Modification of the chemical structure of an environmentally‐friendly castor oil lubricant

Widely used mineral‐oil based lubricants are often released into the environment and cause pollution. Therefore importance is attached to developing environmentally‐friendly lubricants. Vegetable oils have been used as lubricants since ancient times. These materials, similar to synthetic esters, have better biodegradability than mineral oils. They are also renewable. Vegetable oils have some shortcomings, such as a higher pour point and a lower viscosity index than synthetic esters. They have a limited viscosity range and lower oxidative stability due to the presence of unsaturated bonds. Therefore vegetable oils as such cannot satisfy all the requirements of modern machine lubrication. In this paper, the chemical structure of natural castor oil is modified by an isomerisation reaction and by extending the carbon chain, so as to lower the pour point and improve the viscosity index. The results show that structurally modified castor oil has very good lubrication characteristics. Its viscosity at 40°C is 150 mm²/s, its pour point is −40°C, and its viscosity index is improved. The antiwear and friction characteristics are better than those of mineral oil of the same viscosity and comparable to pentaerythritol esters and diisocapryl sebacate.     

Lubricant components from vegetable oils of indian origin

A number of transesterified and alkarylated derivatives have been synthesised from available vegetable oils of Indian origin. The viscosity, viscosity index, freezing points, load-carrying characteristics, friction coefficient, and thermo-oxidative stability of these derivatives as compared to highly refined hydrorefined hydrocarbon oils have been studied. It has been found that several esters of fatty acids of these vegetable oils have a high natural viscosity index, low pour points, and high thermooxidative stability, and can meet the requirements as base fluid components for energy-efficient, eco-friendly, long-drain interval, multigrade oils. These oils have markedly lower viscosities at 40°C, higher load-carrying characteristics, and lower friction coefficients than the base fluids of currently marketed multigrade oils. A 50% blend with hydrorefined hydrocarbon oils could prove highly viable. The above results clearly establish the potential for utilising these esters, either alone, or in combination with mineral oils, for formulating cost-effective high-performance, energy-efficient, and environmentally friendly lubricants. Performance characteristics of these oils as engine oils, automotive gear oils, and 2 stroke oil with conventional additives and with alternative additives are under investigation in comparison to the most advanced hydrocarbon based multigrade oil formulations of long-drain interval.     

Mixtures of vegetable oils and di‐2‐ethylhexyl‐sebacate as lubricants

Lubricants based on vegetable oils are growing in popularity in various applications. Environmentally friendly, vegetable oils and their derivatives constitute alternatives to mineral‐based lubricants. Soybean oil, sunflower oil and rapeseed oil have better viscosity indices than mineral oils and even some synthetic oils, are biodegradable and have low production costs. However, vegetable oils have disadvantages, such as poor thermo‐oxidative stability due to the carbon–carbon double bonds and poor low‐temperature properties, which limit their use as lubricant base stocks. This study describes new base‐stock oils obtained from mixture of vegetable oils and di‐2‐ethylhexyl‐sebacate synthetic oil, which become lubricants when additives are introduced. These mixtures offer a large range of kinematic viscosities, while their pour points are under −33°C and their flash points over 240°C. The copper strip corrosion test result is 1a. The diameters of wear scars measured under four‐ball testing (40dyn) are less than 1mm. A differential scanning calorimetry study and a thermo‐gravimetric study under a nitrogen atmosphere for the mixed oils are reported. In the former study two‐endothermic processes were observed between −15°C and −50°C. In the thermo‐gravimetric analysis curve the weight loss is specific for each vegetable and synthetic oil component. From these studies a higher thermal stability was observed for vegetable oils than for ester oils, and it was concluded that the mixtures of vegetable and synthetic oils of diester type are physically homogeneous mixtures. The low production cost of lubricants based on vegetable oils makes them attractive alternatives for mineral oil based lubricants. Overall the mixtures of vegetable and ester oils can be competitive base oils for environmentally friendly lubricants. Copyright © 2006 John Wiley & Sons, Ltd.     

Origins of the friction and wear properties of antiwear additives

Experimental techniques have been developed to measure the friction, antiwear film‐forming and wear properties of lubricants in rolling–sliding contact. Friction measurements show that zinc dialkyldithiophosphates (ZDDPs) and also some other phosphorus‐based additives increase friction in mixed lubrication. Film thickness measurements show that this increase in friction correlates with the thickness of antiwear film. They also reveal some of the drivers of antiwear film formation and removal. A novel wear tester is described which enables the mild wear resulting from ZDDP‐containing oils to be monitored. Copyright © 2006 John Wiley & Sons, Ltd.     

Tribological degradation of two vegetable‐based lubricants at elevated temperatures

Renewable‐based lubricants are being considered as potential alternatives to petroleum‐based lubricants for various reasons, mainly increased environmental sensitivity. However, understanding the tribological performance of such vegetable‐based lubricants under elevated temperatures is critical for their industrial implementation. This study focuses on the friction and abrasion rate characteristics of soybean and sunflower base oils in comparison to a base mineral oil under sliding wear at elevated temperatures. It was found that the abrasion rate and friction were less severe for the vegetable‐based lubricants up to temperatures around 100°C. The observed performance of the vegetable‐based lubricants was verified using a kinetic reaction mechanism model of lubricant degradation. Copyright © 2007 John Wiley & Sons, Ltd.     

Assessing friction characteristics of liquid lubricants

A reduction in friction in tribological contacts will lead to reduced energy requirements. It is therefore important to be able to measure the frictional characteristics of various liquid lubricants. Current laboratory tests measure friction, but the test rigs invariably induce wear between the mating contacts. A new test rig, the wire‐on‐capstan rig, is presented which measures friction without inducing wear between the wire and the rotating capstan. All seven lubricants tested (three without additives and four proprietary hydraulic oils) exhibit a reducing coefficient of friction with increasing temperature. Temperatures ranged from 25°C to 100°C. The mineral‐based oil showed the lowest friction coefficient and the three synthetic ester based oils produced the highest friction coefficient over the complete temperature range. Future testing will extend the range of operating conditions and product types. Copyright © 2006 John Wiley & Sons, Ltd.     

Effect of mechanical shear on the thin‐film properties of base oil‐polymer mixtures

The thickness and frictional characteristics of thin lubricant films are known to affect the fuel economy properties of oils. The base oil and polymer compositions of the lubricant are generally considered to be critical chemical factors that can influence these thin‐film lubricant properties in new oils. However, it is important to produce lubricants with good fuel economy properties that are maintained after the lubricant is degraded. Lubricants in use can undergo oxidation and mechanical shear degradation. The effect of oxidation degradation on thin‐film physical properties has previously been studied. This paper investigates the effect of mechanical shearing on thin‐film properties. Dispersant olefin copolymers are found to reduce thin‐film friction in simple mixtures and in fully formulated oils. In simple mixtures, shearing the dispersant olefin copolymers does not affect the friction‐reducing ability of these polymers. In fully formulated oils, even though shearing diminishes to a degree the friction‐reducing ability of dispersant olefin copolymers, these copolymers can still provide significant friction reduction.     

Film‐forming properties of castor oil–polyol ester blends in elastohydrodynamic conditions

The viscosity and elastohydrodynamic (EHD) film thickness properties of binary blends of castor oil with polyol esters were determined experimentally. Predicted blend viscosities were calculated from the viscosities of the pure blend components. Measured viscosity values were closer to the values predicted using the Lederer model than the Arrhenius model. EHD film thickness data were mostly in agreement with the predictions of the Hamrock–Dowson model. Observed deviations of EHD film thickness were attributed to boundary film formation and thermal effects. Calculated effective pressure–viscosity coefficients, α, displayed a complex relationship with blend viscosity. At 40°C, the addition of 10% polyol esters resulted in a 12–17% drop in α of castor oil. Higher concentrations of polyol esters resulted in an increase of α. At 70 and 100°C, α displayed an almost linear dependence on blend composition. Copyright © 2011 John Wiley & Sons, Ltd.     

Lubricating properties of rapeseed‐based oils

Three commercially available hydraulic/transmission lubricants based on rapeseed oil have been investigated for their lubricating properties. The coefficient of friction, scuffing‐load capacity, and pitting resistance were evaluated, and the results compared with a corresponding commercial mineral‐based oil. The results showed in general a substantially lower coefficient of friction and better pitting resistance for rapeseed‐based oils than for the mineral oil. Scuffing load capacity was, with one exception, the same for all oils. As a result of lower shear stresses during contact, and a higher viscosity index, the temperatures in the gearbox were lower for the rapeseed oils tested than for the mineral oil. Insufficient antiwear behaviour at high loads was found to be a major drawback of these vegetable oils.     

Tribological Performance and Mechanism of Phosphate Ionic Liquids as Additives in Three Base Oils for Steel-on-aluminum Contact

Butylammonium dibutylphosphate and tetrabutylammonium dibutylphosphate ionic liquids (ILs) were evaluated as antiwear additives for steel-on-aluminum contact in three different base oils, a polyalphaolefin, an ester oil and an IL 1-methy-3-hexylimidazolium hexafluorophosphate, respectively, with similar viscosity and different polarities. The friction experiments were carried out on an Optimal SRV-IV oscillating reciprocating friction and wear tester at room temperature. Results indicate phosphate ILs can effectively improve the tribological properties of the base oil, especially the antiwear property, as additives for steel/aluminum contacts. For the base oils PAO10 and PAO40 with different viscosities, the higher viscosity of PAO40 can be beneficial to reducing the friction coefficient. The worn surface morphologies and chemical compositions of wear scars were analyzed by a JSM-5600LV scanning electron microscope and PHI-5702 multifunctional X-ray photoelectron spectrometer (XPS). The XPS analysis results illustrate that the phosphate IL additives in the base oils with different polarities exhibit the same tribological mechanism. A synergy exists between the adsorbed layers and boundary-lubricating films generated from the tribochemical reaction of IL and the substrate surface, which may reduce the friction coefficient and wear volume of the friction pairs.     

Comparison of boundary lubrication films formed under four‐ball friction test conditions with different AW/EP Additives‐an X‐ray photoelectron spectroscopy study

The antiwear and extreme‐pressure properties of six different types of additive (molybdenum dialkyldithiophosphate, dibenzyl disulphide, molybdenum dialkyldithiocarbamate, zinc dialkyldithiophosphate, chlorinated paraffin wax, and triaryl phosphate) were evaluated by standard four‐ball friction and wear tests. This was followed by scanning electron microscopy (SEM), X‐ray photoelectron spectroscopy (XPS), and X‐ray photoelectron imaging (XPI) analyses of the worn surfaces to determine the structure of the boundary lubrication film and the mechanism of the tribochemical reaction occurring during the friction process. The presence of the additives in the base oil significantly increased the weld load and drastically reduced the wear‐scar diameter, suggesting antiwear and extreme‐pressure properties of the additives. The enhanced antiwear and loadcarrying capacity of the additive‐containing oils was attributed to the formation of a complex boundary lubrication film formed between the surfaces during the friction process as a result of the tribochemical reaction. The antiwear and extreme‐pressure properties of the additives were explained based on the XPS data. The studies indicated that the lubricating properties of the additives depend on their chemical nature and reactivity with metal surfaces.     

Borate esters used as lubricant additives

Borate esters possess friction‐reducing, antiwear, and anti‐oxidant characteristics when blended in lubricating oils. However, borate esters are susceptible to hydrolysis. The formation of a stable five‐member ring structure in the ester molecules, involving coordination of nitrogen with boron, contributes substantially to the resistance to hydrolysis of borate esters. The susceptibility of borates to hydrolysis can be reduced by introducing N,N‐dialkylaminoethyl groups with alkyl radicals containing more than three carbon atoms. X‐ray photo‐electron spectroscopy and X‐ray diffraction reveal that the borate ester can be adsorbed on the rubbing surface, and some of the adsorbed borate film degrades and forms boron nitride. Four‐ball wear tests indicate that the combination of oil‐soluble copper, tin, and cadmium compounds with organoborates gives better antiwear properties than the components separately. An antiwear synergistic mechanism is postulated in which borates with electron‐deficient boron p orbitals catalyse the tribo‐reduction of the metal compounds on the rubbing surfaces, producing elemental metals.     

Lubricant for Heavily Loaded Slow-Speed Journal Bearing

Slow-speed journal bearings subjected to heavy loads operate in a mixed/boundary lubrication regime. Clearance and lubricant play very important roles in reducing the wear and friction in these bearings. In the present article, an experimental study on heavily loaded slow-speed journal bearings with various radial clearances lubricated with three different lubricants is presented. Lubricants with varying viscosities and containing different percentages of antiwear additives have been used. Bearing surface roughness and out-of-roundness are treated as noise parameters. The results of friction coefficient and total wear have been reported. The experimental results suggest that a lubricant with high viscosity and antiwear additives significantly reduces the coefficient of friction and amount of wear under varying bearing clearances, circularity, and cylindricity. The use of such a robust lubricant may obviate the effect of manufacturing uncertainties. This results in reduction of manufacturing and measurement costs.     

Anti‐friction,anti‐wear and load‐carrying characteristics of environment friendly additive formulation

The extreme pressure (EP), anti‐wear and friction‐reducing characteristics of some of the commercial additive formulation and individual components on which these formulations are based have been studied and compared to characteristics of the components synthesised from naturally available non‐traditional vegetable oils and cashew nut shell liquid that have been refined and partially hydrogenated to improve stability. It has been shown that individual components from sulphurised and phosphosulphurised vegetable oils, esters and hydrogenated cardanol (derived from cashews) have better anti‐wear and friction‐reducing characteristics than the sulphurised olefins and alkylaryl phosphorothioates normally used as EP and anti‐wear additives, while the load‐carrying characteristics of a number of the combination of these derivatives are comparable. It has been shown that all these formulations are rapidly biodegradable and non‐toxic in nature as compared to traditional EP, anti‐wear and friction‐reducing additives, which fill in the category of slightly toxic to harmful. It is possible to formulate energy‐efficient EP gear oils that are fully biodegradable and non‐toxic by a combination of vegetable oil‐based additives of sulphurised vegetable oils, phosphosulphurised methyl recinoleate and phosphosulphurised hydrogenated cardanol amine borate, which meet all the performance characteristics of US steel 224 eg 52100, M‐50 AISI 1010 requirements. Copyright © 2007 John Wiley & Sons, Ltd.     

Reduced Phosphorus Concentration Effects on Tribological Performance of Passenger Car Engine Oils

Phosphorus is present in engine oils in the form of the antiwear and antioxidation additive zinc dialkyldithiophosphate (ZDDP). Its effects on wear and friction were studied at different temperatures using a high-frequency reciprocating rig (HFRR). The electrically insulating tribofilm formation was measured using an electrical contact resistance (ECR) technique. The wear and friction performance of a fully formulated fresh oil containing 0.05 wt% phosphorus was compared with the corresponding used oil drained from a vehicle. The results show that the wear performance of fresh oils having phosphorus concentration from 0.02 to 0.1 wt% is very similar. Further reduction of phosphorus concentration below 0.02 wt% leads to high wear. The coefficient of friction increases with increased phosphorus concentration at temperatures above 80°C but decreases with increased phosphorus concentration at temperatures below 80°C. The used oil and the fresh 0 wt% P oil running on the original fresh steel surface exhibit higher wear than when both oils were evaluated on a previously formed film from a fresh oil containing 0.05 wt% phosphorus.     

Tribological properties of phosphate esters as additives in rape seed oil

With the growing concern about environmental pollution, vegetable oils have begun to be applied as base oils for environmentally friendly lubricants. Additives containing phosphorus can be used for their excellent antiwear properties and low toxicity. In view of this, a study of a series of phosphate esters as additives in rape seed oil (RSO) was carried out using a four‐ball tester. The results indicate that phosphate esters possess very good load‐carrying capacity and good antiwear and friction‐reducing properties compared with RSO by itself. Surface analysis of the worn balls was carried out using X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The tribological mechanism of the additives is discussed on the basis of the experimental results.     

A study on the tribological characteristics of graphite nano lubricants

Many researchers have tried to improve the tribological characteristics of lubricants to decrease friction coefficients and wear rates. One approach is simply the use of additives in the base lubricant to change its properties. Recently, nanoparticles have emerged as a new kind of additive because of their size, shape and other properties. A nano lubricant is a new kind of engineering lubricant made of nanoparticles, dispersant, and base lubricant. In this study, graphite nanoparticles were used to fabricate nano lubricants with enhanced tribological properties and lubrication characteristics. The base lubricant used was industrial gear oil, which has a kinematic viscosity of 220 cSt at 40°C. To investigate the physical and tribological properties of nano lubricants, friction coefficients and temperatures were measured by a disk-on-disk tribotester. The surfaces of the fixed plates were observed by a scanning electron microscope and an atomic force microscope to analyze the characteristics of the friction surfaces. The results show that when comparing fixed plates coated with raw and nano lubricants, the plate coated with a nano lubricant containing graphite nanoparticles had a lower friction coefficient and less wear. These results indicate that graphite nanoparticle additives improve the lubrication properties of regular lubricants.     

Influence of fatty acid composition on the tribological performance of two vegetable‐based lubricants

In light of diminishing natural resources, global climatic change and increased environmental sensitivity, renewable‐based lubricants are being considered potential alternatives to petroleum‐based lubricants. Understanding the tribological performance of vegetable‐based lubricants in relation to their chemical composition is essential for their industrial implementation. This study focuses on the friction and abrasion rate characteristics of soybean and sunflower oils in comparison to a base mineral oil under sliding wear at ambient conditions for various applied loads. It was found that the abrasion rate and friction were the least severe for the soybean, followed by the sunflower oil. The observed trends were attributed to differences in their fatty acid compositions, in particular, a lower percentage of linoleic and oleic acids within the soybean oil. Copyright © 2007 John Wiley & Sons, Ltd.     

Reduction in axle oil operating temperatures by fluids with optimized torque transfer efficiencies

Effective axle oils must efficiently transfer torque from the drive‐train to the wheels, while maintaining low axle oil operating temperatures. Previous studies have shown that fluids, which form thicker elastohydrodynamic (EHD) films and have lower EHD friction, have higher torque transfer efficiencies (TE) and lower axle oil operating temperatures (OT). In general, oils with higher viscosities form thicker films and those with lower viscosities have lower EHD friction. Therefore, optimizing oil's rheological properties to maximize TE and minimize OT is difficult. In this paper, we examine two approaches to maintaining high TE while reducing OT. One approach is to minimize boundary friction since previous studies have shown that the boundary frictional properties of oils influence OT and not TE. A second approach is to more thoroughly examine the effect of rheology on film thickness and EHD friction. Film thickness and EHD friction are related to the high temperature high shear viscosity and pressure‐viscosity coefficient of oils. We have found that oils with high pressureviscosity coefficients and low high temperature high shear viscosities will form thick films and have low EHD friction. This optimized combination of physical parameters, along with lowering the boundary friction coefficient of axle oils, results in oils with high TE and low OT. Copyright © 2006 John Wiley & Sons, Ltd.