MECHANISMS FOR POST-COMPACTION CHANGES IN TENSILE-STRENGTH OF SODIUM-CHLORIDE COMPACTS PREPARED FROM PARTICLES OF DIFFERENT DIMENSIONS

The changes in mechanical strength of sodium chloride compacts, prepared from particles of different dimensions, were followed during a post-compaction storage period of up to 4 days at different relative humidities. This revealed that the increase in tablet strength can occur by two different mechanisms referred to as mechanism alpha and beta. Compacts of fine particulate sodium chloride were shown to increase in tablet strength in the presence of moisture by means of a mechanism described as molecular rearrangement at the surface of the particles. For this mechanism, mechanism alpha, the increase in tablet strength could be time delayed, with respect to the formation of the compacts, by storing the compacts at 0% relative humidity. Compacts of coarse particulate sodium chloride were shown to increase in tablet strength at all relative humidities, mechanism beta, but the presence of moisture in the compact slowed down the process of increasing tablet strength. These changes in tablet strength were postulated as due to either a visco-elastic particle deformation (stress relaxation), where the relative positions of the particles in the compacts are changed continuously after ejection from the die, or a stabilisation of bonds which had already been formed during compression.