Thermodynamic analysis of compact formation; compaction, unloading, and ejection. II. Mechanical energy (work) and thermal energy (heat) determinations of compact unloading and ejection.
A compaction calorimeter, previously described (DeCrosta, M.T., Schwartz, J.B., Wigent, J.B., Marshall, K., 2000. Thermodynamic analysis of compact formation; compaction, unloading, and ejection. I. Design and development of a compaction calorimeter and mechanical and thermal energy determinations of powder compaction. Int. J. Pharm. 198, 113--134), was utilized to evaluate the thermodynamics of the unloading and ejection of compacts of Avicel pH102, Emcompress, Fast-Flo #316, Starch 1500, and acetaminophen (APAP). A constant strain waveform, applied by a compaction simulator, enabled the separate thermodynamic evaluation of unloading from compaction. The brittle materials, Fast-Flo #316 and Emcompress, displayed the most unloading work, and the plastic/self-lubricating materials, Avicel and Starch 1500, displayed the least. Unloading heat values were negative for all materials, except APAP. APAP's positive heat values indicated the breaking of bonds during unloading as a result of its highly elastic nature. Positive internal energy changes of unloading, which indicate the net breaking of bonds, were observed for APAP and Emcompress over the compaction forces tested. Negative energy changes for Starch 1500, Fast-Flo #316, and Avicel became positive with increasing compaction forces. Ejection work increased with increasing compaction force for the brittle materials, whereas smaller ejection work values for Avicel, Starch 1500, and APAP remained constant. Increasing negative heat values as a function of compaction force were observed for Fast-Flo #316 and Emcompress. Negative internal energy values for ejection were observed for Fast-Flo #316 and Emcompress, which indicates net bond formation as a result of high shear of the compact with the die wall. Internal energy changes for Starch 1500, Avicel, and APAP, were approximately zero, indicating the absence of net bonding or bond formation during the process.