Study On Hybrid Ziegler-Natta Catalyst For Ethylene Polymerization

Most of the support materials successfully used in Ziegler-Natta catalysts have been inorganic compounds, such as silica gel, magnesium chloride and zeolite. Compared with the inorganic carriers, organic carriers can greatly reduce inorganic residues to improve the performance of polyolefin products. Therefore, inorganic/organic carrier of the composite catalyst has become an important direction of new polyolefin catalysts. The current preparation method of inorganic/organic composite are complicated and introduces a number of other polar substances, not being suitable for the catalyst of olefin polymerization. A phase inversion method was introduced for hybrid Ziegler-Natta Catalyst preparation.This thesis focused on the following three aspects:(1) the preparation of silica/poly (styrene-co-acrylic acid) (PSA) core-shell microspheres; (2) the study of hybrid Ziegler-Natta catalysts for gas phase and slurry phase ethylene polymerization; (3) the improvement of hybrid Ziegler-Natta catalysts for slurry ethylene polymerization. Specific works are as follows:1. Inorganic/organic composite carrier particles were prepared based on phase inversion method. The influences of several factors on the composite surface morphology and particle size were studied, such as evaporation rate of the non-solvent, polymer concentration, the size of silica gel particle and different poly (styrene-co-acrylic acid). The slower of the non-solvent evaporation rate was, the better the dispersion of composite carrier became, and 1 ml/min was chosen as the suitable non-solvent evaporation rate. The influences of other factors on the particle size distribution of composite carriers were quite small. In addition, the specific surface area, pore volume and pore size of the composite supports were significantly reduced compared to silica gel.2. Hybrid Ziegler-Natta catalysts were prepared based on phase inversion method. Ethylene polymerizations were conducted in a gas phase reactor. Composite catalyst has high activity (3000 g polyethylene/g catalyst) (9.0 bar pressure). HC-20 catalyst had a good ethylene polymerization activity and its kinetic curve was stable with an inductive period, while HC-5 catalyst demonstrated a fast decay during polymerization and its activity and kinetic curve both resembled SLC-S catalyst. What's more, the thickness of membrane could affect the hydrogen response of the hybrid catalysts and the molecular weight distribution of resultant polyethylene.3. The hybrid Ziegler-Natta catalysts were employed for ethylene slurry polymerization. The activities of the composite catalysts in slurry polymerization were significantly higher than the activities in the gas phase polymerization. In the slurry polymerization, the kinetic profiles of SLC-S catalyst, HC-5 catalyst and HC-20 catalyst were relatively stable then with a small decay. With the increase of the thickness of hybrid catalysts, Mn and Mw of the obtained polyethylene gradually increased, but their MWD became narrow. Meanwhile, the bulk density of the polyethylene obtained from slurry polymerization was slightly lower than that from gas phase polymerization.4. In order to improve the performance of the catalyst, MgCl2 was replaced by BuMgCl to obtain a series of catalysts. With the increase of the amount of TiCl4, catalyst activity gradually increased with a linearly trend. At the same time, the kinetic profiles were in the gradual transition from the rapid decay type to the slowly decaying type, MWD decreased and the bulk density increased.