Synthetic Functionalization of Colloidal Lignin Particles for Wood Adhesive Applications

Abstract: Functionalizable spherical colloidal lignin particles (CLPs) represent a valuable asset for the valorization of lignin side-streams from the pulp industry. The spherical structure allows for the circumvention of the heterogeneous and poorly dispersible structure of the biopolymer. However, organic solvents and alkaline media degrade the particle structure and dissolve the polymers due to their chemical nature and solubility. The solvents will alter the aggregated polymers into irregular shapes that would correspond to inconsistent physicochemical properties. Then, the material will become unusable for advanced material applications, namely wood adhesives. In this study, a replicable process to yield pH ca. 12 stable CLPs for wood adhesives or further functionalization for other advanced material applications was developed and optimized. Lignin was functionalized with cross-linkers, glyoxal or formaldehyde, and selfassembled into spherical structures in the micro emulsification of the organic solution. The formed colloids were partially rotary evaporated to retain organic solvents within the colloidal structures, and then be cured at 73-76 °C until pH stable and further functionalized for advanced material applications. The functionalization with glyoxal was pursued further for its possibly increased reactivity and the health concerns associated with formaldehyde. The process requires the addition of glyoxal to lignin in an acidic organi cmedia at ambient temperature, and the solution to react at 64 °C. Glyoxal is likely added to the polymer structure in its hydrated and dimerized form, and its attachment to lignin should be analyzed through the behavior of glyoxal in different media. The formed colloids were rotary evaporated to an organic solvent content of 60 wt. % of the spheres to allow the occurrence of the curing reaction. These materials were finally cured by thermosetting them at 73-76 °C until pH stable. The particles can be cured with base-catalysis through the controlled addition of the base NaOH(aq). However, the mode and rate of addition of the catalyst are critically important for a nondegradative infusion of a base into solvent present ot removed particles without morphological changes. Further procedural improvement and larger batches are necessary to conduct CLP adhesive experiments.