Effect of Substituent Groups on the Antioxidant Performance of Diphenylamine Derivatives and Structural Optimization on the Basis of Theoretical Calculations

Lubricating oils that reduces friction and wear are widely used in mechanical engineering but generally suffer oxidation and degradation. In this study, a series of diphenylamine antioxidants with different substituted alkyl groups were synthesized via the Ullmann method. The antioxidative performance and lubricating properties in both nonpolar and polar-based oils were investigated, and their enhancement mechanism was discussed. The results showed that in polar base oils, the oxidation induction time (OIT) was prolonged from 40 min in PAO and 48 min in DIOS to 3235 and 3198 min, respectively, depending on the alkyl groups. As the alkyl chain increases, the polarity of diphenylamines gradually decreases, thus enhancing the compatibility and the ability to trap generated free radicals. Similar enhancement was also achieved in nonpolar oils, showing the maximum OIT of up to 3235 and 1524 min in PAO and liquid paraffin, respectively, but independent of the length of the substituted alkyl chains since the stability of the nitrogen-containing radicals was barely influenced. Besides improved antioxidative performance, a ∼50% reduction in wear loss was determined after the addition of as-prepared antioxidants, which prevented the degradation of lubricating oils and the formation of sludge and carbon deposition. This work provides a theoretical basis for the structure design of antioxidants with high performance and offers experimental guidance for their applications in lubricating oils with different polarities.