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24 Lubricant Additives: Chemistry and Applications

TABLE 1.4

Metal Deactivators for Lubricants

Surface Passivators Basic Structure

Triazole derivative

Benzotriazole

2-Mercaptobenzothiazole

Tolyltriazole derivative

Chelators

N,N′-disalicylidene-1,2-diaminopropane

1.10.6 MECHANISMS OF PRIMARY ANTIOXIDANTS

1.10.6.1 Hindered Phenolics

A representative example of HP antioxidant is 3,5-di-t-butyl-4-hydroxytoluene (2,6-di-t-butyl-4-

methylphenol), also known as BHT. Figure 1.8 compares the reaction of an alkyl radical with BHT

versus oxygen. The reaction rate constant (k

) of alkyl radical with oxygen to form alkyl peroxy

radicals is much greater than that (k

) of alkyl radical with BHT [179]. Hence with an ample supply

of oxygen, the probability of BHT to react with alkyl radicals is low. As oxidation proceeds with

more alkyl radicals being converted to alkyl peroxy radicals, BHT starts to react by donating a

hydrogen atom to the peroxy radical as shown in Figure 1.9. In this reaction, the peroxy radical is

reduced to hydroperoxide, whereas the BHT is converted into a phenoxy radical that is stabilized

through steric hindrance and resonance structures. The steric hindrance provided by the two butyl

moieties on the ortho positions effectively prevent the phenoxy radical from attacking other hydro-

carbons. The cyclohexadienone radical resonance structure can further combine with a second alkyl

peroxy radical to form the alkyl peroxide, which is stable at temperatures <120°C [200]. Without

resonance transformation, an alternative reaction pathway for phenoxy radicals is to combine with

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Antioxidants 25

each other into a chain termination step. As depicted in Figure 1.10, a hydrogen atom is donated

from one phenoxy radical to the other, thus regenerating one BHT molecule and one methylene

cyclohexadienone.

Under higher-temperature oxidation conditions, the cyclohexadienone alkyl peroxide previ-

ously formed is no longer stable. As illustrated in Figure 1.11, it will decompose to form an alkoxy

radical, an alkyl radical, and a 2,6-di-t-butyl-1,4-benzoquinone. As can be expected, the generation

of new radicals will deteriorate the overall effectiveness of the BHT under high-temperature oxida-

tion conditions.

R• +

>> k

R• +

O•

ROO•

+ RH

FIGURE 1.8 Reactivity of BHT with alkyl radical.

O•

+ ROOH

+ ROO•

C OOR

•

FIGURE 1.9 Hydrogen donation and peroxy radical trapping mechanisms of BHT.

O•

FIGURE 1.10 Termination reaction of phenoxy radicals.

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26 Lubricant Additives: Chemistry and Applications

1.10.6.2 Aromatic Amines

A particularly effective class of aromatic amines useful as primary antioxidants is the ADPAs.

The reaction of the antioxidants begins with hydrogen atom abstraction by alkyl peroxy radi-

cal and alkoxy radical, the mechanism of which is illustrated in Figure 1.12. Owing to the rapid

reaction of alkoxy radicals with oxygen, the resulting alkyl peroxy radical is present at higher

concentration, and its reaction with ADPA predominates. The aminyl radical formed can undergo

a number of possible reaction pathways depending on temperature, degree of oxidation (relative

concentration of peroxy radicals versus alkyl radicals), and the chemical nature of the ADPA

[201]. Figure 1.13 shows the low-temperature (<120°C) oxidation inhibition mechanism [179,202],

starting with aminyl radical attacking a second alkyl peroxy radical to form a nitroxyl radical

and alkoxy radical. The nitroxyl radical is stabilized through three possible resonance structures

[182,202], as illustrated in Figure 1.14. Next, a third alkyl peroxy radical reacts with the nitroxyl

radical to form a nitroxyl–peroxide complex, which can further eliminate an ether molecule, form-

ing a nitroxyl cyclohexadienone. Following reactions involve a fourth alkyl peroxy radical being

added to the nitroxyl cyclohexadienone to form nitroxyl cyclohexadienone peroxide, followed by

a dissociation reaction to form 1,4-benzoquinone and an alkylated nitrosobenzene. Therefore, on

OOR

Heat

+ RO• + •CH

FIGURE 1.11 Decomposition of cyclohexadienone alkyl peroxide at higher temperatures.

FIGURE 1.12 ADPA as a hydrogen donor.

RO , ROO

R + ROH, ROOH

•

FIGURE 1.13 Low-temperature function mechanism of ADPA.

ROO

O•

R + RO•

ROO•

−

OOR

−ROR

−

O•

ROO•

−RO•

O+

ROO•

•

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Antioxidants 27

an equal mole basis, in theory, ADPAs can quench twice as many alkyl peroxy radicals as a mono

HP under low-temperature conditions. At high temperatures (>120°C), the nitroxyl radical inter-

mediate can undergo one of two possible reaction pathways by either reacting with a secondary

or a tertiary alkyl radical to form an N-sec-alkoxy diphenylamine [179,186,203] or an N-hydroxyl

diphenylamine intermediate, respectively. These mechanisms are illustrated in Figure 1.15. In the

former case, the resulting alkoxy amine intermediate can thermally rearrange to form a ketone and

regenerate the starting ADPA. In the latter case, nitroxyl radical is regenerated upon reaction of

the hydroxyl diphenylamine intermediate with an alkyl peroxy radical. Thus, at high temperatures,

one molecule of ADPA can catalytically scavenge a large number of radicals before the nitroxyl

FIGURE 1.14 Resonance structures of nitroxyl radical.

−

O•

•

FIGURE 1.15 High-temperature (>120°C) function mechanism of ADPAs.

ROOH

ROO•

O•

CHRR′

ROO•

Radicals

C R′

RR′IIC•

•

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28 Lubricant Additives: Chemistry and Applications

radical is destroyed. It has been reported that such regeneration process can provide ADPAs with

a stoichiometric ef ciency of more than 12 radicals per molecule [186].

1.10.7 MECHANISMS OF SECONDARY ANTIOXIDANTS

1.10.7.1 Organosulfur Compounds

Organosulfur compounds function as hydroperoxide decomposers through the formation of oxi-

dation and decomposition products. The mechanism is illustrated in Figure 1.16 for an alkyl sul-

 de. The antioxidant action starts with the reduction of an alkyl hydroperoxides to a less reactive

alcohol, with the sul de being oxidized to a sulfoxide intermediate. A preferred mechanism for

the subsequent reaction of sulfoxide intermediate is the intramolecular beta-hydrogen elimination,

leading to the formation of a sulfenic acid (RSOH), which can further react with hydroperoxides to

form sulfur-oxy acids. At elevated temperatures, sul ninc acid (RSO

H) may decompose to form

sulfur dioxide (SO

), which is a particularly powerful Lewis acid for hydroperoxide decomposition

through the formations of active sulfur trioxide and sulfuric acid. Previous work has shown that

one equivalent of SO

was able to catalytically decompose up to 20,000 equivalents of cumene

hydroperoxide [204]. Further enhancing the antioxidancy of sulfur compounds is that, under certain

conditions, the intermediate sulfur-oxy acids (RSO

H) can scavenge alkyl peroxy radicals, thus giv-

ing the sulfur compound primary antioxidant characteristics:

RSO H RSO ROOH

ROO

•

→ ⫹

1.10.7.2 Organophosphorus Compounds

Phosphites are a main class of organophosphorus compounds being used as secondary anti-

oxidants. Phosphites decompose hydroperoxides and peroxy radicals following the reaction

ROOH

−ROH

RSOH + H

C=CR

ROOH

−ROH

RSO

ROOH −ROH

RSO

RH + SO

ROOH

−ROH

ROSO

ROOH

ROO

C=O

Decompose more

hydroperoxides

FIGURE 1.16 Antioxidation mechanism of alkyl sul de.

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Antioxidants 29

mechanisms. In these reactions, phosphite is oxidized to corresponding phosphate, with the

hydroperoxide and the peroxy radical being reduced to less-reactive alcohol and alkoxy radical,

respectively.

(RO) P R OOH O R OH

⫹⬘ ⫽ ⫹⬘→ (RO) P

(1.17)

(RO) P

⫹⬘ ⫽⫹⬘ROO (RO)P O RO

••

→

(1.18)

Phosphites with certain substituted phenoxy groups may also behave as peroxy and alkoxy radical

scavengers as shown in Figure 1.17. The resulting phenoxy radicals can further eliminate peroxy

radicals upon resonance transformation to cyclohyxadienone radical as discussed earlier. Owing to

the steric hindrance provided by the alkyl groups on the ortho-positions, these phosphites tend to be

more stable against hydrolysis and are preferred for use in moist lubrication environment.

1.10.8 ANTIOXIDANT SYNERGISM

Antioxidant synergism describes the effect or response of a combined use of two or more antioxi-

dants being greater than that of any individual antioxidant. Synergistic antioxidant systems offer

practical solutions to problems where using a single antioxidant is inadequate to provide satisfactory

results, or where the treatment level has to be limited due to economic or environmental reasons.

Three types of synergy have been proposed for lubricant antioxidants [205]: (a) homosynergism,

(b) heterosynergism, and (c) autosynergism.

Homosynergism occurs when two antioxidants acting by the same mechanism interact,

generally in a single-electron-transfer cascade. A classic example is an ADPA in combination with

a HP antioxidant. ADPA is initially more reactive than HP in scavenging alkyl peroxy radicals. As

illustrated in Figure 1.18, the amine is  rst converted to an aminyl radical, which is relatively less

stable and will accept a hydrogen atom from the HP to regenerate the alkylated amine [179,182].

In consequence, the HP is converted to a phenoxy radical. The driving force for this regeneration

cycle to occur is the higher reactivity of the ADPA compared to the HP and the greater stability of

the phenoxy radical relative to the aminyl radical [201]. After the HP is consumed, the aromatic

amine antioxidant begins to deplete. By regenerating the more reactive amine, the overall effective-

ness of the system is enhanced, and the useful antioxidant lifetime is extended.

* With permission from Dong, J. and C.A. Migdal, Synergestic Antioxidant Systems for Lubricants. 12

Asia Fuels and

Lubes Conference Proceedings, Hong Kong, 2006.

OR P(OR′)

R′′O•

R′′O P(OR′)

R′′OO•

O•

FIGURE 1.17 Alkoxy and peroxy scavenging mechanisms of phenyl phosphite.

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30 Lubricant Additives: Chemistry and Applications

Heterosynergism occurs when antioxidants act by a different mechanism and hence comple-

ment each other. This type of synergy usually takes place when a primary antioxidant and a sec-

ondary antioxidant are present in one lubricant system. The primary antioxidant scavenges radicals,

whereas the secondary antioxidant decomposes hydroperoxides by reducing them to more stable

alcohols. Through these reactions, chain propagation and branching reactions are signi cantly

slowed or inhibited. A representative example of a heterosynergism is an aminic antioxidant in

combination with a ZDDP.

Autosynergism is a third type of synergistic response that results from two different antioxidant

functions in the same molecule. Usually, antioxidants having functional groups that provide radical

scavenging and hydroperoxide decomposing functions exhibit autosynergy. Examples of this type

of antioxidants are sulfurized phenols and phenothiazines.

1.11 OXIDATION BENCH TESTS

Oxidative degradation of lubricants can be classi ed into two main reactions: bulk oil oxidation

and thin- lm oxidation. Bulk oil oxidation usually takes place in a larger oil body at a slower rate.

The exposure to air (oxygen) is regulated by the surface contact kinetics, and the gas diffusion is

limited. The oxidation leads to increases in oil acidity, oil thickening, and, to a more severe extent,

oil-insoluble polymers that may manifest as sludge when mixed with unburned/oxidized fuel com-

ponents, water, and other solids. Thin- lm oxidation describes a more rapid reaction in which a

small amount of oil, usually in the form of a thin- lm coating on metal surface, is exposed to ele-

vated temperatures and air (oxygen). Under these conditions, hydrocarbons decompose much more

quickly and the polar oil oxidation products formed at the oil–metal interface can rapidly build up

on the metal surface, leading to the formation of lacquer or deposits.

Over the years, many oxidation bench tests have been developed and proven to be valuable tools

for lubricant formulators, particularly in the screening of new antioxidants and the development of

new formulations. Most bench tests attempt to simulate the operating conditions of more expensive

engine and  eld tests, when taking into consideration the oxidation mechanisms described earlier.

In addition, a third mechanism based on oxygen uptake in a closed system has been employed in

some bench tests, such as the RPVOT [206].

RO• , ROO• ROH, ROOH

O•

•

FIGURE 1.18 Mechanism of synergism between ADPA and hindered phenol.

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Antioxidants 31

Owing to the limitation of laboratory setup, a single bench test cannot address all oxidation

aspects of a real world scenario. The large variation in test conditions, particularly test tempera-

ture, use of catalyst, performance parameter, oxidation mechanism, and targeted oxidation stage,

etc., makes it rather dif cult or even impractical to correlate one test with another. It is therefore

a common practice to run multiple tests at a time when characterizing a lubricant formulation

and its additives. This section selectively reviews oxidation bench tests more closely related to the

characterization of antioxidants. These tests have been standardized by some of the international

standardization organizations such as ASTM and the Co-ordinating European Council (CEC),

etc., and are more widely used in the industry. It is important to note that there are a number of

custom- tailored test methods designed for speci c needs that have been proven to be advantageous

in certain circumstances. The value of these tests should not be underestimated.

1.11.1 THIN-FILM OXIDATION TEST

1.11.1.1 Pressurized Differential Scanning Calorimetry

Differential scanning calorimetry (DSC), including PDSC, is an emerging thermal technique

for rapid and accurate determination of thermal-oxidative stability of base oils and performance

of antioxidants. PDSC has been a more sought after technique for two main reasons. First,

high pressure elevates boiling points, thus effectively reducing experimental errors caused by

volatilization losses of additives and light fractions of base oil; second, it increases the saturation

of the reacting gases in sample, allowing the use of lower test temperature or shorter test time at

the same temperature [207].

PDSC experiments can be run in an isothermal mode to measure oxidation induction time

(OIT) corresponding to the onset of oil oxidation or in a programmed temperature mode to measure

the onset temperature of oxidation. The temperature technique has been utilized to study deposit-

forming tendency of  ve engine oils, and the results obtained were consistent with their engine test

ranking [208]. The OIT technique, however, is more commonly used for its simplicity and speed.

Its early use can be traced back to the 1980s when Hsu et al. [209] tested a number of engine oils

and found the induction periods of the samples to be indicative of the sequence IIID viscosity break

points. Soluble metals consisting of lead, iron, copper, manganese, and tin together with a synthetic

oxidized fuel were included as catalysts to promote oil oxidation.

The CEC L-85 and the ASTM D 6186 [210,211] are two standard methods that are based on OIT

technique. Key test conditions of the methods are listed in Table 1.5. The CEC L-85 test method

was originally developed for European Association des Constructeures Europeens de l’Automobile

(ACEA) E5 speci cation for heavy-duty diesel oils and has been incorporated in the current E7

speci cation. The test is capable of differentiating between different quality base oils, additives,

indicating antioxidant synergies and correlating with some bulk oil oxidation tests [212]. With

appropriate modi cations to the standard methods, PDSC has been successfully utilized in the

characterization of various lubricants in addition to automotive engine oils. These include, but not

limited to, base oils [213,214], greases [215], turbine oils [214], gear oils [216], synthetic ester lubri-

cants [217], and biodegradable oils [218,219]. Using PDSC to study the kinetics of base oil oxidation

[220] and antioxidant structure–performance relationship [221] has also been reported.

1.11.1.2 Thermal-Oxidation Engine Oil Simulation Test (ASTM D 6335; D 7097)

The TEOST was originally developed to assess the high-temperature deposit-forming characteris-

tic of API SF quality engine oils under turbocharger operating conditions [222]. The original test

conditions were speci ed as the 33C protocol and subsequently standardized in the ASTM D 6335

method [223]. In this test, oil containing ferric naphthenate is in contact with nitrous oxide and

moist air and is cyclically pumped to  ow past a tared depositor rod. The rod is resistively heated

through 12 temperature cycles, each going from 200 to 480°C for 9.5 min. After the heating cycle

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32 Lubricant Additives: Chemistry and Applications

TABLE 1.5

Conditions of Oxidation Test Methods

Test

Designation

Oxidation

Regime

Temperature

(°C) Gas

Gas Flow

or Initial

Pressure Catalyst Sample Size EOT

Parameter

Measured

PDSC D 6186 Thin  lm 130, 155,

180, 210

500 psi,

100 mL/m

None 3.0 mg Occurrence of

oxidation

exotherm

OIT

PDSC CEC L-85 Thin  lm 210 Air 100 psi,

no  ow

None 3.0 mg 120 min

maximum

OIT

TEOST 33C D 6335 Bulk 100, 200–480 N

moist air

3.6 mL/min Fe naphthenate 116 mL 12 Programmed

cycles

Deposits

TEOST MHT D 7097 Thin  lm 285 Dry air 10mL/m Oil-soluble Fe,

Pb, Sn

8.4 g 24 h Deposits,

volatile

TFOUT D 4742 O

uptake, thin

 lm

160 O

90 psig Fuel, naphthenates

of Fe, Pb, Cu,

Mg, and Sn, H

1.5 g Sharp pressure

drop

OIT

TOST D 943,

D 4310

Bulk 95 O

3.0 L/h Fe, Cu, H

O 300 mL ∆TAN = 2.0

1000 h

TAN, sludge,

metal weight

loss

IP48 IP48 Bulk 200 Air 15 L/h None 40 mL 6 h × 2 Viscosity,

carbon

residue

IP 280/CIGRE IP 280 Bulk 120 O

1 L/h Cu, Fe

naphthenates

25 g 164 h Volatile acids,

oil acidity,

sludge

RPVOT D 2272 O

uptake 160 O

90 psig Cu, H

O 50 mL ∆P = 25 psi OIT

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Antioxidants 33

is complete, deposit formed on the depositor rod is determined by differential weighting. The 33C

protocol was found capable of discriminating engine oils with known ability in resisting deposit

formation in critical areas of engines [222].

The successful use of high-temperature deposition test to characterize engine oils has led to the

development of a TEOST mid-high temperature (MHT) protocol, a simpli ed procedure for the

assessment of oil deposition tendency in the piston ring belt and under-crown areas of  red engines

[224]. Thin- lm oxidation condition was thought to be predominant in these areas, and accordingly,

the depositor assembly was revised to allow the oil  ows down the rod in a slow and even manner

to obtain a desired thin  lm. To better re ect the thermal-oxidative conditions of the engine zone of

interest, a continuous depositor temperature of 285°C together with modi ed catalyst package and

dry air is employed. The test runs for 24 h, and afterward, the amount of deposits formed on the

tared depositor is gravimetrically determined [225]. Since introduction, the TEOST MHT has been

incorporated in the International Lubricant Standardization and Approval Committee (ILSAC)

gas fuel (GF)-3 and GF-4 engine oil speci cations with an upper limit of 45 and 35 mg, respectively.

Aside from being a thermal-oxidation test, TEOST can also be used to characterize neutral and

overbased detergents of automotive engine oils [226].

1.11.1.3 Thin-Film Oxidation Uptake Test (ASTM D 4742)

The TFOUT method was originally developed under the U.S. Congress mandate to monitor batch-

to-batch variations in the oxidative stability of re-re ned lubricating base stocks [227]. The test

stresses a small amount of oil to 160°C in a high-pressure reactor pressurized with oxygen along

with a metal catalyst package, a fuel catalyst, and water to partially simulate the high- temperature

oxidation conditions in automotive combustion engines [228]. Better oxidative stability of oil cor-

responds to a longer time it takes to observe a sharp drop in oxygen pressure. TFOUT can be

carried out in a RPVOT apparatus upon proper modi cation to the sampling accessories. Based on

the results obtained from testing a limited number of reference engine oils, qualitative correlation

between TFOUT and the sequence IIID engine dynamometer test has been established [229]. Since

being adopted as an ASTM standard method, there has been a wider utilization of the TFOUT to

screen lubricants, base stocks, and additive components before sequence III engine testing [227].

1.11.2 BULK OIL OXIDATION TEST

1.11.2.1 Turbine Oil Stability Test (ASTM D 943, D 4310)

The turbine oil stability test (TOST) has been widely used in the industry to assess the oxidative

stability of inhibited steam turbine oils under long-term service conditions. It can be used on other

types of industrial lubricants such as hydraulic  uids and circulating oils and in particular on those

that are prone to water contamination in service. The test runs at relatively low temperature (95°C)

to represent the thermal-oxidative conditions of real steam turbine applications. Two versions of the

TOST, namely, ASTM D 943 and D 4310 [230,231], have been developed. Both the methods share

some common test conditions including test apparatus, catalysts, sample size, temperature pro le,

and gas, with minor differences in the test duration and target oxidation parameters to be moni-

tored. The ASTM D 943 measures oxidation lifetime, which is the number of hours required for

the test oil to reach an acid number of 2.0 mgKOH/g or above. The ASTM D4310 determines the

sludging and corrosion tendencies of the test oil by gravimetrically measuring oil-insoluble prod-

ucts after 1000 h of thermal and oxidative stresses. The total amount of copper in oil, water, and

sludge phases is also determined.

A modi ed TOST method that operates at higher temperature (120°C) and in the absence of

water has been proposed [232]. The procedure requires RPVOT as a monitoring tool and is spe-

ci cally suitable for the determination of sludging tendencies of long-life steam and gas turbine

oils formulated with the more stable groups II and III base stocks and high-performance aminic

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