Sustainable Polyurea Greases Based on Epoxidized Soybean Oil: Influence of Ureido Structure on Performance

In this study, sustainable polyurea greases were prepared using epoxidized soybean oil (ESO) as bio-based base oil, with octadecylamine (ODA) reacted with three diisocyanates: 4,4′-diphenylmethane diisocyanate (MDI), 1,6-hexamethylene diisocyanate (HDI), and toluene diisocyanate (TDI). The diisocyanate structure dominated the thickener microstructure: MDI-ODA formed a compact short-rod fibrillar network with strong hydrogen bonding and π–π stacking, endowing the grease with the highest consistency (256), dropping point (262 °C), lowest oil separation (2.7%), and optimal thermal stability (T5% = 278 °C). HDI-ODA showed a lamellar structure with moderate performance, while TDI-ODA presented a loose porous network. Rheological tests confirmed MDI-ODA/ESO possessed the highest yield stress and structural recovery (79.5%). Tribological tests showed MDI-ODA/ESO delivered the lowest friction coefficient and wear scar diameter. Compared with non-epoxidized soybean oil (SO), ESO significantly enhanced grease performance. This improvement is attributed to the ring-opening reaction between the N–H of the ureido group and the epoxy groups of ESO, which improves thickener–oil compatibility. In addition, the polar epoxy groups promote the formation of stable lubricating films. This work verifies that diisocyanate structure and base oil epoxidation are critical for high-performance sustainable polyurea greases.