Among all of the large-scale disinfection and/or decontamination technologies available, vaporized hydrogen peroxide (VHP®) is of particular interest due to its rapid sterilization, easy usage, intrinsic environmental friendliness (i.e. simple by-products composed of only water and oxygen), and compatibility with many materials and systems.

Wire Board (Figure a), Interdigitated Board (Figure b), Pad Board (Figure c), Active Board (Figure d), and Aviation (MILSPEC) Wire (Figure e)

VHP® technology has been investigated for possible usage in aircraft applications. These studies used vaporized hydrogen peroxide concentrations in the range of 150 - 600 ppm and cycle times of 80 - 120 min. Maximum concentrations of hydrogen peroxide vapor were carefully controlled to avoid condensation in cool locations within the aircraft cabins. Although these studies did not evaluate the compatibility of the various cabin materials with exposure to vaporized hydrogen peroxide, analysis of the collected data showed that VHP® did not seem to have any visible effect on the materials.

A typical VHP® process cycle consists of an initial dehumidification step, then a conditioning phase followed by the actual sanitization/decontamination process. Finally, an aeration phase is employed to remove residual hydrogen peroxide vapor. During the dehumidification phase, warm, dry air flows into the enclosure to lower the relative humidity to less than 10%. This allows a higher concentration of hydrogen peroxide vapor to be injected into the enclosure without causing condensation. Hydrogen peroxide liquid concentrate (35% liquid H2O2 with a pH ~ 3) is then flash vaporized and injected into the enclosure during the initial conditioning and sanitization/decontamination phases. The purpose of the conditioning phase is to rapidly increase the hydrogen peroxide concentration to minimize the overall cycle time.

During the sanitization/decontamination phase, a steady concentration of hydrogen peroxide vapor (typically 250 - 450 ppm) is maintained. This produces the desired sanitization/decontamination effect that is often measured by the 6-log kill (i.e. 106 reduction) of a commercial biological indicator (BI) spore population of Geobacillus stearothermophilus. Once the sanitization/ decontamination phase is completed, the enclosure is aerated with fresh air while any residual hydrogen peroxide vapor breaks down into environmentally benign water and oxygen.

Many polymeric materials are known to be susceptible to absorption of moisture. The small water molecules diffuse into the polymer matrix and force apart the polymer macromolecules, causing swelling. Increases in the distance between the polymer chains reduce the strength of the secondary intermolecular bonds and increase the softness and ductility of the polymer. However, the highly cross-linked epoxies used in aerospacegrade fiber composites minimize moisture absorption. Thus, these materials exhibit good resistance to degradation in wet environments.

While molecules of H2O2 vapor should be absorbed even less by epoxies than H2O molecules, the intermolecular crosslinks might be degraded by oxidation from the hydrogen peroxide. The extensive usage of fiber/epoxy composites in aerospace structures and avionics dictates that the compatibility of these materials with hydrogen peroxide vapor be examined. Therefore, a decision was made to design dummy and active printed circuit boards. The dummy circuit board had no active components. The active board had operational components including voltage level converters, a microcontroller, resistors, capacitors, and connectors. In addition to these boards, avionics wires were also used as test samples, as shown in the accompanying figure.

This work was done by Sin Ming Loo, Josh Kiepert, Derek Klein and Michael Pook of Boise State University; Shih-Feng Chou and Tony Overfelt of Auburn University; and Jean Watson for the Federal Aviation Administration. FAA-0002

This Brief includes a Technical Support Package (TSP).
Evaluation of the Effects of Hydrogen Peroxide on Common Aircraft Electrical Materials

(reference FAA-0002) is currently available for download from the TSP library.

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This article first appeared in the February, 2020 issue of Aerospace & Defense Technology Magazine.

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