Highly Transparent Cross-Linked Cellulose Nanocrystal Coatings with High Gas Barrier and Enhanced Water Resistance for Sustainable Food Packaging

Cellulose nanocrystals (CNCs), owing to their high crystallinity, thermal stability, and strong mechanical performance, serve as promising candidates for sustainable, high gas barrier packaging materials. However, their inherent moisture sensitivity and poor water resistance limit their practical applications. This study presents a strategy to overcome this limitation by constructing a network within CNC coating layers using 1,2,3,4-butanetetracarboxylic acid (BTCA) as a cross-linker and sodium hypophosphite (SHP) as a catalyst. The resulting cross-linked network had improved water resistance, enabling the coatings to maintain structural integrity during water immersion without redispersing. Furthermore, cross-linking concurrently enhanced oxygen and water vapor barrier properties, overcoming the common trade-off between the two. For the coating formulation with 30% cross-linker, the oxygen barrier performance was improved by nearly 2 orders of magnitude compared to plain polylactic acid (PLA), while the water vapor transmission rate was also significantly lowered by 63% under dry cup conditions. In addition, a substantial portion of the enhanced barrier properties were retained after water immersion, showing great potential for its application in real-world packaging scenarios where the films may be exposed to high humidity or wet environments. Importantly, the enhanced oxygen barrier performance was demonstrated in a guacamole storage test where the cross-linked CNC coatings effectively slowed oxidative browning. This work demonstrates a viable path for developing water-resistant, high barrier, and sustainable packaging materials.