Citric Acid-Based Esters as Potential Synthetic Lubricants: A Study of Their Synthesis, Rheological Properties and Thermal Stability

AbstractThe failure of mineral oils to work in machinery and engines under intense pressure, heat, and corrosion has prompted researchers to consider the development of synthetic lubricating oils that are appropriate for these demanding conditions. This research was conducted to synthesize new esters by reaction of citric acids (CA) with mono-alcohols (hexanol, octanol, decanol and 2-ethylhexanol), then the products were reacted with propanoic acid, valeric acid, and octanoic acid separately using 1:1 molar ratio. The resulting esters were characterized using nuclear magnetic resonance spectroscopy (NMR), Fourier-transform infrared spectroscopy (FTIR), and thermal gravimetric analysis (TGA). The rheological properties were studied, and it was found that the prepared esters have high thermal stabilities as the degradation takes place in the range of 175ºC - 425ºC. The prepared esters show Newtonian behaviors as their viscosities do not change by changing shear rate. The physicochemical characteristic of prepared esters were also determined which showed improved low temperature properties (PP) -36ºC for ester (2R)-1,2-bis(2-ethylhexyl) 3-nonyl 2-(pentanoyloxy)propane-1,2,3-tricarboxylate (COEV) and viscosity index (VI) at 195 for ester (2R)-1-decyl 3-(2-ethylhexyl) 2-heptan-3-yl 2-(propionyloxy)propane-1,2,3-tricarboxylate (CDEP). The stabilities of the prepared esters were confirmed via the computational analysis and their stabilities were ranked as CDEP > COEP > COEO > COEV.Keywords: synthetic lubricantscitrate estersrheologyviscosity indexpour pointDisclaimerAs a service to authors and researchers we are providing this version of an accepted manuscript (AM). Copyediting, typesetting, and review of the resulting proofs will be undertaken on this manuscript before final publication of the Version of Record (VoR). During production and pre-press, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal relate to these versions also.