Thermo‐oxidative stability of new‐generation base oils

Thermo‐oxidation of base oils is the primary cause of lubricant degradation and engine failure during use. Degradation is mainly due to high‐temperature oxidation and thermal decomposition near the piston ring zone, forming oxygenated compounds that polymerise to form high‐molecular‐weight insoluble deposits. New‐generation base oils are found to be more stable towards oxidation and deposit formation due to the absence of aromatics and polynaphthenes. However, compatibility with antioxidants and other additives is now of greater concern because of the poor solvency of these oils. With the increase in the purity of the oil, sometimes the oxidation performance is poor in comparison to group I oils, mainly due to the removal of sulphur compounds, which act as natural antioxidants. Thermal techniques, such as differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), are emerging as fast and accurate methods for determining the thermo‐oxidative stability of base oils and their additive blends, making it possible to measure the oxidation induction time, incipient oxidation temperature, and deposit‐forming tendency. The objective of this work is to evaluate the thermo‐oxidative stability of new‐generation group II/III base oils without antioxidant additives, using DSC and TGA. The kinetics of base oil oxidative degradation are studied using different heating rates. The data obtained from thermal techniques are correlated with the micro‐oxidation data obtained from the Penn State Micro‐Oxidation (PSMO) test. The response of a typical antioxidant additive, zinc dialkyldithiophosphate, towards oxidative degradation of base oils has also been studied.     

Major pathways for used oil disposal and recycling. Part 1

One of the main concerns with lubricating oil relates to used oil management for both industrial and engine oils, although the environmental impact of gasoline and diesel engine oils is the most critical. Provided that efficient management systems are in place, most used oil should not reach the environment, so, the major question is how to dispose of collected used oil. The first option lies in burning it as a fuel, the second in recycling (reclaiming, reprocessing, re‐refining). The latter allows recovery of mineral base oils, which are valuable constituents of crude oil. Mobile (on site) and fixed plants for industrial oil recycling will first be discussed, and the paper will look at the most modern re‐refining processes that produce base oils of as high quality as virgin base oils. Based on current re‐refining experience, the quality of finished lubricants blended from re‐refined base stocks is also noted. Re‐refining today may be of significant benefit to the economy and can, of course, protect the environment. All modern re‐refining technologies produce small amounts of by‐products in which toxic materials may have been concentrated. A final aspect of reprocessing used oil is to integrate it, after hydrogen treatment, into existing refineries. This valuable raw material can then be directly routed to a lube oil unit or even to a cracking unit for conversion to gasoline. The integration of used oil treatment processes into selected refineries may be the most effective pathway to used oil disposal. In this first part, the author looks at the nature of the problems associated with used oil, its use as a fuel, and simple recycling. He then goes on to look at major re‐refining processes, starting with hydrogenation (KTI, Mohawk, BERC/NIPER, and PROP technologies). Part 2 will describe other processes, including a range of vacuum distillation/clay treatment technologies.     

Performance properties of oil‐soluble synthetic polyalkylene glycols

Conventional polyalkylene glycols (PAGs) that are derived from the polymerization of ethylene oxide and/or propylene oxide provide many performance benefits such as excellent friction control, good low temperature properties, high viscosity indices and excellent deposit control. One limitation is their poor miscibility in hydrocarbon oils. A new range of oil‐soluble PAG base oils has been developed using butylene oxide as one of the precursors, and these provide superior miscibility in hydrocarbon base stocks. The new base oils offer many of the traditional advantages of PAGs such as high viscosity indices, good low temperature properties and good traction behaviour. In addition, they can be used as a co‐base oil or additive in hydrocarbon base stocks to improve deposit control and provide good film‐forming behaviour. Examples of their structure property relationships will be discussed and also aspects of how this technology may enable application expansion of PAGs in the future. Copyright © 2012 John Wiley & Sons, Ltd.     

Suitable additives for vegetable oil‐based automotive shock absorber fluids: an overview

Formulation of a cost‐effective, high‐performance and eco‐friendly lubricant, largely depends on the base oil quality, then the selection of suitable additives and their proportions. Vegetable oils, identified to be eco‐friendly, renewable, future‐available and cost‐effective treasures for lubricant formulation, apart from processing, will rely much on suitable additives to meet the performance requirements for automotive shock absorber (ASA) fluids. Additives that will guarantee performance, longevity and eco‐friendliness of formulated vegetable‐based functional fluids have to be uncommonly effective, resistant to depletion, non‐toxic and highly biodegradable. Their selection in these regards will require skills and experience, which will harness the various arms of synergism as effective tools to succour the known weaknesses of the base oil. This is a review on additives that could be used in formulation of vegetable oil‐based (ASA) fluids. The outcome shows that there are customary and novel additives that are suitable for formulating vegetable oil‐based ASA fluids. Copyright © 2016 John Wiley & Sons, Ltd.     

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.     

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.     

Improvement of thermooxidative stability of non‐edible vegetable oils of Indian origin for biodegradable lubricant application

For environmental reasons, as well as the dwindling source of petroleum, a new class of environmentally acceptable and renewable lubricants based on vegetable oils is available. Even though vegetable oils possess excellent lubricant‐related properties, there are some concerns about using it as lubricant base oil. Still, unmodified and modified varieties of soybean, rapeseed, sunflower and canola oils have been in use in the USA and Europe. In India, with the shortage of edible oil, alternate sources of vegetable oils stocks are being explored. With this aim, a comprehensive study has been conducted earlier in the authors' laboratory. In this study, numerous options of non‐edible vegetable oil sources were explored, and a few potential vegetable oils were studied in the laboratory. It was found that even though the oils performed much better in comparison with other vegetable oils, it still required improvement in thermooxidative stability. Therefore, in the later part of the study, different options were explored to improve thermooxidative stability. With a background on the initial studies of the authors as described above, the present paper deals with the studies on improvement of these non‐edible candidate vegetable oils of Indian origin for lubricant by treating with selected antioxidants for applying them in lubricants. Copyright © 2010 John Wiley & Sons, Ltd.     

两种润滑脂中基础油提取方法的比较

为研究不同提取方法对锂基润滑脂中基础油提取效果的影响,采用12-羟基硬脂酸体系稠化基础油制备锂基润滑脂,然后采用石油醚提取法和压力分油提取法分别提取制备的润滑脂中的基础油,并分析2种不同方法提取的基础油与原稠化基础油在黏度、族组成、低温性能和组分构成方面的差别。结果表明：2种不同方法提取的基础油的红外谱图与稠化基础油的红外谱图基本一致,族组成和苯胺点也与稠化基础油基本一致;压力分油提取法提取的基础油的黏度、黏指、倾点和蒸发损失与稠化基础油一致;石油醚提取法提取的基础油的黏度与混合基础油中的小黏度基础油基本一致,黏指和倾点与润滑脂稠度相关,蒸发损失略大于原稠化基础油。对于润滑脂的剖析,2种方法结合使用可以得出较为准确的结果。     

Effect of base stocks on the automobile engine bearing

The government has required that engine oil viscosity be lowered and that highly refined base stocks be used in engine oils, with the aim of promoting low fuel consumption and developing long-life oils.To improve the reliability of automobile plain bearings by means of engine oils, we have developed a bearing test apparatus that uses a half-metal. Oil can be supplied through the oil supply port in the same way as in actual engines. In this apparatus, variable loads can be applied to the opposite side of the oil supply hole of the bearing by hydraulic pressure that is synchronized with the rotational speed of the shaft. The influence of the base stocks used for engine oils on the contact between the rotating shaft and the bearing metal, and on the bearing rotating torque generated by rotation of the shaft, were investigated.We found that the contact ratio and the bearing rotating torque varied with different types of base stock compositions, even if the base stocks had the same viscometric properties. Therefore, it is necessary to develop countermeasures to avoid adverse effects when lowering engine oil viscosity and using highly refined base stocks.     

Tribological behaviour of three phosphate esters containing the benzotriazole group as additives in rapeseed oil

Since the 1970s more attention has been paid to biodegradable lubricants as a way of reducing the pollution caused by lubricants with a mineral oil base. This concern has promoted the use of vegetable oils as alternative base oils. This paper reports on a study of the effect of phosphate esters containing the benzotriazole group as additives in rapeseed oil (RSO), tests being carried out using a standard four‐ball tester. The results show that these additives produce good antiwear performance and load‐carrying capacity compared with RSO alone. Surface analysis of the worn halls was carried out using X‐ray photoelectron spectroscopy. The tribological mechanism is discussed on the basis of the experimental results.     

The influence of lubricating fluid type on the properties of biodegradable greases

The lubricating ability of a grease depends on both the base oil and the thickener. As a result of their intrinsic properties and/or because of their com‐patibility with thickeners and specific additives, base fluids have different influences upon the properties of grease formulations. It is well known that mineral oils are the most widely used lubricant bases due to their inherent lubricity and lower cost, but recent environmental concern has led to consideration of the use of vegetable oils and readily biodegradable synthetic fluids as raw materials in lubricating grease formulations. As well as the base materials, the additives for biodegradable greases should also be biodegradable. This requirement limits the kind of products that may be used in environmentally friendly greases. This paper presents comparative data concerning the tribological and physico‐chemical properties of biodegradable greases formulated with certain vegetable oils, such as rape seed oil, castor oil, and soybean oil or their mixtures, and synthetic esters. The improvement of the load‐carrying properties of biodegradable greases and the antioxidative effect of some suitable additives have also been studied, and the results are presented here.     

Friction‐reducing and antioxidant capabilities of engine oil additive systems under oxidative conditions. II. Understanding ligand exchange in a molybdenum dialkyldithiocarbamate/zinc dialkyldithiophosphate additive system in various base oils

The ligand‐exchange reactions between molybdenum dialkyldithiocarbamate, Mo(dtc)₂, and zinc dialkyldithiophosphate, Zn(dtp)₂, have been investigated during inhibited oxidation in a model hydrocarbon (hexadecane) and in a series of Group I–IV base oils at 160°C. These investigations revealed that the ligand exchange leads to formation of single‐exchange products, Mo(dtc)(dtp) and Zn(dtp)(dtc), and double‐exchange products, Mo(dtp)₂ and Zn(dtc)₂, and that the extent of the exchange is significantly affected by oxidation and inhibition reactions involving the original additives, the ligand‐exchange products, the base oil, and base oil‐derived oxidation products. It is concluded that there are two reaction sequences that control the product distribution and additive consumption during oxidation in different base oils. The first sequence involves the formation of peroxy radicals and hydroperoxides, and the decomposition of hydroperoxides by Zn(dtp)₂ and Zn(dtp)(dtc). One of the most important factors affecting this sequence is the base oil oxidisability. The second sequence involves inhibition of oxidation by peroxy radical‐trapping antioxidants naturally present in or added to the oil, or formed during the oxidation. Zn(dtc)₂ and Zn(dtp)(dtc) appear to be very effective in this respect and are, therefore, preferentially consumed. This preferential consumption shifts the ligand‐exchange equilibrium towards the formation of Mo(dtc)(dtp) and Mo(dtp)₂. The combined effect of the two sequences depends on base oil properties. The first sequence predominates in base oils exhibiting high oxidisability (e.g., in Group I oils) and the second in paraffinic oils, such as Group III and IV oils, having low oxidisability and no sulphur or aromatics. the retention of friction‐reducing capability with the Mo(dtc)₂/Zn(dtp)₂ additive system during oxidation appears to be tied to the first sequence, which leads to consumption of Zn (dtp)₂, since the friction‐reducing capability ceases when Zn (dtp)2 is consumed and hydroperoxides can accumulate in the system. Consequently, the best retention of friction‐reducing capability is achieved in bases oils with low oxidisability.     

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.     

Molecular spectroscopic studies of the effect of base oils on additive—additive interactions

The effect of different types of base oil of API Groups I to V, such as mineral based (solvent refined, hydrofinished, hydro‐cracked/ wax isomerised) and synthetic based (polyalphaolefin and ester), on the interaction of zinc dialkyldithiophosphate (ZDDP) with polyisobutylene succinimide (PIBS) has been studied by variable‐temperature (VT) IR and ³¹P NMR spectro‐scopic techniques. Since ZDDPs are known to exist in monomeric and polymeric forms and as neutral and basic salts, and their functioning is temperature dependent, the structural changes observed in the NMR and IR spectra of the ZDDP—PIBS system in different base oils have been addressed with this in mind. The strength and composition of ZDDP—PIBS complexes has been explained in terms of the degree of solvation provided by the medium (base oil) in which they are present, and thermodynamic parameters ΔH and ΔS estimated from the VT IR spectra. The results indicate that the complexes are stronger and stable in polar base oils, such as from Groups I and V. The equilibrium shifts towards the neutral form of ZDDP in polar base oils. The results have been substantiated by correlating the variation in the shift and width of IR and ³¹P NMR signals to the stability of the complexes and low values of ΔH and ΔS. Polar solvents, such as methanol and tetrahydro‐furan, also favour the formation of the neutral form of ZDDP and stronger complexes between PIBS and ZDDP, in a manner similar to that observed for polar base oils.     

Assessment of the useful life of current long‐drain and future low‐phosphorus engine oils

The assessment of engine oil useful life is an important step in the development of future low‐phosphorus, catalyst‐compatible, and long‐drain engine oils. This paper describes the development and application of a new laboratory screening test, the Ford oil ageing test (FOAT), for the assessment of engine oil useful life. FOAT simulates the Sequence IIIE engine dynamometer test and evaluates oils on the basis of viscosity increase. It correlates well with both single‐ and double‐length Sequence IIIE test results. FOAT allows for inexpensive screening of candidate oils prior to engine tests and is currently being used in the evaluation of future low‐phosphorus engine oils. This comparative study has demonstrated that low‐phosphorus, catalyst‐compatible engine oils can be formulated to provide a similar useful life to current commercial long‐drain oils.     

Maximising the fuel efficiency of engine oils: The role of tribology

Improvement of engine fuel efficiency is one of the most important goals of current automotive development. Maximising the contribution of engine oils to fuel efficiency is a very important part of this process. Engine friction modelling, based on fundamental tribological considerations, has shown that further engine friction improvements are possible through engine oil reformulation. This reformulation should minimise friction under hydrodynamic conditions through modification of the rheological properties of oils, and also minimise friction under mixed and boundary lubrication conditions through changes in the chemical composition of the oils. These improvements can be achieved by appropriate selection of a base oil as well as by the use of effective friction‐reducing additive systems. A very important consideration in formulating these highly fuel‐efficient oils is their ability to retain their fuel efficiency during the entire oil service interval. This paper describes the role of tribological research in the development and introduction of advanced fuel‐efficient engine oils.     

New Lubricating Oils by Graphite Treatments of Petroleum Distillates

High-surface-area graphites, that can be prepared by grinding synthetic graphites in vibratory ball mills, can be used in a novel refining technique which can markedly reduce pour points of various petroleum distillates and can be used to prepare oil fractions with high viscosity indices. The treatment also lowers the content of aromatic and sulfur compounds and completely eliminates polycyclic aromatics.

Some of the oils obtained by selective adsorption on graphite may well have some of the desirable properties of naphthenic oils and also constitute improved base stocks for multigrade lubricating oils, oils for gas-turbine engines, hydraulic oil and transmission lubricants. Results obtained to date indicate that graphite refining can produce oils similar to those that can be obtained by super-refining treatment i.e., high pressure hydrogenation and deep dewaxing. Otherwise, the process can be used to prepare good quality base oils in one step, thus eliminating furfural extraction, dewaxing and ferrofining or clay contacting.

A notable feature of base oils prepared by this new process is then relatively low viscosity at low temperatures (0°F) which makes them good candidates for the preparation of lubricating oils with fuel saving properties.     

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.     

Total vegetable oil greases

A grease has two major constituents, namely, a lubricant, that performs the function of lubrication; and a gellant, that provides a solid continuous phase, occludes the lubricant, and gives apparent physical structure to the grease. Generally, the gellant is 5–30% and the lubricant 65–90%, additives and fillers making up the rest. In conventional greases, the gellant is a vegetable oil soap, and the lubricant is a liquid oil of petroleum origin or is a synthetic. Such greases have limited biodegradability, because the major constituent, i.e., the lubricant, is normally not biodegradable. In total vegetable oil grease, both the gellant and the lubricant are derived from vegetable oils, giving a grease of potentially high eco‐compatibility. Esters, dibasic acid esters, and alkylated esters of vegetable oil are known to be high‐quality lubricants. These can be used with soap stocks prepared from vegetable oils to give a grease of total vegetable oil origin. The vegetable‐oil based lubricants and soaps are prepared separately and combined in appropriate proportions to give a grease of the required specifications. Alternatively, esterification and saponification can be carried out simultaneously to give a grease of the desired specifications, where an alkali will be the catalyst for esterification, and reactant for saponification. In this paper, the process parameters, and kinetics of these simultaneous reactions are discussed. The results of experimental evaluation of some of these greases are also presented.     

Carbonyl peak index technique in the study of lubricant oxidation

A technique known as the Carbonyl Peak Index (CPI) is used to assess the relative oxidation stabilities of two competing grades of commercial automotive crankcase oils, their base oils, and the oxidation characteristics resulting from oil/metal interactions. The CPI provided empirical data from the infrared spectra which enabled a more lucid explanation of the oil soluble acidity and sludge deposition characteristics of the oils. A combination of these parameters has confirmed that the hydrocarbon type composition of the oils gave some stability to the fully formulated monograde oil. Consequently, the monograde base oil was found to possess a greater stability towards oxidation than the multigrade base oil under the conditions of test. However, the fully formulated oils were found to be more stable than the base oils, mainly due to the added antioxidants. Metals in the oils acted as antioxidants initially, but later turned to pro-oxidants. The reversal of metal action from inhibition to autooxidative catalysis was also found to be base-oil hydrocarbon-type dependent and this has been related to the natural antioxidant capacity of the oils.