Zinc dithiophosphates

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. 

Zinc dithiophosphates, which serve as antioxidants (qv) and antiwear agents in lubricants, are prepared by reaction of amyl alcohol and phosphoms pentasulfide followed by treatment with 2inc sulfate (43). 

Sulfur can replace 30-50% of the asphalt in the hlends used for road construction. Road surfaces made from asphalt-sulfur hlends have nearly double the strength of conventional pavement, and it has been claimed that such roads are more resistant to climatic conditions. The impregnation of concrete with molten sulfur is another potential large sulfur use. Concretes impregnated with sulfur have better tensile strength and corrosion resistance than conventional concretes. Sulfur is also used to produce phosphorous pentasulfide, a precursor for zinc dithiophosphates used as corrosion inhibitors. 

Zinc dithiophosphates act as anti-oxidants by promoting the decomposition of hydroperoxides. The mechanism of this reaction is complicated involving hydroperoxides and peroxy radicals192,193 and is also affected by the other additives present in the lubricant oil.194 However the first step is thought to be a rapid initial reaction of the zinc dithiophosphate and hydroperoxide to give a basic compound [Zn4(/i4-0)(S2P(0R)2)6] (Equation 88 Figure 9).141... 

A combined addition of a chain-breaking inhibitor and a hydroperoxide-breaking substance is widely used to induce a more efficient inhibition of oxidative processes in polyalkenes, rubbers, lubricants, and other materials [3 8]. Kennerly and Patterson [12] were the first to study the combined action of a mixture, phenol (aromatic amine) + zinc dithiophosphate, on the oxidation of mineral oil. Various phenols and aromatic amines can well serve as peroxyl radical scavengers (see Chapter 15), while arylphosphites, thiopropionic ethers, dialkylthio-propionates, zinc and nickel thiophosphates, and other compounds are used to break down hydroperoxide (see Chapter 17). Efficient inhibitory blends are usually prepared empirically, by choosing such blend compositions that induce maximal inhibitory periods [13],... 

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. ... 

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... 

Zinc Zinc dithiophosphate-antiwear/antioxidant additive in most engine oils and industrial oils... 

Phosphorus Zinc dithiophosphate orphosphonate compounds phosphorus is also found in gear oil additives... 

The zinc dithiophosphates (7) appear to be active antioxidants they produce sulfur acids by reduction of hydroperoxides and also act as radical scavengers (B-79MI11502). 

Polar functional groups of the additives are adsorbed on metal surfaces and this provides a barrier to prevent corrosive materials from contacting the metallic surfaces. Zinc dithiophosphates, dithiocarbamates and benzotriazoles are commonly used to protect copper-lead bearings from the type of corrosion present at concentrations of 50 to 300 mg/kg of oil. Rust inhibitors derived from sulfonates and amines also form absorbed films on ferrous metal, which prevents corrosive material from reaching the metal surface. 

A field test was conducted to evaluate the valve train wear in a 2.3 L OHC (over-head cam) engine with new technology crankcase lubricants these oils also passed the V-D test (Haris and Zakalka, 1983). Oils formulated with secondary alkyl zinc dithiophosphate (ZDDP) wear inhibitor provided significantly better wear protection than two different primary alkyl ZDDPs. Secondary alkyl ZDDP demonstrated good wear protection at a phosphorus content as low as 0.07 (wt%). 

Additive (AW), normally zinc dithiophosphate (ZDDP), which is added to lubrication formulation to prevent scuffing of the moving parts. Antiwear agent (forming tribofilm with a metal surface during friction process). 

Zinc dithiophosphates The dominating position of ZnDTPs as additives for lubricating oils is due to their multifunctional performance. Not only do they act as antioxidants, but they also improve the wear inhibition of the lubricant and protect metals against corrosion. ZnDTPs are mainly used to formulate anti-wear hydraulic fluids and engine oils. 

Zinc dithiophosphates Under service conditions, ZnDTPs undergo various chemical transformations and after 2,000-3,000 km they cannot be detected. However, 35% of the degraded ZnDTP products containing P-O-C bonds remain after 10,000 km and the antioxidant and anti-wear performance of the lubricant is still satisfactory [91]. Thus the antioxidant acidity up to 2,000-3,000 km is dominated by ZnDTPs and is subsequently governed by products resulting from their thermal cleavage. The contribution of ZnDTPs to the antioxidancy activity may be summarised as follows ... 

Antiwear additives. These reinforce the antiwear action of the lubricant. The main family of antiwear additives are alkyl-zinc dithiophosphates and numerous phosphorus derivatives. 

Figure 11-11. Thermal decomposition of di-n-butyl and diisobutyl zinc dithiophosphates. (a) Zinc di-n-butyl dithiophosphate. (b). ...

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