The growing interest shown b y 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 42l has given the characteristics of many light hydrocarbons from Clo 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 cornpromise 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 b y a very branched chain, or b y 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 ve y favourable, but the best compromise is achieved b y polyalkoxy-ether-esters and polyalkoxy diesters. Unhappily these are miscible to hydrocarbons only if the hydrocarbon chains are sufficiently long. Of course for fornulation 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 JSL (1) 201Using the data of Project API42, comparisons were made, for equivalent chain lengths, between the following hydrocarbon structures, which will be examined here in the order given below: 0 normal alkanes 0 alkanes branched by one or more alkyl chains or by a ring 0 cycloalkanes 0 aromatics
Normal alkanesNormal alkanes or normal paraffins cannot be used in oil compositions due to their high pour point (PP) even with their high viscosity indexes (VI). Table 1 gives the characteristics of a number of normal alkanes between C20 and C M . The viscosity indexes vary according t o the number of carbon atoms, and reach a peak value of 226 for C35. Their pour points rise steadily between + 36 and + 86°C.
Alkanes branched by alkyl chainsSide chains lower the pour point, viscosity and viscosity index, as shown by the examples in Table 2. When the side chains are linear, a long branch in mid-chain is preferable to several short branches [ l o , 111; if the branch is close to the end of the chain its influence is weaker [12, 131; if the number of branches of equal length is doubled, the VI falls considerably, but the PP varies slightly [ 141. If the side chains are themselves branched, it appears that a certa...
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