(563a) Flowability Modification of Fine Powders by Plasma Enhanced Chemical Vapor Deposition

Within the scope of this work, a novel process (patent pending) to improve the flowability of fine powders is investigated. Therefore, nanoparticles are generated by favoring homogenous gas-phase reactions in a plasma enhanced chemical vapor deposition (PECVD) process and are simultaneously attached to the surface of the substrate particles. These nanoparticles act as spacers between the substrate particles and thus increase the distance between their surfaces. According to Hamaker's law, this leads to a reduction of the Van der Waals forces [1]. Additionally, under certain process conditions like low pressures, the substrate particles are coated by heterogeneous gas-phase reactions. The roughness of these coatings has a comparable effect on the Van der Waals force like the attached nanoparticles.

The novel process shows some significant advantages compared to alternative methods [2,3] for increasing the flowability of bulk solids. The process is a combination of the two process stages of nanoparticle formation and attachment in a single process. Thus, extra handling of nanoparticles, associated with additional adhesion effects, is not required. Compared to alternative treatments, like nanoparticle attachment by mixing [1], remarkable time and cost savings can be obtained. Moreover, the application of a non-equilibrium plasma provides the opportunity to treat temperature sensitive materials.

To investigate the effect of particle treatment in the PECVD process, 5 µm-particles (C-H-O-compound) as a model substrate were treated in a downer reactor. The substrate particles were conveyed through the plasma zone of the reactor by the process gas mixture, consisting of O2, Ar and Hexamethyldisiloxane (HMDSO). The organosilicon monomer was used as a reactant for the formation of SiOx nanostructures on the substrate particle surface. The influences of varying process parameters like system pressure, plasma power and gas composition on the flowability were investigated by means of a ring shear tester.

The study demonstrates that the developed process enables to increase the flowability of the bulk solid up to a factor of 300 % compared to the untreated material. It has been demonstrated that the flow factor is increased by elevating the plasma power and the monomer flow rate respectively. No significant change of the flowability was observed by varying the oxygen to monomer ratio from 5 to 30.

[1] I. Zimmermann, M. Eber, and K. Meyer, Z. Phys. Chem. 218 (2004), 51-102

[2] S. Jonat, S. Hasenzahl, A. Gray, P.C. Schmidt, J. Pharm. Sci. 93 (2004), 2635-2644

[3] J.H. Werth, M. Linsenbühler, S.M. Dammer, Z. Farkas, H. Hinrichsen, K.-E. Wirth, D.E. Wolf, Powder Technol. 133 (2003), 106-112