Concentration effects of chlorinated paraffin and zinc di-ethylhexyl dithio phosphate on boundary lubrication properties were tested in vegetable and mineral base stocks. Solvent refined low sulfur paraffinic mineral oil (150 N oil) and conventional food grade soybean oil (soy oil) with EP additive concentration of 0-20% (w/w) were used in ASTM D2783 four-ball extreme pressure (4-ball EP) and Twist Compression Tribotests (TCT). Weld points in 4-ball EP and times to failure in TCT at 200 MPa showed that 150 N oil needed more than double treat levels of EP additives to achieve similar boundary lubrication performance as their 5% blends in soy oil. Also, incorporation of 20% soy oil into 150 N oil-based EP additive blends improved the performance to nearly the same level as soy oil only blends of corresponding additives. Boundary lubricity of some soy oil samples was similar to that of a commercial straight oil chlorinated metal forming lubricant. Several suggestions are provided to explain such pronounced influence of the base stock type on EP additive response. The findings suggest that soy oil and other farm-based oils may provide strategies for formulating cost effective industrial fluids and other lubricants.

Twist compression tribotester (TCT) and 4-ball extreme pressure (EP) methods were used to investigate commercial polysulfide (PS) and biobased 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), 4-ball EP produced a set of preweld 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 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. Thus, 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 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.