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14 Lubricant Additives: Chemistry and Applications

TABLE 1.2

Structure, Physical Properties, and Typical Applications of Commercial Hindered Phenols for Lubricants

Phenols Structure

Melting

Point (°C)

Solubility (in

mineral oils) Applications

2,6-Di-tert-butyl-phenol

36–37 >5

Industrial oils, power transmission

 uids, greases, fuels

2,6-Di-tert-butyl-4-methylphenol (BHT)

69 >5

Industrial oils, power transmission

 uids, food grade lubricants, greases

(3,5-Di-tert-butyl-4-

hydroxyhydrocinnamate)methane

OCH

110–125 0.1–0.5

Industrial oils, food grade lubricants,

greases

Octadecyl 3-(3′,5′-di-tert-butyl-4′-

hydroxyphenyl)propionate

50–55 >5 Industrial oils, eco-friendly oils

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Antioxidants 15

3,5-Di-tert-butyl-4-hydroxyhydrocinnamic

acid, C

–C

alkyl ester

O C

−C

Liquid at

25°C

Engine oils, power transmission  uids,

industrial oils

3,5-Di-tert-butyl-4-hydrocinnamic acid,

–C

alkyl ester

− C

Liquid at

25°C

Engine oils, power transmission  uids,

industrial oils

4,4′-Methylene bis(2,6-di-tert-butylphenol)

OHHO

154 NA

Engine oils, industrial oils, food grade

lubricants, greases

2,2′-Methylene

bis(4-methyl-6-tert-butylphenol)

128 2–5 Engine oils, industrial oils, greases

2-Propenoic acid, 3-[3,5-bis(1,1-

dimethylethyl)-4-hydroxyphenyl]-1,6-

hexanediyl ester

CH CH C

OHO

105 <1

Industrial oils, greases, food grade

lubricants

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16 Lubricant Additives: Chemistry and Applications

reacting alkyl or alkenyl succinic acid anhydride with an appropriate amine may impart such mul-

tifunctionalities. Product made by reacting a polyalkenylsuccinic acid or anhydride  rst with an

aromatic secondary amine, then with an alkanol amine, was found to provide appreciable antioxid-

ancy, dispersancy, and anticorrosion effects to engine oils as tested in a Caterpillar engine test [141].

A more recent U.S. Patent literature [142] discloses materials made from the reaction of alkyl or

alkenyl succinic acid derivative with a diamino naphthyl compound for use as antioxidant, antiwear,

and soot dispersing agents for lubricating oils. By fusing a HP moiety to an alkenyl succinimide

domain, a novel dispersant having antioxidant property was obtained [143]. The product improved

the performance of engine oils in the sequence VG, an industry recognized sludge test to evaluate

the ability of a lubricant in preventing the formation of sludge and varnish deposits in a  red engine.

U.S. Patent 5,075,383 [144] describes novel antioxidant–dispersant additives obtained by reacting

amino-aromatic polyamine compound, including aromatic secondary amines, with ethylene–pro-

pylene copolymer grafted with maleic anhydride. Engine oils containing the additives displayed

improved performance characteristics in laboratory oxidation and sludge dispersancy tests, as well

as in the sequence VE and the MWM-B engine tests.

1.7 COPPER ANTIOXIDANTS

The ability of copper compounds to function as oxidation inhibitors has been of interest to the

lubricant industry for years. Copper is usually considered to be an oxidation promoter, and its

presence is of a concern in lubricants such as power transmission oils, where  uid contact with

copper-containing bearings and sintered bronze clutch plates takes place [145]. It has been sug-

gested that copper corrosion products, originating from surface attack of copper metal, are generally

catalysts that accelerate the rate of oxidation [146], whereas oil-soluble copper salts are antioxidants

[147]. To maximize the full antioxidant strength of a copper compound, the initial concentration

needs to be maintained at an optimum range, normally from 100 to 200 ppm [145,147]. Below this

range, the antioxidant effect of the copper compounds will not be fully realized, whereas above the

range, interference with antiwear additives may occur, leading to pronounced increase in wear on

high-stress contact points [148].

Examples of oil-soluble copper antioxidants developed in early years were a group of copper–

sulfur complexes, obtained by sulfurizing certain types of unsaturated hydrocarbons in the presence

of copper [149–151]. A more recent patent describes lubricant compositions that are stabilized with

a zinc hydrocarbyl dithiophosphate (ZDDP) and 60–200 ppm of copper derived from oil-solu-

ble copper compounds such as copper dihydrocarbyldithiophosphate or copper dithiocarbamates

[148]. Oxidation data are given for fully formulated engine oils containing the ZDDP and various

supplemental antioxidants including amines, phenolics, a second ZDDP, and copper salts. Only the

blends with copper salts passed the oxidation test. With the other additives, the viscosity increase

was excessive. Organo-copper compounds including copper naphthenates, oleates, stearates, and

polyisobutylene succinic anhydrides have been reported to be synergistic with multiring aromatic

compounds in controlling high-temperature deposit formation in synthetic base stocks [147].

FIGURE 1.5 Examples of commercial sulfur-bridged phenolic antioxidants.

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Antioxidants 17

More complex compounds obtained from further reactions of copper salts have also been

reported to be effective antioxidants in various lubrication applications. For example, copper car-

boxylate or copper thiocyanate was reacted with a mono-oxazoline, bis-oxazoline, or lactone oxa-

zoline dispersant to form coordination complexes, wherein the nitrogen contained in the oxazoline

moiety is the ligand that complexes with copper. The resulting products exhibit

improved varnish

control and oxidation inhibition capabilties [152]. Reaction products of a copper salt (acetate, car-

bonate, or hydroxide) with a substituted succinic anhydride derivative containing at least one free

carboxylic acid group are effective high-temperature antioxidants and friction modi ers. When

incorporated in an engine oil formulation, the oil passed rust, oxidation, and bearing corrosion

engine tests [153]. In another patent [154], a HP carboxylic acid was used as the coupling reagent.

The resulting copper compounds are reported to be effective in the controls of high-temperature

sludge formation and oil viscosity increase when used alone or in synergistic mixtures with a con-

ventional aminic or phenolic antioxidant.

1.8 BORON ANTIOXIDANTS

The search for more eco-friendly additives to replace ZDDP has led to renewed interest in boron

esters owing to their ability to improve antioxidation, antiwear, and antifriction properties of lubri-

cants when used alone or in combination with other additives. The complex tribological behavior of

boron compounds in formulated lubricants depends on their particular chemical structures and the

interactions between boron and other active elements such as sulfur, phosphorus, nitrogen, or their

combinations when present [155,156].

A number of boron–oxygen-bearing compounds have been reported to be effective oxidation

inhibitors in terms of prevention of oil viscosity increase and acid formation at elevated tempera-

ture (163°C) [157–161]. Representatives are boron epoxides (especially 1,2-epoxyhexadecane) [157],

borated single and mixed alkanediols [158], mixed hydroquinone-hydroxyester borates [159], phe-

nol esters of hindered phenyl borates [160], and reaction products of boric acid with the condensates

of phenols with aromatic or aliphatic aldehydes [161].

Borate esters with nitrogen are known for their antioxidant activity and improved antiwear

properties probably due to the formation of additional boron nitride  lm on rubbing surface [162].

Borated adducts of alkyl diamines with long-chain hydrocarbylene alkoxides and low-molecular-

weight carboxylic acids have been reported to have antifriction properties and high inhibition abil-

ity especially at elevated temperatures [163]. Appreciable oxidation inhibition effect has also been

reported for borate esters of hydrocarbyl imidazolines [164], borates of mixed ethyoxyamines and

ethoxyamides [165], and borates of etherdiamines [166].

Synergistic antioxidant effect of borate esters with ADPAs or with zinc dithiophosphates has

been established. When tested at 180°C in a PAO using a pressurized differential scanning calo-

rimetry (PDSC), strong synergistic antioxidant action was observed between borate esters and a

dioctyl diphenylamine at a 1:1 (w/w) blending ratio [167]. Similar effect was observed in the mix-

tures of borate esters and a ZDDP [155]. The synergism with ZDDP is of practical importance as

it allows reduced phosphorus level in a  nished lubricant without sacri ce of oxidative stability.

The catalytic effect of boron in enhancing antioxidant performance has led to the development of

phenolic-phosphorodithioate borates, obtained from coborating HP and alkyl phosphorodithioate-

derived alcohol. The borates were found to possess exceptional antioxidant and antiwear proper-

ties. Both the HP moiety and the phosphorodithioate alcohol moiety were believed to provide the

basis for the synergy each of which are subsequently enhanced by the integral boron coupling

moiety [168].

Despite many tribological and antioxidation bene ts that borate esters can offer, large use of

the chemistry for lubricant applications has not taken place. One serious drawback with most borate

esters has been their high susceptibility to hydrolysis, a process that liberates oil-insoluble and

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18 Lubricant Additives: Chemistry and Applications

abrasive boric acid. Following attempts have been made to address the issue with varying degrees

of success:

1. Incorporation of HP moiety to sterically inhibit the boron–oxygen bonds from hydrolytic

attack. Commonly used HPs are 2,6-dialkyl phenols [169], 2,2′-thiobis(alkylphenols) and

thiobis(alkylnaphthols) [170].

2. Incorporation of amines that have nonbonding pairs of electrons. The amines coordinate

with the electron-de cient boron atom, thus preventing hydrolysis. U.S. Patents 4,975,211

[171] and 5,061,390 [172] disclose the stabilization of borated alkyl catechol against

hydrolysis by complexing with diethylamine. Signi cant improvement in hydrolytic sta-

bility was reported for borate esters incorporated with a N,N′-dialkylamino-ethyl moi-

ety [156]. It was hypothesized that the formation of a stable  ve-member ring structure

in molecules involving coordination of nitrogen with boron substantially inhibited the

hydrolytic attack from water.

3. Use of certain hydrocarbon diols or tertiary amine diols to react with boric acid to form

stable  ve-member ring structures [173].

1.9 MISCELLANEOUS ORGANOMETALLIC ANTIOXIDANTS

More recently, a number of oil-soluble organometallic compounds, for example, organic acid

salts, amine salts, oxygenates, phenates and sulfonates of titanium, zirconium, and manganese

have been claimed to be effective stabilizers for lubricants [174,175]. Some of the compounds are

essentially devoid of sulfur and phosphorus, therefore, suitable for modern automotive engine

oils where lower contents of the two elements are desired. In one example [174], lubricating oils

having 25 to ~100 ppm of titanium derived from titanium (IV) isopropoxide exhibited excellent

oxidative stability in the high-temperature (280°C) Komatsu hot tube test and ASTM D 6618

test evaluate engine oils for ring sticking, ring and cylinder wear, and the accumulation of piston

deposits in a four-stroke cycle diesel engine. In another example [175], titanium (IV) isopropoxide

was used to react with neodecanoic acid, glycerol mono-oleate, or polyisobutenyl bis-succinimide

to form respective titanated compounds. These compounds, when top-treated in a SAE 5W30

engine oil to result in 50 to ~800 ppm of titanium in oil, improved the deposit control capability

of the oil as tested by using the TEOST (ASTM D 7097). Similar antioxidant effect was observed

for neodecanoates of zirconium and manganese in the same oil.

Oil-soluble or dispersible tungsten compounds, more speci cally, amine tungstates and tungsten

dithiocarbamates, have been attempted as antioxidants for lubricants and found to be synergistic

with secondary diarylamine and alkylated phenothiazines. The mixtures, when added to an engine

crankcase lubricant to result in ~20 to 1000 ppm of tungsten, were highly effective in controlling oil

oxidation and deposit formation [176].

Sulfur-free molybdenum salts such as molybdenum carboxylates have been attempted as anti-

oxidants and found to be synergistic with ADPAs in lubricating oils [177,178]. The synergistic

mixtures improved oxidation stability of crankcase lubricants while providing additional friction

modi cation characteristics.

1.10 MECHANISMS OF HYDROCARBON OXIDATION

AND ANTIOXIDANT ACTION

It is now understood that oxidation of hydrocarbon-based lubricants undergoes autoxidation, a process

that leads to the formation of acids and oil thickening. To a more severe extent, oil-insoluble sludge

and varnish may be formed, causing poor lubrication, reduced fuel economy, and increased wear.

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Antioxidants 19

Antioxidants are essential additives incorporated in lubricant formulations to delay the onset of

autoxidation and minimize its impact. The mechanisms of lubricant degradation and its stabiliza-

tion by antioxidants are discussed in the following sections.

1.10.1 AUTOXIDATION OF LUBRICATING OIL

The well-documented autoxidation mechanism involves a free-radical chain reaction [179–181]. It

consists of four distinct reaction steps: chain initiation, propagation, branching, and termination.

1.10.1.1 Initiation

R H R HOO

⫺⫹→ ii

(1.1)

RR R R

Energy

⫺⫹→ ii

(1.2)

The initiation step is characterized as the formation of free alkyl radicals (R

•

) from the breakdown

of hydrocarbon bonds by hydrogen abstraction and dissociation of carbon–carbon bonds. These

reactions take place when hydrocarbons are exposed to oxygen and energy in the form of heat, UV

light, or mechanical shear stress [182]. The ease of homolytic cleavage of an R–H bond follows this

order, as determined by the C–H bond strength and the stability of the resulting radical [183]: phenyl

< primary < secondary < tertiary < allylic < benzylic. Thus, hydrocarbons containing tertiary

hydrogen or hydrogen in an alpha position to a carbon–carbon double bond or aromatic ring are

most susceptible to oxidation. The reaction rate of chain initiation is generally slow under ambient

conditions but can be greatly accelerated with temperature and the presence of catalytic transition-

ing metal ions (copper, iron, nickel, vanadium, manganese, cobalt, etc.).

1.10.1.2 Chain Propagation

RO ROO

ii⫹ →

(1.3)

ROO RH ROOH Rii⫹⫹→

(1.4)

The  rst propagation step involves an alkyl radical reacting irreversibly with oxygen to form an

alkyl peroxy radical (ROO

•

). This reaction is extremely fast, and the speci c rate is dependent on

the radical’s substituents [179]. Once formed, the peroxy radical can randomly abstract hydrogen

from another hydrocarbon molecule to form hydroperoxide (ROOH) and a new alkyl radical (R

•

Based on this mechanism, each time a free alkyl radial is formed, a large number of hydrocarbon

molecules may be oxidized to hydroperoxides.

1.10.1.3 Chain Branching

1.10.1.3.1 Radical Formation

ROOH → RO HO

ii⫹

(1.5)

RO RH ROH Rii⫹⫹→

(1.6)

HO RH H O R

ii⫹⫹→

(1.7)

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20 Lubricant Additives: Chemistry and Applications

1.10.1.3.2 Aldehyde or Ketone Formation

RR HCO RCHO R⬘⫹⬘

ii→

(1.8)

RR R CO RR CO R⬘⬙ ⬘ ⫹ ⬙→ i

(1.9)

The chain-branching steps begin with the cleavage of hydroperoxide into an alkoxy radical (RO

•

)

and a hydroxy radical (HO

•

). This reaction has high activation energy and is only signi cant at tem-

peratures >150°C. Catalytic metal ions accelerate the process. The resulting radicals will undergo a

number of possible reactions: (a) the alkoxyl radical abstracts hydrogen from a hydrocarbon to form

a molecule of alcohol and a new alkyl radical according to reaction 1.6, (b) the hydroxyl radical fol-

lows the pathway of reaction 1.7 to abstract hydrogen from a hydrocarbon molecule to form water

and a new alkyl radical, (c) a secondary alkoxyl radical (RR′HCO

•

) may decompose through reac-

tion pathway 1.8 to form an aldehyde, and (d) a tertiary alkoxy radical (RR′R″CO

•

) may decompose

to form a ketone (reaction 1.9).

The chain-branching reaction is a very important step to the subsequent oxidation state of

the oil as not only will a large number of alkyl radicals be formed that expedites the oxidation

process, but also the lower-molecular-weight aldehydes and ketones generated will immediately

affect the physical properties of the lubricant by decreasing oil viscosity and increasing oil vola-

tility and polarity. Under high-temperature oxidation conditions, the aldehydes and ketones can

undergo further reactions to form acids and high-molecular-weight species that thicken the oil

and contribute to the formation of sludge and varnish deposits. Detailed mechanisms will be

discussed in Section 1.10.3.

1.10.1.4 Chain Termination

RR RR

ii⫹⬘ ⫺⬘→

(1.10)

R R OO ROORii⫹⬘ ⬘→

(1.11)

As oxidation proceeds, oil viscosity will increase due to the formation of high-molecular-weight

hydrocarbons. When oil viscosity has reached a level that diffusion of oxygen in oil is signi cantly

limited, chain termination reactions will dominate. As indicated by reactions 1.10 and 1.11, two

alkyl radicals can combine to form a hydrocarbon molecule. Alternatively, an alkyl radical can

combine with an alkyl peroxy radical to form a peroxide. This peroxide, however, is not stable and

can easily breakdown to generate more alkyl peroxy radicals. During the chain-termination pro-

cesses, formation of carbonyl compounds and alcohols may also take place on the peroxy radicals

that contain an extractable α-hydrogen atom:

RR CHOO RR CHOOOOHCR R

RR C O RR CH OH

⬘⬘⬘

⬘⫽ ⫹ ⬘ ⫺

⫺

i ←→

→

(1.12)

1.10.2 METAL-CATALYZED LUBRICANT DEGRADATION

Metal ions are able to catalyze the initiation step as well as the hydroperoxide decomposition in

the chain-branching step [184] through a redox mechanism illustrated in the following section. The

required activation energy is lowered for this mechanism, and thus, the initiation and propagation

steps can commence at much lower temperatures.

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Antioxidants 21

1.10.2.1 Metal Catalysis

1.10.2.1.1 Initiation Step

MRHMHR

(n 1) n⫹⫹ ⫹ ⫹

⫹⫹⫹→ i

(1.13)

MOM O

(n 1)

⫹⫹⫹⫺

⫹⫹→

(1.14)

1.10.2.1.2 Propagation Step

M ROOH M H ROO

(n n⫹⫹⫹

⫹⫹⫹

→ i

(1.15)

M ROOH M HO RO

n(n⫹⫹⫹⫺

⫹⫹⫹→

(1.16)

1.10.3 HIGH-TEMPERATURE LUBRICANT DEGRADATION

The preceding discussion provides the basis for the autoxidation stage of lubricant degradation

under both low and high-temperature conditions. The end result of low-temperature oxidation is the

formation of peroxides, alcohols, aldehydes, ketones, and water [185,186]. Under high-temperature

oxidation conditions (>120°C), breakdown of peroxides including hydroperoxides becomes pre-

dominant, and the resulting carbonyl compounds (e.g., reactions 1.8 and 1.9) will  rst be oxidized

to carboxylic acids as shown in Figure 1.6. As an immediate result, the oil acidity will increase. As

oxidation proceeds, acid or base-catalyzed Aldol reactions take place. The reaction mechanism is

illustrated in Figure 1.7 [187]. Initially, α,β-unsaturated aldehydes or ketones are formed, and fur-

ther reaction of these species leads to high-molecular-weight products. These products contribute to

oil viscosity increase and eventually can combine with each other to form oil-insoluble polymeric

products that manifest as sludge in a bulk oil oxidation environment or as varnish deposits on hot

metal surface. Oil viscosity increase and deposit formation have been identi ed to be the principal

oil-related factors to engine damages [188].

1.10.4 EFFECT OF BASE STOCK COMPOSITION ON OXIDATIVE STABILITY

Mineral base stocks used to formulate lubricants are hydrocarbons that are originated from crude

oils and essentially contain mixtures of n-paraf ns along with isoparaf ns, cycloparaf ns (also

called naphthenes), and aromatics having about 15 or more carbon atoms [189]. In addition, small

amounts of sulfur-, nitrogen-, and oxygen-containing species may be present depending on the

re nery techniques employed. In the American Petroleum Institute (API) base oil classi cation

system, mineral oils largely fall into the groups I, II, III, and V, with some distinctions shown in

Table 1.3 in terms of saturates, sulfur contents, and viscosity index. Group I base oils still dominate

the base oil market, accounting for more than 50% of global capacity. Groups II and III base stocks

OOH

C OH +

−ROOH

ROO

−R

2 R

•

2 R

FIGURE 1.6 High-temperature (>120°C) lubricant degradation leading to the formation of carboxylic acids.

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22 Lubricant Additives: Chemistry and Applications

are on the horizon, and their use is expected to grow in large scale in the coming future, especially

after the completion of nearly a dozen new group II/III oil re nery plants worldwide [190].

It has been widely recognized that base oil composition, for example, linear and branched

hydrocarbons, saturates, unsaturates, monoaromatics, polyaromatics, together with traces of

nitrogen-, sulfur-, and oxygen-containing heterocycles, etc., plays an important role in the oxida-

tive stability of the oil. There have been quite extensive research activities attempting to establish

correlations between base stock composition and oxidative stability [191–195]. However, owing

to the large variations in the origin of the oil samples, the test methods, test conditions, and the

performance criteria employed, the conclusions are not always consistent and in some cases

contradictory to each other. In general, it has been agreed that saturated hydrocarbons are more

stable than the unsaturated toward oxidation. Of the different saturated hydrocarbons found in

mineral oils, paraf ns are more stable than cycloparaf ns. Aromatic compounds, due to their

complex and large variation in the chemical makeup, play a more profound role. Monocyclic

aromatics are relatively stable and resistant to oxidation, whereas bi and polycyclic aromatics

are unstable and susceptible to oxidation [196]. Alkylated aromatics oxidize more readily due to

FIGURE 1.7 High-temperature (>120°C) lubricant degradation leading to the formation of high-molecular-

weight hydrocarbons.

(CH

)

C OR′

(CH

)

CHCOR′ + H

(CH

)

COR"'

(CH

)

R CHCOR'

C(CH

)

n−1

CHCOR′

(CH

)

Acid

or base

COR"'

−H

Sludge

precursors

TABLE 1.3

API Base Oil Categories

API Category Percent Saturates Percent Sulfur Viscosity Index

Group I ≤90 ≥0.03 ≥80 and ≤120

Group II ≥90 ≤0.03 ≥80 and ≤120

Group III ≥90 ≤0.03 ≥120

Group IV PAOs

Group V Includes all other base oils not included in the  rst four groups

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Antioxidants 23

the presence of highly reactive benzylic hydrogen atoms. Kramer et al. [193] demonstrated that

the oxidative rate of a hydrocracked 500N base oil doubled when the aromatic content increased

from 1 to 8.5 wt%. Naturally occurring sulfur compounds are known antioxidants for the inhi-

bition of the early stage of oil oxidation. Laboratory experiments have shown that mineral oils

containing as little as 0.03% of sulfur had good resistance to oxidation at 165°C over sulfur-free

white oils and PAOs [145]. In hydrocracked oils that are essentially low in aromatics, better oxi-

dative stability was found with elevated sulfur concentration (>80 ppm) versus a level at 20 ppm

or lower [192]. It has been proposed that sulfur compounds act as antioxidants by generating

strong acids that catalyze the decomposition of peroxides through a nonradical route or by pro-

moting the acid-catalyzed rearrangement of arylalkyl hydroperoxides to form phenols that are

antioxidants [145,179]. Contrary to sulfur, nitrogen-bearing compounds, especially the hetero-

cyclic components (also called “basic nitrogen”), accelerate oil oxidation even at relatively low

concentrations [197]. In highly re ned groups II and III base stocks that are essentially devoid

of heteroatom-containing molecules, aromatic and sulfur contents are considered as the main

factors which in uence the base oil oxidative stability [192,193]. It has been shown that oxida-

tive stability of a given base stock can be enhanced when the combinations and concentrations

of base stock sulfur and aromatics are optimized [194].

1.10.5 OXIDATION INHIBITION

The proceeding mechanistic discussion makes clear several possible counter measures to control

lubricant oxidation. Blocking the energy source is one path. However, this is only effective for

lubricants used in low shear and temperature situations. A more practical approach for most lubri-

cant applications is the trapping of catalytic impurities and the destruction of alkyl radicals, alkyl

peroxy radicals, and hydroperoxides. This can be achieved through the use of a metal deactiva-

tor and an appropriate antioxidant with radical scavenging or peroxide decomposing functionality,

respectively.

The radical scavengers are known as primary antioxidants. They function by donating hydrogen

atoms to terminate alkoxy and alkyl peroxy radicals, thus interrupting the radical chain mechanism

of the auto-oxidation process. The basis for a compound to become a successful antioxidant is that

peroxy and alkoxyl radicals abstract hydrogen from the compound much more readily than they do

from hydrocarbons [198]. After hydrogen abstraction, the antioxidant becomes a stable radical, the

alkyl radical becomes a hydrocarbon, and the alkyl peroxy radical becomes an alkyl hydroperoxide.

HPs and aromatic amines are two main classes of primary antioxidants for lubricants.

The peroxide decomposers are also called secondary antioxidants [180]. They function by

reducing alkyl hydroperoxides in the radical chain to nonradical, less-reactive alcohols. Organo-

sulfur and organophosphorus compounds and those containing both elements, such as ZDDPs, are

well-known secondary antioxidants.

Since transitional metals are present in most lubrication system, metal deactivators are usually

added to lubricants to suppress the catalytic activities of the metals. Based on the functioning mech-

anisms, metal deactivators for petroleum products can be classi ed into two major types: chelators

[180] and surface passivators [199]. The surface passivators act by attaching to metal surface to form

a protective layer, thereby preventing metal–hydrocarbon interaction. They can also minimize cor-

rosive attack of metal surface by physically restricting access of the corrosive species to the metal

surface. The chelators, however, function in bulk of the lubricant by trapping metal ions to form an

inactive or much less-active complex. With either mechanism, metal deactivators can effectively

slow the oxidation process catalyzed by those transitional metals, which in turn lends metal deacti-

vators an antioxidant effect. Table 1.4 lists examples of metal deactivators that are commonly found

in lubricant formulations.

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