Some synergistic antioxidants for synthetic lubricants

Abstract

Oxidation stability is a major requirement for all lubricants. It is even more important for synthetic lubricants used in aircraft gas turbine engines, owing to the high oil temperature and air exposure. Two basic means are available to improve oxidation stability of synthetic lubricants: choice of base fluid and use of antioxidants. It is well known that fluids such as polyphenyl ethers are much more oxidatively stable than fluids such as dibasic acid esters. Discussions of the effect of the nature of the C-H bond on oxidation stability have been made in our previous paper and by other authors such as Ingold and Sniegoski. However, the choice of base stock is severely limited by cost and other considerations such as volatility, flow characteristics, etc. When the choice of base fluid has been made, use of a better antioxidant is the only avenue available to improve the oxidation stability of the lubricant. This paper describes some of the antioxidant systems which were found effective for ester type synthetic lubricants. The esters used included those based on dibasic acids, neopentylpolyols and complex esters. The systems discussed include one or two primary antioxidants and a synergistic antioxidant. The primary antioxidant used included N-phenyl-..capmore » alpha..-naphthylamine (PANA) and p,p'-dioctyldiphenyl amine (DODPA). Synergistic antioxidants which were found to improve the effectiveness of the primary antioxidants include alkali metal compounds, aromatic diamines, heterocyclic amines, hydroxybenzophenones, and certain S and P compounds.« less