Moisture adsorption and tabletting. II. The effect on tensile strength and air permeability of the relative humidity during storage of tablets of 3 crystalline materials

Abstract

The effect of moisture adsorption on mechanical strength and air permeability of tablets during storage was studied. Three crystalline substances were used, two water-soluble (saccharose and sodium chloride) each represented by two size fractions and one practically insoluble (calcium hydrogen phosphate). Tablets were stored in a series of RH (33-100%) before measurement of tablet strength and permeability. Tablets were also stored for an additional period of time at a low RH whereafter the tablets were characterized again. For saccharose, an increase in RH during storage resulted in an increase in tensile strength up to a certain level and to a decrease in tablet surface area. This is probably a result of a rearrangement of molecules at the particle surfaces by the action of adsorbed water leading to a formation of solid bridges. A less pronounced effect was obtained for the finer fraction of sodium chloride, whereas no such effect was obtained for the coarser fraction and for calcium hydrogen phosphate. At high RH, the tablet strength decreased in most cases probably because condensed water disturbed intermolecular attraction forces between particles in the tablets. Storage at low RH of transferred tablets did not change the tablet strength or the tablet surface area, i.e. the tablets were at this RH unaffected by adsorbed moisture. Exceptions were transfer of saccharose and sodium chloride tablets from an RH in which the materials had begun to deliquesce. In these cases dissolved material crystallized in the tablet and the formation of solid bridges increased the tablet strength.