This project was undertaken using advanced instrumentation techniques and methods for conducting an exploratory investigation on chemical degradation and mechanical penetration of coatings to reduce maintenance costs.

Degradation of aircraft coating systems is of vital importance from a safety standpoint as well as aesthetically, and results in significant maintenance cost to the aircraft industry. Early detection of degradation is critical to prevent complete failure and delamination. Development of methods based on spectroscopic techniques are required because of their potential to be used for in-situ assessment. Aircraft coating systems, usually containing polyurethane, are prone to degradation through microbes and harsh weathering conditions. The broader goals in this project were three-fold:

1. isolate, optimize fungal growth, and identify various potential fungal species from degraded coats;
2. investigate physical penetration of isolated fungi using Atomic Force Microscopy (AFM) and Environmental Scanning Electron Microscopy (ESEM); and
3. assess chemical degradation using Raman and Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy techniques.

Fungal penetration and chemical degradation of in-house and U.S. Army prepared model coating systems was reported. They first optimized growth conditions for the two fungal species, Aureobasidium pullulans and Fusarium spp. on Potato Dextrose Agar medium. Second, model coating systems were prepared on aluminum (Al)-coated glass slides with (A) white Universal bonding primer (Rust Oleum®) coating; and (B) white Universal bonding primer + a clear top coat (Dupli-Color®). Third, fungi and the two coating systems were characterized using Attenuated Total Reflectance (ATR)-FTIR spectroscopy to obtain corresponding reference spectra. An accelerated fungal-biodegradation study was also conducted by inoculating the two coating systems with pre-grown fungi isolates at 28°C and 80% humidity for approximately eight weeks.

The AFM and confocal laser scanning microscope scans showed the tested fungal isolates grew well, penetrated, and produced chemical and topological changes to the model coating system. In addition, the ATR-FTIR analysis of the fungal exposed samples showed a decrease in the bands’ intensities corresponding to various polyurethane bands (1726 cm-1 Carbonyl (C=O) species;1466 cm-1 Aliphatic (CH2) stretch polyurethane backbone; 1240 cm-1 (C=O) + (O-CH2) stretch; 1147 cm-1 C-O-C stretch). Control coating systems were similarly incubated without being exposed to the fungi, so they did not show any fungal degradation. Fungal exposed samples showed an increase in Amide-I band (1653 cm-1) intensity indicating biomass growth. At a later stage in the project, a spore suspension of each of A. pullulans and a consortium of fungi were used to inoculate U.S. Army coating systems. Data from ESEM showed fungal physical penetration, cracking, and damage to U.S. Army coating systems. It was intriguing to observe that the clean samples of U.S. Army paint borne coating systems were contaminated with several mold fungi such as Penicillium, Aspergillus, Phycomycetes, and other unidentified isolates of fungi. 

Further work on the project is likely to result in improved maintenance, new protocols, and substantial reduction in maintenance costs to the U.S. military. The results from this study established the basis for detailed studies, thus knowledge gained will have broad implications in an array of industries.