ABSTRACT
During engine tests, it has been observed that the combined use of molybdenum dithiocarbamates (MoDTC) and methylene-bis(dithiocarbamates) (MBDTC) in formulated engine oils resulted in better fuel efficiency, keeping the friction coefficient stable at low values for a longer period of time as compared to the same oil devoid of MBDTC. Therefore, the interactions between MBDTC and MoDTC have been investigated at the molecular level. The qualitative and quantitative evolution of MoDTC in two engine oils similarly formulated, but with and without MBDTC, were compared during engine tests using a specifically developed high performance liquid chromatography-mass spectrometry (HPLC-MS) analytical method. Parallel to the molecular study, the evolution of the friction coefficients of both lubricants as well as the evolution of the fuel consumption of the engine were determined. The combined use of MoDTC and MBDTC was shown to exhibit better fuel efficiency and to maintain a relatively low friction coefficient for longer periods of time as compared to the oil devoid of MBDTC. It could be determined that the enhanced performances observed were presumably related to an extension of the lifetime of MoDTC in the engine oil containing MBDTC. Since the MoDTC remaining at the end of the engine test in oil containing MBDTC exclusively bear ligands corresponding to the dithiocarbamate moieties of MBDTC, it can be concluded that the prolonged existence of MoDTC was due to the progressive replacement of the degraded dithiocarbamate ligands on MoDTC educts by those released from MBDTC during engine functioning. As a result, the concentrations of MoDTC could be maintained at a useful level for a longer period in the engine oil containing MBDTC, leading to better fuel consumption performances.
ABSTRACT
Molybdenum dithiocarbamates (MoDTC) are widely used in automotive industries as lubricant additives to reduce friction and to enhance fuel economy. Sulfur-containing additives such as zinc dithiophosphates (ZnDTP) are proposed to play a key role in the improvement of friction reducing properties of MoDTC in formulated lubricants by facilitating the formation of MoS2 tribofilm at the rubbing contacts. This study focuses on the interactions between MoDTC and ZnDTP under conditions comparable with those prevailing in operating engines. The capacity of ZnDTP to sulfurize MoDTC in solution in a hydrocarbon base oil could be demonstrated. Sulfurized Mo complexes bearing one or two additional sulfur atoms (1S-MoDTC and 2S-MoDTC, respectively) which have replaced the genuine oxygen atom(s) from the MoDTC core were detected and quantified using a specifically developed HPLC-MS analytical method. A possible sulfurization mechanism relying on the higher affinity of phosphorus from ZnDTP for oxygen could be proposed. In parallel, the evolution and molecular transformation of the prepared 2S-MoDTC in hydrocarbon base oil under thermal and thermo-oxidative conditions were followed using HPLC-MS and compared with the evolution of their friction coefficients. 2S-MoDTC complexes were shown to exhibit a better retention of friction reducing capability under oxidative conditions than the "classical" MoDTC, although they did not seem to significantly reduce the friction coefficients of lubricants as compared to the "classical" MoDTC. Therefore, sulfurization of MoDTC by ZnDTP might contribute to delaying the progressive consumption of MoDTC and the loss of their friction-reducing efficiency in lubricants under thermo-oxidative conditions.
ABSTRACT
Molybdenum dithiocarbamates (MoDTCs) are a class of lubricant additives widely employed in automotives. Most of the studies concerning MoDTC take into account the dimeric structures because of their industrial relevance, with the mononuclear compounds usually neglected, because isolating and characterizing subgroups of MoDTC molecules are generally difficult. However, the byproducts of the synthesis of MoDTC can impact the friction reduction performance at metallic interfaces, and the effect of mononuclear MoDTC (mMoDTC) compounds in the lubrication has not been considered yet in the literature. In this study, we consider for the first time the impurities of MoDTC consisting of mononuclear compounds and combine experimental and computational techniques to elucidate the interaction of these impurities with binuclear MoDTC in commercial formulations. We present a preliminary strategy to separate a commercial MoDTC product in chemically different fractions. These fractions present different tribological behaviors depending on the relative amount of mononuclear and binuclear complexes. The calculations indicate that the dissociation mechanism of mMoDTC is similar to the one observed for the dimeric structures. However, the different chemical properties of mMoDTC impact the kinetics for the formation of the beneficial molybdenum disulfide (MoS2) layers, as shown by the tribological experiments. These results help to understand the functionality of MoDTC lubricant additives, providing new insights into the complex synergy between the different chemical structures.
ABSTRACT
The tribological performances of engine oils have been shown to be enhanced by the synergistic interactions between Mo dithiocarbamates (Mo(DTC)2) with other additives, and notably Zn dithiophosphates (Zn(DTP)2). Being two key components in formulated lubricants, a detailed understanding of the mechanisms involved between these two types of additives is needed to develop engine oils with enhanced friction reduction performances, and improved fuel economy. In this context, we report here the investigation at the molecular level of the interactions between Mo and Zn complexes with DTC and DTP ligands using laboratory experiments. Our analytical approach comprised NMR spectroscopy (1H, 13C, 31P) allowing direct investigation of both homoleptic and heteroleptic Mo and Zn complexes as well as a specifically-developed HPLC-MS method for the investigation of the different DTC species formed during lubricant ageing experiments. The results showed that ligand exchange reactions between Mo(DTP)2 and Zn(DTC)2 complexes strongly favor the migration of the DTC ligands from Zn to Mo, illustrating the higher affinity of Mo for DTC ligands. In the case of binary mixtures involving Mo(DTC)2 and Zn(DTP)2 - a combination of additives frequently used in formulated lubricants - the formation of mixed complexes (Mo(DTC)(DTP)) resulting from ligand exchange reactions could be directly evidenced for the first time by the analytical methods used. These species could account, at least to some extent, for the synergistic effect of Mo(DTC)2 and Zn(DTP)2 on the friction reducing properties of engine oils. However, they were formed in significantly lower proportions than those previously reported in the literature using indirect methods.
ABSTRACT
Molybdenum dithiocarbamate (MoDTC) is a well-known lubricant additive, which, in tribological conditions, is capable of forming layers of MoS2 with excellent friction reduction properties. Despite being widely employed in commercial engine oils, a comprehensive theoretical description of the properties of MoDTC is still lacking. In this work, we employ density functional theory to study the structural, electronic, and vibrational properties of MoDTC. We investigate the relative stability of different isomers, different hydrocarbon terminations, and oxidized complexes. Oxidation was found to be energetically favorable for a wide range of conditions, and the most favorable position for oxygen atoms in MoDTC turned out to be the ligand position. These results, along with the calculated reaction energies for different dissociation paths, can be useful to better identify the elementary steps of the decomposition process of MoDTC.
ABSTRACT
This study investigated the impact of the oxidation process on the concentration of polycyclic aromatic hydrocarbons (PAH) in blown bitumen and identified some key contributing parameters. The U.S. Environmental Protection Agency's PAH list was used for this study. PAHs are considered a good toxicological marker, and measurement of PAHs in bitumen can be performed easily. The results of PAH content in blown bitumen and the corresponding feedstock was determined from the limit of detection up to 120 mg/kg for 24 samples. Compared to PAH levels in coal tar pitch, PAH levels in bitumen are very low. Measurements were performed by three laboratories using different methods to allow robust conclusions. The results highlight the difficulties in measuring PAHs in bitumen with accuracy for values below 30 mg/kg; therefore the discussion is based on summary statistics by adding concentrations of PAHs with common ring sizes. Incorporation of flux oil in the feed of the blowing bitumen unit tends to increase PAH content in feed stock and in blown bitumen, particularly the 4- to 6-ring PAHs, which are the most carcinogenic as identified by an animal skin painting test. The amount of PAH content from blown bitumen with flux oil can be at least three times higher than the amount in blown bitumen without flux oil, depending on the quality and quantity of the flux oil used. This study shows that the blowing process does not produce PAHs in bitumen. Conversely, it appears to reduce them in the final product. Close to 10 to 30% of PAHs are probably stripped from the liquid phase of bitumen during the blowing operation.
