Dithiophosphates, effect

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. 

When the operating temperature exceeds ca 93°C, the catalytic effects of metals become an important factor in promoting oil oxidation. Inhibitors that reduce this catalytic effect usually react with the surfaces of the metals to form protective coatings (see Metal surface treatments). Typical metal deactivators are the zinc dithiophosphates which also decompose hydroperoxides at temperatures above 93°C. Other metal deactivators include triazole and thiodiazole derivatives. Some copper salts intentionally put into lubricants counteract or reduce the catalytic effect of metals. 

Flotation was carried out at an alkaline pH, controlled by lime. A xanthate collector with cyclic alcohol further (pine oil, cresylic acid) was shown to be the most effective. The use of small quantities of a dithiophosphate-type collector, together with xanthate was beneficial. 

The type of collector and flowsheet configuration play an important role in gold recovery from these ores. With a flowsheet that uses bulk Cu-Pb flotation followed by Cu-Pb separation, the gold recovery is higher than that achieved with a sequential Cu-Pb flotation flowsheet. In laboratory tests, and Aerophine collector type, in combination with xanthate, had a positive effect on gold recovery as compared to either dithiophosphate or thionocarbamate collectors. Table 17.10 compares the metallurgical results obtained with an Aerophine collector to those obtained with a dithiophosphate collector. 

Laboratory studies conducted on Falconbridge ores, also from the Sudbury Region, during 1980 [16] showed that PGM recovery can be improved with the use of a secondary collector. Figure 18.5 shows the effect of level of secondary collector on PGM recovery in a Cu-Ni bulk concentrate. The highest PGM recoveries were achieved using isobutyl dithiophosphate (Minerec 2087) as the secondary collector. 

Although the coupling of glycosyl dithiophosphates using an excess methyl triflate as an activator in the presence of 2,6-di-tertbutylpyridine proceeded in a satisfactory yield (70%), the application of dimethylsulfonium triflate resulted in the formation of disaccharides in a considerably more effective way (94% yield). 

Zinc dialkyl/diaryl dithiophosphates are widely used as antiwear additives in engine oils to protect heavily loaded engine parts from excessive wear. They are also used as antiwear agents in hydraulic fluids. The salts are effective oxidation and corrosion inhibitors, they also act as detergents. During friction, these salts form anion-radicals that are successfully cleaved (Kajdas et al. 1986). 

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 radical (RO)2PS2 in indene would thus effectively act as a chaincarrying radical and is therefore not considered to be an intermediate in chain-breaking inhibition by dithiophosphates. A two-stage mechanism was therefore proposed, involving a stabilized zinc salt-peroxy complex, either a radical or an ion pair as illustrated in structures V and VI. 

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... 

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... 

As a rule, however, the final formulation of products must be subjected to time-consuming practical tests. This is particularly true for motor oils, where only the practical test in the engine can assess, for instance, high temperature efficiency of dithiophosphates and the frequently antagonistic effects of dispersants and oxidation inhibitors in a given additive combination (Klamann, 1984 Mortier and Orszulik, 1997). 

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.

The salts of alkyl xanthates, iV.AT -di-substituted dithiocarbamates and dialkyl dithiophosphates [43] are effective peroxide decomposers. Since no active hydrogen is present in these compounds, an electron-transfer mechanism was suggested. The peroxide radical is capable of abstracting an electron from the electron-rich sulfur atom and is converted into a peroxy anion as illustrated below for zinc dialkyl dithiocarbamate [44]... 

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]. 

S.2.3.3. Induction effect in molecules of thiophosphates Derivatives of thiophosphates include monothio and dithiophosphate with the following... 

TABLE 9-9. EFFECT OF DIISOPROPYL DITHIOPHOSPHATES ON WEAR AND FRICTION... 

Table 9-9, from Rowe and Dickert s investigation of solutions of diisopropyl dithiophosphates in n-hexadecane [19], lists the rates of wear in parallel with the coefficients of friction for steel against steel and copper against steel. A strong effect from the presence of dithiophosphate is seen for steel on steel in the sharp decrease in both the wear rates and the coefficients of friction relative to that with uncompounded hexadecane as the lubricant. For copper against steel, the wear rate decreases significantly, particularly with the metal salts of diisopropyl dithiophosphate, but the coefficient of friction is not altered systematically. Table 9-10 shows the frictional behavior of hardened alloy cast iron in the presence of decalin solutions of triphenyl phosphate and diphenyl phosphate [15]. The mechanisms governing the action of phosphates and dithiophosphates are discussed in Chapter 11. 

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