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 had been modified by various metal coatings, has been measured using a centrifuge technique. The surfaces were characterised by surface roughness and surface free energy measurements, whereas the particles, which had a particle size of 32-45 microns, 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 to a change in this surface property. However, there was a major difference in the surface free energy parameters of the surfaces. The surface free energy varied from a nearly non-polar character to strong Lewis-base. The adhesion force of pregelatinised starch and lactose monohydrate particles increased with the increasing base character of the surfaces. In principle this behaviour could also be seen for calcium carbonate particles. However, here the order of the adhesion forces to the surfaces tested was disrupted on one occasion because of a pronounced influence of particle and surface hardness on this property. In general, the ranking of the adhesion forces matched observations made during capsule filling on a Bosch GKF-400 tamp-filling machine. It was therefore concluded that a metal coating of the tamping pins would be able to reduce powder adhesion. In order to identify the best coating, centrifugal adhesion force measurements could be undertaken, or surface free energy measurements could be made. The results found suggest that a surface finish by means of chromium nitride coating provided, in most cases, a significant reduction of the powder adhesion. For very hard powder particles such as inorganic excipients the hardness of the surfaces must also be increased. In these cases plasma-coating of chromium appears helpful.