Mass transport and kinetics in the heterophasic copolymerization of propylene

Scope of the PhD work is a fundamental experimental and modeling study of reaction kinetics and mass transfer kinetics in the polymerization of high impact polypropylene (hiPP) under industrial relevant conditions. The polymerization of hiPP is a two stage process, in which at first a semicrystalline, isotactic homo-PP matrix is produced. In the second stage, an amorphous, elastomeric ethylene-propylene copolymer is formed, which is immiscible with the homo-PP matrix phase. Changes in particle morphology, as e. g. pore clogging and a decrease in porosity, were often speculated to cause diffusion limitations in the second stage of the polymerization. Aim of the work is the derivation of an experimentally justified reaction-transport model and its application for analysis of the balance between reaction and diffusion during the polymerization process. Gas-phase polymerization experiments were carried out in a 5 liter horizontal stirred tank reactor in semi batch mode. By analyses of the gas composition during copolymerization, individual consumption rates for each monomer can be monitored. Additionally, sorption measurements are carried out in a high pressure sorption balance in a non-reacting system in order to measure solubility and transport properties decoupled from reaction kinetics. Sorption measurements are carried out with both copolymer powders as well as with pressed polymer films, the later one in order to diminish the influence of powder morphology. From film sorption experiments, effective diffusion coefficients for the heterophasic material can be derived. Knowing the diffusion properties of the material itself, conclusions about morphological developments of the polymer powders can be drawn from powder sorption experiments. Based on these experimental findings, a mass transport model is proposed, which can describe equilibrium gas solubilities as well as sorption rates for the different copolymers. A combined mass-transport reaction-kinetic model was derived in order to investigate the balance of monomer transport and consumption. Main focus in model derivation is set on the quantitative correct description of experimental determined mass-transport and monomer consumption rates. By model based analyses of the two step process, possible diffusion limitations can be detected and consequences for the polymer can be discussed.