USE OF X-RAY MICROSCOPY AND SYNCHROTRON MICROTOMOGRAPHY TO CHARACTERIZE POLYETHYLENE POLYMERIZATION PARTICLES

Many models have been proposed for heterogeneous polymerization kinetics that make varying assumptions regarding the catalyst fragment sizes and their distribution within the growing particles. An understanding of the processes of catalyst fragmentation, fragment dispersion, and particle agglomeration is crucial in the design of heterogeneous, olefin polymerization catalysts. Each of these phenomena can profoundly influence the course of the polymerization and the properties of the final polymer product. The phenomena and their influence on catalyst disintegration within the growing polymer particle have not, heretofore, been studied directly. We have employed high-resolution (to 5 μm) synchrotron computed microtomography, using the X26C beamline at Brookhaven National Laboratory, to directly image the distribution of larger silica fragments and voids within single polymer particles at varying polymer yields. These samples had been prepared by gas-phase ethylene polymerization over silica-supported chromium catalysts to yields of 11-200 g of polymer/g of catalyst. We come to several conclusions from these studies: (1) the catalyst fragment distribution is not uniform within the particles (the discernable larger fragments are concentrated near the periphery of the particles), (2) large fragments (>30;μm) are still evident at yields up to 200 g/g, and (3) the particles comprise at least 20-30% voids of dimensions >50 μm. © 1990, American Chemical Society. All rights reserved.