Anti-wear additive

Bisimidazolium-based ionic liquids with different anions have been evaluated as the anti-wear additives in poly (ethylene glycol) at room temperature (22). The results showed that bisimidazoUum-based ionic liquids could effectively reduce the friction and wear of sUding pairs. 

The excellent tribological properties of these additives were attributed to the formation of high-quality physical adsorption films and tribochemical products during the friction and the good miscibility of ionic liquids with the base oil (22). 

Castor oil tris (diphenyl phosphate) was synthesized using an environmentally friendly and renewable resource, i.e., castor oil (23). 

The results indicated that castor oil tris(diphenyl phosphate) as the additive could effectively reduce the friction and wear of sliding pairs in the two base greases. The tribological performances were also better than the commonly used zinc dialkyldithiophosphate-based additive package in hthium 12-hydroxystearate greases and also in lithium complex greases. 

It was formd that boundary lubrication films composed of Fe(0H)0, Fe304, FeP04 and compounds containing P-O bonds were formed on the worn surface, which resulted in an excellent friction reduction and also anti-wear performance (23). 

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The increasing demands being made on equipment by the requirement for increased output from smaller units create problems of lubrication, even in systems where full-fluid film conditions generally exist. For instance, at start-up, after a period of rest, boundary lubrication conditions can exist and the mechanical wear that takes place could lead to equipment failure. Anti-wear additives, by their polar... 

Despite the introduction of new, improved methods of refining it has been necessary to enhance the performance of lubricants by the use of additives, either to reinforce existing qualities or to confer additional properties. Once additives were regarded with some suspicion —an oil that needed an additive was necessarily an inferior oil today they are an accepted feature of lubricants. Almost all quality lubricants on sale today contain one or more additives. An enormous range of additives are available for use in lubricants " , some produced by the oil companies and others provided by specialist manufacturers. Additives are usually named after their particular function, but many additives are multifunctional. Thus, an anti-wear additive may also protect a surface against corrosion. The main types of additives that can enhance the anti-corrosion behaviour of lubricants are listed in Table 2.22. 

RT-ILs are also selected as lubricant additives. Usage of ionic liquids as boundary lubricant additives for water has resulted in dramatically reduced running-in periods for silicon nitride materials [70]. When ILs were mixed into a neat mineral oil, the mixture has proven to produce lower wear on aluminum flat than either the oil or the ionic liquid alone [72], which indicates that a small amount of ILs in the mineral oil may function as an anti-wear additive. 

Organophosphate esters are among the most widely used classes of synthetic compounds in hydraulic fluids. They are used as anti-wear additives in mineral oil hydraulic fluids and are significant components in certain fire-resistant hydraulic fluids (FMC 1991c, 1991d, 1992a, 1992b Wills 1980). 

Overall, this work highlights how quantum chemical methods can be used to study tribochemical reactions within chemically complex lubricant systems. The results shed light on processes that are responsible for the conversion of loosely connected ZP molecules derived from anti-wear additives into stiff, highly connected anti-wear films, which is consistent with experiments. Additionally, the results explain why these films inhibit wear of hard surfaces, such as iron, yet do not protect soft surface such as aluminum. The simulations also explained a large number of other experimental observations pertaining to ZDDP anti-wear films and additives.103 Perhaps most importantly, the simulations demonstrate the importance of cross-linking within the films, which may aid in the development of new anti-wear additives. 

Lett. 24, 105 (2006). Interpretation of Experiments on ZDDP Anti-Wear Additives and Films through Pressure-Induced Cross Linking. 

Entrapped oxygen and water vapor in lubricants can act as anti-wear additives to form protective surface films. Metals are known to catalyze decomposition of certain lubricants, and decomposition temperatures may be reduced by 60°C or more. This is particularly true of bearing surfaces, on which the surface energy may be increased by stress-induced dislocations and by freshly exposed metal surfaces. An example of the way a tribochemical surface is affected by a polymerization process is vinyl chloride. If the load is increased from 0.1 to 0.5 kg in the presence of a vinyl chloride atmosphere, the Auger spectra of iron oxide surface shows a marked increase in the concentration of vinyl chloride on the surface (Buckley, 1981). 

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. 

The effect of operating conditions in the formation of these in situ films is likely to be complex. In 1972 Forbes showed that even under fairly mild conditions with an anti-wear additive such as tricresyl phosphate surface films are formed which contain both phosphorus and organic fragments. Similarly Cann showed that a cross-linked thick surface film is formed from zinc dialkyidithiophosphate under mild rubbing conditions. It seems probable that at least some of the soluble molybdenum compounds would also experience partial breakdown under mild rubbing conditions, and that these would encourage the formation of protective surface films. 

These results must be treated with caution. ZDDP s are anti-wear additives, as well as anti-oxidants and corrosion inhibitors, but are not particularly effective in increasing load-carrying capacity, so that the comparison between the two additives in this case is not a very demanding one. On the other hand, the highest concentration of molybdenum disulphide used was only 1 %, well below the concentrations normally used for load-carrying performance, which would typically be greater than 5%. 

Generally, the friction coefficient decreases as the load of filler increases but there is a critical quantity above which the friction coefficient decreases. The correct amount of anti-wear additive for a particular material and a particular applied load can be determined by simple morphological observation of the surface. The expected wear pattern forms surface debris whereas if the part is not wearing well its surface will melt and become shiny. 

Cai MR, Zhao Z, Liang YM et al (2010) Alkyl imidazolium ionic liquids as friction reduction and anti-wear additive in polyurea grease for steel/steel contacts. Tribol Lett 40 215-224... 

Lubricity Ester groups are polar and therefore affect the efficiency of anti-wear additives. When a base fluid is used which is too polar, it and not the anti-wear additives will be adsorbed onto, and cover, the metal surfaces, giving higher wear characteristics. Consequently, although esters have superior lubricity properties compared to mineral oil, they are less efficient than anti-wear additives. Esters are classified in terms of polarity or non-polarity by the van der Waal formula [48], Equation (2.1) ... 

Phosphate esters have been produced commercially since the 1920s and now have important applications as plasticisers, lubricant additives and synthetic-based fluids for hydraulic and compressor oils. Their first use in lubrication was as anti-wear additives. Later developments in aircraft hydraulic control systems, particularly during the Second World War, introduced phosphate esters as less flammable hydraulic fluids. As esters of orthophosphoric acid they have the general formula OP(OR)3, where R represents an aryl or an alkyl group or, very often, a mixture of alkyl and/or aryl components. The physical and chemical properties of phosphate esters can be varied considerably depending on the choice of substituents [59, 60], selected to give optimum performance for a given application. Phosphate esters are particularly used in applications that benefit from their excellent fire-resistant properties, but compared to other base fluids they are fairly expensive. 

Emulsions are in the HFAE and HFB groups of hydraulic fluids. The HFAE oil-in-water emulsions are more important, having a high water content, with very low flammability and are widely used due to their low cost. However, oil-in-water lubricants have a reduced wear protection compared with pure oils and, in addition, rust/corrosion and bacterial attack give problems. Improvement in these properties is achieved by using mst/corrosion inhibitors, bactericides and, if necessary, anti-wear additives. Suitable emulsifiers ensure good stability of the emulsions. 

Slow moving slides and tables in machine tools are subject to jerky motion due to alternating slipping and sticking of the sliding surfaces. Consequently, specially developed lubricants are required to prevent and/or reduce the stick-slip phenomenon. Slideway oils usually contain polar surface-active compounds, mostly fatty acid derivatives, together with anti-wear additives and oxidation inhibitors. The polar surface-active agents form oriented boundary layers which prevent adhesive friction. 

Chains used in industry are lubricated with heavy oil or grease and should have sufficiently low viscosity and ability to maintain lubricating qualities under different temperatures, moisture, etc. [102]. In extremely dusty environments a solvent containing anti-wear additives is used. If the chain is running through and/or is immersed in water, a combination of penetrating oil and tacky grease is the best method. 

Furey, M.J., Kajdas, C. and Kempinski, R. (2003) Recent developments in environmentally friendly anti-wear additives and lubricants from tribopolymerization. Tribologia - Finnish J. Tribol. 22 21-30. 

API GL-2 Lubricants intended for automotive worm gear axles operating under such conditions of load, temperature and sliding velocity that lubricants satisfying API GL-1 service will not suffice. Products suited for this type of service contain anti-wear additives or film-strength improvers specifically designed to protect worm gears. 

Anti-wear and load-carrying additives work by reacting with ferrous metal surfaces. The metal surfaces have to be sufficiently reactive themselves for the additive to work, which is the case with traditional steels such as M50 and M50 NiL but problems are encountered with more corrosion-resistant steels. These steels are designed to be chemically less reactive to inhibit corrosion but this affects the ability of the anti-wear additive to react with the metal surface. It is desirable to use these corrosion-resistant steels in engine design and the quest for an additive system that works with corrosion-resistant steels, without adversely affecting other areas of performance, is currently the subject of much research in the aero-engine lubrication community. 

Some additives used as anti-wear agents can also have anti-corrosion properties such as TCP, sebacic acid and azelaic acid. However, corrosion inhibition is a fine balance of competition with the anti-wear/load-carrying capability of the lubricant. Like the anti-wear additive, the corrosion inhibitor is designed to react with the metal surface. Some corrosion inhibitors specifically aimed at preventing static water corrosion of ferrous metals compete with the anti-wear additive for the metal surface to the extent that the resulting lubricant has poorer anti-wear properties than non-corrosion-inhibited lubricants. 

Nye grease with anti-oxidant and anti-wear additives... 

Additive Loss in Lubricants Additive loss is detrimental to the performance of lubricants. Some additive levels can be monitored by infrared spectroscopy. For instance, anti-wear additives, zinc dithio-dialkyl (diaryl) phosphate (ZDDP) and tri-cresyl phosphate (TCP) contain a common phosphate functional group that can be measured by infrared. The P-O-R (where R = alkyl/aryl) stretch shows a strong IR absorbance for all of these compounds and is used to trend the anti-wear level. The P-O-R stretch area is measured over the region of 1020-960 cm using the general baseline of 2000-600 cm. ... 

Anti-wear additives are but one of a number of additive types formulated into base oils - there are also anti-oxidants. Chapter 4, and anti-acid, detergents anddis-persants. Chapter 7, lubricity, anti-wear, extreme pressure, pour point depressants, anti-rust and anti-foam additives. Chapter 6. Viscosity index improvers, VIIs, are high-molecular weight polymers which alter the temperature dependence of the base oil viscosity. Chapter 5. Taken altogether, the additive mass percentage of a formulated lubricant can be as high as 15-20%, a veritable chemical soup but one which is very carefully formulated and tested. The additives are often multi-functional, thus some VII compounds have a pour point depressant function. Chapters 5 and 6. Some anti-oxidants have anti-wear and also anti-acid functionality. Chapters 4, 6 and 3. Given these cross-interactions, formulation of a final lubricant product is a complex and skilled activity. Chapters 8-13. 

ANTI-WEAR ADDITIVE - A lubricant additive to reduce wear. 

A major need within this Department has been the determination of commercial additives based on dimer acid (dilinoleic acid), used as pipeline corrosion inhibitors and anti-wear additives in aviation turbine fuel [47]. Quantitative determination of the 15-25 ppm level in fuel was solved by aqueous alkaline extraction from the fuel, followed by acidification and extraction into chloroform. After washing with water and evaporation of the bulk of the chloroform, the residue was made up to 10 mL volume and injected on to a single 100A Styragel column. A detection limit of Ippm additive (c 0.5 ppm dimer acid) was easily achieved using refractive index detection. 

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