Development of eco-friendly RETO–PPO-based grease reinforced with hybrid fillers: Physicochemical, thermal, and tribological characterization

Growing interest in sustainable tribological solutions has encouraged the development of grease derived from recycled and biodegradable raw materials. This study aimed to develop and evaluate an eco-friendly grease formulated from recycled electrical transformer oil (RETO), polypropylene oxide (PPO), organic acids, and hybrid fillers, and to assess its tribological, thermal, and environmental performance.FTIR, SEM/EDX, and TGA tests showed extensive casting, good uniform nanoparticle distribution, and thermal safety of formulations under inert conditions of about 300°C. Experiment No. 2 containing 20 g of malic acid showed the best balanced performance (load 255 kg = EP, drop point = 190, stress stability <6 100/psi h), and low friction tolerance (μ = 0.089). The study showed that the lubricants composed of RETO, PPO, organic acids and hybrid fillers have a good opportunity to develop environmentally effective greases that are in line with the science of green tribology and the circular economy. The reduction in carbon dioxide emissions and energy consumption, compared to oil-based greases based on mineral oil were estimated through Life Cycle Assessment (LCA), at around 58% and 42%, respectively. The interaction of RETO, PPO, organic acids, and hybrid fillers represents a good prospect towards up-scaling eco-efficient greases in line with green tribology and circular economy.