Video microscopy studies on growth kinetics of single catalyst particles during olefin polymerization

A new mini-reactor concept for the applied video microscopy technique is presented. The mini-reactor was built by the company Premex, Switzerland and is characterized by a high reactor tightness, fast heating, good heat removal out of the reacting particles, and improved optical contrast of images. The technique of video microscopy was employed to study the growth and fragmentation processes of single catalyst particles. A commercial analysis software from Olympus was used to evaluate the photos. The studied Ziegler-Natta catalysts were synthesized and characterized by Borealis Polymers Oy and are based on titanium, magnesium, and electron donors. Three catalyst systems were studied: self-supported, MgCl2supported, and silica-supported catalysts. Polymerization experiments were performed in gas phase at a partial propylene pressure of 5 bar and temperatures between 25-70 °C. The distribution of single particle activity was very narrow in the case of the self-supported and MgCl2-supported catalysts, while rather broad in the case of the silica-supported catalyst. Averages of single particle activities were used to describe the activity profile of the studied catalyst systems. Video microscopic analysis of polymer melt showed that fragmentation occurs in the whole catalyst matrix already from the very beginning of polymerization. The process and speed of catalyst fragmentation was determined to depend on the catalyst preparation method as well as on the kind of support. The rate of increase in surface area of the catalyst fragments with time during polymerization was correlated to the catalyst activation period during the initial stage of polymerization. Scanning electron microscopy studies provided information on morphology evolution and shape replication of catalyst particles into polymer particles. The compactness and intra-particle homogeneity of the polymer particles was observed to also depend on the kind of support and catalyst synthesis method. Polymer particles were compact in the case of the self-supported catalyst while they were porous in the case of MgCl2-supported and silica-supported catalysts. Using a simple chemical Model and neglecting mass and heat transport effects, the kinetics of propylene polymerization, and the molecular weight and molecular weight distribution of the produced polymers were well modeled.