Melt-Blown Fibres vsElectrospunNanofibres as FiltrationMediaDr. Fabrice Karabulut RD & Implementation Scientist — Revolution Fibres, Auckland, New Zealand

17th August 2020

Filtration plays an important role in pu-rifying and decontaminating two life ne-cessities: water and air. As awareness of

the related health issues has increased, the de-mand for protection from air-borne pollutionand disease has also increased. From this per-spective, we explain the unique and enhancedcapabilities that electrospun nanofibres pro-vide when used as an active layer in facemasks. When compared to common melt-blown filters, electrospun nanofibres providebetter protection against air particles, bacte-ria, and viruses such COVID-19.

1. Introduction

This paper aims to provide a high-level comparisonbetween melt-blown (MB) filters (commonly usedfor N95 face masks), and electrospun (ES) nanofi-bre (NF) filters. Nanofibres are widely acceptedas particularly effective in stopping submicron andnanometre contaminants with a minimal impact onpressure drop. The filtration mechanisms by whichthe two filters function will be discussed, as well asthe differences in their properties (fibre diameter,surface area, relative strength, filtration mechanism,breathability, and reusability)..

1.1 What is MB fibre?

Melt blowing is a commercially successful andlow-cost process for producing filtration microfibres.Melt blowing typically produces fibres with diam-eters in the range of 1-10 micrometres (µm).[1, 2]The structure of a MB filter is like a non-uniformfishing net. Its pores are 1 − 3 µm in diameter -much larger than bacteria, and certainly viruses(0.1 µm).

MB filters are usually made of melted polypropylene(PP) passed through a small nozzle. The resultingfilaments are blown out at a high temperature andspeed, then cooled in the air. A disadvantage of thisprocess is that only thermoplastic polymers can beused. In addition, it is difficult to control fibre sizein the melt blowing process. PP can lead to unevenflow of melt, which can make the fibre thicknessuneven, affecting the breathability and filterability.[2, 3]

The filtration mechanisms for MB filters arehighly dependent on electrostatic deposition. Thismeans that when the electrostatic charge is lost, thefiltration efficiency drops. Such electrostatic chargescan be lost due to moisture in the environment.As discussed below at 3.2, the moisture-capturing

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nature of MB N95 masks have been reported tocause headaches and create a favourable environmentfor viruses and bacteria.

1.2. What is Electrospun Nanofibre?

Electrospinning is widely considered the mosteffective method for producing nanofibres. Thisis due to ES’s versatility and ability to use awide variety of polymers both in lab and massscale.[4] Historically, nanofibres were not able tobe produced in large enough volumes and at lowenough cost to be commercially viable. Espe-cially when competing with existing alternativessuch as melt-blowing. Worldwide, the nanofibremarket is continuing to grow. Recent technologyadvancements mean that production rates of ESNFs are close to that of the conventional MB process.

Electrospinning uses electrostatic forces todraw charged threads from a polymer solution tocreate fibres. The diameters of the fibres rangefrom 10 - 300 nanometres (nm).[5] The fibre andpore diameters of the ES NF filters can be easilycontrolled.

There has been a recent interest in green electro-spinning, which involves water-based solutions.[6]Only a few academic groups and Revolution Fibreshave investigated aqueous solution electrospinningand other methods of fabricating green electrospunnanofibres.

NF fabrics produced by electrospinning haveattracted attention for use in filtration. This ispartly because the diameter of nanofibres are 10- 100 times smaller than that of conventional MBmicrofibers. The higher surface area in nanofibresinduces better filtration efficiency, largely becausesurface interaction is the dominant driving forcein air filtration. ES NF is dependent on multiplefiltration mechanisms (discussed below at 2). Assuch, it is not affected by electrostatic attractionto the same degree as MB filters. In addition, theelectrospinning process offers opportunities to finetune the surface functionality through polymerchemistry, blending and nanofiller incorporationduring processing.

FunctionalityThe electrospinning process can generate functionalnanofibres, which provide enhanced properties andlower surface areas compared to MB fibres. The

Figure 1: Scanning electron microscopy images of MBfibre and ES NF.

high surface area of ES NFs makes it possible tofunctionalise the nanofibres in a range of differentapplications. This includes filter media, catalysis,super absorbents, scaffolds for tissue engineeringand wound dressings, energy storage, and electronicapplications.[5] The ES NFs can be produced fromvarious combinations of natural and syntheticbiocompatible polymers at room temperature,potentially lowering energy costs for production.Because electrospinning involves a solution process,it is easier to incorporate antimicrobial, antiviral, bio-cide and virucide agents compared to MB filters.[5, 7]

StrengthsA recent study reported that ES NF nylon andpolyurethane (PU) showed significantly higherstrength MB fabrics of the same material.[8] It wasfound that the strength of ES NF nylon fabric wasup to 10 times that of the MB material, and for PUfabric, 2.5 - 3 times that of the MB material.

2. How does mask filtration work?

To understand how ES NFs enhance filtrationperformance, it is important to understand theparticle capturing mechanism. Particles can beblocked by a filter via five different mechanisms:Sieving, Interception, Inertia Impaction, Diffusion,and Electrostatic Attraction (Figure 2). Gravity canaid the filtration process but is often considered neg-ligible for particles smaller than 600 nm. Particlescan be classified in different sizes as shown in Figure2.

Particles larger than the pore size of the fil-ter are captured by the sieving mechanism.When the filter is charged, oppositely chargedparticles are attracted and deposited on the filterby electrostatic attraction. Smaller particles notcaptured by these mechanisms are filtered accordingto inertial impaction, interception, and diffusion.

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Figure 2: Concept of size of particles present in nature(top),[9] and the mechanism associated withmask filters (bottom).

Inertial impaction works on particles between 300– 600 nm, which follow the airflow. These particlesare heavier than the air fluid surrounding them.As the air flow splits in different directions whenentering the fibre pore, the particles continue in astraight line and then impact and deposit on the fibre.

Diffusion is very efficient on the smallest particles(< 300 nm). Such particles are not held in placeby air fluid and diffuse randomly within the airstream. As the particles traverse the flow streamwith random motion, they hit the fibre and deposit.

Direct interception works on particles thatare not large enough to have inertia and not smallenough to diffuse within the airflow stream. Thesemid-sized particles follow the air stream as it bendsthrough the fibre spaces. Particles are interceptedwhen they collide with a fibre.

Because of the various mechanisms by whichfiltration occurs, the smallest particles are typicallynot the most difficult to filter. Most filters havea region of lower filtration efficiency somewherebetween 0.1 − 0.5 µm.[10] Particles in this rangeare large enough to be less effectively captured bydiffusion, but small enough to be less effectivelycaptured by interception or impaction. The most

penetrating particle size (MPPS) will depend on thefilter media, air flow, and electrostatic charge on theparticle.

3. Why are electrospun masksbetter?

Most toxic particulate compounds are smallerthan 1 micrometre in diameter. Conventionalmechanical fibrous filters (such as MB filters) removemicrometre-sized particles with high efficiency.However, for particles in the submicron range, ESNF are considered better as they offer enhancedfiltration performance. This is due to their highsurface area and small pore diameter.[11]

Electrospun nanofibres are characterised by avery large surface area, which significantly increasesthe probability of the particles depositing onthe fibre surface - thereby improving the filterefficiency. In addition, ES NFs have low basisweight, high permeability, and tight pore size thatmake them appropriate for a wide range of filtrationapplications.[12] ES NF filters have a thinner fibrediameter (10 - 300 nm), and a smaller and moreuniform pore size than common MB N95 facemasks, which are made of PP fibres with diametersin the range of ∼ 500 - 1000 nm. The followingsections will discuss why ES NF filters allows forbetter filtration performance, breathability, and thepossibility of reusability.

3.1. Filtration

In general, air filtration is primarily based on depthfiltration via the combined effects of sieving, inertialimpaction, interception, diffusion, and electrostaticinteractions. Figure 3 shows the typical captureefficiency curves for particles captured by fibrousfilters as a function of particle diameter.

As discussed above, some particles in the nano range(∼ 100 - 500 nm)are difficult to filter as they do notbehave entirely according to one capture mechanism.Filtration of MPPS particles require uniformmultiple nanofibre layers, which defer the particlesso that they obey one of the mechanisms. Multilayerfilters are often hindered by poor breathabilityand high pressure drop, which is undesirable forfilters. However, electrospinning enables control ofthe porosity, packing density, fibre diameter andsurface area of the nanofibres. Modification of these

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Figure 3: Filtration efficiency as a function of particlediameter for single-fibre mechanisms and totalfiltration efficiency (left).[13] Filtration effi-ciencies of four fibrous filters with differentfibre sizes (right). Figure from reference.[14]

parameters allow them to filter higher amountsand a wider range of particles/contaminants thanMB filters. As shown in Figure 3, the smaller thefibre diameter, the higher the overall filtrationefficiency for NF filters. Electrospun nanofibresallow optimisation of filtration performance and thepossibility of tuning the pressure drop/breathability.

3.2. Breathability

Breathing comfort is commonly associated withpressure drop. However, moisture transportation isanother important factor to consider. A recent studyconducted breathability tests through N95 MBand ES NF masks to evaluate their water vapourtransmission rates (WVTR).[15] It was observedthat the WVTR of ES NF filters was superior.MB filters have sponge-like structures which resistmoisture. Therefore, moisture takes longer to passthrough the filter.

Figure 4 shows the CO2 emission mechanism forboth types of filters. It can be observed that MBfilters exhibit poor emission as compared to ES NFfilters. Recently, a survey reported on the risk ofheadaches to healthcare providers from wearingN95 facemasks.[16] They concluded that this is dueto higher humidity levels, breath resistance, andaccumulation of heat inside the micro-climate ofthe masks. Other consequences of humidity werediscussed in another study. Researchers found thatMB N95 masks provide a favourable environmentfor viruses and bacteria due to its moisture-lovingnature.[17] On the other hand, ES NF filters have afiner structure and more uniform morphology andpore diameter. This allows the water vapour tobe passed through the filter more consistently andefficiently. This makes ES NF an excellent candidateto replace MB filters.

Figure 4: Evaluation of breathing comfort by infraredthermal camera of ES NF filter and MB fil-ter: (top) air and moisture transmission and(bottom) CO2 transmission. Figure from ref-erence.[15]

3.3. Reusability

The current pandemic has amplified the need formasks to be reusable but retain effectiveness. Astudy was performed on the reusability of MB andES NF filters when cleaned with ethanol (sprayedand dipped).

The results showed that MB filters are onlyeffective for single use due to the steep reductionof filtration efficiency after ethanol cleaning (to∼ 64%). This is because the electrostatic chargeof MB filter is lost when cleaned, leading to adramatic drop in performance. MB filters lose staticelectricity when exposed to water and moisture,diminishing their filtering effect to almost half theoriginal performance.

In stark contrast, it was found that ES NFfilters can be successfully reused multiple times aftercleaning with ethanol as the filtration efficiencyremains consistent (∼ 97-99%).[15]

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4. References

1. Luo, C.J., et al., Electrospinning versus fibreproduction methods: from specifics to technologi-cal convergence. Chemical Society Reviews, 2012.41(13): p. 4708.

2. Moyo, D., A. Patanaik, and R.D. Anandjiwala,12 - Process control in nonwovens production, inProcess Control in Textile Manufacturing, A. Ma-jumdar, et al., Editors. 2013, Woodhead Publishing.p. 279-299.

3. Bo, Z., Production of polypropylene melt blownnonwoven fabrics: Part II -Effect of process parame-ters. Indian Journal of Fibre and Textile Research,2012. 37: p. 326-330.

4. Seeram Ramakrishna, K.F., Wee-Eong T.,Thomas Yong, Zuwei Ma and Ramakrishna Ramase-shan, Electrospun nanofibers; solving global issues.Materials today, 2006. 9(3): p. 40 - 50.

5. Xue, J., et al., Electrospinning and ElectrospunNanofibers: Methods, Materials, and Applications.Chemical Reviews, 2019. 119(8): p. 5298-5415.

6. Lv, D., et al., Green Electrospun Nanofibers andTheir Application in Air Filtration. MacromolecularMaterials and Engineering, 2018. 303: p. 1800336.

7. Santiago-Morales, J., et al., Antimicrobial ac-tivity of poly(vinyl alcohol)-poly(acrylic acid) elec-trospun nanofibers. Colloids and Surfaces B: Bioint-erfaces, 2016. 146: p. 144-151.

8. Tsai, P.P., W. Chen, and J.R. Roth, Investiga-tion of the Fiber, Bulk, and Surface Properties ofMeltblown and Electrospun Polymeric Fabrics. In-ternational Nonwovens Journal, 2004. os-13(3): p.1558925004os-1300306.

9. Tebyetekerwa, M., et al., ElectrospunNanofibers-Based Face Masks. Advanced Fiber Ma-terials, 2020.

10. Hinds, W.C., Aerosol technology : properties,behavior, and measurement of airborne particles.1999, New York: Wiley.

11. Poudyal, A., et al., Electrospun NanofibreFilter Media: New Emergent Technologies and Mar-ket Perspectives, in Filtering Media by Electrospin-ning: Next Generation Membranes for SeparationApplications, M.L. Focarete, C. Gualandi, and S.Ramakrishna, Editors. 2018, Springer InternationalPublishing: Cham. p. 197-224.

12. Qin, X. and S. Subianto, 17 - Electrospunnanofibers for filtration applications, in ElectrospunNanofibers, M. Afshari, Editor. 2017, WoodheadPublishing. p. 449-466.

13. Yoon, K., B.S. Hsiao, and B. Chu, Functionalnanofibers for environmental applications. Journal

of Materials Chemistry, 2008. 18(44): p. 5326-5334.14. Podgorski, A., A. Ba lazy, and L. Gradon,

Application of nanofibers to improve the filtrationefficiency of the most penetrating aerosol particles infibrous filters. Chemical Engineering Science, 2006.61(20): p. 6804-6815.

15. Ullah, S., et al., Reusability Comparison ofMelt-Blown vs Nanofiber Face Mask Filters for Usein the Coronavirus Pandemic. ACS Applied NanoMaterials, 2020. 3(7): p. 7231-7241.

16. Lim, E.C., et al., Headaches and the N95 face-mask amongst healthcare providers. Acta NeurolScand, 2006. 113(3): p. 199-202.

17. Ba lazy, A., et al., Do N95 respirators provide95% protection level against airborne viruses, andhow adequate are surgical masks? American Journalof Infection Control, 2006. 34(2): p. 51-57.

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