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Experimental Study on Charge Decay of Electret Filter Due to Organic Solvent Exposure
Abstract
The electret filter is a potential component to remove airborne particles due to its high collection efficiency and low pressure drop. However, its filtration performance is gradually decreased by exposure to organic solvents, which limits the application of electret filters. The effect of ethanol exposure on the filtration performance of polypropylene electret filters was investigated experimentally to clarify the charge decay phenomenon in this study. Experimental results revealed that filter performance is strongly dependent upon the challenged mass and existing state of an ethanol solvent. The filter performance was drastically degraded by exposure to ethanol droplets generated from a solution with ethanol concentrations above 30%; however, it was maintained during exposure to ethanol vapors. This tendency was also seen in the surface potentials of the exposed filter media. In addition, we found that the critical challenging amount of ethanol droplets was in the vicinity of 0.045 g/cm² to neutralize a tested electret filter in this study.
Copyright 2015 American Association for Aerosol Research
... However, it is also known that the initially high filtration efficiencies of these new electret filters can decrease significantly over operating time due to exposure to particles [7,[10][11][12][13][14][15], organic solvents [16,17], or high temperatures and/or humidity [18,19]. During this filter ageing process, the original charge decays and the filter becomes less effective in filtering particles. ...
... According to Choi et al. [17], the aggregate state of the solvent can have a significant influence. It was shown that the extent of discharge by immersing in liquid solvent can depend on the immersion duration [17], potentially leading to an incomplete discharge [16,[22][23][24]. ...
... According to Choi et al. [17], the aggregate state of the solvent can have a significant influence. It was shown that the extent of discharge by immersing in liquid solvent can depend on the immersion duration [17], potentially leading to an incomplete discharge [16,[22][23][24]. It was further speculated that the evaporation of the liquid solvent during drying may induce charges again [23,25]. ...
Electret filters are electrostatically charged nonwovens which are commonly used in aerosol filtration to remove fine particles from gases. It is known that the charge and thus also the filtration efficiency can degrade over time. Thus, many testing standards require to remove the charge by treatment with liquid isopropanol (IPA) or IPA-saturated air. However, the parameters influencing this discharge have not been completely clarified yet. The aim of this work was, on the one hand, to experimentally investigate the influence of the IPA treatment on different electret filters and, on the other hand, to show the optimization potential of electret filters with respect to efficiency and long-term stability by numerical simulations. The experiments revealed that the air permeability is a central influencing parameter. Small pores lead to a reduced discharge efficiency using liquid IPA, while both treatment methods are suitable for larger pores. The simulations showed that a homogeneous charge distribution within the filter depth is advantageous for the initial performance. In contrast, charge penetrating deeper in the filter medium delays the charge decay and thus increases the operating time, with the trade-off of a lower initial performance.
... The charge retention capacity is generally dependent on the electrical conductivity and the energy level of localized trap sites [5,[13][14][15]. The mechanisms of charge decay are mostly explained by the relaxation of polarized state of dielectrics, spatial distribution of dipoles and free charges, and the neutralizing effect of dielectric conductivity [10,11,[16][17][18]. ...
... Thus, electret filters tend to give higher quality factor (QF), which is the relative efficiency to a unit pressure drop [6,22,23]. The limitation of electret filter comes from the charge decay, which occurs through environmental aging that causes electric conduction or charge carrier mobility [8,[16][17][18]. It has been demonstrated that the exposure of an electret filter to heat, humidity, and solvent can cause deterioration of filtration performance, as a result of charge loss [17,22,24]. ...
... The environmental factors that deteriorate the electret filtration have been studied rather frequently [8,17,18,24], probably because it is associated with the practical use conditions; on the other hand, the material aspects that influence the charge decay have rarely been studied. While various material options were examined to attain the high-quality factor filters [10,25,26], it lacked scrutiny on the material attributes that enable the long-term charge retention. ...
Electret filters as opposed to mechanical filters display the enhanced ability to capture airborne particles with the electrostatic attraction. However, the environmental aging during shelf-life or use may cancel its benefit by dissipating the charges. This work investigates the polymeric attributes influencing the charge decay and the electrostatic filtration of electret filters, employing polymers with different dielectric constants (r) and wettability. As accelerated aging, high temperature (120 °C) or high humidity (25 °C, 90% RH) was applied to the electret filters for 48 h. For the humidity aging, wetting property of material was a critical factor affecting the charge decay and the filtration performance, as the absorbed water increases the electrical conductivity. For the thermal aging, the material with the highest r deteriorated the electric potential and the filtration performance by the largest extent, due to the lower band gap energy for charge transfer. The results of this study implicate that r and wettability are important material parameters influencing the electric conductivity and chain mobility, and they can be used as convenient predictors for charge retention capacity affecting the robust electrostatic filtration performance.
... Long-term use and reuse of electret filters as face masks has become prominent during the COVID-19 pandemic (Ullah et al., 2020), highlighting the importance of the lifetime of air filters in our daily (Jang and Kang, 2020;Wong et al., 2020). However, electrostatic charges, which are key properties of electret filters, may deteriorate upon exposure to specific environments, such as humid ones (water vapor) (Motyl and Łowkis, 2006;Xu et al., 2014) and those in which organic chemicals are present (Choi et al., 2015;Kim et al., 2009). If chemicals present in the environment reduce filtration performance, resulting in the penetration of airborne pollutants, this could directly lead to adverse health outcomes in filter users. ...
... However, their impact was likely to be small, because organic solvents exert their neutralizing effects in solvent form, not as vapor. Choi et al. (2015) reports that filtration efficiency was maintained during exposure to ethanol vapors, while it was sharply reduced by exposure to ethanol solution. Further research is needed to reveal the major causative agent that reduced filtration efficiency among the myriad of substances contained in cigarette smoke. ...
The role of air filters is becoming increasingly important due to the threat of air pollution to public health. Understanding the lifetime of air filters is essential for assessing air pollution exposure. However, the effects of common environmental chemicals on filter performance have not been explored. Air filters in ventilation systems and air purifiers are commonly exposed to cigarette smoke aerosols. Moreover, due to the coronavirus pandemic, people are more likely to be in close proximity with smokers while wearing face masks, such that their masks will be exposed to cigarette aerosols. In this study, we applied a stepwise approach to analyze the effects of cigarette smoke on the filtration performance of electret melt-blown filter media that are commonly used to create face masks. We found that cigarette aerosols dramatically reduced filtration efficiency, while standard test particles of a similar loading weight did not affect filtration efficiency. After loading up to 204 μg/cm² of cigarette smoke on 100 cm² of electret filter medium, the filtration efficiency of some filters decreased from 92.5% to 33.3% (−Δ59.2%). Interestingly, we founded no changes in pressure drop following cigarette smoke exposure despite the reduction in filtration efficiency, suggesting that cigarette smoke aerosols significantly impact the electrostatic charge properties of the filters. Our results indicate that the lifetime of commonly-used air filters may be much shorter than expected and that people may unknowingly be directly exposed to airborne pollutants.
... Complete suppression of bacterial growth after 5 min immersion [22] NA [7,19] Complete bactericidal efficiency of S. subtilis and E. coli [18] Isopropanol Decrease in particle filtration efficiency (over 5%) NA [11,17] Significant drop in oily aerosol filtration efficiency after 2 min immersion (up to around 40%) NA [9] Detergent -laundering Significant drop in particle filtration efficiency after soap solution soaking for 2 min (up to around 39%) NA [15] Polymers 2021, 13, 45 3 of 16 ...
... The particle filtration of an electret media is attributed to the mechanical capture and the electrostatic attraction of particles [31,32]. Many previous studies associated the deteriorated performance of disinfection-treated respirators with the loss of electrostatic charges [7,9,11,15,[17][18][19]22,25], yet the direct evidence of charge deterioration was often missing. Additionally, it has hardly been examined for the effect of treatment on the structural integrity of the filter media, which may affect the resistance of respirators. ...
In the outbreak of COVID-19, the extended wear of single-use, disposable respirators was inevitable due to limited supplies. As a respirator is front-line protection against particulate matter, including bioaerosol and droplets, a comprehensive understanding for the reuse strategy is needed. In this study, eight different disinfection methods commonly applied for the reuse of respirators were compared for their influence on the filtration and bactericidal/bacteria removal performance, with in-depth discussion on the cause of effects. Treatments including oven-dry, ultraviolet irradiation (UV), microwaving, laundering with and without detergent, and immersion in hypochlorite, isopropanol, and ethanol were performed to respirators. Immersion in ethanol or isopropanol was effective for inactivation and removal of bacteria, yet such a treatment significantly deteriorated the filtration efficiency in about 20–28%, dissipating the surface charges. Laundering, while effective in removing the attached bacteria, triggered physical damage, leading to a possible reduction of filtration performance. A short-term oven-dry, UV irradiation, and microwaving mostly preserved the filtration performance, yet the drawback lied in the incomplete bactericidal efficiency. This study would contribute to the public health and safety by providing scientific background on the effect of disinfection treatment methods for respirators.
... The mechanism of charge decay is generally explained by the relaxation of polarized state of dielectrics and the spatial distribution of dipoles and free charges [54][55][56][57]. Environmental factors such as heat, moisture, or solvent exposure can cause electric conduction, facilitating the charge carrier mobility, and leading to charge dissipation [57][58][59][60][61]. ...
... The mechanism of charge decay is generally explained by the relaxation of polarized state of dielectrics and the spatial distribution of dipoles and free charges [54][55][56][57]. Environmental factors such as heat, moisture, or solvent exposure can cause electric conduction, facilitating the charge carrier mobility, and leading to charge dissipation [57][58][59][60][61]. In Cho et al.'s study [36] where the charge retention of polycarbonate electrospun web was investigated, the charge decay was aggravated at above the glass transition temperature (T g ). ...
With increasing air pollution and sporadic outbreaks of epidemics, there is ramping attention on the filtration devices. The main constituents of airborne pollutants are particulate matters of solid particles, liquid aerosol, bioaerosol/bio-droplets, and gas/vapor. With the growing demand for high-performance filters, novel materials and functionalities are being developed applying advanced technologies. In this paper, recent developments of fiber-based particulate filters are reviewed, with a focus on the important performance parameters and material properties. Trends in technology and research activities are briefly reviewed, and the evaluative measures of filtration performance are reported. Recent studies on the advanced filter materials are reviewed in the aspect of polymers and the fabrication process of fibrous assembly. The characterization method including 3D modeling and simulation is also briefly introduced. Multifunctional filters such as antimicrobial filter and gas and particulate filters are briefly introduced, and efforts for developing environmentally sustainable filters are noted.
... The effect of a filter was studied by utilizing a fresh film-type electret filter (FF) and a neutralized film-type electret filter (NF). The NF was prepared by soaking an electret filter in an ethanol solution and drying it at room temperature for 72 hours (Lee et al., 2002;Choi et al., 2015). ...
... Therefore, the performance of a filter was affected by the electrostatic force condition on the filter. Previous research showed that ethanol treatment to neutralize a filter significantly decreased the particle collection efficiency (Lee et al., 2002;Choi et al., 2015). The effect of diffusion and field charging did not significantly affect particle collection efficiency for the combination of the unipolar charger and electret filters because the short distance of 25 mm does not improve the collection efficiency. ...
Electret filters have been used widely to collect particles because of their low pressure drop and high collection efficiency due to the electrostatic attraction between the particles and filter. However, the collection efficiency of these filters decreases over time due to charge degradation. Therefore, a unipolar charger using a corona discharge was combined with an electret filter to overcome this decrease. The highest particle collection efficiency for 70 nm, uncharged particles occurred with a fresh electret filter was 34.0%, whereas a neutralized filter had a collection efficiency of 32.6%. However, when fresh and neutralized electret filters were used with a unipolar charger (electret field: 1.9 kV cm –1), the highest particle collection efficiencies of 99.8% and 98.2%, respectively, were obtained and appeared at the electrode-to-mesh distance, a distance similar to the duct diameter. The distance between the tip of the discharge electrode and filter, which can be predicted using Sigmond's law, affected the charge density value on the filter surface. The ozone concentration was negligible under optimum conditions for particle collection. Therefore, using an electret filter combined with a corona discharge has good potential for improving air purifiers.
... Hence, the polarization effect and Coulomb interactions increase while the fibers' diameter decreases [18]. However, the initial filtration efficiency of electrets is reduced by temperature, humidity, and organic solvents [19][20][21][22]. Electrets tend to lose their charge quickly, so, at the production stage, it is worth using additives to ensure that the charge is retained. ...
This study explores the fabrication of electret nonwoven structures for high-efficiency air filtration, utilizing the blow spinning technique. In response to the growing need for effective filtration systems, we aimed to develop biodegradable materials capable of capturing fine particulate matter (PM2.5) without compromising environmental sustainability. Polylactic acid (PLA) was used as the primary polymer, with the addition of SiO2 and MoS2 to enhance the fibers' charge retention and filtration performance. The fibers were charged electrostatically to improve particle capture efficiency. The experimental results showed that fibers containing 5% MoS2 exhibited the highest filtration efficiency, surpassing those with SiO2, despite MoS2 being a semiconductor and SiO2 a die-lectric. Furthermore, the addition of MoS2 improved the filtration efficiency across a range of particle sizes (0.2-1 µm) while maintaining a manageable pressure drop. These findings suggest that incorporating MoS2 in electret nonwoven structures can significantly improve filtration performance, making it a promising material for advanced air filtration applications. This study contributes to the development of eco-friendly filtration materials with high performance, essential in reducing exposure to airborne pollutants.
... The non-fluid treatments show better performance than fluid-based treatments in filtration efficiency tests. The possible explanation can be because of the neutralization of electric charges of polypropylene filtration layer after using organic solvents (Choi et al., 2015). ...
An experiment coupled with a computational analysis was conducted to investigate the effectiveness of surgical masks, which include KF94 and ASTM Level 1, in an indoor environment. The KF94 mask sample shows the highest filtration efficiency (99.9%) in the analysis. The simulation is consistent with the experimental results as the concentration of sodium chloride (NaCl) droplets is < 4% on average in the room. The ultraviolet-C (UVC) irradiation and dry heating samples are shown to retain the highest filtration efficiencies (> 97%) after 3 disinfection treatment cycles. All methods (boiling, steaming, ethanol treatment, and dry heating) effectively reduce the S. aureus load by 99.99%. The UVC irradiation shows exposure to 450 µW cm–2 for 10 minutes can effectively eliminate all S. aureus on the mask materials. Simulation shows the reduction in overall NaCl and carbon dioxide (CO2) levels is directly proportional to the filtration efficiency and the effectiveness of reused masks is also directly proportional to the filtration efficiency. The proxy indicator (CO2) for aerosol particles demonstrates that very fine respiratory droplets can penetrate the mask after reaching a steady state. The CO2 concentration increase shows that aerosol particles are accumulated under adequate ventilation and further pose the risk of infection. The NaCl droplets (2%) simulation shows that respiratory droplets have infiltrated the mask, but the mask demonstrates a higher ability to block the NaCl droplets and prevent their penetration. The findings suggest a need to revise the existing regulation of the control policy.
... However, the commercial electret filter actually used in HVAC systems and air purifiers are pleated structure with the pleat height of several centimeters in order to improve the dust holding capacity. In addition, the real face velocity of filter in air purifiers was typical in the range of 0.5-2 m s −1 , which was much higher than the filtration test value used in previous laboratoryscale studies [22][23][24][29][30][31]. The reports on the actual performance of EES operated in air purifiers were insufficient and there are still several important issues which lack sufficient attention. ...
The filtration efficiency deterioration over operating time due to exposure to particles, organic solvents and other factor is an obvious defect of electret filter, especially during the coronavirus pandemic. Electrostatic enhanced method has been proposed as a promising way to improve the filtration efficiency and service time of electret filters without increasing the pressure drop. In this work, the effect of discharge electrode structure and operation mode of the electrostatic enhanced structure(EES) on the filtration efficiency of commercial pleated electret filters were studied firstly, then the EES was installed in a special designed prototype air purifier, two key indicators of air purifiers(clean air delivery rate (CADR) and cumulate clean mass (CCM)) were tested to evaluate the actual performance improvement of electret filters by the electrostatic enhanced method. It was found that the discharge electrode structure had significant influence on the filtration efficiency and multi-wire array electrode was more suitable for the discharge electrode to pleated filter to ground mesh structure used in this paper. The decayed electrostatic charges of pleated electret filter cannot be recharged again through the EES in actual operation condition. The filtration efficiency improvement of the test pleated electret filters was dominantly contributed by particle charging. The filter polarizing induced by the external electric field was helpful in increasing the filtration efficiency when the particles were charged while its effect on uncharged particles was almost negligible. Besides, the actual performance of the prototype air purifier indicated that the EES can alleviate the filtration efficiency deterioration of test electret filters and extend the service life of H11 and H13 filters by more than 3 and 1.5 times respectively according to the CCM test results.
... These electrets require poling [27]. Nonconductive electrets are also used for computer memories [28][29][30][31][32][33][34], gas filters [35][36][37][38][39][40][41] and radiation dosimeters [42,43]. ...
An electret refers to a permanent electric dipole. Nonconductive electrets (polymers and ceramics) have long been studied and utilized, but conductive electrets (metals and carbons) are emerging. This paper introduces the concept of the electret-based capacitance C′ to elucidate the discharge behavior of conductive electrets in the form of unpoled solder and other conductors (metals). The C′ is defined as the electret charge Q′ (determined by integrating the discharge curve) divided by the DC electret voltage V′. The conventional permittivity-based capacitance C (measured) is lower than C′ by orders of magnitude. For Sn–4Ag lead-free solder, C′ (given by Q′/V′) is 620 F, whereas C is 175 pF. The electret stems from the interaction of a small fraction of the carriers (free electrons) with the atoms, as supported by DC polarization asymmetry. For Sn–4Ag, this fraction is 3.5 × 10–9. Electret discharge (V′ decreasing to zero) occurs upon short circuiting. Self-charge (V′ restored) occurs upon subsequent open circuiting, being slightly more sluggish than discharge. The electret discharge/charge amounts to discharge/charge of C′. The discharge/charge time constant is found to approximately equal RC′ (R = resistance) and the discharge/charge energy is found to approximately equal ½ C′V′², as expected for the discharge/charge of C′ and shown for Sn–4Ag, copper (0 and 37% cold work) and low-carbon steel. This two-fold agreement between theory and experiment for each of the metals studied strongly supports the C′ concept. Strong correlation occurs among high fraction of carriers that participate, high participating charge density and high discharge time constant.
... However, uncharged particles are also polarized by the spatially inhomogeneous electric field surrounding the fibers and thus are increasingly filtered without increasing the pressure drop [21]. Although electret filters typically reach a high initial filtration efficiency, it is known that they can drastically degrade with time due to the exposure to particles [23,24], organic solvents [25,26], or air humidity [27,28]. This effect also concerns mobile air cleaners, as shown in previous studies [29,30]. ...
Indoor air cleaners can contribute to reducing infection risks by the filtration of virus-carrying droplets. There are various national standards to test indoor air cleaners that determine the clean air delivery rate (CADR), but typically only as a size-integrated value for particles >0.3 μm. Thus, a test method using potassium chloride (KCl) and paraffin as surrogate particles in the size range of viruses and exhaled droplets was developed. We show that air cleaners with fibrous and electrostatic filters are generally capable of reducing the airborne particle concentrations. However, for electret filters, the performance can strongly degrade over time by being loaded with particles. By comparing filters with different efficiencies in the same air cleaner, we demonstrate that the use of high-efficiency filters can be even at the expense of the cleaning efficacy. We developed a mathematical model to estimate the inhaled dose of viruses and show that the combination of natural venting and an air cleaner can lead to a substantial reduction of the infection risk.
... Electrets have been widely applied in the fields of air filter materials with high efficiency and low resistance [1][2][3][4], due to the long decaying time of charges. The filtering efficiency of electret filter materials is usually higher than that of common filter materials. ...
With the rapid development of the society, the severe haze weather caused by particulate matters (PMs) occurs frequently. Electret air filters have been playing an important role in purifying the air and alleviating the pollution. In this work, electret air filters were fabricated based on the polypropylene (PP) materials to investigate their filtering performance to the PMs. First, the electret performance of the PP-based films was mainly focused on. The charge storage stability of the electret films was found to be associated with the material recipe and the electret technique. Introducing 1% of tourmaline into the isotactic PP can realize the best charge storage stability, while adding the graphene can weaken the charge stability. And the electret techniques including the high-temperature pre-aging and polarization at elevated temperature were adopted to optimize the charge storage performance. Then, based on the optimized electret materials and electret technique, the purification performance of the electret filter elements dependent on the polarization condition was evaluated. The present work can provide an optimization strategy to improve the performance of electret materials and electret air filters.
... The filter functions are predominantly based on the electret charge effect, which is effective under limited conditions, requiring an oil-or alcohol vapor-free environment. The deterioration of electrostatic force and filter efficiency have been observed as a charge loss when the MB filter is exposed to an organic solvent, such as isopropanol, ethanol, and acetone [26][27][28]. Furthermore, the non-reusability of MB filters is a recent concern owing to the extensive need for masks to prevent the spread of coronavirus (COVID-19) [29]. ...
The significant public health concerns related to particulate matter (PM) air pollutants and the airborne transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have led to considerable interest in high-performance air filtration membranes. Highly ferroelectric polyvinylidene fluoride (PVDF) nanofiber (NF) filter membranes are successfully fabricated via electrospinning for high-performance low-cost air filtration. Spectroscopic and ferro-/piezoelectric analyses of PVDF NF show that a thinner PVDF NF typically forms a ferroelectric β phase with a confinement effect. A 70-nm PVDF NF membrane exhibits the highest fraction of β phase (87%) and the largest polarization behavior from piezoresponse force microscopy. An ultrathin 70-nm PVDF NF membrane exhibits a high PM0.3 filtration efficiency of 97.40% with a low pressure drop of 51 Pa at an air flow of 5.3 cm/s owing to the synergetic combination of the slip effect and ferroelectric dipole interaction. Additionally, the 70-nm PVDF NF membrane shows excellent thermal and chemical stabilities with negligible filtration performance degradation (air filtration efficiency of 95.99% and 87.90% and pressure drop of 55 and 65 Pa, respectively) after 24 h of heating at 120 ˚C and 1 h immersion in isopropanol.
... In this paper, we found that paint workers dressed in normal labor protection (gas mask and protective suits) in hangars face occupational health risks when they are exposed to high concentrations of VOCs. We also found that PP filter purification efficiency will be affected by VOC pollutants (Jasper et al., 2007;Choi et al., 2015;Sachinidou et al., 2018). Therefore, the source accounted for 13% of anthropogenic emissions in China, and will approach 5673 kt in 2020 with an assumed absence of regulation (Wei et al., 2009;Klimont et al., 2002). ...
To achieve efficient ventilation and purification in the high-ceiling painting workshop is faced with the contradiction between effect and energy consumption. Understanding the characteristics of gaseous pollutants is of paramount importance for ventilation and purification system design. The composition and concentration of volatile organic compounds (VOCs) emitted from aircraft painting workshop were sampled by Tenax-TA tubes and analyzed with gas chromatography-mass spectrometry (GC-MS). Approximately 50 types of VOCs (detection rate > 50%) were detected in the aircraft spraying workshop, with percentages of 36.3% for esters, 31.9% for aldehyde ketones, 28.5% for biphenylenes, 1.9% for alcohols and 1.4% for alkanes. The TVOC concentrations in the workshop were 48.6 mg/m³ and 132.4 mg/m³, during the varnish painting and the finish painting processes, respectively, and these are several times higher than those observed in other industries (automobile painting and wooden furniture painting). The field test data of spraying workshops from 197 spraying factories in five industry sectors were collected from field measurement and existing literature. Some VOCs components are the general pollutants in the painting workshops, such as Acetic acid, butyl ester, Toluene, and 2-Butanone. A novel ventilation model using multiple target purification units is proposed to eliminate the pollutants in an aircraft spraying workshop. Compared with the original trench exhaust system, the air volume of the proposed targeted ventilation system is reduced by 75%, and the energy consumption is reduced by 45,000 kW·h per aircraft.
... [7,13] However, these melt-blown mask filters are known to suffer from issues, such as poor reliability for long term usage and filtration performance degradation due to charge loss under exposure in organic solvent, such as ethanol, isopropanol. [14,15] Electrospun nanofibers have been considered as the most feasible alternative owing to the simplicity of processing and compatibility with several polymers; fibers with a wide range diameter can be produced, which contributes to the diversity of fabrication systems. [16][17][18] Electrospinning is a novel method that facilitates the construction of ultrathin nanofibrous-web membranes with a significantly high air permeability and relatively low pressure drop, owing to the well-known slip effect that occurs when the fiber diameters are similar to the size of air molecules. ...
Current global emergencies, such as the COVID-19 pandemic and particulate matter (PM) pollution, require urgent protective measures. Nanofibrous air filter membranes that can capture PM0.3 and simultaneously help in preventing the spread of COVID-19 are essential. Therefore, a highly efficient nanofibrous air filter membrane based on amphiphilic poly(vinylidene chloride)-graft-poly(oxyethylene methacrylate) (PVDF-g-POEM) double comb copolymer was fabricated using atomic transfer radical polymerization (ATRP) and electrospinning. Fourier transform infrared spectroscopy, X-ray diffraction, proton nuclear magnetic resonance, transmission electron microscopy, differential scanning calorimetry, and thermogravimetric analysis were employed to successfully characterize the molecular structure of the fabricated amphiphilic PVDF-g-POEM double comb copolymer. The nanofibrous air filter membrane based on amphiphilic PVDF-g-POEM double comb copolymer achieved a low air resistance of 4.69 mm H2O and a high filtration efficiency of 93.56 % due to enhanced chemical and physical adsorption properties.
... Generally, non-fluid treatments perform better than fluid-based treatments in filtration efficiency test. The plausible reason is that the electric charges of polypropylene filtration layer were neutralized after the treatment, especially with the use of organic solvents [13]. ...
The possibility to extend the lifespan or even reuse one-off personal protective equipment, especially for N95 respirator and surgical mask become critical during pandemic. World Health Organization has confirmed that wearing surgical mask is effective in controlling the spread of respiratory diseases in the community, but the supply may not be able to satisfy all the demands created all over the world in a short period of time. This investigation found that dry heat and UVC irradiance could effectively disinfect the mask material without creating significant damage to surgical mask.
... Isopropyl alcohol with a minimum purity of 99.5% is used to dis- Besides that, studies showed that unsaturated vapors of organic solvents do not lead to any discharge of the fibrous material. 21,26 To explain and understand the discharging mechanisms of electret fibers in detail, experimental evidence and further research are needed. ...
Electret filters are widely used in HVAC systems to decrease particulate matter in indoor environments. The previous standard in Europe for testing air filters for general ventilation was EN 779. In July 2018, it was replaced by the new international standard ISO 16890. One major change is the discharging process: it is now performed by treating the filters with saturated isopropyl alcohol (IPA) vapor. The process is intended to simulate a worst-case scenario of the filtration efficiency due to the reduction of the electret effect. These minimal efficiencies are a principal part of the filter classification. Therefore, two round robin tests with different filter classes (F9 and F7 according to EN 779) and up to eleven participants were carried out to evaluate the new test method by comparing the filtration efficiencies and pressure drops before and after the IPA treatment. Pressure drop measurements showed no mechanical altering of the material due to the discharging process. The calculated filter classes had a maximum deviation of 5 %. Even with different equipment, the results indicate that the new ISO 16890 seems to be a viable test standard and a decent replacement for previous national standards.
... The other is to increase the charge capacity of the filter fibers. Promising results in enhancing filtration efficiency have been shown when escalating charge capacity of filter fibers is combined with lower pressure drops (Choi et al. 2015;Li and Jo 2010;Sambudi et al. 2017). ...
A new porous fiber shape was produced, one with a longitudinal notch. This new shape increases the surface area and therefore the overall number of pores, which in turn leads to a higher per-fiber surface charge density. Filters with charge generally show advantageous lower pressure drops when compared to the same filtration efficiencies. Although previous studies have hypothesized that increasing the surface area of porous fibers can improve charge capacity, but few experiments have proven this concept. In this study, two different micron-sized fibers were fabricated via electrospinning: smooth (“normal”) fibers and the novel notched fibers with surface pores. The fiber profile and voltage drop of the two fibers were measured, and the penetration rate of the two types of fibers before and after charging was measured with aerosol particles ranging from 25 nm to 478 nm in size. The results showed that the specific surface area of the notched fiber was 47.44 ± 0.67 m² g⁻¹, larger than that of the smooth fibers (2.64 ± 0.08 m²g⁻¹). The notched fiber was charged by negative corona discharge, and the surface capacitance was measured to be -51.2 μC m⁻² when the thickness was 0.19 mm, which was also larger than that of the smooth fibers (-38.2 μC m⁻²). When charged, the value of the average quality factor of the notched fibers increased from 0.030 Pa⁻¹ to 0.068 Pa⁻¹. Overall, the notched fibers demonstrated better filtration efficiency. We concluded that this novel fiber shape delivers both low penetration rates and low pressure drops.
Copyright © 2019 American Association for Aerosol Research
... Although vapor treatment was reported to be better, Choi et al. [35] found that the exposure of an electret filter to ethanol vapors did not lead to filter neutralization. Therefore, it was necessary to investigate the detailed discharging process of IPA vapor treatment. ...
Electret filters with electrostatic charges added on fibers are widely used in HVAC system of residential and commercial buildings to mitigate PM2.5 exposure in indoor environments. Filtration applications require the knowledge of minimum filtration efficiency of electret filters under worst condition. To determine the minimum efficiency, different electret discharging methods are used in test standards to eliminate electrostatic effects. This study evaluated the isopropanol (IPA) liquid discharging method in EN 779 and the IPA saturated vapor treatment in ISO 16890. It was found that although both IPA liquid immersion and saturated vapor treatment can significantly remove fiber charge, saturated vapor treatment showed better discharging performance. The charge density of media discharged by IPA liquid immersion and saturated vapor treatment were 2 μC/m² and 0 μC/m², respectively. It demonstrated that IPA saturated vapor treatment can completely remove fiber charge, while undesirable fiber charge occurred after being treated by IPA liquid due to interface electrification. The test results of unsaturated vapor treatment indicated that IPA in the phase of vapor was not capable of removing fiber charges. The fact that saturated vapor treatment was more effective than liquid immersion was due to the different discharging process. For saturated vapor treatment, vapor condensed on fiber surface and formed a liquid layer with thickness of 15 nm. This ultra-thin layer of IPA liquid can discharge the fiber charge completely without inducing extra charge after liquid evaporation. This study concluded that discharging method in ISO 16890 should be used in electret filter test.
... Investiga- H. et al., 1987) tion about its collection mechanisms has been carried out from various view points such as collection mechanisms by , Pich J. et al. (1987), Romay F.J. et al (1998), filtration performance of various particles by Baumgartner H.-P. and Löffler F. (1986), , Otani Y. et al. (1992Otani Y. et al. ( , 1993 2015), Kilic A. et al. (2015). Time change of electret fiber was investigated by Kanaoka C. et al. (1984b), Lee M., et al. (2002), Choi H.-J. et al. (2014Choi H.-J. et al. ( , 2015 and electrification method and process was by Kawabe M. et al. (2009), Kilic A. et al. (2015. Fig. 15 shows η at different charging states by a rectangular electret fiber . ...
Researches relating dust collector using fiber as particle collection body i.e., air filter, cartridge filter and nonwoven bag filter, were reviewed. Their filtration process was classified into 3 stages, i.e., Stage 1.inner filtration I, Stage 2.inner filtration II and Stage 3.surface filtration. Collection mechanisms of fresh circular fiber have been well understood and so-called classic filtration theory is applicable except single nano-particle, where discussion about the possibility of thermal rebound is necessary. In Stage 1, effects of shape of both fiber and particle, and filter structure, non-uniformity are the important issues. In Stages 2 and 3, filtration process becomes very complicated because of many affecting parameters. Main target in Stage 2 is to develop effective scheme to describe the phenomena and to find filter structure having a large holding capacity. Most important issue in Stage 3 is to develop the effective cleaning technique to minimize the dust emission based on rational but not empirical scheme.
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... Angadjivand et al. (1996) suggested employing a hydrocharging effect to enhance the performance of a corona-charged electret filter, and showed that the hydrocharged filter media offered a higher quality factor than the corona-charged filter. Choi et al. (2015) and Im and Hong (2014) also prepared MB filter media using a hydrocharging process. When comparing fractional efficiency and pressure drop, we confirmed that MB filter media performed better than glass fiber filter media for both parameters (Table 4). ...
The purpose of this study was to compare the performance efficiency of melt-blown and currently used glass fiber filter media under the same environmental conditions. To evaluate filter efficiency, filter class was determined according to ISO and European standards (EN 1822-1:2009) using an automated filter tester (0.3 μm size), taking into account particle filtration, fractional efficiency for negative pressure devices, and consumption of electrical power. The average fractional efficiency, quality factor (QF), fractional efficiency by dust loading amount, pressure by dust loading amount, and QF by dust loading amount were higher in the case of melt-blown media than in the case of glass fiber filters. The fractional efficiency of hydrocharged melt-blown filters was higher than that of uncharged media. Based on performance efficiency, melt-blown filters are more effective high efficiency particulate air filters than glass fiber media.
© 2018 American Association for Aerosol Research
Aerosol filters comprised of electrostatically charged bipolar fibers are referred to as electret filters. A novel computational model is developed in this work to study the impact of droplet deposition...
The harmful effects of air pollution on humans are multifaceted and are closely linked to outbreaks of various infectious viruses. Thus, design of high‐performance air filters that are lightweight, flexible, and easy to prepare is urgent. The electret air filter can utilize the electrostatic force of charge to capture dust particles, making it efficient and convenient, which is highly favored by people. In this study, a summary of the research on electret filters for highly efficient air purification is presented. First, this study outlines the mechanisms of air filtration methods and discusses three factors that evaluate filter performance. The methods of manufacturing electret air filters are then presented, and the advantages and limitations of different methods are discussed. Then, electrification, including corona discharge, friction electrification, electrospinning, thermal electrification, and water electrification as methods, enhances the physical charge adsorption performance of materials, while the use of fluorinate polymer, non‐fluorinate polymer, and biobased polymer as functional substrates exhibits varying degrees of electrification effects. Different materials and manufacturing techniques give the filters additional advantages, such as being self‐degradable and antibacterial. Finally, the challenges that remain in the development of electret filters for use in air purification are also discussed in terms of material type and fabrication technology. It is expected that highly efficient air purification with low energy consumption can be achieved by electret filters with multifunctional capabilities.
Air pollution caused by fine particulate matter (PM0.3) has drawn increasing attention as an overwhelming threat to public health. Electret treatment is commonly used to improve the filtration performance of commercial fibrous filter materials by enhancing the electrostatic adsorption effect, but it is greatly affected by environmental factors (especially humidity). Moreover, filter materials are generally non-degradable and non-recyclable, causing serious environmental pollution. Herein, a strategy to manufacture fully degradable polylactic acid (PLA) filtration composites based on porous PLA nanofibers prepared by electrospinning was investigated in this study. Porous, bead-on-string and conventional PLA nanofibers could be obtained by adjusting spinning condition parameters. The porous PLA nanofibers exhibited 9.8 times greater specific surface area (24.01 m² g⁻¹) and 18 times more cumulative pore volume (0.108 cm³ g⁻¹) than conventional PLA nanofibers. More importantly, fibrous filtration composites based on porous PLA nanofibers possessed a high PM0.3 filtration efficiency (99.9989%), low pressure drop (90.35 Pa) and high air permeability (72.4 Pa⁻¹) at an air flow rate of 32 L min⁻¹ without electret treatment. The fibrous filtration composites based on conventional or bead-on-string PLA nanofibers also exhibited excellent filtration performance (>99.99%), but the associated high pressure drop and low air permeability limited their application.
Nonwoven media used as electret air filters are often embedded with charges to improve particle capture efficiency. These charged filters are invariably exposed to low surface tension fluids such as oils and alcohols leading to charge loss. In this study, filtration media are endowed with charge protection through increased surface repellency using melt additives that can migrate to the surface during processing. Nonwovens containing fluorochemical melt additives are produced, and examined to determine the relationship between surface chemistry, isopropyl alcohol (IPA) repellency, resultant charge retention, and filtration characteristics. Surface fluorine/carbon (F/C) ratios of ≈0.2 are sufficient to protect filtration performance from vapor discharging methods. Samples with bulk additive loadings of 1.2% or higher are found to achieve the necessary repellency to resist discharging independent of the migration state of the sample, while samples loaded at the 0.6% level required sufficient migration to achieve the requisite F/C ratio of 0.2 in order to be protected. Samples that achieved the necessary surface chemistry to provide significant IPA repellency retained > 80% of electret charge and corresponding filtration performance. These results have special significance in the design of filtration media relevant in global healthcare and other industrial settings.
The effect of the surface charge density of an electret filter on its collection efficiency degradation via isopropyl alcohol (IPA) species exposure is discussed in this study. Five electret filters with different electrostatic properties and identical physical properties were prepared and exposed to IPA species. Furthermore, the filtration performance was evaluated. Filters exposed to IPA droplets showed a drastic reduction in the collection efficiency. In addition, the electret filter with a higher surface charge density had a higher tolerance to IPA droplets, that is, the challenged IPA droplet amount that neutralized the electret filter with a high surface charge density (= 5.0 × 10-5 C/m2) was 2.5 times higher than that of an electret filter with a low surface charge density (= 1.9 × 10-5 C/m2). The results of this study revealed that there was a positive relationship between the surface charge density and challenged amount required to neutralize an electret filter.
The performance of disposable face masks in blocking pathogenic microbes mainly relies on the ability of melt-blown fabrics to electrostatically adsorb air droplets. While their easy invalidity in electrostatic effect highly limits their use recyclability, leading to a heavy burden on the surrounding environment due to their decomposing difficulty. Thus, it is urgent to develop reusable air filtration fabrics. In this work, we report a washable electret air-filtering fabrics with negligible cytotoxicity by combining spontaneous polarizing photochromic molecule with electrospinning electret technology. The electrostatic charge and filtration efficiency of the composite fabrics were highly improved compared to the commercial polypropylene melt-blown fabrics. Moreover, due to its electrostatic charge induced by self-polarization, the photochromic Spiropyran(SP)/polystyrene(PS) electret fabrics can maintain high filtration performance (over 95%) towards PM0.3 particles and low air flow resistance (<343.2 Pa) even after 5 times of washing (by 75% ethanol)/drying cycles. The combination of self-polarization photochromic molecule and electret material with negligible toxicity would create high-performance reusable air filtering fabrics, thereby SP/PS composite fabrics have a potential application in eco-friendly recyclable air purification and isolation and protection of pathogenic microbes.
Airborne particulate matters (PM) pose serious health threats to the population, and efficient filtration is needed for indoor and vehicular environments. However, there is an intrinsic conflict between filtration efficiency, air resistance, and service life. In this study, a two‐stage electrostatically assisted air (EAA) filtration device is designed and the efficiency‐air resistance‐filter life envelope is significantly improved by a thin coating of polydopamine (PDA) on the polyethylene terephthalate (PET) coarse filter by in situ dopamine polymerization. The 8 mm thick EAA PDA‐140@PET filter has a high filtration efficiency of 99.48% for 0.3 µm particles, low air resistance of 9.5 Pa at a filtration velocity of 0.4 m s⁻¹, and steady performance up to 30 d. Compared with the bare PET filter, the penetration rate for 0.3 µm particles is lowered by 20×. The coated PDA is of submicron thickness, 10⁻³ × the gap distance between filter fibers, so low air resistance could be maintained. The filter shows steadily high filtration efficiency and an acceptable increase of air resistance and holds nearly as many particles as its own weight in a 30 day long‐term test. The working mechanism of the EAA coarse filter is investigated, and the materials design criteria are proposed.
Luftreiniger werden derzeit häufig als mögliches Hilfsmittel zur Minimierung des Infektionsrisikos im Rahmen der COVID-19-Pandemie diskutiert. Dabei taucht oft die Frage auf, ob Luftreiniger grundsätzlich in der Lage sind, Viren oder virenbeladene Tröpfchen abzuscheiden. Ziel dieses Artikels ist es, die wesentlichen Grundlagen der Wirkungsweise von Luftreinigern und Filtern zu beschreiben, Methoden zur Messung der Reinigungswirkung im Größenbereich von Viren und virenbeladenen Tröpfchen aufzuzeigen sowie typische Szenarien zum Betrieb von Luftreinigern in einem mathematischen Modell zu erfassen. Darauf basierend können die Möglichkeiten und Grenzen von Luftreinigern für reale Anwendungsfälle besser eingeschätzt werden.
The following common filter materials were characterized and evaluated for filtration performance: glass fiber (GF), polytetrafluoroethylene (PTFE), and polypropylene (PP). The pressure drop of GF and PFFE was found to be approximately 500–600 Pa at 0.10 m/s face velocity, and the pressure drop for the PP is lower at only 30–80 Pa. The filtration efficiency of PTFE and GF was determined as 99.067% and 98.234%. Results show the filtration efficiency of PP is between 95.836–98.732%, but the filtration efficiency of PP filter materials will decrease 20–60% after electrostatic removal. The contribution of various particle collection mechanisms to the overall efficiency of the filter was calculated using the single-fiber filtration efficiency model. The diffusion of the aerosol particles at submicron size was the main collection mechanism, and the contribution of the gravity sedimentation mechanism to the measured particle size range was very low or even negligible.
As the prevalence of COVID-19 in all the area of China and continuous spread to many other countries, it is in urgent need of medical mask for community people. We have reported a simple method to inactivate artificially influenza virus-contaminated medical mask without affecting the filterability of mask by heating with hair dryer for 30min. In this study, we extended our studies to optimize our protocols and evaluate the filterability of mask after several-rounds of heat treatment. We found that baking at 70℃ for 30min almost completely inactivated virus as hair dryer treatment for 30min described before. One-round of heat treatment with hair dryer for 30min or baking at 70℃ for 30min did not affect filterability of mask whilst two-rounds of treatment slightly but significantly reduced filterability of mask. Thus, heating with hair dryer for 30min or baking at 70℃ for 30min can inactive contaminated medical mask and the treated mask can be reused at least once.
Nanofiber filter media can potentially reduce exposure to PM2.5 in indoor environments because of the filters’ high particle-removal efficiency. To facilitate such filter use, this study conducted a series of experiments to understand the evolution of wetting liquid aerosols, taking incense particles as an example, on nanofiber filter media and the influence of this evolution on PM2.5 removal efficiency and pressure drop. Scanning electron microscope images were also taken to observe the nanoscale interactions between incense particles and the nanofiber network. The results show that the PM2.5 removal efficiency at first decreased as the loading mass increased, because interactions between the particles and the nanofiber network enlarged the pores. The evolution of pressure drop may consist of two stages, i.e., a first stage with a linear relationship, and a second stage with a steep increase in pressure drop with the loading mass. When the pore size became small enough, in addition to inertial impaction and Brownian diffusion, the capture mechanism of interception also became significant. Consequently, the second stage, with a steep increase, tended to occur. Finally, methods for establishing empirical equations for PM2.5 removal efficiency and pressure drop as a function of loading mass were proposed.
Electret filter materials have attracted wide attention because of their low pressure drop and high filtration efficiency. Few studies have measured the filtration performance and the charging status of the charged filters during soot particle loading. In this study, the polytetrafluoroethylene (PTFE) needle felt filters were charged by a corona discharge method. The filtration efficiency and surface potential were investigated during the soot particle loading process. The effects of NaCl particles, soot particles and A2 dust on surface potential and filtration efficiency were investigated. The reusability of charged filters and the effect of temperature on surface potential were studied. The results indicated that the initial filtration efficiency of charged PTFE needle felt filters (PNFFs) was greatly improved compared with that of uncharged PNFFs. The attenuation rate of the surface potential at 100 °C was faster than that at room temperature. The surface potential decreased with the start of particle deposition on the filters, correspondingly, the filtration efficiency of the charged PNFFs decreased. The efficiency of charged PNFFs loaded by soot particle decreased first and then increased, with the transition point corresponding to 20 Pa of pressure drop growth. With the same increment of pressure drop, soot particles with higher conductivity caused more efficiency degradation than NaCl particles. The reusability of the charged filters was affected by the effectiveness of dust cake removal and the particle conductivity.
The purpose of this study was to improve the efficiency of filters used in asbestos control systems, such as those used at asbestos removal sites. We evaluated the melt-blown (MB) filter media for their asbestos removal efficiencies. The filter grades were based on the ISO and European standards (EN 1822) of E12 (≥99.5% collection efficiency) and H13 (≥99.95% collection efficiency) with a size of <0.2 μm asbestos diameter. Based on test chamber experiments, the asbestos removal efficiency of the grade H13 MB filter (99.974%) was higher than that of the E12 grade MB filter (97.120%). In addition, the lowest level of pressure drop was observed in the case with a 3.8 mm pitch interval. The concentrations of airborne asbestos based on phase contrast microscopy in the sites with asbestos concentrations presenting high risks before turning on the asbestos control system was 0.038 fiber cm⁻³ at demolition site A and 0.027 fiber cm⁻³ at demolition site B. Chrysotile asbestos was detected at both demolition sites A and B before turning on the system, but were not detected after using the system. Therefore, MB filters present an efficient alternative to current commercial filters and should be considered for use in asbestos removal applications.
© 2018 American Association for Aerosol Research
Dielectrics are extensively applied in several fields due to their strong charge retention properties. Filters, for respirator and ventilation applications, are commonly made of dielectric materials due to the increased charge retention which enhances their efficiencies. However, when they are exposed to organic solvents their charge retention drops significantly causing charge degradation. Several mechanisms were hypothesized to trigger this phenomenon. In this study, we investigated the discharge of dielectric filters composed of polymers, focusing on several hypotheses including the surface charge neutralization caused by ions assumed to be present during organic solvent evaporation, the ability of the organic solvent molecules to act as ion scavengers and the charge de-trapping due to the swelling of the polymer filter exposed to different organic vapors. It was observed, experimentally, that there were no measureable ions in the vapors of the organic solvents and that the organic molecules could not act as ion scavengers. An in-situ discharging and monitoring setup enabled us for the first time to study the degradation kinetics of the surface potential of polymer dielectric filters, which generally included three phases for the majority of the investigated polymer filter samples. The surface potential was steady for a certain period then it decreased following a profile close to an exponential function, and finally it approached zero volt. It was assumed that a critical swelling degree should be reached to initialize the charge de-trap. Overall, solubility of the polymer filter in different solvents, organic vapor diffusion coefficient and equilibrium concentration were found to be the rate limiting factors for polymer dielectrics discharge.
Mobile air cleaners, in particular such with fibrous electret filters, are widely used to improve the indoor air quality. We investigate the particle size dependence of the air cleaning efficiency and conclude that recent testing standards overestimate the cleaning performance with respect to relevant indoor aerosols if considering only particles larger than 0.3 µm. Furthermore, we artificially age the filters with cigarette smoke to evaluate their long-term stability. Finally, we compare the results to an aging method proposed by a recent testing standard and find a very distinct aging behavior.
The slip flow effect is often brought out to explain the reduction in pressure drop for nanofiber filters. Kirsch, Stechkina, and Fuchs (1973) studied the slip flow effect on the pressure drop of fibrous filters consisting of micron fibers, and proposed an empirical equation to predict the dependence of the dimensionless drag, F, on the Knudsen number, Kn, with considering non-uniformity of fiber packing. However, their empirical equation was derived based on the experiments with filters consisting of micron fibers so that the empirical equation is not yet verified for nanofiber filters. In the present work, we used various commercially available nanofiber filters with various physical properties, and the pressure drop was measured at low pressures in order to examine the validity of the empirical equation. As a result, we found that the empirical equation is valid even for nanofiber filters with a large inhomogeneity factor at a large Kn up to 20.
Electret filters are widely used in applications requiring high-filtration efficiency and low-pressure drop. These filters rely on electrostatic particle capture mechanisms in addition to the conventional mechanical capture mechanisms. This study reports experimental data collected on the performance of three types of commercially available fibrous electret filters: corona-charged fibrillated split-fiber media (type A), triboelectrically charged mixed-fiber media (type B) and corona-charged meltblown media (type C). The filtration efficiency of these filters was measured as a function of particle size (i.e., 0.05 to 0.5 μm) and charge state (i.e., singly charged and neutral) for two face velocities. The same experiments also were performed on discharged electret filters to obtain information on the pure mechanical capture mechanisms. The magnitude of the effective surface electric field for each tested electret filter was estimated from a simplified Coulombic capture model, giving information on the ranking of the charge level for the tested electret filters. The single-fiber efficiencies for Coulombic and dielectrophoretic capture mechanisms were isolated by assuming negligible interaction among the different particle capture mechanisms. The single-fiber efficiencies were fitted by power law expressions to a combination of parameters suggested by dimensionless numbers derived from theory. The experimentally obtained power law exponents were in good agreement with those predicted by theory (Brown, 1981; Emi et al., 1987) using simplified charge configurations. No direct semi-empirical correlations in terms of dimensionless parameters could be obtained due to the lack of reliable information on the charge density of the tested electret filters.
Collection performance of an electret filter with rectangular fibers was studied experimentally for cases in which electrostatic effect and Brownian diffusion are predominant by using particles from 0.02 to 0.4 μm in diameter and at different charging states. A single fiber collection efficiency ηED was found to be expressible as a function of dimensionless parameters of Peclet number Pe, and Coulombic and induced force parameters, Kc and KIn as, here, A, B, C, and D are the numerical constants depending upon the charging density of electret fiber. Indices of each dimensionless parameter determined through the experiment coincided with the previous theory. A maximum penetration of particles appeared in the transition region of predominant collection mechanisms, i.e., between Brownian diffusion and induced force effect, ranging smaller than 0.1 μm in diameter for uncharged particles, and between Coulombic and induced force effects, ranging larger than 0.1 μm in diameter, for charged particles. Semiempirical expressions for a single electret fiber collection efficiency and a most penetrating particle size, applicable to particles in any charging state, were obtained taking account of Brownian diffusion, and induced and Coulombic force effects simultaneously. Undulation of the penetration observed in the filtration of particles in charge equilibrium was explained by using the semiempirical expression for a single fiber efficiency and charge distribution on a particle.
Electret filters are frequently used in air cleaners because they have a lower pressure drop than the mechanical filters at the same collection efficiency. Manufacturers of electret filters have tried to increase the electrical charge density of electret fibers in order to improve the collection performance. Through these efforts, high-performance electret filter (HPEF) is developed and a question is raised on whether the previous prediction equations for collection efficiency are applicable to these HPEF because the prediction equations were never tested for such high charge density electret filters. In the present work, we measured the collection efficiencies of recently-developed HPEF and studied the applicability of previous prediction equations for collection efficiencies. As a result, HPEF had the electrical charge density twenty times as high as that of the previously-studied electret filter and the single fiber efficiency was close to the maximum limit determined by the packing density of fibers. Furthermore, it is found that the electrical charge of HPEF is fairly stable against organic solvent, leaving one-third of initial charge after soaking it with ethanol. In predicting the collection efficiency of HPEF, the conventional equation can be applied to the collection by induced force, however that by Coulombic force is not applicable because the single fiber efficiency due to induced force and Brownian diffusion is high and close to the upper limit without the collection by Coulombic force.
Performance degradation of electret filter media when exposed to xylene was investigated using a custom-made exposure apparatus. Three types of electret filter media were exposed to xylene in liquid and gas phases. Experimental data indicated that the penetration percentage of di-octyl-phtalate (DOP) aerosols through electret filter media was not influenced by exposure to xylene vapors for periods up to 8h. The filtration performance for all three types of electret filter media decreased over 30% when the samples were exposed to liquid xylene. Pressure drop changes of the filter media prior and after exposure to liquid xylene were statistically negligible. Neither morphological changes nor degradation of the polymer fibers were observed after exposure of the filter media samples to liquid xylene. Thermodynamic simulations were performed to determine the molar flux of liquid and gas phases through the filter media specimens. Experimental results obtained via capillary gas chromatography were within 4% of the predicted values. The decrease in performance was attributed to changes in the density and spatial distribution of the electret charges on the surface of the polymer fibers.
Three charging techniques (viz., corona charging, tribocharging, and electrostatic fiber spinning) were used to charge fibers or fabrics of different polymer types. Corona charging is suitable for charging monopolymer fiber or fiber blend, or fabrics. Tribocharging is only appropriate for charging fibers with dissimilar electronegativity. Electrostatic fiber spinning combines the charging of polymer and the spinning of the fibers as a one-step process. It was observed that two dissimilar fibers following tribocharging had higher filtration efficiency than the corona-charged polypropylene fibers. An electrostatic spinning process produced nanofibers exhibiting extremely high efficiency by mechanical filtration mechanisms. Little charge was retained in electrospun polyethylene oxide fibers; however, polycarbonate and polyurethane retained a great amount of charge.
This study was undertaken to determine the effects of using an electret filter on aerosol penetration. Various factors, including particle size (0.05 to 0.5 micro m), aerosol charge state (neutral and single charge), face velocity (0.1, 0.3, 0.5 and 1.0 m/s), and relative humidity (RH 30% and RH 70%), were examined to assess their effects on aerosol collection characteristics. The results presented here demonstrate that the electric fields of the electret and discharged filter were -1.53 x 104 and -1.3 x 102 (V/m). The penetration through the electret filter with singly charged aerosol and neutral aerosol ranged from 0.4% to 13% and 14% to 29%, respectively. According to these results, the coulombic capture force was dominant for the smaller aerosol and the dielectrophoretic capture mechanism was considered important for the larger aerosol. The level of penetration through the electret filter increased with increasing face velocity and relative humidity. The temperature did not affect the penetration through the electret. Furthermore, from the regression analysis conducted during the operating conditions of this work, the aerosol charge was shown to exert the greatest influence on aerosol penetration.
In this study, a melt-blown nonwoven filter for medical masks was manufactured using an online hydro charging system. The effects of basis weight (gsm) and die to collector distance (DCD) on the breathing resistance and filtration efficiency were investigated. Hydro charging of the nonwoven web seemed to improve the filtering efficiency. However, it did not affect breathing resistance. It was demonstrated that the breathing resistance and filtration efficiency increased with increasing gsm but decreased with increasing DCD. With a DCD of 19 cm, when the gsm increased from 20 to 50 gsm, breathing resistance increased from 5.14 to 8.33 mmH_2O and filtration efficiency increased from 81.39 to 94.98%. With a basis weight of 25 gsm, when the DCD increased from 15 to 25 cm, breathing resistance decreased from 7.38 to 4.27 mmH_2O and filtration efficiency decreased from 89.56 to 68.28%. In addition, it was demonstrated using stability tests that the hydro charging effect lasted for 13 weeks.
Melt-blown polypropylene electret fabrics are widely used as air filter media. However, its filtration efficiency gradually decays in application process. This paper is to investigate the correlation between filtration efficiency decay and solvent effect. Experimental results show that filtration efficiency displays a regular decrease when polypropylene electret fabrics are exposed to solvents in the sequence of water, ethanol, isopropanol and acetone. The results can be correlated to solubility parameter difference between polypropylene and solvent according to the Flory–Huggins swelling theory. Smaller solubility parameter difference leads to greater decay of filtration efficiency owing to greater affinity between polypropylene and solvent.
Electrospinning is a simple and versatile process to produce polymer nanofibers, which are useful for ultrafine particle filtration. In this study, a polyurethane filter with an average fiber diameter of 150-250 nm was prepared through the electrospinning process and its filtration characteristics were investigated. We found that the electrospun fiber diameter was highly dependent on the polyurethane concentration, electric field, and tip-to-collector distance. As the polyurethane concentration, electric field, and tip-to-collector distance under the same electric field increased, the fiber diameter increased. We also found that the produced filter media had a minimum collection efficiency at particles sizes from 80 to 100 nm, which implies an electrostatic attraction between the filter and the test particles. Furthermore, we observed that interception was a predominant collection mechanism at Peclet numbers higher than 10 in nanofiber filtration for ultrafine particles.
A polyurethane nanofiber filter with excellent mechanical properties was prepared, and the effect of operating conditions on fiber morphology was examined. The filter fabricated by an electrospinning process is charged and has high filtration efficiency due to electrostatic force. Therefore, it can be a good alternative to control hazardous ultrafine particles.
This review summarizes the research progress made so far on electret air filters used for separation of airborne particles from complex air stream. A set of different categories of these filters are delineated and the methods of manufacturing of these filters are described. The principles and mechanisms of filtration and modeling of pressure drop by these filters are analyzed. The filtration performance of these filters is discussed along with their specific advantages and limitations. The reliability of these filters is reported in terms of their charge storage capability. Many interesting inventions as reported on electret air filters in the recent patents are discussed. The latest regulations on air filters are described and how they are driving electret filter research is indicated. Specific research needs are highlighted with a view to further improve the filtration performance and long-term effectiveness of these filters.
This study deals with the performance of an electret filter towards submicronic aerosol and particularly the dependence of its penetration with the charge distribution of the aerosol and a neutralization effect. An experimental set-up was developed to determine simultaneously the charge density of the aerosol and the filter penetration. It appears that the penetration of electret filters is all the more low that the charge per unit surface of a particle is important. The effect of a neutralization of the filter damages dramatically the performance of the filter.
Recently the use of high-efficiency mechanical filters in respirators has been shown to be potentially hazardous, because under certain circumstances the respirable fibres they contain can be shed; electrically charged filters are therefore likely to become even more widely used than at present. In general these perform well, because their high internal electric field enhances filtration efficiency without causing any increase in airflow resistance, but they do have a possible drawback in that their performance may deteriorate when they are exposed to aerosols. The practical importance of this depends on its extent. Both this and the nature of the process of deterioration have been investigated by exposing respirator filter materials, on site, to industrial aerosols, comprising aerosols produced during foundry fettling operations, foundry burning, lead component manufacture, lead smelting, silica sand quarrying, refractory brick production, coking and asbestos textile manufacture. In addition the effect of coal dust has been investigated by loading the filters in a dust tunnel. The exposures were carried out in increments in order to obtain quantitative data on filter performance as a function of the mass of aerosol deposited. Initially filter efficiency usually decreases with increasing aerosol load, but at higher loads clogging may compensate, causing filter resistance to increase. Equal masses of different aerosols are found to cause widely different changes in performance. In general dry dusts cause little loss of performance, but foundry aerosols are more damaging to all types of filter, and coal-tar fume to some. A simple theory of degradation is outlined, predicting that loading filters with homogeneous aerosols causes the penetration to increase exponentially, with a constant of proportionality that quantifies the ability of the aerosol to degrade the filter. Aerosols sampled during static experiments give results that support this prediction; though results on respirators themselves suggest that these collect a heterogeneous aerosol, which can obscure the exponential relationship. In general, different types of filter material are degraded at about the same rate, and this is shown to be consistent with a charge-screening process. The correlation between the behaviour of different materials means that it should be possible to devise a laboratory test to enable filter performance to be estimated. The behaviour of coal-tar fume does not fit the simple pattern, and its effect is consistent with solvent action on soluble polymers, which makes quantitative prediction more difficult. Corrective measures are suggested for any situations in which loss of performance is unacceptable.
Solvent-induced performance deterioration in corona charged polypropylene electret filter media was studied. Electret media, exposed to isopropanol (IPA) in the liquid phase, exhibited reduced filtration efficiency with negligible changes in the pressure drop across the media. Scanning electron microscopy imaging indicated no observable morphological changes after IPA exposure. Solvent-induced efficiency deterioration of electret filter media was hypothesized to originate from charge deterioration rather than charge masking. The charge of the individual fibers in filter media was characterized using electrostatic force microscopy (EFM). EFM investigation produced evidence that exposure to IPA in the liquid phase affected the electrostatic charges on fibers. Exposure to organic solvents such as xylene, toluene, and ethyl benzene has been reported to increase charge mobility in polypropylene fibers, thereby reducing the electrostatic charge and the ability of the fibers in the electret filter media to capture particulates.
Unlike ordinary filters, the initial high collection efficiency of the electret filter (E) might briefly fall as dust load increases. Here, the electrical enhancement factor of the electret fiber (λE) under dust-loaded conditions for uncharged and charged particles is obtained from published experimental results. It is found that the collection efficiency of the electret fiber (η) can be approximated as the sum of the electrical and mechanical collection efficiency. The predicted values of E are shown to agree well with reported experimental observations for both uncharged and charged particles.
Electret filters are composed of permanently charged electret fibers and are widely used in applications requiring high collection efficiency and low-pressure drop. We tested electret filter media used in manufacturing cabin air filters by applying two different charging states to the test particles. These charging states were achieved by spray electrification through the atomization process and by bipolar ionization with an aerosol neutralizer, respectively. Polydisperse solid NaCl particles with 0.1%, and 1% solutions or liquid dicotyl sebacate (DOS) particles were generated from an atomizer, and they were loaded on the filter media. The amount of charge, the mean particle size, and the particle material significantly affected the collection performance of the electret filter media for submicron particles. The collection efficiency of the electret filter media degraded as more particles were loaded, and showed minimum efficiency at steady state. The electret filter media captured the highly charged particles more efficiently during the transient state. At steady state, the filter media loaded with smaller NaCl particles showed lower collection efficiency. The filter media loaded with liquid DOS particles showed collection efficiency much lower than those loaded with solid NaCl particles.
Polyacrylonitrile (PAN) fibers with mean diameters in 270-400 nm range were prepared by electrospinning for use as a filter media. Compared to commercial filters made of polyolefin and glass, the fibers of electrospun filters were more uniform in diameter. The performance of electrospun filters was evaluated by measuring the penetration of monodisperse NaCl nanoparticles (below 80 nm in size) through the filters. It was found that electrospun filters could be made which had nanoparticle penetration values comparable to commercial filters but with substantially less filter mass. The penetration of nanoparticles through the electrospun filter media could be reduced by increasing the filter thickness, which is controlled by the collection time during the electrospinning process. Nanoparticle collection by electrostatic forces was found to be negligible for electrospun filters. Filter quality factors and single fiber collection efficiencies were found to be independent of filter thickness for electrospun filters, and the penetration of nanoparticles through electrospun filters was in better agreement with theoretical predictions than was the measured penetration through a commercial filter. This study shows that electrospinning is a promising technology for the production of high performance nanoparticle filters. (C) 2007 Elsevier Ltd. All rights reserved.
Method of Charging Electret Filter Media. United States Patent
- S A Angadjivand
- M E Jones