23rd International Colloquium Tribology - January 2022 169 Polymers as important additives in E-drive fluids Dmitriy Shakhvorostov New Mobility Manager, Oil Additives, Evonik Operations GmbH, Germany Corresponding author: dmitriy.shakhvorostov@evonik.com Stephan Wieber New Mobility Manager, Oil Additives, Evonik Operations GmbH, Germany Roland Wilkens New Mobility Manager, Oil Additives, Evonik Operations GmbH, Germany Andreas Hees New Mobility Manager, Oil Additives, Evonik Operations GmbH, Germany 1. Introduction The fully electrical or partially electrical (hybrid) power train for passenger vehicles is the currently preferred solution to achieve the GHE reduction targets (1-2). Both pure battery electrical and fuel cell electrical vehicles have very similar drive trains. Optimization of the latter is ongoing with the purpose of range extension. All components (mechanical transmission, electric motor, power electronics) need to achieve all together the maximum efficiency at given durability (1). The efficiency improvement can contribute to CO 2 reduction during use phase and also extend EV range. Alternatively, or in addition it can be used to reduce cost and CO 2 emissions during production (e.g. by means of battery size reduction). New concepts of the drive train combine all three elements - transmission, motor and power electronics in one housing. Such topology has the advantage of having no electrical interfaces and saving space and weight, as well as a better heat dissipation, since the lubricant is in direct contact with the motor Cu windings and electronic elements of the power electronics. The latter are desired to be dimensioned as small as possible and providing high specific power which requires very efficient cooling. The new lubricant has to withstand elevated tribological loads and provide more efficient heat transfer, while being compatible with all new materials in the system. In this paper we discuss how the alkyl methacrylate-based polymers can be useful for these seemingly mutually exclusive requirements. 1.1 Viscosity Since the introduction of the first CO 2 emission limits a trend to reduce the kinematic viscosity of the driveline lubricants could be seen (10 years ago was the kinematic viscosity at 100°C (KV100) 9-30 cSt, now 4-9 cSt (2) (3)). The reduction of KV100 occurs to reduce the operating viscosity during the certification cycle, which can be done alternatively/ additionally with a viscosity index improver (VII). The formulation with an VII can also provide specific rheological properties which are beneficial at high rpm or high temperature (for instance enabling sufficiently high viscosity to obtain a sufficiently stable lubricant film protecting from excessive wear, see Figure 1). Figure 1: Comparison of three formulations based on pure base stock, including conventional polyalkyl methacrylates (PAMA) and comb polymers. Viscosity index improvers are required to have sufficient shear stability and optimal thickening efficiency at low (-40…40°C, typically as low as possible) and at high (80…150°C, typically as high as possible) temperatures. For mineral oils it is also important to have good wax compatibility/ inhibition behavior. Modern comb polymers suffice these conditions quite well - they possess a polar back bone and relatively long side chains. The thickening mechanism is described by the solvation of the backbone and side chains. Both can be optimized to achieve a better combination of shear stability and thickening efficiency to formulate oils with far less temperature dependent viscosity or in other words with higher 170 23rd International Colloquium Tribology - January 2022 Polymers as important additives in E-drive fluids viscosity index (VI) as seen in Figure 1. The thickening of these polymers at low temperatures is quite insignificant and allows the use of relatively thick base stocks. The shear stability of the polymers defines the stability of the viscosity profile (optimized for the start of service) during the lifetime. Shear induces mechanical degradation of polymers and thickening becomes less, which resembles itself in the viscosity reduction of the formulation (see figure 2). The shear loss (viscosity reduction in %) is usually measured after tapered roller bearing in a 192 h test. With comb polymers it is possible to formulate oils with high VI and still pass severe requirements on shear loss, even with relatively high KV100 = 6 cSt, as seen in figure 2. The low temperature viscosity must be kept as low as possible for efficient oil pumping, efficient lubrication and cooling at starting conditions. This is realized at best with the synthetic base stocks and comb technology (see Figure 3). Figure 2: Comparison of shear stability of the formulations with different base stocks (API Gr III or polyalphaolefins-PAO) and corresponding VII technology. Comb polymers enable high VI formulations with sufficient shear stability. All formulations are defined at fresh KV100 = 6cSt and same Dispersant/ Inhibitor-additives. Figure 3: Comparison of Brookfield viscosity of the formulations (KV100 =6 cSt) with different base stocks (API Gr III or PAO) and VII technology. 1.2 Transmission Efficiency The mechanical transmission efficiency is dependent on several parameters, but specifically on the operation cycle and used lubricant. The newest requirements for the transmission efficiency are defined based on the Worldwide harmonized Light vehicles Test Procedure (WLTC). We have used an electrical highly integrated axle transmission with 150 kW power and maximum speed of 16000 rpm from a European manufacturer to study the influence of the operating viscosity or VI, VII type and the base stock composition. The formulation parameters are summarized in table 1. These represent the currently often used targets on kinematic viscosity and low temperature viscosity requirements. The transmission was operated at the characteristic torque and rpm combinations, replicating the WLTC cycle. In Figure 4 the final results on reduction of mechanical transmission losses are shown. In these results it can be seen the benefit of operating viscosity reduction/ high VI of the oil, the use of comb polymers, polymer functionalization and base oil type. Moreover, there is a benefit of using a higher viscosity grade of Gr III base stock in these formulations. Table 1. Example formulations with conventional polyalkylmethacrylates (PAMA) and comb polymers. All formulations have included same Dispersant/ Inhibitor-additives and exhibit similar KV100. Figure 4: Reduction of the mechanical losses in transmission upscaled to the WLTC operation mode with use of different formulations from table 1. Several parameters such as operating viscosity or VI, used base stock composition and polymer functionalization impact the efficiency. 23rd International Colloquium Tribology - January 2022 171 Polymers as important additives in E-drive fluids This has also an additional benefit of improved oil evaporation loss and flash point. Such a formulation strategy can only be followed if a polymer can provide very low thickening at low temperatures to suffice Brookfield requirements. Use of fully synthetic polyalphaolefin base stock compositions provide further significant losses reduction in the range of 20%. Figure 5: Electrical conductivity of the pure Gr III 4cSt base fluids treated with 10% of the different VIIs. The shown data represents the polymer contribution to the electrical conductivity of the formulation at nearly maximum treat rate. Values are shown as function of the kinematic viscosity, which varied by temperature increase from room temperature to 150°C. 1.3 Compatibility The challenge for the formulator is to provide a fluid with increased tribological performance and less aggressive behavior against materials such as elastomers, Cu and insulation materials as well as low electrical conductivity. The polyalkylmethacrylates and comb polymers are the ingredients, which can slightly increase the electrical conductivity of the base oil, especially if functionalized (Figure 5). This electrical conductivity increase is though not critical since the currently acceptable level of electrical conductivity is in the range of >10 nS/ m at 100°C. For these requirements the formulator has no limitation in the choice of the polymer for the desired functional properties in the formulation.The polymers have no negative impact on the Cu corrosion either as seen in table 2. Functionalized polymers can even provide a protective function as seen in the pure base stock API Gr III + 10% polymer blends. Table 2: Cu-corrosion rating after the ASTM D130, 180°C, 3h. Results show that polymers do not harm Cu, or even provide protective function if functionalized. 1.4 Wear and Durability The well-known property of E-motors is the availability of significant torque (>40 Nm) already at relatively low rpm (<800 rpm) compared to internal combustion engines. Figure 6: Results of the four-ball test ASTM D 4172 with API Gr III base stock and blends with 10% polymer. Two effects can be seen, the effect of operating viscosity and the effect of functionalization with F-PA- MA2 polymer. The latter delivers wear scar diameter comparable with formulations having full set of AW/ EP additives. This property is great for the efficient acceleration of the vehicle but brings additional care for the durability of the transmission due to reduced lubricating film thicknesses at above-described conditions. Functionalized PAMAs can help maintaining the sufficient lubricating film thickness at the desired lubricating conditions (which can be seen in Figure 6). In four ball test experiment it is recognizable that blends of 10% polymer in API Gr III oils can provide wear scars (0.6 mm) comparable with fully formulated fluids containing full set of AW/ EP additives. In addition to the wear in the four-ball apparatus one can see that pitting damage in both gear and bearing elements can be reduced and double the lifetime (see Figure 7). Figure 7: FZG PT C/ 9/ 90 FVA2/ IV and FE 8 pitting tests with 3 repetitions each show doubling of the lifetime with a formulation containing 6% of the functionalized F-PAMA2. Both formulations have KV100=6 cSt and full set of AW/ EP additives. 172 23rd International Colloquium Tribology - January 2022 Polymers as important additives in E-drive fluids 2. Conclusion New requirements to the compatibility of the lubricant with the electrical components force formulators to look for new ways to formulate fluids and use existing additives. Owing to the good no-harm performance of the polyalkylmethacrylates and comb polymers these polymers can be useful in several aspects: • Comb polymers enable the use of thicker base stocks in the formulation with severe requirements on shear stability and low temperature performance • Comb polymers and especially functionalized types are very suitable to improve efficiency of the electrical power train. • Functionalized PAMA and Comb can provide benefits in durability of gear and bearing elements. In addition, these can be useful for Cu protection. • Both linear PAMA and COMB structures are fully compatible with E-drive components. References [1] Bundesministerium für Umwelt, Naturschutz und nukleare Sicherheit. [Online] 01. September 2021. [Zitat vom: 02. 09 2021.] https: / / www.bmu.de/ themen/ klimaschutz-anpassung/ klimaschutz/ nationale-klimapolitik/ klimaschutzplan-2050. [2] UNECE Vehicle Regulations. [Online] https: / / unece.org/ transport/ vehicle-regulations. [3] Electric Axle Drive Module for high Speeds. Domian, Hans-Jörg, Ketteler, Karl-Hermann und Scharr, Stephan. Berlin: Springer, 2013, ATZ Worldwide, Bd. 13, S. 10-13. [4] Wincierz Christoph, Michael Mueller, Aidan Rose. Corporate experience of Evonik Oil Additives. [Befragte Person] Dmitriy Shakhvorostov. 6. 12 2011. [5] The Effect of Viscosity Index on the Efficiency of Transmission Lubricants. Vickerman, Richard J., Kevin Streck, Elizabeth Schiferl, and Ananda Gajanayake. 2010, SAE International Journal of Fuels and Lubricants, S. 20-26.