23rd International Colloquium Tribology - January 2022 161 A Novel Class of Biobased Organic Friction Modifiers Revealing the Superlubricity Effect: Tribology and Application Experience in Motor Oil and e-Fluid Formulations Arthur Coen OLEON s.a., Compiègne, France Karima Zitouni OLEON s.a., Compiègne, France Ward Huybrechts OLEON s.a., Compiègne, France Philippe Blach OLEON s.a., Compiègne, France Anne-Elise Lescoffit OLEON s.a., Compiègne, France Boris Zhmud BIZOL Germany GmbH, Berlin, Germany Corresponding author: zhmud@bizol.de 1. Introduction Increased lubricant performance requirements drive a steady growth in the market share for synthetic lubricants. Combined with proper additives, synthetic base oils can be used to manufacture top-tier energy saving lubricants. One type of additives deserves special attention in this connection: friction modifiers that are an indispensable tool for “smart lubricant” engineering. In this study, the tribological properties of a novel biobased friction modifier comprising polymerized fatty acid glycerol ester (PFAGE) are described. PFAGE forms gel-like adsorption layers that contribute to surface repulsion. As a result, the additive reveals very interesting tribology at low sliding speeds. In particular, it has the ability to maintain full-film lubrication in a loaded contact in the zero sliding speed limit, the behaviour ass °Ciated with the superlubricity effect. This tribological behaviour is rationalized using a dynamic adsorption model. 2. Lubricant formulation experience Friction modifiers can roughly be grouped into two major categories: (1) particulate systems (PTFE, graphite, graphene, MoS 2 , WS 2 , IF-WS 2 , nanoboric acid, copper/ copper oleate nanoparticles, etc.); and (2) adsorption layer forming systems, which in their turn, can be monomolecular (glycerol mono-oleate, sorbitan mono-oleate, esters of hydroxycarboxylic acids, phosphate esters, borate esters, fatty acids, fatty amides, fatty amines, ethoxylated fatty amines, ammonium phosphate ionic liquids, etc.) and polymeric (methacrylates, polyesters, polyethers, polyamides, polymerized vegetable oils, etc.). The main advantage of particulate systems is their relatively high chemical stability, while their chief disadvantage being the propensity for separation. Particulate systems tend to render lubricant formulation opaque in appearance, which is not always acceptable. Adsorption layer forming systems are numerous: there are hundreds of commercial products on the market. Despite the many undisputed advantages over their API Group I mineral counterparts, modern “synthetic” base oils of API Group III and IV are characterized by low solvent power and may cause elastomer compatibility and miscibility issues [1]. This necessitates deployment of solubility improvers, such as esters and alkylated naphthalenes, in lubricant formulations. Organic friction modifiers (OFMs) are expected to have borderline solubility in order to achieve adequate affinity to metal surface while maintaining a certain bulk reserve of the additive. 162 23rd International Colloquium Tribology - January 2022 A Novel Class of Biobased Organic Friction Modifiers Revealing the Superlubricity Effect: Tribology and Application Experience Table 1: Examples of solubility improvers commonly used in lubricant formulations *Not soluble means that more than 20% solubility improver is needed Another important requirement is that OFMs will not interfere with EP/ AW additive action at high loads. Reactions of EP/ AW additives such as ZDDP are activated by combination of pressure and shear forces. Since OFMs are surface active, they may form compact adsorption layers that passivate EP/ AW additives. The chief difference between monomolecular and polymeric friction modifiers is the compactness of adsorbed layers. Whereas monomolecular OFMs tend to produce dense “brush-like” molecular layers, polymeric OFMs produce “gel-like” adsorption layers. These layers cause repulsion between the surfaces, contributing to the socalled “superlubricity” effect [2]. Surface-gel-forming OFMs are less likely to engage into competitive adsorption with EP/ AW additives. This allows one to develop formulations combining favorable antiwear properties with improved fuel economy. Another important practical consideration is the effect retention. As oil is ageing, friction modifiers may lose their effect. For instance, MoDTC-doped lubricants tend to quickly lose their efficiency due to oxidation. In contrast, PFAGE-doped oil reveals much better effect retention. This tendency can be demonstrated in a simple MTM test, see Fig. 1. (a) (b) Fig. 1: MTM test data comparing MoDTC and PFAGE in API Group III base oil after (a) 2 h ageing at 100 °C, and (b) 8 h ageing at 100 °C followed by additional 8 h at 130 °C. Two current standards, Sequences VIE and VIF (ASTM D8114 and D8226), commonly used to measure fuel economy performance of passenger car motor oil mandate that fuel economy at two different aging stages is determined: FEI1 after 16 hours (fresh oil) and FEI2 after 109 hours (aged oil) [3]. It is always important to balance formulations in order to exploit potential synergisms and minimize possible antagonistic effects between different additives. For instance, PFAGE efficiency proved to be strongly affected by common detergency additives present in commercial additive packages. 23rd International Colloquium Tribology - January 2022 163 A Novel Class of Biobased Organic Friction Modifiers Revealing the Superlubricity Effect: Tribology and Application Experience Fig. 2: MTM test data comparing MoDTC and PFAGE used as top-up additives in a commercial motor oil after 2 h ageing at 100 °C. In the presence of surface-gel-forming friction modifiers, lubricant films may reveal complex non-Newtonian rheology. Fig. 3 Rheology of lubricant films in the presence of surface-gel-forming OFMs. References [1] B. Zhmud, M. Roegiers, New Base Oils Pose a Challenge for Solubility and Lubricity. Tribology and Lubrication Technology 65 (2009) 34. [2] B. Zhmud, A. Coen, K. Zitouni, Fuel Economy Engine Oils: Scientific Rationale and Controversies, SAE Tech. Paper 2021-24-0067. [3] P. Lee, B. Zhmud, Low Friction Powertrains: Current Advances in Lubricants and Coating, Lubricants 9 (2021) 74.