Modeling particle growth and morphology of impact polypropylene produced in the gas phase.

摘要

A gas phase reactor system using on-line FTIR for controlled composition olefin polymerization experiments with gaseous or liquid monomers has been designed and constructed in this work. Using this equipment, a comprehensive study of the kinetics, particle growth and morphological development of impact polypropylene produced in-situ with a TiCl{dollar}sb4{dollar}/MgCl{dollar}sb2{dollar} catalyst has been conducted. The catalyst was found exhibiting a decay type behavior for ethylene and propylene homopolymerization but an activation effect was observed when both monomers were present together. Hydrogen was also seen to boost the rate of propylene polymerization but not ethylene, and increased the rate of catalyst deactivation during propylene polymerization. Microscopy analysis of the particles over a range of copolymer content (up to 70 wt. %), copolymer composition, reaction temperature and hydrogen levels reveal how the copolymer phase segregates from the homopolymer and grows within the homopolymer matrix. A model for particle growth is proposed. A computer model for the study of the effects of changing morphology for polyolefins produced in multistage processes has been developed and used to investigate the role of monomer diffusion limitations during polymerization using the experimental data found in this work. To study the effects of residence time distribution in multistage continuous processes for impact polypropylene, population balance models have been developed for multistage processes consisting of gas and liquid phase reactors. The effects of catalyst size distribution and monomer diffusion limitations can be incorporated into the models. It is shown that commercial impact polypropylene consists of a broad distribution of polymer properties as a consequence of reactor residence time distribution issues. Implications for product homogeneity, particle sticking and process productivity are discussed.