The use of kraft lignin to enhance nanocellulose film properties

The pressing need to replace petroleum-based plastics with renewable and biodegradable alternatives has sparked growing interest in biopolymers derived from lignocellulosic biomass as sustainable solutions. Among these, nanocellulose stands out as a versatile product capable of forming strong, transparent, and flexible films. However, these films lack active properties like antioxidant, antibacterial and UV-shielding capacity, essential for applications such as food packaging. To address this, incorporating lignin, a byproduct of lignocellulosic biorefineries, offers a promising route to enhance the functionality of nanocellulose films. In line with this idea, this work studies the incorporation of kraft lignin into nanocellulose films by two different protocols: the first protocol involves directly mixing a cellulose nanofiber (CNF) suspension with an aqueous lignin suspension; the second protocol uses lignin dissolved in acetone : water (9 : 1) which is transformed into lignin nanoparticles (LNPs) <i>via</i> solvent shifting when mixed with the aqueous CNF suspension. The resulting suspensions of CNFs and lignin were subsequently used to produce casting films. It was found that incorporating lignin into the CNF film not only conferred UV-shielding capacity, but also enhanced barrier properties without compromising the mechanical properties, particularly when lignin was introduced as LNPs (even at 10-20% LNP content). However, adding bulk lignin at a high concentration (20%) negatively affected water vapor permeability and mechanical properties. Antioxidant and antibacterial capacities correlated with lignin content, showing greater enhancement when lignin was present as nanoparticles compared to bulk lignin. These results indicate that forming LNPs <i>in situ</i> within the CNF suspension is a more effective approach to optimize the properties of nanocellulose films. Thus, the obtained films presented good active properties with mechanical properties comparable to those of traditional plastics, but significantly lower barrier properties.