Silylation of lignin as an effective tool for the development of bio-based oil structuring materials

Abstract

In this work, silylation process was employed as straightforward approach to develop biolubricants based on kraft lignin. The silylation reaction was conducted using tert-butyldimethylsilyl chloride (TBDMSCl) in the presence of dimethylformamide and imidazole as solvent and catalyst, respectively. By adjusting reaction times (2-24 h), several levels of physicochemical modification were achieved. Structural analysis via FTIR and two-dimensional 1H–13C HSQC NMR spectroscopy confirmed the successful incorporation of silane groups into lignin chemical structure, improving wettability and dispersibility. Additionally, SEM revealed notable changes in lignin particle morphology, particularly in particle size and aggregation. These physicochemically modulable amphipathic particles, containing hydrophilic groups and hydrophobic silane segments, were incorporated into castor oil and significantly impacted the rheological and tribological properties of the oleo-dispersions. Two distinct rheological behaviours were observed: liquid-like (at low reaction times <5 h) and gel-like (at high reaction times, > 16 h), where the silane groups, which act as a “bridge” for interaction with castor oil, and the area/volume ratio of lignin particles played a key role. The anti-friction and anti-wear properties were excellent, similar to or better than those obtained with castor oil or traditional lithium lubricating greases, attributed to the mending effect of the silylated lignin particles. This work offers a fast and straightforward compatibilization strategy to produce sustainable oil structuring agents, particularly relevant in the lubricant industry.