Electrospun Recycled Materials

With concerns about finite resources on earth, it is vital to consider ways to recycle discarded waste products for new applications. Plastic, in particular, has been the target of various recycling initiatives as many of them contributed to the pollution problem due to their extremely slow degradation rate under natural condition. Electrospinning is a very robust process for spinning fibers and does not require high purity feed material. This reduces the amount of processing to purify recycled raw material prior to usage. Further, tts output may be used in high performance and high value applications.

Polystyrene foam has been shown by several researchers to be easily convertible to useful products using electrospinning [Shin 2005]. Ezzatzadeh et al (2017) was able to construct a nitrate absorbent membrane by electrospinning iron oxide nanoparticles loaded nanofiber made of polystyrene. Disposable food container made of polystyrene foam was selected as raw material for this application. Foam polystyrene cannot be easily reformed using re-melting method but it can form solution using suitable solvents. Since electrospinning typically uses solution as the feed material, this makes foam polystyrene an appropriate candidate for electrospinning nanofiber production. By optimizing the electrospinning parameters, Ezzatzadeh et al (2017) was able to prepare polystyrene Fe₃O₄ composite nanofibers with diameters in the range of 50 to 300 nm. Their study showed that the resultant membrane was effective in absorbing nitrate and the amount of nitrate removal increases with increasing Fe₃O₄ loading and membrane thickness.

Polyethylene terephthalate (PET) is a common plastic for making disposable drinking water bottle. Known for its toughness, it has also been used as filter material. Strain et al (2015) used recycled PET without plasticizer for electrospinning into fibrous membrane for use as smoke filters. Membrane made out of 400 nm diameter nanofibers showed the most effective smoke filtration, capturing 43x its own weight in smoke residuals. For optimum mechanical robustness and absorption capacity, membrane with fiber diameter of 1 µm was preferred with absorption capacity of (32x) its own weight. In comparison, original cellulose acetate cigarette filter tip absorbed only 2.7 times its own weight. Baselga-Lahoz et al (2022) investigated the use of recycled PET for the production of PET nanofiber filter media in facemask. Diameters of the electrospun PET fibers can be varied easily by changing the concentration of the solution. Tests showed that smaller fiber diameters increases filter performance but also increases pressure drop which is undesirable. Higher diameter fibers have lower pressure drop but the filter performance is not as good. Therefore, Baselga-Lahoz et al (2022) used a combination of smaller and larger fiber diameters in the construction of the filter media. With electrospinning, they are able to stack layers of fibers with different diameters using the same process but different solution concentration. A facemask was constructed with the middle filter layer having an average fiber diameter of 1.24 µm and thickness greater than 500 µm, sandwiched between thicker fiber layers with average fiber diameter of 3.18 µm. The constructed facemask has a retention efficiency of more than 98.2% against particles between 0.5 and 10 µm, and 100% against particles of 3 µm with a pressure drop of 0.36 mbar. This facemask showed better retention ability for fine and coarse particles compared to commercial surgical masks. A concern of using recycled PET in the construction of filter media is its degradation over time especially with the higher surface area of fine electrospun fibers. Storage of the electrospun masks in room condition for 4 months showed a drop in filtration efficiency of less than 2.2% with the lowest efficiency at 97.8%. Zander et al (2016) used electrospun recycled PET nanofibrous membrane for microfiltration in water treatment. To reduce biofouling, the nanofibrous membrane was loaded with quaternary ammonium and biguanide biocides. The electrospun membrane demonstrated rejection efficiency of more than 99% of beads size as small as 500 nm. Loaded with antibacterial agent, the membrane achieved 6 log reduction for both gram negative and gram positive bacteria. While pure recycled PET may be electrospun into fibers for many applications, additives may be loaded to further improve its performance or change its properties. Santos et al (2018) selected lignocellulosic biomass, a renewable resource for improving the mechanical strength of electrospun PET membranes. The lignocellulosic fibers were blended into PET solution for electrospinning into a composite fiber. Pure PET aligned fibers exhibited a tensile strength of 8.8 MPa in the direction of fiber alignment. With 40% lignocellulosic fibers added, the tensile strength increased to 15.7 MPa, an almost 44% increase. Since lignocellulosic fibers are hydrophilic, having a greater composition in the composite is likely to reduce the water contact angle. With pure PET electrospun fibers, the water contact angle was 134.6 °. With 40% lignocellulosic fibers added, the resultant composite membrane have a water contact angle of just 32.5°.

Photographs of fibre mats (a) before and (b) after smoke filtration testing (1.0 ?m diameter), conducted according to the scheme shown in (c). IR-spectroscopy (d) of a clean fibre mat compared to that of smoke-exposed fibre mats with average fibre diameters of 0.4, 1.0 and 4.3 µm. [Strain et al 2015]

Cellulose acetate is a commonly used material in consumer products. It is also used in disposable items such as cigarette butts. Hemamalini et al (2019) showed that it is possible to recycle cellulose acetate in used cigarette butt as raw material for electrospinning into fibers. However, the presence of nicotine in a used cigarette butt was found to hinder electrospinning of high quality fibers. Pre-production process was needed to extract these trace amount of nicotine before dissolving the cellulose acetate (CA) in a mixture of acetone and dimethyl formamide. The nicotine free CA solution could be electrospun to form nanofibers. Silver nitrate has also been added to the CA solution to give the resultant fibers antimicrobial property. The silver nitrate loaded electrospun CA fibers were able to inhibit the growth of gram positive E. coli and gram negative S. aureus.

Electrospinning offers an attractive method for processing recycled materials into higher value and higher performance products. Various recycled common plastics such as polystyrene, polycarbonate, PET and their blends has been shown to be electrospinnable [Zander et al 2015]. In application such as air and water filters, the performance of the filtration membrane from recycled plastics are comparable to those from fresh raw materials. This may encourage collection and recycling of more materials.