Phosphorus reductions came in with three-way catalytic exhaust converters, which were installed in cars after fuel economy improvements and a schedule of emissions reductions were mandated in the United States in the 1970s. Higher levels of ZDDP led to the formation of zinc pyrophosphate, a glass-like compound that blocked the catalyst bed and prevented it from reducing emissions as intended.

The article explained that because phosphorus accumulation makes the deactivation of the catalyst irreversible, engine manufacturers often have to design emissions control systems that tolerate cumulative levels of zinc and phosphorus over the vehicles operating life.

Thus, there is significant interest in developing a new lubricant additive that is ashless, has less impact on engine emissions control catalysts, and reduces friction and wear, the article emphasized.

The current limit on phosphorus-which comes from ZDDP-in passenger car engine oils that meet ILSAC GF-5, API SN and the future ILSAC GF-6 standard is between 600 and 800 parts per million (0.06 to 0.08 percent by weight).

Additive companies and oil formulators have explored a variety of routes to replace the amount of ZDDP that must be sacrificed to meet these and earlier limits, and some have worked to develop new varieties of ZDDP that they claim are not harmful to catalysts. It should also be noted that there has always been some debate about the harmfulness of the original ZDDPs.

Enter Ionic Liquids

An alternative that has stimulated much research and development activity in recent years is ionic liquid additives. They are defined as salts, but because of their large and asymmetric cations and anions that prevent them from forming stable crystals, they remain in liquid form even at unusually low temperatures, noted Thomas Schubert, managing director and founder of Heilbrom, Germany-based Iolitec Ionic Liquid Technologies.

The manifold combinations of cations and anions provide a large number of liquid materials with (tunable) unique properties such as high conductivity, high thermal and electrochemical stability, negligible vapor pressure, non-flammability [and] beneficial rheological and tribological properties, Schubert said in an abstract submitted to the 21st International Colloquium Tribology held at the Technische Akademie Esslingen in January.

Ionic liquids have high surface affinity for metals, very low volatility relative to their viscosity-which minimizes evaporative losses-and do not form ash when combusted in engine oil. Some of their performance benefits include thermal, oxidative and hydrolytic stability; wear protection; friction reduction and corrosion protection.

Jun Qu, distinguished research and development staff scientist for the Materials Science and Technology Division at Oak Ridge National Laboratory, has conducted research on ionic liquids to determine their potential to serve as engine oil additives. He presented results of a study in which ionic liquids were used to replace half the amount of ZDDP in motor oils at the Society of Tribologists and Lubrication Engineers annual meeting in Minneapolis.

Friction and Wear Performance

Qu measured friction and wear performance of formulations of 4 centistoke polyalphaolefin containing constant 800 ppm of phosphorus, but with different ratios of a phosphonium alkylphosphate ionic liquid and a ZDDP mixed in.

The oil samples reflected the phosphorus content found in engine oils meeting ILSAC GF-5 and future GF-6 standards, which is between 600 and 800 ppm. They were tested using a ball-on-flat reciprocating sliding tribotest, meaning a steel ball slid against a grey cast iron flat surface.

A 1:1 molecular ratio of the ionic liquid and ZDDP provided the best friction and wear reduction benefits in oils with phosphorus concentrations of 800 ppm, he told attendees at the industry gathering in May. A 1:1 ratio contains 0.52 percent by weight of ionic liquid and 0.40 percent by weight of ZDDP.

Such an ionic liquid and ZDDP combination provided an additional 30 percent friction reduction and 70 percent wear reduction compared with using the ionic liquid or ZDDP alone at the same level of phosphorus content in the oil, Qu later explained to LubesnGreases.

Friction and wear performance of the oil samples with additional amounts of phosphorus in the ZDDP and in the ionic liquids were also tested by analyzing their tribofilms. Qu showed friction coefficient results of the ZDDP-only samples at 400, 800, 1,200 and 3,200 ppm of phosphorus and noted that surfaces become rougher and the tribofilm becomes unstable as concentrations increase, which lead to the formation of cracks, possibly due to the higher concentrations of sulfide and decreasing amounts of phosphorus, as discovered by surface chemical analysis.

Sulfide is more brittle, so therefore we see this kind of micro-cracking and micro-fracturing on the surface, he expanded.

In contrast, with the combined ZDDP and ionic liquid formulations, where both additives were evenly split at 200, 400, 1,600 and 3,200 ppm each, the level of friction stayed relatively flat. The phosphate based compounds were dominant in the tribofilm even at much higher concentrations, and the sulfide compound was kept low. As a result, the tribofilm remains stable, Qu found.

Fuel Economy and Emissions

Formulators are steadily dropping engine oil viscosity to achieve higher fuel economy, and additive chemistries are scrambling to keep up wear protection without compromising efficiency gains.

Qu told LubesnGreases that his past research, done in collaboration with Shell Global Solutions in 2015, tested fuel economy performance of an ionic liquid and ZDDP combination in an experimental SAE 0W-16 engine oil using the ASTM D8114 Sequence VIE engine dynamometer test. The results were a 2.1 percent fuel economy improvement against an SAE 20W-30 reference oil.

In the current ongoing effort with GM and Driven Racing Oil, we decided to move forward with the even lower viscosity of SAE 0W-12 to seek further fuel economy benefits and test out the wear protection capability of the ionic liquid and ZDDP combo for a lower viscosity oil, and under more stressed operation conditions as offered by an engine dyno test designed for racing cars, Qu explained.

The results from this fuel economy measurement showed an improvement of 9.9 percent for SAE 0W-12 engine oils against an SAE 5W-30 reference oil. For two similar SAE 0W-12 oils containing the ionic liquid and ZDDP combo or the ZDDP alone at the same 800 ppm phosphorus content, the combo oil improved the fuel economy by 4.4 percent and reduced cam lobe wear by 40 percent compared with the ZDDP version, he noted.

What about ionic liquids effect on emissions? The journal article from Catalysts-of which Qu was one of the authors-detailed emissions tests on three-way catalytic converters, aged using ZDDP and ionic liquid additives.

The results showed that overall phosphorus content was 33 percent lower in the catalyst aged with ionic liquid compared to the ZDDP-aged catalyst. In addition to the improved fuel economy described above, an ionic liquid has demonstrated less adverse impact than ZDDP on the commonly used three-way catalyst for controlling exhaust emissions, said Qu.

A Pipe Dream?

BASF is one chemical manufacturer that has significantly invested in research and development of ionic liquids as additives for applications such as compressor fluids, but Matthias Fies, technical marketing manager for lube oil additives at the companys Fuel and Lubricant Solutions unit, said ionic liquids still have a number of hurdles to overcome before market acceptance.

There are different chemistries for ionic liquids and the cost of the raw materials influences the cost of the ionic liquids. Ionic liquids as additives for engine oil formulations compete with other chemistries and technologies, Fies told LubesnGreases.

Formulators will select components based on performance, price and availability. So far, we have not seen tremendous commercial interest in these materials despite the promising performance claims that have been published by others, he added.

In the automotive engine oil market, the hurdles might seem even bigger, since ZDDP is relatively inexpensive to manufacture and has decades-long, field-proven antiwear performance.

Developing any component as an antiwear agent and actually replacing ZDDP in a commercial application is an enormous undertaking that will take tens of millions of dollars in development costs alone, and I believe [it is] unlikely to happen without a mandate to remove phosphorus from the formulations, industry consultant Steve Haffner told LubesnGreases.

Ionic liquid interaction with other additives in the engine oil must also be considered. When introducing an ionic liquid and ZDDP combo to a fully formulated engine oil, compatibility with other polar compounds in the oil, specifically detergent, dispersant, antioxidant and friction modifiers, has to be taken into consideration to allow optimal performance, Qu stressed.

However, Iolitecs Schubert later said to LubesnGreases that the price of ionic fluids could be comparable to that of a more traditional additive such as ZDDP, but it will depend on the formulation and demand. But they may not have to compete on price alone. If the performance is the same and you can avoid the use of ZDDP, which amount is the customer willing to pay? Schubert mused.