The objective of this work was to develop an understanding of the degradation mechanisms of coatings materials induced by fungi, primarily those induced by mold and mildew. Recent fleet inspections have revealed damage induced by fungi that may be tied to the use of newer resin systems and non-chromate pigments. A successful project identified the primary constituents of the coatings that are susceptible to attack, the specific means of attack, and a path to develop solutions to mitigate or eliminate the problem. Proposed solutions did not compromise the environmental benefits of the target materials or degrade their performance.

In many geographic areas, the unwanted growth of mold, mildew, and other fungi on DoD weapons systems is a serious problem. Growth of mildew on military aircraft has been found to cause corrosion and operational damage, in addition to creating potential health hazards to flight crews and maintenance personnel. Traditional products for addressing fungal growth on surfaces are corrosive as well as environmentally hazardous. As regulations become increasingly more stringent, environmentally benign materials are becoming more prevalent. However, the use of more environmentally friendly finishing systems may facilitate the growth of mold and mildew because the toxic nature of the older materials hindered fungal growth. Such organisms may find that the new, lower toxicity materials are a viable nutrient source. Understanding how fungi feed on coatings and their constituents will lead to the development of resins and additives that allow environmentally friendly materials to perform as designed without the added problems associated with microbial attack. This ability is paramount to the ultimate implementation of environmentally friendly finishing systems throughout the DoD.

Fungal growths cause detrimental effects on materials through direct attack, indirect attack, and physical interference. Direct attack occurs when the fungus breaks down a nonresistant material and uses it as a nutrient source, resulting in deterioration that affects the physical properties of the material. Some examples of nonresistant materials are animal- and vegetable-based 2 adhesives, certain polyurethanes, and paints or varnishes that contain susceptible constituents. Indirect attack causes damage to materials when fungal growth on surface deposits of dust, grease, perspiration, and other contaminants accumulates on materials during manufacture or in service. Damage is caused to the underlying material, even though that material may be resistant to direct attack. Damage to optical systems results primarily from indirect attack. Physical interference occurs on electrical or electronic systems where the damage may result from either direct or indirect attack. For example, fungi can form undesirable electrical conducting paths across insulating materials or may adversely affect the electrical characteristics of critically adjusted electronic circuits. Fungi can adversely affect light transmission through the transparency, block delicate moving parts, or change non-wetting surfaces to wetting surfaces, leading to a loss in performance.

In addition, metabolic waste products (i.e., organic acids) excreted by a fungus cause corrosion of metals, etching of glass, or staining or degrading of plastics and other materials. Fungus on materials can cause physiological problems (e.g., allergies) or be so aesthetically unpleasant that the users will be reluctant to use the material.

Even legacy systems that are primed with chromate primers have experienced fungal issues. The H-53 Fleet Support Team (FST) at Cherry Point has reported persistent mildew in the bilge area. Additionally, microbial-induced corrosion has been reported in the F-16 cockpit area and mold has been found in the cargo compartment under insulation blankets on some C-130 aircraft. These instances show that even chromate systems do not totally control fungal growth in certain environments. Looking forward, beach testing has shown that fungi grow more readily on some newer non-chromate coatings, suggesting that constituents other than the pigments in the new coatings are more susceptible to fungal attack. The potential for increased susceptibility to fungi will undoubtedly be a concern with the transition of more environmentally friendly, non-chromate coatings into routine use, especially on interior surfaces.

To meet the objectives of this SEED SON, proposals should not exceed $200,000 in total cost and approximately one year in duration. Work performed under the SEED SON should investigate innovative approaches that entail high technical risk and/or have minimal supporting data. At the conclusion of the project, sufficient data and analysis should be available to provide risk reduction and/or a proof-of-concept. SEED projects are eligible for follow-on funding if they result in a successful initial project.