Tribological Properties of Quaternary Ammonium Salt Polymeric Ionic Liquids as Water-Based Lubricant Additive

As a kind of environmentally friendly material, water can be used widely due to low cost. At present, the water-based lubricant has been used for metal working fliud and hydraulic transmission fluids. Instead of oil-based lubricant, the diverse kinds of aqueous lubricant have attracted more and more attention due to environmental regulations. However, there are many challenge from physicochemical properties of the aqueous lubricants, such as the viscosity, the corrosion and the film-forming ability. To consider the above disadvantages, diverse kinds of lubricant additives have been used to adjust aqueous lubricant’s physical-chemical properties and improve their tribological performance. In the past, water-soluble functional materials with small molecules and polymers had been always chosen as lubricant additive, although many of them can only serve for the single functional application. For traditional lubricant additive, it is not easy to solve poor lubrication performance and low viscosity simultaneously. The polymeric ionic liquids have been derived from polymers with ionic liquids structure. They might have the advantages of both ionic liquids and polymers. It might be the potential additives toimprove both poor lubrication performance and low viscosity. A series kind of water-soluble polymeric ionic liquids (Note as PPM) with the structure of quaternary ammonium cations have been sythesizedby free radical polymerization. As the monomers, poly(ethylene-glycol) methyl ether methacrylate (PEGMA) and (2-(methacryloyloxy)ethyl) trimethylammoniumchloride (METAC) provided the functional blocks for improving viscosity and lubricity after the polymerization process. After obtaining the polymeric ionic liquid with the anion of choloride (PPM-Cl), butyric acid (Ba), hexanoic acid(Ha) and octanoic acid (Oa) were used for ion exchange to get three kinds of water-soluble polymeric ionic liquids (PPM-Ba, PPM-Ha and PPM-Oa). Meanwhile, the water-soluble commercial tackifier APE30 was selected as the reference to assess three kinds of ploymeric ionic liquids. Being taken as lubricant additives, APE30 and polymeric ionic liquids were dissovled in deionized water to attain a series kinds of aqueous lubricant. The corrosion resistance of aqueous lubricantswas evaluated by a cast-iron flake immersion test. It was found that three kinds of polymeric ionic liquids owned excellent anti-corrosion properties, whereas the APE30 only provided an awful corrosion-resistance property. The kinematic viscosity and rheological behaviors of aqueous lubricants were investigated to evaluate the viscosity improving effect. The results showed that three kinds of polymeric ionic liquids provided higher viscosity than APE30 tackifiers for aqueous lubricants under the same conditions. Tribological behavior of aqueous lubricant were performed on SRV testing system, and then the worn surfaces were observed on the non-contact surfaces optical three-dimensional profiler and the scanning electron microscope. The polymeric ionic liquids showed excellent tribological performance and protected the mating surface from the adhesive wear and the abrasion. The chemical state of worn surfaces were analyzed on an X-ray photoelectron spectrometer to explore the lubricating mechnism of aqueous lubricant with the polymeric ionic liquid. The results showed that there were the complex tribochemical reaction when the metal substrate met with polymeric ionic liquid in friction process. The formation of chemisorption film and tribofilm containing N elements on the surface played an important role for friction reduction and anti-wear ability of lubricant additive. A series of polymeric ionic liquids might be the potential water-based lubricant additive for industrial applications.