Tagged: oxidation

What are Viscosity Index Improvers?

Viscosity Index Improvers (VIIs) are additives that help maintain the viscosity of lubricating oils across a wide temperature range, ensuring consistent performance.

This article will explore the nature of viscosity index improvers and their role in industrial and automotive lubricants. We will also look at their impact on lubricant efficiency, innovations involving this type of additive, and future trends.

Before discussing the nature of viscosity index improvers, we need to understand the role of viscosity. Essentially, this is one of the most critical functions of a lubricant, as it directly affects its flow rate and ability to keep the two interacting surfaces apart.

By nature, all base oils have an assigned viscosity based on their blend. However, other properties are required when we’re creating finished industrial or automotive lubricants. For instance, we may need the oil to withstand higher temperatures while still maintaining a particular viscosity, which not only provides wear protection for the equipment but also flows at a rate that does not incur frictional losses. Those are a lot of functions!

Typically, as temperature increases, viscosity decreases, and as the temperature decreases, the viscosity increases. One example is the state of water: when heated, it can turn into a gas (lower viscosity), or when frozen, it can transform into ice (higher viscosity). However, depending on the type of material, there will be varying rates of viscosity change with temperature. The viscosity/temperature relationship is called the viscosity index (VI).

As per Mortier, Fox, & Orszulik (2010), the kinematic viscosity of oil is measured at 40°C and then at 100°C. The viscosity change is then compared with an empirical reference scale initially based on two sets of crude oils: a Pennsylvania crude arbitrarily assigned a VI of 100 and a Texas Gulf crude assigned a VI of 0.

The higher the VI, the less effect that temperature has on the oil, which means that the oil can maintain a particular viscosity for a longer time at a more extensive temperature range. This is ideal for lubricants in environments experiencing temperature changes. However, not all oils have a high viscosity index. Typically, paraffinic oils can have a very high viscosity index. On the other hand, naphthenic oils have a low or medium viscosity index. The table below gives an overview of the viscosity index for various oils.

Table 1: Viscosity index of API Groups I-III
Table 1: Viscosity index of API Groups I-III

When trying to manage or alter the viscosity index of the oils above, the use of Viscosity Index Improvers (VII) can help by adding that property to an oil to allow it to have other beneficial properties. As per (Mortier, Fox, & Orszulik, 2010), viscosity index improvers consist of five main classes of polymers:

  • Polymethylmethacrylates (PMAs).
  • Olefin copolymers (OCPs).
  • Hydrogenated poly (styrene-co-butadiene or isoprene) (HSD/SIP/HRIs).
  • Esterified polystyrene-co-maleic anhydride (SPEs)
  • A combination of PMA/OCP systems.

What Happens When Defoamants, Dispersants & Detergents Are Used Up?

For the three additives we spoke about earlier, each of them is sacrificial in one way or another.

Defoamants get used up when they are called upon to reduce the foam in the oil. On the other hand, detergents and dispersants use their characteristics to suspend contaminants in the oil.

In all of these scenarios, each of these additives can be considered to become depleted over time. While performing their functions, they will undergo reactions that reduce their capability to perform them more than once.

Hence, it can be concluded that these additives become depleted over time even though they may not have physically left the oil but now exist in a different form.

The air release property of the oil is affected by the loss of defoamants. This value will see a significant rise, indicating that it takes longer for air to be released from the oil. As such, air remains in the oil in either a free, dissolved, entrained, or foam state.

Consequently, this impacts the ability of the oil to lubricate the components properly and can even result in microdieseling and increased oil temperatures in the sump.

On the other hand, as the detergents and dispersants are reduced, the capacity of the oil to hold contaminants also decreases.

Therefore, one will begin noticing that deposits may start forming on the equipment’s insides, leading to valves sticking (especially in hydraulic systems) or a general increase in the system’s temperature as these deposits can trap heat.

With the introduction of an increased temperature, the oil can begin oxidizing, leading to more deposits being formed and possibly even varnish.

Essentially, these additives are essential to the health of the oil in your system. The detergents and dispersants can help to keep your system clean (free from contaminants such as soot).

The defoamants can even reduce the risk of wear, increased temperatures to the lube system, the potential to form varnish, or the possibility of succumbing to microdieseling.

References

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

2 Mang, P., & Dresel, D. (2007). Lubricants and Lubrication – Second Edition. Weinheim: WILEY-VCH Verlag GmbH & Co KGaA.

4 Mang, P.-I., Bobzin, P.-I., & Bartels, D.-I. (2011). Industrial Tribology Tribosystems, Friction, Wear and Surface Engineering, Lubrication. Weinheim: WILEY-VCH Verlag & Co KGaA.

3 Mortier, D. M., Fox, P. F., & Orszulik, D. T. (2010). Chemistry and Technology of Lubricants – Third Edition. Dordrecht Heidelberg: Springer.

Do Detergents Really Clean?

Traditionally, detergents were given their name as it was assumed that they provided cleaning properties to the oil, similar to laundry detergents. However, these metal-containing compounds also provide an alkaline reserve used to neutralize acidic combustion and oxidation by-products.

Due to their nature, these compounds disperse particulate matter, such as abrasive wear and soot particles, rather than removing them (in a cleaning action). There are four main types of detergents: phenates, salicylates, thiophosphate, and sulfonates4.

Calcium phenates are the most common type of phenate. They are formed by synthesizing alkylated phenols with elemental sulphur or sulphur chloride, followed by neutralization with metal oxides or hydroxides. These calcium phenates have good dispersant properties and possess a greater acid-neutralization potential.

Salicylates have additional antioxidant properties and a proven efficacy in diesel engine oil formulations. They are prepared through the carboxylation of alkylated phenols with subsequent metathesis into divalent metal salts. These products are then overbased with excess metal carbonate to form highly basic detergents.

Thiophosphonates are rarely used today as they are an overbased product.

Sulfonates generally have excellent anticorrosion properties. The neutral (or over-based) sulfonates have excellent detergent and neutralization potential. These neutral sulfonates are typically formed with colloidally dispersed metal oxides or hydroxides.

Calcium sulfonates are relatively cheap and have good performance. On the other hand, magnesium sulfonates exhibit excellent anticorrosion properties but can form hard ash deposits after thermal degradation, leading to bore polishing in engines. Barium sulfonates are not used due to their toxic properties.

Detergents in ATFs are used in concentrations of 0.1-1.0% for cleanliness, friction, corrosion inhibition, and reduction of wear3. However, these values are a bit higher in manual transmission fluids, at 0.0 – 3.0%. On the other hand, no detergents are required for axle lubricants!

Why Are Dispersants Important?

Quite often, detergents and dispersants are grouped together mainly because their functions can complement each other. As noted above, the significant difference is that dispersants are ashless, while detergents are more metal-containing compounds.

However, some ashless dispersants also offer “cleaning” properties, so the two are not mutually exclusive.

A large oleophilic hydrocarbon tail and a polar hydrophilic head group can categorize detergents and dispersants. Typically, the tail solubilizes in the base fluid while the head is attracted to the contaminants in the lubricant.

Dispersant molecules envelop the solid contaminants to form micelles, and the non-polar tails prevent the adhesion of these particles onto the metal surfaces so that they agglomerate into larger particles and appear suspended.

Ashless dispersants are, by definition, those that do not contain metal and are typically derived from hydrocarbon polymers, with the most popular being polybutenes (PIBs).

For example, dispersants are typically required in concentrations of 2-6% in ATFs and are used to maintain cleanliness, disperse sludge, and reduce friction and wear3. These values in manual transmission fluids and axle lubricants vary from 1-4%.

Are Defoamants Necessary?

Defoamants, also called antifoam additives, are found in many oils. Most oils need to keep foam levels to a minimum, and it is very easy for foam to form in lube systems due to their design and flow throughout the equipment.

When foam enters the oil, it can affect its ability to provide adequate surface lubrication. This can lead to wear occurring at the surface level, damaging the equipment.
Many oils require defoamants to provide various functions and in differing ratios depending on their application. In automatic transmission fluids (ATFs), defoamants are usually needed in concentrations of 50-400ppm to prevent excessive foaming and air entrainment3. On the other hand, for manual transmission fluids and axle lubricants, defoamants are required in slightly lower concentrations, between 50 and 300 ppm.

However, OEMs must verify these concentrations. If the concentration of defoamants is too high, this can actually increase foaming. Additionally, defoamants must be properly balanced with the other additive packages to ensure they do not negatively counteract another additive.

There are two main types of defoamants: silicone defoamers and silicone-free defoamers. Silicone defoamers are considered the most efficient defoamants, especially at low concentrations of around 1%. These defoamants are typically pre-dissolved in aromatic solvents to provide a stable dispersion.

However, there are two significant disadvantages associated with silicone defoamers. Due to their insolubility, they can easily transition out of the oil and have a powerful affinity to polar metal surfaces.

On the other hand, silicone-free defoamers are another alternative, especially for applications that require silicone-free lubricants. Such applications include metal-working fluids and hydraulics, which are used close to silicone-free ones, and even those involved in applying paints or lacquers to these pieces.

Some silicone-free defoamers include poly(ethylene glycol)s (PEG), polyethers, polymethacrylates, and organic copolymers. Tributylphosphate is also another option for defoamers4.

Defoamants, Dispersants, and Detergents in Lubricants – What’s the Difference?

Additives can enhance, suppress, or add new properties to oils. Defoamants, dispersants, and detergents are no exceptions. This trio of additives can be found in most finished lubricants, albeit in varying ratios.

Let’s discuss the main differences among these three, why each is so important, and ways to confirm their presence.

What’s the Difference?

While they are all additives (which begin with the letter D), their functions are distinctively different. They all work to protect the oil from various types of contaminants.

For instance, defoamants reduce the air bubbles in the oil. At the same time, detergents keep the metal surfaces clean, and dispersants encapsulate the contaminants so they are suspended in the lubricant.1 This is illustrated in Figure 1.

Figure 1: Defoamants, detergents and dispersants explained.
Figure 1: Defoamants, detergents and dispersants explained.

From our last article on Lubricant Additives – A Comprehensive Guide, here are some detailed descriptions of how each of these additives functions.

Defoamants

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.

Dispersants

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.

Detergents

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 essential, as metal-containing detergents produce ash (typically calcium, lithium, potassium, and sodium)1.

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.
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

  1. Bruce, R. (2012). Handbook of Lubrication and Tribology Volume II Theory and Design. Boca Raton: CRC Press.
  2. (2022, July 20). Why Choose RULER? Retrieved from Fluitec: https://www.fluitec.com/why-choose-ruler/
  3. 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/
  4. Mang, T., & Dresel, W. (2007). Lubricants and Lubrication. Weinheim: WILEY-VCH Verlag GmbH & Co. KGaA.
  5. (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
  6. Mortier, R. M., Fox, M. F., & Orszulik, S. T. (2010). Chemistry and Technology of Lubricants. Dordrecht: Springer.
  7. Pirro, D. M., Webster, M., & Daschner, E. (2016). Lubrication Fundamentals, Third Edition, Revised and Expanded. Boca Raton: CRC Press.
  8. (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
  9. 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
  10. Stachowiak, G. W., & Batchelor, A. W. (2014). Engineering Tribology. Butterworth-Heinemann.

Types of Antioxidants

Different additives have successfully suppressed the degradation of finished lubricants6, 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.

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.

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.
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.

Want to read the entire article? Find it here in Precision Lubrication Magazine!