| Summary: | The implementation of new legislation mandating the use of biodegradable and renewable lubricants increased the demand for vegetable-based lubricants worldwide. To avoid food crisis in the future, palm stearin (PS) oil, a byproduct of the palm oil refining process, was utilised as a potential vegetable-based lubricant. However, poor oxidation stability hinders the direct application of pure vegetable oil in real mechanical systems. This study aims to improve this drawback through chemical modification, namely transesterification and epoxidation reactions. Optimising the transesterification reaction using one-factorat- a-time (OFAT) methodology results in the optimal yield of palm stearin methyl ester (PSME) of 95.26% under the following conditions: 1:4 oil-to-alcohol molar ratio, reaction temperature of 60 °C, reaction time of 8 hours and 6.0wt.% of lipase concentration. While, response surface methodology (RSM) integrated with Box- Behnken model was employed to optimise the reaction parameters of epoxidation. The optimal yield of epoxidized palm stearin methyl ester (EPSME) was found to be 98.9% at a carbon double bond-hydrogen peroxide molar ratio of 1:1.5, reaction time of 4.34 hours, temperature of 52 °C, enzyme concentration of 6.416wt%, and carbon double bond-acetic acid molar ratio of 1:0.163. The evaluation of physicochemical properties revealed that the pour point of PSME and EPSME improved by 50.3% and 12.8%, while their viscosity index increased by 2.3% and 0.6%. Rheological property results demonstrated that all lubricants exhibit a similar trend of flow behaviour where a transition from non-Newtonian fluid with shear thinning to Newtonian behaviour and then to non-Newtonian shear thickening fluid is observed at 100 °C. A pin-on-disc tribometer was employed to assess the tribological performance of palm stearin-based lubricants as a function of sliding speed and applied load. The results have shown that, with an increase in sliding speed from 1.5 ms-1 to 7.5 ms-1, EPSME exhibited the lowest coefficient of friction (COF), wear scar diameter (WSD) and surface roughness (Ra) values compared to PS and PSME, with reductions of about 57.43%, 20.19%, 23.52%, respectively. As the applied load rose from 2 kg to 4 kg, the COF, WSD, and Ra values of EPSME were augmented by 113.93%, 44.50%, and 41.44%, respectively, but were still remained lower than those of PS and PSME. At 7.5 ms-1, EPSME forms the thickest carbonaceous lubrication film at the contact zone between aluminium and steel surfaces compared to the PS and PSME. However, as the applied load increases, the thickness of this protective film gradually decreases. Thus, this work successfully demonstrated that chemical modification reactions not only improve the physicochemical properties but also significantly enhance the tribological properties of the lubricant.
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