Evaluation on synergistic antioxidation of molybdenum dialkydithiocarbamate with arylamine antioxidant

Abstract

PurposeMolybdenum dithiocarbamate (MoDDC) has good antiwear and friction‐reducing properties in lubricants, and can be protected against thermo‐oxidative degradation. However, MoDDC‐containing lubricants may lose their ability to reduce friction over time, which may result from its concentration in oils below a critical concentration owing to oxidative degradation of MoDDC. Thus, in order to enhance the durability of the low friction performance of MoDDC, the paper aims to investigate good synergistic antioxidants with MoDDC.Design/methodology/approachThe antioxidation properties of molybdenum dialkyldithiocarbamate (M 807) – and p,p‐dioctyldiphenylalmine (V 81) – or mixed octylated and butylated diphenylalmines (V 961)‐containing poly‐α‐olefin (PAO)‐derived lubricants were evaluated by differential scanning calorimetry (DSC) and modified penn state micro‐oxidation test (PMOT). DSC test measures incipient oxidation temperature (OT) and oxidation induction time (IT) of the lubricant at high temperatures and the oxidation stability of oil weight loss is measured by PMOT. Moreover, the oxidized samples were analyzed by Fourier transform infrared (FTIR) and electron spin resonance (ESR).FindingsA DSC test shows that OT and IT of V 81‐ or V 961‐containing PAO were improved significantly by M 807 addition. A PMOT indicates that when combining with V 81 antioxidants, M 807 can also effectively reduce the increase in weight loss of PAO and deposits formed in oils. These results suggest that the M 807 shows a good antioxidative synergism with alkylated diphenylamine antioxidants. In addition, FTIR results from a PMOT confirm that the addition of M 807 can significantly enhance the oxidation IT of oils containing V 81 and inhibit formation of oxidation products including carbonyl bonds or hydroxyl group. ESR analysis from a PMOT indicates that M 807 may form stable radicals with arylamine by the coordination effect.Originality/valueThis paper provides simple and quick methods to evaluate synergistic antioxidation properties between different types of additives, and a mechanism of the inhibition involving a synergy was proposed. They can offer practical help in industrial applications and to an individual starting out on an academic career.