Indentation Test for Free-Flowable Powder Excipients

Abstract

Indentation of a sphere into nonconsolidated powder excipients is a suitable complementary method for the evaluation of flow properties of the free-flowable noncohesive powders. To use the standard penetrometry method to indentation of nonconsolidated powder layer by a glass sphere of 16 mm in diameter, the suitable geometry of the measurement container is required to prevent undesirable moving in a direction opposite to that in which the indenting sphere moves. Thus, the powder indentation by a sphere seems to be similar to indentation by the Brinell hardness tester. In this work, the depth of indentation was measured for five pharmaceutical powder excipients: sodium chloride, sodium citrate, potassium citrate, sorbitol, and boric acid with the three size fractions in the range of 0.315–0.630 mm. To express powder flowability, the uniform mass and/or volume flow rate from the 10-mm cylindrical outlet of the cylindrical testing hopper was used. The relationships between the powder flow rate and its bulk density and/or the different characteristics of indentation were modeled by using linear regression. Out of the indentation characteristics, the mass of indentation calculated as the multiple of the volume of indentation and the powder bulk density could be recommended to estimate the powder mass flow rate due to the linear regression with the coefficient of determination equal to 0.973 and the accuracy of 4.3%. In conclusion, the indentation by the sphere could be recommended as a quick simple method for the comparison of flow properties of the nonconsolidated, noncohesive powder excipients.

Keywords:

• powders
• excipient
• indentation
• flow rate