Metal dialkyl dithiophosphates

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) ... 

The present paper reports the results of a kinetic study of the inhibition of the azobisisobutyronitrile-initiated autoxidation of cumene at 60 °C. and of Tetralin at 70 °C. by zinc diisopropyl dithiophosphate, undertaken to test the validity of the chain-breaking inhibition mechanism proposed above. In addition, the effectiveness of several metal dialkyl dithiophosphates as antioxidants in the autoxidation of squalane... 

Peroxide Decomposition Mechanism. Since virtually no work has been reported which concerns only the mechanism by which zinc dialkyl di-thiophosphates act as peroxide decomposers, it is pertinent to discuss metal dialkyl dithiophosphates as a whole. The mechanism has been studied both by investigating the products and the decomposition rates of hydroperoxides in the presence of metal dithiophosphates and by measuring the efficiency of these compounds as antioxidants in hydrocarbon autoxidation systems in which hydroperoxide initiation is significant. 

We have carried out a limited study of the effect of metal dialkyl dithiophosphates on a hydroperoxide-autocatalyzed oxidation system. Table III summarizes induction periods for the oxidation of squalane at 140 °C. These results do not unambiguously reflect the peroxide-decomposing property of each dithiophosphate radical capture also occurs. 

Table III. Effect of Metal Dialkyl Dithiophosphates, [ (RO)2PS2]a.M, (at 4 X 10 n gram atoms of Phosphorus per liter) on the Oxidation of Squalane at 140°C.

The catalytic nature of the action of metal dialkyl dithiophosphates in the decomposition of cumene hydroperoxide at room temperature has been clearly shown by Holdsworth, Scott, and Williams (11) They... 

No readily acceptable mechanism has been advanced in reasonable detail to account for the decomposition of hydroperoxides by metal dialkyl dithiophosphates. Our limited results on the antioxidant efficiency of these compounds indicate that the metal plays an important role in the mechanism. So far it seems, at least for the catalytic decpmposition of cumene hydroperoxide on which practically all the work has been done, that the mechanism involves electrophilic attack and rearrangement as shown in Scheme 4. This requires, as commonly proposed, that the dithiophosphate is first converted to an active form. It does seem possible, on the other hand, that the original dithiophosphate could catalyze peroxide decomposition since nucleophilic attack could, in principle, lead to the same chain-carrying intermediate as in Scheme 4 thus,... 

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 ...

Watkinson [1988] lists some surfactants used for their dispersing action in organic liquids. He includes amongst them organic and metal sulphonates metal phenolates metal dialkyl dithiophosphates sodium dialkyl sulphosuccinates polyoxyethylene alkyl and alicyclic amines monethanol ammonium phosphate salts co-polymers of N-substituted formamide fatty acid phosphates... 

Metal dialkyl dithiocarbamates inhibit the oxidation of hydrocarbons and polymers [25,28,30,76 79]. Like metal dithiophosphates, they are reactive toward hydroperoxides. At room temperature, the reactions of metal dialkyl dithiocarbamates with hydroperoxides occur with an induction period, during which the reaction products are formed that catalyze the breakdown of hydroperoxide [78]. At higher temperatures, the reaction is bimolecular and occurs with the rate v = k[ROOH][inhibitor]. The reaction of hydroperoxide with dialkyl dithiocarbamate is accompanied by the formation of radicals [30,76,78]. The bulk yield of radicals in the reaction of nickel diethyl dithiocarbamate with cumyl hydroperoxide is 0.2 at... 

Physical Form, brown to black oily liquid new mineral-based crankcase oil contains petrochemicals (straight-chain hydrocarbons, aromatic hydrocarbons, and polyaromatic hydrocarbons or PAH) plus stabilizers and detergents including zinc dithiophosphate, zinc diaryl or dialkyl dithiophosphates (ZTDP), calcium alkyl phenates, magnesium, sodium, and calcium sulfonates, tricresyl phosphates, molybdenum disulfide, heavy metal soaps, cadmium, and zinc. ... 

The mechanisms of inhibition by peroxide decomposers, metal deactivators, and ultraviolet absorbers are fairly well understood in general terms, although many details of the individual reactions remain to be elucidated. Classifying a preventive antioxidant into one of the three categories above will only rarely describe its entire function. The dual behavior of dialkyl dithiophosphates in the liquid phase has been mentioned. Many other phosphorus- and sulfur-containing antioxidants commonly classified as peroxide decomposers can also act as chain breakers. Similarly, the structure of many metal deactivators and ultraviolet absorbers indicates that they must also have some chain-breaking activity. 

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-... 

Figure 11-12. Influence of the metal ion on the additive action of dialkyl dithiophosphates. Four-ball wear test at 15 kg load, 1500 rpm. Additive furnishes 4 mmoles P per 100 gms white oil solution. a Bi(IIl). b Sn(II). c Sbdil). d Pbdl). e Ag(I). f Fedll). g Nidi), h Cd(ll). k Zn(ll). Data by Allum and Forbes [58].

For extreme pressures, additives that react with metal surfaces are used. They contain sulfur and phosphorus such as dialkyl dithiophosphates. 

Industrial lubricants include metalworking lubricants, industrial greases, industrial lubricants, transformer oils, hydraulic oils, refrigeration oils, turbine oils, compressor oils, rock drill lubricants, paper machine oils, way lubricants, and railway journal box oils. The required additive package for each of these industrial oils is different depending on its specific application. Unlike lubricants used in automotive lubrication, industrial lubricants typically do not use metallic detergents and ashless dispersants as additives to keep metal surfaces clean and prevent insoluble materials from formation of deposits on metal surfaces [23]. However, in recent years the use of overbased detergents to supplement performance of the antiwear additive zinc dialkyl dithiophosphate has been reported [24]. 

Figure 2 Simplified antioxidant mechanism of metal dithiolates (dithio-phosphates, dithiocarbamates, xanthates) exemplified with nickel dialkyl dithiophosphate.

Zinc dialkyl dithiophosphates (ZnDTPs) are widely used as extreme pressure and antiwear additives in many different kinds of engine and industrial lubricants. It is known that ZnDTP forms tribological films on rubbing metal surfaces it has been proposed that these films consist of amorphous polyphosphates, but the exact chemical composition of the different polyphosphates in the ZnDTP tribofilm is not known, and a generally accepted reaction mechanism has not emerged to date. Most authors believe that thermal decomposition is the major mechanism of ZnDTP tribofilm formation as a result only tribological experiments conducted at elevated temperatures (60-200 °C) are, typically, reported in the literature. 22 All the evidence obtained so far for substantiating the amorphous polyphosphate model has been based on ex situ experiments. 

Metal complexes, first of all, dialkyl dithiophosphates and dialkyl dithiocarba-mates of such metals as Zn, Ni, Ba, and Ca, are widely used for the stabilization of polymers and lubricants. Inhibitors of this type are inferior to phenols in efficiency at moderate temperatures (350-400 K) but exceed them at higher temperatures (430-480 K). The mechanism of action of inhibitors of this type is complicated. The reaction of these inhibitors with hydroperoxide plays a very important role in the complex mechanism of inhibition. 

The metal complexes of diethyl dithiophosphate were first investigated in 1931,134 when it was shown that the transition metal complexes [M((EtO)2PS2 ] (n = 2 or 3) were similar to those of xanthates and dialkyl dithiocarbamates, containing a four-membered XSMS (X = C or P) ring. A... 

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