Synthetics basics: Polyalphaolefins — base fluids for high-performance lubricants

Polyalphaolefins or PAOs are saturated oligomers manufactured via the catalytic oligomerisation of alphaolefins. An oligomeric mixture is obtained which is then hydrogenated and distilled to produce a range of PAOs. These synthetic fluids are differentiated and graded by their viscosities at 100°C. PAOs are mainly used as high-performance functional base fluids in engine lubricants, gear oils, hydraulic oils, automatic transmission oils, compressor/pump oils, and greases. Automotive engine (crankcase) lubricants represent the biggest market for PAOs and here they compete with petroleum-based oils. When compared to mineral oils, PAOs exhibit a number of inherently superior physical and chemical properties, such as greater fluidity at low temperature, lower volatility, a higher viscosity index, a lower pour point, better oxidative/thermal stability, low toxicity, and biodegradable low viscosity grades. This combination of properties means that PAOs enhance the formulator's ability to produce high-performance lubricant oils. Some examples of performance benefits obtained from PAO-based formulations have been shown to match and even exceed the performance of higher cost traditional ester-based oils when tested under the new Japanese Automobile Standards Organisation (JASO) FC two-stroke specifications. Low viscosity automotive engine oils, which can only be formulated with PAOs, can produce a fuel saving of >2% when tested under US Federal Test Procedure (USFTP) 75.     

Alkylated naphthalenes as high‐performance synthetic fluids

The physical and chemical properties for a series of structurally well‐defined, alkylated naphthalenes have been compared. Structural characteristics, such as the alkyl chain length, alkyl chain branching, and number of alkyl groups on the naphthalene ring, have been correlated with physical properties, such as viscosity, viscosity index, pour point, and aniline point. The thermo‐oxidative stability of the alkylated naphthalenes was evaluated by the rotating pressure vessel oxidation test (RPVOT), a bulk oxidation test (BOT), and pressure differential scanning calorimetry (PDSC). Although all of the alkylated naphthalenes showed better thermo‐oxidative stability than other traditional hydrocarbon‐based fluids, the relative performance of the structurally different alkylated naphthalenes was found to be test dependent. Initial RPVOT studies with a linear polyalkylated naphthalene show that it gives good additive response to the incorporation of traditional antioxidants.     

Improving the performance of synthetic base fluids with additives

Because of their improved performance characteristics over mineral oil based lubricants, partly and fully synthetic fluids are being used more open, the main types of synthetic base fluids being esters and polyalphaolefins (PAOs). The response of additives, such as anti-oxidants, anti-wear additives and viscosity index improvers, to mineral oil, eslers or PAOs will obviously differ. This paper reports an attempt to find the most effective types of such additives for esters and PAOs, in order to optimize lubricant performance. PAOs have deficiencies in terms of a number of peformance characteristics, particularly with respect to additive solubility, and they are thus blended with esters to improve these properties.     

A comparison of fire-resistant hydraulic fluids for hazardous industrial environments. Part I. Fire resistance and lubrication properties

Fire-resistant hydraulic fluids have been developed since the Second World War. In addition to phosphate esters, there are various other fluids (e.g. oil-in-water emulsions, water glycols) available. This paper provides a technical comparison of the different fluid types, in terms of fire resistance, lubricating properties, and viscosity. The second part of the study examines the corrosion properties, thermal and oxidative stability, shear stability, materials compatibility, and other physical properties.     

A comparison of fire-resistant hydraulic fluids for hazardous industrial environments. Part II. Stability,fluid life,and environment

Fire-resistant hydraulic fluids have been developed since the Second World War. In addition to phosphate esters, there are various other fluids (e.g. oil-in-water emulsions, water glycols) available. This paper provides a technical comparison of the different fluid types, in terms of fire resistance, lubricating properties, and viscosity. The second part of the study examines the corrosion properties, thermal and oxidative stability, shear stability, materials compatibility, and other physical properties.     

Synthesis and Characterization of Alkyl-Bridged Bicycloheptanes as Traction Fluids

The traction fluid is a critical component of a toroidal-continuously variable transmission (T-CVT). As the medium that transmits power through the toroids, the traction fluid needs to provide a high traction coefficient and retain low dynamic viscosity at cold temperatures; this is a challenging combination of properties. A comparison of a variety of fluids shows a broad correlation between the traction coefficient and the fluid's low temperature viscosity, or pour point. This work investigated a series of novel compounds as traction fluids through chemical synthesis and the measurement of their relevant physical properties. Specifically, four new alkyl-bridged bicycloheptane fluids have been synthesized and refined to high purity. Their traction coefficients, measured with a ball-on-disc traction apparatus, are comparable to those of commercial fluids over the relevant range of temperature and pressure. Their dynamic viscosities at low temperature, however, are higher than the viscosity of commercial fluids and exceed the value of 3 × 10 ⁴ cP at ?40°C. These bridged bicycloheptanes also exhibit a correlation between their low-temperature viscosity and traction coefficient. The reasons for this correlation are discussed, and the effect of the molecular structure on viscosity and traction coefficient is investigated. This analysis finds semi-quantitative relationships between fluid properties and the molecule's volume, stiffness, and ring structure.     

A laboratory evaluation of partial synthetic automatic transmission fluids

Blending synthetic fluids with conventional base stocks gave large reductions in low temperature viscosity without appreciable reductions in high temperature viscosity or deterioration in properties such as volatility or flash point. Ten percent blends of a polyalpbaolefin or an ester synthetic reduced the viscosity at — 27.9°C by approximately 50%. These blends gave improved oxidation stability as measured by the Aluminum Beaker Oxidation Test, but poorer antiwear performance as measured by the Four Ball Test. The latter result indicated that the antiwear additive package should be optimised to accommodate composition differences between conventional fluids and conventional/synthetic blends.     

Film-forming properties of estolides

Estolides are biobased materials obtained from the synthesis of ingredients derived from agricultural products. They are oligoesters obtained by the reaction of fatty acids and/or methyl esters with a double bond. By varying the chemistries of the starting materials and the reaction conditions, estolides of varying chemical structures (e.g., branching), and physical properties (e.g., mol wt, viscosity, pour point, cloud point) are obtained. Estolides have been found to have suitable properties for some lubrication applications. In this work, the effect of estolide physical/chemical variability on film thickness and pressure–viscosity coefficient (pvc) were examined. The results showed that estolides have lower pvc than the non-polar hydrocarbon PAO, but much higher than seed oils (e.g., soybean, jojoba, canola), which are used as feedstock in estolide synthesis. The film thickness and pvc properties of estolides were also found to be dependent on the structure (e.g., homo- versus co-oligomer) and purity of the estolide oils. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable.     

Development of Al-base lubricating grease thickeners

In this paper are presented the results of work on the development of poly-oxo–aluminium acylates that are dissolved in synthetic lubricants, polyalphaolefins (PAOs) andpolyisobutenes (PIBs), and one pharmaceutical mineral oil in order to produce a range of greases that can be food-grade, and have chemical stability. The method of synthesis is described, together with the results using different base oils (two different PAOs, four different polybutenes, and a pharmaceutical grade IS0 VG 46 white mineral oil. These greases are characterised here in terms of their mechanical stability and physical properties. As well as being of food grade, these greases are also shown to be chemically stable.     

Current and future polyalphaolefins

Polyalphaolefins currently have a market share of less than 1 per cent in Europe, but this is expected to double in the next 5 years. This paper looks at additive response, rheology, volatility, and oxidation resistance of PAOs; of particular importance are their low temperature capabilities. The results of various tests are given, in comparison with mineral-based oils. In conclusion, research is under way to investigate the possibilities of ‘tailor-making’ PAOs by varying production process variables and using a variety of starting olefin streams.     

Behavior of perfluoropolyalkylethers under simulated dynamic bearing conditions

Perfluoropolyalkylether (PFPAE) fluids are of interest to the United States Air Force as potential high temperature liquid lubricants in gas turbine engines. PFPAE fluids have desirable thermal and oxidative stability, and favorable temperature/viscosity characteristics. However, their performance depends on the specific base fluids, additives, bearing material used as well as contact conditions and environments. Screening tests using a modified ball‐on‐rod type rolling contact fatigue (RCF) tester were conducted to study the effects of the above variables and lubricant circulation on fatigue life, wear and performance of PFPAE. Post test lubricant samples were analyzed for changes in physical and chemical properties. Traditional testing for viscosity, acid number and weight changes was performed. Fluid degradation was studied using Fourier transform infrared spectroscopy (FTIR), gas chromatography with atomic emission detector (GC‐AED) and supercritical fluid chromatography (SFC). Elemental analysis of the deposits formed at the tribocontact were determined by Auger electron spectroscopy (AES). This revised version was published online in July 2006 with corrections to the Cover Date.     

Modeling and characterization of piezoelectric cantilever in fluids at different temperatures

This paper introduces measurement and modeling of a piezoelectric beam used as a sensor in different types of liquids. It is immersed in different fluids at temperatures increasing from 20 °C to 50 °C. The working principle is based on detecting different resonance frequencies of the cantilever in different solutions. The oscillation of piezoelectric beam is measured using a vector network analyzer. An electrical equivalent circuit derived from a resonator model is used to simulate the experimental data. These calculated circuit constants have been related to physical properties of liquids under test. The combination of these liquids which includes non-conducting and conducting solutions, exhibiting low and high viscosity covers a good range of common physical properties of fluids. Main focus of this research is to explore the capability of piezoelectric cantilever as a liquid sensor with the influence of temperature. The equivalent circuit model has been proved to be viable to fit experimental data in non-conducting solution but less effective in conducting solution.     

The Relation of Fluid Properties and High Temperature Hydraulic Performance

The performance of hydraulic systems at high temperatures is particularly sensitive to the properties of the hydraulic fluids. The effects of stability, viscosity, compressibility, volatility, gas solubility, and lubrication on hydraulic component and system performance are discussed. These properties singly and in combination affect many aspects of hydraulic system operation. The reaction of fluids to high temperatures causes significant changes in hydraulic performance.     

Dynamic and Static Irradiation of Nuclear Power Plant Lubricants

Proper choice of lubricating greases for use in machinery exposed to high energy radiation requires not only a sound estimate of the service the radiation dose, but also careful consideration of the normal lubrication variables. Interaction of all the variables may be complex. Static and dynamic test results are given which illustrate this point, and which reveal utility of some commercial mineral oil greases and a new high temperature radiation resistant grease.

Irradiation tests of turbine oils show that maximum expected radiation dose in current and projected power plants over a twenty year period does not change the physical properties of the oil. Oxidation stability and other properties are therefore still of most importance in turbine oil selection. Some experimental radiation-resistant fluids are discussed which have potential for fluid lubricant use where radiation resistance may be required.     

Low pour point polyalkyl benzene oils for refrigeration compressors

In this paper, the results of fractionation of polyalkyl benzene residues from monoalkyl benzene production, using a molecular evaporator to obtain fractions which are usable as lubrication oils for refrigeration compressors with a non-polar refrigerant, are presented. Two of the distillion fractions pre-pared correspond to international viscosity classes I S 0 VG 22 and 46. Their yields of 45 and 15% respectively from the original residues are economically interesting and they are of a higher quality than the previously used mineral oil based lubricants. The polyalkyl benzenes have improved properties, such as viscosity index, flash point temperature, and pour point. The lower thermo-oxidative stability of the fractions is compensated for formulations with appropriate antioxidants.     

Hydrolysis of phosphate-based aviation hydraulic fluids

High-temperature in-service failure of phosphate ester-based aviation fluids is primarily due to acid-catalysed hydrolysis. The reaction is controlled with epoxides, but careful attention to the other additives is needed to avoid including species which catalyse hydrolysis. From an airline's point of view, water concentration is the critical factor, since it is the only variable that it can control. From a system designer's point of view, minimising fluid temperature should be a goal. Plots relating fluid life to temperature and water level for typical commercial fluids are included.     

Tribological behaviour of polyalphaolefins: Wear and rolling‐contact fatigue tests

Polyalphaolefin (PAO) fluids have become widely accepted as high‐performance lubricants and functional fluids due to certain inherent, and highly desirable, characteristics. One of these characteristics is their low toxicity, which, combined with excellent viscometrics and lubricity, have made low‐viscosity PAO fluids an important component in lubricant formulations. Typical data found in product specifications for lubricants are the kinematic viscosity and the viscosity index. These values do not give enough information with which to choose the optimum lubricant for a lubricated contact. In mechanical systems, rolling, sliding, and rolling/sliding contacts occur, and lubricants have to work optimally under these operating conditions. In this study the rolling‐contact fatigue lives (L₅₀ and L₁₀) of PAOs of different viscosities were experimentally determined. The tests were carried out using a four‐ball machine. Wear tests were also carried out using another four‐ball tester in order to measure the wear‐scar diameter and the flash temperature parameter. The lubricants were characterised by infrared spectroscopy, and the pitting of the balls was observed using scanning electron microscopy.     

Effect of Base Oil Structure on Elastohydrodynamic Friction

The EHD friction properties of a wide range of base fluids have been measured and compared in mixed sliding–rolling conditions at three temperatures and two pressures. The use of tungsten carbide ball and disc specimens enabled high mean contact pressures of 1.5 and 2.0 GPa to be obtained, comparable to those present in many rolling bearings. The measurements confirm the importance of molecular structure of the base fluid in determining EHD friction. Liquids having linear-shaped molecules with flexible bonds give considerably lower friction than liquids based on molecules with bulky side groups or rings. EHD friction also increases with viscosity for liquids having similar molecular structures. Using pure ester fluids, it is shown that quite small differences in molecular structure can have considerable effects on EHD friction. The importance of temperature rise in reducing EHD friction at slide–roll ratios above about 5% has been shown. By measuring EHD friction at several temperatures and pressures as well as EHD film thickness, approximate corrections to measured EHD friction data have been made to obtain isothermal shear stress and thus EHD friction curves. These show that under the conditions tested most low molecular weight base fluids do not reach a limiting friction coefficient and thus shear stress. However, two high traction base fluids appear to reach limiting values, while three linear polymeric base fluids may also do so. Constants of best fit to a linear/logarithmic isothermal shear stress/strain rate relationship have been provided to enable reconstruction of isothermal EHD friction behaviour for most of the fluids tested.     

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.     

Tribological Properties of Vegetable Oils Modified by Reaction with Butanethiol

Corn, canola, and castor-lauric estolide oils were chemically modified by photochemical direct reaction of butanethiol with the double bonds on the hydrocarbon chains. The effect of chemical modifications on viscosity, viscosity index (VI), pour point (PP), cloud point (CP), oxidation stability (RPVOT), 4-ball anti-wear (AW), and extreme pressure (EP) were investigated. The sulfide modified (SM) corn and canola oils showed increased viscosity, increased RPVOT time (more than one order of magnitude), reduced PP (9?C18 °C), and reduced VI. The SM estolide displayed similar trends in VI and RPVOT but showed no change in viscosity or cold flow. The SM oils, along with commercial mono- and polysulfide additives were also investigated as additives, at 0.6% (w/w) S concentration, in corn and polyalphaolefin (PAO) base fluids. In both fluids, the additives resulted in minor changes of PP, CP, coefficient of friction, wear scar diameter (WSD), and weld point (WP). Only the commercial polysulfide EP additive displayed large WP increases in the fluids. The additives resulted in no change of oxidation stability of corn oil, but displayed big improvement in the oxidation stability of PAO (8 to 16-fold increase in RPVOT time). The difference in the effect of the additives on the oxidation stability of PAO versus corn oil was attributed to the difference in the reactive hydrogen contents in the two base fluids relative to those in the additives. An empirical equation, for correlating change in RPVOT time with change in bond dissociation energy of reactive protons before and after chemical modification, is proposed.