Potential applications of electrospun fibers in aviation industry

The aviation industry comprises of commercial and military flights, unmanned vehicles and drones. There are several aspects of the aviation industry where electrospun fibers may contribute to its development. These include structural enhancement, maintaining clean cabin air and noise control.

Several studies have shown that having an electrospun fibers layer between composite laminates will significantly improve its fracture properties. A nanofiber layer (average diameter of 195 nm) used in composite with thickness of 27 µm and surface density of 1 g/m² which amounts to just 0.1% volume of unidirectional carbon fiber-reinforced epoxy composute was able to show 54% improvement in its flexural modulus and 47% increase in initiation energy on impact properties [Molnar et al 2014]. Finer diameter fibers (< 500 nm) were better able to improve the flexure strength of the composite compare with larger diameter fibers. Comparing the effect of fiber layer thickness of 40 µm, 70 µm and 105 µm, the smallest thickness has the highest flexural strength and elastic modulus although the thicker layers were better able to arrest crack propagation. A study by Liu et al [2006] using nanofiber layer (of materials nylon 6, Epoxy 609 and thermoplastic polyurethane) from 0.3 mm to 0.8 mm thickness showed similar flexural strength to unreinforced polymer which is consistent with other studies that thinner membrane layer is better for improving flexural strength. In this case, 0.3 mm layer is probably too thick for flexural strength improvement.

Since a thin layer of electrospun membrane is very light, there has been early attempt to use it as a micro-air wing (MAW) skin. Pawlowski showed that it is possible to electrospin fibers on a MAW wing frame to create a bird wing-like texture. Using piezoelectric copolymer of PVDF and trifluoroethylene (TrFE), they anticipate that actuation of the wing may be carried out by application of a voltage. Preliminary trial showed perceptible vibration of the wings upon excitation.

Electrospun fibers are been used commercially as air filtration media due to its ability to trap small particles. A thin layer electrospun fibers is able to significantly improve the filtration performance of a filter media. This is particularly useful in commercial aircrafts this would reduce the weight and space taken up by the filter media. Electrospun fibers have also been used to remove microbes and this will reduce health hazards posed by re-circulating air within the confines of commercial aircraft cabins. Natural microbial inhibiting materials such as chitin and chitosan and its derivatives may be electrospun into fibers [Seyam et al 2012; Nawalakhe et al 2012; Jung et al 2012]. Inorganic materials such as TiO₂ also exhibited anti-microbial capability especially when it is exposed to infrared red light [Aboelzahab et al 2012]. Electrospun cyanoethyl chitosan [Seyam et al 2012] and iminochitosan [Nawalakhe et al 2012] was found to be effective against Escherichia coli (Gram negative), Pseudomonas aeruginosa (Gram negative), Staphylococcus aureus (Gram positive) and Bacillius subtitis (Gram negative).

Electrospun fibers have been used in air and water filtration for removal of particles and have been demonstrated to be very effective for this purpose. Similar, small pore size of electrospun fibrous membrane may also be effective as a barrier against bacteria entry. Chaudhary et al (2014) used an electrospun polyacrylonitrile-silver composite filter media to cover a nutrient media in room condition and passes ambient air through the filter media. When compared to the negative control which is without the protective filter media, the nutrient media protected by the nanofibrous filter remains free of bacteria growth after two months while the unprotected nutrient media show microorganism growth.

Electrospun fibers may also be used as a sound barrier or sound absorbance layer to reduce sound generated by motor and engine. Electrospun fibers were successfully used on an unmanned air vehicle (UAV) to control the direction of the propeller noise such that it is much quieter below it. Tests on electrospun nanofiber mesh have supported its use as sound absorbance material. Comparing the sound absorption coefficient of electrospun silica fibers of different diameter to glass wool, Akasaka et al (2014) found significant improvement in sound absorption of electrospun fibers over glass wool at frequency of 1600 to 6400 Hz. There is also a general improvement in sound absorption with smaller fiber diameter from 8.24 µm of glass wool down to electrospun fibers of various diameters with the best absorption at diameter of 670 nm. Noise reduction material used in aircraft interior needs to be light and occupy a small space while having good sound absorbance property. Asmatulu et al (2009) tested the sound absorbance property of electrospun polyvinyl chloride mat of different thickness and with fiber diameters ranging from a few hundred nanometers to a few microns. Fiber diameters of about 200 to 500 nm with thickness of 0.5 mm were better able to absorb sound at the higher frequency (>5000 Hz). As the thickness of electrospun fiber increases, the sound absorbance shift towards the lower frequency. However, the absorption coefficient drops too. When the fiber diameter goes beyond 500 nm, the sound absorbance shift towards the lower frequency with thicker mesh but absorption coefficients remains the same.