Is Replacement of Aromatic Compounds with Alkyl Ones Really the Effective Way toward Developing Greener Lubricants?

Abstract

Abstract The present work designs a series of cycloalkyl ester oils and reports a comparative study on their physicochemical and tribological properties, biological toxicity and degradability. From the perspective of molecular structure, the factors affecting their physicochemical, tribological properties, biological toxicity and degradability were discussed, including steric hindrance, group polarity and so on. The results demonstrated that cycloalkyl ester oils had better thermal stability and viscidity-temperature capability than the reference oils. Additionally, their friction reducing and anti-wear performance was superior to the reference samples. Unexpectedly, the cycloalkyl ester oils were found to have higher toxicity to the experimental plants (wheat and pak choi) due to the introducing of the cycloalkyl group, which enhanced the lipophilicity of the corresponding oils and then strengthen the damaging of the oils to plant cell. Some of the cycloalkyl ester oils, on the other hand, were proved to be more biodegradable than the reference samples. On the whole, developing green lubricants by replacing aromatic rings is not entirely successful. Lubricants' biodegradability can be improved with this replacement, but their ecotoxicity cannot be reduced. Anyway, the present work provides a specific benchmark for the subsequent development of environmentally friendly, high-performance synthetic ester lubricants.