Tribology International 40 (2

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triacylglycerides. Triacylglycerides, also termed triglycer-

but the fatty acids are almost exclusively ricinoleic acid, an18-carbon monounsaturated fatty acid with a hydroxybranch at the 12th carbon. Jojoba oil is a monoester

also a potential source of fatty acids.

use as lubricants. Finally, the strong intermolecularinteractions whilst providing a durable lubricant lm alsoresult in poor low-temperature properties.

ARTICLE IN PRESSVegetable oils have displayed excellent lubricationproperties in laboratory investigations. Composition andselected properties of vegetable oils commonly investigated

0301-679X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.triboint.2006.10.001

Corresponding author.E-mail address: gws@mech.uwa.edu.au (G.W. Stachowiak).ides, are glycerol molecules with three long chain fattyacids attached at the hydroxy groups via ester linkages. Athree-dimensional representation of a triglyceride moleculeis displayed in Fig. 1. The fatty acids in vegetable oiltriglycerides are all of similar length, between 14 and 22carbons long, with varying levels of unsaturation. Someparticularly interesting variations to the normal vegetableoils are castor and jojoba oil. Castor oil is a triglyceride,

The triglyceride structure is also the basis for theinherent disabilities of vegetable oils as lubricants. Un-saturated double bonds in the fatty acids are active sites formany reactions, including oxidation, lowering the oxida-tion stability of vegetable oils. Another concern is thesusceptibility of the triglyceride ester to hydrolysis. Thesimilarity in all vegetable oil structures mean that only anarrow range of viscosities are available for their potential1. Introduction

Stronger environmental concerns and growing regula-tions over contamination and pollution will increase theneed for renewable and biodegradable lubricants. Anannual growth rate of 710% for environmentally favour-able lubricants is expected on the US market over the nextfew years compared to a rate of only 2% for the overalllubricant market [1]. Vegetable oils are a viable andrenewable source of environmentally favourable oils.The majority of vegetable oils consist primarily of

vegetable wax, with both long chain monounsaturatedalcohol and fatty acid, either 20 or 22 carbons. Both castorand jojoba oils display properties quite distinct to all othervegetable oils due to the quirks in structure [2,3].The triglyceride structure of vegetable oils provides

qualities desirable in a lubricant. Long, polar fatty acidchains provide high strength lubricant lms that interactstrongly with metallic surfaces, reducing both friction andwear. The strong intermolecular interactions are alsoresilient to changes in temperature providing a more stableviscosity, or high viscosity coefcient. The entire base oil isVegetable oil-based lubrica

N.J. Fox, G.W

Tribology Laboratory, School of Mechanical Engineering, U

Received 20 July 2006; received in revised fo

Available online

Abstract

Vegetable oils are being investigated as a potential source o

biodegradability, renewability and excellent lubrication perform

properties and narrow range of available viscosities, however, li

addresses oxidation as a limitation of vegetable oil-based lubrican

along with methods used to monitor and analyse the products

lubrication performance is discussed. A brief discussion of method

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Vegetable oil; Lubricant; Oxidation; Oxidation stability; Review007) 10351046

tsA review of oxidation

Stachowiak

ersity of Western Australia, Crawley, WA 6009, Australia

9 October 2006; accepted 11 October 2006

November 2006

vironmentally favourable lubricants, due to a combination of

e. Low oxidation and thermal stability, poor low-temperature

their potential application as industrial lubricants. This review

The basic mechanism of vegetable oil autoxidation is presented,

oxidation. The potential impact of such oxidation products on

ed to assess and improve oxidation stability completes the review.

www.elsevier.com/locate/triboint

being considered. Belluco and De Chiffre evaluated theperformance of a range of mineral and vegetable oil-basedcutting uids in a range of machining operations [23].Vegetable-based oil formulations displayed equal or betterperformance than the reference commercial mineral oil inall operations. Jojoba oil has been considered as a two-

ARTICLE IN PRESSogyas potential lubricants are displayed in Table 1. Withoutadditives vegetable oils outperformed mineral base oils inantiwear and friction [2,4], scufng load capacity [5] andfatigue resistance [6]. Fully formulated vegetable oillubricants, in comparison to mineral oil counterparts,display a lower coefcient of friction, equivalent scufngload capacity and better pitting resistance, but also poorerthermal and oxidative stability [712]. At extreme loadsvegetable oil-based lubricants become signicantly lesseffective [13].Vegetable oils are particularly effective as boundary

lubricants as the high polarity of the entire base oil allowsstrong interactions with the lubricated surfaces. Boundarylubrication performance is affected by attraction of thelubricant molecules to the surface and also by possiblereaction with the surface. Biresaw et al. examined the levelof attraction between lubricant and surface, calculating theGibbs free energy of adsorption using appropriate models[17,18]. Based on the free energies of adsorption triglycer-ides should outperform the equivalent monoester and thelevel of unsaturation of the fatty acids should play littleeffect on the lubrication performance. Polyunsaturatedfatty acids, however, have displayed greater lubrication

Fig. 1. 3-D representation of a triglyceride molecule.

N.J. Fox, G.W. Stachowiak / Tribol1036properties at higher temperatures and loads than lessunsaturated counterparts. Raising the concentrations ofoxygen in the oil increased this improvement to lubrication,displaying the importance of reactions on boundarylubrication performance [19].Lubricant formulations are being developed based on

the benets and limitations of vegetable oils. The majorityof published data is on the use of vegetable oils ashydraulic uids. Honary provided valuable baseline datafor the eld, investigating the performance of soybean oilat various degrees of rening in hydraulic uid bench testsand suggesting soy-based hydraulic uids were viable.Field trials of fully formulated vegetable oil-based hydrau-lic/transmission uids have also been positive [20,21].Remmele and Widmann, in particular, presented acomprehensive, long-term examination of a commerciallyavailable rapeseed oil based hydraulic uid [22]. Perfor-mance, biodegradability and ecotoxicity of the rapeseedlubricant were monitored during eld trials in a variety ofagricultural machinery over a period of 6 yr. By the end ofthe test period, the machines had operated for over35,000 h and more than 21,000 km. The hydraulic uidcaused no damage and there was no more leakage thanwith a mineral oil. On average 60% of the machines oilreserve was replaced during the 6-year eld experiment.The uid also remained biodegradable with low toxicitythroughout at all stages of its life.Whilst the potential of vegetable oils as hydraulic uids

has been the major topic of investigation other avenues are

Table 1

Selected properties of a variety of common vegetable oils [2,1416]

Fatty acid % Palm Soybean Sunower High oleic

sunower

Castora

C 14:0 1.5

C 16:0 43 11 6 2.5 8

C 16:1 Trace Trace

2

C 18:0 5 4.5 3 2 4

C 18:1 40 23 17 89.5 86

C 18:2 10 55 74 6

C 18:3 Trace 6

C 20:0 0.5 Trace

C 20:1

C 22:0 0.5 Trace

Viscmm2/s, 40 1C 40 31 33 41 252Viscmm2/s, 100 1C 8.3 7.4 8.3 8.8 20Viscosity index 190 218 242 202 90

Pour point, 1C 9 15 12 24

C XX:Y represents a fatty acid chain of XX carbon atoms and Y double

bonds.aCastor oil contains 8090% ricinoleic acid, C18:1 12 OH.

International 40 (2007) 10351046stroke engine lubricant [24]. Formulations of jojoba oilwith commercial lubricants, displayed good miscibilitywith petrol, comparable scufng and deposit formingtendencies and improved wear performance. Permuswanexamined the performance of high-stability vegetable oilsin a modied engine test rig [15]. The oils were effective aslubricants with no failures, but did increase in viscosityover time and caused deposits on the bore.In summary, vegetable oils display many desirable

characteristics, which make them very attractive lubricantsfor many practical applications. However, the mainweakness is their relatively poor oxidation stability. Thisreview addresses the issue of the oxidation of vegetable oillubricants. The mechanism of vegetable oil autoxidation ispresented along with techniques used to analyse oxidisedvegetable oils and how oxidation compounds may inu-ence lubrication performance. Finally methods used toassess and improve the oxidation stability are discussed.

2. Mechanism of vegetable oil autoxidation

Oxidation stability of triglyceride-based vegetable oils isprimarily limited by the degree of unsaturated doublebonds. Unsaturated carboncarbon bonds function asactive sites for many reactions, including oxidation [25].A majority of triglyceride-based vegetable oils containunsaturated fatty acids and are susceptible to oxidation.The greater the level of unsaturation, that is, the moredouble bonds, the more susceptible the oil becomes tooxidation [103]. The mechanism for the autoxidation ofvegetable oils is well studied and a classical representationof the oil autoxidation mechanism is shown in Fig. 2[2628].Vegetable oil oxidation is initiated by formation of free

radicals. Free radicals can easily be formed from the

Hydroperoxides, once formed, can break down toproduce more free radicals. This branching step leads tothe proliferation of radicals that can go back and aid in thepropagation of more hydroperoxides. Not all free radicals,however, will propagate the oxidation process; some mayreact with and terminate each other. Hydroperoxidescontinue to build up in the oil via the propagation andbranching steps. At some point the collection of hydro-peroxides no longer remain stable and decompose into amyriad of volatile and non-volatile secondary oxidationcompounds. They can also be involved in polymerisationreactions, leading to deposits.The vegetable oil autoxidation mechanism presented

here is a simplication of a complex series of reactions. Theprocess is further complicated by variations in conditions,such as ultraviolet rays, temperature, pressure and oxygen

the lubricant either in storage or during application.

ARTICLE IN PRESS

H

OO

O +

O2

O2

O

R

-R

ous l

ous h

N.J. Fox, G.W. Stachowiak / Tribology International 40 (2007) 10351046 1037removal of a hydrogen atom from the methylene groupnext to a double bond. Free radicals rapidly react withoxygen to form a peroxy radical. The peroxy radical canthen attack another lipid molecule to remove a hydrogenatom to form a hydroperoxide and another free radical,propagating the oxidation process.The abstraction of a hydrogen atom by the peroxyl

radical to generate a hydroperoxide is the rate-limiting stepof vegetable oil autoxidation [27]. The rate constant for therate-limiting step depends primarily on the strength of thecarbonhydrogen bond being broken. The strength of acarbonhydrogen bond next to a carboncarbon doublebond is lowered and the hydrogen can be removed easily,thus those oils containing double bonds are moresusceptible to autoxidation. As the number of doublebonds increases there become more sites susceptible to theabstraction of a hydrogen atom and the autoxidationprocess can occur at a faster rate. Vegetable oils containinga high percentage of monounsaturated fatty acids willtypically autoxidise only at high temperatures, whereasthose oils containing polyunsaturates, such as linoleic andlinolenic acid, readily autoxidise at room temperatures.

Initiation RH R +

Propagation R + O2 R

ROO + RH

Branching ROOH R

RO + RH +OH + RH +

Termination ROO + RO

ROO + R

R +R R

Peroxide decomposition ROOH vari

Polymerisation ROOH variFig. 2. Classical representation of tThe physical and chemical changes that occur within theoil during oxidation are likely to have an impact onthe lubrication performance. The following section out-lines the major compounds produced during vegetableoil oxidation, methods that can be used to monitorand analyse the compounds and their potential impacton lubrication. A summary of the section is displayed inTable 2.

ROOH + R

OH

ROH + ROO

H2O + ROO

ROOH + O2

OOH

ower molecular weight compounds

igher molecular weight compounds availability, or by the presence of other compounds, suchas antioxidants, chelating agents and metals. Metals, forexample, act as a catalyst for the oxidation of vegetable oilsspeeding up degradation and the production of freeradicals [29]. Iron and tin are particularly effective catalystsof the oxidation process, whereas copper and lead havevery little impact. Ultraviolet rays decompose hydroper-oxides in the process of photo-oxidation [30].

3. Oxidation compounds

Vegetable oils will decay during the lifetime ofhe oil autoxidation mechanism.

ARTICLE IN PRESS

lub

que

enc

eso

ros

niq

rap

rap

nce

y/m

ress

s sp

hi

y [5

nce

y [5

hro

nce

y [6

uid

tic

ogyTable 2

Summary of vegetable oil oxidation compounds, analytical techniques and

Stage Compound Analytical techni

Primary Hydroperoxide Titration [31] Chemiluminesc Electron spin-r Infra-red Spect Combined tech

Secondary Volatile Gas chromatog Gas chromatog

[4648]

Non-Volatile High-performachromatograph

Atmospheric pionization/mas

High molecular weight Gel permeationchromatograph

High-performachromatograph

Free Fatty Acid Titration [68] Capillary gas c High-performa

chromatograph

Supercritical Nuclear magne

[71]

N.J. Fox, G.W. Stachowiak / Tribol10383.1. Primary oxidation compounds

3.1.1. Hydroperoxides

Triacylglyceride hydroperoxides are the primary oxida-tion compounds of triacylglyceride-based vegetable oils. Asthe primary oxidation compound it is important to be ableto analyse hydroperoxides and understand of the impact ofsuch compounds on the lubrication process. Fully under-standing the manner in which hydroperoxides affectlubrication will allow the development of more effective,and durable, vegetable based lubricants. Hydroperoxidesare particularly important, as they are the primaryoxidation compounds; all secondary oxidation compoundsderive from hydroperoxide decomposition.

3.1.1.1. Analysis. Peroxide value, or PV, is commonlyused as an indication of the level of peroxides in the systemand is determined using AOCS Ofcial Method Cd 8b-90[31]. It is chemical method, based on an iodimetrictitration. The oil sample is dissolved by solvents and addedto a solution containing potassium iodide. Hydroperox-ides/peroxides in the oil oxidise the iodide to iodine and thelevel of iodine determined via a titration against thiosul-fate. By back calculation the concentration of peroxidesis then determined. The method is time consuming andthe titration end point can be questionable, but it is astandard analytical method that can be performed by anylaboratory.rication impact

s Lubrication impact

e [32,33]

nance spectroscopy [32]

copy [3437]

ues [33]

Prowear effect [3841]

hy [49]

hy/mass spectrometry

Likely negligible

liquid

ass spectrometry [52]

ure chemical

ectrometry [53]

Epoxideshigh viscosity, high-oxidationstability, similar boundary lubrication

[54,55]

Majority of compounds uncertaingh-performance liquid

6]

size-exclusion

7,58]

Increased viscosity Uncertain impact on lubrication

matography [68]

liquid

9]

chromatography [70]

resonance spectroscopy

Improve boundary lubrication properties[13,51,72,73]

Lower oxidation stability [74,75]

International 40 (2007) 10351046Instrumental methods of analysis are being developedfor the study of peroxides in vegetable oils. The aim is toproduce rapid and reliable instrumental methodologiesthat can be used by different laboratories with lowvariation in results. Instrumental methods are generallyadvantageous over chemical methods, as they requiresmaller sample volumes, involve less sample preparationand have lower reliance on fresh reactants. The majordisadvantage, of course, is the need for the instrument. Notonly must the instrument be available it must also beregularly maintained and calibrated.Chemiluminescence, electron spin-resonance spectro-

scopy, ultraviolet spectroscopy and infra-red (IR) spectro-scopy have all been utilised in studies of peroxides invegetable oils. Chemiluminescence trials involve reaction ofhydroperoxides/peroxides in the oil sample with a photo-active compound, luminol for example, in the presence of acatalyst. The light produced from the reaction is detectedvia a photomultiplier to give the levels of peroxide [32,33].Electron spin resonance (ESR) spectroscopy studies the

spin state of radicals. Radicals are reaction intermediatesand examining the type and level of radicals in a samplecan provide information regarding the chemical reactionsoccurring. Compounds, called spin traps, are addedto react with radicals present in a system, in order tostabilise them long enough to be examined by ESRspectroscopy. Researchers have demonstrated that lipidhydroperoxides oxidise 2,2,6,6-tetramethyl-4-piperidone to

ARTICLE IN PRESSogystable nitroxide radicals. The level of nitroxide radicals andhence hydroperoxides can then be determined by ESRspectroscopy [32].IR spectroscopy has been the tool used in a number of

studies examining hydroperoxides in vegetable oils. IRspectroscopy examines vibrations of chemical bonds uponapplication of IR beams, with emission frequency relatedto bond type. Early IR studies utilised spectra in the midIR region4000200 cm1 [34]. More recent studies utilisethe near IR region12 8004000 cm1 [3537]. The analy-sis of peroxides via IR spectroscopy is not a simple one-peak method. It requires mathematical analysis of thechanges occurring in the spectra during oxidation alongwith calibration against compounds that can confuse theperoxide spectra, such as alcohols, free fatty acids,moisture and glycerides [34,35]. Once a calibrationprocedure is established IR spectroscopy methods becomeroutine and effective.The instrumental methods described above are primarily

concerned with the level of peroxides in oxidised oilsamples. These detection methods can be used in combina-tion with a chromatographic technique to provide in-formation regarding the type of hydroperoxides presentas well as the amount. The chromatographic technique,such as high-performance liquid chromatography (HPLC),separates the hydroperoxides in the solution based ontheir interaction with stationary and mobile phases.The detection method then determines the level ofthe separated compounds. HPLC coupled with a chemilu-minescence detection system has been used to characterisethe hydroperoxides formed during autoxidation andphotosensitised oxidation of vegetable oils [33]. Monohydroperoxides were predominantly observed in theearly to middle stages of oxidation, with bis OOHand tris OOH apparent at high levels after prolongedoxidation.

3.1.1.2. Impact on lubrication. Very little work has beenpublished examining the impact of hydroperoxides on thelubrication properties of vegetable oils. Increased levels ofhydroperoxides have been linked to increased wear duringboundary lubrication trials of oxidised sunower oil.Increased wear was also linked to degradation of thetriglyceride fatty acids. It was uncertain as to which factorhad the most impact-presence of hydroperoxides, degrada-tion of triglyceride fatty acids or both [38].The impact of hydroperoxides on the lubrication

performance of mineral oils has been the subject of a fewstudies. Newley et al. monitored the lubrication perfor-mance and peroxide levels of an ester base stock duringboundary lubricated fretting wear tests [39]. A closerelationship was observed between increased wear andperoxide accumulation. Addition of antioxidants, peroxidedecomposers and radical scavengers, to the lubricantreduced both peroxide accumulation and wear.

N.J. Fox, G.W. Stachowiak / TribolHabeeb and Stover examined the role of hydroperoxidesin engine wear [40]. A commercially available engine oil,with a ZnDDP antioxidant package, was subjected to bothmotored and red 2.3 L engine dynamometer tests and eldtrials. In all cases fresh oils displayed little wear. Additionof hydroperoxides to the fresh oils caused a dramaticincrease in wear, correlated to the overall concentration.Initially the authors believed that addition of t-butylhydroperoxide caused excessive wear by degrading the oiland in the process, generating strongly corrosive com-pounds. However oil samples from the experiments showedthat although signicant wear had occurred, relatively littleoil oxidation had taken place. The results suggested thatthe hydroperoxides themselves were responsible for theobserved wear.Rounds presented a general examination of the impact

of hydroperoxides on wear [41]. Tests were conductedusing a four-ball wear rig, with a napthenic mineral oil thesubject of most the study. Addition of hydroperoxides tothe base oil at low to medium loads increased wear, withthe prowear effect related to the concentration. WithZnDDP additives present hydroperoxides had to reach athreshold concentration before the prowear effect wasobserved. At high loads addition of hydroperoxidesreduced wear, suggesting some extreme pressure properties.A synthetic diester base oil was also examined. In thediester, hydroperoxides acted as antiwear agents at lowconcentrations suggesting that the hydroperoxide hydro-lysed the ester-forming acids that either helped protect thesurface or reacted with the hydroperoxide remaining in theoil. This is an interesting nding in relation to vegetableoils, which are primarily triesters. The prowear effect ofhydroperoxides was observed when using low alloy steelballs. When stainless steel balls were utilised wear wassignicantly reduced.In these mineral oil studies the impact of hydroperoxides

on lubrication performance was inuenced by a number offactors including; base oil, lubricated surfaces, lubricationconditions, additives and type of hydroperoxide. Conduct-ing similar studies using vegetable oil lubricants dosed withknown levels and types of hydroperoxides would improveour understanding of the lubrication process, particularlyduring the early stages of oxidation. Triacylglyceridehydroperoxides, the primary oxidation compounds ofvegetable oils, can be synthesised. Hui et al. describemethods for producing monohydroperoxides of threedifferent triglycerides [42]. Gargouri and Lagoy describethe bioconversion of polyunsaturated fatty acid triglycer-ides to the corresponding hydroperoxides [43]. It is alsopossible to synthesise hydroperoxides of individual fattyacids [44,45].

3.2. Secondary oxidation compounds

3.2.1. Volatiles

Volatile organic compounds are produced during thesecondary stages of vegetable oil oxidation. They are

International 40 (2007) 10351046 1039formed following the decomposition of the triglyceridehydroperoxides.

spve

ARTICLE IN PRESSogy104techniques for analysing volatile organic compounds arewell established along with knowledge of the major volatilecomponents of oxidised vegetable oils. Volatile compo-nents of oxidised vegetable oils are generally analysed viagas chromatography (GC) and gas chromatography-massspectrometry (GC-MS) [4648]. Common gas chromato-graphic methods for measuring the volatile organiccompounds in fats and oils are described in AOCSRecommended Practice Cg 4-94 [49].During the oxidation of vegetable oils polyunsaturated

fatty acids in the triglyceride breakdown rst and producethe greatest amount of volatiles. Linolenic acid (C18:3)decomposes primarily into 2,4-heptadienal and propanal,with linoleic acid (C18:2) producing mainly pentane,pentanal, hexanal and 2 heptenal. Monounsaturated fattyacids in the triglyceride decompose at a much slower rateand release minimal levels of volatile compounds whencompared to their polyunsaturated counterparts. Oleate(C18:1) rich oils primarily produce octanal and nonanal,with some heptanal [47,48].

3.2.1.2. Impact on lubrication. Volatiles formed duringvegetable oil oxidation are primarily short-chained hydro-carbons and alcohols. Polarity and chain length are criticalfactors to consider for wear/friction modiers [50]. The lowpolarity and short chain length of these volatile compoundsmeans they are unlikely to have any impact on lubricationperformance. With a study suggesting long chain alcohols,1218 carbons, to have virtually no effect on the wearresistance and load-carrying capacity of vegetable oils [51], itbecomes even less likely that short chain volatile compoundswill signicantly impact lubrication performance.

3.2.2. Non-volatiles

Decomposition of hydroperoxides leads to short chainvolatile compounds along with the corresponding chain-shortened, non-volatile compounds.

3.2.2.1. Analysis. A method to analyse the non-volatileorganic compounds formed during the oxidation ofvegetable oils has been developed by SteenhorstSlikkerv-eer et al. [52]. Normal phase HPLC was used to separatethe oxidation products and a mass spectrometer (MS) wasused for identication. Vast ranges of non-volatile organiccompounds were identied in the oxidised oil, but theycould be separated into three major categories.

Triacyglyceride with intact fatty acid chains, one ofthem containing an oxygen, hydroxy, epoxy or peroxygroups.

Triacyglyceride with intact fatty acid chains, two ofin.1.1. Analysis. Volatile compounds are primarily re-onsible for the rancid avours and odours of oxidisedgetable oils. The food industry is particularly interestedthe avour and quality of vegetable oils and as such,3.2N.J. Fox, G.W. Stachowiak / Tribol0them containing an oxygen, hydroxy, epoxy or peroxygroups.oli Triacyglyceride with intact two fatty acid chains and oneshortened chain ending in an oxygen or hydroxy groups.

Different vegetable oils decomposed into distinctlydifferent variations of the major categories.Byrdwell and Neff also developed a technique to

characterise the vast range of potential oxidation productsof triglycerides [53]. The technique, termed atmosphericpressure chemical ionisation mass spectrometry (APCI-MS), involves liquid chromatographic separation of intactoxidised TAGs, followed by online mass spectrometricdetection. The most commonly observed non-volatileproducts of vegetable oil oxidation were hydroperoxidesand expoxides. The primary hydroperoxides that remainedintact were those formed from oleic acid and to a lesserextent linoleic acid. Hydroperoxides formed from linolenicacid and most of those from linoleic acid decomposedfurther to yield other secondary compounds. Epoxidesformed stable and long lived species and resulted mainlyfrom the oxidation of monounsaturated fatty acids.

3.2.2.2. Impact on lubrication. It would be near impos-sible to determine the impact of all non-volatile oxidationcompounds on lubrication performance, there are simplytoo many, often with short lifetimes. Epoxides, as long-lived products of oxidation, could impact on lubricationperformance. The lubrication performance of vegetable oilepoxides has been examined [54,55]. Epoxidised vegetableoils have a higher viscosity, greater oxidation stability,lower deposit forming tendencies and display similarboundary lubrication properties when compared to thecorresponding vegetable oil.

3.2.3. High molecular weight compounds

High molecular weight compounds are the product ofcyclisation and polymerisation reactions that occur at hightemperatures and pressures and represent the nal stages ofthe oxidation process.

3.2.3.1. Analysis. AOCS Ofcial Method Cd 22-91 out-lines the analysis of polymerised vegetable oils by gelpermeation, HPLC [56]. The method provides a distri-bution of compounds based on molecular size, withresults presented as a percentage of high-molecularweight compounds. No specic information is obtainedregarding the chemistry of the high-molecular weightcompounds.Greater separation and identication of high-molecular

weight compounds has been obtained through the use ofhigh-performance size-exclusion chromatography (HPSEC)[57]. Gomes et al. extended the ability to analyse oxidisedcompounds by combining HPSEC with an initial separa-tion of polar material [58]. Compounds with a polaritygreater than unaltered triglycerides, including triglyceridegopolymers, oxidised triglycerides, partial glycerides and

International 40 (2007) 10351046fatty acids, were rst separated from the oxidised vegetableoil by silica gel column chromatography. The polar

ARTICLE IN PRESSogycompounds were then examined, with a high degree ofseparation, by HPSEC.The resistance to polymerisation of vegetable oil

lubricants has been examined using the Penn StateMicroreactor, a technique described in greater detail laterduring discussions on the assessment of oxidation stability[2,4,59]. The level of unsaturation was determined to be themajor factor in the degree of oxypolmerisation experienceby the oil-increased unsaturation led to increased poly-merisation. The level of polymerisation of vegetable oilshas also been assessed by changes in sample weight overtime [60].

3.2.3.2. Impact on lubrication. Polymerisation is generallyconsidered an undesirable aspect of lubricants, leading tothe formation of deposits and lacquers on lubricatedsurfaces and substantial increases in viscosity. But poly-merisation does not necessarily lead to failure of thelubrication mechanism. In a study of oxidised vegetable oil,extended oxidation led to polymerisation and release ofacids but also to reduced friction and wear under boundarylubrication conditions [38]. It was uncertain, however, as towhich aspect led to the improved performance, increasedviscosity, free acids or polymerisation.Polymerisation may even play a part in the lubrication

mechanism of vegetable oils under extreme conditions.Murakami et al. observed an improvement in the lubrica-tion performance of vegetable oils containing polyunsatu-rated fatty acids at high temperatures, suggesting theformation of friction polymers as a possible reason [19].Hsu et al. examined the tribochemistry of copper and steelsurfaces lubricated by stearic acid and suggested thetribochemical reaction products, including metal stearates,were similar to those observed during thermally inducedreactions [61].

3.2.4. Fatty acids

Free fatty acids appear during the degradation ofvegetable oils. Fatty acids are primarily released from thetriglyceride by b hydrogen elimination and hydrolysis.Both b hydrogen elimination and hydrolysis are notoxidation reactions but are likely to occur coincidentallywith oxidation as another degradation process.In triglycerides, there is a lone hydrogen on the 2nd, or b,

carbon. This b-hydrogen is readily susceptible to elimina-tion. If the b-hydrogen is removed, the middle carbonoxy-gen bond grows weak, and a free fatty acid will form [62].Hydrolysis is the reaction of an ester with water

producing an alcohol plus an acid. In the case of vegetableoils water reacts with the triglyceride removing a fatty acidand leaving an alcohol group.

3.2.4.1. Analysis. The level of free fatty acids in vegetableoils can be determined using the AOCS Ofcial Method Ca5a-40 [63]. The method is a simple titration against sodium

N.J. Fox, G.W. Stachowiak / Tribolhydroxide, with results reported as percentage by weight ofoleic acid. IR spectroscopy has been suggested as ainstrumental tool for rapid determination of free fatty acidlevels [6467]. Each IR method was based on comparisonof oil samples with calibration standards containing knownlevels of fatty acids using one peak [67] or a wavelengthrange [66].AOCS Ofcial Method Ca 5d-01 describes the analysis

of free fatty acids in vegetable oils by capillary gaschromatography [68]. The vegetable oil rst undergoessilylation, which derivatizes free fatty acids but leavestriglycerides intact. The silyated fatty acids are thendetected and measured by the capillary gas chromato-graph. This method allows both type and level of fatty acidto be assessed. HPLC [69], supercritical uid chromato-graphy [70] and nuclear magnetic resonance spectroscopy[71] have also been successfully used to provide informa-tion on both free fatty acid type and levels.

3.2.4.2. Impact on lubrication. The impact of free fattyacids on the lubrication performance of vegetable oils hasbeen the subject of a number of studies [13,51,72,73]. Eachstudy suggests free fatty acids can improve the boundarylubrication properties of vegetable oils.Vizintin et al. examined the boundary lubrication

properties of rapeseed oil dosed with oleic acid using ahigh-frequency, linearly oscillating friction and wear testmachine [13]. The addition of oleic acid (C18:1) to rapeseedoil improved the wear performance up until a 3.17GPaload. At higher loads the blend and the base oil displayedequal performance. The concentration of oleic acidconsidered for optimal improvement was 5% by weight.A four-ball machine was used by Cao and Yu to examine

the effect of saturated free fatty acids on the antiwear andextreme pressure properties of rapeseed oil [51]. Fatty acidsimproved wear resistance and the improvement was relatedto chain length. The 12, 14 and 16-carbon chain fatty acidsall provided similar improvements. The 18-carbon chainfatty acid (stearic) provided the greatest improvement outthose tested.Minami et al. conducted experiments on high oleic

sunower oil using a pin on disc machine at bulktemperatures of 50 and 75 1C and at a variety of loads[72]. At 50 1C, the high oleic sunower oil performedextremely well with additives playing little effect. Only at abulk temperature of 75 1C and high loads did the additionof stearic acid (C18:0) improve the lubrication properties.Fox et al. examined the effect of stearic (C18:0), oleic

(C18:1) and linoleic (C18:2) acids on the boundarylubrication performance of sunower oil using a recipro-cating ball on plate rig [73]. Stearic acid was the mosteffective boundary lubrication additive in sunower oil,reducing wear and providing a steady reduction thecoefcient of friction. Increasing the level of unsaturationin the fatty acid had a negative inuence on theperformance as a boundary lubrication improver, withthe addition of linoleic acid displaying little to no

International 40 (2007) 10351046 1041improvement. The stearic acid/sunower oil blend wasvery effective until temperatures approached 150 1C.

ARTICLE IN PRESSogyWhilst free fatty acids may provide some improvementto the lubrication performance they may reduce theoxidation stability [74]. Frega et al. evaluated the impactof free fatty acids on the oxidation stability of the vegetableoils using a Rancimat device [75]. Addition of free fattyacids shortened the induction time of all the vegetable oilstested.

4. Oxidation stability

Low oxidation stability is one of the major factorshampering industry acceptance of vegetable oil-basedlubricants. The oxidation stability of vegetable oils can beimproved. Selective breeding programs and genetic mod-ication can increase stability by reducing the level ofunsaturated fatty acids in the oil [25,76,77]. Similarly,stability can be increased by chemical modication of theoil structure by techniques such as blending, interesterica-tion, hydrogenation and epoxidation [54,78,79]. Wagner etal. have presented a thorough review of the techniquesavailable to modify and improve vegetable oil character-istics from a lubricant industry perspective [80]. Aftermodication to the base oil, the stability of the formula-tions can also be improved by addition of antioxidantpackages.A balance, however, must be met between improving the

oxidation stability and preserving the lubricating propertiesof vegetable oils. Too many modications may destroy theproperties that make vegetable oils useful in the rst place.Reducing the level of unsaturated fatty acids, for example,whilst increasing the oxidation stability, also reduces theeffectiveness of the lubricant at low temperatures. Majorchemical modications to the base oil would alsosignicantly raise the cost of the lubricant.Vegetable oils containing a large percentage of mono-

unsaturated fatty acids are the most likely candidates forvegetable-based lubricants. They have a greater oxidationstability than polyunsaturated oils, and also remain as auid over a much larger range of temperatures than fullysaturated oils. In common oilseed crops, the mostabundant monounsaturated fatty acid is oleic acid, an 18-carbon atom chain.

4.1. Antioxidants

Antioxidants are used to improve the oxidation stabilityof vegetable oil formulations and there are vast numbers ofpapers assessing their effectiveness [2,4,19,57,60,8187].Reviews discussing the effectiveness of a variety ofsynthetic and natural antioxidants in limiting vegetableoil autooxidation are available from both a food [28,8890]and lubricant perspective [91]. Becker and Knorr presenteda particularly comprehensive evaluation of antioxidanteffectiveness in vegetable oils at elevated temperatures [92].There are two major classes of antioxidant; chain

N.J. Fox, G.W. Stachowiak / Tribol1042breaking radical scavengers and peroxide decomposers.Chain breaking antioxidants react with radicals to formstable compounds and prevent propagation of the oxida-tion reaction. The most effective of the type quench theinitial peroxy and hydroperoxy radicals as well as thealkoxy and hydroxy radicals formed during the branchingstages. Some less effective free radical scavengers onlyquench the alkoxy and hydroxy radicals formed duringthe branching stages. Commonly utilised chain-breakingantioxidants include butylated hydroxy anisole (BHA),butylated hydroxy toluene (BHT), mono-tert-butyl-hydro-quinone (TBHQ), propyl gallate (PG) and the naturallyoccurring tocopherols. Amine-based antioxidants, such asdiphenylamine, are not very effective in vegetable oils.Bond dissociation energies can be used for the selection

and design of chain-breaking antioxidants. Zhu et al.discussed a method used to determining the bonddissociation energy of acidic HA bonds, along with thepotential benets to predicting antioxidant effectiveness[93]. Bond dissociation energies inuence the effectivenessof hydrogen atom transfer reactions from the antioxidantmolecules to the reactive radical intermediates such ashydroxyl, alkoxyl, peroxyl and hydroperoxyl radicalsformed during the degradation reactions. For an antiox-idant to efciently quench all destructive radical inter-mediates its bond dissociation energy would have to belower than that of the peroxyl and hydroperoxyl radicals.Peroxide decomposers are the other major class of

antioxidant. The role of peroxide decomposers is to reactwith and decompose hydroperoxides. Generally hydroper-oxide decomposition produces more free radicals thatpropagate and branch the oxidation process. Hydroper-oxide decomposition by peroxide decomposers, however,results in stable compounds and thus limiting oxidation.Common examples of peroxide decomposers used invegetable oils are catalase and glutathione.Some antioxidants display multiple functionality. Zinc

dithiophosphates (DTP) and dithiocarbamates (DTC), forexample, act as both radical scavenger and peroxidedecomposer. Ascorbic acid, a natural antioxidant, acts byscavenging oxygen, getting preferentially oxidised to waterand dehydroascorbic acid. Reaction with reduced glu-tathione can then regenerate the ascorbic acid. Ascorbicacid can also acts as a metal chelator.Metal-chelating agents and UV absorbers are not strictly

antioxidants, but can assist in the prevention of oxidation.UV absorbers, such as phenylsalicylate and hydroxyben-zophenone, prevent decomposition of hydroperoxides byUV exposure. Metal-chelating agents, such as citric acidand ethylene diamine bind metal ions that can catalysehydroperoxide decomposition. These compounds oftencomplement the action of antioxidants.Certain combinations of antioxidants can result in

synergism, where the result of the whole is better thanthe two parts. Butylated hydroxy anisole with butylatedhydroxy toluene or propyl gallate is a synergistic combina-tion of antioxidants for storage at ambient conditions, but

International 40 (2007) 10351046combining butylated hydroxy toluene and propyl gallateresults in negative synergism. At higher temperatures

synergism is displayed with combinations of phenols withsulphides or aromatic phosphates. Combining phenols andaromatic amines display slightly negative effects.Antioxidant effectiveness is affected by several factors

including the base oil composition, environmental condi-tions and the presence of other additives. The presence ofhigh levels of polyunsaturated fatty acids in the vegetablebase oil severely reduces the benet of any addedantioxidants. Temperature is a major contributing factor.Some antioxidants decompose at higher temperatures, e.g.propyl gallate, others simply become less effective, e.g.tocopherols. Becker and Knorr established that protection,or hindering, of the active hydroxyl group was critical forcontinued effectiveness of free radical scavengers at hightemperatures [92].

4.2. Assessment of oxidation stability

The methods used to assess the oxidation stability ofvegetable oil formulations must be carefully matched to theintended application in order to obtain realistic estimates.Hsu has reviewed laboratory bench tests for automotiveindustry, including those used for estimating oxidation andhigh-temperature stability [94,95]. Hamblin later discussedHsus review of assessing lubricant stability with the viewof selecting tests suitable for vegetable oils [91]. Frankel

the oxidation stability of mineral oil formulations aredirectly applicable to vegetable oils. Of particular concernis the presence of water in the trials, especially if the endpoint is determined by acidity. Vegetable oil triglyceridesare susceptible to hydrolysis, breaking down with waterinto glycerol plus free fatty acids. Assessing just acidity asthe end point of an oxidation trial may give a falseestimate. Effective methods of assessing the oxidationstability of vegetable oil formulations are discussed belowand summarised in Table 3.The Schaal oven test, used by the food industry, has the

fewest limitations associated with it when considering theshelf life of vegetable oils. The test conditions are describedin the AOCS Recommended Practice Cg 5-97; OvenStorage Test for Accelerated Aging of Oils. Samples,stored in a dark, forced draught oven at a temperaturebetween 60 and 70 1C are periodically monitored forchanges in primary and/or secondary oxidation com-pounds. The test is ended when a dramatic change isobserved in the oil. The test is most accurate when bothprimary and secondary compounds are monitored. Themajor advantage of the test is that the conditions, and thusresults, are truly representative of storage at ambientconditions. The concerns are that the method is slow andrequires a large number of analyses.Increasing sample temperatures and automating the

ARTICLE IN PRESS

l fo

Adv

Est

Sc

Elu

R R

a

S A C

c

S A

N.J. Fox, G.W. Stachowiak / Tribology International 40 (2007) 10351046 1043reviewed the techniques used by the food industry to assessoxidation stability [96]. Not all techniques used to estimate

Table 3

Summary of methods used to assess the oxidation stability of vegetable oi

Oxidation trial Description

Schaal oven test [97] Long-term storage at elevatedtemperatures

Monitor products of oxidation

Field trials Assess lifetime and performanceduring application in eld

Monitor products of oxidation/variation in performance

Active oxygen

method/Rancimat

[98]

Bubble air through oil held at 98 1C Monitor volatile acids and/or

peroxides

Large variations in acidity/peroxidessignies end point

Penn State

microreactor [2,4,59]

Small sample oxidised over xedperiod of time

Oxidised sample washed withTHFTHF/oxidation solution analysed

by gel permeation chromatography

Differential scanning

calorimetry

[25,82,99,100]

Small sample subjected to temperatureprogram

Thermodynamic variations monitored

Oxidation observed as rapid release of

energy

Ainanalysis can accelerate oxidation stability trials. A com-monly used accelerated test for vegetable oil stability is the

rmulations

antages Disadvantages

xcellent representation of

orage

imple experimental

onditions

Long trial periods Large number of analytical trials

xcellent representation of

bricant service lifetime

Long-term trials Complex Expensive

educed trial times

ancimat is a commercial,

utomated variation

Rapid loss of volatiles or degradationof peroxides can make end point

questionable

Accelerated trials may not representreality

mall volume

ccelerated trials

haracterises oxidation

ompounds

Requires access to /establishment ofmicroreactor

Accelerated trials may not representreality

mall volume

ccelerated trials Requires access to calorimeter

ccurate thermodynamic

formation

Accelerated trials may not representreality

ARTICLE IN PRESSogyactive oxygen method (AOM). The AOM involvesbubbling air through oil held at 98 1C. The Rancimat iscommercial, automated version of the AOM that measureslarge variations in volatile acids or peroxides as theendpoint. Tests can range in length from less than an hourto above 300 h using the AOM and up to weeks with theSchaal oven test [98].The Penn State Microreactor is an effective technique

that provides estimates of vegetable oil volatility, oxidationstability and deposit-forming tendencies [2,4,59]. Accelera-tion of the oxidation process is achieved through acombination of small volumes, large surface areas, elevatedtemperatures, catalysis and plentiful availability of oxygen.A small sample volume, 40 uL, is injected onto a heatedcatalyst surface and held at a constant temperature with acontrolled supply of dry air for the test duration. At theend of the trial the catalyst is washed with THF and thesolution analysed by gel permeation chromatography todetermine the degree of polymerisation. The catalystcontainer is weighed before and after sample addition todetermine sample weight. After the test, but beforewashing, the container is weighed again to determine anyloss or gain in the sample. The container is weighed for thelast time after washing with THF to check any weightchange from possible attacks on the surface. The PennState Microreactor is effective as its rapid and provides avariety of information regarding the oxidation degradationprocess.Differential scanning calorimetry (DSC) is another

technique that has been used to examine the oxidationstability of vegetable oil formulations. DSC is an analyticaltechnique used to measure the thermal transitions oc-curring within a test sample, whilst subjected to a preciselyprogrammed temperature change [99]. Tan and CheMan presented a comprehensive review of the potentialof DSC in studying vegetable oil oxidation [100]. Estimatesof oxidation stability and antioxidant effectiveness byDSC compare well with other accelerated techniques.As changes in energy are accurately measured duringDSC trials, the technique is also well suited for examiningkinetic studies. A combination of PDSC/NMR wasrecently used to examine the oxidation kinetics ofgenetically modied vegetable oils [25]. Quantitative H1

and C13 NMR provided structural information regardingthe vegetable oils, which were statistically compared tokinetic data from the differential scanning calorimeter toelucidate behaviour.The major concern with all temperature accelerated

oxidation tests used to consider storage life is that the testconditions are no longer representative of storage atambient conditions. One difculty is that the mechanismfor vegetable oil autoxidation alters as temperatures riseabove 80 1C. At high temperatures side reactions involvingpolymerisation and cyclisation become more important.Hydroperoxides also decompose more rapidly at elevated

N.J. Fox, G.W. Stachowiak / Tribol1044temperatures into secondary compounds, so using mea-surements of peroxide level in determining the extent ofoxidation become limited. Alterations in effectiveness, orpossible decomposition, of antioxidants also need to beconsidered. In order to predict performances at lowertemperatures, accelerated oxidation tests should be con-ducted at multiple temperatures to assess the impact oftemperature on results.Correlating results of oxidation stability trials of

lubricants to actual application lifetimes is even moredifcult than predicting storage life. The conditionsthe lubricant is subjected to during the application shouldbe determined and trial conditions established to mimicthe application as close as possible. Conditions wouldinclude, but not be limited to, oxygen availability,presence of catalysts, pressure, temperature and lubricantreplenishment. How oxidation impacts on lubricationperformance should also be understood in order todetermine what indicators would be best used to assesslubricant lifetime. Chen and Hsu recently presented anattempt to combine bench tests and computer models topredict lubricant performance over time [101,102]. Achemical kinetics model, using constants derived frombench tests, was combined with a nite differenceprogramme, simulating engine-operating conditions, topredict lubricant performance in diesel engines. Thesimulations displayed good agreement with engine dynam-ometer trials.

5. Summary

Low oxidation stability hampers the acceptance ofvegetable oils as a potential source of environmentallyfavourable lubricants. The mechanism of vegetable oiloxidation is well studied along with methods for improvingthe oxidation stability, such as antioxidant addition ormodication of oil composition. Oxidation has an impacton the lubrication performance of vegetable oils and quitepossibly plays a role in the lubrication process. Correlatingstudies of oxidation with lubrication performance will leadto greater understanding into the role of oxidation in thelubrication mechanism of vegetable oils.

Acknowledgements

The authors wish to thank the School of MechanicalEngineering, University of Western Australia for itssupport during the preparation of this paper.

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ARTICLE IN PRESSN.J. Fox, G.W. Stachowiak / Tribology International 40 (2007) 103510461046

Vegetable oil-based lubricants--A review of oxidationIntroductionMechanism of vegetable oil autoxidationOxidation compoundsPrimary oxidation compoundsHydroperoxidesAnalysisImpact on lubrication

Secondary oxidation compoundsVolatilesAnalysisImpact on lubrication

Non-volatilesAnalysisImpact on lubrication

High molecular weight compoundsAnalysisImpact on lubrication

Fatty acidsAnalysisImpact on lubrication

Oxidation stabilityAntioxidantsAssessment of oxidation stability

SummaryAcknowledgementsReferences