Effect of Moisture on Compaction and Compression

The effects of moisture on the flow properties, tensile strength, Heckel plot (particle rearrangement, yield pressure), energies involved in compaction (gross, plastic, and elastic energies), and elastic recovery are reviewed. he identification and quantification of the numerous parameters that affect the compaction process are vital for product uniformity. For example, moisture adsorption plays an important role in physical and chemical stability, in the properties of solid dosage forms and excipients, and in polymers for sustained-release formulations. The vapor pressure of water in the atmosphere is quantified by the percent relative humidity (% RH). The moisture content at which a solid material produces a water vapor pressure equal to that of the surrounding environment is defined as the equilibrium moisture content (EMC). The solid’s resultant weight gain at a specified temperature and % RH is expressed as a percentage of its initial dry weight. For a drug known to undergo hydrolysis in the presence of moisture, it is important to study EMC and hygroscopicity. In terms of powders’ hygroscopic behavior, studies find that five factors determine the moisture adsorbance rate: (a) the pressure gradient between the vapor pressure of water in the atmosphere over the sorbed moisture layer of the drug substance, (b) temperature, (c) the surface area of solid drug exposed to the water vapor, (d) the velocity of moist air’s movement, and (e) a reaction constant that is characteristic of the solid. Mikuliniskii and Rubinshtein studied the kinetics of magnesium sulfate’s moisture uptake (1). They concluded that the kinetics depended on: (a) surface adsorption, occurring at a rate proportional to the difference between the partial pressure of water vapor in the atmosphere and that of the saturated salt solution, and (b) water diffusion into the crystal at a rate dependent on the product of the diffusion coefficient and the concentration gradient of water. The effect of moisture on tablets’ chemical and physical stability is outside the scope of this article.