A potential bio-antioxidant for mineral oil from cashew nutshell liquid: an experimental and theoretical approach

The objective of the present work was to evaluate the antioxidant potential of a mixture of saturated, monoene, diene, and triene cardanols derived from the cashew nutshell liquid in naphthenic mineral oil. A mineral naphthenic oil sample was doped with the cardanols mixture at concentrations of 500, 2000, and 5000 mg/kg and evaluated using differential scanning calorimetry (DSC) and the accelerated oxidative method (PetroOXY), following ASTM standards (E2041-18, E1970-16, E537-12, and E698-18). The addition of cardanols increased the oxidative stability of the mineral oil by a factor of 4 to 5. To evaluate the antioxidant potential of each particular cardanol present in the mixture, structural analysis and specific antioxidant mechanisms were investigated by density functional theory (DFT). Each molecular structure was optimized with the hybrid functional B3LYP with a basis set 6-31G (d, p), and the antiradical mechanisms (HAT, SPLET, and SET-PT) were evaluated. The HAT was the best observed mechanism, standing out for the cardanol monoene that showed presented better antiradical activity. Concerning the global reactivity study, it was concluded that the increase of the unsaturations in the side chain of the molecules contributes significantly to their increased general reactivity. When evaluating the Fukui index, it was confirmed that, for the cardanol monoene, the reactivity prevails in the aromatic ring with an emphasis on oxygen and carbon 4, without the interference of the carbon chain. The results showed that the cardanol mixture may be applied as bio-antioxidant for naphthenic mineral oils.