The wall friction properties of pharmaceutical powders, blends and granulations
Data from wall friction testing and physical property characterization of over one hundred pharmaceutical powders, blends, and granulations has been analyzed. The analyses focused on data for stainless steel surfaces with the most common finishes for pharmaceutical powder processing equipment, either a 2B cold rolled mill finish or an electropolished 2B surface. Active pharmaceutical ingredients (APIs) exhibited the highest friction against these surfaces, whereas active granulations exhibited the least friction. The typical (median) wall friction angle for an active blend on 2B stainless steel was 22 degrees versus 18 degrees for an active granulation. Typical wall friction values on electropolished 2B surfaces were about 17 degrees and 12 degrees for active blends and granulations, respectively. Blends typically exhibited larger wall friction angles than the granulations suggesting that simple blends will usually require hoppers or bins with steeper walls to achieve mass flow. Lower wall friction angles were consistently observed against the smoother electropolished 2B surface, and, thus, the wall surface finish should be considered when designing bins and hoppers for use with pharmaceutical powders. The wall friction angles of blends and granulations did not show any definite trend as the percentage of API increased.