23rd International Colloquium Tribology - January 2022 51 Liquid Amides - Novel, High Performance Base Oils Dr. Claire Ward Croda Europe Ltd., Goole, UK Corresponding author: claire.ward@croda.com 1. Introduction Synthetic esters are widely used within the lubricants industry. Their chemistries can easily be tuned to achieve a broad range of properties, making them beneficial in numerous industrial and automotive applications. However, in systems where there is the possibility of water contamination combined with elevated temperatures, certain esters can be prone to hydrolysis which compromises the performance and lifetime of the ester base fluid. Amides can be an attractive alternative for such appli-cation areas and Croda has developed a new synthetic base stock to fulfil this unmet need. Unlike primary and secondary amides, tertiary amides can be designed to remain liquid at low temperatures so they can be used as (co-)base fluids. This research exploits the benefits of tertiary liquid amides and shows that through carefully controlling the chemistry of the amide building blocks, a wealth of beneficial properties can be achieved. This paper discusses the properties and performance of one novel liquid amide derivative. 2. Outline This research showcases the benefits of the amide within fully formulated oils for a number of industrial and automotive applications, especially those where water ingress can be detrimental to performance. The amide is investigated as a neat base oil or a co-base oil at 2.5-10 % dose and the performance is compared to other Group V base oils, including commercially available esters base fluids. 3. Stability benefits of amides Base oil is the major constituent of lubricant formulations, so inherent hydrolytic and oxidative stability of this component can play an important role (alongside the additives) in improving the service life of the formulation in an application. To showcase these stability features, a steam turbine formulation was investigated as this application is exposed to the challenging conditions of a high temperature environment with water contamination and high-speed mixing from the rotating machinery. An ISO VG 46 formulation was made with several different base oils and a standard turbine & compressor oil add pack (0.6 % recommended dose) for polyalphaolefin (PAO) and mineral oil systems. The hydrolytic stability was tested using the beverage bottle method (ASTM D2619, Cu, H 2 O, 93 ºC) and an extended 240 hour experiment was completed as well as the standard 48 hour test [1]. Even over the extended time-period, the liquid amide only experienced a baseline change in acid value, comparable to the PAO system, while the di-ester base oil showed significant change in acid value due to hydrolysis (Figure 1). This is in line with the trends seen by the hydrolytic stability of the neat, untreated base oils in the SS 155181: 2012 test [2]. Figure 1: Steam turbine formulation hydrolytic stability Figure 2: Steam turbine formulation oxidative stability The oxidative stability of the steam turbine formulations was tested using the turbine oil oxidation stability test (TOST) method (ASTM D943, Cu & steel coils, H 2 O, O 2 , 95 ºC) [3]. Figure 2 shows the liquid amide system 52 23rd International Colloquium Tribology - January 2022 Liquid Amides - Novel, High Performance Base Oils had significantly better oxidative stability than the di-ester formulation, and gave comparable performance to the Group III mineral oil system, which reached the maximum 2.0 mg KOH/ g total acid number (TAN) around 10 000 h. The PAO formulation was the only system to exceed the 10 000 hours maximum test time with a low TAN (0.4 mg KOH/ g), but it is worth noting that the anti-oxidant package was specifically designed for PAO and mineral oils. 4. Amides as solvency boosters One of the main uses of the liquid amide is as a co-base oil due to enhanced polarity vs. comparable esters, giving it excellent solubility properties for polar additive packages in formulations. The liquid amide is compatible with a wide range of fuels and base oils from Group I, II, III, IV and V, but it is not water-soluble. This is important for a co-base oil as it facilitates the blending of stable formulations to meet the desired application physical properties as well as the target costs. Co-base oils are especially useful in severely hydrotreated base oils such as PAO and gas-to-liquid (GTL) because they only contain fully saturated, non-polar hydrocarbon species, which dramatically decreases the solvent power [4]. Figure 3 shows the room temperature (RT) stability of two PAO-based industrial formulations over 3 months. With no co-base oil, both additives are incompatible with the PAO fluid, showing separation or sedimentation. A typical polyol-ester can help to give a clear, stable formulation. However, the liquid amide co-base oil solubilises the additive packages more consistently and at a lower dose rate of 5 % to give clear, bright formulations [5]. Figure 3: PAO solvency booster 3 month study at RT Figure 4 shows a similar stability experiment looking at the solubility of a polymeric friction modifier in a GTL system. Without a co-base oil, the friction modifier is insoluble in GTL. This can be remedied with a co-base oil such as a polyol ester at 5 %, but the liquid amide shows superior solubility booster powers by achieving the same stabilising effect at a lower dose of 2.5 %. Figure 4: GTL solvency booster 3 month study at RT 5. Conclusion This paper examines a tertiary liquid amide which has been structurally designed for use as a novel, high performance base oil in industrial and automotive applications. The hydrolytic and oxidative stability benefits of the new liquid amide base oil could offer enhanced product performance and lifetime over some conventional esters in challenging high temperature environments with the potential for water ingress. The intrinsic differences in amide and ester polarity also expand the solubility properties of this new base oil, helping to create stable formulations with some challenging components. References [1] ASTM D2619-21, Standard Test Method for Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method), ASTM International, West Conshohocken, PA, 2021, www.astm.org [2] SS 155181: 2012, Petroleum and related products - Determination of the hydrolysis characteristics of oils and fluids - Hydrolytic stability test method, Swedish Institute for Standards, Stockholm, Sweden, 2021, www.sis.se [3] ASTM D943-20, Standard Test Method for Oxidation Characteristics of Inhibited Mineral Oils, ASTM International, West Conshohocken, PA, 2020, www.astm.org [4] Zhmud, B., Roegiers, M., “New base oils pose a challenge for solubility and lubricity”, 2009, 89, 21-24. [5] Chen, X., Kurchan, A., Shen, Z., 2015, “LUBRI- CATING OILS”, WO 2015/ 175778.