Electrospun filtration media properties

Electrospun nanofibers membrane are known to be good air filter media. To optimize the membrane for commercial application, the performance of electrospun air filter media should exceed conventional air filter media. Factors such as its durability and added functionality must also be taken into account for commercial usage.

Personal N95 facemask

Nanofiber layer thickness

In a study by Heikkila et al (2008), using 120 nm diameter nylon fiber media coated on a nonwoven substrate, a filtration efficiency of 95% or above was obtained for particle size of 0.16 µm and above when the coating is 0.5 g/m². The thickness of the fiber coating on the substrate is approximately 10 µm. Raising the coating thickness to 1 g/m² only increases the pressure drop (from 200 Pa to 450 Pa) without any corresponding improvement in filtration efficiency. Gibson et al (2001) showed that a 0.12 g/ m² nylon nanofiber coating on polyurethane foam was able to increase the filtration efficiency (based on particle size of 2 to 3 µm) of the foam from about 75% to almost 95%. 100% efficiency was recorded when the coating weight was 1 g/m². Between coating between 0 to 2 g/m², the air flow resistance was found to increase linearly. A separate study by Yun et al (2007) using polyacrylonitrile electrospun fiber membrane showed that its quality factor and single fiber collection efficiency for filtering particles less than 80 nm did not show any significant difference for coating thickness of 4 microns and 18 microns although there is a corresponding increase in pressure drop. Similar result is shown by Li et al (2013) where nanofiber layer thicker than 6.3 µm did not show significant improvement in filtration efficiency. It has been recommended that an approximately 0.02 - 0.07 g/m² layer of nanofibers with a diameter of 100 - 400 nm on a substrate is optimum as air filtration media [Jaroszczyk et al 2009].

The thin layer of nanofibers was also found to allow easy diffusion of water vapor although air permeability is low. Resistance to water vapor diffusion is lower than commercially available membrane laminates such as Sympatex and Gore-Tex. This makes electrospinning a potential method for improving filtration properties of protective clothing while having fabric breatheability that is better than commercial membranes. The coating duration to obtain 95% filtration efficiency is just 30 s (0.12 g/ m²)[Gibson et al 2001].

Filtration media arrangement

Most research on the filtration performance of nanofiber coated filter media is based on a single fiber-substrate layer. With electrospun fibers, layered approach often showed better filtration performance compared to a single thick membrane. A thin layer of nanofiber (2.3 µm electrospun nanofiber layer from 10 s of deposition) on a polypropylene substrate has been shown to significantly increase the filtration efficiency of its base substrate from 17.5% to above 92% for filtering 75 nm diameter NaCl aerosols [Li et al 2013]. In commercial application, just as important is the packing of the filter media and its influence on filtration properties. Filtration performance can be improved simply by having the electrospun nanofibrous layer sandwiched between two microfibrous substrate instead of on a single microfibrous substrate. Vinh et al (2016) tested the air filtration performance of filter media comprising of electrospun polyacrylonitrile (PAN) nanofibers (200 to 300 nm diameter) on a nonwoven fabric and the same nanofibrous layer sandwiched between tow nonwoven fabrics. A lamination process using heat and pressure was used to bind the layers together. Their study showed that the filtration efficiency for 0.3 µm NaCl particles increases from 91% to 95% when the nanofibers are sandwiched between two nonwoven fabric with a corresponding pressure drop increase from 22 mmH₂O to 25 mmH₂O. This improvement in efficiency may be due to reduction of airflow speed as it passes through the first nonwoven fabric layer thus allowing greater opportunity for collision of the particles on the nanofibers. Zhang et al (2010) showed that quality factor of electrospun polyacrylonitrile fiber filter (Quality factor 0.0625 Pa^-1) performed better than any commercial HEPA filters (Millipore glass fiber HEPA, LydAir MG High Alpha HEPA filter) when it is stacked based on three thin layers. The performance of stacking more thin layers of electrospun membrane is much better than stacking thicker layers of electrosppun fiber membranes. Li et al (2013) showed similar results from having three composite layers (2.3 µm electrospun nanofiber layer on polypropylene substrate) stacked on top of one another demonstrated filtration efficiency of 99.95% which is the same as commercial HEPA filter but with a much better quality factor (0.063 for stacked composite versus 0.028 for HEPA filter). Molaeipour et al (2014) also demonstrated better filtration efficiency without corresponding pressure drop when double layer of electrospun nonwoven filter membranes were used for tar removal in cigarette filter tip compared to single layer thicker fiber membrane. Different filtration media packing designs have been developed to reduce pressure drop, size of filter and clogging by the industries [Jaroszczyk et al 2009]. Although pressure drop is lower for nanofiber due to slip flow effect, this advantage diminishes with increasing particle deposits. Given that the permeability of nanofiber web is much higher than conventional filter media, pressure drop and clogging process will be longer in comparison [Jaroszczyk et al 2009] thereby increasing the durability of the filtration media.

A substrate is often used to support thin electrospun nanofibrous membrane even when it is arranged in layers. However, this may not be necessary as a sufficiently thick membrane can be self-supporting and yet show good performance compared to commercially available filter media. Nicosia et al (2015) electrospun Polylactide/polyhydroxybutyrate (PLA/PHB) nanofibers with thickness of 35 µm and basis weight of 0.61 mg/cm² and stacked three layers together. The resulting filter media showed a pressure drop of 120 Pa, efficiency of 95.7% at 0.3 µm and a quality factor of 0.026 Pa^-1 which is comparable to commercial HEPA filter (AX2221HD from Lydall with quality factor between 0.023 and 0.029 Pa^-1).

Patterned/Textured Fiber Membrane

For standalone electrospun nanofiber layer as air filter, it is important that the nanofiber layer is able to withstand the airflow without tearing. To improve the tear resistance of the nanofiber layer, patterned/textured fiber membrane has been constructed using a patterned or grid collector. The resultant membrane been tested for its mechanical and filtration performance. Gibson et al (2004) showed that patterning and grid spacing has no obvious effect on air flow resistance and aerosol filtration properties of electrospun thermoplastic elastic polyurethane (TPU) membrane. Damage on patterned samples when it fails were also less on patterned membrane compared to non-patterned membrane. In patterned samples, high air pressure may create a small punctured hole on the membrane that relieves the stress instead of catastrophic crack propagation across the whole membrane in non-patterned membrane.

Anti-bacteria property

Air filter media especially those used in ventilation system may harbour bacteria or fungus growth as the air carrying their spores pass through the membrane. Having a membrane with anti-bacterial property and anti-fungal property may prevent proliferation of microbes and fungus on the surface of the membrane. Electrospun air filtration membrane has been loaded with anti-bacterial substances and shown to be effective in inhibiting bacteria growth while maintaining good filtration performance. Wang et al (2016) loaded poly(lactic acid) with titania nanoparticles and this combination exhibits high antibacterial activity of 99.5 against Staphylococcus aureus with high filtration efficiency of 99.996% and relatively low pressure drop of 128.7 Pa. Using electrospun Polylactide/polyhydroxybutyrate (PLA/PHB) nanofibers loaded with ammonium-based ionic liquid (IL) which is a quaternary ammonium compounds (QACs), the resultant fibrous mat was able to show inhibition against fungus, Aspergillus niger and Chaetomium globosum. Victor et al (2021) conducted a bacterial filtration efficiency of antibacterial membrane through an Andersen sampler. The prepared filtration membrane was electrospun polyvinylidene fluoride (PVDF) blended with titanium nanotubes (TNT) and sandwiched between polypropylene (PP) nonwoven sheets. The filter media was first shown to exhibit antibacterial properties through zone of inhibition on agar gel cultivated with Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Bacterial filtration efficiency (BFE) was tested using S. aureus aerosol where aerosols of particle size in the range of 0.1-10 µm were passed through the prepared filter membrane. Optimized filter membrane showed a bacterial filtration efficiency of 99.88% with good antibacterial properties against both gram-positive and negative organisms.

SEM images of the PLA/TiO2 fibrous membranes loaded with 1.75 wt% TiO2 Nanoparticles at 60% relative humidity [Wang et al. Journal of Nanomaterials, vol. 2016, Article ID 6272983, 17 pages, 2016. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

In an epidemic, facemasks may be worn for a long duration through the day. The Covid-19 pandemic has shown that people have had skin issues on the face where the mask is in constant contact with the skin after wearing it for long hours. One contributing factor may be propagation of microbes in the warm and moist interface between the mask and the skin. Therefore, development of facemasks with antibacterial properties may improve the hygiene of mask wearing. Qiu et al (2021) selected polyvinyl butyral (PVB) as the material to be electrospun into nanofibers. An advantage of PVB is that the solvent used is ethanol which is commonly used for antibacterial purposes and is less toxic to humans. Berberine hydrochloride (BH), a plant-based chemical was used as the antibacterial agent that is loaded into PVB solution and electrospun to give antibacterial nanofibers. The electrospun PVB/BH nanofibers were collected on an hydrophilic polypropylene spunbond nonwoven base. The surface with the electrospun PVB/BH nanofibers was shown to be hydrophobic with a water contact angle of about 140° while the uncoated spunbond surface remains hydrophilic. The membrane with PVB/BH exhibited antibacterial properties with zone of inhibitions on staphylococcus aureus agar plate. The composite filter membrane also showed good air permeability with air filtration efficiency of 96.4% for PM 0.3 and 100% for PM2.5 and a pressure drop of 108 Pa.

Electrostatic property

The filtration efficiency of an air filter may be improved with higher electrostatic potential. Guo et al (2021) compared the filtration efficiency of electrospun polyethylene terephthalate (PET) and electrospun PET and thermoplastic polyurethane (TPU) fibers mixture. The electrospun efficiency of PET nanofiber membrane was less than 90% while PET/TPU nanofiber mixture composite membrane was more than 90%. In the PET/TPU fiber mixture, TPU with beads were deliberately produced. Examining the electrostatic potential of the PET/TPU fibers mixture found that there is a much higher electrostatic potential of 120 to 165 mV around the TPU beads compared to the surrounding electrostatic potential of 30 to 80 mV. This may be due to the higher dielectric constant of TPU compared to PET which gives it greater charge storage potential. Discharging of the filter media resulted in a drop of average filtration efficiencies of 6.5% and 2.5% for PET/TPU fiber mixture composite membrane and PET membrane respectively. Hence there is a greater contribution of filtration efficiencies by the electrostatic potential on PET/TPU fiber mixture composite membrane compared to pure PET fibers.

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Content [Air Filtration]