Two months ago, I presented readers with a primer on lubricants and lubrication. That begins a discussion that can go one of two ways. We can either explore the chemistry and formulation of our lubricants, or we can discuss the function of the various common components that the lubricant supplier (their chemists and chemical engineers) develops, manufactures, and deploys. I will focus on the function of the lubricant as this is most important for us to understand.
To start, there are a couple of baseline concepts. Viscosity is mostly determined by the base oil. Remember, a lubricant is the combination (both chemical and mixture) of a base oil and an additive package. Viscosity is one common way to monitor the health of a lubricant and is a key indicator to the status and effectiveness of the lubrication system.
Now on to the additives side of the equation. Additives to a lubricant are organic or inorganic compounds dissolved or suspended as solids in the base oil. In our industry it is common to have as little as 0.1 percent by volume of an additive package to as high as 30 percent by volume. The amount, selection, and specification of the chemistry of the additives is solely a function of the system they are to protect. So, what are the additives, what functions do they perform, and how do they perform these functions?
Additives have three basic roles (Figure 1):
- Enhance existing base oil properties with antioxidants, corrosion inhibitors, anti-foam and demulsifying agents.
- Suppress undesirable base oil properties with pour-point depressants and viscosity index (VI) improvers.
- Impart new properties to base oils with extreme pressure (EP) additives, detergents, metal deactivators, and tackiness agents.
Antioxidants or Oxidation Inhibitors
These prevent the oxidation of the base oil which increases the useful life of the lubricant. Oxidation occurs at all temperatures however, as the temperature increases so does the rate of oxidation. Ultimately oxidation precipitates an acidic condition in the lubricant (which promotes corrosion) and sludge (which results in surface deposits and causes the viscosity to increase) to form. Oxidation inhibitors, as they are also called, are used to extend the operating life of the lubricant. The additives are sacrificial and are consumed while delaying the onset of oxidation, thus protecting the base oil.
The presence of wear particles, water, and other contaminants also promote oxidation of the lubricant. This mechanism of oxidation also generally leads to formation of acids and sludge. The presence of acid (or acidic pH) will promote corrosion of the metal components in the system. Sludge formation increases the viscosity of the lubricant, thus reducing its ability to move debris out of the contact zone it is trying to protect, as well as reduce the efficiency of thermal transport (meaning the contact zone will heat more under the same loading conditions). Examples of antioxidant additives are zinc dialkyl dithiophosphates, hindered phenols, sulphurized phenols, and aromatic amines. These compounds decompose peroxides and terminate free-radical reactions that occur in the lubricant. The mechanism by which these types of additives work are sacrificial in nature thus, as they do their job, the quantity of them in the lubricant is reduced. It is not uncommon to have to replenish these additives either by changing the lubricant or periodically adding more of the additive compounds.
Corrosion and Rust Inhibitors
These work to reduce or eliminate corrosion by neutralizing acids and forming a protective film, either adsorbed or chemically bonded on the metal surfaces. Preferential adsorption of polar constituents on metal surface forms the protective film that prevents corrosive materials such as organic acids from reaching and attacking the metal. These are usually compounds having a high polar attraction toward metal surfaces such as succinates, alkyl earth sulfonates, metal phenolates, fatty acids, amines, as well as zinc dithiophosphates. Some of these inhibitors are specific to protecting certain metals, thus an oil may contain several types of corrosion inhibitors. (Figure 2)
Anti-Foam Agents or Defoamants
Anti-foam agents are additives that either prevent the lubricant from foaming and / or reduce the foam if it does form. Foam forms in part due to the constant mixing of the lubricant in the presence of air (or other gases) which leads to gaseous entrapment. Foam reduces the ability of the lubricant to transport thermal energy as it is now a mixture with a gas that has much lower heat-transport capacity. Foaming also reduces the load-carrying capacity as it becomes much more difficult to develop and maintain an elastohydrodynamic shear layer when part of the fluid film layer is compressible. The chemicals in this additive group possess low interfacial tension, which weakens the bubble wall and allows the foam bubbles to burst more readily. They have an indirect effect on oxidation by reducing the amount of air-oil contact. Silicone polymers at a few parts per million and organic copolymers at higher concentrations are widely used in mineral-based oils. The antifoaming agents are essentially insoluble in the lubricant hence they need to be finely dispersed. They work by attaching themselves to the entrapped air bubbles and aid in forming bigger bubbles (via coalesce), which in turn cause the larger bubbles to disassociate from the liquid part of the lubricant and burst as bubbles do. (Figure 3)
Demulsifying Agents or Dispersants
These are used mainly with detergents to keep surfaces clean and free of deposits. Dispersants keep insoluble particles and the precursors of deposits finely dispersed or suspended in the lubricant even at high temperatures. These suspended particles are subsequently removed by oil filtration or an oil change. Generally, polymeric and ashless dispersants are used today such as polymeric alkyl thiophosphonates, alkyl succinimides, succinic acid esters / amides, as well as organic complexes containing nitrogen compounds.
Demulsifier additives prevent the formation of a stable oil-water mixture or an emulsion by changing the interfacial tension of the oil so that water will coalesce and separate more readily from the lubricant. This is an important characteristic for lubricants exposed to steam or water so that free water can settle out and be easily drained off.
Emulsifiers, on the other hand, are used in oil-water-based metal-working fluids and fire-resistant fluids to help create a stable oil-water emulsion. The emulsifier additive can be thought of as a glue binding the oil and water together, normally they would like to separate from each other due to interfacial tension and differences in specific gravity.
Pour Point Depressants
Pour point depressants are typically polymers that allow lubricants to flow at very low temperatures without heavy wax formation at these temperatures. Thus, they enable the lubricant to flow as intended (within the stated viscosity limits) throughout its stated applicable temperature range. When a lubricant — more specifically, the base oil — gets colder, waxes begin to form and coagulate within the fluid. This phenomenon is called the cloud point. When this occurs, the crystals start to impede the flow of the lubricant, thereby effectively increasing the viscosity beyond the stated working range. Of course, this adversely affects the flow of lubricant into the contact zone and thus diminishes its effectiveness.
Pour point depressants do not lower the temperature at which wax crystals begin to form, or the amount of wax that is formed. Instead, they work by altering the crystal shape and size, which inhibits lateral (large) crystal growth. There are two known methods by which this may be achieved: surface adsorption and co-crystallization. Alkylaromatics and aliphatic polymers are two types of pour point depressants that are commercially available. Most commercially available pour point depressants are organic polymers, but nonpolymeric substances such as phenyltristearyloxysilane and pentaerythritol tetrastearate may also be effective.
Viscosity Index Improvers (VII)
Viscosity index improvers, also known as viscosity modifiers, are additives that prevent the lubricant from losing its viscosity at high temperatures. They are specifically designed to promote better lubricant flow at low temperatures. In addition, VI improvers are used to achieve high-VI lubricants for improved start-up and lubrication at low temperatures.
To visualize how a VI-improver additive works, think of a coil spring. In this case it stays tightly coiled when the lubricant is cool (relative) and then as the temperature increases, the additive ‘uncoils’ or expands which prevents the lubricant from thinning at high temperatures. (Figure 4)
Unfortunately, VI improvers do have a few drawbacks. The additives are large (high molecular weight) polymers, which makes them susceptible to being mechanically broken by shearing forces within the contact zone. Gears are notoriously hard on VI-improver additives. Permanent shearing of the VI-improver additive can cause significant viscosity losses.
There are a large number of chemical compounds that can be used to provide this functionality, too many to list. They are generally soluble flexible polymer molecules that uncoil and spread out as temperature increases. This has the effect of increasing the viscosity of the lubricant as temperature rises, which is counter to the natural tendency of the base oil (which is generally as reduction in viscosity as a function of temperature increase). They also possess or are structurally made up of numerous branches that entangle with those of other neighboring molecules, which in turn creates macromolecular structures. These macromolecular structures trap and conglomerate smaller oil molecules, thus increasing the effective viscosity of the lubricant.
These keep surfaces free of deposits and neutralize corrosive acids formed due to oxidation. These molecules are chemical bases consisting of a polar substrate and a metal oxide or hydroxide. Metal-organic compounds of calcium and magnesium phenolates, phosphates, salicylate, and sulfonates are recommended. Over-based detergents are used in lubricants expected to see high humidity environments and are there to neutralize the large amounts of acidic components produced by lubricant oxidation. Deposit precursors are insoluble in the lubricant and tend to have greater affinity for the detergent molecules. The additive molecules cling to the surface of the particle and envelop it, thereby also acting as dispersants and prevent those particles from agglomerating and later causing them to settle in sump out of the lubricant replenishment stream. A detergent additive is normally used in conjunction with a dispersant additive.
Tackifiers are stringy materials used in some lubricants and greases to prevent the lubricant from flinging off the metal surface during rotational movement. To be acceptable to lubricant formulators and end users, the additives must be capable of being handled in conventional blending equipment, stable in storage, free of offensive odor, and be nontoxic by normal industrial standards. Since many are highly viscous materials, they are generally sold to the lubricant formulator as concentrated solutions in a base oil carrier.
These are used to alter the coefficient of friction that would be experienced between sliding surfaces when only the base oil is present. (Figure 5) Organic and sulfurized fatty acids, amines, amides, imides, high molecular weight organic phosphorus, and phosphoric acid esters are added volumetrically in the range of 0.1 percent and 1.5 percent. They preferentially adhere (adsorb) very strongly to metal. The head of the friction modifier is attracted to the metal surface and the long tail (typically with at least 10 carbon atoms) remains solubilized in the lubricant. The chemical structure and the polarity of the molecules play a major role in the friction reduction. The polarity of head group provides for strong surface adsorption. The physical, chemical, and tribological properties of ionic liquids can be tailored to suit a wide variety of applications ranging from its use as polymer brushes in biological application, or as water soluble or oil soluble lubricant additive. They also work, as in the case of Automatic Transmission Fluid (ATF), to reduce the difference between static and dynamic coefficients of friction. In addition, the additives used to create ATF promote a better friction curve. Remember that in a friction-based system (i.e. a wet clutch pack, etc.) we want the interaction between the lubricant (ATF) and the chemistry of the friction material to generate a friction response curve with a positive slope, and to maintain that positive slope over the expected service life of the clutch. As well, we do not want the friction modifier (enhancer) to provide more stable friction system response at the cost of lubricating and cooling our gears and bearings — a very interesting compromise requiring precise chemistry and formulation, to be sure. (Figure 5)
Anti-wear and Extreme Pressure Additives
These additives are used to cause metal wetting. This is when additives anchor to metal surfaces, which is what they are supposed to do. They attach to the interior of the gear casing, gear teeth, bearings, shafts, etc. Additives that perform this function are rust inhibitors, anti-wear (AW) and EP additives, oiliness agents, and corrosion inhibitors. Anti-wear additives work specifically to protect metal surfaces during boundary conditions. They form a ductile, ash-like film at moderate to high contact temperatures (150 to 230 degrees F) and under boundary lubrication conditions, the film shears instead of surface material.
Anti-wear additives and extreme pressure agents form a large group of chemical additives that carry out their function of protecting metal surfaces during boundary lubrication by forming a protective film or barrier on the wear surfaces. As long as the hydrodynamic or elastohydrodynamic oil film is maintained between the metal surfaces, boundary lubrication will not occur, and these boundary lubrication additives will not be required to perform their function. When the oil film does break down and asperity contact is made under high loads or high temperatures, these boundary lubrication additives protect the wearing surfaces.
Let’s now investigate how additives work within a base oil. Most work through the principle of polarity and are typically described under the banner of Polar Additives. Additive polarity is defined as the natural directional attraction of additive molecules to other polar materials in contact with oil. In simple terms, it is anything that water dissolves or dissolves into water. It’s important to note that additives are also sacrificial. Once they are gone, they’re gone.
Polar mechanisms, such as particle enveloping, water emulsifying, and metal wetting should be discussed in the context of delivery of the additive package to the load zone. Particle enveloping means that the additive will cling to the particle surface and envelop it. These additives are metal deactivators, detergents, and dispersants. They are used to peptize (disperse) soot particles for the purpose of preventing agglomeration, settling, and deposits, especially at low to moderate temperatures.
When using additives, more is not always better. As more additive is blended into the base oil, sometimes there isn’t any more benefit to be gained, and at times the overall performance of the lubricant can be degraded. In other cases, the performance of the additive does not improve, but the duration of service does improve. In addition, increasing the percentage of a certain additive may improve one property of the lubricant, with an adverse effect on another particular additive. When the specified concentrations of additives become unbalanced, overall lubricant quality can also be affected. Some additives compete with each other for the same space on a metal surface. If a high concentration of an anti-wear agent is added, the corrosion inhibitor may become less effective. The result may be an increase in corrosion-related problems. In lubrication terms, this is known as additive depletion.
Lubricant Additive Depletion
It’s very important to understand that most of these additives get consumed and depleted by:
- Decomposition or breakdown.
- Adsorption onto metal, particle, and water surfaces.
- Separation due to settling or filtration.
The adsorption and separation mechanisms involve mass transfer or physical movement of the additive. For many additives, the longer the lubricant remains in service, the less effective the remaining additive package. When the additive package is depleted, viscosity increases, sludge begins to form, corrosive acids start to attack bearings and metal surfaces, etc. Generally speaking, the higher the quality (i.e., purity, cleanliness, etc.) of the base stock oil, the longer the lubricant will last. You really do get what you paid for.
By way of references, I would like to thank Debashis Puhan for the excellent article titled, “Lubricant and Lubricant Additives,” presented June 8, 2020, reviewed on August 31, 2020, and finally published November 27, 2020, in the journal Tribology in Materials and Manufacturing – Wear, Friction and Lubrication. And as always, I would like to thank my friends at Afton for all their support, guidance, and the knowledge they shared over the years.