Most of todays lubricants contain some combination of additive components which are designed to provide the proper performance in a given application. Some lubricants may need only a single component, such as a rust inhibitor, but others require complex mixes of a number of different components. Engine oils are a particularly intriguing mix of chemistries which, taken together provide the protection and performance modern engines require to do their job.
Broadly, an additive does at least one of three things: It protects the base oil, it enhances base oil properties or it protects the surfaces that it contacts. There are some which are multifunctional, providing more than one facet of the protection and enhancements required for a lubricant.
Over the years a number of different engine oil marketers have touted their additive systems as unsurpassed, unequaled or some other superlative in an effort to capture a greater share of the marketplace. Consider, for example, the old Pennzoil Z-7 program which dates to the early 1950s. For those of you too young to remember, Z-7 was shorthand for a set of performance components in the oil: 1) pour point depressant, 2) oxidation inhibitors, 3) anti-wear, 4) rust and corrosion inhibitors, 5) detergent/dispersants, 6) friction modifier, and 7) antifoam.
Those basic tools are still in place, but a modern-day passenger car engine oil has some additional goodies which improve fuel economy as well as oxidation resistance. To gain a better understanding of additive systems in general and engine oil additives specifically, heres a look at the components in a typical additive package for blending ILSAC GF-5 engine oils, the newest industry specification.
Viscosity Index Improvers
As the name suggests, these boost the viscosity index (V.I.) of the oil. V.I. is a measure of the change in viscosity of an oil with temperature. Higher-V.I. oils do not change viscosity (thicken or thin) as much as lower-V.I. oils over an arbitrary standard temperature range of 40 to 100 degrees C.
V.I. improvers or modifiers are polymers and work by a process of temperature-related solubility. At lower temperatures, the polymer is less soluble in the oil, which results in lower relative viscosity. At higher temperatures, the polymer is more soluble, resulting in higher relative viscosity. The exact thickening mechanism is open to discussion. Some believe that the polymer shape changes in the oil from a coiled structure to an elongated one with increasing temperature. Others believe that the solubility is simply temperature dependent, without any structural changes.
One property which is of importance is shear stability, which is the amount of thickening power lost due to mechanical shear in the engine. As the polymer is subjected to high-shear-rate environments in the engine, it can break (shear down), causing smaller, less effective molecules to be formed. There is also a temporary shearing effect which is the result of the polymer aligning with the direction of shearing force and causing a temporary loss of viscosity. When the shearing stress is gone the polymer regains a more random alignment in the oil, with the result that viscosity goes back to what it was originally.
In order to meet viscosity limits related to the newest engine oils (such as SAE 0W-20), formulators are finding it necessary to use very low viscosity base oils. As even lower viscosity engine oils find their way into the marketplace, the base oils will continue to be lower and lower in viscosity – resulting in other needs that will have to be addressed, some by refining and others by additive chemistry.
Pour Point Depressants
A lubricants pour point is the lowest temperature at which it will continue to flow – the lower the better if you live in a cold climate. When chilled, waxy molecules in the oil will stiffen and clump together, preventing the fluid from flowing. So pour point depressants are a special part of the engine oil additive system: They work by modifying wax crystals in paraffinic base oils.
At older API Group I base oil refineries, traditional solvent dewaxing removes wax on the basis of melting point, with the normal target being about 0 degree F. Newer API Group II and III oils are catalytically dewaxed using hydrotreating and isomerization processes to alter the wax found in the oil, rather than remove it. The difference in processes leads to differences in the PPD used.
Solvent dewaxing leaves a range of wax structures in the base oil, and so a broader spectrum PPD is needed. In the case of catalytically dewaxed oils, the PPD must be more specific to the wax structures found in the particular oil. In fact, selecting the appropriate PPD becomes almost base stock-, process- and viscosity-specific. Oil formulators and experts in the PPD area know which materials work in specific base stocks and select them according to the needs of the formulation. In general, low-viscosity oils have different requirements than do higher-viscosity oils, due to differences in wax structure and amount of wax present in the base stock.
The DI Factor
Additive packages for engine oils are commonly referred to as DI packages. DI stands for dispersant-inhibitor.
By volume, dispersants are the main additive component in engine oils, often accounting for up to 50 percent of the total additive package. As their name implies, dispersants function by preventing or inhibiting the concentration of various contaminants in the oil. Contaminants are typically the result of engine exhaust gases, which bypass the rings and get into the oil. Once in the oil, they cause other phenomena to occur such as sludge and varnish formation and oxidation. By containing these bad actors (called blow-by), dispersants minimize their negative impact on the engine oil and the engine surfaces.
In chemical terms, a dispersant functions via chelation, catching the contaminating particles in reactive locations within the dispersant. Its like the pincers of a crab grabbing on to the particles. In fact, the word chelation is derived from a Greek word meaning claw.
The most common dispersant chemistry uses large nitrogen-containing molecules, with the nitrogen sites being the active points that attract and hold the contaminants. (Some dispersants are derived from non-nitrogen bases.) It is these molecules that darken as the oil is used, and which are finally completely used up. Thats why oil changes are necessary.
Detergents and Base
The next-most-common component in the DI package is a detergent. The name is a bit of a misnomer since these compounds do not clean up anything. They act primarily on the metal surfaces, providing a protective film which minimizes the formation of deposits. A strong bond forms between the detergent molecule and the metal surface, effectively preventing blow-by and oxidation products from collecting on the surface. Detergents were introduced in World War II to increase the operational life of engines, particularly in submarine service, and soon proved their value in surface vehicles too.
Chemically, detergents are most often alkyl metallic sulfonates, alkyl metallic phenates, or alkyl metallic salicylates. They are normally neutralized by metal oxides such as magnesium, calcium or occasionally barium, to form low-base detergents. Alternatively the material can be neutralized with a large excess of metal oxide and then treated with carbon dioxide, to form what we call overbased detergents.
Low-base detergents normally are alkyl sulfonates. They provide a soap-like structure which is an effective means of controlling deposits that can build up on piston surfaces and in ring grooves.
Overbased detergents, in addition to their soap contribution, provide a means of neutralizing acids formed by the combustion process. This is especially important when using high-sulfur fuels, because when sulfur is involved in the combustion process, it forms sulfur oxide compounds when in contact with water, these go on to form sulfuric or sulfurous acids. It is critically important to neutralize these compounds in the oil before they can attack the various metal surfaces.
The use of one type of detergent or another is normally the choice of the additive formulator, and may depend on the materials each company produces. Additive suppliers have long-established histories with their particular detergents, and can optimize them in their packages. Often, a combination of low-base and overbased detergents is used to get peak performance in engine tests and field use.
The next major component of an engine oil additive package is an antiwear agent. Historically, zinc dialkyldithiophosphate (ZDDP) has provided wear protection in high-load areas of the engine, preventing the loss of metal.
ZDDP was first patented in 1944 by Herbert Freuler of Unocal, and over six decades later is still going strong. By one estimate, some 300 million pounds are manufactured in the Western hemisphere each year. These compounds contain both phosphorus and sulfur, which react with metal surfaces to create a thin film of metal sulfides and phosphides, which wears away and prevents welding of the mating surfaces.
ZDDP has additional properties which make it valuable as an antioxidant and a corrosion inhibitor. When used with classic metal-free ash-less antioxidants, it often gives a synergistic response in oxidation tests. It is also very protective of many bearing materials, including copper, lead and aluminum metal alloys.
There is a dark side to ZDDP however: Its phosphorus and sulfur content must be reined in as much as possible to prevent any negative impact on vehicle exhaust systems. Phosphorus from engine oil that gets past the piston rings and into the exhaust can react and create tenacious, glassy deposits on the catalytic converter, deactivating the device. For that reason, other compounds have been harnessed to aid wear prevention without as much impact on catalysts. Molybdenum-containing materials are being used increasingly for this purpose. Another approach is to use less-volatile phosphorus compounds, which stay anchored in the engine oil instead of decamping to the exhaust system.
Another class of antiwear agents is the metal dithiocarbamates. These compounds contain sulfur but not phosphorus, and are effective anti-wear agents as well as antioxidants and corrosion inhibitors. Zinc, molybdenum and antimony metals are most often used to make these.
Oxidation, Friction and Foam
Antioxidants are another key part of the additive system. These compounds are most often derivatives of either phenol or phenyl amines. They work by interfering with the oxidation process, which can create highly reactive materials called free radicals. By interfering with the formation of free radicals, oxidation inhibitors retard the degradation of the base oil. The process by which ZDDP retards oxidation is by decomposition of one of the intermediates in the oxidation process known as peroxides.
The need to maximize the fuel economy contribution of the engine oil has caused friction modifiers to become a major ingredient (although not dosage-wise) in modern engine oils. Their contribution to friction reduction is small in an individual engine, but carries weight in the greater world of Corporate Average Fuel Economy requirements. Chemically, friction modifiers are surface-active agents which reduce the drag on metal parts moving over each other. Most of the friction in an engine is the result of piston-ring-to-cylinder-liner contact.
Antifoam agents are a small but vital part of the engine oil additive system. Usually found in oils at parts-per-million levels, these materials change the interfacial tension of the oil, which aids in releasing air from the oil. This is an important need as entrained air can accelerate oxidation and effectively reduce the film strength of the oil, causing more metal-to-metal contact. Simply stated, bubbles dont lubricate very well.
The science and technology of engine oil additives has been developed over at least 75 years. There is little doubt that additive technology will continue to improve – perhaps not radically, but as new requirements surface, new or modified chemistry will emerge to fill the need.