Junbing Yao, General Manager

Modern engine oils must meet more and more stringent challenges from oxidative degradation; the use of one or two types of antioxidant alone is becoming less and less effective. The synergistic combination of several types of antioxidants will become increasingly more important to meet the stringent challenges. Several commercial passenger car engine oils were analyzed, which determined that four types of antioxidants are generally employed in various combinations in modern engine oils. The four general antioxidant types are hindered phenol, aromatic amine, sulfur compounds, and organomolybdenum compounds. According to oxidation mechanisms, these antioxidants can be classified according to three major modes of action: (1) hindered phenol and aromatic amine antioxidants are recognized as radical scavengers, (2) sulfur compounds are classified as hydroperoxide decomposers, and (3) molybdenum compounds are known as antioxidation synergists.

A pressure differential scanning calorimetry test (PDSC), a rotary pressure vessel oxidation test (RPVOT) and a thermo-oxidation engine oil simulation test (TEOST) were employed to measure the synergistic effects of using these antioxidant types in combinations. The study indicated that antioxidants with the same action mechanism, e.g. radical scavengers, are synergistic (homosynergism), and that the synergism between two antioxidants with different action mechanisms, e.g. radical scavengers and hydroperoxide decomposers, is much stronger (heterosynergism). Especially strong is the heterosynergism between a dithiocarbamate sulfur-containing derivative and an alkylated diphenylamine, which can serve as a very effective high temperature, ashless antioxidant combination. Additionally, while molybdate esters do not exhibit strong antioxidation properties on their own, they can greatly improve the antioxidation properties of radical scavengers like alkylated diphenylamines.