Hydroxylated derivatives of methyl oleate as biobased tribological additives for lubricants

Two hydroxy derivatives of methyl oleate (MeO), viz. methyl 9(10)-hydroxy stearate (MeHS) and methyl trans-9,10-dihydroxystearate (MeDHS), were prepared and characterised. The tribological properties of MeO, MeHS and MeDHS in lubricants were evaluated, and their triological mechanisms were analysed. Results showed that MeO, MeHS and MeDHS were efficient in improving tribological properties of paraffin oil and rapeseed oil, of which MeDHS was the best candidate. Furthermore, MeO, MeHS MeDHS provided more powerful lubricating abilities in paraffin oil than in rapeseed oil. The enhanced tribological properties of lubricants by MeO, MeHS or MeDHS were evidenced by increased maximum non-seizure loads, decreased wear scar diameters and friction coefficients. Finally, excellent lubricating abilities of MeDHS and MeHS were attributed to formation of a strengthened tribofilm due to adsorption of their molecules through strong bonding of the extra polar hydroxyl group(s), and the ester group of the parent methyl oleate, onto the metal surfaces.