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High impact polypropylene structure evolution and impact on reaction

: Plata, Miguel Angel Acevedo
: Bartke, Michael

Halle-Wittenberg, 2018, 148 pp.
Halle-Wittenberg, Univ., Diss., 2018
Fraunhofer IAP ()
Polypropylene; polymerization; morphology; prepolymerization; heterophasic copolymers; mass-transfer

This work focuses on the effect of reaction conditions, in particular prepolymerization conditions, on resulting particle morphology and how in turn particle morphology influences process kinetics in the multistage polymerization of heterophasic polypropylene. Experimental studies were carried out using two industrial Ziegler-Natta catalysts in gas-phase and bulk phase polymerizations using a number of different lab-scale setups. After establishing reproducibility and reference activity levels, initially a broad screening of the effect of homopolymerization and prepolymerization conditions on particle morphology was performed. The results clearly showed for both catalysts studied, that prepolymerization is essentially required in order to obtain good catalyst activity and that addition of small amount of ethylene during prepolymerization is an effective handle to adjust particle morphology. By addition of ethylene, bulk density of the resulting polymer is reduced and correspondingly porosity (measured by Hg-porosity measurements) is increased. Sorption measurements in a high-pressure sorption balance reveal for the samples with lower bulk density faster mass-transfer rates. Polymerization activity in the matrix stage (either performed as gas-phase or bulk-phase polymerization) is for both catalysts studied basically not affected by ethylene in the prepolymerization step. Also crystallinity of the matrix polymers is not affected by ethylene in the prepolymerization step. During the rubber-phase polymerization, the samples with ethylene in the prepolymerization step showed surprisingly lower activities compared to the samples without ethylene in the prepolymerization step. There are indications that ethylene incorporation in the rubber step is slightly higher for samples with ethylene during the prepolymerization step. Simulation studies show that the samples with ethylene in the prepolymerization step reduce the mass-transfer limitations for ethylene in the heco-stage, which helps to better exploit the catalyst capabilities for ethylene incorporation. This research forms part of the research program of the Dutch Polymer Institute (DPI), project #785