An Inverse Analysis Approach to the Characterization of Chemical Transport in Paints

The overall goal of the following experiment is to characterize the transport dynamics and mechanisms of a chemical into and out of a thin paint coating. This is achieved by applying the contaminant of interest to the test substrate, which allows for the interaction of the contaminant with the painted surface. As a second step, the contaminated substrate is placed inside the high vacuum experimental chamber, which characterizes the gas desorption profile of the contaminated substrate using a mass spectrometer.

The results allow the determination of the saturation, concentration, and diffusion constant values for the contaminant transport through the paint based on the analysis of the measured mass flux of the contaminant from the substrate. This method can help answer questions in materials characterization, such as determining the properties that drive the penetration of harm harmful chemicals. Understanding these properties enables the development of chemically hardened materials that has the potential to reduce decontamination burden and minimize health risks to people who have to interact with these contaminated materials.

The main advantage of this approach over existing methods like Desorption and integral techniques, is that it allows for the determination of highly precise mass transport properties in thin film coatings. Then using these properties, it's possible to simulate chemical distributions within materials that enable the evaluation and prediction of health hazards that arise due to absorbed contaminants. Demonstrating the procedure today will be Sean, a co-author on the manuscript, and Jan Lin, a chemist in our lab.

Working with chemical warfare agents is extremely hazardous and should only be done by trained personnel and certified facilities using redundant safety and security precautions such as personal protective equipment, the two man rule, and properly certified engineering controls. First preset the environmental chamber for substrate conditioning to the specified temperature and relative humidity, which is 20 degrees Celsius and 50%place, 0.32 centimeter thick, 5.08 centimeter radius, painted stainless steel discs with a total paint coating thickness of approximately 100 micrometers on stainless steel trays with a test surface to be exposed to a chemical agent facing upwards. Next, cover the substrates with Petri dishes.

Place the trays containing the test substrates into the environmental chamber for at least 60 minutes, but ideally overnight if possible. After doning the appropriate personal protective equipment, obtain the chemical contaminants from cold storage, allowing them to equilibrate to room temperature prior to use. Following this, fit the repeater pipette with a tip and ensure that the tool is set to deliver the appropriate volume of contaminants.

After uncapping the contaminant vial, place the cap on the hood surface threads facing up. After picking up the pipette, slowly lower the tip into the contaminant solution. Load the agent delivery tool with agent in accordance with the manufacturer's directions.

By slowly pulling up the lever on the side of the pipette, gently place the loaded pipette onto the hood working surface, and recap the contaminant vial. Once the trays containing the test substrates have been removed from the environmental chamber, remove the Petri dishes and set them on the hood surface digitally. Photograph each test substrate to record the appearance of each substrate before contamination.

Next, deliver a single droplet of agent onto the first test substrate. Cover the contaminated material with a polystyrene petri dish to minimize evaporation while contaminating additional substrates. Once again, remove the Petri dishes from the test substrates and set them on the hood surface digitally.

Photograph each test substrate to record the initial contaminant material interactions. After covering the test substrates with the petri dishes, place the tray of substrates back into the environmental chamber. Collect pipette confirmation samples before and after dosing of substrates for analysis via chromatography to confirm contaminant mass delivery prior to aging.

The samples vent the stainless steel high vacuum experimental chamber to prepare it for later use. After allowing the contaminated substrates to age in the environmental chamber for the specified duration, place them on the working surface of the hood following removal of the Petri dishes from the test substrates. Digitally photograph each substrate to record the post aging contaminant material interactions.

Double contain the test samples in multiple airtight containers and transfer to the working hood with the high vacuum chamber. After ensuring that the chamber is properly vented, unpackaged the samples and remove the test substrates from the Petri dishes. Then place one test substrate into each temperature controlled substrate holder using stainless steel tweezers.

Seal the vacuum chamber and begin the pump down sequence. Following this, begin recording Selected mass fragment channels as a function of time identified with specific mass per unit charge values. Measure specific background gas species in addition to the primary mass fragments from the molecules of interest in real time at less than 0.25 hertz until the contaminant partial pressure drops below the detection limits of the mass spectrometer.

Collect the emission curves for the duration of the emission of contaminant from the substrate stop recording emission curves with the mass spectrometer. Once the contaminant mass flux has decreased to the chamber pressure baseline, then vent the high vacuum chamber to atmospheric pressure. At this point, open the high vacuum vapor emission chamber instrument and remove the substrate from the chamber using stainless steel tweezers.

Place the substrate into a glass extraction jar and add 20 milliliters of extraction solvent to the jar, cap the jar and swirl the jar three times. After leaving the substrate in the extraction solvent for 60 minutes, swirl the jar three times again and then uncapped the jar using a clean disposable glass pipette. Transfer approximately one to two milliliters of extraction solvent into an analytical vial for analysis via gas or liquid chromatography to measure the contaminant mass retained by the substrates.

Examples of the calculated mass flux of VX and HD from solvent dispersable painted substrates based on time result mass spectrometry for the main mass fragments are shown here. The ionizer was located in close line of sight, proximity to the surface so that species emitted were detected preferentially and pseudo instantaneously. The different boundary conditions associated with constraining the analytical solution for molecular diffusion in a finite thickness absorbing coating are shown here.

The transport model enables simulation of the contaminant absorption during the aging period and the resulting flux during the experiment. Simulation results for the spatial dependent concentration distribution of HD or VX are illustrated here. Although the coating absorbed a higher mass of HD as indicated by a greater saturation concentration, it is more localized near the substrate surface compared with vx.

The calculation and prediction of resulting vapor flux of the contaminants from the paints is shown here based on fitting experimental data. The differences in transport phenomena between HD and VX demonstrate that intermolecular interactions can alter the transport of absorbing molecules through the paint systems to generate different distributions. Please remember that working with chemical warfare agents and stimulants is extremely hazardous and should only be performed at certified facilities using safety and security precautions consistent with what we've demonstrated today.