Impact of lignin as a bioactive additive on thermal stability performance and degradation behavior of biolubricant

Purpose This paper aims to investigate the thermal oxidative stability performance of a triester biolubricant under the addition of a lignin-based additive. Kinetic and thermodynamic analysis is a successful tool for revealing stability and also demonstrating thermal degradation behavior. Design/methodology/approach Biolubricant was synthesized through esterification, following which the purification yielded the trimethylolpropane-based triester lubricant product. Extracted lignin from rice straw biomass was added to enhance thermal stability of biolubricant. In increase in the solubility of lignin in biolubricant was carried out through the acetylation process. Different characteristic temperatures of onset, peak and burnout, kinetic analysis of the Coats–Redfern method and thermodynamic of enthalpy Gibbs free energy and entropy changes were determined using results of thermogravimetric analysis. Findings A concentration of extracted lignin at higher than 10 mg/ml was a limitation for dissolution in biolubricant. Lignin can improve a biolubricant’s stability by increasing all characteristic temperatures. The first reaction order was the best mechanism to describe the evaporation and oxidation, while the reaction shifted to second for identifying combustion. The activation energies enhanced in the evaporation, oxidation and combustion stages with lignin addition. Positive values of enthalpy and Gibbs free energy change were combined with negative entropy magnitudes, indicating that thermal oxidative degradation of biolubricant was associated with endothermic and non-spontaneous reactions. Originality/value This study showcases the possibility of waste utilization as a renewable green lubricant and improves its thermal stability performance using lignin, a natural antioxidant additive.