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4 Lubricant Additives: Chemistry and Applications

1.11.2 Bulk Oil Oxidation Test ............................................................................................. 33

1.11.2.1 Turbine Oil Stability Test (ASTM D 943, D 4310).....................................33

1.11.2.2 IP 48 Method ..............................................................................................34

1.11.2.3 IP 280/CIGRE ............................................................................................34

1.11.3 Oxygen Update Test ...................................................................................................34

1.11.3.1 Rotating Pressure Vessel Oxidation Test (ASTM D 2272) ........................34

1.12 Experimental Observations .....................................................................................................34

1.13 Antioxidant Performance with Base Stock Selection .............................................................37

1.14 Future Requirements ...............................................................................................................38

1.15 Commercial Antioxidants .......................................................................................................39

1.16 Commercial Metal Deactivators .............................................................................................41

References ........................................................................................................................................ 41

1.1 INTRODUCTION

Well before the mechanism of hydrocarbon oxidation was thoroughly investigated, researchers had

come to understand that some oils provided greater resistance to oxidation than others. The differ-

ence was eventually identi ed as naturally occurring antioxidants, which varied depending on crude

source or re ning techniques. Some of these natural antioxidants were found to contain sulfur- or

nitrogen-bearing functional groups. Therefore, it is not surprising that, certain additives that are

used to impart special properties to the oil, such as sulfur-bearing chemicals, were found to provide

additional antioxidant stability. The discovery of sulfurized additives providing oxidation stability

was followed by the identi cation of similar properties with phenols, which led to the development

of sulfurized phenols. Next, certain amines and metal salts of phosphorus- or sulfur-containing

acids were identi ed as imparting oxidation stability. By now numerous antioxidants for lubricating

oils have been patented and described in the literature. Today, nearly all lubricants contain at least

one antioxidant for stabilization and other performance-enhancing purposes. Since oxidation has

been identi ed as the primary cause of oil degradation, it is the most important aspect for lubricants

that the oxidation stability be maximized.

Oxidation produces harmful species, which eventually compromises the designated functiona-

lities of a lubricant, shortens its service life, and to a more extreme extent, damages the machinery it

lubricates. The oxidation is initiated upon exposure of hydrocarbons to oxygen and heat and can be

greatly accelerated by transitional metals such as copper, iron, nickel, and so on. when present. The

internal combustion engine is an excellent chemical reactor for catalyzing the process of oxidation

with heat and engine metal parts acting as effective oxidation catalysts. Thus, in-service engine oils

are probably more susceptible to oxidation than most other lubricant applications. For the preven-

tion of lubricant oxidation, antioxidants are the key additive that protects the lubricant from oxida-

tive degradation, allowing the  uid to meet the demanding requirements for use in engines and

industrial applications.

Several effective antioxidant classes have been developed over the years and have seen use in

engine oils, automatic transmission  uids, gear oils, turbine oils, compressor oils, greases, hydraulic

 uids, and metal working  uids. The main classes include oil-soluble organic and organometallic

antioxidants of the following types:

1. Sulfur compounds

2. Sulfur–nitrogen compounds

3. Phosphorus compounds

4. Sulfur–phosphorus compounds

5. Aromatic amine compounds

6. Hindered phenolic (HP) compounds

7. Organo–copper compounds

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Antioxidants 5

8. Boron compounds

9. Other organometallic compounds

1.2 SULFUR COMPOUNDS

The initial concepts of using antioxidants to inhibit oil oxidation date back to the 1800s. One of the

earliest inventions described in the literature [1] is the heating of a mineral oil with elemental sulfur

to produce a nonoxidizing oil. However, the major drawback to this approach is the high corrosivity

of the sulfurized oil toward copper. Aliphatic sul de with a combined antioxidant and corrosion

inhibition characteristics was developed by sulfurizing sperm oil [2]. Additives with similar func-

tionalities could also be obtained from sulfurizing terpenes and polybutene [3–5]. Paraf n wax has

also been employed to prepare sulfur compounds [6–9]. Theoretical structures of several sulfur

compounds are illustrated in Figure 1.1. Actual compounds can be chemically complex in nature.

Aromatic sul des represent another class of sulfur additives used as oxidation and corrosion

inhibitors. Examples of simple sul des are dibenzyl sul de and dixylyl disul de. More complex

compounds of a similar type are the alkyl phenol sul des [10–15]. Alkyl phenols, such as mono- or

di-butyl, -amyl, or -octyl phenol, have been reacted with sulfur mono- or dichloride to form either

mono- or disul des. As shown in Figure 1.1, the aromatic sul des such as benzyl sul de have the

sulfur attached to carbon atoms in the alkyl side groups, whereas the alkyl phenol sul des have the

sulfur attached to carbon atoms in the aromatic rings. In general, the alkyl phenol sul de chemistry

appears to have superior antioxidant properties in many types of lubricants. Mono- and dialkyl-

diphenyl sul des obtained by reacting diphenyl sul de with C

–C

alpha-ole ns in the presence

of aluminum chloride have been demonstrated to be powerful antioxidants for high-temperature

lubricants especially those utilizing synthetic base stocks such as hydrogenated poly-alpha-ole ns,

diesters, and polyol esters [15]. The hydroxyl groups of the alkyl phenol sul des may also be treated

Sulfurized dipentene

Sulfurized ester

(CH

)

CH CH (CH

)

(CH

)

CH CH (CH

)

Sulfurized olefin

(CH

)

O (CH

)

(CH

)

CCH

(CH

)

CH CH (CH

)

CH CH (CH

)

Dibenzyl sulfide

(S)

Dialkylphenol sulfide

FIGURE 1.1 Examples of sulfur-bearing antioxidants.

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6 Lubricant Additives: Chemistry and Applications

with metals to form oil-soluble metal phenates. These metal phenates play the dual role of detergent

and antioxidant.

Multifunctional antioxidant and extreme pressure (EP) additives with heterocyclic structures

were prepared by sulfurizing norbornene, 5-vinylnorbornene dicyclopentadiene, or methyl cyclo-

pentadiene dimer [16]. Heterocyclic compounds such as n-alkyl 2-thiazoline disul de in combi-

nation with zinc dialkyldithiophosphate (ZDDP) exhibited excellent antioxidant performance in

laboratory engine tests [17]. Heterocyclic sulfur- and oxygen-containing compositions derived from

mercaptobenzthiazole and beta-thiodialkanol have been found to be excellent antioxidants in auto-

matic transmission  uids [18]. Novel antioxidant and antiwear additives based on dihydrobenzothio-

phenes have been prepared through condensation of low-cost arylthiols and carbonyl compounds in

a one-step high-yield process [19].

1.3 SULFUR–NITROGEN COMPOUNDS

The dithiocarbamates were  rst introduced in the early 1940s as fungicides and pesticides [20]. Their

potential use as antioxidants for lubricants was not realized until the mid-1960s [21], and since then,

there have been continuous interests in this type of chemistry for lubricant applications [22]. Today,

dithiocarbamates represent a main class of sulfur–nitrogen-bearing compounds being used as antioxi-

dants, antiwear, and anticorrosion additives for lubricants.

Depending on the type of adduct to the dithiocarbamate core, ashless and metal-containing

dithiocarbamate derivatives can be formed. Typical examples of ashless materials are methylene

bis(dialkyldithiocarbamate) and dithiocarbamate esters with general structures being illustrated in

Figure 1.2. Both are synergistic with alkylated diphenylamine (ADPA) and organomolybdenum

compounds in high-temperature deposit control [23]. In particular, methylene bis(dialkyldithiocar

bamate) in combination with primary antioxidants such as arylamines or HPs and triazole deriva-

tives is known to provide synergistic action in stabilizing mineral oils and synthetic lubricating oils

[24–26]. This material has been used to improve antioxidation characteristics of internal combus-

tion engine oils containing low levels (<0.1 wt%) of phosphorus [27]. In another effort to reduce

phosphorus content in aviation gas turbine lubricants, methylene-bridged bis(dialkyl) or bis(alkylar

yldithiocarbamate) was used as high-temperature antioxidant and antiwear agent to replace tricresyl

phosphates that are of a concern to produce neurotoxic ortho-cresol isomers in trimethylolpropane

triester base oil under high-temperature service conditions [28].

It has been known that metal dithiocarbamates such as zinc, copper, lead, antimony, bismuth,

and molybdenum dithiocarbamates (MoDTCs) possess desirable lubricating characteristics includ-

ing antiwear and antioxidant properties. The associated metal ions affect the antioxidancy of the

additives. Within the group, MoDTCs are of greater interest particularly for engine crankcase lubri-

cants. Certain molybdenum additives posses good oxidation resistance and acceptable corrosion

characteristics, when prepared by reacting water, an acidic molybdenum compound, a basic nitro-

gen complex, and a sulfur source [29,30]. Oil-soluble trinuclear MoDTCs prepared by reacting

FIGURE 1.2 Ashless dithiocarbamates for lubricants.

Bis(disubstituted dithiocarbamate) Dithiocarbamate ester

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Antioxidants 7

ammonium polythiomolybdate with appropriate tetralkylthiuram disul des were found to be supe-

rior to dinuclear molybdenum compounds in terms of providing lubricants antioxidant, antiwear,

and friction-reducing properties [31].

When combined with an appropriate aromatic amine, MoDTCs can exhibit synergistic antioxidant

effects in oxidation tests [32]. As a result, molybdenum dialkyldithiocarbamates (C

7–24

) and ADPAs

are claimed broadly for lubricating oils [33]. More restrictive are claims for molybdenum dialkyl-

dithiocarbamates (C

8–23

and C

3–18

) and ADPAs in lubricating oils that contain <3 wt% of aromatic

content and <50 ppm of sulfur and nitrogen [34]. Molybdenum dialkyldithiocarbamates and HP anti-

oxidants are jointly claimed for lubricating oils that contain 45 wt% or more one or two ring naph-

thenes and <50 ppm sulfur and nitrogen [35]. MoDTC was used to top-treat engine oils formulated

with group I base stocks (>300 ppm S) and an additive package designed for group II base stocks.

The oils passed the sequence IIIF oxidation test, in which the oils would otherwise fail without the

molybdenum top-treatment [36]. Further demonstrated is a combination of ADPAs, sulfurized ole n,

or HP and oil-soluble molybdenum compounds including MoDTC. The mixture is highly effective in

stabilizing lubricants, especially those formulated with highly saturated, low-sulfur base oils [37].

Thiadiazole derivatives, particularly the monomers and dimers, represent another class of

sulfur- and nitrogen-bearing multifunctional additives with antioxidant potency. For example, the

monomeric 2-alkylesterthio-5-mercapto-1,3,4-thiadiazole has been reported to increase oxidative

stability of engine oils under thin- lm oxidation conditions by using thin- lm oxygen uptake test

(TFOUT) [38]. Lithium 12-hydroxystearate grease containing 2,5-dithiobis(1,3,4-thiadiazole-

2-thiol), a dimer, exhibited superior oxidative stability in the American Society for Testing and

Materials (ASTM) D 942 pressure bomb oxidation test [39]. When used in conjunction with ADPA

and organomolybdenum compound, the thiadiazole derivative improved the thermal-oxidation

engine oil simulation test (TEOST) deposition (ASTM D 7097) characteristic of an engine oil from

the control oil containing sulfurized isobutylene instead [40]. In addition to providing antioxidant

bene t, the thiadiazole derivatives have been widely used as ashless antiwear and EP additives.

Some of them can also provide corrosion inhibition and metal deactivation properties to nonferrous

metals such as copper.

Phenothiazines are also well-known sulfur- and nitrogen-bearing antioxidants and have been

used to st abil ize av iation  uids. Recent advances have lead to N-substituted thio alkyl phenothiazines,

having improved antioxidant activities and oil solubility [41]

as well as N-aminopropylpheno-

thiazine that can be used for further derivatization of the N-amino group [42]. For example, alkyl

phenothiazines together with aromatic amines can be attached to ole n copolymers to result in a

multifunctional antioxidant, antiwear agent, and Viscosity index (VI) improver for lubricants [43].

Diamine sul des, including diamine polysul des, can also provide effective oxidation control

when used in conjunction with oil-soluble copper. In demonstration, dimorpholine disul de and

di(dimethyl morpholine) disul de were compared to primary alkyl ZDDP and found to be superior

in controlling oil viscosity increase of engine crankcase lubricants at elevated temperatures [44].

1.4 PHOSPHORUS COMPOUNDS

The good performance of phosphorus as an oxidation inhibitor in oils was identi ed early on in

lubrication science. The use of elemental phosphorus to reduce sludge formation in oils has been

described [45]. However, elemental phosphorus, like elemental sulfur, may have corrosive side effects

to many nonferrous metals and alloys, so it is rarely incorporated in oils in this form, rather oil- soluble

organic compounds of phosphorus are preferred. Naturally occurring phosphorus compounds such

as lecithin have been utilized as antioxidants and many patents have been issued on these materials

for single use or in combination with other additives [46–49]. Lecithin is a phosphatide that has been

produced commercially as a by-product from the processing of crude soybean oil.

The antioxidant property of synthetic neutral and acid phosphite esters has been known for

sometime. Alkyl and aryl phosphites such as tributyl phosphite and triphenyl phosphite are ef cient

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8 Lubricant Additives: Chemistry and Applications

antioxidants in some petroleum base oils, and many patents have been issued on such compositions

[50,51]. Table 1.1 summarizes the patenting activities of the past three decades on the stabilization

of various lubricants with organophosphites. For optimum antioxidant performance, phosphites are

customarily blended with aminic or HP antioxidants that can lead to synergistic effect. For better

hydrolytic stability, tri-substituted phosphites with sterically hindered structures such as tris-(2,4-

di-tert-butylphenyl) phosphite and those based on pentaerythritol as described in the U.S. Patent

5,124,057 [52] are preferred. The aluminum, calcium, or barium salts of alkyl phosphoric acids are

another type of phosphorus compound that displays antioxidant properties [53,54].

1.5 SULFUR–PHOSPHORUS COMPOUNDS

The identi cation of sulfur and phosphorus compounds as powerful antioxidants for protection

of hydrocarbons has led to the development of oil-soluble antioxidants, having both elements

in one molecule. Numerous patents have been issued on such compositions, and a considerable

number have been used commercially [60–67]. In fact, antioxidants containing both sulfur and

phosphorus are usually more effective and ef cient in a wider variety of base stocks than those

containing only phosphorus or sulfur. Many commercial oils have employed one kind or other of

these sulfur–phosphorus-type additives.

One widely used class of sulfur–phosphorus additive is the metal dialkyldithiophosphates,

which are typically prepared by the reaction of phosphorus pentasul de with alcohols to form

dithio-phosphoric acids, followed by neutralization of the acids with an appropriate metal com-

pound. Many types of alcohols such as the aliphatic, cyclic [62], and phenolic derivatives have

been used, and those of relatively high molecular weight (such as lauryl, octyl, cyclohexyl, methyl

cyclohexyl alcohols, and amyl [65] or butyl phenols) are preferred to give suf cient thermal stability

to the  nal products while rendering suf cient solubility in oils. For the second-step reaction, zinc,

TABLE 1.1

Applications of Organophosphites as Antioxidants for Lubricants

Applications Phosphites Supplementary Antioxidants References

Compressor oils Trinonylphenyl phosphite, tributyl

phosphite, tridecylphosphite,

triphenylphosphite,

trioctylphosphite,

dilaurylphosphite

Secondary aminic and hindered

phenolic

Automotive and industrial

lubricants

Triaryl phosphites, trialkyl

phosphites, alkyl aryl phosphites,

acid dialkyl phosphites

Secondary aminic and hindered

phenolic

Automotive and industrial

lubricants

Triphenyl phosphite, diisodecyl

pentaerythritol diphosphite,

tri-isodecyl phosphite, dilauryl

phosphite

Secondary aminic and hindered

phenolic

Hydraulic  uids, steam

turbine oils, compressor oils,

and heat-transfer oil

Steric hindered tributyl phosphite,

bis(butylphenyl pentaerythritol)

diphosphite

(3,5-Di-t-butyl)4-hydroxybenzyl

isocyanurate

Steam turbine oils, gas turbine

oils

Triphenyl phosphite, trialkyl-

substituted phenyl phosphite

Alkylated diphenylamine,

phenyl-naphthylamine

Hydraulic  uids, Automatic

transmission  uids

Trialkyl phosphites Secondary aminic and hindered

phenolic including bis-phenol

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Antioxidants 9

barium, molybdenum, or calcium oxides are usually chosen. For more than 60 years, zinc salts

of dialkylthiophosphoric acids (ZDDP) have been one of the most cost-effective antioxidants and

therefore have been included as a key component in many oxidation inhibitor packages for engine

oils and transmission  uids. In addition, ZDDPs show good antiwear properties, especially in the

valve train area owing to the formation of sul de and phosphate  lms through corrosive reactions

on metal surfaces. These  lms can also provide protection against corrosive attack from the organic

acids formed during the oxidation process. The salts of C

dialkyldithiophosphoric acid are

the most common, but a broad range of other alkyl and aryl derivatives have been developed to

meet special needs, for instance, protection at higher temperatures. The reaction scheme of making

ZDDP is shown in Figure 1.3.

A number of patents describe modi cations to the  rst step of the reactions shown in Figure 1.3; by

conducting preliminary condensation reaction of phosphorus pentasul de with unsaturated organic

compounds such as terpenes, polybutenes, wax ole ns, fatty acids, fatty esters, sperm oil, and so

on to form high-molecular-weight intermediate products [68–89]. During these reactions, hydrogen

sul de is liberated, and the intermediates are usually acidic. The mechanism of the P

reaction

with ole ns in these cases may be one of substitution (replacement of reactive hydrogen atoms) as

well as of addition. In preparing the  nal additives, these acidic intermediates were neutralized by

the treatment with alkaline earth oxides or hydroxides to form metal salts. The calcium, barium, or

potassium salts are the most preferred products. Some additives may also display detergency char-

acteristics. The concept of conducting preliminary condensation reactions provides a facile route

to the synthesis of a wide variety of products from the reaction of phosphorus pentasul de and an

unsaturated organic moiety. Several of these, particularly the terpene and polybutene reaction prod-

ucts, have been used extensively in commercial applications.

To reduce the staining effect of ZDDP on metal parts (especially copper), addition of alkyl or

aryl phosphites during the synthesis has been attempted [90]. For example, triphenyl phosphite is

added to the dialkyldithiophosphoric acid and heated at 110ºC for an hour before the addition of

zinc oxide. In another patent, a novel dithiophosphate with improved oxidation stability is described

[91]. An acid is reacted with a glycol, to give a monoester having a hydroxyl group, which is then

reacted with P

to give the dialkyl dithiophosphoric acid. Zinc oxide is subsequently added to give

the novel dithiophosphates. To improve solubility, the salts can be made of lower dialkyl dithiophos-

phates by utilizing both primary and secondary alcohols, including butyl alcohols in the process

[92]. Mixed metal salts of dialkyl dithiophosphoric acids and carboxylic acids are claimed to have

higher thermal stability [93].

Many descriptions have recently appeared of organomolybdenum phosphorodithioate com-

plexes that impart excellent oxidation stability to lubricants. In certain circumstances, oil-soluble

molybdenum compounds are preferred additives owing to their multifunctional characteristics

such as antiwear, EP, antioxidant, antipitting, and antifriction properties. For instance, several

molybdenum dialkylphosphorodithioate complexes with varying alkyl chain length of amyl,

octyl, 2-ethylhexyl, and isodecyl were reported to exhibit appreciable antioxidation, antiwear,

FIGURE 1.3 Synthesis of ZDDP.

2 ROH + P

SH + H

2 RO

SH + ZnO

2 RO

RO P

Zn + H

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10 Lubricant Additives: Chemistry and Applications

and antifriction properties [94]. Novel trinuclear molybdenum dialkyldithiophosphates prepared

by reacting an ammonium polythiomolybdate and an appropriate bis(alkyldithiophosphoric) acid

possess excellent antioxidant as well as antiwear and friction-reducing properties [31]. Some

molybdenum compounds have been used commercially in engine oils and metal working  uids

as well as in various industrial and automotive lubricating oils, greases, and specialties [95].

The combination of ZDDP with a molybdenum-containing adduct, prepared by reacting a phos-

phosulfurized polyisoalkylene or alpha ole n with a molybdenum salt, has been described [96].

In this case, the molybdenum adduct alone gave poor performance in oxidation tests, but the

mixture with ZDDP provided good oxidation stability. Novel organomolybdenum complexes pre-

pared with vegetable oil have been identi ed as synergist with ADPAs and ZDDPs in lubricating

oils [97].

Owing to increasing concerns on the use of metal dithiophosphates that are related to toxicity,

waste disposal,  lter clogging, pollution, etc., there have been extensive research activities on the

use of ashless technologies for both industrial and automotive applications. A number of ashless

compounds based on derivatives of dialkylphorphorodithioic acids had been reported as multifunc-

tional additives. Upon reacting diisoamylphosphorodithioic acid with various primary and second-

ary amines, eight alkylamino phosphorodithioates with varying chain length from C

to C

were

obtained and found to possess excellent antiwear and antioxidant properties as compared to ZDDP

[98]. Alkylamino phosphorodithioates obtained from reacting heptylated or octylated or nonylated

phosphorodithioic acids with ethylene diamine, morpholine, or tert-alkyl (C

–C

) amines have

been demonstrated to impart similar antioxidant and antiwear ef cacy and superior hydrolytic sta-

bility over ZDDP [99]. Phosphorodithioate ester derivatives containing a HP moiety are also known

to have antioxidant potency. This type of chemistry can be obtained by reacting metal salts of

phosphorodithioic acids with HP halides [100] or with HP aldehydes [101]. Substituting the phenol

aldehydes with hindered cyclic aldehydes, in which the carbon atom attached to the carbonyl carbon

contains no hydrogen atoms, may also result in products having excellent antioxidant and thermal

stability characteristics [102].

1.6 AMINE AND PHENOL DERIVATIVES

Oil-soluble organic amines and phenol derivatives such as pyrogallol, gallic acid, dibutylresorcinol,

hydroquinone, diphenylamine, phenyl-alpha-naphthylamine, and beta-naphthol are early examples

of antioxidants used in turbine oils and lubricating greases [103,104]. In engine oils, these types of

compounds showed only limited effectiveness. Other amines and phenol derivatives such as tetra-

methyldiaminodiphenylmethane and alizarin were used to some degree, rarely alone, but more often

in combination with other types of antioxidants. For example, a mixture of a complex amine with

a phosphorus pentasul depolybutene reaction product has been reported [105]. Another reported

mixture is a complex phenol derivative such as alizarin in combination with an alkyl phenol sul de

and a detergent additive [106]. As technology advances, numerous amine and phenol antioxidants

have been invented, and many of them have become the most widely used antioxidants in the lubri-

cant industry.

1.6.1 AMINE DERIVATIVES

ADPAs are one of the most important classes of amine antioxidants being used today. Owing to their

higher reactivity over the unsubstituted diphenylamine, ADPAs have been workhorse antioxidants

for engine oils and various industrial lubricants for more than two decades. Figures 1.3 and 1.4

illustrate the typical synthesis routes of some commonly used ADPAs. The reactions start with

benzene, which is  rst converted into nitrobenzene [107], followed by a high-temperature reduc-

tion to aniline [108]. Under very high-temperature (400–500°C) and high-pressure (50–150 psi)

conditions, aniline can undergo a catalytic vapor-phase conversion to form diphenylamine [109].

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Antioxidants 11

To make ADPAs, diphenylamine is reacted with an appropriate alkylating agent such as alcohol,

alkyl halide, aliphatic carbonyl compound, or an ole n. The ole ns are preferred for economic rea-

son. The most commonly used are isobutylene (C

), diisobutylene (C

), nonenes (C

), styrene, and

propylene tetramer (C

). Depending on the acidic catalyst, ole n, and other reaction conditions, for

instance, the temperature, the degree of alkylation will vary from mono- to di-alkylation.

Mono-ADPA is generally more effective than the corresponding disubstituted on a weight

basis because additional alkylation substantially reduces the number of moles of diphenylamine

per weight unit. However, in practice, obtaining monosubstituted diphenylamine in relatively

pure format is dif cult because as soon as the diphenylamine is monoalkylated, it quickly pro-

ceeds to dialkylation. Attempt in the preparation of high content of mono-ADPAs has led to

the use of novel clay catalyst with greater selectivity in alkylation reactions and C

–C

linear

ole ns to produce high levels (at least 50 wt%) of mono-ADPAs with lower levels of dialkyl

diphenylamines and undesirable unsubstituted diphenylamine [110]. Alkyl groups of six or more

carbon of mono-ADPA tend to render the material lower yellow color and higher resistance to

discoloration [111].

It was found that monosubstituted diphenylamines more readily oligomerize under various

conditions to produce higher-molecular weight, linear oligomers. Oligomers with 2–10 degrees of

polymerization are desirable antioxidants especially for high-temperature applications. Disubsti-

tuted and polysubstituted diphenylamines, however, are more restricted from forming oligomers

higher than dimers. Oligomeric versions of monosubstituted diphenylamine prepared from reacting

diphenylmine with C

–C

ole ns have been described for use in ester lubricants [112]. The prod-

ucts are claimed to be more effective than simple diphenylamines for extremely high-temperature

applications. Homo-oligomers of alkylated (C

–C

) diphenylamines, styryenated diphenylamines,

FIGURE 1.4 Synthesis routes of ADPA antioxidants.

, St

, C

+ C

olefin

olefin + styrene

olefin

+ HNO

+ H

Catalyst

Benzene Nitrobenzene Aniline Diphenylamine

Catalyst

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12 Lubricant Additives: Chemistry and Applications

or cross-oligomers of the ADPAs with substituted N-phenyl-α(β)-naphthylamine (PNA) are claimed

to possess superior antioxidant ef cacy in synthetic ester lubricants for high-temperature applica-

tions [113]. Oligomeric products derived from thermal and chemical condensation of ADPA and

alkylated PNA in the presence of aldehyde can provide high performance and nonsludging attri-

butes, as evident in the rotating pressure vessel oxidation test (RPVOT, ASTM D 2272) and the

ASTM D 4310 sludging tendency test designed for turbine oils [114].

There appears to be a great number of patenting activities on the process of using isobutylene

derivatives as alkylating agents. Under certain mole ratio range, diphenylamine can be reacted

with diisobutylene at a temperature of 160°C or higher to facilitate chain scission of diisobutyl-

ene [115]. In the presence of an acid clay catalyst, the resulting product has <25% of 4,4′-dioctyl

diphenylamine, which yields a liquid at room temperatures. In another process that involves two-

step reactions [116], a light-colored, liquid product is obtained by  rst reacting diphenylamine with

diisobutene, followed by reaction with a second ole n, preferably isobutene. Speci c mole ratio,

reaction temperature, and reaction duration are critical to obtain the desired ADPAs. To obtain

higher levels (>50 wt%) of monosubstituted diphenylamine content in the  nal product, diisobutyl-

ene is allowed to react at a lower temperature range of 105–157°C in the presence of a clay catalyst.

By carefully controlling mole ratio of the reactants together with reaction duration, the process,

as disclosed, selectively results in a higher proportion of mono-ADPA and a lower proportion of

unsubstituted diphenylamine and disubstituted or polysubstituted diphenylamines [90,117]. U.S.

Patent 6,355,839 [118] discloses a one-step process using highly reactive polyisobutylene oligomers

having an average molecular weight of ~160 to 280 and at least 25% of 2-methylvinylidene isomers

as the alkylating agents to make ADPAs and other types of alkylated diarylamine. The resulting

products are liquid at ambient temperatures.

Several antioxidant patents based on alkylation of benzotriazole compounds have been issued.

One particular bene t of using this class of antioxidant over the ADPAs is their additional activity in

the reduction of copper corrosion. Examples are N-t-alkylated benzotriazoles obtained by reacting a

benzotriazole with an ole n such as diisobutylene [119], and the reaction products of a benzotriazole

with an alkyl vinyl ether or a vinyl ester of a carboxylic acid such as vinyl acetate [120]. Antioxidant

and antiwear properties were reported for benzotriazole adducts of an amine phosphate [121] or an

organophosphorodithioate [122]. The former type also exhibited rust prevention characteristics in

the ASTM D 665 corrosion test.

Aromatic diamines are a broad group of aminic antioxidants suitable for lubricants. 3,5-

diethyltoluenediamines with the amino moieties being located on the 2,4 and 2,6 positions rela-

tive to the methyl group have been claimed to be effective in the prevention of oil viscosity

increase and acid buildup [123]. The additives are relatively noncorrosive to copper and lead

bearings and are compatible with seals at high temperatures and pressures. Substituted ben-

zylamines or substituted 1-amino-1,2,3,4-tetrahydronaphthalene is particularly useful for syn-

thetic lubricants such as polyalphaole ns (PAOs) or polyol esters. Oils bearing these additives

demonstrate very low metal corrosion, low viscosity increase, and low sludge buildup [124].

N,N′- diphenyl-p- phenylenediamines in which the phenyl groups may be substituted with methyl,

ethyl, or methoxy have been claimed as effective antioxidants [125].

A broader range of substi-

tuted p-phenylenediamines has been claimed for crankcase lubricating oils for use in environ-

ments where iron- catalyzed oxidation reactions can take place [126]. 2,3- Dihydroperimidines that

are prepared from the condensation of 1,8-diaminonaphthalenes with ketones or aldehydes show

good oxidation inhibition in the RPVOT (ASTM D 2272). Synergistic behavior of the amines was

also observed when an appropriate phenolic antioxidant is present [127]. Oils containing N,N′-

disubstituted-2,4-diaminodiphenyl ethers and imines of the same ethers have shown low viscosity

increase, low acid buildup, and reduced metal corrosion in bench tests [128,129]. The reaction

product of a hydrocarbyl succinic anhydride and 5-amino-triazole demonstrated antioxidant ef -

cacy in a railway diesel oil composition [130].

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Antioxidants 13

1.6.2 PHENOL DERIVATIVES

Phenols, especially the sterically hindered phenols are another class of antioxidants being extensively

used in industrial and automotive lubricating oils and greases. Based on the chemical structure,

phenols may be customarily categorized into simple phenols such as 2,6-di-tert-4- methylphenol

(also known as BHT) and complex phenols that are typically in polymeric forms having molecular

weights of 1000 or higher. The structures, important physical properties, and typical applications of

some commonly used HPs are given in Table 1.2.

Similar to the alkyl phenol sul des discussed earlier, the combinations of HPs and sulfur chem-

istry have been widely reported. For example, the reaction products of simple phenols such as the

2,6-di-tert-butylphenol listed in Table 1.2 with selected thioalkenes have shown effectiveness in the

prevention of acid buildup and oil viscosity increase, without causing lead corrosion [131]. Another

patent describes a process for preparing hydrocarbylthio-HPs by reacting substituted phenols with

hydrocarbyl disul des using an aluminum phenoxide catalyst [132]. Using a 4,4′-methylene bis(2,6-

di-tert-butylphenol) as reference, the thiophenols were found to be superior in bulk oil oxidation

tests and bench corrosion test on bearings. High oligomeric phenolic antioxidants in the form of

hindered and sulfur bridged have been developed [133]. These compounds have lower volatility, bet-

ter thermal stability, and improved seal compatibility and corrosion properties. In general, sulfur-

bridged HPs are more effective than the conventional phenolics under high-temperature oxidation

conditions and are considered particularly suitable for the lubricants formulated with highly re ned

base stocks [134]. Figure 1.5 shows structures of some commercial sulfur-bridged HPs that have

found use in various lubricant formulations. Thioalkene-bridged hemi-HPs prepared from catalytic

reaction of HP with thioalkene have also been reported to be active in the stabilization of mineral

oils and synthetic oils [135].

1.6.3 AMINE AND PHENOL-BEARING COMPOUNDS

Given the high popularity and effectiveness of amine and phenol derivatives as lubricant anti-

oxidants, the combination of amine and phenolic moieties in one molecule represents a logic

approach to enhance performance. In a prior art [136], fusing amine with a long carbon chain 3,5-

di-tert-butyl-4-hydroxyphenalkyl group that separates the phenol group from the amino nitrogen

leads to novel products with lower volatility, better thermal stability, and higher solubility in oils.

Nelson and Rudnick [137] reacted an ethyoxylated alkyl phenol with an alkyl arylamine in the

presence of an aldehyde. The resulting product had improved antioxidant potency owing to a syn-

ergistic action between the phenolic moiety and the amine, and also showed enhanced solubility

in oils owing to the presence of alkylated aromatic moiety in the molecule. Phenolic imidazo-

lines have been prepared from polyaminophenols and carbonyl compounds [138]. In addition to

providing antioxidant activity, the products also have corrosion inhibition and metal deactivation

properties owing to the cyclic imidazoline moiety.

Multifunctional additives containing sulfur, nitrogen, and phenolic moieties in one molecule

have been reported. In this instance, mercaptobenzothiazoles or thiadiazoles are Mannich reacted

with HP antioxidants to yield oil-soluble compounds with antioxidant and antiwear properties [139].

More complex product having similar functionalities was obtained by reacting a sulfur-containing

HP ester with an ADPA [140].

1.6.4 MULTIFUNCTIONAL AMINE AND PHENOL DERIVATIVES

The industry-wide trend in the reduction of phosphorus and sulfur, in particular, ZDDP in  nished

lubricants has led to increasing activities in the development of novel multifunctional additives

that have combined properties of antioxidancy, antiwear, and to some extent dispersancy, while

having low-to no-sulfur and phosphorus contents. It has been shown that products obtained from

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