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The Georgicks of Virgil, with an English Translation and Notes Virgil, John Martyn Ipsi in defossis specubus secura sub alta Otia agunt terra, congestaque robora, Pierius says it is confecto in the Roman manuscript. And Tacitus also says the Germans used to make caves to defend them from the severity of winter, .

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The major types of esters and their feedstocks are reviewed in Table 2. The simplistic, fundamental reaction is that of Reaction 2. An azeotropic agent such as toluene to remove water can be used. Catalysts used include sulphuric acid, p-toluene sulphonic acid, tetra-alkyl titanate, anhydrous sodium hydrogen sulphate, phosphorus oxides and stannous octanoate.

Polyol esters are made by reacting a polyhydric alcohol, such as neopentyl glycol NPG , trimethylol propane TMP or pentaerythritol PE , with a monobasic acid to give the desired ester. When making neopolyol esters, excess acid is used because the acid is more volatile than the neopolyol glycol and therefore easier to recover from the ester product.

Other, more critical requirements are related to the chemical properties of the lubricant and many of these can be measured satisfactorily only by elaborate and expensive apparatus specially developed to simulate performance. One disadvantage of very long-chain molecules is their tendency to shear into smaller fragments under stress. From the above lists, there is a natural trade-off between viscosity index and pour point, e. Esters made from mixtures of normal and branched acids with the same carbon number have viscosity indices between those of the normal and branched acid esters.

But their pour points are lower than those esters formed separately from either branched or normal acids. Using the above formula it can be seen that as a general rule, increasing molecular weight improves overall lubricity. The thermal stability advantages of polyol esters compared to diesters is well documented. This type of decomposition requires more energy and can occur only at higher temperatures, as in Reaction sequence 2. Reaction sequence 2. One exception to the rule that branched acid esters give poorer stability than linear acids is the branched C9 acid 3,5,5-trimethylhexanoic acid.

The weak tertiary hydrogen is sterically hindered and is more stable than would normally be expected. The structure has fewer secondary hydrogens than the linear form and is hence more stable [49]. Molecular geometry affects hydrolytic stability in several ways. By sterically hindering the acid portion of the molecule, because hindrance on the alcohol portion has relatively little effect, hydrolysis can be slowed down.

For this purpose, geminal di-branched acids such as neoheptanoic acids have been used. The length of the acid chain is also very important, for acids shorter than pentanoic tend not to be used owing to their corrosivity. The hydrolytic stability of neopolyol esters can generally be regarded as superior to that of dibasic esters. Solvency: This can be divided into compatibility with additives and other lubricants, and also elastomer compatibility.

Compatibility with additives and other lubricants: Esters are generally fully compatible with mineral oils, which gives them three major advantages. First, there are no contamination problems and therefore esters can be used in machinery that previously used mineral oil. In addition, they can be blended with mineral oil semi-synthetics to boost their performance. Second, most additive technology is based on mineral oil experience and this technology is usually directly applicable to esters. Elastomer compatibility: Elastomers contacting liquid lubricants undergo an interaction with liquid diffusing through the polymer network.

There are two possible kinds of interaction, chemical rare and physical. During physical interactions two different, and opposing, processes occur: — extraction of soluble components out of the elastomer, causing shrinkage, — absorption of the lubricant by the elastomer, causing swelling. The degree of swelling of elastomeric materials depends on:.

Several polar esters are well-known industrial plasticisers. Non-polar base stocks, such as PAOs, have a tendency to shrink and harden elastomers. By carefully balancing these compounds with esters, lubricants with neutral physical behaviour towards elastomeric materials can be formulated. Environmental aspects: Increasing environmental awareness has raised water pollution to a major issue.

Polyols, polyoleates, C36 dimer esters, diesters WGK 0 Phthalates and trimellitates WGK 0—2 Biodegradability: The general biochemistry of microbial attack on esters is well known and has been thoroughly reviewed. Figure 2. Low-temperature viscosity is arguably the single most important technical feature of a modern crankcase lubricant, for cold starts are a prime cause of engine wear which can be countered only by immediate and effective circulation of the lubricant.

Moreover, vehicles are increasingly required to operate reliably under arctic conditions. Their low volatility is especially important because of the trend towards smaller sump capacities and longer oil change service intervals.

Chemistry and Technology of Lubricants

Two-stroke oils: Esters such as C36 dimer esters and polyoleates have several advantages over mineral oils as the lubricant component of two-stroke engine fuel mixtures. First, their clean-burn characteristics result in less engine fouling with considerably reduced ring sticking and lower levels of particulate deposit build-up.

Ignition performance and plug life are also enhanced. Third, in some applications, low-temperature performance is important, such as snowmobile-type vehicle engines. PAHs are a major contributor to the carcinogenic nature of exhaust emissions. Esters can also be used to reduce the level of smoke emitted by the engine. Compressor oils: This market sector covers a wide range of compressor types used for a number of different gases. Diesters and phthalates have found their major application in air compressor lubricants and are also used in natural gas compressors.

In reciprocating compressors, where oils of rather higher viscosity are preferred, trimellitate esters can be used. Diesters and polyol esters may also be blended with PAOs for use in various compressor types. Their low ecotoxicity and high biodegradabilities also lessen their environmental impact. Diesters generally have high viscosity indices, giving a wide temperature range without needing to use viscosity improvers, which can shear in this application. Esters have a further advantage of their good thermal conductivity which conducts heat away from heat sources more effectively than mineral oils.

Aviation oils: The bulk of aviation lubricant demand is for both military and civilian gas turbine lubricants. Hydrocarbon oils cannot meet the requirements placed on jet engine oils, primarily lubrication, oxidation and ageing stability. Some diesters are still used in less demanding applications such as for small private aircraft and turbo-prop engines.

Due to the better temperature stability of polyols, there is a growing tendency to use them in preference to diesters. For example, polyalkylene glycols can be solid or liquid, water soluble or water insoluble, and can be produced to almost any required viscosity. Section 2. The commonly used alkylene oxides are ethylene oxide and propylene oxide, Fig. Alkylene oxides are very reactive and readily polymerised in highly exothermic reactions. Under basic polymerisation conditions, ethylene oxide gives products with exclusively primary terminal hydroxyl groups, whereas propylene oxide preferentially gives secondary terminal hydroxyl groups, Reaction sequence 2.

Although only a small number of alkoxide ions, i. Products are normally characterised by an average molecular weight or by their viscosity. When polymerisation is complete, the residual catalyst can be i neutralised, e. The polarity resulting from the oxygen atoms also confers quite different gas solubilities. Obvious routes to controlling product properties are the following: — variation of the starting molecule initiator , — variation of the alkylene oxide s used, — the molecular weight.

Initiators: Typical initiators include butanol monofunctional , ethylene or propylene glycol difunctional and trimethylol propane trifunctional. Both monoand difunctional initiators give products with linear chains but the monofunctionalbased products have half of the chain ends capped by an alkyl group.

Tri- and polyfunctional initiators give products with branched chains. Alkylene oxide s : The vast majority of commercialised polyalkylene glycols are based on ethylene oxide only, propylene oxide only or copolymers incorporating the two. Due to the more reactive nature of ethylene oxide, random copolymers will tend to preferentially incorporate propylene oxide units at the chain extremities. Polyalkylene glycols for use in engineering and lubrication applications are usually homopolymers of propylene oxide or random copolymers.

Molecular weight: By controlling the quantity of oxide added, polyalkylene glycols with a wide range of molecular weights and hence viscosities can be selected. Water solubility is primarily governed by the ratio of ethylene oxide:propylene oxide in the polymer, with a higher proportion of ethylene oxide leading to greater solubility.

Typical oxide ratios used for water-soluble copolymers are between and ethylene oxide:propylene oxide by weight. All polyalkylene glycols exhibit inverse solubility in water, i. This is explained by the loss of hydrogen bonding at elevated temperatures. This results from the lateral methyl groups of the propylene oxide unit disrupting crystallisation.

Higher values are obtained for polymers with a high degree of propylene oxide units. The viscosity index tends to be better for products with a low degree of chain branching. Degradation: In contrast to mineral oils, polyalkylene glycols form either volatile or soluble degradation products upon oxidation and do not leave unwanted solid deposits during service.

Lubrication: Polyalkylene glycols show very good frictional behaviour and are inherently excellent lubricants. During the s, PAGs were used in heat treatment foundries to quench metal. Following strong growth during the s and s, PAGs are now used for many applications worldwide and constitute the largest market share within the synthetic lubricant sector. What of the future?

Compressor lubricants: Lubrication in the pressurised area of a compressor presents many problems. Lubricants must not only seal, cool and reduce friction and wear but also cope with the presence of compressed aggressive or oxidising gases. Metal cutting: This operation involves the removal of metal as chips from a workpiece component. Increased industrial demand for high-performance lubricants has resulted in a much wider acceptance of synthetic-based products.

PAGs are extensively used to lubricate calendar gears and bearings within the plastics, rubber and paper industries, being particularly suited for heavily loaded worm gears. Paper and concrete mills are examples of processes where ingress of dust and moisture gives particular problems. Gear oils based on water-soluble PAGs can be used for easier cleaning and extended service intervals. Both water-soluble and water-insoluble types of PAGs can be used in the formulation of synthetic greases.

The clean burn-off characteristic of all PAGs makes them particularly suitable as grease bases incorporating molybdenum disulphide or graphite. The PAGs form volatile oxidation breakdown products, leaving no carbonaceous residues or sludge. A typical application is chain lubrication in very high-temperature ovens. Rubber lubrication: The negligible swelling characteristics of water-soluble and water-insoluble PAGs are used to advantage within the rubber industry.

PAGs can be formulated with solvents and wetting agents, and applied directly by brush or spray. Two-stroke engine lubrication: Synthetic lubricants virtually eliminate engine problems associated with deposition and fouling, commonly seen with mineral oil lubricants. 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. This feedstock is a complex mixture of cresols, xylenols and other materials. This has led to the use of controlled coal tar fractions, where the ortho-cresol and other ortho-nalkylphenol content is greatly reduced.

Low molecular weight trialkyl esters are water soluble and require non-aqueous techniques. For mixed alkylaryl ester production the reactant phenol and alcohol are added separately, the reaction being conducted in a stepwise fashion. Consequently, phosphate esters range from low-viscosity, water-soluble liquids through to insoluble, high-melting solids. The hydrolytic stability of aryl esters is superior to the alkyl esters but increasing chain length and degree of branching of the alkyl group leads to considerable improvement in hydrolytic stabilities.

Increased chain branching leads to a progressive drop in viscosity index. Alkyl—aryl phosphates tend to be more susceptible to hydrolysis than are the triaryl or trialkyl esters. Hydrolysis has potentially serious consequences as it produces acid esters which can lead to corrosion and may also catalyse further degradation.

The thermal stability of triaryl phosphates is considerably superior to that of the trialkyl esters, which degrade thermally by a mechanism analogous to that of the carboxylic esters, Reaction 2. Many triaryl phosphates have fairly high melting points but an acceptable pour point can be achieved by using a mixture of aryl components. Increasing the length of straight-chain alkyl substituents increases viscosity and VI but has a negative effect on low-temperature performance with increased pour points. Increased branching of alkyl substituents, for constant molecular weight, gives lower viscosities and VIs but improved pour point.

Phosphate esters are very good solvents and extremely aggressive towards paints and a wide range of plastics and rubbers. Therefore, careful consideration is required when selecting suitable gasket and seal materials for use with these esters. As an example, ethylene—propylene rubbers and poly-urethane or epoxy-cured paints may generally be used. Indeed, the presence of phosphate esters in a formulation may help to solubilise other additives.

They have extremely good lubricity, as demonstrated by their wide use as load-carrying additives in a range of lubricants. Other requirements such as viscosity or thermal stability can be met by appropriate choice of substituents. They are also chosen for other low-temperature applications for conditions such as those found on North Sea, and similar weather condition, oil rigs. Sullivan, F. Gunderson, R. Seger, F. Garwood, W. Beynon, K. Antonsen, D. Isa, H. White, M. Gates, D. Kashiwa, H. Rev CEER 18, 14— Evans, A. Fontana, C. Plesch, P. Higashimura, T. Brennan, J. Prod Res. Whitmore, F.

Madgavkar, A. Shubkin, R. Reprints 24, Onopchenko, A. Reprints 27, Puskas, I. Corno, C. Ferraris, G. Audisio, G. Chaffee, A. Chen, C. Marcel Dekker, New York, p. Samuel, P. Bulletin of the Catalysis Society of India 2, 82— Souillard, G. SAE Paper Frederickson, M. Hancock, E. Kennedy, J. Wiley, New York, Chapter Fotheringham, J. Noda, K. Callis, G. Naitoh, Y. Kagaya, M. Sugiura, K. Wilelski, K. Van der Waal, G. Krevalis, M. Macrae, A. Wyatt, J. Cerniglia, C.

Randles, S. Presented at a Royal Society of Chemistry seminar, York. Krulish, J. SAE paper No. Cosmacki, E. Wits, J. CFC Halocarbon Conference. Klamann, D. Verlag Chemie, Weinheim. Marino, M.

Booser, E. Abrasive, adhesive, contact fatigue and corrosive wear mechanisms are explained. Interactions between reduced ZDDP levels and molybdenum compounds to maintain wear protection and antioxidancy are considered for the requirements of lower SAPS formulations in future formulations. These laws are attributable to Leonardo Da Vinci [1], then rediscovered by Amontons in [2]. In Coulomb distinguished between static friction, the force required to start sliding, and kinetic friction, the force required to maintain motion [3]. He showed that kinetic friction is lower than static friction and is nearly always independent of the speed of sliding.

To understand the causes of friction, the following must be considered: i Even the most carefully prepared surfaces contain peaks called asperities and valleys which are large compared with molecular dimensions. The solid surfaces contain peaks of these highest asperities, so the area of intimate contact reach between them is very small. This heating effect may cause micro-structural changes to asperities as well as local melting. The frictional force is mainly the force required to shear these junctions.

The classic laws of friction can now be understood since the contact area over which the junctions are formed is almost independent of the apparent, dimensional, area of the sliding surface and is directly proportional to the applied load. With static surfaces, or at low sliding speeds, cold welding is produced by the intense pressure in the region of contact. At higher speeds it is assisted by the high-temperature softening or melting of the metal surface. This is the force Fp required to displace the softer metal of the two dissimilar metals in sliding contact. The total frictional force F1 is given by Equation 3.

From these simple theories, it follows that for low friction the sliding materials must have the following: — low shear strength, — high hardness. These requirements are met if thin low-shear-strength layers are formed on hard metal substances. In sliding contacts this can arise from a number of processes; in order of relative importance they are adhesion, abrasion, corrosion and contact fatigue. The bonded section may be work hardened, and hence strengthened, and shear may occur not at the asperity tip junctions but within the body of the material.

This transfers material from one surface to the other and no overall wear occurs until a secondary process causes the transferred material to break away. Sometimes transferred material resides in a surface and may even transfer back to the original surface. However, groups of particles can be formed which break away as a single entity when the elastic energy just exceeds the surface energy, the latter being greatly reduced by environmental reaction.

Adhesive wear processes can be viewed as a state of dynamic equilibrium with their environment. The fresh surfaces exposed by wear are more reactive than the original surface and must be passivated by additives if wear is not to be catastrophic. Many wear processes commence as adhesive wear but the fact that this mechanism leads to the formation of abrasive debris inevitably means that there is a possible transition of the wear mechanism from adhesive to abrasive.

During abrasion a metal undergoes extensive work hardening. For this reason the initial hardness of the abrasive particle is unimportant if the heat-transformed wear particle is substantially harder than that of the metal surfaces. Should the wear particle present in the contact become softer due to thermal processes generated by friction, then the particle will be deformed under the contact pressure and damage to the sliding surfaces is minimised. Examples of frictional hardening and softening are well documented and can be predicted from the phase diagrams of metals.

Bovington The majority of engineering materials are heterogeneous, composed of phases with considerable differences in hardness. But if the abrasive particle size is comparable to the scale of heterogeneity of the material, then gouging out the hard material from the structure leads to an increased wear rate. Abrasion can be limited by: — increasing the hardness of the rubbing surfaces, — putting soft coatings onto surfaces to embed debris, e. This wear fatigue mechanism causes pitting or spalling of the surfaces.

This type of surface failure manifests itself by the breakout of small, roughly triangular portions of the surface material. It occurs primarily due to high stresses which cause fatigue failure to commence at a point below the surface where the highest combined stresses occur. After initiation, encouraged by impurity inclusions, tangential traction and the hydraulic forces generated by lubricants, propagation occurs parallel to the surface until a structural weakness such as a grain boundary is encountered.

The crack then propagates to the surface. Hydraulic action of the lubricant under contact pressures can help to lever out the triangular pits. A lubricated system contains many corrosive species, such as:. All of these have a tendency to chemically attack metals and rubbing usually enhances such attack by: i removal of surface corrosion product layers, exposing fresh metal to attack, ii raising surface temperatures to speed diffusion and increase reaction rates.

Corrosion can take place uniformly over a rubbed surface or selectively at surface inclusions, grain boundaries and between dissimilar materials, etc. For the former case, a problem results only if corrosion is excessive. Limited corrosion is necessary for anti-wear and extreme pressure protection. Selective corrosion can weaken surface structures and initiate fatigue.

For engines using modern diesel fuels, with relatively low levels of sulphur, corrosive cylinder linear wear is not a problem, provided that adequate overbased detergents are present in the lubricant to neutralise the sulphur acids produced by combustion. Therefore, wear rates increase linearly with concentration and exponentially with temperature.

Corrosion can be limited by: i using corrosion and rust inhibitors, ii using additives of the minimum activity needed to prevent adhesion, iii limiting water access and activity, iv neutralisation by overbased detergents. Boundary lubrication functions by several mechanisms. Bovington which often operate simultaneously, all of which counter one or both of the general causes of friction and hence wear previously described, i.

These layers are a shear strength layer which allows motion without high friction and can also reduce wear. Thick inorganic layer Fig. Bovington v Rehbinder effect: Chemicals can weaken metals by the Rehbinder effect, Fig. The precise mechanisms of this effect are unclear but may involve hydrogen embrittlement.

The effect is to minimise ploughing and hence control friction and wear. Within an engine a range of the above conditions operate. To a reasonable approximation, the kinetics of these processes can be described by the Langmuir isotherm, Equation 3. Provided that a critical minimum surface is maintained, wear and friction can be controlled. The adsorption of dilinoleic acid [4], a series of organic sulphur compounds [5] and a ZDDP zinc dialkyldithiophosphate [6] has been described in these terms.

Bovington 3. Clear evidence of this effect has been shown for molybdenum chemistries [7] and for ZDDPs [8, 9]. These factors include strong dipole interactions or strong hydrogen bonding which aid physical adsorption and the ease of chemical reaction from this adsorbed layer. As an illustration, Fig. Those steels such as stainless steel with high surface concentrations of inert oxides, e. The greater reactivity of pure iron is also shown. Engineers tend to use increasingly harder metallurgies to combat abrasive wear whose surfaces are less and less reactive to anti-wear agents.

Consequently the risk of adhesive wear increases. In addition, frictional heating can generate phase changes in steels, resulting in either hardening, e. In addition to the above, and of increasing interest, rubbing surfaces generate low energy, 1—3 eV, electrons or exoelectrons spontaneously [12], which can promote chemical reactions, i.

If the rate. This excessive corrosion can occur either because the additive used is excessively reactive at the contact temperatures or because the concentration of the additive in the oils is too high. Maintenance of this corrosive adhesive balance is essential and Fig. These compounds include: i oxygen-containing organic compounds that have a polar head group capable of adsorption at surfaces. The carboxylic acids can function by forming metal soaps with the contacting surfaces.

In this case there is evidence that the upper limit for friction reduction coincides with the melting point of the metal soap [10]. There is also evidence that control of chain length, chain matching of the carboxylic acid hydrocarbon backbone and choice of solvent hydrocarbon can minimise friction [10]. Figure 3. The data points to the importance of two parameters, that of adhesive interactions between adsorbate and surface, and lateral interactions between adsorbate molecules on the surface. Whilst polarity in a molecule is a prerequisite for adsorption, dipole moment measurements as a direct measure of polarity are not infallible predictors of friction.

Bovington Fig. For example, whilst nitroalkanes have greater dipole moments than carboxylic acids, 2. The difference between the less polar groups of the known boundary lubricants and the nitro groups is that the former are capable of hydrogen bonding whereas the latter are not [16].

Hydrogen bonding is a special case of a donor—acceptor interaction with an electronegative atom, usually oxygen or nitrogen with an unshared electron pair, functioning as the acceptor and a hydrogen atom, covalently bonded to a second electronegative atom, functioning as the donor. Due to differences in electronegativity of the atoms comprising a hydrogen bonding functional group, as either donor or acceptor, hydrogen bonding groups must be polar, but the reverse is not necessarily true. This is due to a redox reaction involving iron from the steel surface and the metal oleate.

Thus adsorption of hydrogen atom acceptors such as alkyl esters or compounds such as alcohols and carboxylic acids, which can act as either donors or acceptors, leads to friction reduction. A high degree of lateral interaction forms a cohesive adsorbed layer which results in improved adsorption. The radicals are generated by low-energy 1—3 eV exoelectrons [21], emitted spontaneously by solid surfaces when disturbed by plastic deformation, abrasion, fatigue cracking or phase transformation. Film formation depends on the contact temperature and the structure of the monomer additive used.

Low-temperature polymerisation is favoured where additional polymerisation occurs, e. Condensation-type monomer polymerisation, e. Films can be formed from dihydroxydocosanoic acid [23] and appear to be formed as polyesters by intermolecular dehydration to form a network polymer. In contrast, monohydroxydocosanoic acid gives a linear polymer. The reaction of sulphur additives involves a controlled corrosion process;. Nevertheless, certain sulphur-containing compounds have been used as supplementary anti-wear agents in crankcase lubricants and have potential use in low or zero phosphorus application.

Typical of these would be the sulphurised esters of fatty acids, the dimercaptothiadiazoles or the dithiocarbamates. Dimercaptothiadiazoles are widely used as metal passivators and as loadcarrying additives. They have been used particularly as potential replacements for ZDDPs in zero phosphorus formulations [24]. Particularly good friction and wear reduction occurs when a hydroxyl group is attached to the end of a straight alkyl chain.

Dithiocarbamates of zinc, and particularly of molybdenum, show friction- and wear-reducing properties particularly when used in conjunction with ZDDPs and are discussed in the following sections. The importance of the polar impurities present in TCP has been demonstrated [26]; the probable identity of the important impurity is lauryl acid phosphate, found to be 20 times as effective at wear protection compared to pure tricresyl phosphate.

A conclusion is that the ester function of phosphoric acid is necessary only to ensure solubility of the product in oil. TCP can also give rise to anti-rust protection by hydrolysis to phosphoric acid. Long-chain-length mono- and dialkyl phosphonates form protective layers under boundary lubrication conditions.

The phosphate anion formed has reduced oil solubility, leading to accumulation in the boundary layer of oil covering the metal surface.

The phosphonate anion reacts with Fe III created by oxidation in the rubbing contact by bridging rather than chelation. This compound has a hexagonal-type crystal structure where each molybdenum atom is surrounded at equal distances by six atoms of sulphur placed at the corners of a triangular right prism of Each layer consists of two planes of sulphur atoms and an intermediate plane of molybdenum atoms. The distance between the planes of molybdenum and sulphur atoms is The bonds between molybdenum and sulphur are covalent whilst between the sulphur atoms the bonds are van der Waals type.

The low binding energy between the planes of sulphur atoms allows molecular sliding along them. The shearing force further decreases due to lattice defects present in real crystals. VI test was improved between 0. Molybdenumbased oils are the best route yet available to deliver very low friction in service.

The rate of formation increases with temperature [36]. It appears that. As for all additives, interactions with other additives in solution, Fig. For the former, nearly all experiments were conducted in rudimentary tribological tests and on solutions in simple base oils [42, 43]. The clear conclusion is that it is the structure of the alkyl groups that determines the anti-wear potency of the ZDDP. In particular, chain branching and chain length are critical such that short, branched-chain alkyl groups are more reactive than long primary alkyl chains.

However, short-chain, primary alkyl groups can be more reactive than long-chain secondary ones. Similar considerations govern thermal stability [44], and there is an inverse relationship between the temperature of thermal decomposition and the potency of anti-wear protection.

Blends of the two ZDDPs gave good initial wear protection and good sustained wear protection. The initial step of ZDDP reactions at surfaces involves the loss of Zn and the consequent formation of the nucleophilic species [6], Fig. Nucleophilic substitutions of one phosphorus species by another leads to P—O—P structures and zinc mercaptide Zn SR 2 as a reaction intermediate. Reaction of this mercaptide with dithiophosphate leads to trithiophosphates and eventually tetrathiophosphates. Finally, an oil-insoluble deposit is formed of a mixture of zinc thiophosphate and zinc pyro- and polypyrothiophosphates.

Inhibition of volatile alkyl thiol formation due. These complexes are unlikely to be formed by dispersants since co-ordination around Zn atoms is less likely. There is some evidence that. It is also well outside the range of dispersant nitrogen to zinc ratios encountered in current formulations, typically in the range of 0. Hindered phenols and aromatic amines donate a hydrogen atom to the peroxy radical, ROO.

ZDDPs, and also metal dithiocarbonates, act as oxidation inhibitors by peroxide decomposition in a manner which does not produce radicals, thus removing a major initiation source. It seems likely that ZDDPs are sources of DDPAs dialkyldithiophosphonic acids and it is this which is responsible for either ionic decomposition of the hydroperoxide or decomposition by electron transfer, Reaction 3. It is this form which is then oxidised to other products by loss of the RO 2PS2 coordinating group [58].

Therefore wear protection will be reduced or conversely, if the ZDDP is protected from oxidation by using alternative antioxidants, the overall anti-wear performance of the lubricant may be improved. Inhibition of peroxide formation, Reaction 3. Zinc dialkyldithiocarbamates are alternative peroxide radical scavengers and like the usual antioxidants, sterically hindered phenols and aromatic amines, they terminate peroxide radicals by Reaction 3. Of this latter group, the most effective inhibitors of high-temperature oxidation are claimed to be the aromatic amines [56], but these are capable of complexing ZDDPs and hence inhibiting their anti-wear performance.

Careful balance of ZDDP and antioxidant s is required to achieve both wear and viscosity increase control. Principal amongst these are water, and oxides and oxyacids of nitrogen and sulphur. Together with unburned fuel, these are the key constituents of blow-by gases that can interact with the lubricant on cylinder liners, in the crankcase and in the valve train areas. Stable emulsions have small droplet sizes and these give rise to less wear than when unstable emulsions with larger droplet sizes are formed [61].

Wear occurs by corrosive oxidation and partition of ZDDP into the aqueous phase does not occur. Detergent systems which formed unstable emulsions gave rise to high levels of wear, whilst those which gave rise to stable emulsions gave low levels of wear. Bovington of wear, especially of the valve train [62, 63]. Introduction of NOx at a rate of 0. This agrees with general experience from motored cylinder head rig tests. Since NOx and nitrous acids are strong oxidising agents, they cause both local, in contact area, and general, in crankcase, depletion of ZDDP with consequent increases in wear rate.

This latter test was developed because increasing soot levels due to the use of EGR could cause increased levels of engine wear [64], in addition to increased oil viscosity due to soot agglomeration. It was also shown, not surprisingly, that this entrainment is more pronounced at high temperatures. They have lower shear strength than the metal surface and hence are removed, partially or fully, during sliding. Because this process is a form of corrosive attack of the surfaces, careful control of chemical activity and additive concentration are needed if catastrophic corrosive wear is to be minimised.

Successful control of anti-wear and anti-friction properties of oils requires a careful balance of the additives in the formulation. Problems of formulating low-sulphur and -phosphorus lubricants: The need for engine manufacturers to ensure the durability of exhaust after-treatment devices, and hence to maintain exhaust emission standards, has led to a demand for crankcase lubricants which contain reduced levels of sulphur and phosphorus. A major source of sulphur in most engine oils is the base stock into which the additive package is blended.

These base oils are also used as diluents for highly viscous lubricant additives in order to facilitate handling and blending. These basestocks have excellent low-temperature viscometric properties and allow the blending of oils with a wide viscosity range. An enforced reduction in the concentration of ZDDPs and MoS compounds which can be included in additive packages for low SAPs applications means that consideration must be given to how to maintain wear protection and antioxidancy at an acceptable level.

Normal phosphorus levels are of the order of 0. If levels of 0. Paris X — Bovington 4. Dacre, B. Bovington, C. Stipanovic, A. Willermet, P. Plaza, S. Rosenblum, B. Kajdas, C. Monteil, G. Belzer, M. Gorby, W. Ratoi, M. Castle, R. Choo J-H. Stinton, H. Rabinowitz, E. Okabe, H. JSLE Int. Tokyo, p. Hu, Z. China Int. Young Trib. Wei, D-P. Lacey, I. Bieber, P.

Johnson, G. Korcek, S. Kubo, K. Muraki, M. Paliacios, J. Nagoya, pp. Suominen Fuller, M. Fujita, H. Journal Part J. Taylor, L. Yin, Z. Martin, J. Alliston-Greiner, A. Leeds-Lyon Symp. Bec, S. A, — Allum, K. Rounds, F. Rowe, C. Fujitsa, K. Jones, R. Powell, K. Gallopoulos, N. Inoue, K. Kawamura, M. Harrison, P. Kulp, M. Howard, J. Prior, Ed. Schumacher, R. Paddy, J. Emanuel, E. Murakami, Y. Esslingen, Tribology, , 11— Bregent, R.

Chinas-Castillo, F. Mechanisms of these processes are described and discussed. Differences in hydrocarbon reactivity are related to molecular structure. Base oil oxidation stabilities depend upon their derivation, whether solvent neutral, hydrocracked or synthetic, and their response to antioxidant treatment.

Lubricant oxidation control focuses on radical scavengers and hydroperoxide decomposers and their synergistic mixtures. Engine oils increasingly use phenolic and aminic antioxidants as radical scavengers with organometallic complex antioxidants. Sterically hindered phenols substituted at 2- and 6-positions by t-butyl groups are particularly effective, reacting successively with peroxy radicals to form stable cyclohexadieneone peroxides.

Secondary amines as either two aryl or phenyl and naphthyl groups are very effective at eliminating four peroxy radicals per molecule. At higher temperatures a catalytic cycle is suggested as an extended stabilisation mechanism. They catalyse or inhibit oxidation by complexing and decomposing hydroperoxides or can also oxidise peroxy radicals and reduce alkyl radicals to inert products. Organomolybdenum complexes such as molybdenum dialkyldithiocarbamates, MDTC, are increasingly used to stabilise engine lubricants particularly because of synergy with other antioxidants such as alkylated diphenylamines.

Aguilar et al. In contrast, reactions of atmospheric oxygen with hydrocarbon polymers and liquid hydrocarbons lubricants , as well as with certain biological systems, under varying conditions of temperature and oxygen pressure are undesirable processes. Such reactions lead to deterioration of these materials. All oxidative processes with oxygen have a common reaction pattern attributable to the biradical status of oxygen. Its initial stage is characterised by a slow reaction with oxygen followed by a phase of increased conversion until the process comes to a standstill.

Initiation of the radical chain reaction: Under normal conditions, i. For instance, a tertiary alkyl radical reacts 10 times faster with oxygen than does a methyl radical. The rate of Reaction 4. Due to their low reactivity, peroxy radicals are present in relatively high concentration in the system when compared with other radicals, determined via electron spin resonance. A more favourable route of hydrogen abstraction by a peroxy radical [8—10] occurs via an intramolecular propagation outlined in Reaction sequence 4. Reaction Sequence 4. Radical I may also intramolecularly abstract a hydrogen radical, Reaction 4.

Chain branching: During the early stage of autoxidation, various types of hydroperoxides are generated. At low concentrations they may be cleaved homolytically to yield an alkoxy and a hydroxy radical, Reaction 4. Once formed, hydroxy and especially primary alkoxy radicals are so active that they abstract hydrogen atoms in non-selective reactions, Reactions 4. The relationship between hydroperoxide accumulation and oxygen uptake of a hydrocarbon is schematically presented in Fig. As can be seen, during the induction period, hydroperoxides are accumulated; after the induction period, the oxidation is autocatalysed.

Termination of the radical chain reaction: As the reaction proceeds, autoxidation is followed by an autoretardation stage, resulting in a standstill before the hydrocarbon is completely consumed. Termination may be effected by the combination of radical species such as peroxy radicals to yield ketones and alcohols, Reaction sequence 4. In contrast, tertiary peroxy radicals may either combine to give di-tertiary alkyl peroxides or undergo a. In addition, cleavage of a dihydroperoxide II of Reaction 4.

Under metal-catalysed conditions or at higher temperatures, considered in Sections 4. Primary oxidation phase: Initiation and propagation of the radical chain reaction are the same as discussed under low-temperature conditions, but selectivity is reduced and reaction rates increased. At high temperature the cleavage of hydroperoxides plays the most important role. Reaction 4.

Acids are formed by the following two reactions, which start from a hydroperoxy—peroxy radical, see Reaction 4. In a subsequent step they can react with alcohols R1OH, produced by Reactions 4. In addition, when the rate of oxidation becomes limited by diffusion, ethers are formed, Reaction sequence 4. These radicals again contribute to the formation of cleavage products via Reactions 4. The secondary oxidation phase: At higher temperatures the viscosity of the bulk medium increases as a result of the polycondensation of the difunctional oxygenated products formed in the primary oxidation phase.

Further polycondensation and polymerisation reactions of these high molecular weight intermediates form products which are no longer soluble in the hydrocarbon. The resulting precipitate is called sludge. The polycondensation reactions which lead to high molecular weight intermediates sludge precursors can be described as follows. When the reaction becomes diffusion controlled as a result of the increased viscosity of the oil, alkoxy radicals can initiate polymerisation of polycondensation products. This leads to sludge and deposit formation as well as to additional oilsoluble high molecular weight products which contribute to the viscosity increase.

This process can be described as co-polymerisation of two different polycondensation species in which the alkyl groups R, R1and R2 could represent oxo- or hydroxyfunctionalised long hydrocarbon chains Reaction 4. Under high-temperature conditions there is always the possibility of thermal cleavage of a hydrocarbon chain, especially when the availability of oxygen is limited by diffusion, Reaction sequence 4.

The model for high-temperature oxidation is described by Fig. These ions must be present as metal soaps, otherwise they are not catalytically active [16]. Hence the homolytic hydroperoxide decomposition is accelerated at ambient temperatures by small concentrations, 0. The consequence is a high rate of hydroperoxide formation and hence oxidation , as illustrated in Fig. In iron ion-free systems, organocopper salts behave as pro-oxidants. However, from 1, ppm and above they can behave as effective antioxidant systems.

This results in the formation of insoluble sludge as well as a continuous increase in viscosity. The following section deals with the oxidation of base oils, which principally follows the same reaction pattern as the model hydrocarbons but is more complex due to the presence of natural inhibitors and pro-oxidants.

Crude oil is distilled under atmospheric pressure to remove gasoline and distillate fuel components.

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These fractions consist of aromatic compounds and heteroatoms such as nitrogen and sulphur. In hydrocracking, the neutral distillate fraction reacts with hydrogen under high pressure and temperature. In addition, most heteroatoms are removed during hydrocracking. Some of these compounds may act as natural antioxidants, others as pro-oxidants, but overall it is a well-established fact within. It is therefore very unlikely that they can be deactivated by natural or synthetic antioxidants. The use of organometallic complexes of transition metals as antioxidants is becoming more prevalent, especially in engine oils; organomolybdenum compounds are of particular importance.

Sterically hindered phenols: Phenols substituted at the 2- and 6-positions by tertiary alkyl groups are called sterically hindered phenols, the most common substituent is the tertiary butyl group. Based on the fate of 2,6-di-tertiary-butyl-p-cresol in a turbine oil used in a conventional steam turbine, it was concluded that there is a strong possibility that this phenol is consumed not through the inhibition of oxidation, Reactions 4. Replacement of a tertiary butyl group by a methyl group in the ortho position reduces the antioxidant activity considerably, Table 4.

Synthetics, Mineral Oils, and Bio-Based Lubricants: Chemistry and Technology

The major structural types which are commercially available, together with their synthetic routes, are outlined in Table 4. Aromatic amines: Secondary aromatic amines are another important class of antioxidants used in lubricants. The principal substituents of the nitrogen atom are either two aryl or one phenyl and a naphthyl group. Table 4. For a sterically hindered monophenol, this factor is 2. As stated earlier, sterically hindered phenols deactivate only two peroxy radicals per phenol molecule.

Hence, under high-temperature conditions, aromatic amines are far superior to their phenolic counterparts. As shown in Table 4. Due to the longer lifetime of the nitrogen-centred radical XII by resonance stabilisation, dimerisation and oligomerisation take place whilst maintaining the —NH— function. Indications have been found that the products of Reaction 4. Representatives of this class of compounds together with their syntheses are outlined in Table 4.

For the autoxidation and stabilisation of hydrocarbons such as mineral oil, it is proposed that transition metals can either catalyze or inhibit autoxidation by the following set of reactions [37]. In Reaction sequences 4. It has been reported that one in four passenger cars and trucks in the United States contains a particular type of molybdenum compound [40]. Surveying the patent literature indicates that the success of molybdenum as an antioxidant is due to its synergy with other antioxidants, in particular aminic antioxidants such as alkylated diphenylamines.

The rest of this section is dedicated to a patent survey on organomolybdenum compounds and their use in lubricating compositions.