Storage stability of sulphurized sperm oil replacements I: Origin and nature of the sludge

Abstract

Sulphurized sperm oil has been used in numerous lubricant formulations to improve their extreme pressure and antiwear properties. In the beginning of the 1970s, sperm oil became an almost unavailable and expensive raw material. Replacements have been developed, but they are also expensive. Many lubricant manufacturers seem to have turned to sulphurized glycerides, when possible since natural fats and oils are cheaper and more readily available than the synthetic replacements. The greatest limitation on the wider use of sulphurized glycerides in lubricating fluids is their low storage stability in compositions based on paraffinic mineral oils. This has not yet been an object of systematic investigation. The purpose of the experiments presented in the present paper was to examine and suggest the origin and nature of the sludge formed by sulphurized esters of fatty acids in paraffinic mineral oils. The storage stability of sulphurized soya bean oil, lard oil and a mixture of n-butyl and diethylene glycol esters of long-chain fatty acids in three paraffinic base oils was studied. Samples of sludge obtained from the additives after 12 months of storage were characterized. The experimental results confirm earlier observations with model additives that there are less than two sulphur atoms per double bond both in the bulk additive and in the sludge that it forms. The sludge samples were found to have substantially higher acidity than the bulk additives.