Composite structures are usually subjected to a wide range of environmental conditions whereby they can absorb a substantial amount of moisture, oil, or solvent from the environment, thus reducing their mechanical, thermal, electrical, and other physical properties and service times. In this study, hydrophobic barrier films used to prevent moisture ingression into sandwich structures are investigated. Four different hydrophobic thin barrier films-polyvinyl fluoride (PVF), polyether ether ketones (12.5 and 25 μm), polyimide, and polytetrafluoroethylene-were co-bonded to the surfaces of fiber composite sandwich structures to study the changes in mechanical properties of the sandwich structures before and after immersing them into deionized (DI) water and aviation hydraulic fluid (Skydrol®). For comparison and evaluation purposes, the composite coupons underwent two different mechanical tests: sandwich flexure and compression strength compression. Test results confirmed that using barrier films as the outermost ply on composite sandwich structures significantly reduced moisture ingression, and as a result, mechanical properties were considerably increased compared to those coupons without any barrier films.

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Fiber reinforced composites are subjected to a wide range of mechanical loads and environmental conditions, such as wind, high/low temperature, moisture, UV light and aggressive solvents. Compared to other structural materials, polymers can absorb more moisture and UV light from outside environment and lose their material properties (e.g., mechanical, electrical, surface and thermal) and thus the service life. In this study, hydrophobic barrier films including polyvinylfluoride (PVF) and polyether ether ketone (PEEK) were applied on the carbon, Kevlar, and glass fiber reinforced epoxy resin composites (laminate and sandwich structures), and then the effects of moisture absorption on those films were investigated in detail. The coupons were immersed in water for a number of days in order to determine the changes in mechanical properties of the composites. Three point bending tests were applied to the laminate composites, while four point bending tests were conducted on the Nomex sandwich structured composites. We found that moisture absorption could be eliminated by using these hydrophobic films. We found that PEEK film could be an alternative barrier film for the aircraft industry.