July / August 2019 NLGI Spokesman

Initial Steps of Lithium Soap Grease Thickener Agglomeration Paul Shiller The University of Akron Akron, OH 44325 Presented at the NLGI 85th Annual Meeting Coeur D’Alene, ID June 9-12, 2018 Abstract In lithium soap thickened greases, soap molecules form a fibrous network. This fundamental study investigated the arrangement of lithium 12-hydroxystearic acid molecules at the earliest stage of thickening in mineral oil. Experimental data suggested that an initial structure or arrangement of thickener molecules formed when about 4% soap thickener was dispersed in base oil. Rheological measurements showed that at this concentration, the mixture first exhibited viscoelastic properties with its solid-like storage modulus at about the same value as its liquid-like viscous modulus. This concentration was defined as the critical micelle concentration (CMC). Infrared analysis spectra were consistent with results from computer models that showed that the lowest energy arrangement for two lithium 12-hydroxystearic acid (Li-12HSA) molecules was a structure where the tails were opposite the head groups, due to a repulsion between the tail groups, and with the head groups in a planar arrangement.

Introduction: What do we know about grease?

The common understanding of how grease lubrication thickeners work is a “sponge” analogy. It is difficult to determine when this analogy started, but in 1951, Bondi et al. stated that the consistency of grease is due to the thin fibrous shapes of the soap particles forming a latticework that holds the oil by capillary action where the oil can easily flow into the small narrow spaces between the grease fibers.1 In 1954, Boner et al. reviewed the literature discussion about syneresis and bleed.2 Syneresis is the expulsion of liquid as the fibrous lattice structure collapses, and bleed is release of oil due to defects in the network.

These terms (syneresis and bleed) are used interchangeably and are related to the effects of pressure on the lubricating grease; i.e., a sponge that can hold oil and release it under pressure. The fibrous structure of soap thickener in grease was known from the 1950s through electron micrographs produced by Brown, Huddson, and Loring.3 Grease, as it is known today, is a three-dimensional network of thickener particles in oil. These thickener particles interact with each other and the oil, forming a matrix through physical forces. The pores of this network are filled with oil.

Recently, Saatchi et al. developed an oil bleed model that uses the grease thickener and its associated hydrodynamic volume; i.e., the volume of the thickener particles plus the oil bound with them, to calculate oil bleed amounts of grease.4 Saatchi et al. proposed a ‘point of structural opening’ in which the thickener particles, with their associated hydrodynamic volume, are spaced far enough apart to allow open channels and oil bleed. This model fit the data for oil bleed of lithium soap, polyurea, and calcium sulfonate thickened greases. It was particularly useful for predicting the increase in bleed for calcium sulfonate thickened grease. Figure 1 shows the ASTM D6184 data for bleed rate for greases as a function of percent oil in the grease.

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