Variation of drying rate for structural tuning of cellulose nanofibril–clay composite films: Implications for mechanical and barrier performance

Cellulose nanofibrils (CNFs) are promising bio-based nanomaterials for sustainable packaging, yet their high water affinity compromises their barrier performance. While the addition of clay fillers to improve moisture resistance is well established, the role of drying conditions in shaping the mechanical and barrier properties of CNF/clay composites remains underexplored. Herein, TEMPO-oxidized CNFs (TCNFs) were reinforced with bentonite (BT) clay at 8, 15, and 23 wt% and the effect of drying rate on the structural, mechanical and moisture barrier properties of films with the highest clay content was investigated. A fast drying rate (>300 g/m²/h) promoted exfoliated clay structures, and a 65% reduction in water vapor transmission rate compared to neat TCNF films. In contrast, slow drying (~55 g/m²/h) resulted in intercalated and flocculated clay structures with surface agglomerates. Notably, slow drying enhanced mechanical performance, yielding a higher tensile index and specific Young's modulus. The differences can be attributed to the interplay between drying kinetics, colloidal stability, and internal stress development during film formation. This work demonstrates the critical role of processing parameters in determining structure-property relationships and underscores the importance of balancing filler content with drying conditions to tailor the mechanical strength and moisture barrier of CNF/clay composites.