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

Abstract

The 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 175 °C - 425 °C. 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) -36 °C 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 lubricants
• citrate esters
• rheology
• viscosity index
• pour point

Disclosure Statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Conclusion

We might conclude that the key outputs in this study as a fellow:

Different esters were prepared by reacting citric acids (CA) with mono-alcohols (hexanol, octanol, decanol, and 2-ethylhexanol), novel esters were created. The products were then individually reacted with propanoic acid, valeric acid, and octanoic acid in a 1:1 molar ratio. Fourier-transform infrared spectroscopy and nuclear magnetic resonance spectroscopy (NMR) were used to describe the resultant esters. The produced esters had good thermal stabilities since degradation occurs between 175 °C and 425 °C. Because the viscosities of the produced esters do not change as the shear rate changes, they exhibit Newtonian characteristics. Esters' physicochemical characteristics were also established, showing increased low temperature properties (PP) at -36 °C and higher viscosity index (VI) at 195. TANs were ranged from 0.04 to 0.15. The higher flash points indicates the suitability of the prepared compounds to be lubricants. The stabilities of the prepared esters were studied computationally and it was found the stability of the prepared esters were ranked as CDEP > COEP > COEO > COEV > CEP > CHEP, according to their HOMO-LUMO energy gap.