Improvement of the properties of nanocellulose suspensions and films by the presence of residual lignin

The effect of lignin on several properties of nanocellulose suspensions and films, such as degree of mechanical fibrillation, optical transparency, and gas barrier properties is still a matter of study. In the present work, it was investigated the influence of residual lignin on the efficiency of cationization and enzymatic pretreatments to produce lignin-containing nanocelluloses (LCNFs) from unbleached kraft pulps, and, on the properties (mechanical, gas barrier, transparency, antioxidant activity and thermal stability) of the corresponding films. The overall efficiency of the pretreatments was not negatively affected by the presence of lignin (3–4 wt%) in the starting pulps, as measured by the degree of fibrillation, degree of polymerization of cellulose, optical transmittance, and cationic group content (cationization). On the contrary, lignin could even enhance the mechanical fibrillation and the optical transmittance (transparency) of the cationic and enzymatic LCNF suspensions compared to the reference lignin-free nanocelluloses (CNFs) prepared from bleached pulp. Lignin could also improve the optical transparency of the films, which is an important finding of the present work: 64.8% for LCNF-Cationic (-Cat) vs. 56.9% for CNF-Cat, and 74.5% for LCNF-Enzymatic (-Enz) vs. 64.5% for CNF-Enz. Moreover, films with lignin demonstrated higher antioxidant activity, UV-light absorption capacity, larger char residue, and even improved oxygen barrier compared to the analogous CNF films. A remarkable oxygen barrier performance was exhibited by the LCNF-Enz film (oxygen transmission rate below 2 cm³/m².day). Overall, the presence of residual lignin in the cellulose micro/nanofibril production can improve some of the suspension and film properties, particularly the optical transparency.