Lignin-Based Versatile Gel as Green Lubricating Grease

Lubricating grease plays a crucial role in machinery and industrial lubrication, but conventional greases are typically composed of petroleum-derived base oils thickened with metallic soaps, such as lithium stearate, which often suffer from some limitations, e.g., poor biodegradability, oil separation under high shear or temperature, limited structural tunability, and low environmental compatibility. On the other hand, gels provide a new platform for preparing green lubricating greases by forming stable and designable three-dimensional networks with superior oil retention, shear stability, and thermal resistance compared with traditional metallic soap-based systems. Their structural tunability allows precise control of rheological and tribological properties. Lignin is particularly attractive due to its renewability, rich functional groups, and intrinsic antioxidant activity. Previous studies from our group have also demonstrated lignin’s excellent lubricating and anti-wear performance, making it an ideal candidate for constructing sustainable, high-performance gel-based greases. Lignin’s phenolic and aromatic structures endow it with intrinsic antioxidant activity, enabling free-radical scavenging and oxidation inhibition. Its incorporation into lubricants will enhance thermal-oxidative stability and reduce dependence on synthetic antioxidants. This thesis aims to address the limitations of traditional lubricating greases by developing lignin-based gels through a systematic material design, preparation, and characterization approach. Specifically, lignin is utilized and modified to construct gel networks in different base fluids—i.e., vegetable oil, water, and deep eutectic solvents—representing hydrophobic, hydrophilic, and ionic environments, respectively. The prepared gels are tested on their physicochemical, rheological, and tribological behaviors, providing insight into how molecular structure and gel network design influence lubrication performance. In this thesis work, to explore how different gelation mechanisms influence the structure and lubrication performance of lignin-based gels, four types of lignin-based gels were developed using distinct gelation strategies: physical self-assembly, chemical crosslinking, redox polymerisation, and radical inhibition. These lignin-based gels show good thermal stability and excellent tribological performance. The tribological properties were evaluated under controlled conditions (80 °C, 2.72 GPa, according to Standard ASTM D5707–11) to compare with commercial lubricating greases. Under the test condition, the lignin-castor oil gel achieved the lowest coefficient of friction (COF) at ~0.078, lower than that of LGWM 2/0.4 ((lithium/calcium thickener, COF ~0.13) and Uniway grease (calcium-sulfonate thickener, COF ~0.15). And the wear volume of lignin-castor oil gel was 0.78 × 10-4 mm³, which was similar to those two commercial lubricating greases despite the absence of the typical tribological enhancing additives, which are well known to be highly important for reducing wear. The comparable wear volume demonstrates that the continuous lignin-derived network effectively retains the base oil and facilitates stable boundary film formation during sliding. The different gelation strategies were designed not only to examine the influence of synthesis routes but, more importantly, to address the key limitations of traditional lubricating greases. The physically self-assembled oleogels improved the structural reversibility and oil retention under shear; the chemically crosslinked networks enhanced thermal stability and boundary film formation; the redox-polymerized lignosulfonate hydrogels significantly reduced wear under aqueous conditions; and the radical inhibition-controlled gels ensured stable lubricity across a wide temperature range. Collectively, these results demonstrate that lignin-based gels provide a versatile and sustainable platform for overcoming the challenges of oil separation, additive dependence, and temperature sensitivity that restrict conventional greases. In addition to the superior tribological behavior, the lignin-based gels also exhibited remarkable antioxidation performance. The oxidation induction time (OIT) of the lignin-castor oil gel was 651 s, and the chemically crosslinked gel reached 1959 s, which was much better than that of Teboil (520 s) and LGWM 2/0.4 (222 s), indicating lignin can effectively improve the oxidative stability of lubricants. The findings in this thesis highlight lignin-based gels as sustainable, high-performance lubricants that combine strong tribological stability, antioxidation, and adaptability across diverse environments.