Of zinc dialkyl dithiophosphate

Because of the lack of information in the literature on the radical reactions of compounds of quinquevalent phosphorus, it is impossible to postulate a readily acceptable mechanism for the oxidation of zinc dialkyl dithiophosphates. Colclough and Cunneen (7) rejected immediately the possibility of hydrogen abstraction, but in view of the present results serious consideration has been given to this reaction. During this work it was shown (15) that abstraction of hydrogen from trialkyl phosphates, trialkyl phosphonates, and sodium dialkyl phosphates can occur at room temperature in an aqueous medium in the presence of hydroxy radicals. 

Decomposition of ZDDP takes place in the presence of oxygen, either coming from oxygen dissolved in engine oil or from peroxy radicals and hydroperoxides. Solution studies of the oxidation of zinc dialkyl dithiophosphates by peroxy radicals have shown that disulfides are major reaction products (Paddy et al., 1990 Rossi and Imparato, 1971 Willermet, 1998 Willermet et al., 1983 Willermetand Kandah, 1984). 

Figure 4.4 Reprinted from Tribol. Int., Vol. 31, M.L.S. Fuller, M. Kasrai, G.M. Bancroft, K. Fyfe and K.H. Tan, Solution decomposition of zinc dialkyl dithiophosphate and its effect on antiwear and thermal film formation studied by X-ray absorption spectroscopy, pp. 627-644. Copyright 1998, with permission from Elsevier.

Isobutanol use in the manufacture of zinc dialkyl dithiophosphates (ZDDP), anti-wear lube oil additives, represented 13 percent of domestic consumption. Other alcohols used in this application include methylamyl alcohol, primary amyl,alcohol, n-butanol, 2-ethylhexanol and isooctanol. 

Lubricating-oil consumption in modem engines is generally very low (0.1 liters per 1,(XX) km), and their contribution to catalyst deactivation is small. However, with the requirement for extended catalyst durability and extended drain periods for oils, there is considerable interest about the effect of the oil additives on catalyst life. The chief component of the oil that affects catalyst durability is phosphorus, which is usually present in the form of zinc dialkyl-dithiophosphate (ZDDP). Both combusted and uncombusted forms of ZDDP can reach the catalyst, resulting in different effects on activity depending on the temperature of operation. The level of phosphorus in the oil and the amount of alkaline earth metals present (such as calcium) can dictate the extent to which phosphorus can be deposited on the catalyst. However, studies have shown quite clearly that well-formulated lubricants and well-designed catalysts ensure that the antiwear properties of the oils are maintained and that catalyst-equipped vehicles meet the emission standards required [15,16]. 

Investigations of the behavior of metal salts of phosphorodithioate esters along chemical lines have given rise to a number of proposed mechanisms for the action of these substances as lubricant additives. The views of Baumgarten [53] are quite explicit a chemisorbed monomolecular film of zinc dialkyl dithiophosphate is quickly es-... 

Liu and Park " in their study on deactivated automotive catalysts in the presence of zinc dialkyl dithiophosphate (ZDDP) and Pb showed the formation of AIPO4 by reaction wit the support. The deactivation was due to several types of interaction with the alumina support, including the formation of an impervious layer on the washcoat or sintering of the y-alumina particles in the washcoat. 

Oehler, R., Zimmermatm, V., and Jager, G., Preparation of Zinc Dialkyl Dithiophosphates. Reaction and Reaction Products, Erdoel Kohle, Erdgas, Petrochemie, Vol. 40, No. 2,1987, p. 87. 

R.B. Jones, R.C. (Toy, The chemistry of the thermal degradation of zinc dialkyl-dithiophosphate additives, ASLE Trans. 24 (1981) 91-... 

Table 7. Pyrolysis Fragments of Zinc Dialkyl Dithiophosphates (weights per cent of volatile products)...

A series of zinc dialkyl dithiophosphates have been synthesized and their mixtures have been separated by normal-phase HPLC on a 5-pm silica column (68). The zinc dithiophosphates are listed in Table 1.7. 

In the boundary lubrication regime, the antiwear and Mction-reduction processes are associated to the build-up of a tribologic film resulting from the chemical reaction between oil additives and sliding surfaces in contact [1-3]. The case of zinc dialkyl dithiophosphate (ZDDP) additive has been extensively studied by analytical transmission electron microscopy (ATEM) and the morphology, nature, structure and tribologic properties of the tribochemical film were obtained [4-6], The Figures 4.1 and 4.2 summarize the main results. 

Figure 5.16 shows the variations of WSD values with the lubrication of LP containing the as-prepared bimetal alloy nanoparticles under different applied loads [19, 27,28]. The results of zinc dialkyl dithiophosphate (ZDDP, a commonly used commercial lubricating additive), Sn and In nanoparticles are also presented for comparison in Figure 5.16(a). It is clear that the In-Sn alloy nanoparticles displayed an excellent AW property under the applied load range... 

In steel-on-steel lubrication with a zinc dialkyl dithiophosphate additive, a complex surface paste appears to form first of zinc particles and iron dithiophosphate. The iron dithiophosphate then thermally degrades to a brown surface film of ZnS, ZnO, FeO, plus some iron and zinc... 

Phosphorus is a common component of additives and appears most commonly as a zinc dialkyl dithiophosphate or triaryl phosphate ester, but other forms also occur. Two wet chemical methods are available, one of which (ASTM D1091) describes an oxidation procedure that converts phosphorus to aqueous orthophosphate anion. This is then determined by mass as magnesium pyrophosphate or photochemically as molybdivanadophosphoric acid. In an alternative test (ASTM D4047), samples are oxidized to phosphate with zinc oxide, dissolved in acid, precipitated as quinoline phosphomolybdate, treated with excess standard alkali, and back-titrated with standard acid. Both of these methods are used primarily for referee samples. Phosphorus is most commonly determined using x-ray fluorescence (ASTM D4927) or ICP (ASTM D4951). 

Mechanism of Oxidation Inhibition by Zinc Dialkyl Dithiophosphates... 

The kinetics of the zinc diisopropyl dithiophosphate-in-hibited oxidation of cumene at 60°C. and Tetralin at 70°C. have been investigated. The results cannot be accounted for solely in terms of chain-breaking inhibition by a simple electrow-transfer mechanism. No complete explanation of the Tetralin kinetics has been found, but the cumene kinetics can be explained in terms of additional reactions involving radical-initiated oxidation of the zinc salt and a chain-transfer step. Proposed mechanisms by which zinc dialkyl dithiophosphates act as peroxide-decomposing antioxidants are discussed. 

Although zinc dialkyl dithiophosphates, [(RO)2PS2]2Zn, have been used as antioxidants for many years, the detailed mechanism of their action is still not known. However, it is certain that they are efficient peroxide decomposers. The effect of a number of organic sulfur compounds, including a zinc dithiophosphate, on the rate of decomposition of cumene hydroperoxide in white mineral oil at 150°C. was investigated by Kennerly and Patterson (13). Each compound accelerated the hydroperoxide decomposition, the zinc salt being far superior in its activity to the others. Further, in each case the principal decomposition product... 

The inhibition of hydrocarbon autoxidation by zinc dialkyl dithiophosphates was first studied by Kennerly and Patterson (13) and later by Larson (14). In both cases the induction period preceding oxidation of a mineral oil at 155 °C. increased appreciably by adding a zinc dialkyl dithiophosphate. In particular, Larson (14) observed that zinc salts containing secondary alkyl groups were more efficient antioxidants than those containing primary groups. In these papers the inhibition mechanism was discussed only in terms of peroxide decomposition. 

More recently it has been shown (6, 7) that zinc dialkyl dithiophosphates also act as chain-breaking inhibitors. Colclough and Cunneen (7) reported that zinc isopropyl xanthate, zinc dibutyl dithiocarbamate, and zinc diisopropyl dithiophosphate all substantially lowered the rate of azobisisobutyronitrile-initiated oxidation of squalene at 60°C. Under these conditions, hydroperoxide chain initiation is negligible, and it was therefore concluded that inhibition resulted from removal of chain-propagating peroxy radicals. Also, consideration of the structure of these zinc dithioates led to the conclusion that no suitably activated hydrogen atom was available, and it was suggested that inhibition could be accounted for by an electron-transfer process as follows ... 

The conclusion that chain-breaking inhibition by zinc dialkyl dithiophosphates involves electron transfer was reached independently by Burn (6) following a more detailed qualitative study of the inhibition of the azonitrile-initiated oxidation of squalane and cumene and the noninitiated oxidation of indene by metal dialkyl dithiophosphates and related compounds (I to IV) ... 

Kennerly and Patterson (13) studied the effect of several organic sulfur compounds, including thiols, sulfides, a disulfide, sulfonic acids, and a zinc dialkyl dithiophosphate, on the decomposition rate of cumene hydroperoxide in white mineral oil at 150 °C. In each case they found phenol as the major product. They suggested that the most attractive mechanism by which to explain these results involves ionic rearrangement catalyzed by acids or other electrophilic reagents (10) as... 

The carbonyl functionality of MIBK can be hydrogenated over nickel catalysts to yield methyl isobutyl carbinol (4-methyl-2-pentanol or methyl amyl alcohol) [108-11-2]. Industrial processes coproduce methyl isobutyl carbinol during the hydrogenation of mesityl oxide to MIBK. The product ratio of methyl isobutyl carbinol—MIBK during this reaction can be shifted toward methyl isobutyl carbinol by adopting a higher than normal pressure and H2 organic ratio (59). Methyl isobutyl carbinol is used as an ore flotation frother and to produce zinc dialkyl dithiophosphate lube oil additives. It is produced in the United States by Shell and Union Carbide ( 1.12/kg, October 1994). 

In a recent study four independent groups of investigators compared the wear performance of neutral and basic ZDDP neutral [(RO)2P(S)S]2Zn and basic [(R0)2PS2]6Zn40 forms of zinc dialkyl (or diaryl) dithiophosphate, primary di-isobutyl ZDDP, secondary di-isopropyl ZDDP, and aryl di(para-tert-octyl)phenyl ZDDP. 

The TBN values obtained for the fresh, unused, lubricating oil additive package components show results from conductometric and IP 177 (potentiometric) methods being 90% to 98 % and 85% to 90%, respectively, of the corresponding IP 276 (potentiometric) values. The TBN values for some selective products such as zinc dialkyl-dithiophosphate (ZDDP) was observed as an inflection point using the IP 276 (potentiometric) back titration method and also the conductometric method. 

The alcohol also finds use in the manufacture of lube and fuel oil additives and synthetic lubricants (about 6 percent of domestic consumption). The zinc dialkyl dithiophosphate anti-wear additive based on 2-ethylhexanol provides ideal compatability, oil solubility, and high temperature stability in many lube oils for both spark ignition and diesel engines. 

Figure 11-13. Relation between wear testing and thermal stability of metal dialkyl dithiophosphates. (1) Wear of copper pin against steel disk distance-dependent volume-rate, 8 kg load, 10 cm/s, 366 K, 5 hours. Additives zinc dialkyl dithiophosphates in n-hexadecane, 0.04% P. a ...

The simplest concept of a multicomponent additive is a straightforward mixture of individual additives each of which contains a single key active element, and in fact many multicomponent additives are just that. But since it is logically possible to have more than one type of key functional group in the same molecule, this is another kind of multicomponent additive to consider. There is a fundamental difference between a multifunctional additive compound of this kind and the additive structure with more than one key element but only a single functional group, such as is found in the zinc dialkyl dithiophosphates Zn[SP(S) (OR>2]2 dithiophosphate esters of the type RS-P(S) (OR)2 ... 

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