Ultrasonic evaluation of permanent moisture impact on microstructural and micro-viscoelastic properties of...

Moisture exposure can significantly affect the critical quality attributes of pharmaceutical tablets by altering their internal microstructure and micro-viscoelastic properties. In compressed granular materials, moisture-induced changes in chemical state, interparticle bonding, porosity, and residual stress distribution can alter both mechanical integrity and performance-related attributes. In this study, irreversible moisture-induced changes in tablet bulk, microstructural, and micro-viscoelastic properties were investigated and quantified using ultrasonics. As a result of uniform vapor diffusion into the samples (without direct liquid contact), moisture exposure increased tablet mass by approximately 8.3 ± 0.77%. Subsequently, the samples underwent a controlled dry-down phase during which ultrasonic waveforms were acquired using a custom-made rig in pitch-catch mode with paired pressure transducers. Waveforms acquired throughout the moisture cycle were analyzed in the temporal, spectral, and wave-dispersion domains to extract bulk- and microstructure-sensitive response metrics. It was observed that moisture exposure followed by controlled dry-down resulted in only minor residual geometric changes relative to the initial dry state, with diameter, thickness, and mass density showing average increases of approximately 0.97 ± 0.25%, 1.83 ± 0.13%, and 3.69 ± 0.49%, respectively. By contrast, substantial irreversible reductions were observed in pressure wave speed, group velocity, and apparent modulus of elasticity, by approximately 28.87%, 13.20%, and 47.34%, respectively. Equivalent axial residual stress remained approximately 3-4 times higher than equivalent radial residual stress after drying, indicating non-uniform recovery of deformation across directions. Overall, the findings demonstrate that the ultrasonic approach offers a sensitive, non-destructive means of detecting irreversible moisture-induced changes in tablet microstructure that are not captured by observable geometric recovery, highlighting its potential for quality assessment and monitoring applications.