Synthesizing lignin-based gelators to prepare oleogels used as green and fossil-free greases

Traditional lubricating greases are mainly derived from petroleum, which poses major environmental challenges due to their non-biodegradability and pollution issues. This study attempts to synthesize lignin-based green thickeners and explore their potential for developing green and fossil-free greases. A lignin-based gelator was successfully synthesized by reacting malic acid with lignin and epoxidized soybean oil, in which malic acid participated in both the esterification reaction and the ring-opening of the epoxy group. This synthesized gelator was used as the thickener to prepare greases with several different oils, e.g., castor oil, epoxidized soybean oil, rapeseed oil, PAO 15, and paraffin oil. It was found that combining this lignin-based gelator with castor oil and epoxidized soybean oil can be successfully used for preparing greases, while it does not work with other oils. Rheological studied showed that the 35 % gelator-castor oil grease exhibited strong gel-like behaviour, with storage modulus (G', 400 Pa) exceeding loss modulus (G", 40 Pa) and shear-thinning viscosity reducing from 10⁶ to 10⁴ mPa·s under stress. Comprehensive tribological studies on the developed greases show that lignin-based gelators significantly improve lubricant performance (about 20 % lower friction and around 40 % lower wear under a contact pressure of 2.72 GPa, reciprocating speed 0.1 m/s, and 80 °C). XPS analysis further revealed the ability of the developed grease to form a protective film on metal surfaces. This study demonstrates the great potential of lignin as a green thickener and provides new ideas for developing high-performance, green, and fossil-free greases.