Green greases for the future: Biobased materials and emerging gel technologies

Lubricating greases are widely used in bearings, gears, and other heavily loaded machine elements, but conventional lithium- and calcium-soap greases based on mineral oils and metal-containing additives raise increasing concerns regarding toxicity, persistence, and regulatory compliance. This review summarises recent progress in green greases, defined as greases that employ renewable or environmentally acceptable base oils, thickeners, and additives to reduce environmental impact while maintaining functional performance. After briefly outlining the composition and operating principles of traditional greases, we discuss the physicochemical properties, advantages, and limitations of bio-based base oils, with emphasis on vegetable and synthetic esters and on ionic liquids used either as base fluids or co-base oils. We review renewable and biodegradable thickener systems, including modified soaps, polysaccharides, lignin-related structures and mineral or hybrid networks, and relate their molecular structure and gel microstructure to rheology, oil retention and mechanical stability. Particular attention is paid to oleogels, hydrogels and ionogels as emerging gel technologies that enable semi-solid green lubricants. Natural antioxidants, bio-based friction modifiers and environmentally acceptable anti-wear additives are surveyed, and representative formulations are compared with commercial reference greases in terms of friction, wear and other key performance indicators. In addition, green synthesis routes for bio-based gelators and mild, energy-saving processing conditions for grease manufacture are discussed in the context of green-chemistry principles and industrial scalability. On this basis, the environmental performance, application domains and market prospects of green greases are analysed, and the main trade-offs between performance, cost and sustainability are identified. Finally, the remaining challenges, especially high-temperature reliability, long-term stability, standardisation and large-scale implementation, are discussed, and directions are outlined for the rational design and broader adoption of next-generation green greases.