Structure—activity correlations for sulphur EP additives

Abstract A series of new sulphur compounds was synthesised. Mercaptoheterocycles and mercaptoethanol were reacted with C12‐C22 chloroacetates. Mercaptoethanol derivatives were subsequently reacted with trialkyl borates. The solubility of S‐methylcarbalcoxy derivatives in both mineral oil and synthetic esters strongly depended on the alkyl group length, the most soluble being the C12. C14 derivatives. Three tests were used to estimate the antiwear (AW) and extreme pressure (EP) performance of the sulphur compounds: a four ball machine, an LFW‐I machine, and a Falex tester. Despite some differences resulting from the test methods, the best EP characteristics were obtained with the 2‐mercaptobenzothiazole derivatives. This showed that even a strongly bonded sulphur atom, such as that of the thiazolic ring, participates in surface layer formation. Mercaptoethanol derivatives were less soluble in either mineral or ester‐based synthetic oils. Esterification of the hydroxyl groups with trialkyl borate improved their solubility and load carrying performance.