Cellulose nanofibrils (CNF) are explored as polymer reinforcing agents, stabilizers, hydrogels, coatings, and thin films in a wide range of applications, including nanocomposites, tissue engineering, drug delivery, environmental remediation, sensors, and food packaging. As CNF production usually requires great energy demand for mechanical treatments, oxidation of cellulose using (2,2,6,6-tetramethyl-1-piperidinyl)oxidanyl (TEMPO) is a widespread strategy to facilitate cellulose defibrillation, thus minimizing the energy cost. In brief, TEMPO-oxidation inserts carboxylated groups on cellulose surface, increasing electrostatic repulsion and favoring fibril separation. This procedure, however, has a drawback: the yellowing of CNF under heating [1,2], which is an undesired effect that alters CNF visual aspect eventually resulting in poor aging performance. In this work, nanocelluloses (cellulose nanocrystals obtained by acid hydrolysis and TEMPO-oxidized CNF) and lignin nanoparticles (addition of water as non-solvent) were prepared from elephant grass and combined in nanocomposite films containing 0%, 1%, and 10% of nanolignin. Thermogravimetric analysis showed that TEMPO-oxidized CNF have a lower initial decomposition temperature, as compared to pristine cellulose and cellulose nanocrystals. This can be assigned to the presence of thermally unstable sodium anhydroglucuronate units [3]. All the films underwent artificial aging at 40 and 80 °C with different relative humidity (~5 to 75%). The aging effects were more significant at higher temperature and increased relative humidity. The color difference (ΔE) between aged and unaged samples was higher for pure CNF or nanocellulose films when compared to films in which lignin was incorporated. Moreover, the variation of the colorimetric parameter Δb*, which is related to changes from blueish to yellowish, was three times lower for the film containing 10% of nanolignin, as compared to the other samples. Lignin antioxidant capacity could explain this behavior, as these films were ca. 40% more efficient to scavenge the DPPH radical than pure nanocellulose. Therefore, the colorimetric study of artificially aged nanocellulose-based films indicated that the addition of nanolignin can hinder unsuitable aging changes, ensuring longer lifespan to CNF products.

Effect of nanolignin on the stability of nanocomposite films based on tempo-oxidized cellulose nanofibrils against hydrothermal degradation

ACS Spring Meeting 2021

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