Encapsulation of nanoparticles by polymerization compounding in a gas/solid fluidized bed reactor

The first part of project was devoted to encapsulating nanoparticles in a slurry phase reactor to investigate the feasibility of ethylene polymerization on zirconia powder via a Ziegler-Natta catalyst system to obtain zirconia nanoparticles whose surface is uniformly modified by a layer of polyethylene, a few nanometers in thickness. The advantage of Ziegler-Natta catalyst system is to allow the ethylene polymerization reaction under very moderate operational conditions. The thermogravimetry analysis results confirmed the presence of high molecular weight polyethylene on the surface of zirconia nanoparticles. It was shown by X-ray photoelectron spectroscopy that the Ziegler-Natta catalyst is effectively found at the interface between the nanoparticle surface and the polymer. It can therefore be concluded that the polymerization reaction starts at the surface of the substrate. Therefore, the encapsulation of zirconia nanoparticles by polyethylene via polymerization compounding approach is feasible using a simple Ziegler-Natta catalyst. The electron microscopy results demonstrated a thin polymer coat, about 6 nm in thickness, uniformly applied around the nanoparticles. Although the results confirmed that nanoparticles were individually coated by polyethylene, some images also revealed that coating agglomerates of particles agglomerate may also occur. However, the same images could also be interpreted as the result of the late agglomeration of individually coated particles.

In the second part of the research, efforts were focused on the fluidization of zirconia and aluminum nanoparticles. The objective was to understand under what experimental conditions the fluidization of these nanoparticles is uniformly achieved. At first glance, the fluidization of zirconia and aluminum nanoparticles involved limited bed expansion and bubbles rising up quickly through the bed. It also consisted of the non-uniform distribution of agglomerates in the bed where the small agglomerates appear to be fluidized in the upper part while the larger agglomerates move gradually to the bottom of the bed. The fluidized bed expansion was much more limited in the case of zirconia particles than with aluminum powder. In addition, the agglomerates size distribution for zirconia was wider than that in aluminum. The radioactive densitometry and fiber optic results showed that the solid concentration was uniform over the cross section of zirconia and aluminum fluidized bed for a wide range of gas velocity at different heights of bed. In the case of zirconia, it was found that the average solid hold-up over the cross section was almost independent of bed height and the solid hold-up reduction was very small when increasing the gas velocity. However, the average solid hold-up in the fluidized bed of aluminum varied at different bed heights.

The overall solid concentration obtained by the radioactive densitometry technique was in a good agreement with the global measurement obtained by bed expansion data. On the contrary, the overall solid hold-up calculated by fiber optic method was overestimated for higher gas velocities.

In the last part, the polymerization compounding method was applied to encapsulate zirconia and aluminum nanoparticles in a fluidized bed reactor. The main challenge in this part was to graft the catalyst on the surface of nanoparticles, since a only very small amount of catalyst must be fixed on relatively large surface of particles (about 1 μl/m²). When performing the encapsulation process in slurry phase, there is no obstacle to do so since the particles are dispersed in an organic solvent where the catalyst is itself injected and dissolved. Therefore, the particles dispersed in the solvent are uniformly exposed to the catalyst. On the other hand, when encapsulating particles in a gas phase reactor, such as a fluidized bed, there is no liquid media where the catalyst can be dissolved. The procedure applied in this research to deposit the catalyst on nanoparticles consisted of evaporating of Ziegler-Natta catalyst before entering fluidized bed reactor, which was followed by condensing the catalyst vapor on particles surface in the reactor. (Abstract shortened by UMI.)