Mechanical property anisotropy of pharmaceutical excipient compacts

The mechanical property anisotropy of compacts made from six commercially available pharmaceutical excipient powders was evaluated. Uni-axially compressed cubic compacts of each excipient were subjected to pendulum impact testing and transverse tensile testing in several orientations. The pendulum impact test was used to measure the dynamic indentation hardness of each compact face (side, top, and bottom). Transverse tensile testing was utilized to determine the compact axial and radial tensile strength values. The indentation hardness (top>bottom>side) and tensile strength tests (radial>axial) revealed mechanical property anisotropy in all the compacts. The extent of mechanical property anisotropy was quantified by using dimensionless ratios and was found to be significantly different for each material. In general, compacts with a higher degree of compact mechanical anisotropy also exhibited a higher brittle fracture index (BFI). This suggests that the macroscopic flaws intentionally made in the compact for the BFI measurement were similar to the flaws induced in highly anisotropic materials during uni-axial compaction. These results are consistent with the practical observation that brittle materials are more likely to exhibit failure in a plane normal to the compaction axis, i.e. experience tablet capping and lamination phenomena.