In Part 1, we explored the fundamentals of lubricant additives and the key roles they play in enhancing the performance of base oils. In Part 2 of our series, we are diving deeper into some of the most essential additives—those that safeguard machinery, extend lubricant life, and maintain efficiency in even the harshest environments. We will cover antioxidants, anti-wear additives, metal deactivators, demulsifiers, antifoam agents, viscosity index improvers, and pour point depressants. By understanding these components, we can appreciate how modern lubricants keep our machinery running smoothly, despite demanding operating conditions.
Antioxidants: Defending Against Oxidative Degradation
Antioxidants can be split into primary and secondary types. Let’s explore these now.
Primary Antioxidants are also known as radical scavengers. Their main role is to interrupt the auto-oxidation chain reaction that degrades lubricants over time. Oxidation occurs when hydrocarbons react with oxygen in the presence of heat, electrostatic discharge, or radiation, producing free radicals and peroxides. These radicals can further react with the lubricant, accelerating degradation exponentially.
- Amine Antioxidants: These are effective at neutralising free radicals by donating hydrogen atoms to make the radicals inert. The process is complex but powerful; each amine antioxidant can neutralise around 12 radicals before being depleted, making them very effective. They are particularly beneficial at high temperatures, which makes them ideal for use in engine and turbine oils.
- Phenolic Antioxidants: These antioxidants function similarly to amines but work more efficiently at lower temperatures. Phenolic antioxidants can neutralise fewer radicals compared to amines but have a synergistic effect when used together. This combination allows a balance of protection across a range of temperatures, which is why many turbine oils use both types of antioxidants to ensure comprehensive oxidation control.
I personally like to think of phenols as doing most the grunt work of day to day antioxidant tasks, and your amines being like an emergency reserve for the more extreme conditions.
Secondary Antioxidants focus on decomposing peroxides, another byproduct of oxidation. Peroxides are unstable and can break down to form more radicals, further promoting oxidation. Secondary antioxidants like organo-phosphorus and sulphur-based compounds help break down peroxides into more stable products, preventing further damage. Zinc dialkyldithiophosphate (ZDDP), for example, plays a dual role as both an anti-wear agent and a secondary antioxidant, inhibiting peroxide formation while also creating a protective anti-wear layer on metal surfaces.
Lubricant analysis can monitor antioxidant health through advanced techniques such as FTIR (Fourier Transform Infrared Spectroscopy) or RULER (Remaining Useful Life Evaluation Routine). These methods help detect antioxidant depletion, ensuring timely intervention to maintain lubricant stability.
Anti-Wear Additives: Protecting Surfaces Under High Load
Anti-wear additives are essential for reducing metal-to-metal contact in conditions where hydrodynamic lubrication is insufficient and the film thickness allows surfaces to touch—specifically in boundary and mixed lubrication regimes. These additives form protective layers on metal surfaces, preventing wear under high pressure as the last line of defence.
- Zinc Dialkyldithiophosphate (ZDDP): The most commonly used anti-wear additive in the world by far, ZDDP reacts under load to form a protective, glass-like zinc polyphosphate layer on metal surfaces as well as an additional metal sulphide layer too. This sacrificial layer prevents wear by absorbing the pressure, thereby protecting the underlying metal. The thickness of this layer (typically less than 200 nm) is sufficient to protect metal asperities, which are typically the size of microscopic surface irregularities. This mechanism is particularly critical in engine oils, where high loads are present.
- Phosphate Esters and Sulphur-Phosphorus Compounds: These are used in gear oils and industrial lubricants, where high load-carrying capacity is required. Phosphate esters are especially good for protecting surfaces in the mixed lubrication regime, where both fluid film and direct surface contact share the load.
Lubricant analysis can be employed to monitor wear using ferrography or wear particle analysis, allowing early detection of wear metals. A spike in wear metals like iron or copper could indicate the breakdown of anti-wear additives, requiring timely intervention to prevent catastrophic machinery damage.
Metal Deactivators: Preventing Catalytic Oxidation
Metals such as copper and iron are common in machinery and can catalyse oxidation reactions, accelerating lubricant degradation. Metal deactivators are additives that work in two ways to prevent this:
- Surface Passivators: These additives form a protective, inert barrier on metal surfaces, preventing the lubricant from contacting the metal and therefore reducing oxidation. This is akin to placing a shield over vulnerable metals, thus passivating the surface and stopping unwanted reactions.
- Chelating Agents (Chelators): Chelating agents bind with metal ions, stabilising them and preventing them from catalysing oxidation. This process is similar to medical chelation therapy used to treat heavy metal poisoning—chelators essentially ‘trap’ harmful metal ions, rendering them inert. This is critical in machinery with copper components, like coolers, where copper ions could otherwise accelerate oil oxidation.
Lubricant analysis cannot typically detect the presence of metal deactivators directly, but the reduction of oxidation rates in the presence of copper or iron is a good indicator that these additives are functioning correctly.
It has become a somewhat frowned upon practice in recent years for oil manufacturers to dose oils with excessively high copper deactivator additives to gain an advantage on the RPVOT test where copper is used as a catalyst. To me is seems strange but rather than overdosing the antioxidant which would actually have an increase in life of the oil the exceptionally high metal deactivators do not improve oil stability and instead some argue just allows you to cheat the test.
Demulsifiers: Enhancing Water Separation
Demulsifiers help separate water from oil to prevent the formation of emulsions, which can disrupt lubrication by forming thick, gel-like substances. Emulsions can increase viscosity unpredictably, starving lubrication points and reducing overall efficiency.
- Emulsions and Their Problems: Emulsions form when water is dispersed in oil, with surfactant-like molecules stabilising the water droplets. If stable, these emulsions can drastically increase lubricant viscosity, as in the case of mayonnaise—an example of a thick, stable emulsion compared to its thinner constituents of oil and water. Such thickened lubricants can lead to lubrication starvation, especially in critical areas like bearings.
- How Demulsifiers Work: Demulsifier additives work by breaking the emulsion, encouraging water droplets to coalesce into larger droplets that eventually separate from the oil. This allows easy removal of water through settling or draining. Maintaining water separability is crucial in steam turbines or any system exposed to high humidity, as water in the oil can lead to rust and corrosion.
Lubricant analysis for water contamination includes methods like Karl Fischer titration and crackle testing to ensure effective water separation, with demulsifier performance monitored based on how efficiently water is removed with water separation times and demulsibility testing. Here the lubricant is mixed with water and timed how long to separate at various temperatures.
Antifoam Agents: Controlling Air Entrapment and Air Release Issues
Foam can cause operational problems, such as reduced lubrication efficiency and cavitation. Antifoam agents are used to control foam formation by breaking down surface bubbles.
- Different Forms of Air in Lubricants: Air can exist as dissolved air, entrained air, or foam. Dissolved air is generally harmless, but entrained air and foam can lead to problems such as micro-dieseling and cavitation, which damage system components. Poor air release characteristics can also hinder lubricant performance, as excessive entrained air leads to reduced film strength and inconsistent hydraulic performance.
- Causes of High Foam: High foam levels can be caused by several factors including contamination with detergents, oxidation products, or polar contaminants such as grease. Inadequate reservoir design, which leads to turbulence, can also promote foam formation. In addition, high operating temperatures and excessive mechanical agitation can increase foam generation.
- How Foam is Solved: Foam can be controlled using antifoam agents, typically silicone-based compounds that migrate to the air-oil interface and reduce surface tension, causing bubbles to collapse. Effective reservoir design is also critical—using baffles and ensuring sufficient residence time allows foam to dissipate naturally. Proper filtration and the use of correct seal materials can reduce contamination, thereby minimising foam formation.
- Causes of High Air Release Issues: Poor air release can occur due to the presence of high viscosity oils, contamination, or insufficient residence time in the reservoir. Additives like VI improvers that shear under high pressure can also affect air release efficiency.
- How Air Release is Solved: Improving air release involves using base oils with lower viscosity and selecting additives that maintain air release properties. Reservoir design is crucial—having sufficient volume and ensuring minimal turbulence help air bubbles escape. Use of air release agents, additives specifically designed to improve air release characteristics, can also facilitate the quick release of entrained air, ensuring stable lubrication performance.
- How Antifoam Agents Work: These additives, typically silicone-based, migrate to the air-oil interface and reduce surface tension, causing bubbles to collapse. However, silicone antifoam agents can show up in particle count tests, complicating filtration processes. Thus, careful monitoring is required to ensure that antifoam agents are functioning without unintended side effects, like increased entrained air retention. Additionally, effective air release properties are critical, as they determine how quickly entrained air can escape from the oil once it reaches the reservoir. If air release is poor, it can lead to sluggish hydraulic response, reduced lubrication efficiency, and cavitation, which can damage components over time. Careful selection of both antifoam agents and base oil formulation is crucial to balance foam control with efficient air release characteristics.
Viscosity Index Improvers: Adapting to Temperature Changes
Viscosity Index (VI) Improvers are used to stabilise a lubricant’s viscosity across temperature changes. At low temperatures, oils tend to thicken, while at high temperatures, they thin out—VI improvers help maintain a more consistent viscosity.
- How VI Improvers Work: These additives are long-chain polymers that expand as temperature increases, thereby maintaining viscosity. At lower temperatures, they remain tightly coiled, contributing minimally to viscosity. However, in high-shear environments, such as in engines, these polymers can shear down, losing their effectiveness over time.
While VI improvers are beneficial for lubricants, especially in applications requiring consistent viscosity across a wide temperature range, they are not ideal for thermal oils. In thermal oil applications, rapid heat dissipation is a priority, and maintaining a stable viscosity can impede the effective transfer of heat. Thermal oils generally need to flow freely and transfer heat quickly without the thickening effect at high temperatures that VI improvers provide.
Paraffinic vs Naphthenic Oils: VI is an important parameter when choosing between different types of base oils. Paraffinic oils generally have higher viscosity indices, which makes them more suitable for applications where maintaining viscosity over varying temperatures is crucial. They tend to provide better oxidation stability but may be less effective for applications requiring rapid heat dissipation. In contrast, naphthenic oils have lower viscosity indices, which means they experience more significant changes in viscosity with temperature fluctuations. However, their lower viscosity index can be advantageous in applications such as heat transfer, where rapid flow and efficient heat dissipation are required, as they are less likely to resist changes in viscosity, allowing for quicker temperature adjustment.
Lubricant analysis can monitor the condition of VI improvers by measuring viscosity at both high and low temperatures to detect any significant changes that could indicate additive degradation or shear thinning.
Pour Point Depressants: Ensuring Cold Flow Performance
Pour Point Depressants (PPDs) are used to improve a lubricant’s flow properties at low temperatures by disrupting the formation of wax crystals. Lubricants often contain paraffins that crystallise at lower temperatures, forming a gel-like structure that impedes flow.
- How PPDs Work: PPDs are similar in structure to wax crystals but prevent the formation of large, rigid crystals by acting as a disruption. By attaching to these wax molecules, PPDs inhibit the crystalline structure from growing, ensuring that the lubricant remains fluid at low temperatures. This is crucial for machinery operating in cold climates, where maintaining oil flow is vital for component protection.
The pour point test is typically used to verify the efficacy of pour point depressants, ensuring that the lubricant can still flow at temperatures well below the operating conditions. This test is performed by cooling the lubricant in a controlled environment and observing the lowest temperature at which it still flows when tilted. Additionally, cloud point is the temperature at which wax crystals first start to form, which may impair flow but not completely stop it. This test is conducted by gradually cooling the oil and visually detecting the first appearance of cloudiness, indicating wax crystallisation. Cold Filter Plugging Point (CFPP) is another critical parameter, especially for fuels, which indicates the lowest temperature at which the oil will pass through a fuel filter without plugging. The CFPP test involves drawing a sample of the cooled oil through a standardized filter under vacuum to determine the temperature at which the flow becomes restricted. These tests are essential for understanding the cold flow properties of lubricants and fuels, particularly in cold environments.
Conclusion: Additives in Action
Additives are the unsung workhorses that transform basic base oils into sophisticated, high-performance lubricants. From antioxidants that stave off oxidation to demulsifiers that keep water at bay, each additive plays a specific and crucial role in ensuring machinery reliability and efficiency. By understanding their functions and monitoring their effectiveness through lubricant analysis, you can maintain optimal lubricant health, reduce maintenance costs, and extend equipment life.
Stay tuned for future articles that will delve into other specialised additives and their role in modern lubrication technology. There is much more to uncover in the world of lubrication science, and every detail counts when it comes to protecting your assets.