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Air Filtration with Electrospun Nanofibers

There is a wide range of application which an air filter media can be used from industrial to personal and household. Industrial usage of air filter media includes clean room and fossil-fuel power station. Ships and vehicles also require air filter media to reduce particulate emission from their exhaust. Personal and household usage of air filter media includes facemask, air purifier, vacuum cleaners and window filter screen. Air filter is the first commercial application of electrospun product with the industrial production of Petryanov filters in USSR in 1939. It is only until the early 1980s that a company from US (Donaldson) begins making and selling filter media with electrospun nanofiber fibers. Little is known about the enhanced performance of the air filtration media with the addition of nanofibers in the public domain until the early 2000s when researchers start to carry out experiments in this area of application. In a demonstration of the filtration effectiveness of electrospun nanofibers membrane, a thin coat of electrospun nanofibers over a metal mesh has been shown to collect more than 95% percent of PM2.5(Particulate matter, 2.5 µm) in a polluted city environment while allowing 90% percent transparency [Liu et al 2015].

To understand how electrospun fibers significantly enhance the filtering performance of the filter media, it is important to understand the particle capturing mechanism. Depending on the size of the particles, there are four main mechanisms which the filter media removes them. Particle size larger than the pore size of the filter media are simply captured by sieving mechanism. For smaller particles, the remaining three mechanisms are inertial impaction, interception and diffusion. Inertial impaction occurs when the inertia of the particle as it is carried by the air stream hits the fiber although the air stream has flowed round the fiber. When the particle is sufficiently small to flow with the air stream without leaving its path, it may still come into contact with the fiber surface as it attempts to flow round it and be captured (intercepted) by the fiber. For very small particles (< 100 nm), its movement is dominated by Brownian motion and it is captured by the fibers through random collision.

The efficiency of nanofibers in capturing nanoparticles from various smoke sources by collision was shown in a study by Kusumaatmaja et al (2016). Using electrospun polyvinyl alcohol nanofiber membrane, smoke from cigarette, waste combustion and vehicle combustion was passed through it. Observation under SEM showed clearly, nanoparticles adhering to the surface of the nanofibers. Removal of the smoke particles is not through size exclusion as the particles captured are much smaller than the pore size.

SEM images of electrospun PVA membrane (a) before filtration; after filtration of (b). cigarette smoke, (c) smoke from waste combustion, (d) smoke from vehicle combustion [Kusumaatmaja et al 2016].
Description Illustration
Sieving / Size exclusion Mechanism

Particle size:
Larger than pore size of membrane

Particle flight path:
Influenced by air stream but does not closely follow its path due to inertia
Particle filtration by size exclusion
Inertial Impaction Mechanism

Particle size:
Micrometer range

Particle flight path:
Influenced by air stream but does not closely follow its path due to inertia
Particle filtration by inertial impaction
Interception Mechanism

Particle size:
Low micrometer to sub-micrometer range

Particle flight path:
Follows air stream
Particle filtration by interception
Diffusion Mechanism

Particle size:
Nanometer range

Particle flight path:
Brownian motion
Particle filtration by diffusion

Fiber diameter on filtration efficiency

An important difference between nanofiber and larger diameter fiber filter media is the phenomena of slip flow. For larger diameter fiber, the air molecules colliding on the surface of the fiber will acquire the velocity of the fiber (stationary) plus its random motion due to their thermal energy. This results in a pressure drop due to the presence of the fibers. For nanofibers, less air molecules actually impact the fiber surface thus they are able to better maintain their initial flow velocity which is known as slip flow. This slip flow results in less pressure drop compared to larger diameter fibers. It has been shown that pressure drop across nanofiber membrane in relation to its solid volume fraction is less than model prediction without slip flow [Hosseini et al 2010]. Studies have also shown significantly less pressure drop for nanofiber filter media compared to microfiber filter media of the same filtration efficiency [Park et al 2005]. With slip flow, more particles will travel near to the fiber resulting in higher diffusion, interception and inertial impaction. Significant improvement in dust reduction has been recorded when a standard cellulose filter media was coated with a nanofiber layer [Graham et al 2002]. While filtration efficiency increases with greater solid volume fraction (of fibers), this also leads to a greater pressure drop despite slip flow effect of nanofibers. Zhao et al (2016) showed that electrospun polyacrylonitrile (PAN) diameters between 60-100 nm was the most effective in facilitating the slip flow effect which coincides with the mean free path of air molecules at 65.3 nm. To determine the optimum balance between these factors, a Figure of Merit (or Quality Factor) is used.

FOM = -In(P)/Δp

Where P is the penetration of particle; Δp is the pressure drop.

The penetration of particle, P = (particle concentration downstream of the filter) / (particle concentration upstream of the filter).

Using Figure of Merit to determine the performance of the filter media, several studies have shown that smaller fiber diameter (< 1 micron) is better for filtering particles of diameter more than 100 nm. For particles of smaller diameter, the performance from larger diameter fiber is better [Wang et al 2008, Hosseini et al 2010]. A study by Matulevicius et al (2014) using electrospun polyamide 6 (PA 6) and polyamide 6/6 (PA 6/6) nanofibers to demonstrate the effect of fiber diameter on the filtration performance on 100 nm and 300 nm diameter polystyrene latex particles. For 300 nm particles, filtration efficiency was highest (91%) on electrospun PA 6/6 nanofibers which has diameter of about 65 nm. For electrospun PA 6 fibers with diameter of about 230 nm, filtration efficiency was similar for both 100 nm and 300 nm diameter particles at close to 87%. It is possible that with smaller fiber diameter, the pore size is smaller and this improves its ability to trap larger size particles. When the particle size is smaller, size exclusion by the membrane pore size is no longer effective and thus there is less difference in the filtration efficiency between fiber diameter less than 100 nm and fiber diameter less than 300 nm.

When the average fiber diameter increases to 300 nm, the filtering efficiency of the membrane drops significantly [Li et al 2006]. Model simulation using a 3-D structure resembling nanofiber (diameter less than 200 nm) filter media, low air flow and no-slip boundary conditions, nanoparticles less than 100 nm are shown to be mostly captured through Brownian diffusion while larger particles are captured through interception with the fiber. Cake formation for nanoparticles is found mainly in the middle of the nanofibrous membrane since Brownian diffusion is the dominant capture mechanism [Maze et al 2007]. Experimental results using fibers from meltblowing showed that with smaller fiber diameter, the quality factor increases for all particles from 10 to 100 nm. It is only for particles smaller than 30 nm where the quality factor for 1.3 µm diameter fibers membrane is lower than the base layer [Podgorski et al 2006]. In the study of tar removal efficiency by Molaepour et al (2014) using electrospun fiber membrane, filtration efficiency by fibers with diameter of 280 nm is significantly (10% and greater) better than fibers with diameters of 370 nm and 620 nm. Another model using 300 nm diameter fibers also showed that factors that lead to reduced Brownian motion intensity (increased air velocity, viscosity and air pressure) results in a reduction in filtration efficiency for particles size less than 200 nm while increase in air temperature and particle-fiber friction coefficient increases filtration efficiency through increased Brownian motion intensity and reduced particle slip respectively [Sambaer et al 2011]. A study by Ahn et al (2006) using 200 nm diameter fiber showed that its filtration efficiency drop marginally from 99.97% to 99.96% for 300 nm particle when the face velocity increases from 3 m/min to 6 m/min.

Filter particle size

Most air filter media is made out of nonwoven microfibers. Due to their larger pore size, initial filtration efficiency is low and is ineffective for particles size in the range of 100 to 800 nm. With nylon nanofiber (diameter 120 nm) coating of 0.1 g/m2, the filtration efficiency of particles less than 1 microns rise to more than 80% as compared to less than 50% for non-nanofiber coated filter media [Li et al 2006].

Fiber diameter Weight / Thickness of fiber Filtration Efficiency Pressure drop Reference
300 nm diameter fiber 16.48 g/m2 98% for 0.3 µm particles 13.27 mmH2O at 3 m/min of face velocity Park et al 2005
Glass fiber filter media 81.46 g/m2 98% for 0.3 µm particles 37.05 mmH2O at 3 m/min of face velocity Park et al 2005
HEPA filter 500 µm, 78.2 g/m2 99.97% for 0.3 µm particles < 25 mmAq at 3 m/min Ahn et al 2006
Nylon 200 nm diameter fiber 50 µm, 5.75 g/m2 99.97% for 0.3 µm particles > 25 mmAq at 3 m/min Ahn et al 2006
Nylon 200 nm diameter fiber 100 µm, 10.75 g/m2 99.993% for 0.3 µm particles 80 mmAq at 3 m/min Ahn et al 2006
91 nm diameter fiber on filter media 2.3 µm thickness nanofiber 92% for NaCl aerosols with median diameter of 75 nm 40 mmH2O at 32 L/min Li et al 2013
120 nm diameter fiber on filter media 0.1 g/m2 ~88% for 1 µm particles 7.87 mmH2O at 0.3 m/min Li et al 2006
120 nm diameter fiber on filter media 0.1 g/m2 ~60% for 0.3 µm particles 7.87 mmH2O at 0.3 m/min Li et al 2006
300 nm diameter fiber on filter media 0.1 g/m2 ~35% for 1 µm particles 7.87 mmH2O at 0.3 m/min Li et al 2006
Original filter media NA 50% for 1 µm particles NA Li et al 2006
Original filter media NA ~11% for 1 µm particles NA Li et al 2006
120 nm diameter fiber on filter media 0.1 g/m2 ~74% for 0.3 µm particles ~50 Pa at 6 m/min Heikkila et al 2008
120 nm diameter fiber on filter media 0.1 g/m2 >90% for 1 µm particles ~50 Pa at 6 m/min Heikkila et al 2008

Pore Size

A less investigated parameter on slip flow is the pore size of the membrane. The pore size of electrospun membrane is formed by overlapping fibers. Many studies have shown that a reduction in fiber diameter would cause a corresponding reduction in pore size. While reduction of fiber diameter has the positive effect of facilitating slip flow and thus reduce pressure drop, Zhao et al (2016) has shown that when electrospun polyacrylonitrile (PAN) nanofibers diameter is below 60 nm, the pressure drop increased significantly. This has been attributed to air flow distribution around the periphery of the fibers where interference of air flow from neighbouring fibers become significant.

Other forms

Electrospun nanofibers for used in air filtration need not be restricted to the form of 2D membrane. Sponge-like filter media may be used as depth filters for greater particulate holding capacity. This is particularly useful in building filtration system where the filter media are replaced every few months. There are several methods for fabricating 3D block fibrous structure. Deuber et al (2016) used a combination of short nanofibers suspension and controlled freeze drying to construct a sponge comprising of macropores from the freeze drying and micropores from the distance between nanofibers. Their preliminary study showed that the filtration efficiency increased from 91% to 99.96% when the macropores were reduced from 122.6 µm to 15.2 µm. However, the most penetrating particle size (MPPS) remained from 134 to 202 nm across the sponges with different macropore sizes.

Another way of increasing filtration efficiency is to give the filter a surface charge. Zhang et al (2017) investigated the surface charge characteristic of polyacrylonitrile (PAN) and PAN with TiO2 and its effect on particle removal. With the presence of TiO2 in electrospun PAN fibers, the charge retention of the filter were significantly better than pure electrospun PAN filter membrane. Surface voltage of electrospun PAN fibers with 2% TiO2 showed a voltage decrease by 38% after 96 hrs but pure electrospun fibers had a voltage decrease of 57%. With increasing amount of TiO2, the voltage decrease were reduced. Zhang et al (2017) hypothesized that the reduction in fiber diameter with increasing TiO2 content may have helped in the charge retention. KCl particles of 0.3-0.5 µm penetration was also reduced with greater surface voltage.

Published date: 01 Apr 2014
Last updated: 07 Nov 2017



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