Submitted to: Wear
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/28/2006
Publication Date: 9/15/2006
Citation: Kurth, T.L., Byars, J.A., Cermak, S.C., Sharma, B.K., Biresaw, G. 2007. Non-linear adsorption modeling of fatty esters and oleic estolides via boundary lubrication coefficient of friction measurements. Wear. 262(5-6):536-544. Interpretive Summary: Expanding on our previous work to develop and apply advanced modeling techniques to friction derived adsorption data, we chose to compare and contrast the behaviors of simple and complex bio-based materials as lubricant additives. This work demonstrates the failure of previously applied methods and models to adequately account for subtle features of the data. It was found that the more massive and complex materials exhibited improved lubricity at low concentration which may be partially accounted for by increased mass and/or multiple adsorption interactions of each additive molecule. This work extends our understanding of the complex interactions of lubricant additives and provides a theoretical basis for the future development of bio-based lubricant additives.
Technical Abstract: The frictional behaviors of a variety of fatty esters (methyl oleate (MO), methyl palmitate (MP), methyl laurate (ML), and 2-ethylhexyl oleate (EHO)) and oleic estolide esters (methyl oleic estolide ester (ME) and 2-ethylhexyl oleic estolide ester (EHE)) as additives in hexadecane have been examined in a boundary lubrication test regime using steel contacts. Critical additive concentrations were defined and used to perform novel and simple Langmuir analyses that provide an order of adsorption energies: EHE greater than or equal to ME > EHO > MP > MO greater than or equal to ML. Application of Langmuir, Temkin, and Frumkin-Fowler-Guggenheim (FFG) adsorption models via non-linear fitting demonstrates the necessary inclusion of cooperative effects in the applied model. Fits of the steady-state Coefficient of Friction (COF)-concentration data for EHE, ME, and EHO indicate slight cooperative adsorption. MO, MP, and ML data require larger attractive interaction terms (alpha less than or equal to -2.5) to be adequately fit. Primary adsorption energies calculated via a general adsorption model are necessarily decreased while total adsorption energies correlate well with values obtained via critical concentration analyses. To account for multiple surface-site coverage a multiple-site model was defined. The intuitive assumption of multiple-site coverage of more massive components suggests deceptively increased calculated adsorption energies for typically applied models (e.g. FFG, Langmuir).