Sulfurized methyl esters of soya fatty acids: Synthesis and characterization

Authors: Biresaw, Girma; Bantchev, Grigor; LANSING, JAMES - Orise Fellow; Harry O Kuru, Rogers; CHEN, YUNZHI - South China University Of Technology

Submitted to: Tribology Letter
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2020
Publication Date: 4/16/2020
Citation: Biresaw, G., Bantchev, G.B., Lansing, J., Harry-O'kuru, R.E., Chen, Y. 2020. Sulfurized methyl esters of soya fatty acids: Synthesis and characterization. Tribology Letter. 68. Article 61. https://doi.org/10.1007/s11249-020-01292-y.
DOI: https://doi.org/10.1007/s11249-020-01292-y

Interpretive Summary: To obtain the full benefit of biobased lubricants requires that all ingredients in the formulation, base oils and additives, are biobased. Such formulations not only will provide maximum environmental benefits but also maximum economic benefit by increasing the utilization and demand for farm-based raw materials used in lubricant formulation. However, achieving the goal of fully biobased lubricant formulation is currently hampered by the lack of commercial biobased lubricant additives that serve as anti-wear, extreme pressure (EP), antioxidant (AO), etc. In this work, polymercaptanized soybean oil, commercially supplied under the trade name PM-358, was chemically modified to fatty acid methyl ester disulfide (FAME-S2). The disulfide structure is chosen because it is known to provide AO, EP and other additive properties. FAME-S2 was evaluated as an additive in petroleum (polyalphaolefin, PAO-6) and biobased (high oleic sunflower oil, HOSuO) base oils. Blends of FAME-S2 in PAO-6 displayed increasing density and viscosity index with increasing concentration. Evaluation of FAME-S2 as an extreme pressure additive on a 4-ball tribometer showed increasing weld point with increasing concentration, to a maximum of 180 and 220 kgf in PAO-6 and HOSuO, respectively, at 10% w/w concentration. The result shows that fatty acids, methyl esters and other derivatives of soybean and many other vegetable oils can be chemically modified for application as effective biobased lubricant additives. The knowledge developed in this work is of considerable importance for scientists and engineers engaged in biobased additive and biobased lubricant formulation development.

Technical Abstract: Soy-based fatty acid methyl ester disulfide (FAME-S2) was synthesized in good yield by oxidation of polymercaptanized soybean oil fatty acid methyl ester (PM-FAME). The chemical structure of FAME-S2 is of interest because of its potential as a biobased multi-functional additive in lubricant formulations. Neat FAME-S2 and their blends (1–10% w/w) in polyalphaolefin (PAO-6) and high oleic sunflower oil (HOSuO) were characterized for its chemical, physical and tribological properties. Blends of FAME-S2 in HOSuO relative to similar blends of PM-FAME displayed higher oxidation onset temperature (>'15 °C) that remained constant with increasing concentration. Evaluation of FAME-S2 as an extreme pressure (EP) additive on a 4-ball tribometer showed increasing weld point (WP) with increasing concentration to a maximum of 220 and 180 kgf in HOSuO and PAO-6, respectively, at 10% w/w concentration. The results indicate that FAME-S2 has both anti-oxidant and EP properties and can be applied as a multi-functional biobased additive in lubricant formulation. This work demonstrates an encouraging progress towards the development of effective biobased lubricant additives.