Cover: Plasma surface processing is used to overcome agglomeration of dry find powders and meet the demand for homogenously functionalized powder particle surfaces. The cover shows a scheme for a down‐stream reactor designed for efficient surface modification and functionalization of particulates. Further details can be found in the article by A. Sonnenfeld, A. Spillmann, C. Arpagaus, and Ph. Rudolf von Rohr* on page 170.
The surface of microscopic lactose particles was modified by gas phase reactions occurring during a PECVD process to improve the flow behavior of the powder. The particulates were exposed to a capacitively coupled O2/Ar/HMDSO RF discharge in a down stream reactor by varying the gas composition and the RF power. The flowability and the surface free energy of the powders were determined by a ring shear tester and by a tensiometer, respectively. Based thereon, the change in surface free energy was determined not to be the dominating factor for the significantly enhanced flowability of the plasma‐treated powder. Rather, the flowability improvement is supposed to be affected principally by the plasma‐induced change in particle surface roughness.
The surface of 5.5 µm lactose particles was modified by gas phase reactions in a plasma enhanced chemical vapor deposition process to increase the flowability of the bulk solid. The particles were treated in the radio frequency plasma zone (RF) of a downer reactor, using a process gas mixture of oxygen, argon, and hexamethyldisiloxane (HMDSO). By increasing the flow rate of HMDSO (17–103 sccm) and the RF‐power (50–200 W), respectively, the flowability can be increased up to a factor of 2 compared to the untreated substrate material. No significant change in flowability was observed by varying the oxygen‐to‐HMDSO ratio from 5 to 30.
The plasma‐assisted improvement of wettability and flowability of fine powders is described. The plasma down‐stream reactor designed for an adequate treatment of particulates allows efficient surface modification and functionalization of particulates. It is discussed that the resulting rapidity of the processes is due to the homogeneous dispersion of particulates in the reactor. This enables intense particle/plasma interaction presumably due to the formation of floating potential sheaths around each single particle immersed in the plasma. Thus, although the overall energy influx applied is in the range of 10 to 100 J · m−2 of the particulate surfaces, the treatment time in the order of a tenth of a second instead of hours compared to conventional processes.
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