Selective surface functionalization of Kraft lignin nanoparticles and their stability

In recent years, industries are increasingly more concerned about sustainability and climate change. This and the depletion of natural resources turned waste biomasses into an attractive and sustainable alternative to fossil-based materials. Lignin is one of the three main components of lignocellulosic biomass and it is the most abundant aromatic biopolymer on the planet. However, its intrinsic recalcitrance and highly heterogeneous nature hinder its conversion into high value-added products. Lignin nanoparticles (LNPs) have attracted particular attention since their size and higher surface area could help on the promotion of lignin valorisation. Efforts are made to improve their stability at high pH values (>9) and in organic solvents, and to increase compatibility with polymer matrixes. In this study we present a novel method to obtain surface functionalized LNPs by taking advantage of their self-assembly mechanisms. Firstly, two fractions of Kraft lignin are obtained by organic solvent treatment. The low molecular weight phenolic rich soluble fraction is then reacted with epichlorohydrin to produce glycidyl derivatives. LNPs are then made via a two-step anti-solvent procedure, which allows obtaining particles constituted by a high molecular weight insoluble fraction core and an epoxidized soluble fraction shell. Epoxy groups present on the outer shell of the particles can be then exploited in two different manners. One is the thermal treatment of the suspension which allows tailored cross-linking reactions: intraparticle that helps increase stability in harsh conditions (pH and solvent) and/or interparticle to produce nanoclusters. The second one is to use epoxy groups as a platform for further functionalization through well-known amine-epoxy reactions.