Effect of Excess Thickener Components on the Microstructure and Properties of Lithium Greases

The microstructure of grease is the key factor to determine its performance. The thickener of lithium grease is the main component to form the microfiber of grease and its spatial reticulation structure, and the components of thickener have significant effects on the microstructure of grease. In this paper, the effect of excess thickener components on the microstructure and performance of lithium grease was systematically investigated and the mechanism of the effect was elucidated. In this study, 12-hydroxystearic acid (HSA) and lithium hydroxide monohydrate (LiOH H₂O) were used as raw materials for lithium grease thickeners, and five lithium greases with different molar ratios of thickener raw materials were designed and prepared. Including Li-LH2 (HSA: LiOH H₂O=1.0:1.2), Li-LH1 (HSA: LiOH H₂O=1.0:1.1), Li (HSA: LiOH H₂O=1.0:1.0), Li-HA1 (HSA: LiOH H₂O=1.1:1.0) and Li-HA2 (HSA: LiOH H₂O=1.0:1.2). The chemical structures of the prepared greases were characterized using Fourier transform infrared spectroscopy (FT-IR), and the characteristic functional groups were analyzed. The microstructure of the five lithium greases was characterized by scanning electron microscopy (SEM). The test results showed that the excess thickener had a significant effect on the microstructure of the lithium greases, and the thickeners of samples Li-LH2, Li-LH1, Li and Li-HA1 were all spiral fibers entangled in a mesh structure. The thickener of sample Li-HA2 was a mixture of fibers and block structure, and did not form a spiral fiber entangled mesh structure, which was due to the excess of more HSA that affected the arrangement of thickener molecules and hindered the formation of thickener fibers and their mesh structure. The effects of microstructure of five lithium greases on their cone penetration, dropping point, colloidal stability, mechanical stability and rheological properties were systematically studied. The results showed that the lithium greases prepared with 20% and 10% excess of lithium hydroxide and 10% excess of HSA had higher consistency, better colloidal stability and higher structural strength, and the mechanical stability of the lithium greases prepared with 20% excess of HSA was better. Based on the results of the study, the mechanism of the effect of excess thickener components on the microstructure and properties of lithium grease was elucidated. Excess lithium hydroxide monohydrate acts as the nucleus in the microstructure formation of lithium grease, attracts with the polar end of lithium 12-hydroxystearate molecules, arranges to form uniform spiral fibers, and further combines to form a spatial mesh structure with a high degree of entanglement. When HSA excess 10%, HSA was attracted with thickener molecules polar end, which increased the intermolecular arrangement of the thickener spacing, formed a uniform spiral fiber and a lower degree of entanglement of the mesh structure. When the excess of HSA was 20%, the excess of HSA hinders the arrangement and combination between the molecules of the thickener, resulting in the thickener not being able to form spiral fibers and spatial mesh structure. The spatial reticulation structure made of uniform spiral fiber entanglement had high force with base oil, strong binding ability with base oil, good thickening ability, good colloidal stability and high structural strength. This research work provided an important theoretical basis for improving the process level and performance of lithium grease.