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34 Lubricant Additives: Chemistry and Applications

antioxidants. The “dry” TOST method is a potential alternative to the original methods that have

found to be less discriminatory on such high-performance turbine oils.

1.11.2.2 IP 48 Method

The Institute of Petroleum (IP) 48 is a high-temperature bulk oil oxidation test originally designed for

the characterization of base oils [233]. The test stresses a 40 ml of oil sample in a glass tube at 200°C,

along with air bubbling at 15 L/h, for two 6 h periods with a 15–30 h standby period in between. Oil vis-

cosity increase and the formation of carbon residue are determined after the oxidation. The test is con-

sidered unsuitable for additive-type oils (other than those containing ashless additives) or those which

form solid products or evaporate more than 10% by volume during the test. However, successful assess-

ment of engine oils using a modi ed IP 48 method with four 6 h cycles has been reported [233,234].

1.11.2.3 IP 280/CIGRE

The IP 280, also known as the CIGRE test, was designed to assess the oxidative stability of inhib-

ited mineral turbine oils, targeting formations of volatile acid products (through water absorption),

sludge, and increase of oil acidity [235]. The IP 280 and the TOST D 943 are similar to each other

in terms of the oxidation regime employed. However, their test conditions are different, and it is a

common practice to conduct both tests because in some internal turbine oil speci cations, the limits

for both the tests are stipulated. The IP 280 test was found to be more suitable for discriminating

performance of additive packages, whereas the D943 is more suitable for comparative evaluation of

base oils derived from different crude source and processing techniques [236].

1.11.3 OXYGEN UPDATE TEST

1.11.3.1 Rotating Pressure Vessel Oxidation Test (ASTM D 2272)

The RPVOT, originally known as the rotating bomb oxidation test (RBOT), was designed to monitor the

oxidative stability of new and in-service turbine oils having the same composition. It can also be used to

characterize other types of industrial lubricants, for example, hydraulic  uids and circulating oils. The

test utilizes a steel pressure vessel where sample oil is initially pressurized to 90 psi with oxygen and

thermally stressed to 150°C in the presence of water and copper coil catalyst until a pressure drop of

25 psi is observed [237]. The test temperature was chosen to promote measurable oil breakdown in a

relatively short time. However, such temperature causes a lack of representation to most steam turbines

that operate below 100°C and to the combustion turbines that operate at much higher temperatures [238].

Owing to its sensitivity to speci c additive chemistries, RPVOT  nds limited use in comparing differ-

ently formulated oils. In addition, the test is more suitable for the determination of remaining useful life

of in-service turbine oils rather than the quali cation of new oils. Attempting to correlate RPVOT to the

lengthy TOST D 943 on steam turbine oils has been successful, suggesting that the results from RPVOT

may be used to estimate the relative lifetime of turbine oils in the TOST D943 [239].

1.12 EXPERIMENTAL OBSERVATIONS

The following two experiments demonstrate (a) performance behaviors of aminic antioxidant ver-

sus HP that is in agreement with the mechanisms discussed earlier, and (b) how proper selection and

combinations of antioxidants can lead to synergy that further enhances performance.

In the  rst experiment, two turbine oils, each formulated with a base oil selected from an API

group I or group IV base stock, a standard additive package of metal deactivator and corrosion inhibi-

tor, and 0.8 wt% of antioxidants of interest were tested by using the TOST D 943 lifetime method. The

aminic antioxidant was an ADPA containing a mixture of butylated and octylated diphenylamines.

The HP was a C

–C

branched alkyl ester of 3,5-di-tert-butyl-4- hydroxyhydrocinnamic acid.

As can be seen from Figure 1.19, in either oil, the HP signi cantly outperformed the ADPA by

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Antioxidants 35

providing longer protection time against oxidation. Mixtures of the ADPA and the HP at 0.4 wt%

each in the oils provided even stronger protection, leading to an extended lifetime of ~5000 h for the

group I turbine oil and well over 8000 h for the group IV turbine oil. Thus, under the low-tempera-

ture test conditions, the HP was superior to the ADPA. Proper mixing of the two additives produced

synergy, in this case a homosynergism that led to the maximum protection.

In the second experiment, a turbine oil formulated with an API group I base stock, a metal deac-

tivator, a corrosion inhibitor, and a 0.5 wt% of the same antioxidants as before was tested by using

the RPVOT (ASTM D 2272). The results are graphically presented in Figure 1.20. At the higher

500

1000

1504

2008

2512

3016

3520

4024

4528

5032

5536

6040

6544

7048

7552

8064

Time (h)

TAN (mgKOH/g)

HP, GI

ADPA, GI

HP + ADPA, GI

HP + ADPA, GIV

ADPA, GIV

HP, GIV

3.0

2.5

2.0

1.5

1.0

0.5

0.0

FIGURE 1.19 TOST results of turbine oils containing group I or IV base oil and 0.8 wt% of antioxidant.

100

200

300

400

500

600

700

800

HP HP + ADPA ADPA

RPVOT OIT (min)

FIGURE 1.20 RPVOT results of turbine oil containing a group I base oil and 0.5 wt% of antioxidant.

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36 Lubricant Additives: Chemistry and Applications

test temperature (150°C), the OIT of the blend containing ADPA was ~600 min, while the ADPA

was depleted. The HP protected the oil for ~300 min, indicating that the HP is only half as effective

as the ADPA under the same test conditions. A mixture of ADPA and HP with 0.25 wt% of each

additive present provided a protection for over 700 min. Therefore, in contrast to the TOST results,

under high-temperature conditions, the ADPA was superior to the HP. Similar to what was observed

in the TOST, a synergistic mixture of the two additives provided the maximum protection.

The superiority of ADPA over HP and the bene t of antioxidant synergy for maximum oxi-

dation protection have been further demonstrated in a GF-4 prototype passenger car motor oil

(PCMO). The oil contained an API group II base oil, a low level (0.05 wt%) of phosphorus derived

from ZDDP, and a number of other additives (detergents, dispersant, viscosity index improvers,

pour point depressant, etc.) that are commonly found in engine oil formulations. The ADPA, HP,

and their mixture were tested at 1.0 wt% in the oil on a TEOST MHT apparatus, using the ASTM D

7097 standard procedure. The results are presented in Figure 1.21. The baseline blend, which con-

tained all other additives except the antioxidant, produced a fairly high level (130 mg) of deposits.

With the addition of the HP, the deposit was substantially reduced to ~80 mg, with the ADPA, down

to ~55 mg. By properly mixing the two antioxidants while keeping the total level constantly at

1.0 wt%, the deposit was further reduced to ~40 mg. The TEOST results con rm the superior per-

formance of ADPA and further demonstrate the bene t of antioxidant synergy for high-temperature

oxidation conditions.

The antioxidant mechanisms discussed earlier well explain the experimental results and can

serve as a foundation to guide lubricant formulators in the selection of correct antioxidant(s) for

a particular end use. To obtain a successful formulation, other factors such as cost performance,

volatility, color, solubility, odor, physical form, toxicity, and compatibility with other additives

also need be taken into consideration. From a performance standpoint, HPs are excellent primary

antioxidants for their stoichiometric reactions with free radicals under lower-temperature con-

ditions. In contrast, ADPAs are excellent primary antioxidants for high-temperature conditions

owing to their catalytic radical scavenging actions. The homosynergism facilitated between the

FIGURE 1.21 TEOST results of a prototype PCMO containing a group II base stock and a total of 1.0 wt%

of antioxidant.

100

120

140

Baseline HP ADPA ADPA + HP

Deposits (mg)

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Antioxidants 37

ADPA and the HP is powerful in the inhibition of different stages of oil oxidation as demon-

strated. It is, however, important to note that the generation and the magnitude of an antioxidant

synergy are dependent on the formulation, base oil, and test method used. The ADPA/HP synergy

appears robust as it was successfully reproduced in two oil formulations and tests that vastly dif-

fer from each other in terms of base oil makeup, additive type and complexity, test conditions,

and oxidation regimes. In fact, this type of synergy has been used in a wide range of lubricants.

In a more recent development, a methylene-bridged HP was utilized and found to be synergistic

with ADPA in low-phosphorus engine oils [240]. Several instances of other types of synergy have

been demonstrated and discussed in greater depth elsewhere. These include, but are not limited

to, synergy between sulfur-bearing HP and ADPA antioxidants for hydro-treated base stocks

[134,241], synergy between aminic antioxidants [242], and synergy between primary antioxidants

and oragnophosphites [57].

1.13 ANTIOXIDANT PERFORMANCE WITH BASE STOCK SELECTION

Driven by escalating environmental and performance requirements, the lubricant industry is rapidly

changing for the better with the advances of additive and base oil technologies. One notable change

from a formulation point of view is that the conventional solvent-extracted base oils (group I) are

gradually being replaced by high-quality, high-performance groups II and III base stocks made

from hydrotreated (hydrocracked), hydrotreating, and hydrocatalytic dewaxing processes. These

processes provide oils with low sulfur, high degree of saturation, and viscosity index (Table 1.3).

Lubricants formulated with these base stocks generally have improved performance characteristics

such as superior oxidative stability, lower volatility, improved low-temperature properties, longer

drain intervals, and improved fuel economy. Because of these bene ts, the API group III base oils

are becoming a serious challenge to synthetic PAOs for top-tier oil formulations.

Many efforts have been made to understand the relationship between the base oil composition and

the response to added antioxidants. Such knowledge is extremely important for lubricant formulators

when comes to the selection of an appropriate antioxidant system for a given oil. Figure 1.22 shows

the RPVOT test results of four base oils with and without the presence of an antioxidant. Each oil

FIGURE 1.22 RPVOT results of HP and ADPA in API groups I–IV base oils.

200

400

600

800

1000

1200

1400

1600

1800

API group I API group II API group III API group IV

RPVOT OIT (min)

No AO

ADPA

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38 Lubricant Additives: Chemistry and Applications

represents an API group from I to IV. The HP and the ADPA are the same as before. Clearly, with-

out the protection of antioxidant, all oils performed equally poor. A 0.5 wt% of the HP antioxidant

gave modest levels of protection that marginally increase from API group I to IV. When the base

oils were treated with the same level of the ADPA, a drastic performance boost is seen across the

board. The performance responses of the highly re ned groups II, III, and the group IV to the added

ADPA appear to be particularly strong.

The superior antioxidant response of the groups II and III base oils over the conventional group I

base oils may be attributed to the removal of aromatic hydrocarbons and polar constituents and the

large presence of saturated hydrocarbons in the oils [243,244]. One school of thought hypothesized

that oxygen-, sulfur-, and nitrogen-containing polar species may exist in the form of micellar aggre-

gations in base oil. When an antioxidant is added, some of the natural polar molecules may interact

physically and chemically with the additives, leading to a reduction in additive effectiveness in

some circumstances. In those highly re ned base oils where the natural compounds are essentially

low or absent, the added antioxidant is able to exert its maximum effectiveness [245].

ZDDP, another important class of antioxidant/antiwear agent, has been studied by others, and

the results indicated that its antioxidant performance is dependent on the base oil aromatics, alkyl-

substituted aromatics, average chain length of hydrocarbons, and the relative presence of normal

paraf ns and isoparaf ns [196]. In group I base oils, ZDDP gave good responses to highly saturated

hydrocarbons characterized with normal paraf ns having shorter chain length. Isoparaf ns were

found to decrease the antioxidant activity of ZDDP due to the steric hindrance of the side chains,

which restricts the additive molecules from interacting with the hydrocarbons. In oils with higher

monoaromatic hydrocarbons, ZDDP tends to perform better, which was believed to be related to

improved solvency.

1.14 FUTURE REQUIREMENTS

The need for antioxidants in future lubricants will continue and the demands for both quality and

quantity will likely to increase to meet new environment and performance requirements. Although

such trend is inevitably to take place in the entire lubricant industry, three speci c areas may see

more rapid and dynamic advancements: (a) modern engine oils, (b) biodegradable lubricants, and

New engine designs and engine oil formulations are being frequently rolled out. The primary

driving force is environmental in nature: the requirements for less oil consumption, better fuel econ-

omy, extended drain intervals, and lower emissions (particulates, hydrocarbons, CO, and NO

). To

meet these requirements, new automotive engines are designed lighter and smaller but are required

to operate under more severe conditions for higher output and speeds, which lead to higher engine

and oil temperatures. It has been well established that every 10ºC of temperature increase will

approximately double the rate of oil oxidation. Therefore, to maintain satisfactory service lifetime

in a more severe service environment, increasing the level of antioxidant and using those suitable for

high-temperature conditions, such as aminic antioxidants, are expected. Modern catalytic converters

are highly effective in reducing the emissions. However, they are vulnerable to the deterioration

effects of sulfur, phosphorus, and ash derived from engine oils and fuels. Accordingly, initiatives

have been made to reduce the ZDDP content in engine oils. The current ILSAC GF-4 speci cation

has limited the sulfur and phosphorus levels to a maximum of 0.7 and 0.08 wt%, respectively, and

these numbers are likely to be even lower for future engine oils. With the ZDDP being reduced, it

is expected that the uses of ashless, primary antioxidants as well as secondary antioxidants (as a

substitute for ZDDP) will increase.

To meet the increasing performance requirements set for modern engine oils, high- quality

groups II and III base stocks are emerging to replace the conventional solvent-re ned group I oils.

The hydrotreating and isodewaxing processes that are used to make these oils signi cantly lower

the unsaturated hydrocarbons and polyaromatics, which contribute to poor oxidative stability.

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Antioxidants 39

However, the naturally occurring sulfur species that can function as antioxidants have also been

largely removed. Previous discussion has clearly demonstrated that the superior oxidative stability

of these oils can only be realized when an appropriate synthetic antioxidant is used. Therefore, as

lubricant formulators increase the use of hydrotreated and synthetic base stocks, the requirements

for antioxidants in lubricants are expected to rise.

The environmental and toxicity issues of petroleum-based oils as well as their rising cost related

to a global shortage have led to renewed interest in the use of vegetable oils, such as soybean

oil, canola oil, sun ower oil, and coconut oil as lubricants and industrial  uids. The industry has

already seen vegetable oils being utilized to make environmentally friendly automotive engine

oils, two-cycle engine oils, hydraulic  uids, total loss lubricants, and marine lubricants. Vegetable

oils generally possess some excellent lubrication properties, for example, good inherent lubricity,

low volatility, high viscosity index, excellent solvency for lubricant additives, and easy miscibility

with other  uids. However, vegetable oils are known for their poor oxidative stability as compared

to mineral oils. Research has found that typical soybean oil formed polymers at a rate an order of

magnitude faster than mineral oils [246]. To overcome this drawback, it is expected that vegetable

oil–based lubricants will need more antioxidants to meet the performance requirements set for

mineral oil–based lubricants. Owing to the unique hydrocarbon composition of vegetable oils, antioxi-

dant response in these new  uids will be different from mineral oils. Recent work [218] has indicated

that ADPAs that have proven track of high-temperature performance in mineral base oils are not as

effective in stabilizing soybean oil at elevated temperatures (e.g., 170°C). In addition, environmentally

friendly lubricants need to use additives that satisfy biodegradation and bioaccumulation standards.

Antioxidants suitable for mineral oils may become problematic for use in vegetable oil–based lubri-

cants. As such, the development of new classes of biodegradable antioxidants may be needed.

The increasing use of biofuels such as biodiesel methyl esters derived from oil seeds, animal

fats, and reclaimed cooking oils, represents a new challenge to the stabilization of engine oils that

operate on such fuels. Recent work [247] has discovered that biodiesels of various vegetable sources

(canola, soybean, palm oils, coconut, etc.) promote the oxidation of in-service engine oils even at a

low dilution level, primarily shortening inhibited period, and leading to rapid oil viscosity increase.

Further aggravating the situation is the high and narrow boiling points of biodiesels, which make

them more persistent than mineral diesels after entering the crankcase. For an engine oil to perform

satisfactorily with these new fuels, antioxidant level has to be maintained at an effective level to

counteract the strong degradation impact from the biofuels.

Overall, future lubricants will favor antioxidants having high-performance, cost-effective, ashless,

multifunctional, and environmentally-friendly attributes. The ultimate driving force is environmental in

nature while taking into account the emerging base oil technologies and performance speci cations.

1.15 COMMERCIAL ANTIOXIDANTS

Product Company Chemistry

Ethanox

310 Albemarle Tetrakismethylene (3,5-di-t-butyl-4-hydroxyhydrocinnamate)methane

Ethanox 323 Albemarle Nonylphenol disulfur oligomer

Ethanox 376 Albemarle 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C

alkyl ester

Ethanox 4701 Albemarle 2,6-Di-t-butyl phenol

Ethanox 4702 Albemarle 4,4′-Methylene bis(2,6-di-t-butyl phenol)

Ethanox 4703 Albemarle 2,6-Di-t-butyl-alpha-dimethylamino-p-cresol

Ethanox 4716 Albemarle 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C

–C

alkyl ester

Ethanox 4733 Albemarle Mixture of mono-, di-, and tri-t-butyl phenols

Ethanox 4735 Albemarle Mixture of t-butyl phenols

Ethanox 4755 Albemarle Boron containing derivatives of Ethanox 4702

Ethanox 4872J Albemarle Multiring t-butyl phenol, 53% active

(Continued )

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40 Lubricant Additives: Chemistry and Applications

Continued

Product Company Chemistry

Ethanox 4827J Albemarle Multiring t-butyl phenol, 30% active

Ethanox 4777 Albemarle Alkylated diphenylamine

Additin

7010 Rhein Chemie Oligomerized 1,2-dihydro-4-trimethylquinoline

Additin 7130 Rhein Chemie Phenyl-alpha-napthylamine

Additin 7110 Rhein Chemie 2,6-Di-t-butyl-p-cresol

Additin 7120 Rhein Chemie 2,6-Di-t-butyl phenol

Additin 7115 Rhein Chemie Phenol derivative sterically hindered

Additin 7135 Rhein Chemie Styrenated diphenylamine

Naugalube

15 Chemtura 2,2′-Thiodiethylene bis(3,5-di-t-butyl-4-hydroxyphenyl)propionate

Naugalube 16 Chemtura 4,4′-Thiobis(2-t-butyl-5-methyl phenol)

Naugalube 18 Chemtura 2,2-Thiobis(4-methyl-6-t-butyl phenol)

Naugalube 22 Chemtura Mixture of t-butyl phenol

Naugalube 32 Chemtura Tetrakismethylene (3,5-di-t-butyl-4-hydroxyhydrocinnamate)methane

Naugalube 37 Chemtura 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C

alkyl ester

Naugalube 38 Chemtura 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C

13–15

alkyl ester

Naugalube 531 Chemtura 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C

–C

alkyl ester

Naugalube 438 Chemtura Dioctyl diphenylamine

Naugalube 438L Chemtura Dinonyl diphenylamine

Naugalube 635 Chemtura Styrenated diphenylamine

Naugalube 640 Chemtura Butylated-, octylated-diphenylamine

Naugalube 680 Chemtura Octylated-, styrenated-diphenylamine

Naugalube AMS Chemtura Alpha-methystyrenated DPA

Naugard

PANA Chemtura Phenyl-alpha-napthylamine

Naugalube APAN Chemtura Alkylated PANA

Naugalube TMQ Chemtura Oligomerized 1,2-dihydro-4-trimethylquinoline

Naugalube 403 Chemtura N,N′-di-sec-butyl-p-phenylenediamine

Naugalube TPP Chemtura Triphenyl phosphite

Irganox

L 01 Ciba Dioctyl diphenylamine

Irganox L 06 Ciba Octylated PANA

Irganox L 57 Ciba Butylated, octylated diphenylamine

Irganox L 67 Ciba Dinonyl diphenylamine

Irganox L 101 Ciba Tetrakismethylene (3,5-di-t-butyl-4-hydroxyhydrocinnamate)methane

Irganox L 107 Ciba 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C

alkyl ester

Irganox L 109 Ciba Hindered bis-phenol

Irganox L 115 Ciba 2,2′-Thiodiethylene bis(3,5-di-t-butyl-4-hydroxyphenyl)propionate

Irganox L 118 Ciba High MW liquid hindered phenolic with thioether

Irganox L 135 Ciba 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C

–C

alkyl ester

Irganox E 201 Ciba Liquid di-alpha-tocopherol (vitamin E)

Irgaphos

168 Ciba Tri-(di-t-butylphenyl) phosphite

Vanlube

AZ RT Vanderbilt Zinc diamyldithiocarbamate in oil

Vanlube EZ RT Vanderbilt Zinc diamyldithiocarbamate and diamyl ammonium

Diamyldithiocarbamate

Vanlube NA RT Vanderbilt Nonylated, ethylated diphenylamine

Vanlube PCX RT Vanderbilt 2,6-Di-t-butyl-p-cresol

Vanlube RD RT Vanderbilt Oligomerized 1,2-dihydro-4-trimethylquinoline

Vanlube SL RT Vanderbilt Octylated, styrenated diphenylamine

Vanlube SS RT Vanderbilt Octylated diphenylamine

(Continued )

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Antioxidants 41

Continued

Product Company Chemistry

Vanlube 81 RT Vanderbilt Dioctyl diphenylamine

Vanlube 7723 RT Vanderbilt Methylene bis(dibutyldithiocarbamate)

Vanlube 869 RT Vanderbilt Zinc dithiocarbamate/sulfurized ole n blend

Vanlube 8610 RT Vanderbilt Antimony dithiocarbamate/sulfurized ole n blend

Vanlube 887 RT Vanderbilt Tolutriazole compound in oil

Vanlube 887E RT Vanderbilt Tolutriazole compound in ester

Vanlube 9317 RT Vanderbilt Organic amine in synthetic ester

Vanlube 961 RT Vanderbilt Butylated, octylated diphenylamine

Vanlube 996E RT Vanderbilt Methylene bis(di-butyl-dithiocarbamate) and tolutriazole derivative

1.16 COMMERCIAL METAL DEACTIVATORS

Product Company Chemistry

Ethanox

4705 Albemarle N,N-disalicylidene-1,2-diaminopropane

Irgamet

30 Ciba Triazole derivative

Irgamet 39 Ciba Tolutriazole derivative

Irgamet 42 Ciba Water-soluble tolutriazole derivative

Irgamet BTZ Ciba Benzotriazole

Irgamet TTZ Ciba Tolutrizole

Cuvan

303 RT Vanderbilt N,N-bis(2-ehtylhexyl)-ar-methyl-1H-benzotriazole-1-methanamine

Cuvan 484 RT Vanderbilt 2,5-Dimercapto-1,3,4-thiadiazole derivative

Cuvan 826 RT Vanderbilt 2,5-Dimercapto-1,3,4-thiadiazole derivative

NACAP

RT Vanderbilt Sodium 2-mercaptobenzothiazole, 50% active

ROKON

RT Vanderbilt 2-Mercaptobenzothiazole

Vanchem

NATD RT Vanderbilt Disodium, 2,5-dimercaptothiadiazole, 30% active

Vanlube 601 RT Vanderbilt Heterocyclic sulfur–nitrogen compound

Vanlube 601E RT Vanderbilt Heterocyclic sulfur–nitrogen compound

Vanlube 704 RT Vanderbilt Proprietary blend

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