Effects on the yield stress of lithium grease from the adsorption of surfactants on fillers

Abstract

Various materials classed as additives or fillers may be added to greases to improve their service properties; these two types of materials differ greatly in their effects on the grease structure [i]. The desirable effects of the added materials are generally accompanied by undesirable side effects on the grease structure and properties. Most of the additives used to improve the lubricity and protective properties of greases are highly surface-active substances, and when used at the working concentrations, they soften the greases to an extent that cannot be tolerated [2, 3]. This same sort of softening occurs when greases are oxidized under the conditions of service and storage, as a result of the formation and accumulation of oxygen-containing surfactants, primarily acids and alcohols [4]. An effective method for improving the functional effects of additives and eliminating the undesirable side effects is the joint use of additives and fillers in the optimal ratio. The favorable results to be obtained by the use of such combinations are apparently due to adsorption of the additives on the surface of the filler particles. The presence of fillers also prevents any extreme softening of the greases when they are oxidized [5]. This effect can probably be attributed to adsorption of the oxidation products on the filler particle surface, so that the oxidation products have much less effect on the grease structure, and thus there is no sharp change in ~he properties of the grease when it is oxidized. In order to confirm this hypothesis, we investigated the adsorption of surfactants on fillers (graphite, mica, and MoS~--20-40 ~m fractions); the surfactants included certain individual straight-chain fatty acids and alcohols (Cs-C16) and certain additives that are widely used in greases--Khlor~f-40, KINKh-2, and DF-II.* The heat of adsorption, which is a measure of the energy of surfactant molecule interaction with a solid surface, was used to judge the adsorption activity of the fillers. The heats of adsorption of the alcohols, acids, and additives were determined by means of flow microcalorimetry on solutions in heptane, simulating the hydrocarbon dispersion medium in a grease. The microcalorimeter was of the type described by Groszek [6]. The tests were performed at room temperature, with a 0.2-g sample of the filler as the adsorbent. The heptane flow rate was chosen within the range of 1-2.2 ml/min, depending on the adsorption process kinetics. The content of surfactant in the solvent was in the range from I0 -s to I0 -6 M, the additives being supplied in the form of 10% solutions and the alcohols and acids in the form of 0.25 M solutions. The integral heat of adsorption of the acids and alcohols on the fillers depends on the number of carbon atoms in the surfactant molecule (Fig. i). For all of the surfactants investigated, the heat of adsorption varies only slightly as the chain length is increased up to C12. Further increases in the molecular weight of the surfactant lead to increases in the heat of adsorption on MoS= and particularly on graphite. For the mica, these changes are very minor. The adsorptivity of the graphite and MoS= for the acids is somewhat greater than for the alcohols. The mica differs from the other two fillers in that the heats of adsorption of alcohols on the mica surface are greater than those of the acids. The most likely explanation is that these surfactants are adsorbed on the mica mainly by the polar part of the molecule, and chemisorption influences the heat of adsorption. On the surface