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Tablets & Capsules May 2020 33 will be closer to each other than in a poorly flowing powder. Using the bulk and tapped density values, the researchers determined the Carr's compressibility index and Hausner ratio for each material. A compressibility index value greater than 38 percent is considered "poor," while 16 to 20 percent is considered "excellent." As Figure 2 indicates, the SMCC grades demonstrated increased flowability compared to the MCC grades. The flow-test data (Figure 3) and ring-shear-test data (Figure 4) also indicate that SMCC is a free-flowing material that allows for tablets with excellent weight and content unifor- The researchers measured the angle of repose in degrees, with an angle greater than 66 degrees indi- cating "very, very poor" flowabil- ity and 25-30 degrees indicating "excellent" flowability. As Figure 1 indicates, the SMCC grades demon- strated increased flowability com- pared to the MCC grades. The researchers then measured the materials' bulk and tapped densities using a bulk and tap density apparatus (Electrolab, India) and compared the values. Tapped density is a material's bulk density after mechanical tapping of a sample in a graduated measuring cylinder. In a free-flowing powder, there is less interparticle friction, so the bulk and tapped density values angle of repose, Carr's compressibility index, and powder density (bulk and tapped). In addition, they conducted advanced flow tests using a pow- der rheometer (FT4, Freeman) and ring shear tests, as well as scanning electron microscopy (SEM) to char- acterize the excipient's powder mor- phology. The researchers then used a compaction simulator—an instru- mented single-punch hydraulic tab- let press that simulates the operation of a rotary tablet press—to explore whether the co-processed excipient can help to formulate more-robust tablets in direct compression than the traditional excipient (Avicel MCC, DuPont). Powder flow tests The researchers used a powder flow tester for the angle of repose test, a characteristic test to deter- mine interparticle friction based on the three-dimensional angle made by a cone-like pile of material. For the procedure, researchers used a 100-millimeter-diameter base plate and fixed a funnel 40 millimeters above the plate, allowing the powder to fall on the base plate through a 10-millimeter funnel orifice, without any vibration to the funnel or base. The quantity of powder was sufficient to form a cone, with the excess falling over the edge of the base plate. The height of the cone was mea- sured with a calibrated digital Vernier, and the angle of repose was calculated with the following formula: tan (a) = height (0.5 base) Figure 1 Angle of repose for different SMCC grades versus MCC 40 101 102 302 50 90 HD 90 MCC SMCC Material type and grade Angle of repose 36 35 33 32 33 Figure 2 Carr's compressibility index for different SMCC grades versus MCC 36.0 101 102 302 50 90 HD 90 MCC SMCC Material type and grade Compressibility index 26.9 29.7 30.1 26.7 21.5 Figure 3 FT4 powder rheometer flow-test data for different SMCC grades versus MCC Above FFC 10 indicates free flowing MCC SMCC Material type and grade Flowability (FFC) 6.6 7.5 23.2 32.6 17.9 101 102 302 50 90 HD 90 Figure 4 Ring-shear-test data for different SMCC grades Non flowing Very cohesive Cohesive Easy flowing Free flowing Unconfined yield strength (pascals) 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0 Maximum principle stress (pascals) 0 5,000 10,000 15,000 20,000 Avicel SMCC 50 Avicel SMCC 90 Avicel SMCC HD90