Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 24, 2001
Publication Date: N/A
Interpretive Summary: Vegetable oils have excellent chemical, physical, and environmental properties that make them ideal ingredients for lubricant formulations. These properties allow them to be used as base oils and/or as additives for improving the friction and wear properties of lubricants. Vegetable oils also provide biodegradability to lubricants, which is an important consideration in the treatment and disposal of effluents from lubrication operations. However, successful incorporation of vegetable oils into lubricant formulations requires a thorough understanding of those vegetable oil properties that affect certain critical lubrication requirements. In this work, the friction and adsorption properties of normal and high oleic soybean oils were investigated and compared with that of methyl laurate. These two vegetable oils were selected because of the drastic differences in the degree of unsaturation of their fatty acid residues. The results showed that the two vegetable oils adsorbed to friction surfaces much stronger than methyl laurate. Comparison of the two vegetable oils indicated that their difference in the degree of unsaturation did not cause significant changes in their friction and wear properties.
Technical Abstract: Most vegetable oils are amphiphilic triglycerides, liquid at room temperature and capable of solubilizing a variety of lubricant ingredients. In addition, vegetable oils are biodegradable and abundantly available from renewable agricultural resources, making them preferred candidates in lubricant formulation. However, widespread use of vegetable oils in lubrication requires overcoming some of their shortcomings (e.g. poor oxidation stability) as well as a thorough understanding of their boundary and hydrodynamic properties. In this work, the steel/steel boundary friction properties of soybean oil (SBO), high oleic soybean oil (HOSBO) and methyl laurate (ML) were investigated using a ball-on-disk configuration. SBO and HOSBO are triglycerides with radically different degrees of unsaturation of their fatty acid residues. Changes in degrees of unsaturation affect the lateral interaction between adsorbate molecules, which in turn affects the adsorption and, hence, the boundary lubrication properties of the adsorbate. To investigate this possibility, the free energy of adsorption (DELTA Gads) of SBO, HOSBO and ML were estimated from the analysis of the boundary coefficient of friction data using both the Langmuir and Timken adsorption models. The result showed a stronger adsorption for the vegetable oils than for ML, an indication of multiple interactions between the ester groups of the triglycerides and the steel surface. The result also showed no difference in the DELTA Gads values of SBO and HOSBO using either the Langmuir or Timken models. This was interpreted as an indication of the lack of appreciable net lateral interaction between triglyceride adsorbates.