Brown grease-derived ester base oils: tunable viscosity and cold-flow behaviour

Brown grease (BG) was converted into fatty acid methyl (FAMEc), ethyl (FAEEc), and isopentyl (FAPEc) esters, then epoxidized or hydroxylated to generate seven bio-based ester base oils: FAPEc, h-FAPEc, h-FAEEc, h-FAMEc, e-FAPEc, e-FAEEc, and e-FAMEc. Thermal stability (T _d,5%), kinematic viscosity (ν ₄₀, ν ₁₀₀), viscosity index (VI), pour point, and crystallization behavior (DSC) were evaluated. Transesterification reduced viscosity (ν ₄₀ of 7.51-11.7 mm² s^-1) and improved cold-flow performance relative to crude BG, with FAPEc pour point of -6 ± 3 °C, consistent with suppressed crystallization observed by DSC. Subsequent hydroxylation and epoxidation enabled viscosity tuning across ISO VG 15-460 grades. Hydroxylated derivatives increased viscosity (ν ₄₀ of 12.6-30.7 mm² s^-1) with moderate pour point penalties (-3 to 8 ± 3 °C), whereas epoxidized derivatives produced substantial viscosity increases with ester cap-dependent cold flow performance. Notably, e-FAPEc achieved ISO VG 68 while maintaining a pour point of 3 ± 3 °C, demonstrating improved cold-flow performance compared with previously reported epoxy-BG systems. Tribology (ASTM D4172) testing showed ester-platform-dependent behavior, with h-FAEEc giving the lowest average friction, and FAPEc the poorest wear resistance. These results demonstrate that ester platform selection provides a practical design handle for tuning viscosity grade and low-temperature performance in brown grease-derived base oils.