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Journal of the Science of Food and Agriculture J Sci Food Agric 86:1769–1780 (2006)

ReviewPlant-oil-based lubricants and hydraulicfluidsManfred P Schneider∗FB C – Bergische Universitat Wuppertal, D-42097 Wuppertal, Germany

Abstract: It is estimated that, at present, approximately 50% of all lubricants sold worldwide end up in theenvironment via total loss applications, volatility, spills or accidents. More than 95% of these materials arecurrently mineral oil based. In view of their high ecotoxicity and low biodegradability, mineral oil-based lubricantsconstitute a considerable threat to the environment. In contrast, most lubricants and hydraulic fluids based on plantoils are rapidly and completely biodegradable and are of low ecotoxicity; moreover, lubricants based on plant oilsdisplay excellent tribological properties and generally have very high viscosity indices and flashpoints. However, inorder to compete with mineral-oil-based products, some of their inherent disadvantages must be corrected, suchas their sensitivity to hydrolysis and oxidative attack, and their behaviour at low temperatures. Various methodsto improve the undesirable properties of native plant oils will be discussed. In parallel, government regulationsthat encourage or enforce the use of bio-based fluids, at least in ecologically sensitive areas, will help to increasetheir market share. Using the numerous possibilities for selective breeding and/or chemical improvement of thedouble bond systems of natural fatty acids by increased R&D, the major obstacles regarding the use of plant-basedraw materials for the production of lubricant base fluids can be overcome and bio-based fluids should expect afuture with increasing market shares. 2006 Society of Chemical Industry

Keywords: plant oils; high oleic sunflower oil; rapeseed (canola); oxidative attack; biodegradability; ecotoxicity;chemical modifications; additives; eco-labels; lubricants; hydraulic fluids

INTRODUCTION: LUBRICANTS AND THEENVIRONMENTThe environment must be protected against pollutionby lubricants and hydraulic fluids based on mineraloils. This is, of course, best done by preventingundesirable losses and by reclaiming and reusinglubricants. Alternatively, environmentally acceptablelubricants and hydraulic fluids should be usedwhenever and wherever possible. According toliterature reports, varying amounts of lubricants atpresent end up in the environment – they ‘disappear’.It is claimed that in the EU alone in 1990 13%(661 × 106 L of a total of 4959 × 106 L) were lostin the environment.1,2 In the USA 32% (432 × 106

gallons of 1351 × 106 gallons) of lubricating oils endedup in landfills or were dumped. It is claimed that50% of all lubricants sold worldwide end up inthe environment via total loss application (chainsawoils, two-stroke engines, concrete mould release oils,exhaust fumes in engines and metal cutting andforming processes) spillage and volatility.3 Estimatesfor the loss of hydraulic fluids are as high as70–80%.4,5 Most problematic are uncontrolled lossesvia broken hydraulic hoses or accidents whereby largequantities of fluids escape into the environment.6 They

contaminate soil, surface-, ground- and drinking waterand also the air. In Table 1 the estimated amounts oflubricants used in Germany,7 the EU8 and worldwide9

are listed, together with the (estimated) losses.In contrast to mineral oils, lubricants and hydraulic

fluids based on plant oils are generally rapidlyand completely biodegradable and are also of lowecotoxicity. At present the use of pure native plantoils is limited to total loss applications (lubricantsfor chainsaws, concrete mould release oils) andthose with very low thermal stress. Hydraulic fluidsare of increasing importance for applications inenvironmentally sensitive areas where a potential totalloss could be encountered, such as excavators, earth-moving equipment and tractors, in agricultural andforestry applications and in fresh water (groundwater)sensitive areas.10,11 Mineral oils are toxic for mammals,fish and bacteria. Considering the sump capacities ofsuch machinery (up to 1000 L) the ecological impact isobvious – as are the economics of the resulting clean-up operations.

Although it seems obvious that the increased useof rapidly biodegradable lubricants would be ofconsiderable ecological and economical advantage, thepresent market share of these materials is relatively

∗ Correspondence to: Manfred P Schneider, FB C – Bergische Universitat Wuppertal, D-42097 Wuppertal, GermanyE-mail: [email protected]/grant sponsor: German Ministry of Agriculture (BVMEL)(Received 18 November 2005; accepted 31 March 2006)Published online 3 August 2006; DOI: 10.1002/jsfa.2559

2006 Society of Chemical Industry. J Sci Food Agric 0022–5142/2006/$30.00

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Table 1. Consumed lubricants and (estimated) losses (in tons per

annum)

Germany EU World

Amount consumed 1 000 000 5 000 000 30–40 000 000Losses 400 000 2 500 000 20 000 000Losses in % 40 50 55

Table 2. Market share of bio-lubesa in Europe by geography for 2000

and expected for 2006

Year Germany France UK BeneluxScand-inavia Italy

Switzer-land

Austria

2000 4.0 0.1 0.2 2.9 9.0 1.3 5.72006 15.1 0.3 0.4 4.9 10.8 2.0 8.4

a Biodegradable lubricants derived from renewable resources includingplants and animals.

small12 (2% in the EU and worldwide, with anestimated growth rate of 5–10%), as shown in Table 2for a selection of European countries.13 (It shouldbe noted that the Frost and Sullivan market surveyand prediction is probably overoptimistic and thata 10% increase per annum would be consideredalready a success by oil manufacturers. Increasesof 5–10% are in fact observed in reality; see alsoWhitby.14)

For hydraulic fluids this amount is increasing morerapidly, with estimates ranging from 25% to 75%.5,15

In order to increase this market share, the acceptabilitymust be improved. Technical performance, acceptableprice and ecological compatibility will constitute thebasis for future developments along these lines. Froman EU-sponsored study it seems that biolubricantsare already available for the majority of applicationsand that the technical performance is comparableand sometimes even better than for conventionallubricants. Furthermore, the price of mineral oil willcontinue to increase over the years to come and thepresent-day advantage of low cost for mineral oilproducts may be completely lost in a decade fromnow.16,17

PLANT-OIL-BASED LUBRICANTS ANDHYDRAULIC FLUIDS: PROPERTIESIn practical applications lubricants and hydraulicfluids have multiple functions. They are needed forlubrication, transmission of energy, protection againstcorrosion, wear (attrition) and heat removal.

Plant-based fluids meet these requirements ideally.They display:

• excellent tribological properties (ester functions stickwell to metal surfaces);

• lower friction coefficients than mineral-oil-basedfluids;

• lower evaporation (Noack) – up to 20% less thanmineral-oil-based fluids;

• higher viscosity index (multi-range oils);• excellent biodegradability;• high flashpoints;• low water pollution classification.

Their technical properties are thus largely com-parable with mineral-oil-based fluids. However, theyare thermally less stable than mineral oils, sensi-tive to hydrolysis and oxidative attack, and theirlow-temperature behaviour is frequently unsatisfac-tory – properties which can be corrected by a numberof measures:

• exchange of glycerol by other polyols, in particulartrimethylolpropane (TMP);

• avoiding or modifying multiply unsaturated fattyacids;

• suitable additivation.

For the development of lubricants and hydraulicfluids on the basis of renewable resources such asnative plant oils one will always have to make acompromise between the performance based on thechemical structure and the desired biodegradabilityand ecotoxicity. All lubricants and hydraulic fluids arecomposed of so-called base fluids and additives.

BASE FLUIDSNative oilsThese are triglycerides, esters composed of variousnatural fatty acids and glycerol. The most importantfatty acids contained in plant oils are unsaturatedmolecules such as oleic acid (C18:1), linoleic acid(C18:2) and linolenic acid (C18:3), as well as(saturated) palmitic acid (C16:0) and stearic acid(C18:0). At present, the most important sources fornative oils are rapeseed (canola), sunflower (variousqualities) and soybean. The fatty acid compositions ofpotential plant oils are summarized in Fig. 1.18

The structures of fatty acids – both chain lengthand degree of unsaturation – are directly related totheir operational stabilities and lubricant propertiessuch as viscosity, viscosity index and low-temperaturebehaviour (e.g. pour point). Thus the oxidativestability of native oils increases with decreasingamounts of polyunsaturated acids (compare oxidativeattack below). Since at least one cis- (Z) doublebond is essential for the required low-temperaturebehaviour, a high content of oleic acid is optimal.The best compromise at present among the abovefactors is high oleic sunflower oil (HOSO) with ≥90%oleic acid <2% C18:0, <3.5% C18:2, no C18:3(HOSO has now become commercially availablein sufficient quantities:www.tutoc.de). Clearly, bothrapeseed and soybean oil of standard quality do notmeet these criteria and would thus require extensivechemical modification of the polyunsaturated fattyacids, including selective hydrogenation or epoxidation(see chemical modifications below) in order to becomesuitable or would need additivation. Thus at the

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0

10

20

30

40

50

60

70

80

90

100

C16:0 C18:0 C18:1 C18:2 C18:3

Fatty Acids

[%]

safflower oil

high-oleic safflower oil

high-linoleic safflower oil

sunlower oil

sunflower oil (80%)

sunflower oil (90%)

rapeseed oil

soybean oil

high-oleic soybean oil

corn oil

cottonseed oil

Figure 1. Fatty acid composition of various vegetable oils (worldwide).

present time we have two options: (a) to providesufficient quantities of native oils with a high contentof oleic acid by breeding or GMOs; or (b) to modifyrapeseed oil (canola) or soybean oil by a variety ofchemical measures (see below).

Synthetic estersNative oils, i.e. triglycerides, contain glycerol as thealcohol component. They are inherently sensitivetowards hydrolysis and thermal degradation. Muchimproved performance is achieved in products wherethe glycerol has been replaced by other polyols such astrimethylolpropane (TMP),19 neopentyl glycol (NPG)or pentaerythritol (PE)20,21 (Fig. 2).

Some of these esters display extraordinary thermalstabilities. NPG esters are used as lubricants in jetengines. Several of such derivatives are used underarctic conditions. PE derivatives of carboxylic acidsC5 –C9 are employed in modern gas turbine engines.TMP esters of oleic acid are among the most widelyapplied materials for hydraulic fluids at present. Thesebio-based esters show very good biodegradability,moderate oxidative stability and have a moderate pricelevel. They are generally of high viscosity and displaygood shear stability. With some additivation they arethe optimal choice for medium to heavy applications. Asubgroup of these materials is called complex or oligoesters, resulting from the above polyols and mixturesof mono-, di- and tricarboxylic acids (for an examplesee Bondioli et al.22) (Fig. 3).

Ester of trimethylolpropane Ester of pentaerythritol Ester of neopentylglycol

O O

O

O O

OOR

R

R

RR

R

R

O

OO

O O

OO

R O R

O O

O

Figure 2. Esters of trimethylolpropane (TMP), pentaerythritol (PE) andneopentylglycol (NPG).

O

O

O R3R1

O

OO

OR2

O

n

Figure 3. An oligo- or complex ester (schematic).

DiestersOnly partly or to a lesser degree derived fromrenewable resources are synthetic diesters preparedfrom dicarboxylic acids and monovalent alcohols. Theemployed dicarboxylic acids can be derived either fromnatural sources such as azelaic acid (ozonolysis of oleicacid), sebacic acid or dimeric fatty acid, isostearicacid or from purely petrochemical sources such asadipic acid or maleic acid. The alcohols are generallybranched for better low-temperature performancesuch as 2-ethylhexanol (‘isooctanol’), isodecanol orguerbet alcohols (Fig. 4). Also branched fatty acids,e.g. 12-hydroxystearic acid (derived from rhizinoleicacid), can be employed.

O

O

O

O

H H

HOOH HO

OO

O

O

O

O

+ 2

Figure 4. Diesters from adipic and maleic acids and 2-ethylhexanol.

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Base fluids: structure and propertiesThere is a definite relationship between the structureof the molecules and their properties such as viscosity,viscosity index (VI), low-temperature performancesuch as pour point and the oxidative stability.

Viscosity of the base fluid generally increases withthe chain length of the carboxylic acid and the alcohol.However, if polyols are employed the viscosity alsodepends on the number of hydroxy functions present,and base fluids with identical fatty acid residues wouldshow the following series of viscosities depending onthe employed polyol:

PE (40) > TMP (27) > Glycerol (20) > NPG (12)

Viscosity index describes the dependence of viscosityon the temperature. A high viscosity index meansthat there is little change over a wide temperaturerange. It increases with increasing chain length ofcarboxylic acid and alcohol, while branching in eitherthe carboxylic acid or the alcohol results in a loweringof the viscosity index. Base oils based on natural fattyacids in general are known for their high viscosityindices and can be considered multi-range oils.

Good low-temperature properties require a low con-tent in saturated fatty acids (C16:0 and C18:0) and/orshortening of the chain length. In contrast, unsaturatedfatty acids display excellent low-temperature proper-ties. Thus, the pour points of simple esters derivedfrom saturated and unsaturated fatty acids of iden-tical carbon atom number are dramatically different(Fig. 5).

-40

-20

0

20

40

60

80

100

6:0 8:0 10:0 12:0 14:0 16:0 18:0 18:1 18:2 18:3

Fatty acid

Po

urp

oin

t [°

C]

Figure 5. Dependence of pour point on fatty acid structure.

Also, the pour point is lowered (improved) withincreasing branching and shortening of the chainlength.

Oxidative stability requires a low content in polyun-saturated fatty acids (C18:2 and C18:3), while onedouble bond such as in oleic acid (C18:1) is essen-tial for good low-temperature properties and doesnot negatively influence the oxidative stability (seebelow). Fully saturated esters exhibit excellent oxida-tive stability,23,24 while partially unsaturated systemsneed improvement for applications in automotive(engine, transmission fluids), diesel and industrial(hydraulic, compressor) lubricants.

Fully saturated diesters in general show good low-temperature performance (branched alcohol) and ahigh viscosity–temperature index. By adjusting thechain length of the dicarboxylic acid the viscositycan be modified. These oils are highly stable towardsoxidation (saturated compounds).

Hydrolytic stability is strongly dependent on the esterstructure. In general, saturated esters with straight-chain components are more stable than unsaturated orbranched equivalents. The most stable derivatives aresaturated dicarboxylic esters. This is usually attributedto steric effects although there is some debate over this.

Base fluids: chemical structure and performanceAs already pointed out above, native plant oils(triglycerides) are sensitive towards hydrolysis, thermaldegradation by elimination and oxidation (caused bythe multiply unsaturated fatty acids). In order to turnthem into lubricants or hydraulic fluids with improvedperformance and stability, chemical modifications,special additives or even de novo syntheses may berequired.

Thus native oils, ‘straight out of the barrel’, canonly be employed for low-performance applications,at least without suitable additivation.

Bio-based synthetic esters – in which the glycerolmoiety has been replaced by polyols such astrimethylolpropane (TMP), neopentylglycol (NPG)or pentaerythritol (PE) – are suitable for higherperformance requirements. While some mineral-oil-based materials, including poly-α-olefins (PAOs),clearly show even better performance and stability,fully synthetic esters – in which the acid or alcoholcomponent can be derived either from petrochemicals

Table 3. Various base fluids for lubricants and hydraulic fluids: properties

Base fluidViscosity,

40 ◦C (cSt)Pour point

(◦C)oxidativestability

Biodegrad.(CEC)a

Biodegrad.(OECD)a

rape seed oil (canola) 33 −21 Moderate 100 >70high oleic sunflower 39 −21 Good 100 >70soybean 31 −9 Poor 99 >70TMP trioleate 50 −50 Moderate 90 >60TMP trioleate, high oleic 47 −45 Good 90 >60PAO-8 47 −50 Good <50 30Mineral oil (150N) ISO 32 −12 Excellent 15 5

a For definitions see biodegradability section.

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(directly or indirectly) or from native oils – frequentlyoutperform mineral-oil-based fluids.

However, the degree of biodegradability – in gen-eral terms – decreases with increasing stability andperformance as increasingly non-natural componentsare employed (Table 3). From Table 3 it becomesclear that biodegradability decreases as oxidative sta-bility increases. Consequently, a compromise betweenperformance and biodegradability will always be nec-essary. Regardless of the employed base oil, additivesagainst corrosion, wear and tear (attrition) and oxi-dation are usually needed, with the exception of totalloss applications. Also the low-temperature perfor-mance (viscosity and pour point) is usually improvedby additivation. A good selection of currently availablebase fluids and their properties has been summarizedby LR Rudnick.25

ADDITIVESAs outlined above, lubricants and hydraulic fluids arematerials which are composed of a base fluid andso-called additives. These again are usually mixturesof compounds. Their composition is usually a tradesecret and the mixtures are developed largely on atrial and error basis. Their effects can be agonistic orantagonistic and the interplay between the moleculesis little understood in scientific terms. In hydraulicfluids the amount of additives is usually small (1–2%).However, in motor oils usually 10% of additives arepresent. For special applications (e.g. transmissionoils) these can reach up to 30%. Commonly usedadditives are antioxidants, rust (corrosion) inhibitors,demulsifiers, anti-wear additives, additives for pourpoint depression and, more recently, carbodiimides,which largely inhibit the hydrolysis of ester bonds bytrapping the produced water and fatty acids.

In most cases, although claimed to be safe, thetoxicological properties of many additives are stillsomewhat obscure. In summary, at the moment onlyvery few additives are available which could pass therequirements for low toxicity. In Table 4 some widelyused additives are summarized.

Natural additivesUnfortunately, to the best of our best knowledge,there are at present no bio-based additives beingused in practice. Tocopherols α–δ (vitamin E) arenatural antioxidants well known for their ability toscavenge radicals. Their presence in plant oils isusually greater than in mammalian fats. They thereforeconstitute a natural protection system within the plantagainst oxidative attack. Another natural antioxidantis L-ascorbic acid (vitamin C; E300), which canbe added to plant oils in lipid modified form, e.g.as palmitate (E304) and will generally amplify theantioxidant properties of tocopherols. Esters of gallicacid (e.g. of lauric alcohol, dodecanol; E312) would beinteresting and acceptable alternatives (E311, E312)to the more toxic phenolic antioxidants mentioned

Table 4. Commonly used additives

Additive Compound

Waterpollutionclassa

Antioxidants BHT and otherphenols

1

Alkylsubstituteddiphenylamines

1

Deactivators forCu, Zn etc.

Benzotriazoles 2

Corrosion inhibitors Ester sulfonates 1Succinic acid esters 1

Anti-wear additives Zn dithiophosphate 2(3)Pour point

depressantsMalan styrene

copolymersNot identified

Polymethacrylates Not identifiedHydrolysis

protectionCarbodiimides Not identified

a 0, no danger; 1, little danger; 2, danger; 3, strongly endangering.

above. One could also envisage the application of citricacid derivatives or lipid-modified EDTA derivatives assynthetic metal scavengers.

Natural antioxidants are typically unstable underthe harsh conditions in which machinery, includinghydraulic systems, operate. In view of the doubtfulecotoxicity of currently used additives, researchregarding the development and use of bio-basedadditives is definitely recommended.

BIODEGRADABILITY AND ECOTOXICITYMajor arguments for environmentally acceptableplant-oil-based lubricants and hydraulic fluids are(a) high biodegradability and (b) low ecotoxicity.

BiodegradabilityOne has to distinguish between primary and ultimatebiodegradation. Primary degradation describes the dis-appearance of the parent organic compound and mayor may not indicate that the substance will biode-grade completely. By this method the disappearanceof IR bands of C–H bonds (CH3: 2958 cm−1; CH2:2928 cm−1) is measured. The method is known as testmethod CECL-33-A-93 (CEC, Coordinating Euro-pean Council; www.cec.org).26 Since the exact fateof the products remains obscure, this test may notbe acceptable for the certification of environmen-tally acceptable fluids under future, more stringentEuropean regulations.27 Ultimate or total degradationmeans conversion of the tested material into CO2

and H2O by microbial action within 28 days. Thetest method OECD 301 B (OECD, Organisation ofEconomic Cooperation and Development) has foundworldwide acceptance. Numerous standardized testmethods are available and have been reviewed.28,29

The biodegradability of plant-oil-based lubricantshas been tested under operating conditions.24,30,31 Itseems that biodegradability is not affected by usageand that antioxidant additives have a positive effect.

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EcotoxicityAquatic toxicity tests measure the extent to whicha fluid will poison selected environmental speciessuch as algae (OECD 201), Daphnia magna (OECD202-12), fish (flathead minnows Oncorhynchus mykissor Brachydanio rerio (OECD 203-13) and bacteria(Pseudomonas putida) (OECD 209). Mammaliantoxicities are determined using rats (Sprague-Dawley)according to OECD 401.

Typical criteria for environmentally friendly basefluids are: biodegradability (CECL-33-A-93) >80%after 28d, (OECD 301B) >60% after 28d; aquatictoxicity LD50 >1000 ppm.

Yet in spite of these criteria the term environmentallyacceptable lubricants must be properly defined, espe-cially for formulated oils. A formulated oil consistingof a highly biodegradable base fluid in combinationwith toxic additives environmentally is not acceptable:while the bulk of fluid is indeed biodegradable, theoverall lubricant is not.

It should be noted that the criteria for biodegradableoils according to the ISO 15 380 classification aremuch less stringent. Also, regarding the disposalof oils there is no defined regulation. It is left tothe corresponding country to regulate the relevantprocedure. It seems, however, that there is a Europeaninitiative under way based on the ISO 15 830document.

Several European countries are providing eco-labelsfor lubricants, which can be termed environmentallyacceptable,5,10,32 such as the Blue Angel (Germany)and Nordic Swan (Nordic countries); recently intro-duced was a European eco-label called Euromargerite.

AGEING PROCESSES: LOSS OF PERFORMANCEThe ageing of plant-based oils or bio-based fluidsin general is related to the chemical structuresdescribed above whereby three processes dominate:(a) oxidative attack at the allylic positions, mainlyof polyunsaturated fatty acids – modulated also byheavy metals such as Cu, Pb or Sn; (b) hydrolyticdegradation of the ester bonds caused by water;(c) thermal elimination.

Oxidative attack: chemistryOxidative attack on unsaturated fatty acids is initiatedby a one-electron transfer process leading to a

radical intermediate which reacts with molecularoxygen in a so-called radical chain reaction. Theresulting hydroperoxides are further degraded with theproduction of water, aldehydes, ketones, carboxylicacids and further condensation, resulting in increasingviscosity of such oils and even formation of sedimentsby polycondensation. The most sensitive positionsfor such processes are bis-allylic C–H groups, whichare present in multiply unsaturated fatty acids suchas linoleic (C18:2) and linolenic acid (C18:3). Yetoxidative attack also occurs with C–H bonds in normalhydrocarbons as present in mineral oils and relatedcompounds. The relative reactivities of selected C–Hbonds are qualitatively shown in Fig. 6(a).

The differences in reactivity between these positionsare quite large. Thus, while oleic acid with only onedouble bond is quite stable towards oxidation, linoleicand linolenic acids with two and three double bonds,respectively, are much more sensitive in this respect(Fig. 6b);33 for mechanistic studies see Porter et al.34

The differences are even more dramatic if saturatedsystems are included (Fig. 6c).

From this it becomes clear that polyunsaturatedfatty acids are deadly regarding oxidative stability,which increases as the number of double bondsdecreases. Thus HOSO and polyol esters derivedtherefrom have considerably higher stabilities (life-time) as compared to materials that contain multiplyunsaturated fatty acids.

Test methodsThe oxidative stability of native vegetable oils and bio-based fluids can be tested in a number of standardizedways, such as: Baader test (DIN 51554 part 3); TOSTtest (DIN 51587); rotary bomb test (ASTM D 2112-93); ASTM D-943.

The hydrolytic stability ‘beverage (coke) bottle’ testis described in ASTM D 2619 and the TOST test isdescribed in ASTM D-943.

Further tests can be carried out using special testrigs.24,35,36 A detailed description of all these methods,unfortunately, is beyond the scope of this article.

Structure and performance: summaryIn summary, the performance of lubricants andhydraulic fluids in terms of stability/lifetime is stronglyrelated to the chemical structures of these materialsand the relative stabilities are as follows:

<< <<<<

rel = 1 rel.100 rel. 1200 rel 2500

rel = 1 rel = 12 rel = 25

<< <

H H H HRCH2 - H < R2C-H < R3C-H < <<(a)

(b)

(c)

Figure 6. (a) Relative reactivities of selected C–H bonds towards oxidative attack. (b) Relative reactivities of unsaturated systems towardsoxidative attack. (c) Relative reactivities of hydrocarbon moieties towards oxidative attack.

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Bio-based esters (native vegetable oils) < Bio-basedsynthetic esters (e.g. TMPs) < Mineral-oil-basedsaturated synthetic esters ∼ Mineral oils ∼ PAOs

The lower the degree of unsaturation, the longer isthe lifetime. Native oils in which the glycerol moietyhas been replaced by polyols such as TMP, NPGor PE show improved lifetime. Saturated syntheticesters are clearly the most stable ester materials atpresent. However, in many cases the biodegradabilityis less than satisfactory. At present it seems that thebest compromise between performance, price andbiodegradability are high oleic oils.36 These can beobtained either by cultivation (selective breeding),with high oleic sunflower oil (90%+ oleic acid)being the most prominent example, followed in thefuture possibly by high oleic rapeseed and soybean(selective breeding or GMO). Alternatively, chemicalmodifications of commodities such as rapeseed oilor biodiesel would be an interesting alternative. Basefluids with high contents of oleic acid also displayexcellent low-temperature properties with pour pointsof −35 ◦C (HOSO) and −50 ◦C (TMP, trioleate higholeic).

As outlined above, the products resulting from suchmaterials in combination with polyols such as TMPdisplay excellent properties as hydraulic base fluids.37

In fact, TMP esters of oleic acid are among the mostwidely applied materials for hydraulic fluids at present.They are largely bio-based (i.e., they contain naturalfatty acids), and show very good biodegradability,moderate oxidative stability and moderate price level.With some additivation they are an optimal choice formedium to heavy applications. The properties of sucha material are summarized in Table 5.38

At present, HOSO is available only in limitedquantities and for a comparatively high price.However, this may change rapidly as the developmentof environmentally friendly fluids increases. Moreover,new species (GMO) of high oleic rapeseed andsoybean are at the brink of development, while higholeic safflower may not make it to the market placeowing to low productivity of the plant.

In order to find a compromise between availability,price and performance one could consider thefollowing possibilities:

Table 5. Properties of high oleic (90% +) TMP trioleate

Property Data

viscosity, 40 ◦C (cSt) 47Viscosity, 100 ◦C (cSt) 9.5Viscosity index (VI) 190Pour point (◦C) −45Flame point (◦C) 300Evaporation rate (%) 1.3Oxidation stability (Baader) Middle to goodBiodegradation CEC (%) 95Biodegradation OECD (%) 90

1. With low-priced rapeseed oil or biodiesel as startingmaterial one could (a) remove the polyunsatu-rated fatty acids by selective hydrogenation and(b) exchange the glycerol moiety with polyols suchas TMP.

2. With high-priced high oleic sunflower oil one wouldjust have to replace the glycerol moiety by TMP.

The cost of chemical transformations in rapeseedoil is estimated to be approximately 0.5 euro per stepand per kilogram and could therefore compete withthe high price of high oleic sunflower. It would be aquestion of economy to choose the best route. If thecost of native high oleic sunflower oil comes down,this would clearly be the better and faster route tohigh-performance base fluids.

IMPROVEMENT OF NATIVE OILS BY CHEMICALMODIFICATIONSAs pointed out above, native oils (triglycerides) are,depending on their chemical structures, of only limitedstability and prone to oxidative attack, hydrolysisand thermal decomposition. Based on their generalstructure there are basically only two positions innative plant oils (triglycerides) where a chemicalmodification can be effected39–41 (Fig. 7): (a) the estermoieties, especially the critical β-hydrogen in nativetriglycerides; and (b) the double bonds along the fattyacid chains.

Modifications of the ester moietiesTriglycerides contain glycerol as the alcohol compo-nent and have the critical β-H position, which rendersthe molecules sensitive to thermal degradation by elim-ination. As already pointed out above, much improvedperformance is achieved in products where the glycerolis replaced by other polyols such as TMP, NPG or PE.Recent research results seem to indicate, however, thatboth the hydrolytic and oxidative stability of native oilsis considerably increased if the fatty acid portion con-sists entirely of oleic acid. Thus native HOSO displaysa similarly high stability as the combination of stan-dard oleic acid (68–72%) with TMP. This has to bestudied in greater detail, since it would bring the useof neat vegetable oils without any modification a giantstep forward.

O

O

O RR

O

OO

Figure 7. Critical positions in triglycerides susceptible to chemicalmodifications.

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Modifications of the double bond systemAs outlined above, multiple double bonds areextremely sensitive towards oxidative attack. Incontrast, fatty acids with only one isolated cis doublebond such as oleic acid (C18:1) are stable towardsoxidation by an order of magnitude or more.

Selective hydrogenationIdeally all unsaturation should be removed, e.g. bycatalytic hydrogenation. Yet, this ‘hardening’ leads tosaturated fatty acids such as stearic acid and wouldturn the (liquid) oil into (solid) fat – useless for appli-cations as lubricants and hydraulic fluids. Thus at leastone isolated cis double bond is required in order toobtain oils with good (low-temperature) performance(viscosity, pour point). Therefore selective hydrogena-tion of multiply unsaturated fatty acids to materials inwhich all but one unsaturation is removed would pro-vide the ultimate solution to this problem and linolenicacid would lead to oleic acid and its positional isomers,each of which carries only one isolated cis double bond(Fig. 8).

Catalysts achieving such transformations alreadyexist42–45 yet have not been applied on a technicalscale. Previous attempts along these lines by industrywere largely unsuccessful. While the amount ofC18:2 and C18:3 acids was indeed reduced, a largeproportion of the cis C18:1 had isomerized to theundesirable trans C18:1 (elaidic acid), which hassimilar undesirable properties to stearic acid (C18:0),thus rendering this process obsolete. Clearly theindustrial development of this process would allowimprovement of readily available base fluids such asrapeseed oil and rapeseed oil methylester (RME), i.e.‘biodiesel’.

HO

O

HO

O

HO

O

HO

O

HO

O

H2 / selective catalyst

+

+

Figure 8. Selective hydrogenation of multiply unsaturated fatty acids(schematic).

The double bonds in unsaturated fatty acidsoffer numerous other possibilities for chemicalmodifications. One must always be aware, however,that a compromise between the following criteria mustbe found:

• Performance (tribological properties)• Stability towards oxidation• Low-temperature performance• Biodegradability

Branched fatty acidsClearly the best way to improve the stability againstoxidation would be the complete removal of alldouble bonds. Yet in order to maintain the low-temperature properties branched substituents must beintroduced. These, however, typically cause a loweringof the biodegradability. Base fluids synthesized frombranched fatty acids (e.g. isostearic acid) displayexcellent low-temperature behaviour such as lowpour point, low viscosity, high chemical stabilityand high flashpoint. Commercial isostearic acid isa mixture resulting from the thermal isomerisationof polyunsaturated C-18 fatty acids, followed byhydrogenation. Branching points are concentrated inthe centre part of the molecule.46,47 Alternatively, 12-hydroxystearic acid, derived from rhizonoleic acid byhydrogenation, can be employed.

Chemical modificationsA number of examples for the modification of doublebonds are summarized in Fig. 9.21,48 These includealkylation,49 radical addition,49–52 acylation,53 ene-reaction,54 hydroformylation–oxo-synthesis,55–57 co-oligomerisation,58,59 hydroaminomethylation60 andacyloxylation (addition of carboxylic acids).61,62

Epoxidation and derivatives derived therefromEpoxidation of native plant oils transforms multiplyunsaturated functions into oxiranes (epoxides), whichare susceptible to ring opening reactions. Epoxidationscan be achieved via classical, chemical methodsusing peracids63 (for industrial applications see alsoWestfechtel and Giede64) and also enzymatically.65,66

This allows the production of fully saturated esterswith improved stability against oxidative attack andthus an innovative approach for the conversion ofnative oils,67 including epoxidized soybean oil intouseful base fluids for lubricants (Fig. 10).68

The alcohols used were so-called Guerbet alcohols(Jarcol I-18T), a mixture of 2-octyldecanol, 2-hexyldodecanol (C16 15–20%; C18 46–54%, C20

27–33%). Employed fatty acids were C2 –C6. Theresulting fluids, developed in the US Departmentof Agriculture (USDA), are currently being testedby a number of companies regarding their practicalproperties.

In summary, one can state that many of theproducts resulting from the above transformationsshow interesting properties for practical applications,

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OR

O10 9

OR

O

+ 9-regioisomer

10OR

O

O + 9-regioisomer

10

OR

O

1:1 adduct

OR

O

1:2 adduct

OR

OO

9

8

+ 10,11 - regiosiomer

OR

O

10

O H

+ 9-regioisomer

OR

ONH

+ 9-regioisomer

10

OR

OOH

9

8

+ 10,11 - regiosiomer

OR

OOO

+ 9-regioisomer

10

Et3Al2Cl3

O Cl

O Mn(OAc)3O EtAlCl2 Cl

O

H2 / CORh-catalyst, Ph3P

[Rh(COD)Cl]2p(CO/H2) 70bar

NH2CH2 CH2

CH2 CH2

2. RhCl3 x 3H2O

1. Isomerization

CH2OPivalic acid

Oleic acid estercatalyst

Figure 9. Chemical modifications of oleic acid derivatives (clockwise from upper left): (a) alkylation; (b) radical addition; (c) acylation; (d) ene-reaction; (e) aminoalkylation; (f) co-oligomerization; (g) hydroformylation; (h) acyloxylation.

O O

O

O

OO

O O

O OO O

CO2R

OH(R)

OH(R)(R)HO

OH(R)

CO2ROH(R)

O(R)HO

O

R′

O

R′

O

R′ O R′

O O

epoxidized soybean oil (schematic)

R-OHH+

3

(Guerbet alcohols)Jarcol I-18T

R′= C1 - C5

Figure 10. Base fluids from epoxidized soybean oil.

yet frequently biodegradability has not yet beenproperly addressed. In any case, the field is wide openfor innovative research.

GOVERNMENT REGULATIONS ANDLEGISLATUREIt seems that Portugal was the first country to makeuse of biodegradable outboard two-stroke engineoils obligatory in 1991. Yet the source of the basefluid was not further specified and only a 66%

minimum requirement of biodegradability accordingto CEC-L-33-T-82 was imposed. In Austria theuse of plant-oil-based lubricants for chain bar sawsis mandatory (Federal Regulation). In all othercountries there are no compulsory legislative measuresregarding the use of lubricants for special applications.Recommendations exist in the UK69,70 and Canada.71

Numerous state (county) or community regulationsare in (voluntary) force requiring contractors andsubcontractors to use environmentally acceptablelubricants and hydraulic fluids. This obligation

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becomes part of the contract and must also be includedin the financial offer by the contractor.

Increasing legislative pressure would promote theuse of plant-based lubricants.14 For this, however,politicians would have to decide whether theyconsider the release of large quantities of lubricantsinto the environment tolerable or a danger inthe sense of pollution. Unless there is a politicaldecision of some sort there is little or no pressureon the consumer, industrial companies, contractorsetc. to use plant-based lubricants or hydraulicfluids. At present they will base their decision onprice and performance (lifetime) first, followed byenvironmental considerations. Partly it is a questionof cost versus environment.

Another major obstacle for the introduction ofplant-based oils is the fact that manufacturers ofmachinery, including hydraulic equipment, are noteasily convinced to clear their machines for the use ofplant-based oils. Yet this is definitely required by thecustomer, who needs to know that the manufacturergives full warranty for his machinery under theseconditions. Although the required changes in mostcases are relatively minor, there is still considerablereluctance both with manufacturers and customers.The reasons are relatively simple:

1. With a market share of presently 2–5% none ofthe manufacturers sees the need for acting towardsthe use of plant based oils. If the market sharereaches 10-15% this problem would be solvedautomatically.

2. An oil change requires the following activities:(a) Change of elastomers, gaskets etc. for very little

cost (penny articles). This is generally only forolder equipment (before 1995), while modernmachines already have high-quality elastomersbuilt in, such as viton, which will withstand allkinds of lubricants.

(b) Flushing of the system with two loads of plant-based oil in order to reduce the amount ofresidual, conventional oil to <2%. These twofillings have to be discarded.

(c) Introduction of one new filling with plant-based oil.

In summary, the change of elastomers (with olderequipment) is required and three fillings of oil areneeded. In view of the considerable sump volumes insome equipment, the cost for three fillings for one oilchange is quite expensive for the user, who is unlikelyto do that unless some form of government incentiveis provided. Although the elastomers to be changedare of very low cost, the machine has to be dismantledand reassembled, which is a labour-intensive process.In this sense it would certainly help if the governmentwere to pay for the first filling with environmentallyacceptable fluids. However, in view of the small marketshare manufacturers are unlikely to stock such fluidsseparately at the end of their assembly line.

The German Market Introduction Programme(MIP)In order to promote the increased use of plant-basedlubricants and especially hydraulic fluids the Germangovernment has introduced a programme in which alarge part of the initial cost for the switch is coveredby the taxpayer. The programme covers, in particular,(a) expenses for the oil and (b) labour cost requiredfor the change.

The MIP has been well accepted and in the firstyear 4000 machines (equivalent to ca 1000 tons ofconsumed oil) have been changed over to plant-based (environmentally acceptable) oils. 90% of thesewere hydraulic fluids and here mostly (90%) bio-based synthetic esters (e.g. TMP oleates), with onlyca 1% of native oils (a positive list of acceptedfluids can be found in www.pflanzenoel-initiative.de;www.fnr.de; www.rwth-aachen.de). Very few damagesand/or problems were encountered, and only fivemachine malfunctions have been reported. It looksas if the programme will turn out to be successful,mainly in the sense that the public and manycustomers are becoming increasingly aware of theenvironmental aspects of such changes. Also theprogramme allows the accumulation of experiencesregarding the performance of such fluids over a 5-yearperiod.

ECONOMIC SITUATION AND FUTUREDEVELOPMENTIt can be stated in general terms that the costfor biodegradable lubricants and hydraulic fluidsis – depending on the product quality – frequentlyhigher than that of comparable mineral-oil-basedproducts. Since the price level is also dependenton the corresponding additivation and the amountspurchased it is difficult to define the exact differences.Generally, the price of rapeseed oil is much lowerthan that of synthetic esters, with differences of up to200%. However, the price differences have to be seenalso in relation to differences in quality and extendedlifetime (i.e., extended periods between oil changes).The current differences in prices between bio-basedand mineral-oil-based fluids may, however, becomeirrelevant if one looks into the very near future. Itcan be predicted that mineral oil production willreach its peak (Hubberts peak) in ca 2003–2006(right now!)16 and decline after that.17,72–74 Thiswill – as demonstrated presently at your own fillingstation – cause a tremendous increase in the price levelfor all mineral-oil-based products, including lubricantsand hydraulic fluids. A price advantage for mineraloils may therefore no longer exist in the foreseeablefuture. Moreover, with increasing bidding for thedwindling reserves there may be a supply shortage.Since biolubricants are already available for numerousapplications, with performance being well comparableand sometimes even better than those of mineral oilproducts – combined with the methods for chemical

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derivatizations increasingly being developed – plantoils are an attractive alternative as raw materials forlubricants.

ACKNOWLEDGEMENTSWe thank the German Ministry of Agricul-ture (BVMEL), administered by the FachagenturNachwachsende Rohstoffe (FNR), for the generousand continuous support of our work. We also acknowl-edge numerous and fruitful discussions with our sci-entific partners of many years, the IFAS Institute inAachen (Prof. Dr H Murrenhoff and Dr H Theissen)and Fuchs Petrolub AG, Mannheim (Angela Kesseler,Rolf Luther and Prof. Dr. Theo Mang).

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