Green Ionic Polymer Additives for Multifunctional Integration in Water-Based Lubrication Systems: Lubricity, Anti-Corrosion and Viscosity-Enhancing Properties

Environmentally benign water-based lubricants are attracting increasing attention as alternatives to conventional oil-based lubricants that may cause environmental pollution. Herein, three protic poly(ionic liquid)s (PPILs) are synthesized by combining a copolymeric cation with carboxylate anions of different alkyl chain lengths. The PPILs are employed as additives in aqueous lubricants, and their viscosity-enhancing capability and tribological performance are systematically evaluated. Quartz crystal microbalance (QCM), electrical contact resistance (ECR), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS) are used to elucidate the lubrication mechanism. The results demonstrate that PPILs significantly improve the viscosity, corrosion resistance, and tribological properties of water-based lubricants. Notably, the kinematic viscosity of a 6% PPD-Oa aqueous solution increases by two orders of magnitude compared with deionized water, while the corresponding wear volume and coefficient of friction are reduced by approximately 77% and 70%, respectively. Mechanistic investigations indicate that interfacial adsorption and friction-induced chemical processes jointly generate a protective layer at the sliding interface, thereby imparting excellent lubricity and load-bearing capability to the lubricant. Moreover, the three sulfur-free and halogen-free PPILs represent environmentally benign additives with strong potential for application in metalworking fluids and hydraulic systems.