Disperants, Detergents and ZDDP

Lubricating oils are made up of many different items to assure their effectiveness for the application they are designed for. To learn more about these items we went to lubrication expert, Mark Barns from Noria and asked a few questions.

Precision lubrication is a simple concept - at least at face value. Simply put, it requires the right lubricant, be it oil or grease, to be put in the right place, at the right time; that the lubricant and equipment it is lubricating be kept clean, dry and cool; and that rotating equipment be kept appropriately aligned and balanced.

While sound lubrication engineering principles can be used to ensure that lubrication selection is appropriate, used oil analysis’ role, to a large extent, is to ensure that the properties for which the lubricant was selected are still intact and appropriate for the stated application.

Many of the important properties of a lubricant are dependent on the base oil. Its ability to support a dynamic load, its ability to control and dissipate heat and its ability to provide motive power in hydraulic systems are just a few of the important properties. However, in many instances, it is the additives in the lubricant that are just as important - in some instances, almost more important - than the base oil in providing appropriate lubricant functionality. Despite this, many oil analysis programs focus simply on measuring base oil degradation, using tests such as viscosity and acid number, and fail to address the condition of the additives.

Dispersants
In most engine oil formulations, the additive present in the highest concentrations are dispersants, typically present in the 3 percent to 6 percent by weight range. Dispersant additives are designed to hold sludge and other contaminants such as soot in suspension, until they can be filtered out, or otherwise removed through an oil change. The chemistry of a dispersant consists of a polar head group with a long hydrocarbon tail. This acts something like laundry soap, trapping the dirt into what is commonly called micelle.

The basic functionality of the dispersant additive is simple. As the dirt, soot or sludge enters the oil, it is trapped in the core of the dispersant’s micelle. This trapping action prevents contaminants from depositing on engine parts like rings and valves and causing premature equipment failures.

The most common chemistry of a dispersant is a polyisobutylene succinimide. The polyisobutylene’s molecular weight is greater than 1,000 and forms the hydrocarbon tail. The succinimide portion of the molecule contains a polyamine and forms the polar head. In the new oil, this additive can be observed in the FTIR spectrum.

Detergents
In an engine oil formulation, the next most common additive is the detergent. This additive is typically present in the oil at about 2 percent to 3 percent. Detergents are used primarily to control acids formed by the combustion of impurities found in the fuel. However, it does have some ability to “wash” the metal surfaces of organic deposits. There are several types of detergents that are used in these formulations.

The most important of these are sulfonates. The chemistry of the sulfonate is somewhat similar to the dispersant in that it has a hydrocarbon tail and a polar head. The hydrocarbon tail is much shorter, being only a C-16 to C-30 alkyl benzene. The polar head is the salt of a sulfonic acid, typically calcium, magnesium or sodium. In addition to this neutral sulfonate salt, the detergent typically contains an excessive amount of metal carbonate, which is incorporated in the center of the micelle.

ZDDP
Zinc dialkyldithiophosphate (ZDDP), an antiwear and antioxidant additive found in many types of hydraulic and lubricating fluids. Zinc dialkyldithiophosphate, depending on formulation, a common AW hydraulic fluid may contain anywhere from 100 ppm to 500 ppm of ZDDP, as measured by the elemental concentrations of zinc and phosphorus.

Subjecting an oil containing ZDDP to high temperatures and high levels of moisture will likely result in significant additive depletion due to hydrolysis - a chemical reaction between the ZDDP molecule and water. Under such circumstances, the ultimate by-products of the hydrolysis reaction will likely be zinc salts and phosphates, which although no longer chemically ZDDP, may remain in solution in the oil.

The result is that by considering only zinc and phosphorus concentrations, the difference between “good” zinc and phosphorus in the form of ZDDP and “bad” zinc and phosphorus from reaction by-products will be next to impossible to determine.

Don’t forget to stop by our booth in Columbus Ohio at the Lubrication Excellence conference May 16th thru the 18th. http://www.noria.com/conference/le06/splash/

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