Environmentally Resistant Flax Fiber-Reinforced Composites for Aircraft Applications: Aviation Stress Tests with Optical and Mechanical Analyses

Abstract Flax fiber-reinforced composites (FFRCs) must be resistant to environmental conditions to use them for external components in aviation. It was investigated how contact with typical aviation liquids, i.e. water, jet fuel and hydraulic oil, affects the optical and mechanical properties of twill fabric epoxy resin-based FFRCs. These influences were compared to the effect of UV weathering. Samples were exposed to these conditions for up to 28 days at RT. Uncoated samples with different fiber contents served as references and were compared with coated samples. A polyurethane/epoxy-based aircraft coating system and a partially biobased automotive interior coating as a more sustainable alternative were examined. The main damage for the uncoated samples was caused by UV weathering and water. UV light leads to photo-oxidation, causing material erosion. Water is absorbed by the hydrophilic flax fibers. Subsequent swelling leads to deformation and delamination. Thus, the damage was more severe for a higher fiber content. No effect could be demonstrated by immersion in jet fuel and hydraulic oil, as they are hardly absorbed by the fibers. Both coatings showed good protection against UV weathering and delayed water absorption. Nevertheless, mechanical damage was found for the FFRC covered with the fossil coating after 28 days of immersion in water. FFRC with an appropriate coating could therefore be suitable for use in aviation, but prolonged contact with water should be avoided. Protective measures should focus on preventing water uptake and UV irradiation, as these cause significantly more damage than jet fuel and hydraulic oil.