Rheological, thermal and tribological performance of trimethylolpropane esters derived from high oleic non-edible oils: a sustainable biolubricant for industrial applications

The growing demand for eco-friendly and sustainable lubricants has spurred interest in biolubricants derived from renewable sources. This study explores the synthesis and performance of trimethylolpropane esters derived from non-edible karanja and jatropha oils. The karanja trimethylolpropane esters (KTMPE) and jatropha trimethylolpropane esters (JTMPE) were synthesized using a three-step process: acid pretreatment to reduce free fatty acids, base-catalyst transesterification to produce fatty acid methyl ester, and vacuum-assisted transesterification with trimethylolpropane to produce biolubricants. The successful synthesis of biolubricants was confirmed through FTIR, ¹H-NMR, ¹³C-NMR, and GC analyses. Rheological analysis conducted using a modular compact rheometer revealed Newtonian fluid behavior for both biolubricants across a temperature range of 25°C to 100°C and a shear rate of 0 to 100 s⁻¹. The viscosity indices improved from 145 to 163 for KTMPE and from 152 to 250 for JTMPE, while pour points decreased from 3°C to − 4°C for KTMPE and from 4°C to − 8°C for JTMPE. The thermogravimetric analysis confirmed thermal stability up to 210°C for both the biolubricants, supporting their use in high-temperature applications. Tribological evaluation using a four-ball tribometer highlighted the superior performance of JTMPE, characterized by a lower coefficient of friction, smaller wear scar diameter, and greater load-bearing capacity than KTMPE. Surface analysis using FE-SEM and optical microscopy confirmed the formation of a stable lubricating film by JTMPE, with minimum wear debris. These findings underscore the role of trimethylolpropane in enhancing biolubricant properties, positioning KTMPE and JTMPE as sustainable and effective alternatives for diverse industrial applications.