Preparation and tribological properties of zinc dialkyl(aryl)ethoxy dithiophosphates
Purpose ‐ Environmental regulations have called for reduction of phosphorus content in engine oils because this element strongly deactivates the catalyst in automotive catalytic converters. The primary source of phosphorus is zinc dialkyl(aryl)dithiophosphate (ZDDP). Therefore, to protect catalyst ZDDP is replaced with other additives or its amount in engine oil is reduced. Zinc dialkyl(alkylaryl)ethoxy dithiophosphates (ZEDPs), a novel derivative of dithiophosphate additive, soluble in lubricating base oils were synthesized and used to replace containing more phosphorus ZDDP. Design/methodology/approach ‐ Tribological properties of ZEDPs were evaluated using four-ball and the ball-disc tribometers. The ZEDPs thermal stability and worn surface were analysed using TGA, XPS techniques. Findings ‐ ZEDPs significantly reduce the friction and wear under boundary, mixed and hydrodynamic lubrication regimes. Friction and wear properties are dependent on chemical structure of additives, their surface active behavior in tribological system and frictional parameters. The effectiveness of friction and wear reduction increases with the concentration of lubricating oil additive solution and depend on the amount of the additive adsorbed on the surfaces. ZEDPs are much more thermally stable than ZDDPs. The XPS shows that the tribochemical films formed on rubbing surfaces contain zinc and iron phosphates and sulphates. Research limitations/implications ‐ The effect of the number of etoxy groups in ZEDPs molecules on their tribological properties was not investigated. Practical implications ‐ ZEDPs were synthesized and can be a potential replacement for ZDDP. Originality/value ‐ ZEDPs were first investigated in lubrication of steel-steel frictional contact and results indicate that they can be used as additives in combustion engine base oil.
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