Investigations into the reduction of powder adhesion to stainless steel surfaces by surface modification to aid capsule filling

The adhesion force of powder particles to stainless steel surfaces, which h ad been modified by various metal coatings, has been measured using a centr ifuge technique. The surfaces were characterized by surface roughness and s urface free energy measurements, whereas the particles, which had a particl e size of 32-45 mu m, were characterised by their surface free energy only. The roughness of the surfaces was found to be similar, so that changes in the adhesion properties of the powders to these surfaces could not be due t o a change in this surface property. However, there was a major difference in the surface free energy parameters of the surfaces. The surface free ene rgy varied from a nearly non-polar character to a strong Lewis-base. The ad hesion force of pregelatinised starch and lactose monohydrate particles inc reased with the increasing base character of the surfaces. In principle thi s behaviour could also be seen for calcium carbonate particles. However, he re the order of the adhesion forces to the surfaces tested was disrupted on one occasion because of a pronounced influence of particle and surface har dness on this property. In general, the ranking of the adhesion forces matc hed observations made during capsule filling on a Bosch GKF-400 tamp-fillin g machine. It was therefore concluded that a metal coating of the tamping p ins would be able to reduce powder adhesion. In order to identify the best coating, centrifugal adhesion force measurements could be undertaken, or su rface free energy measurements could be made. The results found suggest tha t a surface finish by means of chromium nitride coating provided, in most c ases, a significant reduction of the powder adhesion. For very hard powder particles such as inorganic excipients the hardness of the surfaces must al so be increased. In these cases plasma-coating of chromium appears helpful. (C) 1999 Elsevier Science B.V. All rights reserved.