Synthesis and Tribological Investigation of Lipoyl Glycerides

Abstract

Lipoyl glycerides were synthesized by enzymatic transesterification of lipoic acid with high-oleic sunflower oil in 2-methyl-2-butanol solvent. The synthesis gave a crude product mixture comprising unreacted lipoic acid, free fatty acids, and several lipoyl glyceride structures of varying lipoic acid substitution. A more purified product mixture, devoid of unreacted lipoic acid and free fatty acids, was obtained in 61% yield. The crude and purified product mixtures were thoroughly characterized and their components positively identified. The tribological properties of the product mixtures were further investigated using a variety of methods. The product mixtures displayed significantly improved oxidation stability, cold-flow, and extreme pressure properties over those of the parent high-oleic sunflower oil. The extreme pressure results for the neat products showed a higher weld point for the crude than for the purified mixture. This was attributed to differences in the chemical properties of the components in the two product mixtures.

Tribological Properties of Limonene Bisphosphonates

“…Table 4 compares the OT and PT values of limonene and its bisphosphonate derivatives, along with those of polyalphaolefin (PAO6) and high-oleic sunflower oil (HOSuO) base oils. These two base oils were selected for this study because they have similar viscosity but different polarity [60]. As shown in Table 4, bisphosphonation dramatically improved the OT and PT of limonene above the values reported for HOSuO [60] and close to those measured for PAO6.…”

“…These two base oils were selected for this study because they have similar viscosity but different polarity [60]. As shown in Table 4, bisphosphonation dramatically improved the OT and PT of limonene above the values reported for HOSuO [60] and close to those measured for PAO6. The latter is well known to be a very thermally stable synthetic base oil [6].…”

“…The AW and EP results of the neat oil investigations are summarized in Table 6, where the data for the bisphosphonates are compared to each other and to the base oils (PAO6 and HOSuO). Examination of the neat oil EP results in Table 6 shows that bisphosphonates provide weld points of about double (*200 kgf) the reported values [60,61] for the neat base oils. Among the bisphosphonates, the methyl product gave a higher (260 kgf) weld point than the n-butyl or ethyl products.…”

“…All tests were conducted with the cell pressurized with pure oxygen to 500 ± 25 psig in dynamic mode, i.e., with a positive oxygen flow rate of 100 ± 10 mL/min. Details of the test procedure have been given before [60]. Duplicate runs were conducted, and average values of onset temperature (OT) and peak temperatures (PT) are reported.…”

Investigation of the Physical and Tribological Properties of Iso‐Oleic Acid

“…In this investigation, HOSuO was selected as the model biobased base oils, and PAO‐6 as the model petroleum‐based base oil. These two base oils have similar kinematic viscosity at 40 °C (Biresaw et al, ) but differ vastly in their chemical structure. HOSuO comprises three ester groups in its structure, which makes it relatively more polar than PAO‐6, which is a hydrocarbon with no functional group.…”