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. The 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.
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