Electret Filter Research Articles

Simulation and experimental investigation of electret polypropylene fiber preparation via centrifugal melt electrospinning for enhanced air filtration

Rapid urbanization and industrialization have led to severe air pollution, enabling bacteria and viruses to become suspended in aerosol particles, which can be inhaled and pose a significant threat to human health. Electrostatic filtration offers a solution to enhance filtration efficiency without increasing resistance. However, traditional electret methods involve the usage of toxic solvent evaporation, and electret filter materials like polypropylene (PP) often suffer from rapid charge dissipation. In this study, we combined centrifugal melt electrospinning with Dissipative Particle Dynamics simulations to achieve continuous electret production and investigate the influence of experimental conditions and polymer additives on the charge-trapping capability of PP fibers. The results show that centrifugal melt electrospinning significantly enhances the charge storage capacity of PP fibers. The α crystal form of PP enhances electret performance by generating more effective charge traps, and experimental parameters influence electret performance by altering crystal forms. Additives effectively promote charge stability, resulting in stable voltages up to 11.09 kV with 1.5 wt% electret additive. Moreover, our study elucidates the correlation between crystal structure and electret performance, providing valuable data for regulating electret PP fibers.

A Novel Piezoelectric Nonwoven Fabric for Recoverable High-Efficiency Filters.

Serious haze pollution, mainly caused by fine and ultrafine particulate matters (PMs) and aerosols, poses a significant threat to the public health, especially when the aerodynamic diameter is less than 2.5 μm. Electrostatic capture techniques, such as polymer electret filters and kinetic plasma processes, are widely used instead of mechanical filtration with high removal efficiency and low wind resistance (pressure drop). However, the inability to recharge, coupled with the generation of ozone byproducts, makes it challenging to meet the requirements for both recoverability and highly efficient filtration. Here, we propose an electrostatic filter as an alternative to conventional polymer electrets, aiming to achieve an ultrahigh removal efficiency, long-term performance stability, and reusability. Piezoelectric LiNbO3 (LN) particles are integrated into the polypropylene (PP) matrix through the melt-blown strategy to fabricate the LN/PP nonwoven fabric. Benefiting from the employment of piezoelectric LN particles, the LN/PP nonwovens exhibit an ultrahigh removal efficiency of 99.9% for PM0.3 to PM10. The airflow facilitates the sustained regeneration of piezoelectric charges on the surface of LN/PP nonwovens, thereby maintaining a removal efficiency of approximately 95% for continuous filtration over 11 days. Even after eight cycles of washing, the removal efficiency of the LN/PP nonwovens remains at nearly 90%, demonstrating the excellent reusability. Our proposed strategy offers an ingenious combination of high-efficiency and recoverability for filters, holding great promise for reducing plastic pollution.

An analytical model to predict the mass loading of air cleaners in typical indoor environments and to estimate the service interval from standardized filter loading tests

For indoor air cleaners, especially those using electret filters, it is known that the clean air delivery rate (CADR) can strongly decrease over time due to loading of the filters with particles. Standardized tests like in GB/T 18801 are used to determine the mass of test aerosol particles leading to a reduction of the initial CADR by 50 % (cumulative clean mass), but this method does not allow to draw conclusions on when this reduction is reached in a typical indoor environment. However, a good estimate would help manufactures to give reasonable recommendations in which intervals a service of the air cleaner becomes necessary. Therefore, we developed an analytical model including the most relevant parameters of a typical indoor environment and assumed different courses for the time-dependent decay of the CADR. We show that consistent estimates for the service interval can be derived, which do only slightly depend on the exact choice of the model. However, we partially find pronounced differences between scenarios dominated by either indoor or outdoor sources. We compare the new model to the model of GB/T 18801 and show that the standard overestimates the service interval for a given set of parameters by about 30 %. We finally propose a method for estimating the service interval from only one loading and one discharging step and give perspectives for further applications of the model.

Deep trapped bipolar heterocharges enable electret nanofibrous membranes for high-efficiency PM0.3 filtration

Recently frequent global pandemic outbreak triggers the demand of high-efficiency aerosol filtration materials. Most of traditional electret air filters can hardly achieve high efficiency (>99.99 %) due to limited charge density and inappropriate charge distribution. The influence mechanism of nanofibers bipolar heterocharge configuration on electrostatic capture of airborne particles is still confusing. Herein, a novel strategy to create high-efficiency air filters based on electret nanofibrous membranes with deep trapped heterocharge is reported. Space charges and dipole charges with opposite polarity are formed in electrospun membranes composed of different polymers under in-situ charge injection and electric poling during electrospinning. Nanoscale spatial surface potential mapping on individual nanofiber characterized by Kelvin probe force microscopy demonstrates bipolar heterocharge distribution. Numerical simulation shows that fiber heterocharge distribution endows the electret membrane with greater electrostatic field intensity gradient, resulting in stronger electrostatic force between nanofibers and airborne particles. With bipolar heterocharge configuration and bimodal fiber diameter distribution, the nanofibrous membranes exhibit high-efficiency low-resistance filtration property against 0.3 μm NaCl particles (99.994 %, 97.4 Pa) at 5.3 cm s−1. Furthermore, benefiting from charges with deep trap energy of 1.12 eV, the membranes still maintain 99.972 % filtration efficiency after 40 h exposure in 95 % relatively humidity. The prepared membranes brings new insight in the design of high-efficiency and long-term stability air filtration materials for public health precaution. This study not only reveals influence mechanism of nanofibers heterocharge distribution on electrostatic attraction, but also provides useful guidance for fiber charge configuration design to improve aerosol filtration performance of electret filters.

Effect of pore structure regulation on the properties of 3D cellulose nanofiber electret filtration materials

The properties, electret property and purification performance, of electret filter materials are closely associated with its pore structure, but the effect mechanism of pore structure on its electret property has not been clarified. Herein, the pore structure of 3D electret fiber materials which were prepared by freeze-drying technology and corona polarization technology based on dialdehyde cellulose nanofibers (DAMCC) and ethyl cellulose nanofibers (EC) was regulated by changing the DAMCC/EC mass ratio, solvent system, pre-freezing temperature and fiber concentration to explore the influence of pore structure on its electret properties and purification performance. The results show that the charge trapping and storage properties can be altered by the regulation of pore structure, and the 3D fiber material, prepared under the conditions of DAMCC/EC nanofiber mass ratio of 1/2, TBA content of 10 %, pre-freezing temperature of −40 °C and fiber concentration of 1.5 w/v%, presents a cellular porous structure with a complete fiber mesh wall. This structure is not only beneficial to the migration of space charge to the interior of the fiber material, but also can effectively prevent the escape behavior of space charge inside the material by extending the tortuous pore channel length, thereby improving the initial and stable surface potentials from 0.72 kV and 0.02 kV to 1.19 kV and 0.47 kV respectively. Moreover, the purification time of 3D electret fiber materials can be reduced from 51 min to 13 min owe to the improvement of electret property and pore structure. These will provide guidance for the investigation of pore structure regulation to alter the properties of electret fiber material for air purification.

Experimental and theoretical analysis of loading characteristics of electret filter media for charged particles

The degradation of filtration performance in electret filter media during usage poses a significant challenge. Pre-charging of aerosols has been identified as an effective method to mitigate this issue. However, the effects of particle charging characteristics on the loading characteristics of electret filters still need a comprehensive understanding. In this study, a needle-cylinder corona charger was employed to pre-charge aerosols, and the particle charge state was determined by multiphysics simulation. The effects of particle charge polarity and charge quantity on the loading performance of the electret filter were quantitatively investigated. The results showed that the particle charge polarity had a negligible impact on the loading performance under the condition of the equivalent particle charge quantity. In addition, the charged particles effectively improved the efficiency degradation during the loading process of electret media, with higher charge quantities resulting in more pronounced improvements. The electrostatic attenuation factor showed a negative exponential correlation with the particle charge quantity. This was attributed to the uneven particle deposition on fiber surface due to the attraction of charged particles by the opposite charges on the electret fibers, which alleviated the effect of electrostatic shielding.

Research of electret air filter: A review

AbstractThe 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.

Light stabilizer-modified hydrocharging melt-blown nonwovens with superior charge stability for air filtration

Particulate matter poses substantial risks to human health, air visibility, climate, and ecological equilibrium. Electret materials are instrumental in air purification and personal protection, as they significantly enhance filtration efficiency without increasing pressure drop. Despite these advantages, developing electret filters with both high filtration efficiency and stable performance under thermal conditions and prolonged storage has proven challenging. In this study, we introduce a novel electret melt-blown filter fabricated through hydrocharging technology supplemented with the light stabilizer UV3346. The interaction of water and air molecules with melt-blown fibers generated abundant positive and negative charges, thereby amplifying the electrostatic attraction effect. Concurrently, UV3346 mitigated the formation of reactive free radicals and minimized charge exchanges, thus enhancing charge storage stability. Owing to the incorporation of UV3346, the UV3346/PP-0.8 melt-blown nonwovens, having a basis weight of 22 g/m2, demonstrated a filtration efficiency of 99.55 %, a low pressure drop of 59.45 Pa, and marginal decrease in filtration efficiency after heat treatment at 130 °C. Notably, the filtration efficiency experienced only a 0.30 % reduction after a two-year storage period, markedly outperforming commercial alternatives. This study is expected to inform future work on high-performance air filtration materials suitable for both air purification and personal protection equipment.

Chemically tailored molecular surface modification of bamboo pulp fibers for manipulating the electret performance of electret filter media

The surface chemical composition of materials is essential for regulating their charge trapping and storage capabilities, which directly affect their electret performance. Although chemical modification of materials to alter electret performance has been investigated, the mechanism through which electret properties are regulated more systematically via chemical customization has not been elucidated in detail. Herein, p-phenylenediamine, benzidine and 4,4′-diaminotriphenyl, which have different conjugated strength functional groups, were selected to chemically tailor the surface of bamboo pulp fibers to regulate the electret properties and elucidate the regulatory mechanism more systematically. The results showed that the charge trapping and storage properties of materials could be regulated by introducing functional groups with different conjugated strengths to their surfaces, realizing the regulation of the electret properties. Moreover, the charge trapping and storage ability could be tailored more specifically by regulating the number of functional groups. By chemical customization to provide electrostatic effects to the materials, the purification time was reduced by approximately 45 %–52 %. More importantly, a relatively systematic mechanism was proposed to elucidate the effect of the conjugate group strength on the charge trapping and charge storage properties of the material. These findings will provide guidance for the investigation of chemical modifications to regulate the electret performance of materials.

Designing Multi-Layer electret filters via numerical simulation

The deposition of neutral and charged particles inside an electrostatically charged filter can affect its performance by decreasing its ability to capture additional particles and by increasing its pressure drop. The particle-loaded behavior of a composite filter comprised of three fibrous layers is studied in this paper using a macroscale simulation approach. Each of these fibrous layers were assumed to have fibers with the same amount of charge but different charge polarities of only unipolar positive, only unipolar negative, or only bipolar. The charged layers were stacked on top of one another in three different configurations with respect to the flow direction and their filtration efficiency and pressure drop were compared during particle loading with neutralized particles (having the Boltzmann equilibrium charge distribution), singly-charged particles, and neutral particles. Our simulations revealed that placing the bipolarly-charged layer downstream of the unipolarly charged layers helps to lower the negative impact of particle deposition on pressure drop and filtration efficiency of the resulting composite filter. This study introduces a design tool which can be used to optimize the configuration of a composite filter for enhanced particle capture efficiency and reduced pressure drop increase.

A simple approach to simulate corona-charged electret filters

This paper presents a novel computational model devised to simulate how different ways of corona-charging a fibrous filter can impact its aerosol collection efficiency. The model assigns different bipolar charges to the fibers in the media based on how the corona-generated negative ions impact these fibers they travel from the corona needles towards the grounded surface underneath the media. The charge assignment process is based on observations made from simulating the electrohydrodynamic field around the corona needles. With the fibers virtually charged, the trajectory of positively and negatively charged aerosol particles, ranging in size from 30 to 500 nm, were simulated across the filter thickness to quantify the effect of fiber charge distribution on the filter efficiency. The performance of our virtual filter was found to be better when it was exposed to the corona ions on both the face and back sides. More interestingly, it was found that the filtration efficiency of the filter was dependent on the order by which its face and back sides were charged. The filter was observed to capture smaller particles (less than 150 nm) more efficiently when it was first charged on the face side and then charged on the back side. The opposite effect was observed when the charging order was reversed. This difference in filtration performance was found to arise from the differences in the solid volume fraction of the filter across its thickness.

A sustainable biomass-based electret for face mask and non-volatile transistor memory

An electret filter on the middle layer of disposable medical face masks has a similar function to the charge storage layer on the transistor memory devices. Both applications require a polymer electret to trap charges so that it could establish electrostatic attraction to effectively enhance the filtration efficiency of face mask or modulate the electrical behaviours of transistor memory. By virtue of that very same function, we demonstrate the charge storage performance and memory characteristics of transistor memory using the electret filters of face mask material, from the non-renewable polypropylene (PP) to the biomass-based polymers. We demonstrate dextrin, a biomass-derivative, as a sustainable alternative electret. Due to dextrin being produced natural starch products, it is a renewable material that is biocompatible and eco-friendly. Furthermore, by adding sodium methylsiliconate (SMS), the hydrophobic blended dextrin-SMS transistor memory can maintain inherent prolonged electrostatic charges for over 100 days along with relatively wide memory window of 39.5 V. Ultimately, the non-woven nanofiber electret filter using dextrin-SMS can be successfully produced using an industrial compatible electrospinning technique combined with the addition of polyethylene oxide (PEO) to improve the electrospinability of the mat filter and potentially utilized for future biodegradable face masks and green electronics.

A Novel Polyvinylidene Fluoride/Keratin Electret Filter With Comprehensive Performance and High-Efficiency PM0.3 Removal

Effective and environmentally friendly air filters to preserve public health are essential given the particle matter pollution epidemic that has permanently damaged human health. This study used blending spinning to create a unique polyvinylidene fluoride/keratin electret filter with reliable PM0.3 reduction and comprehensive features. More specifically, adding keratin derived from natural wool significantly increased composite fibrous membranes’ pore structure and surface charge. Keratin and polyvinylidene fluoride work together synergistically to produce a fiber membrane that exhibits good filtration performance with a quality factor of 0.04662 /Pa, a low pressure drop of 127 Pa, and a high particle removal capacity of 99.731%. The development of this electret material provides a new approach for the design of environmentally friendly air filters for efficient removal of PM0.3.

Microscale modelling of electret filters using disordered 2-D domains

This paper reports on a computational study about the impact of fiber dipole orientation on particle collection efficiency of a bipolarly charged filter. The simulations were carried out using ANSYS software in a series of 2-D geometries comprised of randomly distributed fibers. The simulations were enhanced with in-house subroutines to incorporate Coulomb and dielectrophoretic forces in calculating particle trajectories and thereby simulating filter efficiency. The highest and lowest efficiencies were observed for dipole orientations that were perpendicular and parallel to the airflow direction, respectively. The simulation results were also compared with the predictions of the popular semi-empirical correlations from the literature and good general agreement was observed, without the need for any empirical correction factors. In addition, it was observed that the predictions of the semi-empirical correlations better agreed with the simulation results obtained for media comprised of fibers with random dipole orientations when challenged with neutralized particles.

Progress and perspective of polymer electret-based PM2.5 filtration: Efficiencies, regeneration, and energy implications

High-performance PM2.5 filtration technologies are urgently needed for both air purification and energy conservation. As a superior filtration media, the polymer electret filters have attracted wide attention. Herein, the current research status of electret PM2.5 filter media are firstly analyzed from three aspects: (i) the electret properties of various electret filters, (ii) the dynamic filtration performance of typical electret filters, and (iii) the novel reusable electret filters and their regeneration technologies. Then the initial air resistance and lifespan energy consumption of a variety of mechanical and electret filter media are analyzed and compared, disclosing the advantages of charging and material compositing. Based on the analysis, perspectives on electret filter are provided from interdisciplinary views of energy, environmental and material engineering. An in-depth understanding of the charge storage characteristics of electret media at the nm and μm scales is necessary for elucidating the impact of material properties on dynamic efficiency. Meanwhile, the next generation of electret PM2.5 filters should have better dust holding capacity and regeneration performance for energy saving, but also be more versatile and intelligent to meet ever-evolving demands.

The performance evaluation of pleated electret filters enhanced by electrostatic enhanced method

The filtration efficiency deterioration over operating time due to exposure to particles, organic solvents and other factors 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.

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Electrospun Nanofibers Containing Natural Deep Eutectic Solvents Exhibiting a 3D Rugose Morphology and Charge Retention Properties.

In the present study, electrospun nanofibers of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), a biodegradable polyester, containing natural deep eutectic solvents (NADES) were obtained and reported for the first time, exhibiting an unreported 3D morphology and enhanced charge retention properties. Choline chloride (ChCl)/urea/water in a molar ratio of 1:2:1 was used as the NADES model system. Electrospun nanofibers were produced from a 10 wt % solution of PHBV containing 26 wt % NADES with respect to the polymer and were deposited on different substrates, that is, aluminum foil and non-woven spunbond polypropylene (PP). The morphology and charge retention ability were characterized under different conditions and on different substrates. The attained fiber morphology for the NADES-containing mats showed an average fiber diameter of around 300 nm, whereas the pure PHBV polymer under the same conditions produced electrospun fibers of around 880 nm. However, the deposition of PHBV/ChCl/urea/water fibers resulted in a surprising macroscopic rugose 3D surface morphology made of aligned nanofibers when processed at 50% relative humidity (RH). The nanofiber grammages above which this 3D morphology, associated with NADES-induced charge retention, formed was found to be dependent on the substrate used and RH. This morphology was not seen at 20% RH nor when pure PHBV was produced. Charge stability studies revealed that PHBV/ChCl/urea/water nanofibers exhibited lasting charge retention, especially when sandwiched between spunbond polypropylene textiles. Finally, such multilayer structures containing a very thin double layer of PHBV/ChCl/urea/water fibers after corona treatment exhibited improved paraffin aerosol penetration, which was ascribed to the combination of thinner fibers and their charge retention capacity. The here-developed electrospun PHBV fibers containing NADES demonstrated for the first time a new potential application as electret filter media.

A Novel Sustainable Semiconductor/Metal-organic Framework Coated Electret Filter for Simultaneous Removal of PM2.5 and VOCs

A MIL-125-NH2 metal-organic framework (MOF) coated electret filter, named E-MOFilter, was previously developed to simultaneously remove PM2.5 (particulate matter less than 2.5 mm in aerodynamic diameter) and volatile organic compounds (VOCs, e.g., toluene). This E-MOFilter captures toluene primarily through physical adsorption, however, degradation of the adsorbed VOCs is desired for increasing the service life of the filter. In this study, photocatalytic nanosheets Bi2WO6/BiOCl (p-BWO) were synthesized, grafted on the MIL-125-NH2 to be p-BWO@MIL, and then coated to the electret filter to form a new filter named PE-MOFilter. The physical characterizations, including size, morphology, crystal structure, optical properties, and surface area for the semiconductor p-BWO nanosheets, p-BWO@MIL and PE-MOFilter were first examined to find out the optimal p-BWO to MIL ratio and coating wt% of p-BWO@MIL particles on the electret filter. Results demonstrated that the p-BWO nanosheets were successfully coated on the surface of MIL and the p-BWO@MIL retained the surface area and micropore volume of the MIL-125-NH2. The capability of the PE-MOFilter on adsorption and degradation of VOCs and the removal efficiency of PM2.5 was examined. The results showed that this novel PE-MOFilter not only captured but also effectively photodegraded VOC pollutants via the synergistic action of adsorption and photocatalytic oxidation (PCO). The photodegradation efficiency was found to be as high as 68.7% and it depended on the ratio of p-BWO to MIL. The PM2.5 removal efficiency also demonstrated that the coating of p-BWO@MIL particles had a negligible influence on the degradation of fiber charge. This research sheds light on the development of semiconductor@MOF coated electret media to simultaneously remove particulate and gaseous pollutants to improve indoor air quality with low energy consumption.

Particle Collection of Electret Media under Different Filtration Pressures

Electret media have the advantage of enhancing the particle collection efficiency while keeping/reducing the media pressure drop by the electrostatic mechanisms in addition to the mechanical ones. This study investigated the effect of operational pressure on the filtration performance of electret filter media. Experiments was conducted at the pressures in the range of 0.33–3 atm and using DMA-classified particles ranging in electrical mobility sizes of 10–600 nm. The penetration of particles in the neutral, singly charged and Fuchs’ bipolar charge states of two electret media (both charged and discharged) were measured. The single fiber efficiency due to the induced and Coulombic forces, i.e., ηIn and ηC, were obtained. It is found that both efficiencies are a function of the filtration pressure (i.e., fiber Kn#) in addition to those parameters already included in existing models. A simple modification was also proposed for the existing model (based on the single fiber theory), enabling it to predict the particle collection efficiency of electret media operated under various filtration pressures.

High-efficiency retention of ultrafine aerosols by electrospun nanofibers

The versatility of nanofibrous polymeric materials makes them attractive for developing respiratory protective equipment. Ultrafine nanofibers effectively trap the most penetrating aerosols and exhibit consistent performance compared to conventional electret filters. Advanced nanofiber manufacturing technologies such as electrospinning can functionalize filter materials, enhancing them with unique antibacterial, catalytic, sensory, and other properties. Much of the current research in nanofibrous air filtration focuses on using nanofibers for lightweight personal protective equipment such as N95 respirators, but their use for higher levels of respiratory protection required for chemical, biological, radiological, and nuclear (CBRN) protection has not yet been comprehensively explored. In this study, we tested the hypothesis that electrospun filters could provide the particle filtration efficiency and breathing resistance required by the National Institute for Occupational Safety and Health Standard for CBRN air-purifying respirators. Our manufactured nanofibrous filters demonstrated submicron aerosol retention efficiency of > 99.999999%, which is four orders of magnitude better than the requirements of the CBRN standard. They also had a breathing resistance of ~ 26 mmH2O, which is more than twofold lower than the maximum allowable limit. Although the filter material from the gas mask cartridge currently in service with the U.S. military demonstrated a higher quality factor than electrospun filters, the comparative analysis of filter morphology suggested ways of improving nanofibrous filter performance by tuning nanofiber diameter distribution.