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654 Lubricant Additives: Chemistry and Applications

26.2.3.4 Canadian Eco-Label—EcoLogo

If manufacturers, suppliers importers, or dealers of environmentally preferable products and services

participate in the Environmental Choice Program (ECP), they can be awarded with the Canadian

Eco-label EcoLogo

[44] if guidelines for different product categories are ful lled. These cat-

egories include, among others, automotive engine oil (recycled), bicycle chain oil (biodegradable),

chainsaw lubricants (biodegradable and nontoxic), engine oil marine (inboard and outboard), and

industrial lubricants (re-re ned oil, synthetic, and vegetable oil-based). For each category, a differ-

ent set of criteria must be ful lled to qualify for the Canadian EcoLogo. For example, for synthetic

industrial lubricants, biodegradability according to CEC-L33-T82 is required. The lubricant should

not contain more than 0.1% petroleum oil (or additives containing petroleum oil) and any of the

following metals—lead, zinc, chromium, magnesium, and vanadium [45]. In addition, the lubricant

shall not be subject to be labeled according to Class D, poisonous and infectious material [46]. Tech-

nical performance is tested with various ASTM test methods—for example, ASTM D97 for pour

point, ASTM D 665 for rust, ASTM D525 for oxidation stability, and ASTM D-892 for foam [45].

In general, the Canadian EcoLogo concentrates on aquatic toxicity and biodegradability and

technical performance, while the CO

balance and renewability are focused marginally. The

Canadian EcoLogo is one of the few labels that includes a life cycle analysis in its considerations

for awarding products.

26.2.3.5 European Eco-Label

In 1992, the European Union introduced a generally accepted label to help the consumer identify

environmental friendly products and services [47]. It represents a voluntary scheme with a clear

objective to market more environmental friendly products and services and has been developed in

cooperation with industry and nongovernmental organizations (NGOs). After 13 years of existence

of the common label, it was decided by the European Commission on April 26, 2005 to establish

the category of the European Eco-label Lubricants (EEL) [48].

The newly established label covers the  elds of hydraulic  uids, greases, chainsaw oils, and

total loss lubricants such as two-stroke oils and concrete release agents. The label is valid for private

consumers and professional users. Lubricants qualifying for the EEL should meet requirements for

technical performance and should have limited aquatic toxicity, high biodegradability, high bioac-

cumulation, a limited content of dangerous components, and a positive CO

balance. According to

Mang and Dresel, ecological criteria for a product group are normally established for a period of

3 years. This period allows for improvements in the technology used, and changes in the market can

be re ected when the criteria are revised (see Ref. 42, p. 137).

Additives must be evaluated under ecotoxicological perspectives, and additional components

must be proven for acceptability with respect to the following criteria:

R-clauses and impact on formulation

Aquatic toxicity (daphnia and algae)

Biodegradability/bioaccumulation

Not listed on Oslo-Paris-Konvention, Organization for the protection of the Marine

Environment of the North-East Atlantic (OSPAR) [49]

R-clauses have an important impact on the allowed treat levels of additives and have to be seen

cumulatively according to Directive 1999/45/EC [50], the  nished product not being permitted

to carry any R-clause. Evaluation of aquatic toxicity, biodegradation, bioaccumulation, and so on

must be undertaken for all ingredients separately, or for the  nished formulation. This implies

that for all additives, a fully documented toxicological dossier has to be submitted to meet

the requirements stated by the European Eco-label. The technical performance requirements

depend on the intended application; hydraulic  uids, for example, should ful ll ISO 15380 [31].

•

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Eco Requirements for Lubricant Additives 655

In practice, this means that tests such as the Baader test (DIN 51554-3), the steel corrosion test

(ASTM D 665 A), the copper corrosion test (ASTM D 130), and the Forschungsstelle für zahnräder

und Getriebebau (Technical University Munich) (FZG) test (DIN 51354-2) must be passed. Some

criteria only have to be reported or can be agreed with the end user—for example, the dry TOST

test (ASTM D 943 modi ed). As ISO 15380 [31] differentiates technical performance according

to different base oil types, lubricants with differing qualities may be developed.

So far, many of the new European members still have their own label with their individual

speci cations, but many of these countries have declared their intention of adapting the rel-

evant criteria of the European Eco-label to their national label. For example, Hungary and Czech

Republic intend to harmonize their national label with the European Eco-label. Some countries

will only award the EEL in future, some will award their national label with the speci cations

of the EEL, and it is estimated that only a minority will still have only their national label with

their own speci cations.

Figure 26.2 gives a schematic representation of some of the key developments in the biolubri-

cant market and the development of various Eco-label schemes, market implementation programs,

and legislation.

Overall, it can be seen that the general requirements of the various eco-label schemes are quite

similar, as can be also seen in Table 26.2.

However, looking into the details, there are still some disparities, but with the institution of the

European Eco-label, a  rst step toward harmonization has been taken. Only the future will tell if

this will simplify the area of eco-labels or add to the complexity.

An important driver in the development of any market, and in this respect the biolubricant

market is no different to any other, is the economic consideration. Typically, the price level for

biolubricants can be 1.5–5 times (depending on the base  uid chosen) the price of a similar min-

eral oil–based formulation, although this is partially offset by lower costs for disposal and those

incurred as a consequence of accidental leakages and spills. This cost differential may in time

FIGURE 26.2 Historical development of biolubricants and corresponding eco-labels.

Biodegradable two-

stroke outboard

marine engine oils,

snowmobiles

(Scandinavia and

North America) & golf carts

(United States, Australia…)

Increasing environmental

awareness: development

of rapidly biodegradable

base oils for other

lubricants

Austria: ban on

mineral oil−based

chainsaw oils (min.

90% biodegradable)

• 4% of German lubricant

consumption consisted

of biodegradable

products

Germany created the

first environmental

lubricant standard

(RAL UZ 48)

First idea and

introduction of

European eco-label

for various products

and services

American farm

bill start of

federal biobased

procurement

program



also

vald for lubricants

Portugal: first country

to make use of

biodegradable

outboard two-stroke

engines obligatory 

just 66% minimum

requirement of

biodegradability (base

fluid) CEC-L-33-T-

• Environmental criteria

for lubricating greases

and hydraulic fluids in

Sweden: Swedish

standard!

Nordic swan was

introduced as a

voluntary eco-

label

European eco-

label for lubricants

Canada: introduction

of EcoLogo

(second

national ecolabeling

initiative undertaken)

Late 1970s

Beginning

of 1980s

01.05.1992 19981988 1992

26.04.2005

19911989 2002

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656 Lubricant Additives: Chemistry and Applications

reduce, if trends in the price of mineral oil continue, and of course, any accurate analysis of eco-

nomical factors should take into account all costs incurred over the lifetime of the lubricant, includ-

ing but not limited to purchase cost, disposal cost, effects on energy consumption, and servicing

and maintenance costs.

Health and safety considerations make the adoption of biolubricants favorable; typically, higher

 ashpoints at similar viscosities lead to a lower  ammability risk, and with lower hazard ratings, the

potential for harm to the person from inhalation or skin contact is reduced.

26.3 BASE FLUIDS FOR BIOLUBRICANTS

The broad variety of de nitions for biolubricants is also re ected by the large number of suitable

base  uids. A good example to illustrate this is the international standard ISO 6743-4 [51]. In this

norm, the group of hydraulic  uids in which environmentally acceptable  uids are requested is

divided into four base  uid categories: triglycerides, polyglycols, synthetic esters and PAOs, and

related hydrocarbon products. Clearly, good understanding of the different characteristics of these

base  uids is needed to develop and formulate biolubricants.

TABLE 26.2

Comparison of Relevant Parameters of European Eco-Label, German Blue Angel,

Canadian EcoLogo, and U.S. Farm Bill

Eco-Label Applications/Valid for Biodegradation

Aquatic

Toxicity

Health

Hazards Renewability

Technical

Performance

German Blue

Angel

Chain lubricants for power

saws; lubricants and

forming oils; hydraulic

 uids

++ ++ ++ – + +

Swedish

Standard

Hydraulic  uids; greases ++ ++ ++ +/–

–

Nordic Swan Chain oil; mould oil;

hydraulic oil; two-stroke

oil; lubricating grease,

metal cutting  uid and

transmission/gear oil

++ ++ + ++ +

Candian

EcoLogo

General-purpose industrial

lubricants; marine propeller

lubricants; engine oils:

marine inboard/outboard;

chainsaw lubricants;

bicycle chain lubricants;

automotive engine oil

recycled; marine oil

absorbent; hydrocarbon

ab(d)sorbents

++++–+

European

Eco-Label

Hydraulic  uids; greases;

chainsaw oils; two-stroke

oils; concrete release agents

++ ++ ++ ++ ++

Greases: consideration of renewability aspects; Hydraulics: no consideration.

Note: ++, major focus; +, consideration; –, no consideration.

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Eco Requirements for Lubricant Additives 657

26.3.1 TRIGLYCERIDES

Triglycerides are the main components of fats and oils found in plants and animals. Chemically

spoken triglycerides are esters composed of glycerin and three (typically natural) fatty acids. The

most important fatty acids contained in such oils are the saturated acids—palmitic acid (C16:0) and

stearic acid (C18:0)—as well as the unsaturated acids—oleic acid (C18:1), linoleic acid (C18:2), and

linolenic acid (C18:3). The exact fatty acid composition of natural oil depends very much on the

source from whence it is derived. The main fatty acid in a rapeseed oil, for example, is oleic acid

(C18:1), and in a soybean oil, linoleic acid (C18:2) is dominant. Modern agricultural technology such

as breeding new plants or the use of genetically modi ed organisms (GMOs) allows further possi-

bilities to tailor the fatty acid composition. A typical example is the development of new sun ower

oils containing a high amount of oleic acid (C18:1).

Triglycerides have three drawbacks in regard to the technical performance as a lubricant.

First, due to the hydrogen atom in a β position (a), triglycerides can decompose relatively easily

through a low-energy, six-membered ring intermediate. The thermal stability of natural esters is

hence very low. Second, the presence of unsaturated double bonds (b) in the fatty acid moiety

negatively affects the oxidation stability of these compounds. Typically, the bonding energy of a

hydrogen atom in an allylic position is much lower (∆H

(C–H)

= 345 ± 5kJ/mol), compared with a

hydrogen atom in a fully saturated hydrocarbon (∆H

(C–H)

= 411 ± 2 kJ/mol) [52].

A hydrogen atom in an allylic position can therefore be abstracted easily in a radical reaction

initiating the oxidation process. Even worse is the oxidative stability of polyunsaturated fatty acids.

According to Falk and Meyer-Pittroff [53], the difference in the reactivity of oleic acid (C18:1),

linoleic acid (C18:2), and linolenic acid (C18:3) is approximately 1:10:100.

The third drawback of a triglyceride is the low sterical demand of the ester groups (c) when

glycerin is used as an alcohol. This allows water to attack the carbonyl group and hydrolyze the

ester, releasing free fatty acids.

Despite the fact that unsaturated fatty acids are prone to oxidation, the unsaturation also pro-

vides a bene t to the natural oils with respect to their low-temperature  ow properties. As a rule of

thumb, it can be stated that a natural triglyceride needs at least one cis con gured double bond to

show a suitable low-temperature performance. A fully saturated triglyceride would be a solid fat at

room temperature and not be usable for a lubricant formulation.

26.3.2 SYNTHETIC ESTERS

To overcome some de cits of natural esters, the glycerol moiety can be replaced. By using sterically

demanding alcohols not bearing a hydrogen atom in β position for such a transesteri cation process,

NPG TMP PE

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658 Lubricant Additives: Chemistry and Applications

products with considerably greater stability are obtained. Usually alcohols such as trimethylpro-

pane (TMP), neopentyl glycol (NPG), or pentaerythritol are used [54–56].

The obtained polyol esters show improved hydrolytic and thermal stability, the oxidation stabil-

ity however being equal to the used starting material and only dependent on the degree of satura-

tion. Such esters can be classi ed as semisynthetic, and they still have a high degree of biobased

material, as the alcohol part is typically derived from petrochemicals, whereas the fatty acid part

can stem from natural resources.

Synthetic esters are made from synthetic alcohols and fatty acids and can hence be tailored

to the expected or required performance. The use of branched alcohols or fatty acids allows, for

example, for the production of esters with a good oxidative stability, without compromising on

low-temperature performance. A high degree of branching also has a positive effect on the seal

compatibility of such products. Stemming from petrochemicals, such esters would of course not

meet biobased requirements anymore but could still have good biodegradability and a good eco-

toxicological pro le.

26.3.3 POLYGLYCOLS

Polyglycols are synthesized technically from petrochemical starting materials such as ethylene

oxide and propylene oxide yielding a polymeric structure (Figure 26.3).

Depending on the used starting material, polyalkylene glycols can be either water-soluble (poly-

ethylene glycols) or oil-soluble (polypropylene glycols), but using copolymerization, this property

can be further adjusted to the performance requirements. Polyalkylene glycols are biodegradable,

especially water-soluble polyglycols, whereas oil-soluble polyglycols have a typically lower biode-

gradability. However, a high water solubility could be seen as an environmental disadvantage; in the

case of a  uid getting into the environment, water-soluble  uids would migrate rather quickly into

waterways causing an instant contamination. Currently, as polyalkylene glycols are produced from

petrochemicals, they do not meet a biobased requirement. Theoretically, it would also be possible

to produce polyalkylene glycols from natural resources such as lactic acid or sugar, but these are not

yet common industrial processes.

26.3.4 POLYALPHAOLEFINS

PAOs and related hydrocarbons are produced from petrochemical resources and are hence not bio-

based; however, they can have a good biodegradability. PAOs are polymerized from ole ns, in which

the double bounds are in α position. For lubricants, typical starting materials used are 1-decen and

1-dodecen and, despite the name, only oligomers such as trimers and tetramers are used [57].

The technical performance of PAO is good, especially in regard to the oxidative stability. As PAOs

are very nonpolar, the additive solubility can be a concern, and therefore, PAOs are often used in

mixtures with polar esters to overcome this de ciency. Such mixtures can have very good technical

performance, but the variabilities and permutations are so wide that in-depth description would be

out of the scope of this chapter.

FIGURE 26.3 R1 and R2 are typically hydrogen, methyl, or ethyl groups.

Catalyst

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Eco Requirements for Lubricant Additives 659

To formulate a biolubricant, the choice of the correct base  uid is critical, and depending on

the speci c requirements, each of the described  uids could be optimally selected. The advantages

and disadvantages should be balanced and matched with the lubrication needs of the application

(Table 26.3).

Future trends for base  uids used for biolubricants are dif cult to predict. One possible trend

could be the so-called second generation of biolubricants—using biogas to produce liquid hydro-

carbons: biogas to liquid (BTL). If such technology is developed to an industrially feasible process,

this would enable the exploitation of the biomass of complete plants and a wide range of feedstock

such as wood and waste plants. Utilizing gasi cation and Fischer–Tropsch technology, all kinds of

hydrocarbons could be produced. Similar to triglycerides, the products of this technology for use in

lubricants are in strong competition to their demands as fuels; hence, it cannot be predicted when

such technologies could have a substantial impact on the biolubricant market.

26.4 ADDITIVES FOR BIOLUBRICANTS

The performance of a  nished lubricant depends collectively on the base oil, additives, and for-

mulation; and for biolubricants, the selection of base oils and additives is limited by ecological

regulations. The selection of the right additives and their combination has to be seen integrally.

First, it should be established what type of biolubricant is to be formulated and what criteria

it should meet. If the target is to stabilize a biobased  uid without any further requirements,

practically all additives known could be used. However, if the  uid has to be environmentally

acceptable, as a rule of thumb, the formulation has to be ashless (not containing metals) and

only ecotoxicologically harmless additives can be used. Whereas the  rst requirement is rela-

tively easy to achieve, as a broad variety of ashless additives exists on the market, the latter can

be a problem. Currently there are only a limited number of additives available, in which the full

ecotoxicological data have been assessed and disclosed from additive suppliers. For harsh criteria

such as demanded from the European Eco-label, only a few additives are currently approved (van

der Sijp, SMK list, Private Communication, 2007). In addition, the nonmineral base oils used

for biolubricants very often need different additives and treat rates to achieve the performance

known from classical formulations.

TABLE 26.3

Comparison of Selected Parameters for Environmentally Acceptable

Fluids

Parameter Triglyceride Synthetic Ester PAG PAO

Renewability content High Variable Not Not

Biodegradability Good Good Moderate Moderate

Water solubility Low solubility Low solubility Compatible Low solubility

Low-temperature

performance

Weak Good Good Good

Oxidation resistance Weak Good Good Good

Hydrolytic stability Low Medium Good Good

Seal material

compatibility

Good Limited Limited Good

Paint and varnish

compatibility

Good Good Limited Good

Additive solubility Good Good Moderate Limited

Lubricity of base oil Good Good Good Limited

Corrosion resistance Poor Limited Limited Good

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660 Lubricant Additives: Chemistry and Applications

26.4.1 ANTIOXIDANTS

In practice, the oxidative aging of the lubricant is the dominating process, which in uences the

lifetime of a lubricant signi cantly [42]. The oxidative degradation can cause the formation of acids,

varnish, deposits, sludge, and an increase in the viscosity. It is generally accepted that the oxidative

degradation is following a cyclic, free radical mechanism through alkyl and peroxide radicals, as

shown in Figure 26.4.

To prevent oxidation, antioxidants are included in the formulation. Radical scavengers, which

inhibit the propagation of the radical chain mechanism, are described as primary antioxidants.

Sterically hindered phenols and secondary aromatic amines are typical examples of primary

antioxidants.

Despite the fact that aminic and phenolic antioxidants stabilize a lubricant through similar

reaction mechanisms, it has been found that both antioxidant classes act synergistically [58]. Hence,

often, combinations of aminics and phenolics are applied. Unlike phenolic antioxidants, aminic

antioxidants are active at higher temperatures (>120°C) [59]; therefore, it is essential to know the

operation temperature of the intended application. A chemically unmodi ed vegetable oil, which

is used typically only in low-temperature applications, preferably would be stabilized with a high

amount of phenolic antioxidants (>1%), whereas for a high-temperature application, for example, a

PAO-based base  uid, would be optimized with the addition of a synergistic mixture of aminic and

phenolic antioxidants. As primary antioxidants can be regenerated, they are catalytically active in

the stabilization of a lubricant.

Secondary antioxidants react stoichiometrically with peroxides and deactivate these aggressive

chemical intermediates. Examples of secondary antioxidants are phosphorus- and sulfur- containing

compounds, in which the heteroatom has a lower valence state, thus it still can be readily oxidized.

As secondary antioxidants deactivate peroxides, they prevent the chain propagation reaction

initiated when a peroxide is divided into two radicals. A standard secondary antioxidant would be

zinc dialkyldithiophosphates (ZDTPs); however, as zinc compounds are classi ed to be eco-toxic,

and labelled “N” based on toxicity to algae [60], this chemistry could not be used to formulate

Reacts with

product (RH)

Oxygen

Cycle 2

Cycle 1

Catalyst residues

Light

R°

ROOH + R°

ROH

ROH + H

Reacts with

product (RH)

Secondary antioxidants react with

hydroperoxides to yield inactive products

Path of

degradation

Path of

stabilization

ROO°

Energy

light (hv)

Catalyst residues

Free radicals

RO° + OH°

Energy

Primary antioxidants react

with oxygen-centered radicals

to yield inactive products

Booster reacts with

carbon-centered

radicals

FIGURE 26.4 Schematic mechanism of oxidative degradation and the in uence of antioxidants.

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Eco Requirements for Lubricant Additives 661

environmentally acceptable  uids. For such  uids, ashless phosphites and thioesters are much bet-

ter suited. As a consequence of the different additive mode of action, a combination of primary and

secondary antioxidants is very favorable and show synergistic effects.

As an alternative to the aforementioned antioxidants, the use of metal dithiocarbamates is

described in literature [61]. These additives show very good performance in different tests, by far

exceeding the more commonly used zinc dialkyl-dithiophosphates [62]. Unfortunately, the ashless

dithiocarbamates, using an ammonium salt or using a methylene bridge, do not show this perfor-

mance boosting effect. On the contrary, the use of metal-containing dithiocarbamates—such as

bismuth and zinc—cannot be utilized to formulate an environmentally acceptable  uid.

26.4.2 METAL DEACTIVATORS

In various applications, the lubricant can be in contact with parts made of yellow metals—typically

copper—or with dissolved metal contaminants. As copper compounds catalytically decompose

peroxides generating radicals, these metal species are very ef cient pro-oxidatives and need to be

deactivated. Dissolved copper ions are deactivated with chelating agents, whereas the metal parts

are protected and deactivated with  lm-forming metal-passivating agents. The advantage of  lm-

forming agents is the fact that they prevent the progressive oxidation of the metal surface, which

would release copper ions to the  uid. Complexing agents, on the contrary, could even promote the

transfer of copper ions from the metal surface into the  uid. A typical example of a chelating agent

would be disalyclyiden-propylene-diamine, in which  lm-forming agents are very often derivatives

of benzotriazol [63]. Both groups of metal deactivators are used only in very low treat rates—below

0.1%—and are active in all suitable base  uids used for the formulation of biolubricants. A critical

property of some metal deactivators is the low solubility; especially when PAO is used as a base

 uid, this property must be carefully checked.

26.4.3 CORROSION INHIBITORS

Another important class of additives used to protect metal surfaces is corrosion inhibitors; in this

chapter, the term corrosion inhibitor is used synonymously with antirust additives and describes

additives that are used to protect ferrous metals against the attack of moisture, oxygen, and other

aggressive lubricant ingredients. Corrosion inhibitors possess as a general molecule design a long

alkyl chain to improve the solubility in lubricants and a polar group to provide the surface activity.

Corrosion inhibitors are absorbed physically or chemically on the metal surface and form a dense

protective layer. Examples of corrosion inhibitors include metal sulfates, amines, carboxylic acid

derivates, and amine neutralized alkyl phosphoric acid partial esters. Because of their surface activ-

ity, corrosion inhibitors tend to compete with polar base  uids for the metal surface; therefore, it can

be quite complex to improve the corrosion stability of an ester or a polyalkyle glycol (PAG). Another

potential problem associated with corrosion inhibitors is the fact that acids, bases, and metals can

catalyze the hydrolysis of esters.

To prevent the hydrolysis of ester-based  uids, the treat rate of the corrosion inhibitors should be

as low as possible, and in addition, preferably neutral additives such as the neutralized acids should

be applied. It has been described that the response of corrosion inhibitors in different esters can

show signi cant variation [64]. For example, to stabilize a rapeseed oil, twice the amount of a corro-

sion inhibitor was necessary, than was required for a TMP oleate. In addition, it has been found that

at higher treat rates, the use of corrosion inhibitors can have a detrimental effect on the corrosion

protection. This shows the importance of a careful balance of the additive treat rate.

R2 OH

O R2

Catalyst

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662 Lubricant Additives: Chemistry and Applications

For PAGs and PAOs, the corrosion protection is much easier as the hydrolytic stability is not an

issue here. For the nonpolar PAO, additives lowering the surface tension can be used synergistically

with  lm-forming additives.

26.4.4 EXTREME-PRESSURE AND ANTIWEAR ADDITIVES

Extreme-pressure and antiwear (EP/AW) additives are needed in lubricants to protect metal sur-

faces against mechanical damage in case the lubricant  lm is ruptured under critical conditions.

These additives not only absorb onto the metal surface, but also react further in the case of a ther-

mal fragmentation or tribofragmentation reaction, building a chemical reaction layer on the metal.

The formed layers make the tribological contact softer, preventing a direct metal-to-metal contact

of moving metal parts. AW additives are used for applications with medium loads. One model to

explain the mode of action of these additives is to assume that polymer condensation and polymer-

ization reactions are initiated, which provide a protecting layer on the metal. This layer is sheared

off and renewed constantly during the operation [65]. Under very high loads, the performance

of AW additives becomes insuf cient and designated EP additives are needed. These additives

undergo real reactions with the metal surface to form tribolayers consisting of iron phosphite or iron

sul des or iron chlorides. Here, the metal is a part of the protective tribolayer.

As EP/AW additives take part in real chemical reactions, they require certain activation energy

to display the intended performance. The activation energy is provided partly due to the operation

temperature, but more largely due to the local temperature spikes and mechanical forces in the

friction zone. It is hence important to know these conditions for the selection of the correct EP/AW

additives. For low-temperature applications, more active additives might be necessary, whereas for

higher temperatures, it has to be ensured that the additives at least partially stay in grade in the bulk

 uid.

ZDTPs are the most common EP/AW additives, but they are not suitable for environmentally

friendly  uids due to the content of a heavy metal: Zink. For biolubricants, ashless additives such

as triarylphosphates or trialkylphosphates, neutralized alkyl phosphoric acids, and ashless dialkyl-

dithiophosphates are used. All these additives are very reactive in nonpolar PAOs, as they can easily

be absorbed on the metal surface. In polar esters and PAGs, however, the situation is more complex,

as these base  uids interact strongly with metal surfaces and consequently provide an already good

protection against friction and wear, at least under mild conditions. For more demanding applica-

tions, in which the protective reaction layer provided from additives is required, the high surface

activity of the base  uids is detrimental, as the additives are hindered to approach the metal surface

in the  rst stage. Hence, higher treat rates of additives need to be applied, and preferentially more

polar additives have to be used. A further problem is the fact that even ashless sulfur- and phospho-

rus-containing compounds do not necessarily have excellent ecotoxicological pro les. Of course,

this statement cannot be generalized; however, so far, only a very limited number of additives are

available that would be of cially approved for the use in stringent environmentally friendly lubri-

cants such as European Eco-label  uids.

If the application is very demanding and high loads need to be managed, the sometimes still-

used chlorinated products should de nitely be avoided for toxicological reasons. Sulfur carriers,

on the contrary, could be used for biolubricants. For example, sulfurized esters, which can have a

good toxicological pro le as well as a high degree of biodegradability, have been tested and found

to show good performance [64].

26.4.5 POUR POINT DEPRESSANTS AND VISCOSITY INDEX IMPROVERS

Pour point depressants (PPDs) and viscosity index improvers (VIIs) are added to improve the rheol-

ogy of a lubricant.

At reduced temperatures, paraf nic compounds in lubricants can form wax crystals that

agglomerate and crystallize, solidifying the lubricant. To prevent this crystallization, a pour point

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Eco Requirements for Lubricant Additives 663

depressant is added. PPDs are specially designed polymers, which have a bifunctional molecular

structure. Some parts of the molecules are similar to paraf n wax crystals, enabling cocrystal-

lization with the waxes, whereas other parts of the polymer have a structure very dissimilar to

the wax crystals, preventing any further agglomeration and growth of the wax matrices. The

effect is a better  ow behavior and a depression of the pour point. As PAGs, PAO, and most syn-

thetic esters already have good low-temperature behavior, the addition of a PPD is not very com-

mon. Natural esters, however, can crystallize relatively easily—even in some cases at ambient

temperatures—and therefore the demand for a PPD can be high. The amount of PPD used varies

according to the ester type, concentration of the polymer in the  uid, the type of polymer, and the

degree of pour point correction desired.

VIIs are added to minimize the extreme viscosity variation of a given  uid with change in

temperature, especially important for applications working in different temperature regimes. VIIs

have a chemical structure similar to PPD, but the polymers used have a much higher molecular

weight. As base  uids for biolubricants have in general already a quite high viscosity index, a fur-

ther improvement of this property is not very common and only necessary for special applications.

26.4.6 ANTIHYDROLYSIS AGENTS

Antihydrolysis agent is a special class of additives, which is used only for ester-based  uids. As

described earlier, the ester bond can be easily cleaved reacting with water, forming acids and alco-

hol. As this reaction is catalyzed by acids, the reaction has to be seen as autocatalytic, because each

molecule of acid formed during the hydrolysis can itself catalyze a new hydrolysis reaction. To

stabilize an ester, special carbodiimides can be added, which work as acid scavengers and hence

are able to remove the catalyst [66]. The disadvantage of such acid scavengers is that they also read-

ily react with acids intentionally added as a corrosion inhibitor or an EP/AW additive; hence, the

potential for an antagonistic effect is high.

26.4.7 ANTIFOAM AGENTS, DEMULSIFIERS, AND EMULSIFIERS

To improve the surface properties of biolubricants, antifoam agents, demulsi ers, and emulsi ers can

be added. In typical industrial applications, the amount of additive is extremely low (10–100 ppm)

and hence will not be discussed in detail here.

26.5 SUMMARY

Currently, biolubricants are still regarded as niche products. However, in certain applications such

as hydraulic  uids, the use of biolubricants is steadily growing. Government incentives and volun-

tary labeling schemes are aiming to create market drivers, public promotion, and compensation for

potential economical disadvantages. Absolutely mandatory is, of course, suitable technical perfor-

mance of the biolubricants, as well as the lowest possible harm to the environment. To achieve this,

various base oils can be used. To ensure performance requirements are met, additives are needed,

which are tailored for the base  uid used and well suited for the intended application. These addi-

tives need to be carefully formulated, making use of synergistic effects and avoiding any antago-

nism. So far, it is not yet globally accepted, but with a growing regional importance, biolubricants

are required to be ecologically friendly, with criteria such as renewability of feedstocks, low aquatic

toxicity, and a high rate of biodegradability becoming demanded more often. These environmental

factors are not limited to the base oil, but are also valid in a certain degree for the additives; hence,

in future, the demand for environmentally acceptable, high-performing additives will be growing

steadily.

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