The effect of ethoxylated esters on the lubricating properties of their aqueous solutions

Abstract

In many applications strict ecological criteria are crucial. Water is often used in these cases as a lubricant base. Its disadvantageous properties, in particular lubricity, can be modified by introduction of additives. Ethoxylated methyl esters of fatty acids from rapeseed oil were chosen for that purpose. In order to verify tribological properties of aqueous solutions of these compounds, anti-seizure properties under linearly increasing load, friction and wear under 2 kN (four-ball machine) were assessed. Not only radical change of tribological properties as compared to water was observed, but also the results are comparable to the ones obtained for mineral oils, which contained typical, commercial additives. Significant changes in scuffing load (P t), seizure load (P oz), limiting pressure of seizure (p oz) were recorded even at 0.1% wt concentration of the additive. At optimal concentration P t, P oz, p oz, increased even almost 6-, 2- and 3-fold (respectively). The quantities measured increased with ethoxylation degree. Also at constant load, friction (6-fold) and wear (almost 2-fold) were reduced. In this case, however, no influence of concentration and ethoxylation degree on the quantities measured, was observed. The tribological properties can be discussed in terms of results of physical–chemical tests. Ethoxylates are surface active compounds. Within the range of concentrations analyzed, they form micelles in a bulk phase and liquid-crystalline structures in a surface phase. High surface activity of these additives and creation of specific structures in solutions were proved by measuring surface tension, wetting angle and mesophases observation in polarized light. On the basis of the results of the physical–chemical and tribological tests one can conjecture that profitable tribological properties of the lubricating compositions are the result of strong affinity to the surface and their ability to form ordered structures. One can expect that over the cooperating surfaces a deposit is formed. Presence of the deposit increases real contact area and prevents seizure. This hypothesis gives a simple explanation of the unexpected improvement of anti-seizure properties, which accompanies the increase of ethoxylation degree. Ethoxylates with higher number of ethylene oxide attached, reveal smaller surface activity and better solubility in water. On the other hand, they can be characterized by higher dehydratation temperature. This means that at higher temperatures they reveal surface activity. That is why, during seizure tests, at high temperature, under high loads the esters of higher ethoxylation degree protect friction couples against seizure much more efficiently.