Study Of F-and Ti, F-modified Cr-V Bimetallic Phillips Catalyst For Ethylene Polymerization

High density polyethylene (HDPE) is the best choice to replace metallic materials for making automobile fuel tanks from the standpoint of safety, energy saving and environmental conservation as well as product performance such as impact strength, weather resistance and chemical resistance. Nowadays, the domestic market of HDPE for fuel tanks is occupied by foreign producers. The modified Phillips Cr-based catalyst technology for fuel tank HDPE production is monopolized by developed countries. HDPE made from traditional Phillips catalyst could not meet the demands for fuel tanks due to poor short chain branch distribution. In order to develop domestic catalyst technology for HDPE fuel tanks, different F-modified Cr-V/SiO2and Ti, F-modified Cr-V/SiO2catalysts were prepared for ethylene polymerization. The effects of cocatalyst concentration, polymerization temperature,1-hexene and hydrogen concentration on ethylene homopolymerization and copolymerization activity, kinetics, and product performance characteristics were considered.The results showed that under the conditions of85℃,0.15MPa and tri-iso-butylaluminum (TIBA) as cocatalyst with a molar ratio of Al/Cr=2.5for ethylene polymerization or ethylene/1-hexene copolymerization, the effects of F-modification to the Cr-V/SiO2catalysts were shown as follows:F-modification leading to lower activity,25%higher molecular weight, better comonomer incorporation ability, better H2response with40%MW decrease under10ml H2. Under the same conditions, Ti-and F-modification to the Cr-V/SiO2catalysts could lead to even higher MW compared with the F-modified Cr-V/SiO2catalysts. Further increase of Ti contents cloud result in lower MW. The MW decreased30%under10ml H2. Temperature Rising Elution Fractionation (TREF) combined with Successive Self-Nucleation and Annealing (SSA) characterization of the products showed that the amount of chain branches was increased in the higher molecular weight fraction and decreased in the low molecular weight part of polyethylene produced by F-modified Cr-V/SiO2catalyst, which could be benefit for the substantial improvement in the mechanical properties of HDPE product for fuel tanks.