Additives can enhance, suppress, or add new properties to oils. Defoamants, dispersants, and detergents are no exceptions. This trio of additives can be found in most finished lubricants, albeit in varying ratios.
Let’s discuss the main differences among these three, why each is so important, and ways to confirm their presence.
What’s the Difference?
While they are all additives (which begin with the letter D), their functions are distinctively different. They all work to protect the oil from various types of contaminants.
For instance, defoamants reduce the air bubbles in the oil. At the same time, detergents keep the metal surfaces clean, and dispersants encapsulate the contaminants so they are suspended in the lubricant.1 This is illustrated in Figure 1.
From our last article on Lubricant Additives – A Comprehensive Guide, here are some detailed descriptions of how each of these additives functions.
Defoamants
When foam forms in the lubricant, tiny air bubbles become trapped either at the surface or on the inside (called inner foam). Defoamants work by adsorbing on the foam bubble and affecting the bubble surface tension. This causes coalescence and breaks the bubble on the lubricant’s surface1.
For the foam that forms at the surface, called surface foam, defoamants with a lower surface tension are used. They are usually not soluble in base oil and must be finely dispersed to be sufficiently stable even after long-term storage or use.
On the other hand, inner foam, which is finely dispersed air bubbles in the lubricant, can form stable dispersions. Common defoamants are designed to control surface foam but stabilize inner foam2.
Dispersants
On the other hand, dispersants are also polar, and they keep contaminants and insoluble oil components suspended in the lubricant. They minimize particle agglomeration, which in turn maintains the oil’s viscosity (compared to particle coalescing, which leads to thickening). Unlike detergents, dispersants are considered ashless. They typically work at low operating temperatures.
Detergents
Detergents are polar molecules that remove substances from the metal surface, similar to a cleaning action. However, some detergents also provide antioxidant properties. The nature of a detergent is essential, as metal-containing detergents produce ash (typically calcium, lithium, potassium, and sodium)1.
