Submitted to: Chemical Engineering Communications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 1, 2007
Publication Date: May 2, 2007
Citation: Kurth, T.L., Sharma, B.K., Doll, K.M., Erhan, S.Z. 2007. Adsorption behavior of epoxidized fatty esters via boundary lubrication coefficient of friction measurements. Chemical Engineering Communications. 194:1065-1077. Interpretive Summary: Using friction measurements and theoretical modeling, a variety of modified and un-modified bio-based materials have been compared and contrasted. Specifically epoxy methyl oleate (EMO) and methyl oleate (MO), epoxy methyl linoleate (EMLO) and methyl linoleate (MLO), epoxy methyl linolenate (EMLEN ) and methyl linolenate (MLEN)), epoxidized soybean oil (ESBO) and a commercial epoxidized 2-ethylhexyl transesterified soybean oil (VF), as additives in hexadecane. A simple model was used to determine the relative interaction energies of each compound with a steel surface. The order of interaction energies was found to be: ESBO > VF > EMO greater than or equal to EMLO > EMLEN and MLEN greater than or equal to MLO > MO. This ordering agrees with that obtained via a more complex theoretical treatment. The relative data obtained allowed several conclusions to be drawn. The interactions of additive molecules with one another contribute to the measured interaction with a surface. For the epoxidized materials, the additive-additive interactions increase with epoxidation, but the observed interactions with the steel surface decrease. For analogous unsaturated esters the opposite trends are observed. By comparing individual epoxidized materials to the analogous unsaturated methyl esters, it is evident that there is decreasing benefit of epoxidation with increasing degree of unsaturation. This new information has led to greater understanding of additive-additive and additive-surface interactions at the molecular level. Such information will be useful in the development of new products and applications for these and similar bio-based materials.
Technical Abstract: The frictional behaviors of a variety of fatty alkenyl esters and their corresponding fatty epoxide esters (epoxy methyl oleate (EMO) and methyl oleate (MO), epoxy methyl linoleate (EMLO) and methyl linoleate (MLO), epoxy methyl linolenate (EMLEN ) and methyl linolenate (MLEN)), epoxidized soybean oil (ESBO) and a commercial epoxidized 2-ethylhexyl transesterified soybean oil (VF), as additives in hexadecane have been examined in a boundary lubrication test regime using steel contacts. Langmuir critical additive concentrations were determined which provide the following order of negative adsorption energies: ESBO > VF > EMO greater than or equal to EMLO > EMLEN and MLEN greater than or equal to MLO > MO. Thus, for the similar epoxidized materials the greater degree of epoxidation results in less negative calculated total adsorption energies; this trend is reversed for the olefinic parent systems. This ordering agrees with that obtained via a more complex unconstrained cooperative interaction adsorption model. Fits of the steady-state Coefficient of Friction versus concentration data indicate an inverse relation of the obtained interaction parameters with the primary adsorption energies. These results demonstrate the complexity of the adsorption mechanism that is determined both by the adsorbate-adsorbent and adsorbate-adsorbate interactions. The relative adsorption parameters suggest that for the epoxide materials studied, the intermolecular interactions increase with increasing epoxidation, and the total adsorption energies become more negative; this is opposite for the olefinic compounds studied.