Ionic liquids modified CNTs and graphene as additive in vegetable lubricating oil: A route for sustainable tribology

Abstract

The multi-walled carbon nanotubes (MWCNTs) and graphene oxide (GO) were covalently functionalised with imidazolium salt-based ionic liquids. The acid chloride-functionalised MWCNTs and GO were coupled with 1-(3-aminopropyl) imidazole, and then the products were reacted with 1-bromobutane. The bromine anions were exchanged with PF6− and NTf2−. The phase transformation of the MWCNTs was observed after the functionalisation with 1-(3-aminopropyl) imidazole. The black solids were transformed into black liquids, suggesting the imidazolium was chemically grafted onto the MWCNTs. FTIR, XPS, and TGA analyses confirmed the covalent modification and formation of imidazolium salts. SEM and AFM images showed no visual change between the pristine MWCNTs and the functionalised MWCNTs, indicating that the modification processes did not destroy the structural integrity. The solubility of the functionalised MWCNTs with different anions in water was analysed. It was found that hydrophobicity followed the order: NTf2− > PF6− > Br−. The functionalised MWCNTs and GO were employed as additives in vegetable oil at 1 wt.% concentration and the tribological properties were evaluated in a four-ball machine. It was found that vegetable oil with functionalised MWCNTs showed better lubrication performance than pure MWCNTs.

Keywords

• Multi-walled carbon nanotubes
• Graphene oxide
• Ionic liquids
• Lubrication
• Vegetable oil
• Tribology