Meanwhile, ionic liquids lubricating properties and their potential as additives have also been identified, leading to the possibility of ionic liquids becoming the next generation of lubricants additives. But will ionic liquids step out of the lab and into mainstream lubricants use?

Lubricant Development

In 2001, the first papers on ionic liquids as lubricating fluids entered the scientific literature, closely followed by mentions of their potential as additives in metalworking fluids. Although, some researchers claim that ionic liquids werent considered as additives until this decade, when the issue of oil solubility was addressed directly.

Many of the original ionic liquids proposed as additives contained halogens, particularly fluorine, so acceptance for mainstream applications was unlikely due to the hazardous properties of fluorine, which is corrosive and toxic, and many of its compounds. However, as phosphorus and nitrogen-containing cations began to be paired with phosphates, so industry began to look at these compounds. For example, the U.S. Department of Energys Oak Ridge National Laboratory began collaborations with Shell and General Motors to explore their use as additives.

What are Ionic Liquids?

Ionic liquids are defined as salts in liquid form that have a melting point below 100C. Even though they can include metals, those of interest to the lubricants industry do not form ash when combusted in engine oil. Ionic liquids achieve their low melting point by having relatively large and asymmetric cations and anions, which prevents them from forming a stable crystal lattice. The anions are attracted to the cations, so they prevent evaporation. Consequently, ionic liquids have very low volatility relative to their viscosity. Depending on the circumstances, at least one of the ions is attracted to the surfaces in a contact.

Ionic liquids also have a relatively strong affinity for metal (oxide) surfaces. As this is an interaction between an ion and (usually) a thin oxide surface covering a metal, the interaction is stronger than, say, an ester with the same surface. Like an ester, an ionic liquid can be immediately adsorbed onto the surface. It does not require any local heating or exposure of fresh metal due to sliding, so the protection can be applied before the surfaces are damaged by wear. The interaction does not stop with a monolayer, as several studies have shown that multilayers can form.

One of the properties of this class of materials that has caused great excitement, in academia at least, is the potential of such a wide variety of anions and cations. There are millions of cation/anion combinations to play with, leading to the coinage of the term designer solvents. This property opens up the possibility of designing functionality into new additives, and ionic liquids can be made to be biodegradable with many already on the market being non-toxic and environmentally benign.

The downside of the multiplicity of possibilities is that finding the right ionic liquid for a specific application could be difficult. Apart from this, the only notable chemical downside is that many ionic liquids considered for lubrication can be corrosive.

Millions of Combinations

There are two distinct potential advantages to ionic liquids for the lubricants business. First, functionality to address a different performance characteristic could be designed into each of the ions. Second, the newness of these materials has led to a high proportion of studies looking into their interactions with new surface coatings.

The former has not yet been widely reported in the open literature, possibly indicating that we are still in the research phase rather than development. However, there are many studies looking at, for example, incorporating phosphorus into both the cation and anion in the expectation that a low treat rate extreme pressure/anti-wear additive might be discovered.

While dual functionality has not yet been widely addressed, the opportunity is available to design hydrolytic or oxidative stability directly into the ions of the salt, thus potentially producing a much more robust additive that does what it is intended to do without degrading due to other chemical activity.

Several research groups have reported on the interactions between ionic liquids and many of the new surface coatings that have been used in premium-priced equipment for more than 10 years – materials such as titanium nitride, boron nitride, diamond-like carbon and others. The logic is that those additive chemistries that have served the industry well over the past 80 years are unlikely to be as effective in contacts where other materials are replacing steel.

While there is interest in ionic liquids, at least at the research level, the potential performance advantages for the lubricants industry are another sell. We have already seen that they are ash-free, have low-volatility, can be designed for thermal, oxidative and hydrolytic stability, have a high surface affinity and can carry relevant elements/chemistry in both the cation and anion for wear protection, friction reduction, and corrosion protection. In addition, more than one performance characteristic can be designed into the salt, and there is a separate technology push in the search for additives that will work on new non-steel surfaces.

Sweep Out the Ashes

Even where steel is the metal to lubricate, ionic liquids offer a possible route to lowering sulfated ash, phosphorus and sulfur, or SAPS, by replacing zinc dialkyl dithiophosphate anti-wear additives in order
to reduce the effect on engine exhaust after-treatment devices. Ionic liquids offer the possibility of extreme-pressure or anti-wear additives that are free of sulfated ash and sulfur.

Some research groups have tried to load up ionic liquids with a high mass percentage of phosphorus in the search for a breakthrough ashless and sulfur-free salt that could replace ZDDPs or sulfur-containing extreme-pressure additives at low treat rates.

Others, meanwhile, have tried reducing the phosphorus content of the additive by studying ashless salts that contain phosphorus in only the anion, usually with an ammonium-based cation. In this respect, ionic liquids suddenly look less like a disruptive new chemistry than an extension of amine phosphate extreme-pressure additives that have been in use by Lubrizol, Rhein Chemie (now Lanxess) and others, for more than 40 years.

Sparking Commercial Interest

Several companies have been formed to exploit the chemistry of ionic liquids and include lubrication in their portfolio. Iolitec has been in the ionic liquids market since 2004 and offers more than 300 ionic liquid products, including phosphonium phosphates and imidazolium phosphates. Green Lubrication Innovations is more specific about the anti-wear capability of its portfolio, but less specific about the chemistry. The company claims that its additive provides superior wear performance compared to the traditional anti-wear additive, ZDDP. Additionally, its formulations are stable and compatible with existing additive packages used in engine oil, such as antioxidants and detergents, the company claims.

However, this doesnt necessarily indicate imminent widespread commercialization. Indeed, scientific publications would indicate that applications are restricted to niches, such as hard disks and even space exploration. Michael Buttery of U.K. engineering, safety and risk consultancy ESR Technology published some testing work on ionic liquids in space applications using a spiral orbit tribometer, an in-vacuum screening test. Several ionic liquid candidates displayed better volatility and tribological behavior than materials commonly used in space lubricants.

However, these are still early days for space applications. The first ionic liquid to leave the Earth could be on the aperture valve of the Mars Organic Molecule Analyzer mass spectrometer, due to fly on the European Space Agencys 2018 ExoMars mission.

The phosphorus-containing materials were also considered as possible replacements for phosphate-based fire-resistant hydraulic fluids. Preliminary investigations into the most common phosphorus-based ionic liquids have not revealed any significant technical advantages of these compounds over existing neutral or acid phosphates in these applications. As a consequence, it seems unlikely that they will be simple replacements for the current technologies, David Phillips, an independent consultant on these products, told LubesnGreases.

Developers at additive companies and lube marketers also elicited no indications of commercial application, but this could be due to a reluctance to discuss chemistry that could give a competitive edge. There is a school of thought that replacing ZDDPs in crankcase lubricants would require the testing regimes to change radically. The current approval regimes for API, ILSAC or ACEA present barriers that are too high to anyone with a drop-in replacement for ZDDP, said one insider who asked not to be identified.

The additive companies have too much money invested into their read-across matrices to suddenly drop in a new anti-wear additive. It may be easier for an addco or oil company to formulate a single industrial fluid that meets 10 or more OEM requirements than try to formulate a mass-market crankcase lubricant, he said.