The relationships between structure and rheological properties of hydrocarbons and oxygenated compounds used as base stocks

Abstract

AbstractThe growing interest shown by industry in synthetic lubricants for severely stressed engines and machines has led the author's organisation to investigate the best hydrocarbon and oxygenated structures which serve to achieve a good compromise between viscosity index and pour point. Several analyses of structure and property relationships have been published. The American Petroleum Institute's project 421 has given the characteristics of many light hydrocarbons from C10 to C35 belonging to different chemical families. During the Second World War, American and German research centres investigated a large number of esters and selected the best diacid and polyol ester structures.The compromise between the viscosity index and pour point has been studied for hydrocarbons and oxygenated products as to their prospects for use as base stocks. With hydrocarbons having a long main chain which is substituted either by a very branched chain, or by a chain ending with a saturated ring, it is possible to reach a viscosity index of around 160 for a pour point of −20°C. The introduction of ester functions in a hydrocarbon chain improves the compromise between viscosity index and pour point. Diacid esters constituted of linear diacids and branched monoalcohols are very favourable, but the best compromise is achieved by polyalkoxy ‐ether‐esters and polyalkoxy diesters. Unhappily these are miscible to hydrocarbons only if the hydrocarbon chains are sufficiently long. Of course for formulation of lubricants, other properties also linked to chemical structure should be taken into account, such as thermal and oxidative stability, antiwear behaviour, hydrolytic stability, and compatibility with additives.The data presented here and other data derived from research projects conducted in the author's laboratory, some jointly with Produits Chimiques Ugine Kuhtmann, have been analysed to determine the possibilities for main synthetic lubricant base stocks.