Finding Dispersants on Oil Analysis Reports

"Which elements on an oil analysis report are considered dispersants?

On your regular routine oil analysis report, the answer would be none. A dispersant in oil is defined as an additive, usually nonmetallic ("ashless"), which keeps fine particles of insoluble materials in a homogeneous solution. Hence, particles are not permitted to settle out and accumulate. The most common chemistry of a dispersant is a polyisobutylene succinimide.

Dispersants, in addition to antioxidants, viscosity-index improvers and some anti-foam additives are organic molecules. An organic additive molecule contains carbon, hydrogen and possibly oxygen, nitrogen or sulfur, none of which are routinely detected using elemental spectroscopy.

The metallic additives typically monitored via elemental analysis include the following:

• Anti-wear additives — zinc and phosphorus (ZDDP)
• Extreme pressure — phosphorus
• Detergents — calcium, magnesium and barium

However, elemental spectroscopy has two major limitations with respect to tracking additives. First, the technique does not actually measure additives, but rather the individual elements or atoms contained within the additive molecule. While this comment may seem obvious, it has serious implications when talking about trending additive depletion.

To understand the potential problem, consider the fate of one of the most common additives, zinc dialkyldithiophosphate (ZDDP), an anti-wear and antioxidant additive. Depending on formulation, a common anti-wear 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, which is a chemical reaction between the ZDDP molecule and water. Under such circumstances, the ultimate byproducts of the hydrolysis reaction will likely be zinc salts and phosphates, which although no longer chemically ZDDP, they 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 byproducts will be next to impossible to determine.

The second limitation of using elemental spectroscopy for tracking additive depletion is perhaps even more fundamental. Many common additives such as antioxidants, dispersants, VI improvers and some anti-foam additives are organic molecules. Simply stated, an organic additive molecule contains carbon, hydrogen and perhaps oxygen, nitrogen or sulfur. Because none of these elements are routinely detected using elemental spectroscopy, rotating disk electrode (RDE) or inductively coupled plasma (ICP) spectroscopy offers little or no help in monitoring organic additive health.

A more precise tool for measuring additive depletion is Fourier transform infrared (FTIR) spectroscopy. Monitoring the FTIR spectrum will allow for checking of dispersants.