCl2·xBD/PSA/TiCl4) and the corresponding ethylene polymerization were investigated. The chemical composition and particle structure of the obtained catalysts were characterized and discussed. Ethylene polymerizations were then carried out to evaluate the catalysts. The diffusion/barrier effects were the critical factors for study. The results showed that the polyethylene produced by SiO2/MgCl2·xBD/PSA/TiCl4has broader molecular weight distribution, larger melting flow ratio and higher bulk density, compared to the polyethylene produced by SiO2/MgCl2·xBD/TiCl4. Further, the factor of the support composition on ethylene polymerization was carefully analyzed. As the PSA content in polymer-coated particles increased, the activity profile curve of ethylene polymerization varied from decaying style, to increasing-decaying style and consequently to increasing-stable style. The growth of magnesium content in SiCO2/MgCl2·xBD/PSA/TiCl4led to the catalyst showed higher activity and lower average molecular weight. Finally, an optimized chemical composition of SiO2/MgCl2·xBD/PSA/TiCl4was determined. The effects of temperature, pressure, comonomer and hydrogen on the ethylene polymerization process and the resulting polyethylene product by such catalyst were discussed. These results were the bases for pilot-scale ethylene polymerization.Fourthly,"core-shell" polymer-coated particle supported hybrid catalysts the corresponding ethylene polymerization were investigated. Due to the barrier effects of the polymer-coated particles,(n-BuCp)2ZrCl2/TiCl4and TiCl3/TiCl4hybrid catalysts were successfully supported on SiO2/PSA. Slurry and gas phase ethylene polymerization were carried out to evaluate these two hybrid catalyst, respectively. The polyethylene produced by both TiCl3/TiCl4and (n-BuCp)2ZrCl2/TiCl4showed the molecular weight distribution as high as more than8.0. Ultimately, two different kinds of polymerization process, depending on cocatalyst need of the inner active sites in the polymer-coated particles supported catalyst, were deduced based on a few hybrid catalysts which our research group synthesized. It was concluded that the polymer-coated particles could be used as a model carrier for olefin polymerization catalysts.Finally, the pilot-scale production process of the "core-mantle-shell" polymer-coated particles supported catalyst was developed. The amplification equipment for preparation of kilo-class catalysts was designed and built up. SiO2/MgCl2·xBD/PSA/TiCl4was successfully prepared for further study. Ethylene polymerization of the catalyst in10L reactor was carried out in order to collect information for the following pilot experiments, including the hydrogen and comonomer response. Meanwhile, the variations of bulk density, density, melting flow index and the comonomer content of the resulting polyethylene were provided. The process condition in pilot-scale polymerization (300L) for pipe-using polyethylene was determined. The catalyst showed the activity more than7500g PE/g cat in pilot-scale polymerization while the melting flow ratio of the produced polyethylene was over60, implying a broad molecular weight distribution. After several times’adjustments of process condition, polyethylene with density of0.949g/cm3and melting index (5kg) of0.20g/10min was obtained. This was closed to the aim. The results of pilot-scale production indicated that the polymer-coated particles supported Ziegler-Natta catalysts has the potential for industrial application. |
PDF Full Text Request