Synthesizing lignin-based gelators to prepare oleogels used as green and fossil-free greases
Zhipeng Wu, Braham Prakash, Yijun Shi
Abstract
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 10 6 to 10 4 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. • Green and Sustainable Lubricants: Developed a lignin-based gelator using renewable and biodegradable materials like malic acid, lignin, and epoxidized soybean oil to replace petroleum-based lubricants. • Improved Lubrication Performance: Adding lignin-based gelators to castor oil significantly reduced friction by 20% and wear by 40%, highlighting superior lubrication and anti-wear properties compared to conventional lubricants. • Environmentally Friendly Synthesis: The synthesis of lignin-based gelators was achieved through a simplified process without toxic chemicals, emphasizing the eco-friendly nature of this innovation. • Tribological and Rheological Advantages: Rheology and tribology tests demonstrated that the gelator-enhanced lubricants exhibit strong shear-thinning behavior and better film-forming capacity, making them effective for industrial applications. • Protective Surface Layer: XPS and QCM analyses confirmed the formation of a stable protective layer on metal surfaces, reducing metal-to-metal contact and prolonging the life of machinery parts.