INTRODUCTION
The outbreak of coronavirus disease 2019 (COVID-19) and the associated collapse of the global supply chain have resulted in a severe global shortage of personal protective equipment (PPE)—especially N95 respirators and medical face masks—for healthcare personnel, thereby stimulating the search for local supply sources and innovative solutions. Additionally, the use of non-medical masks by the entire population has been imposed by several jurisdictions worldwide in an effort to mitigate the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes COVID-19, and has also led to an increasing demand for masks. Currently, protective face masks are generally made from petroleum-based synthetic fibers, although the use of biodegradable materials with a lower carbon footprint is preferable, provided the protection performance is maintained. The forestry sector, particularly the pulp and paper industry, plays an important role in the context of the crisis caused by the pandemic, as it can supply a wide variety of cellulose-based products for healthcare personnel and the general public. However, few pulp and paper companies in North America produce medical-grade pulp for PPE. For instance, Canada has a single mill producing medical-grade pulp that can be used in the composition of N95 respirators and other PPE by United States manufacturers. This mill has been operating at full capacity since the appearance of COVID-19 (Business Examiner 2020).
For several reasons, cellulose is one of the most promising alternative materials that can be used to partially or entirely replace synthetic fibers traditionally used as filter media in the manufacture of disposable N95 respirators and medical and non-medical face masks. These reasons include its biodegradability, renewability, low cost, abundance, ease of processing, adjustable aspect ratio, strong mechanical properties, and low density, compared to other materials (Tavakolian et al. 2020). Both N95 respirators and medical face masks require a combination of high filtration capacity and good performance against the penetration of pathogens. The latter requires, among other characteristics, water repellency, as liquid droplets may contain viral agents (Shimasaki et al. 2020), and also bioactive properties including mainly an antimicrobial action (Ristić et al. 2011). Face masks should also allow for a low pressure drop (i.e., high air permeability) to facilitate breathing (Osman 2020), while being comfortable, soft, light, and non-allergenic to the wearer’s skin (Zanoaga and Tanasa 2014; Qin 2016), all in accordance to regulatory standards (42 CFR Part 84 1995; NIOSH 2014; ASTM F2100-19e1 2019; NPPTL 2020). Both N95 and medical face masks made of synthetic fibers have a high level of filtration of greater than 95%; however, few masks available on the market have antiviral activities, meaning that they filter microorganisms but do not deactivate them. This in turn increases the risk of cross-contamination (from the contaminated mask to the wearer or the environment) during use and handling and raises further environmental concerns about their disposal after use (Zhou et al. 2020). Cellulose fibers provide desirable properties to filter media (e.g., bulk, permeability, and mechanical strength) and can act as a support structure for the thin and weaker filter media such as meltblown and electrospun matrices (Hutten 2016). Additionally, surface modification of cellulose fibers is considered an excellent option for adding antimicrobial functionality to medical and healthcare products. Although it does not exhibit intrinsic biocidal activities like some biopolymers, e.g., chitosan, the cellulose surface is reactive, so it can be chemically modified to graft certain functionalities into its structure (Tavakolian et al. 2020). Several technologies have been developed to introduce bioactive compounds, including plant-based products (Catel-Ferreira et al. 2015; Gargoubi et al. 2020), animal-derived chitosan and chitosan derivates (Ling et al. 2013; Ahmad et al. 2016), metal and metal compounds (Emam 2019), metal-organic frameworks (MOFs) (an innovative material derived from metal salts) (Nie et al. 2020a), inorganic salts, synthetic polymers (Tavakolian et al. 2020), two-dimensional (2D) nanomaterials such as graphene (Huang et al. 2020), and composite and nanocomposite structures thereof among others, which provide antimicrobial (antibacterial, antifungal, and/or antiviral) properties to cellulose fibers. These antimicrobial materials can be added either during papermaking or as post-treatment coatings to provide effective protection. Furthermore, when cellulose fibers are disintegrated into nanostructures, e.g., nanofibrillated materials, many filter properties and characteristics such as filtration efficiency (Sim and Youn 2016) and surface functionalization are enhanced due to their high specific surface. Additionally, cellulose may be combined with other materials, such as polyester fibers, forming composite structures to meet the requirements for filter media (Pan et al. 2019). However, cellulose fibers are inherently hydrophilic, so they tend to absorb water, which softens and weakens the filter structure, reduces its useful life, and inhibits filtration performance due to fiber swelling (Mukhopadhyay 2014). This characteristic of the material increases the risk of contamination during use, as viruses can penetrate through a humid mask (Zhou et al. 2020), while also providing favorable conditions for the proliferation of fungi and bacteria. Therefore, treatments providing antimicrobial and hydrophobic properties must be applied to cellulose-based masks. In short, the use of cellulose fibers and nanocellulose products from wood pulp offers numerous advantages for N95 respirators and medical face masks, yet it also raises some key technical challenges that must be overcome to meet the requirements for good filter performance.
This paper presents a review of the potential applications of cellulose and nanocellulose materials in the development of biodegradable products to replace the petroleum-based synthetic fibers usually used in the manufacture of disposable N95 respirators and medical face masks. It offers 1) an overview of the structures and compositions of disposable protective face masks, their properties, and standard requirements and particularities of the filter media; 2) descriptions of the cellulose and nanocellulose materials that could be used in the manufacturing of mask filters; and 3) a suite of antimicrobial technologies that could be used for cellulose functionalization and that remain potentially safe for use in face masks. The study is partly a result of a joint effort of researchers and industrial partners of the pulp and paper sector to find innovative solutions for bioproducts obtained from locally and sustainably sourced fiber, which will hopefully contribute to limiting the spread of SARS-CoV-2.
DISPOSABLE FACE MASKS
N95 Respirators
The National Institute for Occupational Safety and Health (NIOSH) approves particulate filtering facepiece (FFP) respirators according to nine filter classes certified under 42 CFR Part 84 (1995): N95, N99, N100, R95, R99, R100, P95, P99, and P100. The N, R, and P designations refer, respectively, to respirators that are non-resistant, resistant, and proof to degradation by oily aerosols. The protection levels of classes 95, 99, and 100 correspond to filtration efficiencies of airborne particles of at least 95%, 99%, and 99.97%, respectively. The N95 is therefore a particulate FFP respirator that is non-resistant to oily aerosols and that filters at least 95% of airborne particles greater than 0.3 μm, which is considered the most penetrating particle size (MPPS). These respirators are classified as protective devices or PPE and are designed to protect the wearer against the inhalation of hazardous airborne particles, including viruses and bacteria. There are two NIOSH-approved N95 FFP types: standard N95 respirators and surgical N95 respirators, which are also referred to as medical/healthcare respirators and cleared by the US Food and Drug Administration (FDA) (NIOSH 2014; NPPTL 2020). North America follows NIOSH specifications for N95 respirators; however, equivalent FFP respirators from Europe (FFP2), China (KN95), and other countries are commercially available in a variety of shapes and sizes.
The N95 respirators have a multilayer composite structure with a central filtering layer displaying electret properties, giving an electrostatic charge to a nonwoven fibrous mat often composed of synthetic fibers. They are generally composed of four layers: two layers (outer and inner) of spunbonded polypropylene of low density (20 g/m2 to 50 g/m2 basis weight); a thick, stiff, and dense nonwoven layer (approximately 250 g/m2 basis weight) that gives support to the meltblown layer and that can be molded to give the shape of the mask; and a meltblown layer with electrostatic properties (Henneberry 2020). The electrostatic charge of the filter media is generally obtained by corona discharge, triboelectrification, or electrostatic spinning. Electret treatments provide many advantages to filter media without increasing structural mass or density (Zhao et al. 2020). These advantages include high initial and sustained efficiency during the filter lifecycle, increased submicron particle capture efficiency, filtration efficiency unaffected by relative humidity and long-term storage at high temperatures (54.5 °C), and lower airflow resistance (Mukhopadhyay 2014). The N95 respirator’s performance depends on the filtration efficiency as well as on face seal leakage, as respirators must fit tightly to the face of the wearer to create a seal (He et al. 2013).
Filtration performance of N95 respirators is evaluated according to 42 CFR Part 84 (1995) and NIOSH standards by polydisperse sodium chloride (NaCl) aerosol testing with an aerosol concentration not exceeding 200 mg/m3 (typical NaCl concentrations range from 12 mg/m3 to 20 mg/m3) at an 85-L/min flow rate using a TSI 8130 automated filter tester (TSI Incorporated, Shoreview, MN, USA) (Rengasamy et al. 2011, 2013), which is a photometric detection method that uses a forward light scattering photometer to measure the flux of light scattering from particles (Rengasamy et al. 2011). This method determines the penetration or filter efficiency and pressure drop in filter media, filter cartridges, and respiratory masks. The NaCl aerosol must have a particle size distribution with a count median diameter (CMD) of 0.075 µm ± 0.020 µm (size distribution can be determined using differential mobility analyzers) and a standard geometric deviation (GSD) of less than 1.86. Additionally, the NaCl aerosol must be at 25 °C ± 5 °C and 30% ± 10% relative humidity and neutralized to the Boltzmann equilibrium state (the remaining charged particles must have a bell-shaped distribution curve with the center at zero). The 42 CFR Part 84 (1995) standard requires pre-conditioning of the N95 respirators before the test (at 85% ± 5% relative humidity and 38 °C ± 2.5 °C for 25 h ± 1 h) because some types of electrostatic filter media can degrade after exposure to high humidity. The filtration efficiency is the percentage of NaCl particles filtered by the material, while the pressure drop corresponds to the air resistance across the filter material (Zhao et al. 2020). The maximum penetration must not exceed 5% for class 95 respirator approval.
Other filtration test methods have also been studied. A comparison between the photometric method and another polydisperse NaCl aerosol penetration method using an ultrafine condensation particle counter (UCPC) to measure the filtration performance of N95 respirators was presented by Rengasamy et al. (2011). The authors showed that aerosol penetrations measured by the UCPC were 2 times to 6 times greater than the levels measured by the photometric method, suggesting the need for the development of a more challenging aerosol testing method for N95 respirator certification. In addition to the NaCl aerosol test, N95 respirators must meet other specifications according to 42 CFR Part 84 (1995) and NIOSH standards, such as inhalation and exhalation resistance to airflow (≤ 343 Pa and ≤ 245 Pa, respectively) for both N95 types, synthetic blood penetration (ASTM F1862/F1862M-17 2017) and biological filtration efficiency for the surgical N95 type, and exhalation valve leakage (leak rate 30 mL/min) and force applied (-245 Pa) for the standard N95 type. Other tests can also be performed, such as surface wetting resistance, microorganism index (bioburden) (Davison 2012), CO2 clearance, and total inward leakage (TIL), which quantifies the respirator’s ability to fit individual facial dimensions to ensure that the respirator fits properly (not a requirement for respirator approval testing) (He et al. 2013, 2014; Ramirez 2015).
Medical Face Masks
Medical face masks, also known as surgical masks or procedure masks, are usually composed of three layers of a spunbonded-meltblown-spunbonded (SMS) nonwoven fabric, where the meltblown middle layer provides higher filtration performance compared to the spunbonded layers (Ghosh 2014; Mukhopadhyay 2014). The outer spunbonded layer is typically blue and hydrophobic to prevent liquid droplets containing viral or other hazardous agents from reaching the wearer (Shimasaki et al. 2020), acting as a prefilter layer for the meltblown middle layer that provides the final filtration (Mukhopadhyay 2014). The inner spunbonded layer is soft, for the wearer’s comfort, and absorbent, to absorb the fluids generated by breathing, coughing, and sneezing, and it usually has no filtration function. The spunbonded layers are generally made of polypropylene, polyester (polyethylene terephthalate, PET), or other thermoplastic fibers (Henneberry 2020). Cellulose fibers such as cotton or rayon fibers, which are made from regenerated cellulose from wood or agricultural residues, and mixes of cellulose/polyester and cellulose/ polyolefin fibers can also be used in the outer and inner layers (Davison 2012; Ciuffreda et al. 2020; Medline 2020). Spunbonding is a well-known continuous nonwoven process in the field of textiles. It is fast and inexpensive and combines spinning, sheet formation, and bonding: Fibers are spun, dispersed by deflectors or air streams to form a web, and finally bonded by hydroentangling, needle punching, or thermal or chemical methods (Lim 2010).
The meltblown filter of the middle layer is composed typically of polypropylene; however, other synthetic fibers can be used, such as poly(butylene terephthalate), a type of polyester, and poly(tetrafluoroethylene) (PTFE), a thermoplastic polymer with inherently hydrophobic properties used to form a porous membrane structure (Solvay 2020). The pore diameter in PTFE stretched films ranges from 0.1 µm to 10 µm, providing high filtration efficiency, lightness, high air permeability (breathability), and long service life; however, they are expensive and cannot completely replace traditional masks (Hendrikx 2019). The melt blowing process is a conventional method for producing micro- and nanofibers from thermoplastic polymers. In the process, a polymer melt is extruded through small nozzles surrounded by high-velocity hot air to produce fibers that are randomly deposited to form a nonwoven web (Hiremath and Bhat 2015). A combination of SMS technologies, known as a multi-denier process, is also employed to form a composite structure and reach the requirements of protective textiles (Lim 2010). Spunbonded fibers are coarser, have higher tensile strength, and have a smaller pressure drop than meltblown fibers (Mao 2016). The polypropylene microfibers of the meltblown layers have diameters of approximately 1 μm to 10 μm and a fabric thickness of 100 μm to 1000 µm, which provides high porosity and a limited capacity to filter fine particles (Zhao et al. 2020). For this reason, other filtration technologies are employed to improve filter performance. Unlike N95 respirators, which are electret-treated to improve filtration efficiency against small aerosols, the meltblown layer of medical face masks has no electret properties. The filtration of particles is only mechanical and therefore less efficient than in N95 respirators. Medical face masks are hence designed mainly to prevent the projection of droplets expelled by the wearer into the environment, e.g., to prevent surgical infections and to protect others from droplets originating from sick people. Consequently, they offer low protection for the wearer against the inhalation of fine airborne particles.
Prather et al. (2020) offer a helpful explanation to distinguish between aerosols and droplets: Aerosols are smaller than 100 μm, remain suspended in the air for long periods (ranging from several seconds to hours), and can be inhaled after traveling more than 2 m and accumulating in poorly ventilated indoor air. In contrast, droplets are larger than 100 μm and fall to the ground in seconds, usually within a distance of 2 m from the source. Transmission of SARS-CoV-2 occurs by both aerosols and droplets (Prather et al. 2020). Although the filtering efficiency of medical face masks is generally tested with nonbiological particles, a recent study has confirmed their efficiency in preventing transmission of human coronaviruses and influenza viruses from symptomatic individuals, suggesting that their use could also help control COVID-19 transmission (Leung et al. 2020). The study showed that medical face masks significantly reduce the environmental emission of the influenza virus in coarse aerosols (particles > 5 µm) and coronavirus in fine-particle aerosols (≤ 5 µm), with a trend to reduce the coronavirus emission in respiratory coarse aerosols. Medical face masks offer more comfort and greater breathability than N95 respirators during use.
Other filtration mechanisms including antiviral technology and nanotechnology have been developed to improve the performances of N95 respirators and medical face masks. The BioFriend™ BioMask™ provides FDA-cleared disposable N95 surgical respirators and medical face masks with antiviral properties. These masks are composed of four layers, with two active layers designed to inactivate viruses through different mechanisms of action. The outer active (first) layer is composed of spunbonded polypropylene treated with a hydrophilic plastic coating that absorbs infectious droplets and inactivates viruses by exposure to citric acid derivates. The inner active (second) layer is composed of cellulose (rayon) and polyester fibers treated with positively charged copper and zinc ions. These active layers inactivate different subtypes of influenza A and B viruses, paramyxovirus (measles), and SARS-CoV-1, which was responsible for the severe acute respiratory syndrome (SARS) outbreak in 2003. The third layer is composed of meltblown polypropylene filter media, while the inner (fourth) layer is composed of spunbonded polypropylene. The antiviral N95 respirators and medical face masks have the same structure, except for the meltblown material used in the third layer of the N95 respirator, which has increased filtration efficiency to meet the required specifications (Davison 2012). The same antiviral technology is used on the Innonix RespoKare® surgical masks, which inactivate many pathogens, including coronaviruses, but they had not yet been proven efficient for SARS-CoV-2 (RespoKare 2017). Additionally, the Inovenso company has developed a new generation of nanofiber face masks, named Inofilter® 95/99, with a three-layer structure made of PET spunbonded nonwoven fabric in the outer and inner layers (35 g/m2 basis weight for each layer) and a nanofiber membrane in the middle layer (0.6 g/m2 to 0.8 g/m2 basis weight) replacing the meltblown middle layer conventionally used in face masks. The nanofiber membrane is made of a thermoplastic fluoropolymer (polyvinylidene fluoride, PVDF) using a patented hybrid electrospinning technology. The PVDF nanofiber membrane has fibers with a diameter of approximately 224 nm and is designed to provide a high mechanical filtration efficiency of 96% to 99%, inhalation protection, and low pressure drop. Another similar version of the nanofiber filter, the Inofilter V®, provides additional properties such as 99.9% viral filtration efficiency, inhalation and exhalation protection, and resistance to liquids like blood and oil (Inovenso Technology 2018).
Medical face masks must meet five specifications for filtering according to the ASTM F2100-19e1 (2019) standard: 1) bacterial filtration efficiency (BFE), 2) submicron particulate filtration efficiency (PFE), 3) differential pressure (ΔP), 4) resistance to penetration by synthetic blood, and 5) flammability. Medical face masks are classified according to three levels of protection: Level 1 (low), Level 2 (moderate), and Level 3 (high), with differing specifications. The BFE tests measure the percent efficiency of the material in preventing the passage of bacteria through the mask. The BFE test is performed with an aerosol of Staphylococcus aureus bacteria (particles of approximately 3 µm) at a constant flow rate of 28.3 L/min (i.e., a normal breathing flow rate) (ASTM F2101-19 2019). Submicron PFE tests measure the initial particle filtration efficiencies of materials used in medical face masks using monodispersed aerosols of latex particles, usually at 0.1 µm in diameter (ASTM F2299/F2299M-03 2017). Both BFE and PFE must meet the requirements of ≥ 95% for low-barrier face masks (Level 1) and ≥ 98% for moderate- and high-barrier face masks (Levels 2 and 3). Differential pressure tests measure the pressure drop across a medical face mask under specific conditions of airflow, temperature, and humidity. The ΔP metric is an indicator of the breathability of the mask, expressed in mm H2O / cm2 or Pa/cm2. The ASTM F2100-19e1 (2019) standard requires a ΔP of < 4.0 mm H2O / cm2 (< 39.2 Pa/cm2) for low-barrier face masks (Level 1) and of < 5.0 mm H2O / cm2 (< 49.0 Pa/cm2) for moderate- and high-barrier face masks (Levels 2 and 3) (Public Works and Government Services Canada 2020). Lower pressures indicate higher breathability or higher air permeability. The breathability of medical face masks depends on several parameters, such as textile type, the applied finish, the thickness, and the number of layers (Rogister and Croes 2013). Resistance to penetration by a synthetic blood is tested according to the ASTM F1862/F1862M-17 (2017) standard, which requires resistance to synthetic blood at pressures of 80 mm Hg, 120 mm Hg, and 160 mm Hg to qualify for the low-, moderate-, or high-barrier classes, respectively. Flammability tests are based on the rate of flame spread on the material. All medical face masks must meet the requirements for class 1 (Nelson Labs 2019).
Filter Media
Protective masks have an air filtration system that can be composed of various filter media. The air filter medium is the part of the filter that separates harmful particles from the air. Air filters are used for a wide range of applications, including vehicle air, commercial and residential indoor air (e.g., heating, ventilation, and air-conditioning systems and high-efficiency particulate air (HEPA) filters), clean rooms, laboratory hoods, large baghouse filters, flue gas scrubbers, industrial dust collectors, vacuum cleaner bags, medical protective devices (e.g., respirators and face masks) (Mukhopadhyay 2014), non-medical face masks, and reusable cloth masks made with a replaceable filter bag. Nonwoven filter media are formed by dry-forming, wet-laid, or combined technologies using many processes depending on the raw material (polymer or fiber source) and filter end use. Dry-forming processes include air-laid web (used to form absorbent materials, normally from cellulose fluff pulp), dry-laid web (used to form many felt materials used for filtration), melt spinning (used to produce spunbonded and meltblown microfibers from melt polymers), and nanofiber spinning (used to produce nanofibers by electrospinning or centrifugal spinning methods) processes. The wet-laid process is used to form webs from wood pulp, vegetable fibers, fibers of polyester, nylon, rayon, or any other material that can be dispersed in water, whereas combined technologies are used to form multi-layer composite structures, as in SMS technology, and nonwoven-membrane materials using different processes and/or materials (Hutten 2016).
Air filter media are designed to filter airborne particles using various filtration mechanisms. There are five filtration mechanisms by which airborne particles (aerosols) can be controlled: interception, inertial impaction, Brownian diffusion, gravitational settling, and electrostatic attraction (used in N95 respirators). In N95 respirators and medical face masks, filters must have a porous structure designed to maximize the space for filtration while keeping the differential pressure sufficiently low to maintain breathability (Vaughn and Ramachandran 2002; Institute of Medicine 2006). Medical face masks comprise a combined filtration mechanism: Inertial impaction and interception predominate in the BFE test for the capture of large particles, while Brownian diffusion predominates in the PFE test for submicron particles (Tong et al. 2016). The N95 respirators are also built to capture particles of different sizes: Large particles (> 1 µm) are captured by interception and inertial impaction, while small particles (< 0.1 µm) are captured by Brownian diffusion, and both small and large negatively charged particles can be captured by electrostatic attraction, which has the advantage of maintaining a low airflow resistance (Institute of Medicine 2006). Protective masks generally use a filtering gradient from the largest to the smallest particles through the mask. This way, the first layer of the filter captures the larger particles and prevents clogging of the pores in the subsequent layers.
Hutten (2016) divided the raw materials used for filter media into four overlapping categories: polymers, fibers, resins and binders, and additives. Polymers used in nonwoven materials include fibers (e.g., meltblown and spunbonded polypropylene, cellulose from wood pulp and vegetable fibers, and regenerated cellulose or rayon), resins, and additives. Fibers comprise natural fibers (e.g., wood pulp, vegetable fibers, and cotton), biofibers (e.g., rayon and lyocell), and synthetic fibers, to name only the most pertinent for mask manufacturing. The most important physical characteristics of fibers for use in filter media are diameter, length, aspect ratio (length-to-diameter ratio), density, linear density (or fiber coarseness, weight in mg per 100 m of fiber), cross-section shape, internal structure (cellular or solid), and strength properties, which include tensile strength, breaking length, stretch or elongation, and stiffness. The ideal properties of the fibers are those that optimize the bulk, air permeability, and pore size of the filter media. When designing filters, the goal is to maximize bulk and air permeability to permit breathability while minimizing pore size to improve filtration efficiency. However, these properties are not directly compatible, as thin fibers provide high density, small pore size, and high filtration efficiency to filter media, but they provide low air permeability. Conversely, coarse fibers provide high bulk and permeability, but they lead to large pore size and thus offer poor filtration efficiency for filter media. In general, the spunbonded web of protective masks is formed by coarse fibers, whereas the fibers are much smaller in the meltblown web. Spunbonded fibers have diameters ranging from 1 μm to 50 μm (ideal range is from 15 μm to 35 μm) and basis weights (grammage) of the filters ranging from 5 g/m2 to 800 g/m2 (typically between 10 g/m2 and 20 g/m2). Their web thicknesses range from 0.1 mm to 4.0 mm (typically between 0.2 mm and 1.5 mm), and they are characterized by high strength-to-weight ratios compared to other structures and high liquid retention capacities. Meltblown fibers have diameters ranging from 0.5 μm to 30 μm (typically between 2 μm and 7 μm) and basis weights of the filters between 8 g/m2 and 350 g/m2 (typically between 20 g/m2 and 200 g/m2) (Hutten 2016). These microfibers have high surface areas for good filtration, smooth textures, and circular cross-sections (Malkan and Wadsworth 1993; Hutten 2016).
The aspect ratio affects the quality and performance in nonwoven webs. In general, synthetic microfibers form highly porous webs and are water repellent, but they are advantageous in filter media because they provide low pressure drop and low wettability, due to a low surface energy. Resins and binders for face masks must provide softness to give greater comfort for skin contact and have skin-friendly properties. Additionally, additives may be used in nonwoven filters, including adsorbent materials (e.g., activated carbon—many replaceable filter cartridges for non-medical face masks are constructed with an activated carbon filter layer), flame retardants (required for cellulose-based filters), water repellents (important for the outer layer of masks), antimicrobial agents, and inks and colorants (used for aesthetic and operational purposes) (Hutten 2016). Debonding and softening agents may be particularly interesting for cellulose-based filters used in face masks. Debonder technology is used in paper products, e.g., tissue paper, to prevent excessive bonding between one cellulose fiber and another and to improve the smoothness of the paper (Furman, Jr. et al. 2013). Debonders provide surface smoothness and increased softness, bulk, and sheet flexibility, among other advantages, to paper sheets (Solenis 2015).
Filter media properties can be affected by several factors related to fiber characteristics (synthetic or natural source, chemical composition, diameter, geometry, specific surface, and density), filter characteristics (thickness, density, bulk, porosity, and pore size distribution) (Vaughn and Ramachandran 2002), filtration velocity (based on flow rate and surface area), filtration mechanisms, and operational conditions (temperature and humidity) (Mostofi et al. 2010). The main properties of filter media are determined according to Eqs. 1 to 6.
The filtration efficiency (E) (expressed in %) is calculated by Eq. 1,
(1)
where Ndown and Nup are the downstream and upstream number concentrations of the aerosol particles, respectively (Long et al. 2018; Lu et al. 2018; Pan et al. 2019).
The pressure drop across the filter (ΔP) (Pa) is determined by the difference between the upstream (Pup) and downstream (Pdown) pressures (Lu et al. 2018; Ma et al. 2018; Nie et al. 2020a).
In addition to standard filtration efficiency and pressure drop tests to determine the filter media properties of N95 respirators and medical face masks, a filtration quality factor (Q) (generally expressed in Pa-1) is commonly used and calculated as shown in Eq. 2 (Mao et al. 2008; Alexandrescu et al. 2016; Long et al. 2018). Generally, large Q values indicate better filter quality, which conjugates high filtration efficiency (low particle penetration) with low pressure drop (high breathability) (Zhao et al. 2020).
(2)
Air permeability is the airflow rate measured through a specific area of the filter media at a specific pressure drop and is generally expressed in cm3/(s·cm2), ft3/(min·ft2), or cfm. The test is performed by an automatic air permeability testing apparatus according to the ASTM D737-18 (2018) standard. Like the pressure drop test, is an indicator of breathability. The airflow depends on ΔP and filter thickness (t) and is measured by the equation proposed by Whitaker (1986) (Eq. 3),
(3)
where v0 is the airflow velocity, k is the permeability constant, and µ is the viscosity of the air (Pan et al. 2019). The air permeability can thus be adjusted by changing the basis weight, thickness, and/or density of the filter. For instance, as filter thickness decreases, air permeability increases (low pressure drop), although the filtration efficiency decreases (Nie et al. 2020a).
The apparent density and porosity both affect the filtration efficiency and pressure drop of the filter media. The density of the filter (ρf) (expressed in g/cm3) is calculated from the ratio of basis weight (BW) and t (Eq. 4),
(4)
where BW is the weight of a unit area, typically measured in g/m2, while t is often measured in μm (Hutten 2016). Filter media used for disposable applications are produced at a low basis weight, e.g., approximately 25 g/m2 in N95 respirators and medical face masks (Zhao et al. 2020).
The bulk (βf) is also used to characterize the filter and corresponds to the inverse of the density, expressed in cm3/g (Eq. 5) (Hutten 2016),
(5)
The porosity (ε) of the filter can be calculated from the basis weight, thickness, and fiber density (ρfiber), as shown in Eq. 6 (Long et al. 2018). The pore properties (size and distribution) are affected by fiber type (synthetic or natural), fiber dimensions (micro- or nano-scale), and, particularly for pulp fiber sheets, drying conditions to remove excess water from wet-formed sheets (Sim and Youn 2016).
Ideally, cellulose-based masks must present antimicrobial properties in the outer and inner layers to prevent pathogenic microorganisms from the environment from reaching the wearer and vice versa.
ACKNOWLEDGMENTS
The authors acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC) by the NSERC Alliance COVID-19 grant (No. ALLRP 554160-20, grantee Alexis Achim).
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