The paper was concerned with the effect of base oil chemical composition on oxidation stability related to both industrial and engine lubricant applications and the questions and discussion covered this ground.

F. T. STRIBLEY (UK), noting that the authors had identified four factors, saturates, alkyl benzenes, sulphur compounds and a pro-oxidant factor (POF), asked if the POF made a small or large contribution to the observed performance in the D2272 and D943 oxidation tests. In reply it was stated that the contribution from the POF was of the same order of magnitude as the contribution from the saturates plus alkyl benzenes or from the sulphur compounds. The numerical values of POF for typical base oils range from 20 to 100 units. A change of this magnitude would result in a reduction of 160 minutes in the D2272 life and 1500 hours in the D943 life.

Replying to questions from S. BULL (France), the author agreed that solvent extraction removes alkyl benzenes and sulphur compounds as well as compounds which contribute to the POF. Those contributing to the POF are the most readily removed and the beneficial sulphur compounds are extracted more readily than the alkyl benzenes. There is an optimum level of extraction but this will differ for each feedstock and the chemical composition of the optimum will not necessarily be the same in each case. There is an added flexibility in refining, through finishing processes, to adjust the POF and the concentration of sulphur compounds.

R. H. PERRY (USA) was interested in the method of measuring POF and the difference between POF and the ‘polar’ content as measured by clay adsorption which has also been reported as important in base stock quality. The author outlined the method which monitors the change in phenolic inhibitor concentration by infra-red spectroscopy over the initial stages of a copper catalysed bench oxidation test conducted at 120°C. The greater the reduction in the concentration of inhibitor in the initial part of the test, the greater is the POF. POF measures the effects of pro-oxidants rather than their concentrations. Because not all ‘polar’ compounds are prooxidants (for example the sulphur compounds can be beneficial as anti-oxidants) one cannot use ‘polar’ content as such as a measure of pro-oxidant species.

G. BRUNTON (UK) questioned the use of total sulphur content and asked for comments on the types of sulphur-containing structures which are the most active and the mechanisms by which they work.

Replying, the author indicated that they had observed differences in the effects on oxidation of different sulphur species but this work was still at too early a stage to make definitive comments. However, although they are aware of differences, they find that the concentration of total sulp

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