24th International Colloquium Tribology - January 2024 53 SAPS-free Bio-based Additives for Lubrication in Next-generation Vehicles Xin He 1* , Christelle Chretien 1 1 Solvay USA Inc., Bristol, United States * Corresponding author: xin.he@solvay.com 1. Introduction All machinery parts consume frictional energy to overcome the resistance to motion due to sliding. This type of energy dissipation and wear caused an economic loss of about 2%-7% of its GDP for countries every year. [1] In order to save energy and be sustainable, lubricants and additives have been developed to minimize friction at the interfaces. Zinc dialkyldithiophosphate (ZDDP) is the most commonly used antiwear additive in boundary lubrication, offering exceptional wear prevention properties. However, ZDDP can present challenges in specific conditions. For instance, it generates deposits during decomposition and is corrosive to certain metals. To achieve sustainable design objectives, it becomes necessary to explore sulfated ash, phosphorus, and sulfur (SAPS)-free alternatives. SAPS-free additives have been proposed to achieve friction modification. Examples include fatty esters and amides, such as glycerol monooleate (GMO) or oleyl amides. However, these SAPS-free additives generally exhibit weaker antiwear properties compared to their metal, phosphorus, and/ or sulfur-based counterparts. In this study, we developed a new family of twin tailed amine derivatives, which not only provide excellent frictional properties but also exhibit antiwear performance. The newly developed additives are fully bio-based and show great potential in the field of engines and electric motors. 2. Methods and Experiments 2.1 Bio-based additives The additives were obtained through a two-step reaction. The initial intermediate was obtained from a decarboxylative ketonization reaction of corresponding fatty acids RCOOH. The twin-tailed intermediate R-(C=O)-R can be further functionalized with amine/ amide groups. The final product will show the following formula: Figure 1: Molecular structure of the SAPS-free bio-based additives Wherein each of R and R’, which are identical of different, is an aliphatic group preferably containing between 5 and 23 carbon atoms. 2.2 Formulation Seven bio-based additives were developed and investigated in this study, including five twin-tailed amine additives and two twin-tailed amides. Four commercial additives: primary-ZDDP, Glycerol monooleate (GMO), molybdenum dithiocarbamate (MoDTC), and oleyl amide were selected as a comparison. The additives were added to the Group III oil (SAE 20, KV @ 100-°C = 8 cSt) with 1 wt% treatment. The formulated oils were mixed at 60-°C for at least 30 min using a mechanical blender. 2.3 Tribology tests Two types of tribology tests were conducted to evaluate the lubrication performance of the newly developed additives. • High-Frequency Reciprocating Rig (HFRR): The friction coefficient was obtained using a ball-on-flat reciprocating configuration in the regime of boundary lubrication. A dead load of 200 g was applied on the ball and the stroke length was 1 mm. The frequency was set as 20 Hz, corresponding to a sliding speed of 20 mm/ s. The friction test was conducted at a constant 40 °C for 15 min and then ramped to 150 °C at 2 °C/ min. The entire process took 70 min. • Four-ball tribometer (ASTM D4172): The wear tests were conducted based on ASTM standard D4172. All candidate oils were studied at 75 °C under a 40 kg load. The instrument was running at a 1200 rpm speed and the test duration was 60 min. An optical microscope with scales was employed to estimate the diameter of the wear scars. 3. Results and Discussions Figure 2 illustrates a comparison of the friction traces between the top contenders (C2, C5, C6) and benchmark additives. Both commercial GMO and MoDTC additives demonstrated a consistent friction coefficient of approximately 0.10 within the designated temperature range. Notably, candidates C5 and C6 exhibited a relatively higher friction coefficient during the temperature ramp-up until reaching 90 °C, after which it dropped. This observation may be attributed to the inferior solubility of the twin-tailed amides in the Group III base oil. However, as the solubility concern became negligible at higher temperatures, the friction coefficients became equivalent to those of GMO and MoDTC starting at 120 °C. Table 1 provides a summary of the friction coefficients at 150 °C, revealing that candidates C5 and C6 achieved lower friction coefficients than GMO and oleyl amide. The remaining products demonstrated comparable or lower values when compared to primary ZDDP. 54 24th International Colloquium Tribology - January 2024 SAPS-free Bio-based Additives for Lubrication in Next-generation Vehicles Figure 2. Friction trace of the selected additives with the temperature ramping from 60 to 150 °C The wear results are presented in Table 1. The wear protection provided by GMO was found to be inferior, resulting in a scar diameter of 0.75 mm. This can be attributed to the ineffective formation of a tribofilm. In contrast, MoDTC, with its sulfur-containing tails, demonstrated quick generation of a protective film during the sliding process, resulting in a significantly lower wear scar of 0.42 mm. It was observed that all the newly developed additives exhibited wear scars ranging from 0.3 to 0.6 mm. Additionally, additives C5 and C6 displayed superior performance compared to all the comparative additives (Figure 3). The wear mechanism of amine or amide-based additives is presumed to involve a passivation effect facilitated by the functional groups. This passivation prevents extensive tribocorrosion at the interface, thereby reducing wear. Table 1: Summary of wear scar diameter (mm) and friction coefficient at 150 °C for the candidate additives and commercial products Tested compound Antiwear: scar diameter (mm) Friction test: coefficient of friction @-150-°C C1 0.53 0.176 C2 0.57 0.127 C3 0.58 0.135 C4 0.54 0.122 C5 0.34 0.085 C6 0.41 0.076 C7 - not tested - 0.171 GMO (COMPARATIVE) 0.75 0.094 Oleyl amide (COMPARATIVE) 0.55 0.105 MODTC (COMPARATIVE) 0.42 0.076 Figure 3. Wear scar diameter of the selected additives Although certain commercial additives have demonstrated excellent lubrication performance, they do not align with the sustainable development trend. Both ZDDP and MoD- TC have ash-related issues, making them unsuitable for various industries. On the other hand, the bio-based GMO additive possesses a hydrolysable nature that tends to degrade under harsh conditions. In contrast, newly developed additives show promising potential in several ways: • The twin-tailed amine/ amide additives are SAPS-free and bio-based • The top candidates showed an equivalent friction to GMO and MoDTC from 120 °C • The top candidates presented superior wear protection compared with ZDDP and MoDTC • The reductive nature of amine and amide group endow the potential low corrosion rate to metals, which expand the application to HEV/ EV field 4. Conclusion The SAPS-free bio-based technology appears to be an excellent alternative for conventional lubricating additives. The leading candidates exhibit remarkable friction and wear characteristics. Considering the growing emphasis on sustainable development and market demands, it is evident that this technology has the potential for broader utilization. Future research will investigate the anti-corrosion and thermal properties, enabling us to explore its potential application in the electric vehicle (EV) industry. References [1] Liu, H., Yang, B., Wang, C. et al. The mechanisms and applications of friction energy dissipation. Friction 11, 839-864 (2023). [2] Robbins, S. J., Nicholson, S. H. Long-term stability studies on stored glycerol monostearate (GMS)-effects of relative humidity. J Am Oil Chem Soc 64, 120-123 (1987).