Additives ZDDP

Solubilization of oxidation products, e.g., organic acids (HA) and some additives (ZDDP) by soft-core reverse micelles and inorganic acids (HS) by hardcore reverse micelles in oil formulations as shown in Fig. 1.3. 

The remaining useful life evaluation routine (RULER) is a useful monitoring program for used engine oils. The RULER system is based on a voltammetric method (Jefferies and Ameye, 1997 Kauffman, 1989 and 1994). The data allows the user to monitor the depletation of two additives ZDDP and the phenol/amineH+ antioxidant. The RULER results were compared to other standard analytical techniques, differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), total base number (TBN), total acid number (TAN), and viscosity to determine any correlation between the techniques (Jefferies and Ameye, 1997 and 1998). The test concluded that the RULER instrument can... 

Thin-layer chromatography (TLC) has been used for estimating the remaining useful life of engine oil including depletion of the multifunctional additive ZDDP. It was found that the soot from the oil did not interfere with the spot intensity since these particles did not travel with the mobile phase. The TLC technique has the potential to be a good supporting technique for estimation of chemical additives depletion (Brook et al., 1975 Coates, 1971). This technique is comparatively easy, very cheap, does not need sophisticated and expensive instruments and takes less than an hour. 

After a short period of use in the average engine, changes start to occur. Initially, a loss of the zinc based antiwear/antioxidant additive ZDDP is observed by negative absorptions at 1000 cm 1 and 715 cm 1. Oxidative degradation of oil follows soon after and this is observed by positive absorptions, represented by carbonyl, hydroxy, nitro and C-O- species. The ER spectroscopy of lubricants can reflect additive depletion and the formation of oxidation products (Coates and Setti, 1984 Coates etal., 1984). 

Sensitivity of exhaust - after treatment devices. It is clear that excessive deposition of phosphorus and sulfur on the catalyst can cause the reduction in system efficiency. Oil phosphorus contaminant comes from the oil additive ZDDP. The reduction in its use adversely affects both antiwear and antioxidation performance. Sulfur comes from the base oil, antiwear additives, detergents, organomolybdenum friction modifiers, and from the fuel. There is strong pressure from OEMs to reduce the sulfur level of the fuel, and to reduce the sulfur contamination of the catalyst, which results from presence of sulfur in oil. 

The authors then checked as to whether tribochemical reactions really existed. which would provide evidence of any friction-induced chemical transformations of the chemisorbed species. Zinc and molybdenum dithiophosphute additives (ZDDP and MoDDP) were chemisorbed onto steel plates prior to the friction test, by a conventional immersion procedure in an additive-containing oil. Two immersion times (2 and 24 h) and one immersion time (5 h) were cho-.sen, respectively, for the ZDDP and MoDDP additives. The chemistry of the treated steel surfaces was investigated by XPS/AES and the friction test was carried out in UHV just after the analysis. At the end of the test, in situ post mortem AES microanalysis was performed both inside and outside the wear scars on both the pin and the flat. Unfortunately XPS microanalysis was not possible with the current equipment, so that complete characterization of chemical bonding could not be achieved. 

In this paper the effects of eliminating the friction modifier and friction modifier plus anti-wear additive ZDDP from the additive package of fully formulated lubricants on friction, wear and wear film forming characteristics are examined. Tests have been conducted under lubricated wear conditions at relatively low (20°C and 50°C) and elevated (up to 100°C) bulk oil temperatures using a reciprocating pin-on-plate tribometer. The wear film has been examined by Energy Dispersive X-Ray analysis (EDX) and X-Ray Photoelectron Spectroscopy (XPS). 

The S and P species for both additives ZDDP and MoDTP, no changes in peak position were detected on the XPS analysis. 

Eleven zinc dialkyldithiophosphates (ZDDPs) in lubricating oil additives were separated by NPLC [723] eight ZDDPs were separated on an ODS column... 

Zinc dialkyldithiophosphates (ZDDPs), which act as antiwear additives in lubricating oils and were postulated to exist in various molecular forms (monomer, dimer or neutral form, and basic form), were studied by multi-edge (Zn K-, P K- and S K-) XAS for structural assessment [311]. Grazing incidence absorption spectroscopy measurements have provided evidence for breakdown of the ZDDP molecule following its adsorption on to a steel substrate surface [312]. XANES and CEMS were used to study the interaction of per-fluoropolyalkyl ether (PFPAE) additives with Fe-based alloys [313],... 

Reactive FFs can only be applied to a few specific cases for which they have been developed, such as the hydrocarbon systems discussed in the first part of this section. For other systems, describing tribochemical reactions requires the use of quantum chemical methods. In recent studies, such methods have been applied to investigate the behavior of zinc phosphates (ZPs) in response to high pressures. ZPs form the basis of anti-wear films derived from zinc dialkyldithiophosphates (ZDDPs), which are additives that have... 

It was also found that cross-linking, which occurs at pressures accessible on aluminum, causes the films to become harder than aluminum. Thus, on aluminum surfaces, one could expect the films to act as abrasives that will induce wear, as has been observed in sliding experiments. The authors of the study suggested that the inability of ZDDP additives to protect aluminum surfaces from wear may be caused by the pressure-induced stiffening of the film. 

Overall, this work highlights how quantum chemical methods can be used to study tribochemical reactions within chemically complex lubricant systems. The results shed light on processes that are responsible for the conversion of loosely connected ZP molecules derived from anti-wear additives into stiff, highly connected anti-wear films, which is consistent with experiments. Additionally, the results explain why these films inhibit wear of hard surfaces, such as iron, yet do not protect soft surface such as aluminum. The simulations also explained a large number of other experimental observations pertaining to ZDDP anti-wear films and additives.103 Perhaps most importantly, the simulations demonstrate the importance of cross-linking within the films, which may aid in the development of new anti-wear additives. 

Lett. 24, 105 (2006). Interpretation of Experiments on ZDDP Anti-Wear Additives and Films through Pressure-Induced Cross Linking. 

Fig. 1.4. The cycle of tribochemical film formations during the tribological mild, more severe, and very severe wear conditions initiated by thermooxidative decomposition of the ZDDP additive in the steel-on-steel combination (not to scale)...

Zinc dialkyldithiophosphates (ZDDPs) function mainly as antioxidants and antiwear additives. Molecules of ZDDPs adsorb on metal surface to participate in surface tribofilm formation under conditions of boundary lubrication. The solid tribofilms are formed at the metal surface to protect even under conditions of coarse contact under load (Bom et al., 1992). 

The use of carbonate-benzenesulfonate hard-core RM additive drastically changes the build-up mechanisms and the resulting structure of the antiwear surface film. Considering these results, the main difference between the antiwear action of the ZDDP molecules and the hard-core RMs is clear. In the case of... 

ZDDP, the antiwear film formation requires that a chemical reaction occurs between the additive and the metallic surface. In the case of hard-core RMs, the mineral material CaC03 is directly introduced in the sliding contact and undergoes small physicochemical changes during the film build-up. Consequently no chemical reaction with the substrate surfaces is required. 

The crown-ether compounds as boundary lubricants and antioxidation additives. On the sliding surface, bromobenzo-15-crown-5 coordinates with ferrous ions and forms a strong reaction layer which protects the underlying metal surface. In the base stock solution, the crown ring can capture the metal ions which catalyze the oxidation of oil formulation (Brois and Gutierrez, 1987, 1989, 1992 and 1994 Le Suer and Norman, 1965 and 1966 Moreton, 1998). Bromobenzo-15-crown has excellent antiwear, antifriction and antioxidation properties, better than the ZDDP tested. 

Mixtures of metallic detergents, such as phenates, sulfonates, phosphonates, and salicylates with ashless dispersants such as succinimides and benzylamine, together with zinc dialkyldithiophosphate (ZDDP), can lead to new effects. The possible interactions between these main additives used in lubricating formulations when dissolved/dispersed in hydrocarbon media are shown in Fig. 2.8 together with an indication of the intensity of those respective interactions. 

Detergent-dispersant interactions at surfaces. In 4-ball wear tests, an ashless dispersant was found to have an adverse effect on ZDDP-sulfonate-carbonate hardcore RM additives. A high molecular weight Schiff base had the worst effect, followed by a bis-PIBS m-PIBS had the least adverse effect. Interactions among additives affects valve train wear. One of the effects is that a succinimide together with other additives increases the decomposition temperature of ZDDP (Ramakamur, 1994 Shirahama and Hirata, 1989). 

A multifunctional additive of carbonate-detergents RMs retarded the decomposition of ZDDP in the ISOT test (Yamada et al., 1992). Mixtures of ZDDP plus carbonate-detergents RMs additive have been reported to have synergistic effects on detergency, see Chapter 3.3 on tribochemical interactions of hard-core RMs and ZDDP (Inoue, 1993 Ramakumar et al., 1994 Willermet, 1995a and 1995b Yin et al., 1997)... 

ZDDP is known to interact with most other additives employed in these formulations e.g., ZDDP is solubilized by soft-core and hard-core micelles and has been considered for reduced antiwear performance (Inoue, 1993 Kapsa et al., 1981 Rounds, 1981 Shiomi etal., 1986 Willermet 1995a and 1995b Yin et al., 1997). Based on the tribofilm formation (polyphosphate) and the presence (or absence) of unchanged ZDDP in the film, we can conclude that the additives compete with the adsorption of ZDDP on the surface (Varlot et al., 2000 Yin et al., 1997a and 1997b). 

The antiwear and antioxidant additive, zinc dialkyldithiophosphate, is a key ingredient in the great majority of engine oil formulations, and other lubricant applications such as hydraulic fluids and gear oils. The ZDDP-derived tribochemical films have been studied by a number of laboratories, but their mode... 

The prediction of antiwear benefits and optimization of the ZDDP dosage has become a very complex task. The chemistry of the mode of action of ZDDP itself is complicated and so the nature of its interactions with other additives needs investigating all the more. Interaction between ZDDP and fatty acids, again in lubricating oil formulations, shows a considerable amount of mechanical test wear. The antiwear property of ZDDP is reduced by fatty acid additive due to the adsorbed layer of fatty acid, and the solubilization process, which disturbs the function of ZDDP (Otsubo, 1975), see Fig. 2.11. 

The reactivity of the lubricating oil ZDDP additives was investigated by molecular orbital techniques (Armstrong et al., 1998). Semi-empirical quantum chemistry methods were used to model the structures of some of the complexes... 

In order to mimic the attack of ZDDP onto the oxide surface (FeO), the structure of the possible complexes formed between an O2 ion and ZDDP was examined. The oxide anion was allowed to interact with the positively charged atoms (zinc and phosphorus), and partially negatively charged sulfur atoms of the additive molecule. The heats of complex formation (Oxide ion + ZDDP -ZDDP Oxide) and total energies determined for each complex were reported (Armstrong et al., 1998). 

The basic form of ZDDP, Zn4[PS2(R0)2]60, has a structure in which the central oxygen atom is surrounded by four zinc atoms in tetrahedral geometry, and the six 0,0-dialkyl-dithionate groups are attached to the six edges of the tetrahedron (Armstrong et al., 1998). For the basic ZDDP, it was found that the lowest energy corresponds to the attack of the oxide ion on one of the sulfur atoms contained in the additive molecule. The attack induces the cleavage of the three bonds, namely two P-S bonds and one Zn-S bond. Overall, the relative stability of the three forms was found to increase in the order monomeric, dimeric, and basic. 

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