Dialkyl dithiophosphates

However, the most widely used materials are the zinc dialkyl-dithiophosphates that have an anti-wear effect in addition to their antioxidant power and, besides, offer an attractive cost/effectiveness ratio. 

Zinc dialkyl dithiophosphates are the primary oxidation inhibitors in combining these functions with antiwear properties in automotive oils and high pressure hydrauhc fluids. Their production volume is followed by aromatic amines, sulfurized olefins, and phenols (22). 

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... 

Dialkyl and diaryl dithiophosphoric acids are the bases of many high pressure lubricants, oil additives (see Lubrication and lubricants), and ore flotation chemicals (see Mineral recovery and processing). Organophosphoms insecticides such as Parathion are made by chlorination of the appropriate diaLkyl dithiophosphate and subsequent reaction of the intermediate dialkyl thiophosphoric chloride with sodium -nitrophenolate according to the following (see... 

The zinc. salts of these acids are extensively used as additives to lubricating oils to improve their extreme-pressure properties. The compounds also act as antioxidants, corrosion inhibitors and detergents. Short-chain dialkyl dithiophosphates and their sodium and ammonium salts are used as flotation agents for zinc and lead sulfide ores. The methyl and ethyl derivatives (RO)2P(S)SH and (RO)2P(S)CI are of particular interest in the large-scale manufacture of pesticides such as parathion, malathion, dimethylparathion, etc. For example parathion. which first went into production as an insecticide in Germany in 1947. is made by the following reaction sequence ... 

Tetraethyltin ZDDP, ZDTP Zinc dialkyl dithiophosphates... 

Mercapto- benzothiazolates Dialkyl- dithiophosphates Dialkyl(aryl)- monothiophosphates Fatty carboxylates... 

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). 

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. 

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) ... 

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... 

RH) by a zinc dialkyl dithiophosphate (ZnP) should ideally be representable by the following simplified scheme (Reactions 1 to 6). 

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. 

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.

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... 

Shopov and his co-workers have recently published two papers on hydroperoxide decomposition by barium dialkyl dithiophosphates. The decomposition rate of cumene hydroperoxide at 140 °C. in the presence of barium dibenzyl dithiophosphate was found (20) not to be described by Equation H. A mechanism, similar to that of Kennerly and Patterson (13) but slightly more detailed was proposed as follows ... 

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,... 

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