Miscibility and tribological investigations of ionic liquids in biodegradable esters

The structural impact of ionic liquids was systematically investigated by assessing their behaviour as lubricant additives in two different biodegradable esters (refined rapeseed oil and synthetic ester). 

Cations were mainly based on derivatives of the tetraalkyl ammonium structure; the anions were alkyl sulfate, methyl sulfonate or bis(trifluoromethylsulfonyl)imide. Miscibility experiments showed that ultrasound treatment yielded the most stable emulsions. X-ray fluorescence was employed to determine maximum concentrations, at which ionic liquids formed stable mixtures in the base oil, either dissolved or as self-stabilized emulsions. Steel/steel contacts lubricated with such saturated rapeseed oil blends were investigated in a reciprocating sliding tester at room temperature and at 100°C. For most anions, a significant reduction of friction and wear was found at ionic liquid concentrations of less than 0.1 weight%. At substantially higher concentrations, a destabilisation of the ionic liquid–ester mixtures was observed, which was accompanied by a loss of wear-preventive properties.