Grease for an electric future

In a quest to make lithium complex greases more sustainable and better suited for use in electric motors, Nynas teamed up with Solvay to study the response of some ionic liquids in lubricating greases.

Mehdi Fathi-Najafi is Chief Specialist at Technical Development and Support, Nynas Naphthenics. He joined Nynas in 2008.

When the hardware changes, the software needs to follow suit. Similarly, the electrification of vehicles heralds changes in the greases used to lubricate their engines.

“The requirements for lubrication in electric vehicles are significantly different from those in combustion engine vehicles. An electric vehicle running on conventional lubricants can lose as much as 30% of its efficiency due to poor lubrication,” says Nynas’ chief specialist Mehdi Fathi-Najafi.

“These new systems require new properties such as electrical conductivity, higher tolerance to temperature, longer life and potentially compatibility to new materials – it is a great opportunity to identify useful solutions” says Eamonn Conrad, Head of Business Development –Phosphorus, at Solvay.

Attempting to find suitable electrification-proof solutions, Nynas and Solvay decided to examine if the properties of a standard type of grease could be improved by new additives. For decades, increasingly advanced additive systems have been developed to maximise the efficiency of vehicles, for instance by reducing the friction between moving parts. The idea was to investigate whether improvements could be made that might benefit both electric and internal combustion engines, leading to greater efficiency and better fuel economy.

Commonly used to lubricate bearings in automotive and industrial applications, lithium complex greases were chosen as the starting point. As for additives, the focus settled on ionic liquids due to their established anti-wear and extreme pressure (AW/EP) performance and the additional requirements around conductivity and lifetime.

“Ionic liquids provide a low coefficient of friction and high wear resistance and have previously been used in neat solution to reduce friction between moving parts. But they can cause corrosive effects and for some applications their cost may prove prohibitive,” explains Mehdi Fathi-Najafi.

The unique conditions of the electric vehicle motor and the market drive to grow these vehicles have led to a situation where ionic liquids can be a viable solution.

In addition to their proven AW/EP properties, phosphonium ionic liquids demonstrate desirable properties such as high thermal stability and the ability to improve electrical conductivity. Importantly, Ionic liquids also meet criteria for electric vehicle engines, such as being metal-free.

“We have identified two promising additive candidates and continue to refine formulations to improve the overall efficiency of these systems. Based on our results so far, we believe that the addition of phosphonium salts has potential,” says Mehdi Fathi-Najafi.

“This work is critical for improving fuel efficiency and energy usage, and for enabling further electrification of motor vehicles,” he adds.

“It is an exciting time; the industry is forced to do something new due to the unique requirements of electrification, and at Solvay we are very fortunate to have a great partner like Nynas to investigate this field,” states Eamonn Conrad.

Promising findings

• Four different lithium complex greases were used in the study. Table 1 shows some of the characteristics of the neat lithium complex greases, in which Azelaic acid was used as complexing agent. Two of the base oils used, T 600 and S 90, are Nynas naphthenic oils.

• Two different ionic liquids were added to these greases – tetrabutylphosphonium bromide (IL3) and tetraoctylphosphonium bromide (IL4) – in concentrations of 5 wt%.

• Figure 1 demonstrates the impact of temperature and the two ionic liquids (IL3 and IL4) on the electrical conductivity. The testing program can be considered as a static test, yet some valuable information was still gained.

• The results shown in Figure 1 suggest that the electrical conductivity of model liquid complex greases can be increased (by 3 to 4 orders of magnitude) through the addition of as little as 5 wt% of the phosphonium ionic liquids used in this study.

• For tribological testing, several standard methods were employed, including ASTM D5706 and ASTM D5707 using SVR and ASTM D2266 and ASTM D2596 using a four-ball machine. Some of the results generated by SVR methods are shown in Table 2. These results reveal a significant reduction in wear and friction coefficient by IL3. However, the lowest wear volume and friction coefficient are achieved using 5 wt% of IL3 in any of the model greases.