|ASADAUSKAS, SVAJUS - Institute Of Chemistry|
|MC CLURE, TED - Tribsys Llc|
Submitted to: Industrial and Engineering Chemistry Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/28/2011
Publication Date: 7/5/2012
Citation: Biresaw, G., Asadauskas, S.J., Mc Clure, T.G. 2012. Polysulfide and bio-based EP additive performance in vegetable vs. paraffinic base oils. Industrial and Engineering Chemistry Research. 51:262-273.
Interpretive Summary: Use of farm-based vegetable oils in lubrication presents a number of potential benefits. Among these are: cleaner environment due to the biodegradability of farm-based oils; safer and healthier work conditions due to the non-toxicity of vegetable oils; improved rural economies due to increased demand for surplus farm crops. However, successful commercialization of bio-based lubricants also requires they be competitive in both performance and price relative to petroleum-based lubricants currently dominating the market. Achieving this goal requires understanding the tribological properties and interactions of the various ingredients used in bio-based lubricant formulations. In the work described here, the effect of chemical structure and concentration of extreme pressure (EP) additives on the performance of soy-based vs. petroleum-based formulations were investigated. EP additives allow the lubricant to perform under high temperature and pressure such as those encountered by engine, metalworking and hydraulic lubricants. The results showed that soy-based formulations performed twice better than petroleum-based formulations, at one-fourth the concentration of the polysulfide EP additive. This result indicates that, compared to petroleum-based oils, soybean oil has better compatibility with critical EP additives commonly used in the wide range of lubricant formulations. The implication of this finding is that soybean oil can be used to develop bio-based lubricant formulations that are competitive to petroleum-based lubricants, both in cost and performance.
Technical Abstract: Twist compression test (TCT) and 4-ball extreme pressure (EP) methods were used to investigate commercial polysulfide (PS) and bio-based polyester (PE) EP additives in paraffinic (150N) and refined soybean (SOY) base oils of similar viscosity. Binary blends of EP additive and base oil were investigated as a function of additive concentration. In addition to weld point (WP), the 4-ball EP method produced a set of pre-weld data, notably peak torque and wear scar diameter, which were found to correlate with WP results. TCT gave a 5-fold larger time-to-failure (TTF) for neat SOY than for neat 150N, whereas 4-ball EP gave similar WP (120 kgf) values for both neat oils. This difference was explained by invoking boundary contribution to TCT but not to the 4-ball EP method. Both additives improved the WP and TTF of the base oils, which further increased with increasing additive concentration. However, the extent of the improvements was highly dependent on the chemistries of the additive and base oil of the blends. At similar concentrations, the WP of PE was higher in the 150N than in the SOY base oil, while the WP of PS was higher in the SOY than in the 150N base oil. Similarly, TTF of 150N was higher with blended PE than PS; whereas for SOY, it was higher with blended PS than PE. This chemistry effect was attributed to the relative compatibility between EP additives and base fluids. The results suggest that a substantial reduction (up to 4-fold) in EP additive usage in commercial lubricant formulations can be achieved through proper selection of compatible base fluids and additives.