Ionic liquids as lubricants

R. David Whitby | TLT Worldwide January 2013

These unique chemicals are used in several applications and have a wide range of viscosities.

 

AN IONIC LIQUID IS A SALT IN WHICH THE IONS ARE POORLY COORDINATED, which results in these chemicals being liquid below 100 C or even at room temperature. At least one ion has a delocalized charge, and one component is organic that prevents the formation of a stable crystal lattice. While ordinary liquids such as water and gasoline are predominantly made of electrically neutral molecules, ionic liquids are largely made of ions and short-lived ion pairs.

These substances are called liquid electrolytes, ionic melts, ionic fluids, fused salts, liquid salts or ionic glasses. The methylimidazolium and pyridinium ions have proven to be good starting points for the development of ionic liquids.

By varying the combination of cations and anions used to make an ionic liquid, their properties can be altered significantly, allowing a huge number of ionic liquids. Some observers put the number of combinations at about 10¹⁸.

During the last 10 to 20 years, ionic liquids have attracted considerable attention because of their five unique characteristics:

• Powerful solvency properties
• Negligible volatility
• Non-flammability
• High thermal stability
• Low melting point.

Ionic liquids have been developed for many applications, including chemical and pharmaceutical manufacturing, cellulose processing, algae processing, dispersants, gas handling and gas treatment, nuclear-fuel reprocessing, solar thermal energy, waste recycling and batteries. In addition, ionic liquids have generally been found to exhibit low acute toxicity and biological activity and ready biodegradability.

The properties of ionic liquids indicate a possible benefit if they are used as lubricants. Different ionic liquids have been found to have a wide range of viscosities and viscosity indices. The first studies into the tribology of ionic liquids were reported 10 years ago. The results indicated reductions in coefficients of friction and wear with several metallic surfaces. Despite the huge number of possible combinations, tribological studies have tended to focus on imidazolium cations combined with hexafluorophosphate or tetrafluoroborate anions.

The tests used to assess the lubricating properties of ionic liquids have tended to focus on laboratory test rigs such as the oscillating friction and wear tester (SRV IV), ball-on- flat reciprocating sliding machine, pin-on-disc unidirectional sliding machine, ball-on-disc unidirectional sliding machine and four-ball friction and wear tester.

In ball-on-disc tests, using aluminium alloy discs and steel balls, some ionic liquids have exhibited lower friction and wear than a 15W-40 engine oil. Similar results have been observed in pin-on-disc tests using pins made from chromium- plated piston rings and discs made of gray cast-iron.

Oil-miscible ionic liquids have been shown to be effective as additives in laboratory antiwear and antiscuffing tests, as well as being non-corrosive to iron and aluminium (1). Conductive greases based on 1-octyl-3-methylimidazolium ionic liquids and polytetrafluoroethylene (PTFE) thickener have better friction-reducing and antiwear properties than a polyalphaolefin-based grease in an Optimol-SRV reciprocating friction tester (2).

Unfortunately, all the tribological tests used to assess the lubricating properties of ionic liquids have involved laboratory test rigs. In my 40 years of industry experience, I’ve learned that the only reliable way to assess whether a lubricant works effectively is to use it in real-life machinery. Laboratory tests, as well as engine or equipment tests that run under controlled conditions, are useful for identifying lubricants that are unlikely to function in real-life.

More R&D work is needed, using extensive field trials, before ionic liquids are able to take their place alongside other high-performance lubricating oils and greases.

REFERENCES
1. Qu, J., Blau, P., Dai, S., Luo, H., Bunting, B., Bansal, D. and Yu, B., “Ionic Liquids as Novel Engine Lubricants or Lubricant Additives,” Presented at the Directions in Energy-Efficiency and Emissions Research (DEER) Conference, Detroit, Michigan, Oct. 6, 2011.
2. Wang, Z., Xia, Y., Liu, Z. and Wen, Z. (2012), “Conductive Lubricating Grease Synthesized Using the Ionic Liquid,” Tribology Letters, 46 (1), pp. 33-42.


David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.