Delivering synthetic performance with VHVI speciality base fluids

Historically, the best lubrication performance has been achieved by the use of synthetic fluids, such as polyalphaolefins or esters. Advanced commercial processing techniques for producing base oils through significant molecular change are now available. In many applications, automotive and industrial products formulated with these high‐quality API Group III speciality base fluids can achieve the same good performance as that from traditional synthetic fluids. This paper represents continued work to understand and demonstrate features of very high viscosity index (VHVI) speciality base fluids. Ultimately, the performance of a finished fluid is the key market requirement. Actual field performance can vary dramatically, even among polyalphaolefin‐based formulations. Several examples are given to show that equivalent high‐level synthetic performance can be delivered through a synergistic balance of VHVI speciality base fluids and additive chemistry.     

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.     

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.     

Composition and oxidative stability of hydro‐cracked base oils and comparison with a PAO

The oxidative stability of nine hydrocracked base oils from seven producers and a PAO was compared using the standard IP 48 test. After a comprehensive characterisation of the oils, an evaluation of oxidative stability was carried out by measuring some common oil parameters, such as viscosity characteristics, carbon residue, pentane insolubles, and acid number. Additionally, the compound‐class composition of the fresh and oxidised oils was determined, and an FTIR spectroscopic analysis was carried out. The oxidative stability of the hydrocracked oils was largely affected by the sulphur and aromatic hydrocarbon concentration in the oils. Oils with an increased sulphur content (above 80 ppm) had better oxidative stability than oils with a low sulphur content (20 ppm and lower), and there was a relatively large variability in the stability of the oils depending on the sulphur concentration. The oxidative stability of most of the hydrocracked oils with a low sulphur content was similar, and matched somewhat the stability of the polyalphaolefin.     

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.     

Biodegradability and toxicity of polyalphaolefin base stocks

Low viscosity polyalphaolefin fluids (2 and 4 cSt ® 100°C) are significantly biodegraded under the conditions of the CEC-L-33-T-82 test procedure. This is contrary to the widespread, but inaccurate, perception that polyalphaolefin (PAO)fluids are generally not biodegradable. In a time-extended CEC-L-33-T-82 test, the 2, 4, and 8 cSt fluids were shown to continue biodegrading well past the 21 -day span prescribed in the standard method. PAO fluids are considered non-toxic and non-irritating to mammals. Moreover, they are not expected to be toxic to aquatic organisms: in the Microtox® test with bioluminescent bacteria, no observable effects were detected for concentrations of 49,500 ppm of the water-soluble fraction.     

The film-forming properties of polyalkylene glycols

It is now recognised that, for many practical applications, an important property of a lubricant is its ability to generate thick, elastohydrodynamic (EHD) films in concentrated contacts. This paper describes a study of the EHD film- forming properties of polyalkylenie glycol lubricants. A wide range of polyglycol structures have been examined, with different monomer types, initiators, and molecular weights. Film thickness has been measured at several different temperatures using both conventional and ultra-thin film interferometry. From the measured film thicknesses, the effective pressure–viscosity coefficients of the lubricants have been evaluated. This has enabled a systematic investigation of the effect of polyalkylene glycol structure on both pressure–viscosity coefficient and EHD film formation.     

Evaluation of oxidation‐mediated volatility of hydrocarbon lubricant base fluids

Lubricant base fluids exhibit differences in volatility based on their molecular structures and the distribution of components. We report work to evaluate the volatilization of hydrocarbon base fluids using thermogravimetric analysis (TGA) and gas chromatographic simulated distillation. Commercial base fluids, including samples from API groups II, III, IV, have been evaluated. Kinematic viscosity was not a good predictor for volatility when all lubricants were considered. Polyalphaolefin (group IV) base fluids were found to have lower volatility than isoviscous group II and group III chemically modified mineral oil base fluids as measured by TGA, simulated distillation and Noack techniques. The advantages of TGA for measurement of lubricant volatility are discussed. Copyright © 2006 John Wiley & Sons, Ltd.     

A critical evaluation of film thickness‐derived pressure–viscosity coefficients

A single parameter, the pressure–viscosity coefficient, α, quantifies the pressure dependence of the viscosity of the liquid in elastohydrodynamic lubrication (EHL). Most published values of α have not been obtained from measurements of viscosity as a function of pressure. Rather, these effective pressure–viscosity coefficients have been derived from the measurement of the EHL film thickness, a more difficult procedure. In this article, five well‐characterized liquids that should be Newtonian in the EHL inlet are identified for which film‐derived coefficients have been reported. These coefficients are compared with coefficients derived from published viscosity correlations and new viscosity measurements. The film‐derived coefficients are found to not be an accurate representation of the piezoviscous response. The procedure of deriving a pressure–viscosity coefficient from a film thickness measurement does not offer an alternative to the simpler and easier viscometer measurement. Copyright © 2014 John Wiley & Sons, Ltd.     

Rheological properties of lithium,lithium complex,and sodium greases

The rheological properties of two lithium, two lithium complex, and two sodium greases, all with the most common NLGI grade two, have been investigated. The greases are based on a mineral and a synthetic oil. The apparatus used was a cone-and-plate rheometer and an impacting-ball apparatus. The impacting-ball apparatus used a steel ball, which impacted a lubricated sintered carbide plate, to measure the shear stress-pressure coefficient, γ, of the lubricant. At pressures found in elastohydrodynamic contacts this coefficient determined the limiting shear stress. The γ-value thus affects the coefficient of friction and consequently, lower γ-value means less friction. The results from the impacting-ball apparatus showed that the γ-value was lower for the greases with a synthetic base oil and that the lithium greases gave a lower γ-value than the corresponding base oils. Results from the cone-and-plate rheometer showed the characteristic shear thinning behaviour of the greases and the influence of shear history and temperature. The results from the cone-and-plate rheometer have also been fitted to a four parameter rheological model.     

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.     

Measurement of oil film thickness in big-end bearings and its relevance to engine oil viscosity classifications

Minimum oil film thickness MOFT measurements have been carried out in big-end bearings of V6 and in-line four-cylinder gasoline engines during engine operation. MOFT decreases with increasing crankshaft speed above 2000 r/min. The most severe practical steady-state operation is high-speed cruising. Maximum shear rates are in the region of 10⁷s⁻¹ at 4000 r/min. the dynamic viscosities at a shear rate of 10⁶s⁻¹ correlate significantly better with monograde MOFT data than with multigrade data; the correlation parameters for mono- and multigrade data are also significantly different. Although the dynamic viscosity measurement correlates with multigrade data better than the low-shear rate kinematic viscosity, the differences are not always significant at the 95% confidence level. Some other rheological parameter or combination of parameters may be better than either kinematic or dynamic viscosities.     

Vegetable and synthetic hydraulic fluids to improve the overall efficiency of a hydrostatic transmission

The purpose of the investigation reported here was to determine whether or not a hydrostatic transmission can act as an alternative to a conventional one, and, if so, to ascertain to what extent the fluid type contributes to an improvement in the efficiency of the hydrostatic transmission. The results presented in this paper are derived from an experimental field test of the hydrostatic transmission of a belt conveyor. This belt conveyor is one part in an ore‐transport line at LKAB mining company in Kiruna. The hydrostatic transmission replaced a conventional transmission consisting of an electric motor, a belt drive and a gearbox. The hydrostatic transmission was operated using three different hydraulic fluids: Shell Tellus TX 68, a conventional mineral oil; Mobil SHC 526, a synthetic fluid; and Binol Hydrap II, a vegetable fluid. All fluids have the same ISO viscosity grade, VG 68. The measurements on the transmission show a 3% overall efficiency improvement when using vegetable and synthetic hydraulic fluids compared with the mineral oil. The current at the start of the transmission was reduced by a factor of 6, compared to that of the conventional transmission. The installed 110 kW electric motor was replaced with one of 55 kW. The test also showed that a closer study of all operating conditions, and a selection of components suited to the size of the load, can improve the overall efficiency of the hydrostatic transmission.     

A comparative life cycle assessment of the manufacture of base fluids for lubricants

A comparative life cycle assessment (LCA) study of three types of base oils used in the manufacture of hydraulic fluids has been carried out, and is reported here. The fluid types investigated are a mineral oil, a synthetic ester, and a rape seed triglyceride oil. The applications chosen for the finished lubricants are in mobile hydraulic systems in forestry operations, some of which are almost 100% total loss applications. The scope and limitations of the LCA model are discussed, and conclusions drawn concerning the application of LCA models as evaluation tools for the development of environmentally adapted lubricants. The methodology used is compatible with the ISO 14000‐type industrial standard.     

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.     

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.     

Elastohydrodynamic study of vegetable oil–polyalphaolefin blends

Two polyalphaolefins, of higher and lower viscosity than vegetable oils, were used to make binary blends of varying compositions with soy bean and canola oils. The pure oils and the blends were used in viscosity and film thickness investigations. The effects of composition and temperature on viscosity were found to agree well with the theoretical predictions of a simple mixing law. The film thicknesses of the various blends under elastohydrodynamic conditions were measured at 20 N load, and varying entrainment speeds and temperatures. From the data, pressure–viscosity coefficients, α, as a function composition and temperature were obtained. The resulting α values were compared with theoretical predictions. Experimental values of α as a function of composition showed a slight negative or no deviation from the values predicted by an ideal mixing model. On the other hand, experimental values of α displayed a mild decrease with increasing temperature, while the model predicted a sharp decrease with increasing temperature. Published in 2008 by John Wiley & Sons, Ltd.     

Pressure viscosity coefficients and traction properties of synthetic lubricants for wind turbine gear systems

Synthetic lubricants are increasingly used to provide equipment reliability for wind turbine gear boxes. The majority of synthetic lubricants used today are based on polyalphaolefins. In gear systems where contact pressures are high, the pressure viscosity coefficient and traction values of the lubricant are important fundamental properties. A comparison of these properties for a wind turbine lubricant based on a polyalphaolefin and two lubricants based on polyalkylene glycols has been undertaken. Pressure viscosity coefficients were calculated from viscosity measurements made using an ultra‐high pressure falling needle viscometer at pressures up to 50 000 psi. Significant differences in properties were observed with both polyalkylene glycol lubricants showing lower pressure viscosity coefficients and much lower traction values. A calculation of the film thickness values in the Hertzian contact zone suggests that polyalkylene glycol lubricants may provide elastohydrodynamic films that are approximately 25% thicker than polyalphaolefin lubricants. Copyright © 2012 John Wiley & Sons, Ltd.     

Performance comparisons of synthetic and mineral oil crankcase lubricant base stocks

The physical and performance characteristics of representative synthetic base stocks are compared with those of mineral oils. Comparisons of the base stocks alone and as fully formulated crankcase lubricants are presented. Bench test results and physical properties of unformulated synthetic and mineral oil base stocks are compared. The volatility characteristics of synthetic base stocks are superior to typical mineral oil base stocks. All the synthetic base stocks displayed greater viscosity indices than did mineral oils. Bench tests designed to measure thermal and oxidative stabilities are used to compare synthetic with mineral oil base stocks. Although synthetics appeared no more resistant to oxidation than did mineral oils, fewer sediments, deposits and fluid viscosity increases were observed with synthetic base stocks. No one type of synthetic base oil showed a distinct advantage in these evaluations. Fully formulated mineral oil-based lubricants are compared with commonly used synthetic-based lubricants containing identical additive treatments. The synthetic lubricants showed superior engine cleanliness, and resistance to viscosity increased in these tests, but were directionally less effective in wear prevention than mineral oil-based lubricants. Proper formulation or addition of an antiwear supplement overcame this deficiency. These comparisons demonstrated that synthetic base stocks are available with a wide range of physical properties. Available synthetic base stocks offer performance advantages at an increased cost over mineral oils, but proper formulation of the synthetic lubricant is necessary to ensure totally acceptable lubricant performance.     

An investigation of the effects of some motor oil additives on the friction and wear behaviour of oil-soluble organomolybdenum compounds

The compatibility of oil-soluble organomolybdenum compounds with some motor oil additives (detergent, dispersant, rust inhibitor and ZDDP) has been investigated on a four-ball machine and a SRV tester. Preliminary results indicated that a combination of two agents might have either synergistic or antagonistic effects on the friction and wear performance of the organomolybdenum compounds, although many combinations were shown to be synergistic. The interactions between two agents were primarily determined by the additive types, additive concentrations and test temperatures. It was also found that the presence of calcium sulphonate detergent was particularly beneficial to molybdenum dithiophos-phate and molybdenum dithiocarbamate in terms of friction reduction and wear reduction at temperatures in a certain range. The induction period of the organomolybdenum compounds was also reduced. Surface analysis results suggested that the synergistic effects were closely related to the formation of thick films, which were rich in molybdenum and sulphur, on rubbing surfaces.