The fundamental molecular structures of synthetic traction fluids

In general, ‘dicyclohexylalkane type compounds’ are known as practical traction fluids. We examined the reason polycyclic compounds have higher traction coeffecients than monocyclic compounds and obtained the following results. (1) In monocyclic compounds, the stiffer the saturated ring, the higher the traction coefficient. (2) The more difficult the rotation of a whole molecule, the higher the traction coefficient. (3) The traction is more affected by molecular size (the rotation of a whole molecule) than by molecular stiffness (the internal rotation of a molecule).     

Molecular structures of traction fluids in relation to traction properties

In order to develop excellent traction fluids, guidelines for molecular design were studied through analyzing correlation between molecular structures and traction properties of traction fluids quantitatively. Some results among various simple compounds as model compounds of traction fluids and among various traction fluids were described in this article. This revised version was published online in June 2006 with corrections to the Cover Date.     

Optimisation of molecular structure for traction fluids

According to the guidelines for the molecular design of traction base fluids obtained from a previous study, more than 100 chemical compounds have been synthesised. Employing 61 naphthenic compounds whose chemical structures have been clearly identified, the optimisation of molecular structure has been studied regarding the compatibility of high‐temperature traction coefficient and low‐temperature fluidity. As a result, the requirements for molecular structures for high‐performance traction base fluids have been clarified.     

Study on the fundamental molecular structures of synthetic traction fluids: Part 2

It is essential to obtain guidelines for molecular design through analysing quantitatively the correlation between molecular structures and traction coefficients of traction fluids for developing an excellent traction fluid with a high traction coefficient. Although it is well known that the hydrogenation of an aromatic compound greatly increases its traction coefficient there are few accounts explaining the reason. We have analysed the influence of the intermolecular force on traction, examined why aromatic and chlorinated compounds have low traction coefficients and obtained the following results. (1) Alicyclic compounds have high traction coefficients because they interlock with each other well under high pressure. (2) Aromatic compounds have low traction coefficients because it is difficult for them to get close to each other due to repulsion of π-electrons and they have little unevenness for interlocking. (3) Chlorocyclohexane has a low traction coefficient due to the repulsion of large chlorine atoms which have a negative charge. (4) The traction property is greatly influenced by intermolecular force and molecular size, and the influence of the molecular stiffness is even smaller.     

Rheology of synthetic lubricating oils

A lubricating fluid's flow characteristics are affected by fluid deformation resulting from the action of mechanical forces: flow resistance, dissipation of mechanical energy, transfer and dampening of mechanical disturbances. These in turn affect the lubricating ability of the fluid through energy absorption, power transfer and other properties. Synthetic fluids can display various thixotropic characteristics and non-Newtonian effects. In this paper, experimental work on these phenomena are reported.     

New synthetic esters with a complex structure considered as tribological fluids

The paper presents results concerning synthesis and characterisation as lubricant fluids of some esters of different monocarboxylic acids with variable chain length, along with aliphatic‐aromatic complex alcohols and/or with superior alcohols (exclusive of aliphatic). The monocarboxylic acids taken into account were n‐butyric, n‐octanoic and oleic, respectively, while the alcohols considered were isodecyl and isotridecyl as purely aliphatic, and 2‐phenoxy‐ethanol, 2‐[(o‐sec‐butyl)phenoxy]ethanol and 2‐[(p‐nonyl)phenoxy]ethanol, as part of the complex aliphatic‐aromatic alcohols group. Under these circumstances, it became possible to study the influence of the structure on the main physico‐chemical properties of these synthetic oils as well as on their tribological representative features. The results recorded certified the validity of the proposed research programme. Copyright © 2009 John Wiley & Sons, Ltd.     

Thermal stability of synthetic oils in aviation applications

The thermal decomposition of selected synthetic base fluids has been investigated. The effects of temperature and time on the rate and extent of thermal degradation were studied in a modified steel bomb test device. The effects of chain length and branching are important factors in the thermal stability of hydrocarbons and similar fluids. In order to overcome the 350°C limit of thermal stability, other classes of chemical have to be used.     

A non-averaging method of determining the rheological properties of traction fluids

Traction machines have been frequently used to study the rheological responses of lubricants in elastohydrodynamic lubrication (EHL) contacts. Fundamental properties are inferred from EHL traction measurements based on the average pressures and temperatures in the contact. This average approach leads to uncertainty in the accuracy of the results due to the highly nonlinear resonse of the fluid such as viscosity to both pressure and temperature. A non-averaging method is developed in this paper to study the elastic and plastic properties of traction fluids operating in EHL contacts at small slide-to-roll ratios. A precision line-contact traction rig is used to measure the EHL traction at a given oil temperature and Hertz pressure. By choosing a sensible pressure-property expression, the parameters of the expression can be determined through the initial slope and peak traction coefficient of the traction measurements. The elastic shear modulus and the limiting shear stress of the lubricant corresponding to a single pressure can then be calculated for a range of pressures and temperatures of practical interest. Two high-traction fluids are studied, and their elastic moduli and limiting shear stresses presented.     

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.     

EHL traction and related rheological parameters under high temperature conditions

An approximate formula is presented for the maximum traction coefficient in EHD conditions, by making some reasonable simplifications of an Eyring fluid model. Experiments are conducted for some mineral and synthetic hydrocarbon oils over a wide range of temperatures. Among the rheological parameters related to maximum traction, the representative stress τ₀, is determined from the experimental traction curve while the effective viscosity-pressure coefficient α is obtained in a high pressure viscometer. When the temperature is raised, α falls and τ₀ rises. The behaviour of τ₀ for some paraffinic oils follows the Eyring theory while the effect of the dissociation of molecular clusters appears with the naphthenic oils. The increase in temperature causes a reduction in the maximum traction coefficient, which can be predicted with sufficient accuracy by the formula using the rheological parameters expressed as a function of temperature.     

两种合成航空润滑基础油高温氧化衰变机理研究

为研究航空润滑油的热氧化安定性,模拟聚α-烯烃(PAO)和酯类油(DE)两种合成航空润滑基础油在发动机内的高温工况,借助傅里叶红外光谱(FTIR)、气相色谱/质谱(GC/MS)联用等仪器对反应油样的黏度和结构组成进行测试与分析.结果表明,PAO具有较差的热氧化安定性能,在200℃时就发生分解,而DE的分解温度可达到300℃.在两种航空润滑基础油的高温衰变中,均有不同的产物生成.PAO衰变产物主要包括烷烃和烯烃,而DE的衰变产物主要是含氧化合物.最后,根据实验结果分析了航空润滑基础油的高温衰变机理.     

The identification and evaluation of natural and synthetic esters in mineral oils

The use of mineral-based lubricating oils blended with vegetable oils or their derivatives has raised the question of the detection, identification, and quantitative evaluation of these substances in formulated products. This paper addresses the problem in a systematic way, and suggests some possible solutions by means of analytical pathways using column chromatography, GLC, and HPLC techniques. The methods suggested allow the identification and evaluation of fatty acid esters with mono- and polyalcohols.     

Viscosity–pressure–temperature behaviour of mineral and synthetic oils

A new high‐pressure viscometer that can measure viscosity at pressures up to 0.8 GPa has been developed in the authors' laboratory. The ‘modulus equation’ has been used to compare the behaviour of mineral and synthetic lubricants. Among the oils investigated there was one ester that biodegraded rapidly both before and after ageing in a long‐term test‐rig operation. To facilitate a comparison or application of the results to other oils, an analysis of the correlation between the viscosity—pressure coefficient and the kinematic viscosity measured at atmospheric pressure has been provided. A prediction of lubricant film thickness based on high‐pressure viscosity data is compared with film thickness measurements in a roller bearing.     

On the rheological behaviour of synthetic fluids thickened by solid additives

A variety of fluid space lubricants, i.e., synthetic oils thickened by solid additives (particles of PTFE and MoS₂), has been developed. These lubricants present a solid-like behaviour under yield stress, and a strong non-Newtonian effect during flow. Measurement of the yield stresses revealed an unexpectedly strong interaction between a synthetic hydrocarbon oil and one type of PTFE particle. Furthermore, the substitution of 1 vol.% PTFE particles by MoS₂ particles introduced a significant change in the yield stress values. This effect is also apparent in viscosity measurements. These lubricants display, through their rheological behaviour, good ability to replenish contacts over a wide range of temperatures, as shown by measurements taken at −20°C and −60°C.     

Studies on the effects of temperature on the chemical characteristics of automotive crankcase oils and their base oils

The effects of varying temperature regimes on the chemical characteristics of commercial mineral-based automotive crankcase oils and their base oils have been studied experimentally. This was achieved by simulating the hot-spot temperatures of parts of the engine surfaces, in an oxidizing atmosphere, using a modified version of the turbine oil oxidation test apparatus developed in our laboratory. The changes in the chemical characteristics of the engine oils and their base oils were studied with infrared spectroscopy, and tests of the oil-soluble acids, the acidity of the volatiles, the carbonyl peak index and the deposit formation.All the oils studied were found to possess high sensitivity to changes in temperatures. The types and nature of the hydrocarbons found in the base oils determined the nature of the volatiles and deposits formed. It has also been found that at temperature above 260°C, polymerization reactions in the fully formulated engine oils were greatly accelerated. In contrast, the base oils showed increases in carbonyl peak index but yielded low deposits. Thus suggested that under these conditions, the products of additive breakdown, may be catalysing the polymerization reactions.     

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.     

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.     

Effects of kinds of oil on power transmission: Efficiency of planetary traction drives

One of the most interesting problems in traction drives is the analysis of their power transmission efficiency. Using an experimental two-disk machine and two planetary traction drives, together with fourteen test oils, it is shown that the power transmission efficiency can increase up to 97% in the case of an experimental planetary traction drive. The synthetic traction oils bring about a lower efficiency than mineral oils when the output torque is small. This is ascribed to their higher traction coefficients which cause larger friction in the rolling bearings which support the rolling elements.     

Low‐temperature characteristics of synthetic fluids

The low‐temperature fluid properties of synthetic base fluids are described, showing how these properties are related to the molecular structure of the base fluid. Examples are given of how the principles of molecular design can be used to improve the low‐temperature properties of synthetic base fluids, focusing on esters derived from both natural and petrochemical raw materials. Results of studies using calorimetric and spectroscopic methods, including differential scanning calorimetry and nuclear magnetic resonance are presented, showing how these techniques can be used to give more detailed insight into the changes occurring in fluids at low temperatures.     

Calculation of Viscous EHL Traction for Squalane Using Molecular Simulation and Rheometry

Recently, remarkable agreement was reported between nonequilibrium molecular dynamics simulation and high-pressure Couette rheometry on squalane. We have utilized the viscosity-strain rate relationship obtained from this unique combination of experimental and simulation data along with high-pressure viscometer measurements to calculate the viscous traction curve in the elastohydrodynamic lubrication (EHL) regime. A comparison with measured traction at 0.57 and 1.29 GPa shows excellent agreement, confirming the validity of the measurements and simulations. Thus, we present for the first time, a successful calculation of EHL traction from the liquid shear response obtained from both molecular dynamics and rheometry.