Calorimetric analysis of powder compression: II. The relationship between energy terms measured with a compression calorimeter and tableting behavior

Abstract

A compression calorimeter was used to study the powder compression behavior of materials exhibiting a wide range of material properties and compactibilities. The calorimetric approach was based on the determination of the heat of compression, the work of compression and the calculation of the internal energy change for the compression phase of the tableting cycle (Lammens, Ph.D Thesis, University of Leiden, Leiden, 1980, pp. 158–259; Coffin-Beach and Hollenbeck, Int. J. Pharm., 17 (1983) 313–324; Wurster et al., Int. J. Pharm., 116 (1995) 179–189). The materials studied were Avicel PH-101, anhydrous lactose, Starch 1500, phenacetin, and polyethylene. Negative compression-induced internal energy changes were observed for all of the materials studied. These results indicated that the compression process lowered the energy states of the powders even though absorption of mechanical energy by the powders is thought to occur during the compression process. Negative ΔE values were also at odds with the measured surface area increases. For Avicel PH-101, anhydrous lactose and Starch 1500, these results did support the previous proposal that a negative internal energy change is indicative of extensive interparticulate bonding (Coffin-Beach and Hollenbeck, Int. J. Pharm., 17 (1983) 313–324). However, in the case of polyethylene and phenacetin, where significant destruction of bonds formed during compression occurred during decompression, the compression energy terms did not reflect the consequences of the complete tableting cycle. For materials with poor compactibility, the compression energy terms were unrelated to the degree of permanent interparticulate bonding. Preliminary attempts were made to address the energetics of decompression so as to make the calculated internal energy change reflective of only the irreversible processes occurring during compression.