PREDICTION OF THE COMPRESSION BEHAVIOR OF POWDER MIXTURES BY THE HECKEL EQUATION

The compression behaviour of plastically flowing microcrystalline cell ulose and pregelatinized corn starch and fragmenting alpha-lactose mon ohydrate and dicalcium phosphate dihydrate, and their binary mixtures was evaluated using the Heckel equation. Powders were compressed using an instrumented eccentric tablet press. The Heckel plots for both pla stically flowing materials and their binary mixtures were linear in sh ape. In contrast, on the Heckel plots of the fragmenting materials and their mixtures, an obvious curvature was seen. The mean yield pressur es were linearly proportional to the mixture composition of two plasti cally flowing materials. When mixed with a fragmenting material, a pla stically flowing component had a greater effect on the compression beh aviour of the mixture than a fragmenting mixture component had. This w as explained by the more pronounced effect of plastic flow on volume r eduction. When two fragmenting materials were compressed, the densific ation of their binary mixture was strongly influenced by the less comp ressible material with higher mean yield pressure. It was concluded th at in the mixtures of two fragmenting materials the particles of the l ess compressible material constituted a firm structure, like a skeleto n, which tended to dominate the compression behaviour of the binary mi xtures. For binary mixtures of fragmenting materials the mean yield pr essures were thus not linearly proportional to the mixture composition . Compression behaviour of powder mixtures seemed to be strongly depen dent on the deformation properties of plain materials to be compressed . On the other hand, one mixture component may dominate the compressio n behaviour of the mixture, and the compressibility may change non-lin early with mixture composition. Therefore, caution is needed when the Heckel equation is applied for predicting compression behaviour of pow der mixtures.