Polyvinyl alcohol-based hard capsules with enhanced mechanical strength and gas barrier properties by montmorillonite...

This study evaluated polyvinyl alcohol (PVA) as a novel shell material for hard capsules. PVA offers high-water solubility, low toxicity, and excellent oxygen- and water vapor barrier properties. However, under humid conditions, PVA exhibits a pronounced loss of mechanical strength. To address this limitation, montmorillonite (MMT), a high aspect ratio filler, was incorporated into PVA, and the mechanical and barrier properties of films and hard capsule shells were evaluated. In unfilled PVA films, increasing relative humidity from 50% to 65% increased water uptake and reduced Young's modulus to approximately one-quarter of its original value. By contrast, PVA-MMT composite films containing 9-17 wt% MMT showed substantial improvement in mechanical performance under high humidity, with a 180% increase in Young's modulus at 9 wt% MMT, consistent with the Halpin-Tsai model. Oxygen- and water vapor permeability were reduced by 5-20-fold, indicating significant enhancement of gas barrier properties. Hard capsules prepared from PVA-MMT composites showed immediate-release dissolution comparable to commercial hypromellose capsules using acetaminophen. Overall, our findings demonstrate that MMT incorporation mitigates the humidity sensitivity of PVA, providing hard capsule shells with balanced mechanical strength and gas barrier properties suitable for pharmaceutical applications.