Lubrication, dithiophosphates

This is an analysis frequently conducted on oil lubricants. Generally, the additive is known and its concentration can be followed by direct comparison of the oil with additive and the base stock. For example, concentrations of a few ppm of dithiophosphates or phenols are obtained with an interferometer. However, additive oils today contain a large number of products their identification or their analysis by IR spectrometry most often requires preliminary separation, either by dialysis or by liquid phase chromatography. 

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. 

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... 

More recendy, molecular molybdenum-sulfur complexes and clusters have been used as soluble precursors for M0S2 in the formulation of lubricating oils for a variety of appHcations (70). Presumably, the oil-soluble molybdenum—sulfur-containing precursors decompose under shear, pressure, or temperature stress at the wear surface to give beneficial coatings. In several cases it has been shown that the soluble precursors are trifunctional in that they not only display antifriction properties, but have antiwear and antioxidant characteristics as weU. In most cases, the ligands for the Mo are of the 1,1-dithiolate type, including dithiocarbamates, dithiophosphates, and xanthates (55,71). 

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... 

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). 

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 ... 

In the area of process monitoring TLC has been used for the study of the thermal decomposition of zinc di-isopropyl dithiophosphate (antiwear additive in lubricating oils) [458]. TLC analysis has been reported as a quality control tool for analysis of dispersing agents (alkylsalicylates, thioalkylphenolates), AOs (dithiophosphates, dialkyldithiophosphates) and their intermediates in lubricating oil (UV detection,... 

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],... 

One approach to obtaining structural information on surface films is reflectance-absorbance infra-red (RAIR) spectroscopy and Fourier Transform IR (Foster, 1999). Model materials were prepared by thermal and thermooxidative decomposition of zinc diisopropyl-dithiophosphate. Lubricant-derived... 

The steel surface of the flat was immersed in the solution of oxymolybdyl dithiophosphate (DDP)2Mo02 at a concentration of 2 wt% in a PAO synthetic lubricant base. The immersion time was 5 hr and the temperature was 100°C (Martin et al., 1996). The X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy analysis of the reaction film indicates that before friction, a relatively thick chemisorbed film composed of phosphorus (P) and sulfur (S) is present on the steel surface. The friction test was carried out in ultra high (UHV) analytical tribometer just after the analysis and in the same conditions as the untreated surface. 

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. 

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). 

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. 

Similarly to bis(octylphenoxy)dithiophosphate, one can obtain other bis(alkylphenoxy)derivatives of dithiophosphoric acid. As mentioned above, these ethers are veiy efficient additives for lubricant oils used in intensive operation engines. An addition of 1-5% of these substances greatly increases thermal oxidative stability and anticorrosion properties of lubricant oils and reduce varnish formation in engines. 

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]. 

The metal (zinc, nickel, etc.) dithiophosphonates are attractive antioxidants in lubricants and plastics,124,125 and vulcanization accelerators (more active than analogous dithiophosphates).126 Many metal dithiophosphonates can be extracted with organic solvents and this property can be exploited for analytical applications, e.g., for copper(II) and bismuth(III) (O-hexyl)butyldithio-phosphonates,127 nickel(II), cobalt(II), and palladium(II) (0-ethyl)methyldithiophosphonates,128 platinum(IV), palladium(II), and gold(III) (O-ethyl)methyldithiophosphonates,129 and noble and rare metal (O-methyl)methyldithiophosphonates.130... 

Synthetic motor oils are made of a synthesized hydrocarbon base oil of hydrogenated polydecene, decanoic acid esters, zinc alkyl dithiophosphate, and synthetic poly alpha olefins. Most synthetic oils also contain additives, detergents, and corrosion inhibitors as well as viscosity modifiers. It is believed that the first synthesized polymeric hydrocarbons were synthesized in 1877, yet it was not until 1929 that the commercial development of synthetic lubricants was undertaken. Because of the availability of commercial petroleum-based lubricants, these synthetic lubricants were ultimately unsuccessful. The advent of commercial jet travel spurred the development of the first commercially successful synthetic lubricant, Mobil 1, in 1975. This lubricant had superior resistance to thermal breakdown and lower friction properties than petroleum-based products. 

In lubricating compositions, these compounds show excellent antioxidant synergy with aromatic amines such as alkylated diphenylamines [43] and sulphur compounds such as metal dihydrocarbyl dithiophosphate, a metal dithiocarbamate, sulphurised olefins, alkyl and aryl sulphides, alkyl and aryl polysulphides, sulphurised carboxylic acid esters, sulphurised alkylphenols, reaction product of an olefin and sulphurised alkylphenol, and phosphosulphurised terpenes or mixtures thereof [43],... 

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. 

Antioxidants. These eliminate or slow down lubricant oxidation, increasing the time between oil changes through improved resistance to high temperatures. Dithiophosphates that are used as antiwear additives are also excellent antioxidants. Other chemical families such as substituted phenols and aromatic amines are also used. 

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