Current and candidate additives for environmentally acceptable lubricants—A review

The world consumes roughly 40 million tons of lubricants annually and nearly half of them end up in the environment because of leaks, discharges, and evaporation, causing significant environmental and economic impacts. The need for moving toward environmentally acceptable lubricants (EALs) is increasingly being recognized. Synthetic ester, polyalkylene glycol, vegetable oil, and water are among the approved base fluids by U.S. Environmental Protection Agency for EALs. However, neat base fluids cannot fully meet the lubrication requirements without incorporating functional additives like anti-wear, antioxidant, corrosion inhibitor, viscosity modifiers, etc. Both the performance and environmental compatibility of the lubricant additives are critical. In this article, the current and candidate EAL additives, both in liquid and solid forms, are reviewed, with a focus on their functionalities in friction and wear reduction, toxicity, and biodegradability. A perspective for future research on EAL additives is discussed at the end.