New methods characterizing avalanche behavior to determine powder flow.

Purpose: To characterize the avalanche behavior of different powders and to compare the results of the strange-attractor and novel characterization approaches.

Methods: The following nine different materials were tested: three lactoses, maltodextrin, two microcrystalline celluloses, sodium chloride, sucrose, and glass beads. Morphology, size, and size distribution, true density, bulk and tap density, angle of repose, flow index, and avalanching behavior were quantified for each excipient by scanning electron microscopy, laser time-of-flight analysis, helium pycnometer, graduated cylinder, fixed-height funnel, Flodex (Hanson Research Corp., Chatsworth, California) method, and AeroFlow (TSI, Inc., St. Paul, Minnesota), respectively. Environmental factors were controlled, and the avalanches were studied at various speeds.

Results: The strange-attractor graph obtained at 1 rotation per 120 s showed that it was difficult to appreciate the flowability differences among 3-mm glass beads, lactose 100, and lactose 325. However, plotting the raw data as a relationship of the time between each avalanche and the inverse of speed revealed a characteristic linear slope for each sample. Furthermore, a new flowability index based on the SD calculated from the raw data gave results that were consistent with Carr's index. A cohesive index also can be determined by avalanche behavior, and it reflects the stability of the rapid particular rearrangements of powder.

Conclusion: A novel method of evaluating avalanche measurements makes it possible to better characterize powder flowability and to predict powder behavior under working conditions.