Protic ionic liquids as additives in water-based ecolubricants

Abstract

Ionic Liquids have demonstrated their ability as lubricants, or as additives for lubricants. Due to their intrinsic properties such as their broad liquid range from as low as -100ºC to as high as 350ºC, they have practically no vapor pressure, and exhibit high thermal stability and decomposition temperatures above 300ºC. Their high polarity induces self-assembly films on metallic surfaces, which enable the ionic liquid to prevent direct contact and reduce friction and wear of the surfaces in contact [1-3]. Protic ionic liquids (PIL) are synthesized in a cleaner and simpler way than conventional ionic liquids and have only organic elements in their composition. They have emerged as being excellent additives to formulate water-based lubricants due to their excellent solubility, low or non-toxic effect and high biodegradability, amongst other properties. [4-6]. In this work, two PIL, bis(2-hydroxy ethyl) ammonium caprylate (DC8) and undecylenate (DC11), are studied as additives at 1, 2, 5, and 10 wt.% in water-based lubricants, in a pin-on-disc configuration using the tribopair sapphire/ stainless steel. The friction coefficients and wear rates data are discussed as a function of the critical micelle concentration of the PIL in the aqueous solution (Figure 1). In addition, the Stribeck curves were studied to display the lubrication regimes, whilst techniques such as XPS, Raman spectroscopy, or electronic microscopy were used to establish the friction and wear mechanisms present in the tribological contacts.