THE EFFECT OF ORIGINAL PARTICLE-SIZE AND TABLET POROSITY ON THE INCREASE IN TENSILE-STRENGTH DURING STORAGE OF SODIUM-CHLORIDE TABLETS IN A DRY ATMOSPHERE

Compacts were formed of three particle size fractions of sodium chloride (425-500, 90-150 and 20-40 pm) at a series of applied pressures. The tensile strength of the compacts was determined directly after compaction and after storage of the compacts in dry air for different periods of time. For compacts formed of the two coarser particles, the compact strength increased during storage and a reduced compact porosity increased the magnitude but decreased the rate of the strength increase. Compacts formed of the smallest particles were mechanically stable during storage irrespective of applied pressure and compact porosity. By stress relaxation measurements it was shown that particles of all three sizes were prone to deform viscous or visco-elastically, However, both porosity-pressure relationships and stress relaxation measurements indicated that particle deformation due to an external applied stress became difficult at low tablet porosities (approx. 5%). Thus, there were differences in the dependence of particle size and compact porosity, between stress relaxation and tablet strength instability. This indicates that time dependent particle deformation is not responsible for the increase in tensile strength of sodium chloride tablets stored in a dry atmosphere.