Oxidation is the most predominant reaction experienced by a lubricant in service, accounting for significant lubricant problems. Oxidation is the major source for viscosity increase, varnish formation, sludge and sediment formation, additive depletion, base oil breakdown, filter plugging, loss in foam properties, acid number increase, rust and corrosion. Therefore, understanding and controlling oxidation is a major concern to the lubricant chemist.

This article discusses oxidation - its sources, reactions and effects.

Overview
More than 140 years ago, A.W. Hoffman1 recognized that the deterioration of rubber under atmosphere condition was

caused by oxygen. This aging process was the first recorded observation of the process of oxidation. Oxidation was defined as a reaction involving combination with oxygen. However, its definition has now been expanded to include any reaction in which electrons are transferred from a molecule. Iron rusting is a form of oxidation, where the reaction is between oxygen and iron. Combustion is another rapid form of oxidation of a hydrocarbon producing water and carbon dioxide. In the case of slower reactions of a hydrocarbon, the typical final product of oxidation is an acid. Hydrocarbon oxidation to an acid involves complex steps where many different compounds are produced.2

The oxidation of a hydrocarbon involves three basic steps: initiation, propagation and termination.

BestPractices_Figure1.gif

Figure 1

Initiation
The faster this step occurs, the higher the degree of oxidation the lubricant will experience. The initiation step consists of the formation of a free radical. A free radical (R) is a molecular fragment having one or more unpaired electrons accessible to easily react with other hydrocarbons (RH). Free radicals are usually short-lived and highly reactive.

There are several sources of free radicals in lubricants. The most predominant is oxygen itself; however, free radicals can also be produced from nitro-oxides (nitrogen dioxide, nitric and nitrous oxide), ultraviolet (UV)-light, pressure-induced thermal degradation and dieseling, and flow electrification (electrostatic discharge).

The types of initiation reactions include:

RH + O2 → R• + HOO•                   (1)
Hydrocarbon   Oxygen     Alkyl Radical    HydroperoxiRadical

R-R + UV-Light → R• + R•                  (2)
Hydrocarbon                          Alkyl Radical    Alkyl Radical

R-R + Electrolytic → R• + R•                 (3)
Hydrocarbon                          Alkyl Radical     Alkyl Radical

In these equations, RH and R-R represent hydrocarbons that are part of the base oil or additives in the lubricant, while R• and HOO• are the free radicals produced. These reactions are relatively slow at room temperature, accounting for the slow rates of lubricant oxidation when the oil is cool. However, like many reactions, the rates significantly increase as the oil temperature increases.

Reaction (1) is oxidation with oxygen.2 If the lubricant is left exposed to sunlight, it has the availability to absorb UV-radiation. This UV-radiation will also initiate oxidation reactions (2).3 This is why one can observe discoloration after a lubricant is exposed to sunlight. This reaction is slower than the traditional oxygen-sourced oxidation due to the low intensity of the UV-radiation. Initiation can also occur as a part of thermal cracking. Thermal cracking occurs in the hot zones of the fluid, but also as a part of micro-dieseling (pressure-induced thermal degradation and dieseling). Reaction (3) results from the sparking observed in filters in turbine and hydraulic applications.4

As these initiation reactions proceed, the concentration of peroxides (ROOH and HOOH) will increase. This leads to a secondary initiation scheme where the peroxides are the source of the free radicals:

ROOH → RO• + HO•           (4)
Alkyl Hydroperoxide  Alkyloxy Radical   Hyrdoxy Radical

The cleavage of peroxide to its free radicals usually requires elevated temperatures; however, it can occur at lower temperatures with the aid of a catalyst, such as the wear metal ion. Thus, the increasing presence of wear metals, often caused by oxidation reaction products, will catalyze the amplified formation of free radicals producing further oxidation:

ROOH + FE+3 → ROO• + H++ FE+2     (5)
Alkyl Hydroperoxide  Ferric Iron  Alkylperoxy Radical Acid  Alkyl Radical

Preventing or slowing the initiation reactions is one method of controlling oxidation. Thus, blocking the initiation sources one at a time and looking for these potential root causes can be a good direction toward lowering oxidation. For example, if one desires to control oxygen - nitrogen blanketing, restricting air entrainment, good antifoam control, or limiting thin film lubricants exposed to open-air conditions - all can control this source. Other sources, such as controlling UV-light exposure, preventing hot spots and controlling filter sparking can also help.

Propagation and Branching
After a free radical has formed, it is capable of propagating the oxidation sequence. This propagation step involves the formation of additional alkyl- or peroxy-radicals and hydroperoxides (Reactions 6 and 7), which will continue this cycle of radical formation.

R• + O2 → ROO•                 (6)
Alkyl Radical Oxygen  AlkylperoxyRadical

ROO• + RH → R• ROOH                (7)
AlkylperoxyRadical Hydrocarbon   Alkyl Radical  Alkyl Radical

Reaction (6) is a rapid reaction to produce the peroxy-radical (ROO•). This peroxy-radical becomes available to react with the base oil or additives in the lubricant to regenerate the alkyl-radical (R•) and restart the cycle (Reaction 7). In addition to the alkyl-radical produced in this cycle, the hydroperoxide (ROOH) can also react with the lubricant (Reactions 4 and 8) to start the production of oxidation compounds - alcohol (ROH) and water (H2O).

RO• + RH → ROH R•              (8)
Alkyloxy Radical Hydrocarbon   Alcohol  Alkyl Radical

The decomposition reaction (Reaction 4) of the hydroperoxide to produce oxy-radicals usually requires higher temperatures to occur at rapid levels (usually 120°C or greater). However, this initiation temperature will be significantly lowered by the presence of metals (Reaction 5) like iron or copper to catalyze the reaction. Therefore, it can occur even at lower temperatures, but at a much slower rate. This is one of the reasons one should be concerned about the lubricant's operating temperature.

Radical decomposition and radical transfer (migration of the radical from one molecule to another) can often be the source of additional oxidation-related products. The most common form of radical decomposition is the source of ketones and aldehydes (Reaction 9).

RR'R"C-O• → RR'C=O + R•           (9)
Alkyloxy Radical                      Ketone         Alkyl Radical

2RC(O)R + 2O2 → 4RC(O)OH           (10)
Ketone                       Oxygen                 Carboxylic Acid

Termination and Antioxidants
The termination step of this oxidation mechanism stops the cycle. The more effective this step becomes, the less oxidation of the lubricant. For this reason, antioxidants are formulated into the lubricants. Antioxidants break into the propagation step of the oxidation mechanism and terminate the process by formation of stable radicals. There are several types of antioxidants:

  • UV absorber
  • peroxide decomposer
  • chain breaking-electron donor
  • chain breaking-electron acceptor

Each of these helps stop the propagation cycle in different ways (Figure 1).

The most common antioxidants (sometimes called primary antioxidants) are the phenolic or aromatic amine types. These are defined as chain-breaking types, because the mechanism for their reaction involves the antioxidant absorbing a free radical to form a stable radical.

ROO• + AH → ROOH + A•            (11)
Alkylperoxy Radical Antioxidant Hydrocarbon Alkyl Hydroperoxide Antioxidant Radical

ROO• + A• → Inert Products           (12)
Alkyl Peroxy RadicalAntioxidant Radical

These primary antioxidants typically have the capability of stopping more than one free radical. The oxidation reaction rates, or ease of reaction, with these antioxidants are typically faster than that with a base oil molecule or other additive, thus allowing them to help protect the lubricant.

Other antioxidant types can also terminate the oxidation propagation. The next most common type is the peroxide decomposer, or secondary antioxidant. Sulfur and phosphorous chemistries typically fall into this category. These include phosphates and thiophosphates (such as ZDDP, alkyl phosphites and alkyl phosphates) or sulfides or polysulfides (such as phenothiazines, dithiocarbamates and sulfurized isobutylenes). The mechanism of these peroxide decomposers is to destroy the peroxides or hydroperoxides into alcohols or water.

ROOH• + RSR → ROH + RS(O)R          (13)
Alkyl Hydroperoxide   Alkyl Sulfide   Alcohol  Alkyl Sulfoxide

It is often said that the peroxide oxidation reaction doesn't occur above about 120°C. This is because it takes the energy of this elevated temperature to cleave the peroxide molecule. Temperature has two effects on any reaction. First, there is a threshold of energy that the reaction needs to occur. If there isn't enough energy in the system to push the reaction up this threshold, it will not occur. This reaction is the peroxide cleavage (H-abstraction of the propagation cycle). Second, if a reaction can occur, then it will about double in rate every 10°C (18°F) in temperature.

When looking at the lubricant temperature, keep in mind that for a sump temperature to be at 120°C, there must be a zone that is hotter to heat the fluid in this sump. The oxidation can be propagated in the hotter zones and the resultant deposits formed in the colder zone. The kinetics of this multiple zone reaction conditions, or flow reactor kinetics, should always be considered instead of a closed reactor model.5 To control the oxidation propagation step, one should therefore study the hotter zones and not just the lower temperature of the sump area.

Ester Formation
In the used oil environment, major reaction products for oxidation are ester. Some of the original FTIR peak height measurements utilized the ester band as the measurement of oxidation. These products are formed by both a primary free radical reaction mechanism (Baeyer-Villiger rearrangement6 - Reaction 14) and a secondary nonradical mechanism (esterification - Reaction 15).

                                                     Acid
ROOH + RC(O)R → ROH + RC(O)OR   (14)
Alkyl Hydroperoxide  Ketone    Alcohol   Ester

ROH + RC(O)OH → RC(O)OR+H2(15)
Alcohol              Carboxylic Acid         Ester        Water

The Baeyer-Villiger reaction typically occurs in the hot reaction zone, where the peroxide can readily decompose to its free radicals. It is catalyzed by the carboxylic acid previously formed in the oxidation cycle. The esterification reaction6 can occur in the hot zone; however, it can also occur in the colder zone.

Condensation/Polymerization
As the oxidation reactions progress, enough material is produced to start additional side reactions. Many of these side reactions are the cause of the increase in viscosity observed from oxidation. Increase in viscosity is the result of increases in insoluble products or high molecular weight products.

As oxidation products increase in size and polarity, the solubility of these materials decreases. The polarity of an organic hydrocarbon increases as oxygen atoms are combined into the molecule. The solubility of polar compounds is poor in nonpolar solutions. If the oxidation components become insoluble, the existence of these poorly soluble materials slow down the Newtonian flow properties of the liquid by adding thickening agents to the liquid - thus increasing the viscosity.

The increase in molecular-sized products (known as oligomers or low molecular polymers) are formed via reactions called Aldol and Claisen condensation reactions.6, 7

                        Acid
3RCH2C(O)CH2R → (RCH2)2C = C(R)C(O)C(R) = C(CH2R)2 + 2H2 (16)
Ketone   Unsaturated Aldol condesation product                       Water

                          RO•
                  AlkyloxyRadical
2RCH2C(O)OR → RCH2C(O)CH(R)C(O)R + ROH   (17)
            Ester            Condesation product           Alcohol

As shown in these reactions, the molecules are joined in the condensation reactions and grow in size. As an additional complication, the unsaturated Aldol products (Reaction 16) can also polymerize, similar to the ethylene reaction to polyethylene, when initiated by the free radicals from the oxidation propagation sequence. These molecules will continue to oligomerize (grow in size) as the oxidation process is allowed to continue - growing into high molecular weight compounds that increase viscosity.

All these polar materials will eventually grow big enough to have poor solubility in the nonpolar hydrocarbons making up the lubricants - thus forming one of the sources of the insoluble materials. As the oxidation process continues, these reactions become increasingly more abundant - to the point where severe oxidation and solidification can occur.

Varnish, sludge, deposit formation and viscosity increase are the ultimate result of these condensation reactions. The reactions are not limited to the base stock hydrocarbons only. The additives are also subjected to these oxidation reactions. Additives formulated to control sludge or varnish, when attacked by oxidation, can lose their effectiveness and become a source of deposit formation.

Oxidation of a hydrocarbon fluid such as a lubricant is a complex mechanism. It has been studied for more than 140 years - but still is not completely understood. As an overall problem to the lubricant, oxidation can be considered a significant source of lubricant failures.

Oxidation is a major source for viscosity increase, acid number increase or corrosion, additive depletions, dispersant failures, base oil deterioration, varnish and sludge formation, filter plugging, oil darkening, as well as many of the wear root causes. For this reason, there have been many tests developed to evaluate the oxidation state of the lubricant. Some of these tests look at the potential lifetime of the lubricant, while others look at the results of the oxidation. None of these tests can fully predict or explain the full array of problems the lubricant experiences from oxidation.

Understanding how oxidation is controlled, measured and potentially stopped can be a major step in the root cause analysis program for any lubricant system.

References

1. A. Hoffman. J. Chem. Soc., 13, 87. 1861.

2. G. Scott. Atmospheric Oxidation and Antioxidants. New York, Elsevier Publishing Company, 1965.

3. C. Migdal. "The Mechanism of Antioxidant Action in Lubricants." 4th Annual Fuels and Lubes Asia Conference, January 1998.

4. A. Sasaki and S. Uchiyama. "Production of Free Radicals and Oil Auto-oxidation due to Spark Discharge of Static Electricity." Proceedings of the 48th National Conference on Fluid Power, Vol. 1, April 2000, I00-9.13.

5. Y. Seijiro, Y. Maeda and T. Meade. Ind. Eng. Chem. Prod. Res. Dev., 20, 530 and 20, 536. 1981.

6. J. March. Advanced Organic Chemistry: Reactions, Mechanism and Structure. New York: McGraw-Hill, Inc. 1968.

7. V. Gatto, W. Moehle, T., Cobb and E. Schneller. Oxidation Fundamentals and its Application to Turbine Oil Testing. ASTM Symposium on Oxidation and Testing of Turbine Oils, December 2005.