Influence of degradation on the film‐forming properties of polyolester oils

The film thicknesses of two polyolester degraded oils were measured over a wide range of temperatures to investigate the influence of degradation on film‐forming properties. The results have been interpreted in the light of the idea that degradation of an ester lubricant can produce two different types of product which have opposing influences on film‐forming properties. One is the formation of smaller, polar molecules, such as acids, which may result in a decrease in effective pressure—viscosity coefficient. The other is the formation of larger, bulky molecules such as sludge, which result in increased pressure—viscosity coefficient. The effective pressure—viscosity coefficient decrease with degradation of the oil thus is made up of an earlier stage involving a series of reactions, followed in a later stage by an increase.     

Friction and film‐forming behaviour of five traction fluids

There is growing interest in the use of toroidal‐type traction drives to provide continuously variable transmissions for use in medium‐ to high‐power automotive engines. These transmissions require the use of specially designed traction fluids to provide high friction in full‐film elastohydrodynamic lubricated contacts. This study has measured the film‐forming and traction properties of five commercially available and developmental traction fluids to provide data needed to predict their performance in traction drives. Some differences in performance between the fluids have been noted.     

Influence of thermo‐oxidative degradation on the biodegradability of lubricant base oils

The paper addresses the problem of how the thermo‐oxidative changes in the chemical structure influence the biodegradability of lubricating base oils. The tests were conducted using synthetic polyolesters, polyalphaolefins (PAO 4) and blended polyolesters with 30 wt % of PAO 4. Oil biodegradability was evaluated in accordance with the International Standards Organization (ISO) 14593 test. The thermo‐oxidative degradation consisted of a 24‐hour exposure of the oil samples to the temperature of 150°C in a universal test apparatus at an air flow rate of 15 L/hour. The results have revealed that the thermo‐oxidative degradation of the chemical structure produces changes in the physico‐chemical properties of the base oils and improves biodegradability. The polyolester oils tested were found to be readily biodegradable both before and after thermo‐oxidative tests. PAO 4 oil and its blends with polyolester oils could not be classified as readily biodegradable. The thermo‐oxidative changes in their chemical structures can exert a positive effect on their biodegradability. Copyright © 2008 John Wiley & Sons, Ltd.     

Performance of synthetic oils in the hydrodynamic regime ‐ II. Generalisation

In Parti I the results of an extensive experimental investigation of the performance of environmentally adapted oils in the hydrodynamic regime were reported. Four oils were tested in a tilting‐pad thrust bearing for different combinations of load, shaft speed, and supply oil temperature. In this second part, details of a generalisation procedure are described. A number of parameters representing the physical properties of an oil, such as viscosity and viscosity‐temperature coefficient, are adopted. The influence of each of these parameters on minimum oil film thickness, maximum temperature rise, and bearing power loss is then analysed. It is shown that viscosity measured at the supply oil temperature is the most important parameter. The effects of the viscosity‐temperature coefficient and oil thermal conductivity are less pronounced and yet significant. It is also shown that it is not possible to select an optimum oil that yields maximised oil film thickness, minimised temperature rise, and minimised power loss at the same time.     

Effect of chemical structure on the hydrolytic stability of lubricating base oils

Base oils differing in chemical structure, representatives of hydrocarbon fluids, ester fluids, and water-insoluble polyalkylene glycols (PAGs), were tested (ASTM D2619) for comparison of their hydrolytic stability, which was found to depend on both their chemical structure and purity. The variations observed after hydrolytic degradation in oil acid numbers, water layer acidity, and copper (Cu) strip weight are indicative of excellent hydrolytic stability of hydrocarbon oils, very good hydrolytic stability of PAG oils, and good hydrolytic stability of polyolester oils (characterized by high purity and an acid number lower than 0.1 mg KOH/g). The positive effect of steric hindrance around the ester bonds on the hydrolytic stability of polyolesters was substantiated by the results obtained for the oils of a pentaerythrite tetracaproate and pentaerythrite tetra(sec-caproate) structure.     

Correlation of structure and properties of groups I to III base oils

The understanding of the relationship between molecular structure and viscosity–temperature behaviour of a lubricant system is a subject of considerable importance. The quantitative distribution and types of different classes of hydrocarbons such as aromatics, paraffins (normal and iso) and naphthenes determine the physico‐chemical behaviour of a lubricant system. The study of molecular structure and molecular alignment of hydrocarbons constituting a lubricant helps in the development of lubricating oil with desired physico‐chemical properties. The present study highlights the application of nuclear magnetic resonance spectroscopic technique for deriving detailed hydrocarbon structural features present in API groups II and III base oils produced through catalytic hydrocracking/isodewaxing processes. The viscosity–temperature and viscosity–pressure properties, such as viscosity index, pour point, elastohydrodynamic film thickness and cold cranking simulator viscosity, were determined. The structural features of these base oils such as various methyl branched structures of isoparaffins and branching index, which are characteristics of high performance molecules, were correlated with the above‐mentioned properties to explain their physico‐chemical properties, particularly low temperature properties. The molecular dynamics parameters such as diffusion coefficient and T₁ relaxation times estimated from the nuclear magnetic resonance spectral studies have provided sufficient evidence for the dependence of these properties on these high performance molecules present in various types of methyl structures of isoparaffins of groups II and III base oils compared with conventional group I base oils. Results are explained on the basis of molecular structural differences of hydrocarbons present in these base oils and diffusion measurement studies. On the basis of the studies, molecular engineering concept for the designing of a high performance base oil molecule is proposed. Copyright © 2012 John Wiley & Sons, Ltd.     

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 vegetable oils and paraffinic oils with unsaturated fatty acids under high‐contact‐pressure conditions in four‐ball tests

The tribological properties of three kinds of vegetable oils of different composition under high‐contact‐pressure conditions were evaluated in four‐ball tests to elucidate the lubrication mechanism of vegetable oils. The types of fatty acids composing the triglycerides and the concentration of dissolved oxygen appeared mainly to control the lubricating performance. Therefore, glycerol and a paraffinic oil containing fatty acids were tested as a comparison. Further, the influences of fatty acid types, antioxidant additives, and dissolved oxygen in the oils on the lubricating performance were examined. It was shown that linolenic acid with three double bonds was the most effective at higher temperature, where the formation of a friction polymer was observed, although the addition of antioxidant additives suppressed the formation of the friction polymer but reduced the load‐carrying capacity.     

The influence of chemical structure on the physical properties and antioxidant response of hydrocracked base stocks and polyalphaolefins

This study was conducted to determine the effect of hydrocracked base stock chemical composition on lubricant properties, oxidation performance, and antioxidant additive response. Fifteen hydrocracked base stocks and polyalphaolefins were analysed by mass spectrometry for paraffinics, and single‐, double‐, and multi‐ring naphthenics. Low levels of aromatics were confirmed in all the base oils. A linear relationship was found between certain naphthenic structures in the base stocks and properties, such as viscosity index, aniline point, and volatility. Additivated versions of the base stocks were also screened in the rotary bomb oxidation test (ASTM D 2272) and the thin‐film oxygen uptake test (ASTM D 4742). A relationship between the types of naphthenic structures in the additivated base stocks and oxidative stability was found. Base oils containing low levels of condensed multi‐ring naphthenic structures exhibited superior oxidation performance, lower volatility, and poorer solvency. Furthermore, antioxidant structure had a profound effect on oxidative stability as the level of multi‐ring naphthenic structures in the base oils decreased. These results suggest that deeper knowledge of chemical composition could help in selecting base stocks and additives to meet future product specifications.     

The influence of some additives on the lubricating properties of semi‐synthetic ester oils

This article presents research on the lubricating properties of semi‐synthetic oils, made as compositions of mineral oil and esters. The esters were synthesized from acid substrates from the oxidation of paraffins. The last non‐seizure load (P_n), the weld point (P_z), the load wear index (I_h), and the limiting wear load (G_oz) were measured, and the effects of additives or mixtures of additives were investigated by their effects on these parameters. It was found that the efficiency of the additives depended on their chemical characteristics and concentration, and also on the chemical nature of the ester component of the semi‐synthetic oils.     

Performance of synthetic oils in the hydrodynamic regime — 1. experimental

An investigation of the performance of environmentally adapted synthetic oils in the hydrodynamic lubrication regime has been carried out. Four oils have been tested: polyalphaolefin and ester based ISO VG46 oils as well as mineral ISO VG68 and VG46 oils. Tests were conducted in a facility containing two identical tilting‐pad thrust bearings typical of the design in general use. The differences between the mineral and synthetic oils in terms of maximum operating temperature, minimum oil film thickness, and bearing power loss have been examined. Substitution of the mineral ISO VG68 oil with an ISO VG46 oil slightly reduces the bearing operating temperature. This is due to a decrease in the basic viscosity. It is concluded that the ester base ISO VG46 oil can be used as an environmentally adapted replacement for the mineral ISO VG68 oil without sacrificing bearing safety. Such a change also offers noticeable energy savings. If the ester based oil is used instead of a mineral oil of the same viscosity grade, bearing reliability is improved by the increased oil film thickness.     

Rheological properties of high‐viscosity‐index mineral base oils for automotive engine lubricants

Viscosity‐pressure‐temperature relations for paraffinic mineral base oils at pressures up to 0.7 GPa and temperatures between 30 and 90°C were determined using a falling‐ball‐type viscometer. The oils used were solvent refined oils, hydrocracked oils, and an oil produced by a wax isomerisation process. The viscosity at pressures higher than those possible with the viscometer was then derived by applying a simplified solution to the traction curves determined using an elastohydrodynamic disc‐on‐ball tester. When the measured viscosity and the calculated viscosity were plotted against pressure, for the oils with a viscosity index higher than 120 the viscosity derived from traction measurements followed the curve extrapolated to the high‐pressure region using either the Yasutomi or Roelands equations (the parameters for which were obtained using the viscometer). However, the calculated viscosity for the lower‐viscosity‐index oils deviated upwards from the extrapolated curve.     

Tribological properties of crown‐type phosphate ionic liquids as lubricating additives in rapeseed oils

The friction and wear properties of rapeseed oils with different concentrations and crown‐type phosphate ionic liquids were studied by an Optimol SRV oscillating friction and wear tester. Crown‐type phosphate ionic liquids have better solubility than conventional ionic liquids in rapeseed oils. The tribological test results showed that the crown‐type phosphate ionic liquids as lubricating additives in rapeseed oil exhibited better tribological performance than the base oil for steel/steel friction pair under various loads. It is noted that the friction pair showed the least friction coefficient and wear volume when the concentration of ionic liquids was 1 wt.%. The better tribological properties of friction pair should be attributed to the effective boundary films formed in the worn surface, on various tribochemical products, organometallic products and iron oxides. Copyright © 2012 John Wiley & Sons, Ltd.     

Phosphate esters with a complex structure of an alkyl‐aryl type considered as base oils

This paper presents results of work carried out to produce phosphate esters of the alkyl‐aryl type. Three new triesters of mixed structure have been synthesised using a special alcohol with an aryl content, e.g., 2‐phenoxy ethanol, and a very long branched alcohol, e.g., isotridecanol. The influence of the aryl content and the effect of the long aliphatic chain on the main tribological properties have been investigated.     

Some comments on micro‐abrasion interactions of pure metals in bio‐oils

An attempt has been made to study the effect of bio‐oils on the two‐ and three‐body abrasion process of pure metals and steel. The equipment used was a micro‐abrasion tester (TE‐66, Phoenix Tribology (Plint)). The materials considered were mild steel, copper, and aluminium, being the most usable and basic materials for industrial applications such as bearings and gear assemblies. The results were used to identify transitions between wear regimes as a function of sliding speed and load. The effect on the abrasion process in the presence of various lubricant oils was also assessed. The mechanisms of abrasion with and without particles were characterised.     

Synthesis and tribological behaviour of phosphorus—nitrogen‐modified rape seed oils as biodegradable lubricant additives

Two types of phosphorus—nitrogen‐modified rape seed oils as biodegradable lubricant additives were synthesised and characterised by infrared spectroscopy. Their tribological properties in rape seed oil and in mineral oil were evaluated in a four‐ball tester. The morphologies of the worn surfaces were analysed by scanning electron microscopy. The results show that the modified rape seed oil additives improve the load‐carrying capacity and the antiwear and friction‐reducing properties of rape seed oil more than they do those of mineral oil. The inferred mechanism of lubrication is that a high‐strength adsorption film and/or tribochemical reaction film forms on the rubbing surfaces, due to the carrier effect of long‐chain rape seed oil molecules and to the high reaction activities of phosphorus and nitrogen and their synergism.     

Effect of the chemical structure of fluorinated esters on the tribological properties of a steel‐on‐steel system

The effect of the chemical structure of fluorinated esters on the friction and wear behaviour of a steel‐on‐steel system was investigated. The friction and wear testing of a steel disc sliding against a counterpart ball of the same steel was carried out using an Optimal SRV oscillating friction and wear tester. The chemical features of the worn steel surfaces were analysed by means of X‐ray photoelectron spectroscopy, and the morphologies and elemental compositions of the worn steel surfaces observed and determined using scanning electron microscopy. The results indicate that a fluorinated ester with methylene groups that are not substituted by fluorine in the acid structure gives the best friction‐reducing behaviour and a keto‐ester shows the best antiwear properties.     

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.     

Experimental investigation of transient and thermal effects on lubricated non‐conformal contacts

In this work, thermal and transient effects on non‐conformal lubricated contacts are investigated through experimental analyses. Experiments between a ball and a plane surface of a disc are described. Friction coefficients and film thicknesses are measured (the film thickness only for the glass‐on‐steel contact). A paraffin base mineral oil is used as a lubricant. First experiments are carried out under steady‐state conditions. To include effects due to different thermal properties of contacting materials, a steel‐on‐steel and a glass‐on‐steel contact with different slide‐to‐roll ratios are tested. If the contacting materials have different thermal properties, as in the case of a glass‐on‐steel contact, thermal effects like the temperature–viscosity wedge action could clearly be shown. It is found that the friction coefficients are influenced by the slide‐to‐roll ratio and the thermal properties of the contacting materials. Under transient conditions, the entraining velocity is varied with a sinusoidal law. Squeeze effects explain ‘loops’ of friction and film thickness found also in previous works. The formation of friction loops is related to the measured film thickness differences. However, also under non‐steady‐state conditions, thermal effects, like the temperature–viscosity wedge action, influence the friction coefficients. Copyright © 2007 John Wiley & Sons, Ltd.     

Influence of surface roughness features on mixed‐film lubrication

An optical technique (three‐dimensional spacer layer imaging) has been developed to map accurately lubricant film thickness in thin‐film elastohydrodynamic (EHD) contacts. This experimental technique has been used to study the influence of surface roughness features, asperity height, and slope on EHD film thickness and pressure. Single ridges transverse to the entrainment direction were used to represent asperities. It was found that the ridges with lower slopes generate films of greater minimum thickness. Below a certain entrainment speed, the minimum film thickness declined at a rate dependent on the ridge slope. At low speeds, the ridges with higher slopes entrapped a larger volume of lubricant ahead of the ridge and along the entrainment direction. For all speeds, the highest ridges entrapped the most lubricant. Both ridge slope and ridge height had a negligible effect on mean film thickness in the contact. Asperity pressure increased with higher ridge slope, but was not influenced by entrainment speed. An increase in pressure was found where lubricant is entrapped upstream of a ridge.