Tagged: lubrication

Testing Methods for Detecting Antioxidants in Lubricants

Since we now have more information about the various types of antioxidants and how they function to suppress oxidation, the next step is to determine whether they are indeed present in our finished lubricants.

The industry tries to identify the presence of antioxidants in a couple of ways. The first way is to measure the rate of oxidation, which does not give the exact value of remaining antioxidants. Instead, it gives the user an idea of how much oxidation has taken place based on other lubricant characteristics.

Then, the user must make an informed decision on the remaining life of the oil. On the other hand, there is one direct test to determine which antioxidants are present in the oil and provide their remaining quantity.

Some common tests in the industry that measure the oxidation rate include RPVOT (Rotating Pressure Vessel Oxidation Test), Oxidation via FTIR, Viscosity, and TOST (Turbine Oil Oxidation Stability Test).

While none of these actually quantify the remaining antioxidants in the oil, they all provide the user with an indication of the rate of oxidation currently occurring in the oil. We will dive deeper into these to understand how they assess oxidation rates.

RPVOT

In the industry, RPVOT has been used for decades to provide users with an idea of the rate of oxidation occurring in their oils. However, this test is performed where the sample is placed in a sealed container with pressurized pure oxygen and rotated at a high speed in a bath with a higher temperature to promote the oil’s oxidation⁸.

As oxidation occurs, there is a pressure drop in the vessel, and the rate of this pressure drop is compared to that of new oil. The final result is given in minutes. Typically, if the value falls below 25% of the original value, the oil is on its way out or almost at the end of its remaining useful life.

But what happens if the value is at 75%? Since the final result is given in minutes, it isn’t easy to correlate that value to a value in the field.

For instance, if the RPVOT result was 800 minutes, we cannot easily correlate that to a particular number of years or months of life remaining for the oil. Hence, this method does not truly measure the remaining antioxidants in the oil.

Oxidation via FTIR

Another way of measuring oxidation is by identifying its presence through FTIR (Fourier Transform Infrared) Spectroscopy. In this type of test, each element produces a unique fingerprint.

As such, oxidation produces a particular peak between 1600-1800 cm-1. There is no absolute reference for oxidation peaks; therefore, these are usually compared against the new oil samples⁹.

This test (ASTM D 7414) is usually used for engine oils rather than industrial oils. However, it still does not provide the user with the remaining antioxidants in the oil. Instead, only that oxidation has already occurred.

Viscosity

In the past, viscosity was usually cited as a method for detecting if oxidation was occurring in the oil. However, due to more recent discoveries and technological evolution, we have noted that the oil’s viscosity only increases after oxidation has occurred.

Therefore, it is not a valid test to identify if oxidation is happening in the oil, as there can be several reasons for the increase in viscosity. In the case of oxidation, the presence of varnish and sludge would account for this increase; however, this test still doesn’t indicate the remaining antioxidants in the oil⁹.

TOST

This test, developed in 1943, evaluates the oil after it is subjected to very specific conditions. Typically, the oil is stressed with high temperatures (203°F / 95°C), gross contamination (17% water), and substantial air entrainment in the presence of iron and copper catalysts⁵.

The oil’s life is measured by the time the sample takes to achieve an Acid number of 2 mg KOH/g.

As such, this test measures the amount of acid produced by an oil under extreme conditions. Again, it does not give us a quantifiable correlation to the oil’s actual field life. It must also be noted that this test is not suited for hydraulic or gear oils but is more tailored to steam turbine oils, which may undergo those simulated conditions.

RULER® test

The RULER test utilizes linear sweep voltammetry to detect the quantity of antioxidants remaining in the oil. It produces a graph showing peaks at the detected antioxidants. Typically, the used oil results are compared against the baseline data to determine the quantity of antioxidants in the oil (Fluitec, 2022).

This method allows users to specifically quantify and trend the decline of antioxidants in the oil over a period of time, as seen in Figure 2. This is very valuable as users can now provide a better estimate of the remaining useful life based on the trend of the decline of antioxidants, and by extension, this helps them to understand the health of their oil.

Figure 2: RULER graph of a standard bearing oil vs. used bearing oil.

Oxidation occurs worldwide on almost every item (food, the human body, and lubricants). It is not going away anytime soon. Hence, there will always be a need for antioxidants to help protect the base oils for finished lubricants.

However, the formulations of these antioxidants may evolve over time as scientists find new, more sustainable alternatives for creating antioxidants.

OEMs also have a role to play as they advance machines and their capabilities; lubricants will have to be engineered for these new applications with varying environmental conditions. As such, there may be a greater need for more advanced antioxidants to help protect the oil.

References

Bruce, R. (2012). Handbook of Lubrication and Tribology Volume II Theory and Design. Boca Raton: CRC Press.
(2022, July 20). Why Choose RULER? Retrieved from Fluitec: https://www.fluitec.com/why-choose-ruler/
Livingstone, G. (2024, February 15). Varnish Deposits in Bearings, Causes, Consequences and Cures. Retrieved from Precision Lubrication Magazine: https://precisionlubrication.com/articles/varnish-deposits-in-bearings-causes-consequences-and-cures/
Mang, T., & Dresel, W. (2007). Lubricants and Lubrication. Weinheim: WILEY-VCH Verlag GmbH & Co. KGaA.
(2016, April 17). Oil Oxidation Stability Test. Retrieved from Mobil: https://www.mobil.com/en/lubricants/for-businesses/industrial/lubricant-expertise/resources/oil-oxidation-stability-test
Mortier, R. M., Fox, M. F., & Orszulik, S. T. (2010). Chemistry and Technology of Lubricants. Dordrecht: Springer.
Pirro, D. M., Webster, M., & Daschner, E. (2016). Lubrication Fundamentals, Third Edition, Revised and Expanded. Boca Raton: CRC Press.
(2024, March 06). Oxidation, the oil killer. Retrieved from SKF: https://www.skf.com/us/services/recondoil/knowledge-hub/recondoil-articles/oxidation-the-oil-killer
Spectro Scientific. (2024, March 06). Measuring Oil Chemistry: Nitration, Oxidation, and Sulfation. Retrieved from Spectro Scientific: https://www.spectrosci.com/en/knowledge-center/test-parameters/measuring-oil-chemistry-nitration-oxidation-and-sulfation
Stachowiak, G. W., & Batchelor, A. W. (2014). Engineering Tribology. Butterworth-Heinemann.

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Lubricant Degradation

Types of Antioxidants

Different additives have successfully suppressed the degradation of finished lubricants^{6, 10}. These include:

Radical scavengers/inhibitors, also called propagation inhibitors
Hydroperoxide decomposers
Metal deactivators
Synergistic mixtures

Each of the above listed performs in a particular way to reduce the oxidation process in the lubricant.

Radical Scavengers

Many applications use radical scavengers as their preferred antioxidant. Radical scavengers are also known as primary antioxidants, as they are the first line of defense in the oxidation process. These are phenolic and aminic antioxidants.

They neutralize the peroxy radicals used during the initiation reaction to generate hydroperoxides⁴. These neutralized radicals form resonance-stabilized radicals, which are very unreactive and stop the propagation process.

Some examples of these additives are¹: diarylamines, dihydroquinolines, and hindered phenols. While they may be known as simple hydrocarbons, they are often characterized by low volatility, used in quantities of 0.5-1% by weight, and have long lifetimes¹⁰.

These primary antioxidants are usually very effective at temperatures below 200°F (93°C)⁷. At these temperatures, oxidation occurs at a slower rate. These primary antioxidants are typically found in applications involving turbines, circulation, and hydraulic oils intended for extended service at these moderate temperatures.

Hydroperoxide Decomposers

The role of hydroperoxide decomposers is to neutralize the hydroperoxides used to accelerate oxidation. Radical scavengers (primary antioxidants) neutralize the free radicals, and these hydroperoxides are formed after the initiation stage. As such, hydroperoxide decomposers are known as secondary antioxidants.

These decomposers convert the hydroperoxides into non-radical products, which prevent the chain propagation reaction. ZDDP is one example of this type of decomposer, although organosulphur and organophosphorus additives have been used traditionally.

Metal Deactivators

Metal deactivators are usually derived from salicylic acid¹⁰. These function by entraining a metal such as copper or iron to inhibit oxidation acceleration. However, when the operating temperatures exceed 200°F (93°C), this is the stage at which the catalytic effects of metals begin to play a more important role in accelerating oxidation⁷.

Therefore, during these conditions, an antioxidant that can reduce the catalytic effect of the metals should be used, such as metal deactivators. These react with the surfaces of metals to form protective coatings. One such example is zinc dithiophosphate (ZnDTP), which can also act as a hydroperoxide decomposer at temperatures above 200°F (93°C).

Ethylenediaminetetraacetic acid is another commonly used example of this type of antioxidant.

Synergistic Mixtures

As explained above, there are different types of antioxidants, but one thing remains the same: They can work better when they work together. Some antioxidants can work in a synergistic manner to provide added protection to a finished lubricant. There are two types of synergism: homosynergism and heterosynergism.

Homosynergism occurs when two different types of antioxidants can be classed under the same category. One example is the use of two different peroxy radical scavengers. These both operate by the same stabilization mechanism but differ slightly in formulation.

On the other hand, heterosynergism is more common and often seen with aminic and phenolic antioxidants. Aminic antioxidants are primary antioxidants and radical scavengers, while phenolic antioxidants are secondary antioxidants and hydroperoxide decomposers.

In this case, the amnic antioxidants react faster than phenolic antioxidants, and as such, they are used up when the lubricant undergoes oxidation. However, the phenolic antioxidant regenerates a more effective aminic antioxidant, which helps suppress the rate of oxidation.

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Lubricant Degradation

How Antioxidants Combat Oxidation in Lubricants

As the name suggests, antioxidants prevent oxidation; thus, it is no surprise that they are also called Oxidation inhibitors. During the refining of the base oil, the natural antioxidants are typically stripped away.

Thus, additional antioxidants must be added to the finished lubricant to ensure it does not oxidize as quickly. It is important to note that antioxidants can reduce the amount of oxidation that occurs but will not stop it completely.

Some of the natural antioxidants in base oils include polycyclic aromatics and sulphur and nitrogen heterocyclics⁶.

During oxidation, acids, and peroxides are typically produced. Antioxidants added to finished lubricants usually contain hindered phenols and ZDDPs (zinc dialkyldithiophosphates). These will suppress the formation of the acids produced in these reactions.

Dedicated antioxidants are amines, phenols, and sometimes ZDDP, which also function as an antiwear additive. Antioxidants account for 3-7% of European and North American diesel and gasoline additive packages for finished lubricants⁶.

As mentioned earlier, antioxidants are not the only additives that have a role to play in protecting the equipment. Antioxidants often work together with detergents to help prevent corrosive wear, especially in engines.

Some moisture and acidic combustion by-products can enter the engine during oxidation and form acids. Detergents can help to reduce corrosive wear caused by these acids. Alkylphenols are often used as a substrate in the preparation of detergents; as such, they also exhibit some antioxidant properties.

It must also be noted that extreme pressure additives containing sulphur or phosphorus may also suppress oxidation. Contrarily, these additives decompose at very moderate temperatures, so their strength as an antioxidant is not generally promoted¹⁰.

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Lubricant Degradation

Understanding Oxidation: The Basis for Antioxidant Use

When speaking about antioxidants, the first thing that comes to mind is oxidation. This is the primary reason that antioxidants exist: to reduce oxidation. But what is oxidation, and why should there be antioxidants?

Oxidation occurs in everything in life (not just finished lubricants). We see oxidation regularly when we leave certain fruits exposed to the atmosphere (think about cut pears or apples). After being in the elements for some time, they are no longer fresh and have degraded slightly.

A similar reaction occurs during the oxidation of finished lubricants. Greg Livingstone provides an excellent summary of the oxidation process in his article, “Varnish, Deposits in Bearings, Causes, Consequences, and Cures.” The oxidation degradation pathway begins with initiation, where free radicals are formed in the presence of heat, wear metals, water, and oxygen as shown in Figure 1.

Afterward, during propagation, the free radicals form hydroperoxides, which can create oxidation by-products (Alkoxy radicals), eventually leading to high molecular weight oxygenated by-products.

During this process, the free radicals can also react with primary antioxidants, or the hydroperoxides can react with secondary antioxidants to slow these reactions. However, they will still form the high molecular weight oxygenated by-products once depleted.

Next in the termination phase is polymerization and agglomeration, followed by the physical and chemical changes to the lubricant. It must be noted that there are various stages to oxidation, and typically, when we see sludge or varnish, oxidation has already occurred.

Figure 1: Summary of the oxidation process.

When oxidation occurs, the oil quickly loses its antioxidants; they can no longer protect the oil. As such, the oil begins to undergo physical changes where sludge and varnish appear, and viscosity usually increases. These oils also experience a rise in acid production after these reactions occur.

Now that we have a better understanding of oxidation, whereby the antioxidants are deployed to help reduce the oxidation rate, we can dive deeper into the world of antioxidants and how they can help fight against oxidation for the finished lubricant.

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Lubricant Degradation

Determining the Root Causes of Oxidation in Lubricants

Finally, we’ve arrived at the point where we can effectively determine the root cause. It is critical that the analyst understands oxidation and has knowledge of the evidence needed before embarking on the root cause journey. As noted in the first part of this article, the question we should ask is, “How could?”.

We hypothesize that oxidation is occurring. In a complete root cause analysis, we should hypothesize the occurrence of all the degradation mechanisms and eliminate them with evidence-based data.

There are two main ways in which oxidation can occur either through the presence of oxygen and temperature over the normal operating temperature of the system or if there is a less-than-adequate presence of antioxidants.

If we follow our line of questioning with the presence of oxygen and temperature and again ask, “How could?” we can get two primary responses. Either there was an air leak in the system, or the system was being pushed beyond its operating limits.

If we further investigate the air leak into the system, we ask, “How could it?” again. There are two main ways: either there are damaged components, or a less-than-adequate system design allows air to enter the system.

If we follow the pathway of investigating “how could” the system be pushed beyond its operating limits, then we can come up with two hypotheses. Either an increase in production was required, or there was a malfunction of the components, which caused strain on the other components.

Both of these hypotheses are physical and can be investigated further, but we will focus on the lubricant aspect of this article. Hence, we will follow the questioning surrounding the less-than-adequate presence of antioxidants.

We begin with the question, “How could we have a less-than-adequate presence of antioxidants?”. From the information gathered in this article, we know this can result from free radicals or less than adequate lubricant specifications.

We will investigate the “Presence of free radicals” first.

“How could we have the presence of free radicals?” Free radicals can emerge as a result of chemical reactions.

“How could these chemical reactions produce free radicals?” There are two main ways in which this can occur. Either the lubricant got contaminated, which introduced catalysts for these chemical reactions, or adverse operating conditions gave rise to these chemical reactions.

Then, we must ask again, “How could we have contamination?” Contamination can occur if leaks are getting into a closed lubrication system or if there is ingress of foreign material from the environment.

Our line of questioning continues when we ask, “How could we have leaks in a closed lubrication system?”. These can result from damaged components or seals allowing leaks into the system or if the system is less than adequately designed.

These are physical attributes of the system, so we will go back to investigating the lubricant aspect.

This is where we get to ask our famous question, “How could we have ingress of foreign material from the environment?”. Ideally, this can be classified in three ways;

There are openings which are allowing materials to enter the system or
Wrong lubricant was placed in the system or
Contaminated lubricant was placed in the system

Let’s investigate all three aspects, starting with the openings allowing foreign material to enter the system. There are two main ways in which this can occur. Either the openings were not closed after use, or the safety latches malfunctioned.

Suppose the openings were not closed after use. In that case, there is a possibility that there were less than adequate inspections to verify that these were closed after use or a less than adequate procedure for the task being completed which required the opening of the hatch.

On the other hand, if the safety latches malfunctioned, this could result from less than adequate checks to verify the functioning of the safety latches.

In these cases, the root causes are not the physical elements but rather the systemic reasons for these procedures not being adequately performed.

Now we investigate the second central hypothesis, “How could the wrong lubricant be placed in the system?” While there are many ways in which this can occur, we have narrowed it down to two main areas.

Either there were less than adequate checks to verify that the technician received the correct lubricant, or there were less than adequate procedures to dispatch the correct lubricant from the warehouse. We will not go further into these two as they are now systemic causes that must be addressed.

Onto the third hypothesis of “How could a contaminated lubricant be placed in the system?”. There are two main avenues for this to occur. Either there were improper storage and handling procedures, or there needed to be more adequate procedures to verify the cleanliness of the lubricant before entering the system.

The other hypothesis stemming from the “less than the adequate presence of antioxidants” is having “less than adequate lubricant specifications.” Let’s investigate this one a bit further.

“How could we have a less than adequate lubricant specification?” Typically, this can result from the lubricant not being blended properly or less than adequate antioxidant levels, which were inappropriate to protect the lubricant.

Now, the line of questioning changes to “Why?” as we have gone past the physical element and some decision-making was involved in this hypothesis. We must ask, “Why wasn’t the lubricant blended properly?”

This can result from less than adequate procedures to ensure the quality of the lubricant by the supplier or less than sufficient checks for the proper blending mix being processed.

These are factors one should consider when receiving any lubricant from their supplier.

On the other hand, if we follow the line of questioning of “How could there be a less than adequate antioxidant level to protect the lubricant?” we can come up with the following.

Either the operating environment caused the antioxidants to be depleted at a higher rate. This would be as a result of a harsh but normal operating environment. In this case, we may be unable to make those environmental changes (without the OEM’s consent).

Or the antioxidants used were not suited to the operating conditions. This is where the line of questioning again shifts to “Why were they not suited?”. This could result from inadequate information in choosing the right lubricant suited for the system.

What Is the Real Root Cause of Oxidation?

From the logic tree that we have created, we can see that there is no sole root cause for oxidation. It can stem from various causes, including physical, human, and even systemic roots. The main takeaway from this exercise is to acknowledge that root causes are not limited to physical causes, such as leaks in the system.

Instead, the actual root causes can be linked to systemic areas of concern where there may not have been enough information to guide the analyst in choosing the most ideally suited lubricant for the application. There are also root causes related to the lubricant not being appropriately blended.

It is critical to thoroughly investigate the real root causes when the lubricant becomes degraded to avoid being stuck in the loop of constantly experiencing degradation.

For more info on other methods, check out the book Bob Latino, and I authored called “Lubrication Degradation – Getting Into the Root Causes,” published by CRC Press.

References:

Ameye, Jo, Dave Wooton, and Greg Livingstone. 2015. Antioxidant Monitoring as Part of a Lubricant Diagnostics – A Luxury or Necessity. Rosenheim, Germany. February 2015.

Latino, Bob, Sanya Mathura. 2021. Lubrication Degradation – Getting into the Root Causes. CRC Press, Taylor & Francis.

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Lubricant Degradation

How do I know if Oxidation is occurring?

What Evidence is needed to prove that oxidation has occurred / is occurring?

However, understanding the oxidation process is just one part of the puzzle. When performing an investigation, we also need to know what factors or characteristics should be present. Additionally, we need to prove that their presence confirms our hypothesis of whether or not oxidation is occurring. This is where the line of evidence-based questioning plays a significant role.

When oxidation occurs, it is usual to see the presence of aldehydes, ketones, hydroperoxides, and even carboxylic acids. These can be confirmed using the FTIR (Fourier Transform Infrared test).

Typically, one will also find some deposits in the system. These deposits can be further characterized and tested to determine their nature using FTIR. Their presence, however, may be confirmed using the MPC (Membrane Path Colorimetry, ASTMD7843) test.

Identifying the presence of the deposits and/or the compounds listed above can lead to the conclusion that oxidation has occurred.

Another critical characteristic of oxidation is the depletion of antioxidants. This can be easily identified by utilizing the RULER® (Remaining Useful Life Evaluation Routine) test. This test quantifies the remaining antioxidants in the oil and gives the value for the amines and phenols (which is very important, especially in synergistic mixtures).

As such, one can detect the trend in the depletion of antioxidants and implement measures to prevent this before they become depleted.

The main tests to assist in determining the presence of Oxidation include:

RULER (Remaining Useful Life Evaluation Routine) levels less than 25% compared to new oil. This value represents the level of antioxidants in the oil. Hence, low levels indicate that the antioxidants are decreasing, possibly due to oxidation. This test can accurately give information on whether oxidation is currently occurring in the oil before deposits are formed.

An increase in acid number indicates the presence of acids resulting from oxidation. However, it must be noted that this change in acid number only occurs after oxidation has taken place. Hence this test is not a good indicator to determine if oxidation is occurring; instead, it is more definitive in letting us know that oxidation has already occurred.

Rapid color changes – darkening of the oil due to the deposits being present. While color is not the best indicator, in some instances, the darkening of the oil can provide a bystander to ask whether something is occurring in the oil. It is not a definitive test for the presence of oxidation.

FTIR test (Fourier Transform Infrared) for the presence of insolubles formed during the oxidation reaction. This can accurately determine the presence of any compound to assist us in determining whether oxidation is occurring.

MPC (Membrane Patch Colorimetry) levels outside the normal range (above 20). This lets us know that insoluble deposits are present in the oil. One must note that there may be instances where the deposits might not appear in the MPC test. As such, this should not be a standalone test to determine the presence of deposits.

RPVOT (Rotating Pressure Vessel Oxidation Test) levels are less than 25% compared to new oil (this is the warning limit). This is the industry standard, but this test does not have a high repeatability value in that if the same test were performed on identical samples, the values would be different. Additionally, the value (reported in minutes) is not easily translated into the environment of the components.

These tests provide us with the evidence we need to determine the presence of oxidation when performing the root cause analysis on the component’s failure.

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Lubricant Degradation

How Can Oxidation Occur in Lubricants?

Typically, when an oil undergoes degradation, the first culprit to be blamed is oxidation. We often hear that the oil has oxidized, producing varnish, leading to its degradation. While this simple statement may seem plausible, it is not the only way oil can degrade.

If an oil has undergone oxidation, the real question we should be asking is not how much varnish has been produced but what caused the oxidation in the first place?

In this article, we will explore the various ways in which an oil can degrade via oxidation. However, as you know from previous articles, other degradation methods exist.

How Can Oxidation Occur?

Before diving further into the root cause of oxidation, one must first fully understand how oxidation occurs. When truly investigating a root cause for a failure, we should start with the question “How could?” rather than “Why?”.

This line of questioning heavily influences the answers. The “How could?” responses stem from a more evidence-based approach.

On the contrary, if we question “Why?” this is more opinionated and can mislead the investigation towards a biased opinion rather than the facts.

This leads us to the main question, “How can oxidation occur?”.

According to Ameye, Wooton, and Livingstone, 2015, oxidation occurs when there is any reaction in which electrons are transferred from one molecule. Ideally, in oxidation, during the initiation phase, free radicals are formed, which in turn produce more free radicals.

A free radical is a molecular fragment with one or more unpaired electrons which are accessible and can easily react with other hydrocarbons, as explained by Ameye, Wooton, and Livingstone, 2015.

After the initiation phase, which has the free radicals, the propagation phase follows, in which the antioxidants react with these free radicals to make them more stable. This is part of the reaction in which there is usually a drastic depletion of antioxidants or where the oil becomes sacrificial.

The antioxidants act as a barrier to protect the base oil from oxidizing. However, they can no longer protect the base oil once they become depleted. This leads to the termination phase, where the remaining free radicals attack the base oil.

As a result, this gives rise to the condensation phase, where we begin to physically notice the changes in the oil’s viscosity and the presence of insoluble by-products. These are the deposits that are known are lube oil varnish to some but can further be defined by their chemical composition.

Understanding how oxidation occurs can assist us in determining the root cause when an oil degrades. It allows us to identify the different stages to further help us determine if it is indeed oxidation that is occurring or not.

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Oil Properties

How Do Lubricant Additives Work?

Each additive works differently to produce its function on the base oil and the overall finished lubricant. This section will explore how each of the lubricant additives works and some of the challenges they may experience.

Pour Point Depressants

As noted above, the pour point depressants help control the flow of the lubricant. This is achieved by modifying the wax crystals present in the lubricant’s base oil. At lower temperatures, the liquid usually has trouble being poured due to the presence of wax molecules in the base oil¹.

There are two main types of pour point depressants, namely;

Alkylaromatic polymers adsorb on the wax crystals as they form, thus preventing them from growing and adhering to each other. This effectively controls the crystallization process and ensures the lubricant can be poured.
Polymethacrylates co-crystallize with wax to prevent crystal growth.

While these additives do not entirely prevent wax crystal growth, they lower the temperature at which these rigid structures are formed. These additives can achieve a pour point depression of up to 28°C (50°F); however, the common range is typically between 11-17°C (20-30°F).

Solubility thresholds may limit the use of this type of additive to achieve the desired effect on the base oil.

VI Improvers

These additives are typically long-chain, high-molecular-weight polymers that change their configuration in the lubricant based on temperature⁴. When the lubricant is in a cold environment, these polymers adopt a coiled form to minimize the effect on viscosity. On the other hand, in a hot environment, they will straighten out, allowing the oil to produce a thickening effect.

While it is more desirable to use high molecular weight polymers (since they provide a better thickening effect), these long-chain molecules are also subject to degradation due to mechanical shearing. Therefore, a balance must be reached between the molecular weight and shear stable service condition.

Another challenge for formulators is to balance the polymer’s tendency to shear with the expected viscosity thickening due to oxidative processes and the viscosity thinning due to the dilution of fuel¹.

Friction Modifiers

These usually compete with the antiwear and extreme pressure additives (and other polar compounds) for surface room. However, they become activated at temperatures when the AW and EP additives are not yet active. Thus, they form thin mono-molecular layers of physically adsorbed polar soluble products or tribochemical friction-reducing carbon layers, which exhibit a lower friction behavior than AW and EP additives².

There are different groups of friction modifiers based on their function. Some are mechanically working FMs (solid lubricating compounds, e.g., Molybdenum disulfide, graphite, PTFE, etc.), adsorption layers forming FMs (e.g., fatty acid ester, etc.), tribochemical reaction layers forming FMs, friction polymer forming FMs and organometallic compounds.

Defoamants (Antifoam)

When foam forms in the lubricant, tiny air bubbles become trapped either at the surface or on the inside (called inner foam). Defoamants work by adsorbing on the foam bubble and affecting the bubble surface tension. This causes coalescence and breaks the bubble on the lubricant’s surface¹.

For the foam that forms at the surface, called surface foam, defoamants with a lower surface tension are used. They are usually not soluble in base oil and must be finely dispersed to be sufficiently stable even after long-term storage or use.

On the other hand, inner foam, which is finely dispersed air bubbles in the lubricant, can form stable dispersions. Common defoamants are designed to control surface foam but stabilize inner foam².

Oxidation Inhibitors

As noted above, antioxidants are usually deployed during the propagation phase to neutralize the scavenging radicals or decompose the hydroperoxides³. There are two main forms of antioxidants: primary and secondary antioxidants.

Primary antioxidants, also known as radical scavengers, remove radicals from oil. The most common types are amines and phenols.

Secondary antioxidants are designed to eliminate peroxides and form non-reactive products in the lubricant. Some examples include zinc dithiophosphate (ZDDP) and sulphurized phenols.

Mixed antioxidant systems also exist where two antioxidants have a synergistic relationship. One example is the relationship between phenols and amines, where phenols deplete early during oxidation while amines deplete later. Another example is using primary and secondary antioxidants to remove radicals and hydroperoxides.

Rust and Corrosion Inhibitors

Rust and Corrosion inhibitors are usually long alkyl chains and polar groups that can be adsorbed on the metal surface in a densely packed formation of hydrophobic layers.

However, this is a surface-active additive, and as such, it competes with other surface-active additives (such as antiwear or extreme pressure additives) for the metal surface. There are two main groups for corrosion additives: antirust additives (to protect ferrous metals) and metal passivators (for non-ferrous metals²).

Rus inhibitors have a high polar attraction to metal surfaces. They form a tenacious, continuous film that prevents water from reaching the metal surface. It must also be noted that contaminants can introduce corrosion into an oil, just as organic acids are produced.

Detergents and Dispersants

Detergents are polar molecules that remove substances from the metal surface, similar to a cleaning action. However, some detergents also provide antioxidant properties. The nature of a detergent is particularly important as metal-containing detergents produce ash (typically calcium, lithium, potassium, and sodium)¹.

On the other hand, dispersants are also polar, and they keep contaminants and insoluble oil components suspended in the lubricant. They minimize particle agglomeration, which in turn maintains the oil’s viscosity (compared to particle coalescing, which leads to thickening). Unlike detergents, dispersants are considered ashless. They typically work at low operating temperatures.

Antiwear Additives

These are typically polar with long chain molecules that adsorb onto the metal surfaces to form a protective layer. This can reduce friction and wear under mild sliding conditions. Usually, these additives are formed from esters, fatty oils, or acids, which can only work at low or moderate levels of stress within the system.

The most common form of antiwear is ZDDP, which is used in engine or hydraulic oils. On the other hand, an ashless phosphorus type of antiwear also exists for systems that require that characteristic, and tricreysl phosphate is the usual choice.

Extreme Pressure Additives

Since extreme pressure additives only become active when higher temperatures or heavier loads are on a system, they have earned the name “Anti-scuffing additives.”

Unlike antiwear additives, extreme pressure additives react chemically with the sliding metal surfaces to form relatively insoluble surface films. This reaction only occurs at higher temperatures, sometimes between 180-1000°C, depending on the type of EP additive used¹.

It must be noted that even with the presence of EP additives in a lubricant, there will still be some wear during the break-in period as the additives have yet to form their protective layers on the surfaces.

EP additives must also be designed for the system they protect as different metals have varying reactivity (EP additives designed for steel-on-steel systems may not be appropriate for bronze systems as they are not as reactive with bronze).

EP additives also contribute to polishing the sliding surfaces as they experience the most significant chemical reaction when the asperities are in contact and the localized temperatures are at their highest. They tend to be created from compounds containing sulphur, phosphorus, borate, chlorine, or other metals⁴.

Do Lubricant Additives Degrade Over Time?

As noted earlier, most additives can deplete over time as they get used up in their various functions. Antiwear and rust protection additives continuously coat the surfaces of the interfacing metals.

This can cause their initial concentrations to decrease over time until it reaches a point where the concentration of the additive is too low to offer any protection. In this case, it has not degraded but depleted.

In earlier years, there used to be prevalent issues with the separation of additives from the finished lubricant due to filtration. However, with the evolution of technology and better practices, this is no longer a common problem operators face.

In the past, operators would notice frequent clogging of their filters and subsequent reduction of additive concentrations, rendering the oil unprotected. It was common to notice additives settling to the bottom of a drum of oil after standing still for some time.

In essence, lubricant additives do not really degrade over time; rather, their concentrations get depleted, which assists in the lubricant degrading faster than a finished lubricant with higher additive concentrations.

Innovation and Future Trends for Additives

What does the future look like for additives within our industry? Will they go away completely?

From my estimations, we’re a long way from that happening. The lubricant industry has evolved over the years, with many advances from the chemical side, which has developed better-suited additives, and the OEM side, which has pushed the chemists to develop lubricant additives that can adapt to equipment changes.

OEMs are creating more components that can withstand higher temperatures, increased pressures, and more demanding environments. Lubricants must also be developed for this specific use, and additive technology will continue to evolve as these boundaries are pushed.

We are also being driven towards more environmentally friendly products, and additives are also on that list. Most of the metals used in the production of additives (such as EP or AW additives) are toxic to the environment, and alternatives are being discovered.

In the field of tribology, there has also been continued research into ways of reducing friction and wear. This is coupled with research into the interaction of varying surfaces and ways lubricants can effectively reduce the coefficient of friction, leading to increased energy efficiency and fuel efficiency in some cases.

Lubricant additives will be around for some time as everything that moves needs to be lubricated, and base oils do not have all the required properties to handle varying temperatures and other conditions that the machine encounters.

While their structure will change to adapt to provide a more environmentally friendly impact, their functions will also evolve based on their future requirements.

References

1 Bruce, R. W. (2012). Handbook of Lubrication and Tribology, Volume II Theory and Design, Second Edition. Boca Raton: CRC Press.

2 Mang, T., & Dresel, W. (2007). Lubricants and Lubrication – Second Completely Revised and Extended Edition. Weinheim: WILEY-VCH GmbH.

3 Livingstone, G., Wooton, D., & Ameye, J. (2015). Antioxidant Monitoring as Part of Lubricant Diagnostics – A Luxury or a Necessity?

4 Pirro, D. M., Webster, M., & Daschner, E. (2016). Lubrication Fundamentals – Third Edition Revised and Explained. Boca Raton: CRC Press.

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Oil Properties

What are the types of Lubricant Additives?

There are many types of lubricant additives, and various formulations exist from different suppliers. In this section, we will cover the most common additives found in finished lubricants.

Pour Point Depressants

All liquids have a particular temperature at which they can effectively flow. The liquid’s viscosity and current temperature determine how quickly it moves. As the name implies, this type of additive can assist in lowering the temperature at which the lubricant flows¹.

VI Improvers

This should not be confused with Pour Point Depressants. Viscosity Index Improvers are also known as Viscosity Modifiers². They assist the lubricant in increasing its viscosity at higher temperatures, allowing lubricants to operate in wider temperature ranges.

Friction Modifiers

When two surfaces rub against each other, friction is formed. Depending on the type and extent of friction, some surfaces can experience welding and even adhesive wear. This is where friction modifiers can help by reducing frictional forces associated with stick-slip oscillations and noises.

Defoamants (Antifoam)

Some lubricants succumb to foam being created in their systems. When foam is made, it significantly impacts the functions of the lubricant and can lead to excessive wear due to lack of lubrication (they disrupt the surface of the lubricant), cavitation (due to the presence of air bubbles), and even increased oxidation (due to presence of air trapped in the system). Foam can also affect the ability of a liquid to transfer heat or cool. Defoamants or antifoam additives reduce the amount of foam being produced.

Oxidation Inhibitors (Antioxidants)

Oxidation occurs in most lubricants. During the oxidation process, free radicals emerge, propagating to form alkyl or peroxy-radicals and hydroperoxides, which eventually react with others to form oxidation by-products. During the propagation phase, antioxidants are usually deployed to neutralize the free radicals or decompose the hydroperoxides³. As such, these additives are sacrificial in nature, as they protect the base oil from oxidation by being depleted.

There are many types of antioxidants, including phenolics and aromatic nitrogen compounds, hindered phenols, aromatic amines, zinc dithiophosphates, and a couple of others.

Rust and Corrosion Inhibitors

If oxygen and water are present at a location containing iron, then rust can be formed. Corrosion affects the non-ferrous metals in the presence of acids in the lubricant¹. Most pieces of equipment succumb to rust and corrosion quite easily, so these inhibitors were developed to mitigate these effects by forming protective layers on the surfaces of the equipment.

Detergents and Dispersants

These two often get confused as they usually work together to prevent deposits from accumulating in the oils. Detergents neutralize deposit precursors (especially in engine oils), while dispersants suspend the potential sludge or varnish-forming materials⁴.

Antiwear Additives

Antiwear additives reduce friction and wear, especially during boundary lubrication conditions. They are designed to reduce wear when the system is exposed to moderate stress².

Extreme Pressure Additives

Extreme Pressure additives are usually confused with antiwear additives, or the names are used interchangeably. However, extreme pressure additives begin to work when the system experiences high stress and try to prevent the welding of moving parts, unlike antiwear additives, which work when the system experiences moderate stress.

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Oil Properties

The Difference Between Antiwear and Extreme Pressure Additives

The terms antiwear additives and extreme pressure additives are often used interchangeably, suggesting that they provide the same functions in a lubricant. This is not exactly true. While there are many similarities in how they function, both additives have distinct functions in protecting lubricants.

Both are film-forming additives (Bruce, 2012). Their functions are to reduce wear between two contacting surfaces or reduce friction to lower the heat produced between the two rubbing surfaces.

They can also be classified as boundary additives that can be temperature-dependent (EP additives) or non-temperature-dependent (Antiwear additives). They both function to mitigate against wear, which is usually caused during boundary lubrication where higher speeds, loads, or temperatures can cause contact with the asperities.

One of the significant differences, as noted by Mang & Dresel, 2007 is that antiwear additives are designed to reduce wear when the system is exposed to moderate stress. On the other hand, EP additives are much more reactive. These are used when the system’s stress is very high to prevent the welding of moving parts.

According to (Bruce, 2012), there are four main groups of commercially available EP additives based on the structures containing phosphorus, sulphur, chlorine, and overbased sulfonates. He explains that the phosphorus, sulphur, and chlorine-containing EP additives are activated by heat over a range of temperatures.

For instance, chlorine-containing EP additives are usually activated between 180-240°C, phosphorus-containing additives are activated at higher temperatures, and sulphur-containing additives operate at 600-1,000°C.

On the other hand, overbased sulfonates contain a colloidal carbonate that reacts with iron to form a thin-film barrier layer between tribocontacts. This protects the surface from direct contact and welding.

As we can see, antiwear and EP additives protect the surfaces between which the lubricant exists. However, they are activated differently and subsequently perform two different functions.

Antiwear additives protect against wear and are not temperature dependent, while EP additives are activated by high stress to prevent the welding of moving parts.

Both functions are essential to protecting the system from additional wear and ensuring it remains operational.

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References

Bloch, H. (2009). Practical lubrication for industrial facilities, Second edition. Lilburn: Fairmont Press Inc.

Bruce, R. W. (2012). Handbook of Lubrication and Tribology, Volume II, Theory and Design, Second Edition. Boca Raton: CRC Press Taylor and Francis Group.

Coyle, C. L., Greaney, M. A., Stiefel, E. I., Francis, J. N., & Beltzer, M. (1991, Feb 26). United States of America Patent No. 4,995,996.

Mang, T., & Dresel, W. (2007). Lubricants and Lubrication, Second, Completely Revised and Extended Edition. Weinheim: WILEY-VCH Verlag GmbH & Co. KGaA.

Mortier, R. M., Fox, M. F., & Orszulik, S. T. (2010). Chemistry and Technology of Lubricants, Third Edition. (C. Bovington, Ed.) Dordrecht Heidelberg: Springer Science+Business Media B.V. doi:10.1023/b105569_3

Pirro, D. M., Webster, M., & Daschner, E. (2016). ExxonMobil, Lubrication Fundamentals, Third Edition, Revised and Explained. USA: CRC Press Taylor and Francis Group.

Zhang, J., & Spikes, H. (2016). On the Mechanism of ZDDP Antiwear Film Formation. Tribol Lett, pp. 1–2.