When it comes to protecting warfighters from exposure to chemical agents that have contaminated combat vehicles, determining how much agent gets absorbed into the material matters. That's what researchers at the U.S. Army Edgewood Chemical Biological Center (ECBC) discovered and helped the Army fix.
The U.S. Army uses more than 1 million gallons of coating, like paint, a year for its combat vehicles. One of the key jobs of a coating, in addition to providing corrosion resistance and camouflage, is protecting its occupants from chemical agent exposure, said vehicle coatings commodity area manager, John Escarsega, at the U.S. Army Research Laboratory. The key to this protection is to make sure vapor and contact hazards are not still present after soldiers and maintenance personnel believe they have decontaminated the vehicle. What the Army needs is a coating that resists chemical agent absorption so that there is no agent left to resurface after decontamination efforts are complete.
For decades, the method for determining the chemical agent resistance of a coating was to place a known amount of chemical agent on a coating sample under engineering-controlled laboratory conditions, wash it with isopropyl alcohol, then measure how much agent vapor re-emitted from the exposed material in the air above it after 22 hours. However, ECBC researchers who specialize in material decontamination had learned that materials like coatings can absorb agent and then re-emit it slowly over a much longer period of time. By carefully reviewing the Army's current chemical agent resistance method, they determined that the vapor collection accounted for only 43 percent of the agent actually in the coating sample. The remaining 57 percent was still trapped in the material and would continue to come out long after the 22-hour measurement window. The researchers recognized this as a significant flaw and notified Escarsega.
"We had to change the Army's perspective on chemical agent resistance from presuming that coatings do not re-emit agent to a realization that they do absorb agent and will re-emit agent later," said lead researcher Dr. Brent Mantooth. "What goes into the coating eventually comes out, so we had to come up with a way to accurately measure retained agent. We also had to figure out and demonstrate how to accurately measure retained agent in a robust and reliable fashion for many different types of materials and agents."
From his study of the interaction of liquid agent droplets with surfaces in his other, related research projects, Mantooth had learned that the time liquid agent is permitted to remain on the surface of a coating is a factor in determining how much agent will permeate into the coating material. Also, in order to accurately measure the amount of agent in the coating, liquid agent remaining on the exposed surface has to be removed in a fashion that will not affect the measurement of the absorbed agent. The current test method caused confusion because it did not include removing the agent on the surface.
Armed with this perspective, Mantooth and his team set a goal of revising the evaluation procedure using a simple and reproducible method for removing the surface-bound liquid agent so that the agent that had absorbed into the material could be measured. The research team investigated a range of different techniques for removal of surface-bound liquid agent from materials. They ultimately arrived at immersing the surface in soapy water along with water rinses. Thus they were able to remove surface-bound liquid agent before using a solvent to extract agent absorbed by the material. The soapy water immersion process emulates the treatment process typically used by soldiers in the field; and actually extracting the agent from the coating, as opposed to relying on off-gassing data, accounts for all the agent that's been absorbed.
The soapy water rinse and solvent extraction procedure is applied to test samples of a vehicle coating for contamination times of five minutes, 60 minutes and 360 minutes on three samples. This approximates the immediate, operational, and thorough decontamination times in a typical exposure event in the field. The amount of agent recovered from the samples determines the degree to which a coating can be considered agent resistant.
"Put another way," said Mantooth, "the less agent the coating retains, the more agent resistant the coating is. This translates to a lower exposure risk to the warfighter and less work decontaminating the vehicle, which is the true measure of agent resistance."
Mantooth and his team are currently finalizing a multi-laboratory verification and validation process to prove the method's repeatability and reproducibility. After that, the method will be codified in the U.S. Military Standard, or Mil Spec, MIL-DTL-64159, which all vendors of coatings to the U.S. armed services will be required to follow.