These packages are added to your lubricants to help them do all that you want them to do.
Designing a lubricant requires the tribology chemist to start with a performance specification sheet outlining parameters and conditions the lubricant must meet and exceed in its finished commercial form. Typical design parameters include:
These typical design parameters are technically known as the “Tribological System” within which the lubricant must perform. Lubricant type, application, quality and operating temperature range are primarily used to determine an appropriate base stock that is then supplemented with a variety of lubricant additives to strengthen or modify the product’s characteristics to meet the finished design specification. Lesser-quality base stocks can be significantly modified to meet specifications with additive packages.
What additives do
Additives play an important role in letting us know when the lubricant is no longer useful, as they are sacrificial in nature: Once depleted, the oil must be replaced or replenished again with the necessary additive(s). By monitoring and comparing the additive package “signature” of the virgin stock oil against a used oil sample through the use of oil analysis, we can tell when oil is degraded and ready to be changed.
Oil additives serve three functions: 1) to enhance; 2) to promote new properties; and 3) to suppress undesirable base-oil properties. Different lubricant types require different formulation packages. Table I depicts which additive performs what function. Additives can be organic or inorganic compounds and, depending on their physical size, will dissolve in the oil (i.e. sub-micron) or remain as suspended solids. These solids are often visible to the naked eye when decanting new oils from one container to another.
Although there are many additive types available to the lubricant chemist, the following 12 are used to make up the core additive package for most commercial lubricants.
1. Anti-Foam Agent. . .
When a fluid is moved quickly through a pumping action, it can entrain small air bubbles in the lubricant. These air bubbles are detrimental to a lubricant as air contains oxygen that will attack the base oil (see Antioxidant). Aerated fluids can also cause pump cavitation. Also known as defoamants, or foam inhibitors, anti-foam agents are designed to increase a lubricant’s surface tension and enlarge the bubble size, allowing them to collapse more easily.
2. Antioxidant Agents. . .
Oxygen is base oil’s primary enemy, especially at higher temperatures, when in combination with contaminants such as water can lead to sludge and viscosity thickening, tar, varnish and corrosive acid formation within the oil and on the bearing surfaces. Antioxidant agents, also known as oxidation inhibitors, can successfully improve oxidation stability by more than 10 times by deactivating catalytic metallic contaminants and by decomposing any formed reactive hydroperoxides within the oil. The most common antioxidant is Zinc dialkyldithiophophate, or ZDDP.
3. Anti-Wear Agents. . .
These types of additives activate when boundary lubrication conditions occur due to metal-to-metal contact under heavy loads and slow rpm. Agents such as ZDDP or Tricresylphosphate (TCP) react chemically with the surface to form a softened ash-like lubricant film.
4. Demulsifiers. . .
Known as emulsion breakers, these types of additives are used where water contamination is expected. They are designed to chemically prevent the formation of any water/oil emulsion by altering the surface tension of the oil, allowing the water to separate easily and be drained off.
5. Detergents. . .
Detergent-type additives are used where combustion takes place. They perform as a chemical cleaner to keep combustion surfaces free from harmful deposits and to neutralize any combustion acids. Developed specifically for crankcase and compressor oils, detergent additives are made up from over-base (alkaline) organic metallic soaps such as barium, calcium and magnesium.
6. Dispersants. . .
Dispersant-type additives are also used in crankcase and compressor oils, often in conjunction with detergents. They chemically disperse and attach themselves to and suspend combustion and contaminant particles like dirt, soot, glycol and depleted additives, to extract them by the oil-filtration system.
7. Dyes. . .
Used in transmission fluids and greases, dye-type additives are used to help identify products and differentiate them from other lubricants.
8. EP (Extreme Pressure) Agents. . .
Sulphur, phopherous and chlorine additives are used to cause a chemical reaction that eutectically softens wear surfaces into a sacrificial metal soap that breaks away from the surface under high-load, extreme pressure conditions (thus reducing the frictional impact of metal-to-metal contact). Solids additives such as Molybdenum Disulphide (MoS2), Polytetraflouroethylene (PTFE) and graphite can be added to a lubricant to act as “sliding agents” and allow contact surfaces to move over one another with minimized contact under severe loading conditions with less surface degradation than the chemical additives.
9. Friction Modifiers. . .
Long-chain polar additives that have an affinity for metal surfaces are added to crankcase and transmission oils to reduce the surface friction of lubricated parts in an effort to increase fuel economy.
10. Pour-Point Suppressants. . .
Additives known as pour-point suppressants are used to prevent the formation of wax crystals in paraffinic mineral oils at low temperatures. This, in turn, allows the oil to pour at lower temperatures.
11. Rust Inhibitors . .
Also known as corrosion inhibitors, this type of additive forms a protective shield against water and corrosive acids to stop the formation of corrosion and rust on ferrous, copper, tin and lead-based metals.
Table II. Typical Oil-Type Additive Packages* (courtesy of EngTech Industries, Inc.)
Click to enlarge.
12. Viscosity Improvers. . .
Viscosity improvers employ long-string polymers that expand as an oil’s temperature increases. This process serves to “thicken” the oil, or increase its viscosity. These additives are used to increase an oil’s serviceability over a wider temperature range in multi-grade form and to bolster lower-quality base oils that have lower viscosity index (VI) ratings.
Table II details which additives are used in various oil types. Note: This table is provided for guideline purposes only! Always consult your lubricant supplier to determine which additives are actually used in the lubricant you have chosen to use. LMT