4.1. Hindered phenolic compounds

composition and degradation of the lubricant [7–10]. Antioxidants are among the most important group of additives, that are normally composed of sterically hindered phenols or aromatic amines [1, 2]. The presence of oxygen and high temperatures within a tribological environment can be important factors for rapid oxidation of lubricants. The quantitative analysis of lubricant antioxidant additives in complex and native base oil matrices has been studied using ESI-MS and MALDI-MS [11, 12]. The rotating pressure vessel oxidation test (ASTM-D2272) is the most common method which can measure the RUL (Remaining Useful Life) of the oil's ability

Antioxidants are one of the most suitable additives to extend the lifetime of lubricants. Furthermore, antioxidants prevent the oxidative degradation of the lubricant oil thickening and the formation of sludge. Aromatic amines (e.g. dialkylated diphenylamine) and sterically hindered phenols (e.g. 2,6-di-tert-butylphenol) are two common antioxidants which are useful in lubricants stabilization to gain synergistic effects [3, 13]. A better understanding of the chemistry of antioxidants and their degradation mechanisms at the molecular level is crucial for developing more efficient lubricants. Lubricants based on mineral oils, are very complex mixtures; therefore, an analytical method with high sensitivity and selectivity to separate the components, and to characterize and quantify antioxidants and their degradation products has been established [14].

Antioxidants are a series of compounds with the capability of controlling oxidation, and consequently preventing oil from breakdown and thickening (increasing viscosity), and helping better performance and longer life of an engine. Natural antioxidants are the chemical compounds that originally present in the mineral oil known as polycycloaromatics and sulfur and nitrogen heterocyclics, or with bio-oil, triglycerides and in biological systems known as tocopherol, astaxanthin, zeaxanthin, lutein, flavonoids, lycopene, etc. In the mineral oil refining process, severe conditions applied in the process strips the base oil of its natural antioxidants [1, 2]. Therefore, the lack of these important group of chemicals should be compensated by supplementation of the base oil using appropriate groups of additives. Three types of antioxidants are generally available, namely, radical scavengers (primary antioxidants), perox-

ide decomposers (secondary antioxidants), and metal passivators/deactivators [15].

Radical scavengers such as phenolic antioxidants, aromatic amines, and sulfur and phosphorus compounds that stop chain propagation by blocking or reacting with free radicals generated in the initiation stage of oxidation. Blocking of the radicals by the scavengers occurs through donation of hydrogen atoms that react with alkyl or peroxy radicals, leading to the formation

Peroxide decomposers such as organosulfur (e.g. dialkyl sulfides and dithiocarbamates) and organophosphorus (e.g. triaryl phosphites and trialkyl phosphites) compounds have the conversion potential of hydroperoxides to non-radical derivatives such as alcohols [1, 2, 15].

Metal deactivators such as benzotriazole and N-salicylidene ethylamine acting as surface filmforming compounds or stable complex-forming agents (chelating agent) function by reducing

4. Activity and classification of antioxidants

of quinones or quinine imines [1, 2, 15].

to resist oxidation.

26 Lubrication - Tribology, Lubricants and Additives

Hindered phenols are a group of (primary) antioxidants that function by scavenging mechanism through hydrogen donation in which the target molecules are peroxy radical intermediates. They are active over a wide range of temperature and they can provide a long-term stability of the lubricant with minimizing viscosity change and discoloration. Synergistic effect may result using a combination of hindered phenols and secondary antioxidants such as thioethers and phosphites. The sterically hindered phenols (I) with 2 and 6 positions on the ring substituted by tertiary alkyl groups (such as butyl) are very active antioxidants reacting with the peroxy radical intermediates (Figure 2). The product of the first reaction (II) is also reactive functioning as the scavenger of the peroxy radicals [15, 18].

The maximal activity of hindered phenolic antioxidants is attainable when both 2 and 6 positions of the aromatic ring are occupied by tertiary butyl groups; with one substituent replaced by methyl instead of tertiary butyl, the relative antioxidant activity may drop by 37.5% as shown in Table 1 [18].

### 4.2. Aromatic amine compounds

This class of antioxidants is more active than the hindered phenols and are available in a wide range of molecular weights and forms. However, aromatic amines contribute more in discoloring

The mechanism of the sequential reaction of the alkylated diphenylamine under low temperatures (<120C) is shown in Figure 4 [1, 23]. At the end of this reactions resulting in the elimination of four peroxy radicals, two compounds are generated, namely; 1,4-benzoquinone

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Comparing a diphenylamine molecule with sterically hindered monophenols, the former has the scavenging potential of four peroxy radicals, while the sterically hindered monophenols

At higher temperatures (>120C), after reaction of the antioxidant with the peroxy radical and formation of the nitroxyl radical in the second step, this compound would have the potential of reacting with (scavenging) a secondary alkyl radical leading to the regeneration of the original alkylated diphenylamine (Figure 5). It has been proven that the performance of the antioxidants based on diphenylamine depends on the substituents in the para position such that stoichiometric efficiencies of over 12 radicals per molecule have been reported in this regener-

Figure 4. Mechanism of sequential reaction of alkylated diphenyl amine with peroxy radical at low temperatures

have the potential of elimination of 2 equivalents of peroxy radicals.

Figure 3. Mechanism of action of aromatic amines on peroxy radicals [resketched from 1, 2, 19].

and an alkylated nitrosobenzene.

ation process [1, 24].

(<120C) [resketched from 1 and 2].

Figure 2. Mechanism of reaction of hindered phenols with peroxy radical.

Table 1. Relative antioxidant activity of phenolic antioxidants affected by the alkyl substituents at the ortho positions [1, 18].

the final product (formulated lubricant) compared to the hindered phenols, especially at higher temperatures or exposure to light [19]. As active hydrogen donors, they can easily transfer the hydrogen atom on nitrogen to peroxy radicals [20, 21]. The typical group in this class of antioxidants are called alkylated diphenyl amines that are substituted amine antioxidants synthesized by the reaction between diphenylamine and alkylating agents. This group of antioxidants are used in lubricants as well as synthetic polymers and rubber vulcanizates [22]. The mechanism of action of aromatic amines can simply be presented as follows Figure 3:

Figure 3. Mechanism of action of aromatic amines on peroxy radicals [resketched from 1, 2, 19].

The mechanism of the sequential reaction of the alkylated diphenylamine under low temperatures (<120C) is shown in Figure 4 [1, 23]. At the end of this reactions resulting in the elimination of four peroxy radicals, two compounds are generated, namely; 1,4-benzoquinone and an alkylated nitrosobenzene.

Comparing a diphenylamine molecule with sterically hindered monophenols, the former has the scavenging potential of four peroxy radicals, while the sterically hindered monophenols have the potential of elimination of 2 equivalents of peroxy radicals.

At higher temperatures (>120C), after reaction of the antioxidant with the peroxy radical and formation of the nitroxyl radical in the second step, this compound would have the potential of reacting with (scavenging) a secondary alkyl radical leading to the regeneration of the original alkylated diphenylamine (Figure 5). It has been proven that the performance of the antioxidants based on diphenylamine depends on the substituents in the para position such that stoichiometric efficiencies of over 12 radicals per molecule have been reported in this regeneration process [1, 24].

Figure 4. Mechanism of sequential reaction of alkylated diphenyl amine with peroxy radical at low temperatures (<120C) [resketched from 1 and 2].

the final product (formulated lubricant) compared to the hindered phenols, especially at higher temperatures or exposure to light [19]. As active hydrogen donors, they can easily transfer the hydrogen atom on nitrogen to peroxy radicals [20, 21]. The typical group in this class of antioxidants are called alkylated diphenyl amines that are substituted amine antioxidants synthesized by the reaction between diphenylamine and alkylating agents. This group of antioxidants are used in lubricants as well as synthetic polymers and rubber vulcanizates [22]. The mechanism of

Table 1. Relative antioxidant activity of phenolic antioxidants affected by the alkyl substituents at the ortho positions

100.0

62.5

action of aromatic amines can simply be presented as follows Figure 3:

[1, 18].

Figure 2. Mechanism of reaction of hindered phenols with peroxy radical.

28 Lubrication - Tribology, Lubricants and Additives

Phenol structure Relative antioxidant activity

the results, the two aforementioned antioxidants were more helpful than other antioxidants but less effective than 2-ethylhexyl nitrate (EHN) which is an accepted NOX-lowering agent. A study was carried out by Mukul et al. [27] to evaluate the importance of chemistry of antioxidants on the oxidative stability and thermo-oxidative properties of gear oil. They conducted the experiments on 4 oil blends, namely AO I (no antioxidant added), AO II (with an amine antioxidant (Irganox L57) added), AO III (with a phenolic antioxidant (Irganox L135) added, and AO IV (with both Irganox L57 and Irganox L135 added). In the high-pressure differential scanning calorimetry (PDSC) test at 160C, the following order obtained for oxida-

A similar trend was also obtained for rotating pressure vessel oxidation test (RPVOT) results for the oil blends tested confirming a good correlation between the two test methods. However, a reverse trend of results for the oxidation level (%) of the aforementioned oil blends was achieved. According to the results, amine antioxidant resulted in a better performance compared to the phenolic antioxidant and synergism of the antioxidants did not have a significant role in delaying the oxidation reactions. The higher performance of amine antioxidant compared with the phenolic antioxidant on the thermo-stability of the lubricant could be its catalytic manner of reaction and regeneration over several cycles of scavenging and breaking

Thermal stability of polyol ester lubricant was affected by different types of antioxidants as reported by Mousavi et al. [28]. Among the systems studied, Phenyl-R-naphthylamine (PAN) showed a remarkable improvement on the thermal stability of the base oil (Figures 6 and 7) indicating less acid and less HMW products generation in this oil blend at high temperature (220C). The greater area in Figure 7 is the representation of the generation of high-molecular

Figure 6. Acid values of original oil (AF) and inhibited oil (using different antioxidants) heated at 220C over time.

weight products (HMW) due to oxidation and polymerization reactions.

AO II > AO IV > AO III > AO I (9)

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tion induction temperature (OIT) of the oil blends:

of chain reactions of oxidation.

Reprinted with permission from [28].

Figure 5. Mechanism of reaction of alkylated diphenyl amine with peroxy radical at high temperatures (>120�C) [resketched from 1].

The performance of aromatic amines on NOx emissions from soybean biodiesel powered DI diesel engine has been investigated [25]. The results indicated that at 75% load for B100 fuel enriched with DPPD (N,N<sup>0</sup> -diphenyl-1,4-phenylenediamine) and NPPD (N-phenyl-1,4-phenylenediamine) as effective antioxidants, reductions of 28.36 and 20.96% were obtained with NO emissions, respectively. For B20, less reduction of NO was achieved with DPPD and NPPD additions compared to B100. The effectiveness of the both antioxidants in B100 and B20 fuels was proved for NO emissions.

In another study by Hess et al. [26], incorporation of antioxidants including butylated hydroxyanisol and butylated hydroxytoluene at 1000 ppm concentration in B20 resulted in the reduction of NOx gases in a single-cylinder engine as shown in Table 2 [26]. According to


Table 2. The influence of additives on NOx emission during combustion [26].

the results, the two aforementioned antioxidants were more helpful than other antioxidants but less effective than 2-ethylhexyl nitrate (EHN) which is an accepted NOX-lowering agent.

A study was carried out by Mukul et al. [27] to evaluate the importance of chemistry of antioxidants on the oxidative stability and thermo-oxidative properties of gear oil. They conducted the experiments on 4 oil blends, namely AO I (no antioxidant added), AO II (with an amine antioxidant (Irganox L57) added), AO III (with a phenolic antioxidant (Irganox L135) added, and AO IV (with both Irganox L57 and Irganox L135 added). In the high-pressure differential scanning calorimetry (PDSC) test at 160C, the following order obtained for oxidation induction temperature (OIT) of the oil blends:

$$\text{AO II} > \text{AO IV} > \text{AO III} > \text{AO I} \tag{9}$$

A similar trend was also obtained for rotating pressure vessel oxidation test (RPVOT) results for the oil blends tested confirming a good correlation between the two test methods. However, a reverse trend of results for the oxidation level (%) of the aforementioned oil blends was achieved. According to the results, amine antioxidant resulted in a better performance compared to the phenolic antioxidant and synergism of the antioxidants did not have a significant role in delaying the oxidation reactions. The higher performance of amine antioxidant compared with the phenolic antioxidant on the thermo-stability of the lubricant could be its catalytic manner of reaction and regeneration over several cycles of scavenging and breaking of chain reactions of oxidation.

The performance of aromatic amines on NOx emissions from soybean biodiesel powered DI diesel engine has been investigated [25]. The results indicated that at 75% load for B100 fuel

Figure 5. Mechanism of reaction of alkylated diphenyl amine with peroxy radical at high temperatures (>120�C) [resket-

nylenediamine) as effective antioxidants, reductions of 28.36 and 20.96% were obtained with NO emissions, respectively. For B20, less reduction of NO was achieved with DPPD and NPPD additions compared to B100. The effectiveness of the both antioxidants in B100 and B20 fuels

In another study by Hess et al. [26], incorporation of antioxidants including butylated hydroxyanisol and butylated hydroxytoluene at 1000 ppm concentration in B20 resulted in the reduction of NOx gases in a single-cylinder engine as shown in Table 2 [26]. According to

Fuel Change in NOx from B20 combustion (%)

B20 + 2-ethylhexyl nitrate �4.5 � 1.0

B20 + citric acid �0.7 � 0.5 B20 + α-tocopherol +0.3 � 0.2 B20 + ascorbic acid 6-palmitate �1.3 � 0.9 B20 + tert-butyl hydroquinone �0.3 � 1.6 B20 + propyl gallate �0.4 � 2.8 B20 + diphenylamine +0.7 � 1.3 B20 + butylated hydroxytoluene (BHT) �2.9 � 1.5 B20 + butylated hydroxyanisole (BHA) �4.4 � 1.0

Table 2. The influence of additives on NOx emission during combustion [26].



enriched with DPPD (N,N<sup>0</sup>

30 Lubrication - Tribology, Lubricants and Additives

ched from 1].

B20 + 2,2<sup>0</sup>

was proved for NO emissions.

Thermal stability of polyol ester lubricant was affected by different types of antioxidants as reported by Mousavi et al. [28]. Among the systems studied, Phenyl-R-naphthylamine (PAN) showed a remarkable improvement on the thermal stability of the base oil (Figures 6 and 7) indicating less acid and less HMW products generation in this oil blend at high temperature (220C). The greater area in Figure 7 is the representation of the generation of high-molecular weight products (HMW) due to oxidation and polymerization reactions.

Figure 6. Acid values of original oil (AF) and inhibited oil (using different antioxidants) heated at 220C over time. Reprinted with permission from [28].

According to Bridgewater and Sexton [30], sulfur dioxide functions as a powerful Lewis acid such that one equivalent can decompose up to 20,000 equivalents of cumene hydroperoxide, i.e., 5–<sup>60</sup> <sup>10</sup><sup>6</sup> mol/l of the sulfur compound decomposed over 50% of the 0.2 mol/l of

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Figure 10. Conversion processes of sulfenic acid by hydroperoxide, and sulfinic acid by heat.

In addition to the sulfur-based acids mentioned above, sulfacids (RSOxH) are also considered organosulfur antioxidants with a mechanism of reaction with peroxy radicals (Figure 11) and

Among organophosphorus compounds, phosphites are the main group of compounds that are used in the formulation of lubricants to overcome the oxidation reactions. They have the potential of reacting with hydroperoxides, peroxy and alkoxy radicals (Figure 12). Therefore, they can be effective on the stability of color and physical and rheological properties of the lubricant. In the reaction with hydroperoxide or peroxy radical, phosphite is oxidized to the corresponding phosphate, while the hydroperoxide and peroxy radical are reduced to a less

If phosphite possesses a phenoxy group in its structure, it would eliminate peroxy and alkoxy radicals through reaction with them and also the generated phenoxy radical from this reaction would be a stable radical with the potential of elimination peroxy radicals (Figure 13). Stability of the generated phenoxy radicals due to their steric hindrance by the two alkyl groups on the ortho positions of the aromatic ring makes them appropriate antioxidant candidates in moist

cumene hydroperoxide in a 6-h period [30].

functioning as a primary antioxidant as below:

reactive alcohol and alkoxy radical, respectively [1, 2].

Figure 11. Reaction of sulfacids with peroxy radicals [resketched from [1, 2].

Figure 12. Reactions of phosphite with hydroperoxide and peroxy radicals [resketched from 1, 2].

4.4. Organophosphorus compounds

systems of lubrication [1, 2].

Figure 7. Peak area (%) obtained with gel permeation chromatography (GPC) for high molecular weight (HMW) chemicals generated at 220C through oxidation/polymerization reactions. Reprinted with permission from [28].

#### 4.3. Organosulfur compounds

Organosulfur compounds function as hydroperoxide decomposers by converting them into non-radical products. Acid-catalyzed decomposition is the most important mechanism of eliminating hydroperoxides in the lubricating system with acid catalysts sourced from organosulfur compounds, as reported by Hawkins and Sautter [29].

Compounds such as dialkyl sulfides (R-S-R) would react with hydroperoxide molecules converting them to sulfoxides as shown in Figure 8:

In the next step, assuming R is an alkyl, sulfoxide molecule can be converted (by heat) to sulfenic acid (RSOH) which is a very reactive acid, as outlined in Figure 9:

Since sulfenic acid is an unstable material, especially in the presence of hydroperoxide, it can be easily transformed to sulfinic acid decomposition occurs that can function as an acid catalyst in the decomposition of hydroperoxides at low temperature. At higher temperatures, sulfinic acid decomposition occurs by thermolysis and is converted to SO2 that functions as the catalyst for hydroperoxide decomposition (Figure 10).

Figure 8. Conversion of hydroperoxides to dialkyl sulfoxide by dialkyl sulfide.

Figure 9. Generation of sulfenic acid from sulfoxide molecule.

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Figure 10. Conversion processes of sulfenic acid by hydroperoxide, and sulfinic acid by heat.

According to Bridgewater and Sexton [30], sulfur dioxide functions as a powerful Lewis acid such that one equivalent can decompose up to 20,000 equivalents of cumene hydroperoxide, i.e., 5–<sup>60</sup> <sup>10</sup><sup>6</sup> mol/l of the sulfur compound decomposed over 50% of the 0.2 mol/l of cumene hydroperoxide in a 6-h period [30].

In addition to the sulfur-based acids mentioned above, sulfacids (RSOxH) are also considered organosulfur antioxidants with a mechanism of reaction with peroxy radicals (Figure 11) and functioning as a primary antioxidant as below:

#### 4.4. Organophosphorus compounds

4.3. Organosulfur compounds

32 Lubrication - Tribology, Lubricants and Additives

Organosulfur compounds function as hydroperoxide decomposers by converting them into non-radical products. Acid-catalyzed decomposition is the most important mechanism of eliminating hydroperoxides in the lubricating system with acid catalysts sourced from organ-

Figure 7. Peak area (%) obtained with gel permeation chromatography (GPC) for high molecular weight (HMW) chemi-

cals generated at 220C through oxidation/polymerization reactions. Reprinted with permission from [28].

Compounds such as dialkyl sulfides (R-S-R) would react with hydroperoxide molecules

In the next step, assuming R is an alkyl, sulfoxide molecule can be converted (by heat) to

Since sulfenic acid is an unstable material, especially in the presence of hydroperoxide, it can be easily transformed to sulfinic acid decomposition occurs that can function as an acid catalyst in the decomposition of hydroperoxides at low temperature. At higher temperatures, sulfinic acid decomposition occurs by thermolysis and is converted to SO2 that functions as the

osulfur compounds, as reported by Hawkins and Sautter [29].

sulfenic acid (RSOH) which is a very reactive acid, as outlined in Figure 9:

converting them to sulfoxides as shown in Figure 8:

catalyst for hydroperoxide decomposition (Figure 10).

Figure 8. Conversion of hydroperoxides to dialkyl sulfoxide by dialkyl sulfide.

Figure 9. Generation of sulfenic acid from sulfoxide molecule.

Among organophosphorus compounds, phosphites are the main group of compounds that are used in the formulation of lubricants to overcome the oxidation reactions. They have the potential of reacting with hydroperoxides, peroxy and alkoxy radicals (Figure 12). Therefore, they can be effective on the stability of color and physical and rheological properties of the lubricant. In the reaction with hydroperoxide or peroxy radical, phosphite is oxidized to the corresponding phosphate, while the hydroperoxide and peroxy radical are reduced to a less reactive alcohol and alkoxy radical, respectively [1, 2].

If phosphite possesses a phenoxy group in its structure, it would eliminate peroxy and alkoxy radicals through reaction with them and also the generated phenoxy radical from this reaction would be a stable radical with the potential of elimination peroxy radicals (Figure 13). Stability of the generated phenoxy radicals due to their steric hindrance by the two alkyl groups on the ortho positions of the aromatic ring makes them appropriate antioxidant candidates in moist systems of lubrication [1, 2].

Figure 11. Reaction of sulfacids with peroxy radicals [resketched from [1, 2].

Figure 12. Reactions of phosphite with hydroperoxide and peroxy radicals [resketched from 1, 2].

The final radical (RO)2PS2

4.7. Organo-copper compounds

4.8. Organo-molybdenum compounds

.

Figure 14. A typical reaction of ZNTP with hydroperoxides.

Figure 15. Reaction mechanism of ZDTP with peroxy radical.

ester and mineral oil lubricants at temperatures below 250C.

ing hydrogenated acid form of this compound with the functionality of an inhibitor.

Copper as a transition metal has been considered an oxidation promoter which may cause damage in the lubricant or lubricating systems; however, copper salts that are soluble in oil are reported to function as antioxidants [34, 35]. Limitation of loading copper within 100 to 200 ppm to obtain the optimal control of oxidation and wear is a drawback for copper-based antioxidants. Over this range, the performance of the anti-wear components in the lubricants would drop due to the reverse impact of copper. Organo-copper antioxidants are effective in

Organo-copper compounds including copper naphthenates, oleates, stearates, and polyisobutylene succinic anhydrides have been reported to be synergistic with multi-ring aromatic com-

According to some other studies, inclusion of oil-soluble compounds of copper in the range of 5 to 500 ppm resulted in improved performance of the automotive crankcase lubricants in

Molybdenum dithiocarbamate has been reported to function as an antioxidant and anti-wear component in the lubricants. However, it would lose its protective properties by time due to dropping its concentration below the critical level of activity [37]. The synergistic application of Molybdenum dialkyldithiocarbamate (MoDDC) with arylamines was tested to improve the durability and low friction performance of MoDDC over time. The DSC (differential scanning calorimetric) results have indicated that the oxidation and induction temperatures for a polyα-olefin (PAO) lubricant would increase by the addition of MoDDC to the formulation. Also, MoDDC would have an antioxidative synergism with alkylated diphenylamine antioxidants

Synergistic mixtures of antioxidants from different groups or classes are generally applied in the formulation of commercial lubricants to provide better stability toward oxidation. In

pounds in controlling high-temperature deposit formation in synthetic base stocks [2].

terms of anti-wear, antioxidant performance and corrosion resistance [36].

(arylamines) such as octyl- and butyl-containing diphenylamine compounds.

5. Antioxidants/additives synergism and antagonism

also has the potential of reacting with hydroperoxide and generat-

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Figure 13. Reactions of phosphite (possessing phenoxy) with alkoxy and peroxy radicals [resketched from 2].

### 4.5. Sulfur-phosphorus compounds

Antioxidants with both sulfur and phosphorus elements are more efficient and effective than those with either sulfur or phosphorus. Metal dialkyldithiophosphates are a group of antioxidants in this class that have been widely used and have been synthesized by the reaction between phosphorus pentasulfide and alcohols (such as aliphatic, cyclic and phenolic, lauryl, octyl, methyl cyclohexyl, etc.) to produce dithio-phosphoric acids followed by a neutralization process using a metal compounds (such as zinc, barium, calcium and molybdenum compounds or oxides). Zinc dialkyldithiophosphate (ZDDP) is one of the well known compounds in this group that have been used as an effective antioxidant and anti-wear component in the lubricant industry for several years [2].
