Fragmentation and disintegration of polymerizing PE particles: role of stress, brittleness and skin

Morphology control is an important issue in the field of polyolefins as well as catalyst development. New generations of supported catalysts often show complex fragmentation behaviour. A single catalyst particle (catalyst on support) is composed of many micro-grains that are packed together. During the first seconds of polymerization, the monomer diffuses through the porous catalyst, leading to the production of polymer, which simultaneously fills up the catalyst pores. This occurs up to a critical degree after which the particle cracks into several sub-grains. Concurrently, a polymer skin also grows around the catalyst particle, and this skin holds the sub-grains together. Ideally, with stable skin formation, one polymer particle results from one catalyst particle. However, a combination of thermal and growth / mechanical stress can lead to so called particle disintegration (“external fragmentation”) – the particle cracks into a number of smaller particles that polymerize and grow, leading to smaller than expected polymer grains (fines). “Fines” in general refer to an arbitrary fraction of particles that are far below the average expected particle size distribution. Generation of fines has been a major problem in olefin (ethylene) polymerizations for a long time now. Production of fines in industrial processes leads to a multitude of problems like – wall sheeting of reactor walls leading to bad heat transfer characteristics; lumping and sometimes clogging of FBR’s due to excessive accumulation of fines within the FBR thus disturbing particle fluidization, circulation and withdrawal; non-homogeneous polymerization leading to off-spec products; wall sheeting and clogging in downstream processes, like heat exchangers, compressors, etc. Hence, elimination / reduction of fines has been an important area of focus in industry. The main objective of this thesis was to develop a semi-quantitative method for characterization of fines generation in ethylene polymerization using MgCl2 - supported Ziegler-Natta catalysts based on a detailed analysis of the polymerization kinetics, molecular weight, crystallinity, particle growth and particle size distribution, pre-polymerization and catalyst pretreatment and their individual and combined impact on internal and external fragmentation. The polymerization rate profiles (reflect the growth stress developing within a growing polymer particle), crystallinity (is an indirect measure of the brittleness of the produced polymer), particle size distribution (gives a direct measure of the particle disintegration) and molecular weight (MW) of the produced polymer (determines the intrinsic viscosity of the polymer matrix) were used as a measure to quantify and understand the degree, extent and mechanism of particle disintegration during polymerization. In this work we have developed a comprehensive semi-quantitative theory (GRAF-S: Growth Rate Accelerated Fragmentation – taking into account the role of the outer polymer Skin) for fines generation that clearly illustrates the underlying processes and their influences on the properties of the produced polymer and consequently on the final morphology of the particle during polymerization. The GRAF-S theory contributes deeply to a more complete understanding of different mechanisms in play, taking into account many factors (individual and combined) and the key influence of the role of the outer polymer skin, during polymerization.