23rd International Colloquium Tribology - January 2022 77 Improvement of tribological performances of MoDTC induced by methylene-bis(dithiocarbamates) in engine lubricants: Enhanced durability of MoDTC and their friction reducing capability under engine operating conditions Yu Min Kiw University of Strasbourg, CNRS, Institut de chimie de Strasbourg UMR 7177, F-67000 Strasbourg, France TotalEnergies Solaize Research Center, BP22-69360 Cedex, France Pierre Adam University of Strasbourg, CNRS, Institut de chimie de Strasbourg UMR 7177, F-67000 Strasbourg, France Philippe Schaeffer University of Strasbourg, CNRS, Institut de chimie de Strasbourg UMR 7177, F-67000 Strasbourg, France Corresponding author: p.schaef@unistra.fr Benoît Thiébaut University of Strasbourg, CNRS, Institut de chimie de Strasbourg UMR 7177, F-67000 Strasbourg, France Chantal Boyer University of Strasbourg, CNRS, Institut de chimie de Strasbourg UMR 7177, F-67000 Strasbourg, France 1. Introduction Energy saving, worldwide concerns over CO 2 emission and the introduction of new engine oil specifications are among the major driving forces to increase fuel economy in internal combustion engines [1-3]. One of the most general trends to improve the fuel efficiency is the use of molybdenum dithiocarbamates (MoDTC) (Figure 1a) as effective friction modifiers in lubricants since they lower significantly the friction coefficient at the tribological contacts under boundary lubrication conditions [4-7]. Despite their excellent efficiency in reducing friction coefficient to relatively low values, the tribological performances of MoDTC in engine oils still highly depends on the nature and chemistry of the other additives present in the formulated oils. The occurrence of synergistic or antagonistic interactions between MoDTC and other lubricant additives plays an important role in minimizing the friction in automobile engines. In this context, our work is dedicated to investigating the impact of methylene-bis(dithiocarbamates) (MBDTC) (Figure 1b) as a lubricant additive on the service lifetime of MoDTC in formulated lubricants. For this purpose, comparative engine tests were performed using the same formulated engine oils containing MoDTC without and with the addition of MBDTC (referred to as oil A and oil A MBDTC , respectively). Oil performances were compared in terms of tribological properties, duration of friction reducing capacity of the lubricants, evolution of fuel consumption of the engines and evolution of the concentrations of MoDTC as a function of time. In addition, laboratory oil ageing experiments were carried out under thermal (non-oxidizing) and thermo-oxidative conditions to study in more details the interactions between MoDTC and MBDTC at the molecular level. The evolution of MoDTC and of their transformation products was followed by HPLC-MS, NMR and Probe-MS analyses. Focus has been put on the evaluation of the effect of zinc dithiophosphates (ZnDTP) (Figure 1c) and oxidative conditions on the interactions between MoDTC and MBDTC. A comprehensive understanding of the underlying chemical transformation of MoDTC and the effects on the tribological performances of lubricants is expected to offer new strategies for molybdenum tribochemistry optimization in real engine systems. Figure 1: Chemical structures of (a) molybdenum dithiocarbamate (MoDTC); (b) methylene-bis(dithiocarbamates) (MBDTC); (c) Zinc dithiophosphate (ZnDTP) R: alkyl chains 78 23rd International Colloquium Tribology - January 2022 Improvement of tribological performances of MoDTC induced by methylene-bis(dithiocarbamates) in engine lubricants 2. Results and Discussion Based on engine tests performed using a formulated engine oil containing MoDTC and MBDTC and, for comparison, the same formulated engine oil devoid of MBDTC, our study has shown that the combined use of MoDTC and MBDTC results in an enhanced preservation of fuel efficiency with ageing time. This was most likely related to the interactions between MoDTC and MBDTC which has a beneficial effect on the functional lifetime of MoDTC in lubricants, allowing the friction coefficient of engine lubricants to be maintained at lower level for longer periods of time and, as a consequence, fuel consumption to be reduced. Indeed, MoDTC could be detected, using a specifically developed HPLC-MS method, in the formulated lubricant containing MBDTC over a longer period of engine test. Since the remaining MoDTC after prolonged engine functioning were shown to exclusively bear ligands corresponding to DTC moieties from MBDTC, it can be assumed that the prolonged existence of MoDTC was due to the progressive replacement of the degraded DTC ligands from MoDTC educts by DTC ligands released from MBDTC during engine functioning (Figure 2). These newly formed MoDTC complexes were progressively consumed (as was the case for the genuine MoDTC species), but they were shown to be concomitantly regenerated and remained in the formulated engine oil at concentrations high enough to ensure the reduction of the friction coefficient to lower levels for an extended duration, as indicated by the tribological measurements. For the MoD- TC educts progressively degraded upon engine functioning, MBDTC thus represents a “stock” of DTC ligands which can be released during engine functioning to regenerate new MoDTC species, leading to better tribological performance of the engine oil. Figure 2: Ligand transfer reactions between MoDTC and MBDTC; R1, R2: alkyl chains We completed these studies by investigating in detail, the interactions between MoDTC and MBDTC at the molecular level in the presence of other additives using laboratory oil ageing experiments under thermal (non-oxidizing) and thermo-oxidative conditions (NO 2 in air). It could be shown that the reactions between MoDTC and MBDTC appeared to be catalyzed by ZnDTP and oxidative conditions (NO 2 in air). The Lewis acid properties of Zn(II) complexes as well as the capacity of Zn(II) to activate dithioacetals have been previously reported in other contexts [8,9]. Therefore, in this study, Zn(II) ions are likely to act as Lewis acids to activate the cleavage of C-S bonds from MBDTC, thereby facilitating the release of dithiocarbamate ligands from MBDTC and thus favouring ligand transfer from MBDTC to MoDTC. Besides, oxidative condition (NO 2 in air) was also shown to induce the release of dithiocarbamate ligands from MB- DTC, even in the absence of ZnDTP or Zn(II) ions. 3. Conclusion The extension of friction reducing properties of engine oils for longer periods of time plays an important role in energy saving and in coping with global environmental problems. The control of thermo-oxidative degradation of Mo-based friction modifiers upon engine functioning remains a key challenge to achieve this objective. In this context, we showed that the interactions between MoDTC and MBDTC has a beneficial effect on the functional lifetime of MoDTC in engine lubricants and on the persistence of their tribological properties resulting in an enhanced preservation of fuel efficiency. This effect has been shown to be associated with DTC ligand transfer reactions between MBDTC and MoDTC and it was assumed that the DTC ligands from MBDTC were progressively transferred to the metal core of MoDTC after oxidative degradation of their ligands. As a result, the concentrations of MoDTC could be preserved at a useful level over extended ageing periods, thus maintaining their friction reducing properties in engine oil. MBDTC thus represents potentially a “stock” of DTC ligands which can be progressively released during engine functioning and can replace the degraded ligands from MoDTC educts under thermo-oxidative conditions following Zn(II) catalyzed and/ or oxidative processes. The results obtained provide insight for future additive design by considering similar delayed ligand transfer mechanisms demonstrated by MB- DTC aimed at extending the functional lifetime of MoDTC. 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