The combined use of diphenylamine antioxidants and hindered phenol antioxidants often exhibits excellent synergistic effects. However, current studies on the relevant mechanisms are primarily focused on non-in situ experimental tests. Therefore, the detailed reaction network and reaction mechanism remain unclear. Chemical reaction molecular dynamics (Reactive Force Field, ReaxFF) has gained widespread application for its advantage of unbiasedly revealing complex reaction processes at the atomic scale without requiring any prior knowledge. However, due to the typical industrial application temperature range of diphenylamine antioxidants and hindered phenol antioxidants, which is approximately 233 K to 493 K, the ReaxFF method, limited to high-temperature chemical simulations (>2000 K), is not applicable. Collective variable-driven hyperdynamics (CVHD) is an accelerated reaction molecular dynamics method combining metadynamics and ReaxFF. It increases the probability of chemical reactions by introducing bias potentials in the non-transition state region. Thus, the simulation of chemical reactions at lower temperatures can be realized effectively.
Under industrial operating temperatures, a study was conducted using CVHD to explore the quantitative structure-property relationship between the molecular structure and oxidation stability of neopentyl polyol ester lubricants. Additionally, the synergistic antioxidative mechanisms of hindered phenolic antioxidants and diphenylamine antioxidants in pentaerythritol ester-based oils were analyzed. The results indicated that the oxidation stability of neopentyl polyol ester lubricants decreased with an increase in the number of ester groups and the length of the alkane chain. However, a higher degree of alkane branching contributes to the addition of more active secondary and tertiary hydrogens, thereby reducing the oxidative stability of synthetic ester lubricants. And under practical operating temperatures (350 and 500 K), the CVHD method reproduced the oxidation process of pentaerythritol ester lubricating oil and the antioxidative mechanisms of different antioxidants. The results indicated that the effective capture of alkyl peroxyl radicals and alkoxyl radicals was crucial for the antioxidative mechanism. Moreover, the key products and reaction pathways aligned well with experimental conclusions. From an atomic perspective, the synergistic mechanism between two types of antioxidants was revealed: benefiting from intermolecular hydrogen transfer, the active hydrogen atom released from the phenol group of 2,6-Di-tert-butyl-4-methylphenol(BHT) at low temperatures promoted the regeneration of diphenylamine (R15) and N-hydroxydiphenylamine (R16), thereby enhancing the antioxidative performance. Notably, the reaction pathway of R16 had not been reported previously. Compared to the easily decomposable BHT at 500 K, 4,4'-Di-iso-octyldiphenylamin(ODA) still exhibited good antioxidative performance. Moreover, the identification of a new potential reaction pathway (R13) suggested that ODA’s superior antioxidative capability over BHT was attributed, in part, to the presence of multiple active reaction sites for capturing peroxyl radicals.
The above study on the synergistic antioxidative mechanisms of the two main antioxidants effectively demonstrated the accuracy and universality of the CVHD method. This method was applicable to various areas such as thermal decomposition, thermal oxidation, and tribological chemical reactions, especially for chemical reaction simulations at lower temperatures. The research provided a clear chemical reaction network and mechanism at the atomic scale for the antioxidation of synthetic ester-based lubricants, offering crucial guidance for formulating synthetic ester lubricants with excellent oxidation stability.
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