Hydrolytic stability of synthetic ester lubricants

Abstract This paper looks in detail at hydrolysis of esters. While this is not normally a problem, hydrolytic stability can be an important consideration, e.g., in air compressors. The paper looks first at the synthesis of esters, which is a reversible reaction of acid + alcohol ↔ ester + water, usually involving a catalyst. Two mechanisms of ester hydrolysis, namely base‐catalysed and acid‐catalysed, are described as being most relevant to lubricant esters, the former being less likely than the latter. Two extended tests are described which have been used in the author's laboratory, a beverage‐bottle test (modified ASTM D‐2619), and a sealed‐tube test, for hydrolytic measurement. The paper goes on to look in detail at the effect of ester structure on rate of hydrolysis, in terms of resonance effects and steric effects, on the effect of impurities, and of acidic and acid catcher additives.