24th International Colloquium Tribology - January 2024 55 Biobased Ionic Liquid for Conductive Lubricants Pieter Struelens 1* , Yen Yee Chong 2 , Micky Lee 2 1 OLEON NV, Evergem, Belgium 2 OLEON Port Klang, Sdn Bhd, Selangor, Malaysia * Corresponding author: pieter.struelens@oleon.com 1. Introduction In the last years, ionic liquids have gained significant attention as promising lubricant additives due to their unique physicochemical properties. Despite the advantages that ionic liquids bring in terms of performance, ionic liquids are commonly linked to having relatively high toxicity and low base stock solubility, limiting their applications. In this study, a novel, sulphurand halogen-free ionic liquid was developed from renewable materials and carefully designed to possess a tailored molecular structure, enabling enhanced solubility and anti-wear characteristics. The conductivity characteristic of the biobased ionic liquid was also investigated to assess its suitability for applications that require electrostatic discharge protection. 1.1 Properties of ionic liquids The properties of the developed product (labelled as L795) are presented in Table 1, and so are those of two benchmarks (labelled as Benchmark 1 and Benchmark 2). The benchmarks selected are marketed phosphonium ionic liquids that have been actively investigated as lubricant additives, where Benchmark 1 is halogen-containing and Benchmark 2 is halogen-free. While the benchmarks are mineral based, L795 can be produced 100 % biobased. Table 1: Properties of ionic liquids Ionic liquid Chemistry Viscosity @ 25-°C, cSt Biobased carbon content (%) L795 Halogen & Phosphonium free 1498 100 Benchmark 1 Phosphonium (contains halogen) 270 Mineral-based Benchmark 2 Phosphonium (halogen-free) 1230 Mineral-based 2. Results and Discussion 2.1 Solubility of ionic liquids To evaluate the solubility and stability of the ionic liquids in base oils (Ester A, PAO, and GIII base oil), 1 wt.% of each ionic liquid was mixed with the base oils and stirred at room temperature for 30 mins. Then, the mixtures were centrifuged at 40 °C, 15000 rpm, 45 mins. Results presented in Table 2 show that L795 and Benchmark 2 were soluble and stable in all the base oils studied. On the other hand, Benchmark 1 was insoluble in PAO and GIII base oil. Based on this observation, two remarks that can be made are: i) Even though both are phosphorus-based, Benchmarks 1-and 2 have different solubility profiles. ii) L795 is as robust as Benchmark 2 in respect of solubility in different groups of base oils. As Benchmark 1 was not soluble in both PAO and GIII base oils, these mixtures were not selected for further evaluation. Table 2: Solubility results of ionic liquids in base oils L795 Benchmark 1 Benchmark 2 Ester A (ISO VG 100) clear clear clear PAO (ISO VG 100) clear cloudy clear GIII (ISO VG 32) clear cloudy clear 2.2 Wear reduction performance of ionic liquids The wear reduction performance of the ionic liquids was evaluated based on ASTM D4172 by comparing the average diameter of the wear scars generated. Figure 1: Average wear scars As depicted in Figure 1, L795 has significantly better performance than Benchmark 2 in reducing wear of PAO and GIII base oil. The addition of 1 wt.% L795 reduced the wear of PAO and GIII base oil by 42.6% and 27.5%, respectively. Although Benchmark 2 performed better than L795 in Ester A, higher wear scars were generated in PAO and GIII base 56 24th International Colloquium Tribology - January 2024 Biobased Ionic Liquid for Conductive Lubricants oil, which limits the feasibility of Benchmark 2 to be used in these base oils. This observation shows that L795 is a more versatile ionic liquid that can be used as anti-wear additives in different base stocks. 2.3 Copper corrosion test results Copper has become an essential metal in automotives as high-performance copper alloy material for foil (battery), winding wires (electric motor), etc. The use of copper is also common in electric vehicles. Hence, the compatibility of lubricants with copper is critical. A modified ASTM D130 was used in this study with testing conditions of 100-°C for 24-h [1,2]. Figure 2: Copper corrosion test in Ester A with 1 wt.% of ionic liquids A study carried out by Yu, Q et. al. (2020) showed that halogen-containing ionic liquids can cause the copper strips to corrode [2]. Nevertheless, such phenomena were not observed in the current study with Benchmark 1 in Ester A (Figure 2). This may be due to the absence of water in the test to hydrolyse the ionic liquid, which can release halogenic gas that corrodes the copper strip. Figure 3: Copper corrosion test in PAO base oil with 1-wt.% of ionic liquids Figure 4: Copper corrosion test in GIII base oil with 1 wt.% of ionic liquids On the contrary, anticorrosion property of L795 and Benchmark 2 was observed in all base oils (Figure 2 - 4). While Ester A, PAO, and GIII base oil created dark tarnish on the copper strips, both L795 and Benchmark 2 reduced the tarnishing of the copper strips. 2.4 Resistivity results The study on ionic liquids to be used in conductive lubricants have captured the interest of many and are being actively investigated for applications that require electrostatic discharge protection. It is worth noting that the targeted resistivity varies for different applications and may not always be beneficial to have a very conductive lubricant (e.g., leak charge). In this study, the resistivity reduction performance of the ionic liquids in base oils were measured based on ASTM D1169 at 25 °C, where the results are tabulated in Table 3. Table 3: Resistivity results of base oils with ionic liquids in GΩ.cm Base oil only L795 Benchmark 1 Benchmark 2 Ester A (ISO VG 100) 1000 60 0.7 20 PAO (ISO VG 100) 6000 70 Not tested (insoluble) 700 GIII (ISO VG 32) 2000 20 Not tested (insoluble) 100 In terms of resistivity reduction, Benchmark 1 was able to reduce the resistivity of Ester A by 4 magnitudes of order. As for the two halogen-free ionic liquids tested, L795 managed to reduce the resistivity of PAO 10 times more than Benchmark 2 and of GIII, 5 times more. As PAO and GIII are conventional base oils used in lubricants, this makes L795 a more versatile ionic liquid to be used in the industry. 3. Conclusion Based on the tests carried out, it can be concluded that compared to market benchmarks, the developed product (L795) shows the best overall performance and versatility with regard to solubility, wear reduction, anticorrosion property and resistivity reduction. Future work will be carried out to investigate the tunability of the performances of base oils with L795 at various dosages. References [1] Fang, Hongling et al. 2021. ‘Lubricating Performances of Oil-Miscible Trialkylanmmonium Carboxylate Ionic Liquids as Additives in PAO at Room and Low Temperatures’. Applied Surface Science 568: 150922. [2] Yu, Qiangliang et al. 2020. ‘Physicochemical and Tribological Properties of Gemini-Type Halogen-Free Dicationic Ionic Liquids’. Friction 9.