Research On Novel And Highly Efficient Modified Hybrid CrO_x-B/SiO₂ Catalyst For Ethylene Polymerization

High density polyethylene (HDPE) based on Phillips catalysts (CrOx/SiO2) has special structural features, such as broad molecular weight distribution and long chain branches. As a reault, the product has unique rheological properties and processing performance which can be widely used to prepare large containers and hollow pipes. However, the content and distribution of short chain branches are unsatisfactory, which has a seriously impact on the performance of pipes, especially on the resistance to slow crack growth. In order to improve the performance of the product, CrOx-B/SiO2 catalysts with different A contents were prepared for ethylene polymerization. The effects of cocatalyst type and concentration, polymerization temperature,1-hexene and hydrogen concentration on ethylene homopolymerization and copolymerization activity, kinetics, and product performance characteristics were considered.The results show that under the conditions of 85℃,0.15 MPa and TiBA as cocatalyst, the addition of A into CrOx-B/SiO2 catalyst is an efficient way to improve the activities of ethylene polymerization or ethylene/1-hexene copolymerization, as well as the performance of polyethylene. The addition of small amount of A can increase molecular weight of the products; however, large amount of A decreases the molecular weight. Besides, the addition of hydrogen decreases the polymerization activity and molecular weight of polyethylene. Temperature Rising Elution Fractionation (TREF) and Successive Self-Nucleation and Annealing (SSA) characterization of the products show that the amount of short chain branches are significant increased in the high molecular weight end and significant decreased in the low molecular weight end of polyethylene produced by CrOx-B/A-SiO2 catalyst, which is benefitial to the performance of PE.