Water-Based Functionalization of Carboxymethylcellulose for Paper Coatings Applications

The growing demand for sustainable alternatives to fossil-based materials in paper packaging requires the development of bio-based coatings that combine effective barrier performance with recyclability. Cellulose-based derivatives represent a promising class of materials to pursue this goal, particularly nanostructured celluloses and water-soluble cellulose derivatives. Among them, carboxymethylcellulose (CMC) stands out due to its extensive industrial use in coating formulations and its excellent film-forming ability. However, the high intrinsic hydrophilicity of the polymer limits its water resistance and barrier performance. The present work aims to increase the hydrophobic character of CMC while preserving its applicability in fully aqueous formulations. In this study, we report a fully water-based and mild strategy for the chemical functionalization of carboxymethylcellulose (CMC) via amidation with octylamine, mediated by 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMTMM). The reaction was carried out in aqueous conditions, mild reaction environments, and the degree of amidation was systematically tuned by varying the DMTMM/carboxylate molar ratio. Functionalization was confirmed by 1H NMR and elemental analysis, yielding CMC-Oct derivatives with degrees of amidation up to 0.24. CMC-Oct films showed a marked reduction in moisture uptake (up to −64%) and water vapor permeability (−60%) compared to unmodified CMC, while maintaining optical transparency. The best-performing formulation was applied as an aqueous coating on paper substrates using up to three deposition layers. Compared to uncoated and native CMC-coated paper, CMC-Oct coatings significantly enhanced surface hydrophobicity (water contact angle up to 105°), water vapor barrier performance (WVP reduction up to 56%), oil resistance (Oil absorption ratio reduction up to 99.8%), and mechanical properties, with increases in both tensile strength (+47%) and elongation at break (+32%). These results demonstrate that amidated CMC prepared under mild conditions is an effective bio-based coating for paper, offering an environmentally friendly alternative to fossil-derived barriers for packaging applications, with promising compatibility with conventional paper recycling processes.