Mitigating the Effects of Moist-Heat Aging on the Properties of Phosphorylated Cellulose Nanofibril Films

Cellulose nanofibril (CNF) films are sustainable materials with promising properties, but their susceptibility to moist-heat aging constrains their long-term applicability. Here, a novel stabilization strategy is proposed based on the incorporation of sodium borohydride (NaBH 4 ) into phosphorylated CNF and lignin-containing CNF suspensions (∼2.5 mmol/g of phosphorus content) prior to film preparation. The NaBH 4 -treated (and neat) films were prepared by solvent casting and subjected to accelerated aging at 80 °C and 65% RH. After eight days of aging, the cellulose degree of polymerization of the films modified with NaBH 4 was approximately twice that of the neat CNF films, which translated into lower tensile strength losses with aging for the NaBH 4 -treated films (33‒41% vs. 63‒67%). The color stability was also improved for the modified films that showed reduced color changes (color differences (ΔE) of 4.8‒5.9 vs. 8.3‒12.2 after eight days of aging). Although the presence of borate species worsened slightly the water vapor barrier performance of the films before aging, all films converged to similar and lower WVTR values after aging (70‒80 g/m 2 .day), consistent with hornification-induced compaction observed by SEM. NaBH 4 treatment increased the film pH, conferred buffering capacity, and, as evidenced by FTIR, led to the formation of borate-cellulose complexes, which contributed to slowing hydrolytic degradation and chromophore generation during aging. These findings demonstrate that NaBH 4 addition is an effective chemical stabilization strategy for enhancing the aging resistance of highly phosphorylated CNF films, expanding their potential use in long-term applications where exposure to hygrothermal stress may occur.