Optimization and stepwise chemical modifications of sunflower oil unsaturated fatty acids for the production of biolubricant base stock

This study focuses on the production of renewable biolubricant base stocks derived from sunflower oil (SFO) as sustainable alternatives to conventional mineral oil–based lubricants. Plant oils offer significant advantages, including renewability, high biodegradability and reduced environmental impact. However, their direct application as lubricant base oils is limited by the low oxidative stability caused by the presence of β-hydrogen on the glycerol backbone and carbon–carbon double bonds (C=C) in the fatty acid chains. To overcome these limitations, stepwise chemical modifications involving esterification, epoxidation and ring-opening reactions were employed to enhance oxidative stability and low-temperature properties. SFO was first hydrolyzed using 2.4 M ethanolic KOH at 51 °C for 1.4 hours to produce sunflower oil fatty acids (SFOFA). The fatty acids were then separated into unsaturated (SFOUFA) and saturated (SFOSFA) fractions using methanol crystallization at a 1:15 oil-to-methanol ratio, −15 °C and 24 hours. The SFOUFA fraction was then esterified with different polyhydric alcohol (neopentyl glycol (NPG), trimethylolpropane (TMP) and pentaerythritol (PE)) under optimized conditions to produce several polyol esters: SFOUFANPGE (molar ratio 3.2:1, 1.37% H2SO4, 149 °C, 4 hours), SFOUFATMPE (molar ratio 4:1, 1.13% H2SO4, 154 °C, 6 hours) and SFOUFAPEE (molar ratio 5:1, 1.08% H2SO4, 177 °C, 6.9 hours). Among these polyol esters, SFOUFATMPE exhibited the highest oxidative stability and was selected for further modification. Epoxidation of SFOUFATMPE was carried out using formic acid and hydrogen peroxide (7:2 molar ratio), with 1.2% H2SO4 at 41 °C for 3.5 hours to reduce the C=C and further improve oxidative stability, producing ESFOUFATMPE. Subsequently, ring-opening of the epoxidized product with oleic acid (4.6 mol, 1.09% H2SO4, 130 °C, 4.9 hours) was performed to introduce molecular branching and enhance low-temperature performance, yielding DOSFOUFATMPE. All reactions were optimized using response surface methodology (RSM) based on a D-optimal experimental design. The final ring-opened biolubricant (DOSFOUFATMPE) exhibited a pour point of −24 °C, flash point of 310 °C, kinematic viscosities of 60.02 cSt at 40 °C and 12.03 cSt at 100 °C, a viscosity index of 150, oxidative stability of 33 minutes and Newtonian rheological behaviour. The stepwise modification successfully overcame the main limitations of plant oils by removing the β-hydrogen through polyol ester formation and reducing the reactivity of C=C through the epoxidation resulting in the improvement of oxidative stability. Overall, this study demonstrates that stepwise chemical modification combined with RSM optimization is an effective strategy for producing a sunflower oil–based biolubricant base stocks with strong potential to replace mineral oil lubricants. The findings also support the development of sustainable and renewable materials in line with the Sustainable Development Goals (SDG), particularly responsible consumption and production (SDG 12) and climate action (SDG 13).