Environmental storage conditions influencing the filtration behavior of electret filters with repeated use

Materials

A commercially available charged polypropylene (PP) meltblown media was obtained from UnionFilTech (South Korea) and used as received. The filter media had the basis weight of 32.1 ± 0.2 g m⁻², the thickness of 229 ± 17 μm, the porosity of 86.5 ± 0.8%, and mean fiber diameter of 5.1 ± 2.2 μm.

Particulate filtration tests

Filtration test was conducted using an automated filter tester (TSI 8130, TSI Inc., USA) with sodium chloride aerosol. An aqueous solution of 2% (w/w) NaCl (Daejung, Republic of Korea) was prepared for generating NaCl aerosol (count median diameter, CMD ∼0.075 ± 0.020 μm). The charges of NaCl aerosol were equilibrated to have zero net charge in Boltzmann distribution. In TSI8130 filter tester, there are built-in light scattering laser photometers. The photometer measures the amount of light scattered by the particles, and calculates the upstream and downstream particle concentrations simultaneously. The penetration (%) of particles is determined by the ratio of downstream to upstream particle concentrations measured by the photometer.

The environmental effect on filtration efficiency was investigated in two different approaches: (1) continuous storage (for 37 days) and continuous particle exposure (up to 40 mg, ∼ 3 h testing), (2) repeated and recurrent particle loading (2.2 mg, 9 min testing) and recurrent storage in between the particle loading (for all day except the particle loading time and 30 min pre-conditioning time) every day, up to 37 days. Environmental storage conditions were determined based on practical applications such as: (1) 23°C, 50% RH simulating common office condition; (2) 40°C, 75% RH simulating a hot and humid weather; (3) 40°C, ∼ 0% RH simulating a hot and dry shelf-life; (4) 90°C, ∼ 0% RH, simulating a disinfection method in oven for reuse of respirator. ¹⁷

Face velocity of 2 cm/s was used for the test, assuming that an adult breathes through a facepiece in 250 cm² at 30 L min^-1 (normal breathing condition). The following calculation was used to determine the face velocity: (30 L/min × 1000 cm³/L × 1 min/60 s) = (250 cm² × 2 cm/s). For the continuous filter test, a filter media area of 100 cm² was exposed to NaCl aerosol in the mass concentration of 20.0 ± 5.0 mg m^-3 at the face velocity of 2 cm s^-1. The penetration and the pressure drop were monitored through the continued mass loading until the penetration was nearly reached to zero (about 40 mg NaCl loading for about 3 h).

For the simulated repeated use-storage test, the filter media was challenged to 2.2 mg of NaCl aerosol per day. The following scenario was considered to determine the exposure mass per day. If a 250 cm² filtering facepiece is used in a harsh particulate level of 300 μg m^-3 for 10 h with 30 L min^-1 of inhalation rate, a total aerosol mass of 22 μg cm^-2 would be challenged to a filtering mask every day.^35,36 For 100 cm² filter area of this study, 2.2 mg would be challenged every day. The aerosol concentration of the test equipment was fixed in 20 mg m^-3, thus it took about 9 min to challenge 2.2 mg of NaCl mass at the face velocity of 2 cm s^-1. Immediately after the aerosol exposure, the filter media was stored in the predetermined environmental condition until the next day test. After filters were stored in those conditions, samples were kept in an ambient condition (23°C, 50% RH) for 30 min before the next aerosol test to challenge 2.2 mg of NaCl. The sequence of aerosol exposure and storage was conducted at an interval of 24 h, and this sequence was repeated until the total mass of 80 mg was challenged to a filter media.

Characterizations

An X-ray computed tomography (Xµ-CT, Xradia 510 Versa, Carl Zeiss Inc., Germany) was employed to analyze the 3D internal structure of filter media. The X-ray source was operated at a voltage of 60 kV with a power of 5.0 W. The field of view was 700 μm, and the corresponding voxel sizes were 0.7 μm. The reconstructed Xµ-CT (3D internal structure) dataset was imported to ORS Dragonfly Pro software (ORS Inc., Canada) for image processing.

The morphology of filter media and captured particles were observed by the field-emission scanning electron microscope (FE-SEM, SM-7800F Prime, JEOL Ltd., Japan), with prior coating with Pt (108auto, Cressington Scientific Inc., UK). An Environmental SEM (ESEM, Quattro S, FEI, USA) was used to observe the morphological change of the deposited particles on the filter media in humid conditions. The particle-loaded filter sample was put on a stage and observed at 2°C and 100% RH.

The tensile test was performed on a rectangular sample of 50 mm × 150 mm using a tensile tester (5 ST, Tinius Olsen Ltd., UK) operated at a crosshead speed of 10 mm min^-1. The surface potential of the meltblown was measured by a non-contacting electrostatic voltmeter (Model 542A, Trek, Lockport, NY, USA). The surface potential probe was placed 2 cm above the web surface, moving across different filter area in 50 cm distance.

Filtration characteristics with varied environmental storage conditions

Figure 2 examined how the storage temperature and humidity affected the aerosol penetration and pressure drop. The filters were treated at the pre-determined storage conditions for 37 days and were tested for continuous particle loading (Figures 2(a) and (b)). The pressure drop and penetration during the continuous aerosol loading, whether the samples were pretreated or not, depicted typical plots showing a maximum penetration as the accumulated aerosol mass increased; after reaching the maximum penetration, the penetration began to decrease as the pores of the filter were clogged. The pressure drop and the penetration of filter media pretreated at 40°C condition were similar to those stored at the ambient condition (23°C, 50% RH). However, 90°C temperature and high humidity (40°C, 75% RH) conditions produced increased penetration (lowered efficiency) and lowered pressure drop compared to the ambient storage condition.OPEN IN VIEWER

As the environmental temperature and humidity could affect the charge stability of electret filter media during the storage, the surface potential of filter media was investigated with different storage conditions (Figure 3). It should be noted that the measurement of surface potential may not directly reflect the surface charges, because an electret filter commonly hold both positive and negative charges, compensating each other. Thus, the observation was focused on the extent of potential variations in estimating the relative charges of the material. For a highly charged media, the surface potential measurement showed large variations, while the surface potential profile was flattened as the charging was deteriorated. The surface potential was measured from 2 cm away from the sample surface, probing across the filter area within 50 cm.OPEN IN VIEWER

The result in Figure 3 is the surface potential of filter media that were stored for 37 days. The surface potential was significantly deteriorated with 40°C, 75% RH condition, indicating that the electrostatic filtration became ineffective as the storage progressed. The pressure drop after 37 days of storage at this condition showed large variations, thus the aging effect at this condition on the pressure drop is inconclusive (Figure 2(a)). It is argumentative whether the environmental moisture affects the filtration efficiency of polypropylene (PP) electret filter media, where the moisture absorption of PP is negligible. From Lee et al.’s study, ²⁶ storage at 20°C, 65% RH for 2 days hardly affected the filtration performance of PP filter media. On the other hand, Motyl and Lowkis ²⁴ reported that storage at 24°C and 78% RH may affect the surface charges of electret filter, but in different extents depending on the filter type. Yet the detailed information is not available on which factors of an electret filter affect the humidity-dependence in surface charges and filtration efficiency. The result in Figure 3 shows that an extended storage at high humidity (75% RH) and warm temperature (40°C) deteriorated the electret charges leading to the reduced filtration efficiency.

At 40°C dry condition, the charge decay was not observed as much as 40°C, 75% RH condition. As a result, the filter efficiency stored at this condition was comparable to that stored at room temperature and humidity at 23°C, 50% RH. The surface potential of the filter media kept at 90°C was nearly zero with little variations, indicating that the reduction of electrostatic charges occurred significantly. Thus, the penetration of the filter stored at 90°C considerably increased for both continuous and intermittent exposures. After the charge decay, the electrostatic particle capture mechanism such as Coulombic attraction and induced polarization can no longer contribute to the overall filtration efficiency; instead, the filtration occurs mostly by the mechanical particle capture mechanism such as diffusion, interception, and inertial impaction. The result implicates that for about a month or longer period of shelf-life, the storage condition needs to be carefully chosen to maintain the filter performance of electret filter.

For the recurrent aerosol exposure and storage conditions (Figures 2(c) and (d)), the pressure drop did not increase as much as the continuous exposure, and it remained below 30 Pa. Especially for the repeated use with storage at 40°C, 75% RH, pressure drop did not increase with days of repeated particle exposure, maintaining the pressure drop almost the same. On the other hand, the pressure drop during the 1-day exposure (2.2 mg of challenging) showed a slight increase. The recurrent test with an ambient storage (23°C, 50% RH) was additionally conducted, and it was confirmed that the trend of filtration performance at the ambient condition was similar to that of 40°C, 75% RH condition. For all tests of recurrent aerosol exposures, a leap in the penetration was observed between the daily test intervals (after storage), implying that the loaded aerosol caused the performance deterioration during storage.

For the repeated exposure-storage test with 90°C storage condition, overall penetration increased steeply with a leap between the testing intervals (Figures 2(c) and (d)). At around the accumulated exposure of 50 mg, there was a sudden discontinuity in the plots both for pressure drop and penetration. This discontinuity in the graph is the result of the fiber breakage occurred at 50 mg exposure (Figure 4(b)). From the result of the tensile test (Figure 4(a)), the stress of filter media that was continuously aged (without particle loading) at 90°C for 25 days decreased from 83 gf cm^-2 to 21 gf cm^-2, showing that the extended storage at 90°C caused the loss of mechanical strength of material. From the repeated recurrent particle test, the filter media could not withstand the weight of the loaded particles at about 50 mg accumulated exposure, causing the breakage of fibers in the web (Figure 4(b)). The test was repeated three times, and the fiber breakage occurred rather consistently at around 50 mg of particle exposure in the recurrent test. The penetration (%) of particles was calculated as a percentage of downstream concentration divided by upstream concentration using a photometer. Thus, the abrupt penetration rise at 50 mg mass exposure is due to the particle leakage through the torn filter. The filtration test was stopped from this point because testing with a torn sample was meaningless.OPEN IN VIEWER

Morphology of filter media with varied storage conditions

Any evidence of morphological changes in internal structures of filter media with 37 days of conditioned storage was investigated by Xµ-CT analysis. Fibers and pores are distinguished by the X-ray intensities, and image processing was performed to reconstruct 3D images of the filter sample. After the reconstruction of 3D image in 500 μm × 500 μm × 500 μm, morphological parameters including filter thickness, fiber diameter, and pore distribution in the web were analyzed. The thickness of filter media after 37 days of storage was the following: filters stored at ambient condition (23°C, 50% RH), 229 ± 17 μm; filters treated at 40°C, 75% RH, 283 ± 17 μm; filters treated at 40°C, 298 ± 12 μm; filters treated at 90°C, 285 ± 14 μm (Figures 5(a) and (b)). The filter media became slightly thicker after every storage condition for 37 days, probably because heat treatment (40°C or 90°C) eliminated the internal stress of the filter media that was imposed during the meltblown manufacturing process. ²⁵ Thus, after heat treatment, the filter media became fluffier. ³⁷ However, this structural change did not have direct influence on the pressure drop and the penetration. Fiber diameter and pore size were further analyzed by Xµ-CT images. The filter media, regardless of environmental storage conditions, displayed their fiber diameter distributions from 1.5 μm to 15 μm, and the average diameter was 5–6 μm (Figures 5(a) and (c)). There was no observable difference in the fiber diameter for different pretreatment conditions, and the fiber diameter was not a cause for the change of filtration performance during the conditioned storage.OPEN IN VIEWER

In Figure 5(d), pore diameter was analyzed to better understand the pore distribution with varied storage conditions. Overall, the pore size distributions of filter media that were treated in different storage conditions were not considerably different. Figure 5(e) shows the cross-sectional image of the web in the varied axis, which analyzes the distribution of fibers and pores in the cross-section. The fiber region was colored in gray, and the pores were colored from yellow to purple depending on the distance from the pore to fibers. For the filter media kept at ambient condition, the yellow color was not seen between fibers in YZ plane and XY plane, meaning that fibers are packed rather tightly with fewer neighboring pores. On the other hand, 90°C pretreated sample showed larger pores (yellow) acting as tortuous channels for direct air flow, and these enlarged pores may affect the mechanical filtration efficiency and pressure drop.

Particle loading characteristics in the filter media

To confirm the accumulated NaCl particle morphology on the filter media, SEM and Xμ-CT images were examined. As particles were continuously challenged to filter media at once, the particles were mostly stacked up as a thick layer at the surface of filters regardless of preconditioning (Figure 6). Since the densely packed NaCl particles blocked the air flow, the pressure drop rose quickly. Unlike the continuous exposure to aerosol, samples with recurrent exposure and storage showed NaCl particles distributed deeper down in the filter media (Figure 7). It seems that the recurrent and repeated air flow self-propelled the particles and enhanced transport through the pore, delaying the clogging. ³⁸ OPEN IN VIEWER

OPEN IN VIEWER

Figure 7.

SEM and Xµ-CT images of 80 mg NaCl loaded filter media with media; repeated filtration test with filter media with recurrent storages at (a) 40°C, 75% RH, (b) 40°C, (c) 90°C. (d) ESEM images of NaCl loaded filter media at 2°C without humidity (left) and under 2°C, 100% RH (right).

For recurrent use conditions (Figure 7), the particles appear throughout the entire depth of the filter media. As a result, the pressure drop was lowered at the expense of increased penetration. In the storage condition at 40°C, 75% RH, the pressure drop maintained almost consistent, and this is attributed to the deliquescence of captured NaCl particles. During the humid storage, large NaCl aggregates of crystals were formed because of the hygroscopic property (Figure 7(d)). The hydration layer on the NaCl surface could have acted as a liquid bridge, and this liquid bridge could induce particle agglomeration.^39,40 In humid conditions, wet NaCl particles spread on the filter fibers, quickly masking the filter charges like liquid aerosol; as a result, the filtration performance may deteriorate rather rapidly in this condition.

ESEM analysis was carried out to observe the effect of humidity on NaCl loading characteristics. From ESEM in Figure 7(d), hygroscopic NaCl deposited on filter fiber was wetted slowly in humid condition, and began to swell, which will lead to masking of charged surface. The filtration efficiency was rapidly deteriorated after NaCl-loaded filters were stored in 75% RH, as shown from the steep increase of penetration (Figure 2(d)). For the thermal storage condition either at 90°C or at 40°C, the captured particles appeared as dendrites and the pressure drop showed an overall inclination with repeated exposure to the aerosol.

Effective lifetime of filter media

The effective lifetime of filter media can be limited by the build-up of pressure drop, deteriorated filtration efficiency, and mechanical damage of material. When filters are not properly replaced at the right time, it may cause a significant build-up of pressure drop or an increased penetration of pollutants due to the charge decay and/or damaged material.^41–45 Figure 8 depicted the mass of particles deposited/retained in the filter media versus the mass challenged to the filter media. The plots filled with dark and light colors indicate that particle mass retained in the filter media and the mass penetrated through the filter, respectively. For the continuous exposure test, the pressure drop increased with a steep slope as the deposited mass increased, while the efficiency was nearly 100% throughout the mass accumulation.OPEN IN VIEWER

The effective lifetime of filter can be determined by a certain level of efficiency and breathing resistance, which are measured by % penetration and pressure drop. In this study, performance levels of 5% of penetration and 12 Pa of resistance (25–30% increase from the initial pressure drop) were used as example criteria of effective lifetime of a well-functioning filter. For the repeated exposure cases with humid storage conditions (40°C, 75% RH storage), the pressure drop maintained to be low, so that the filter users may not notice the increase of breathing resistance. On the other hand, filtration efficiency reached over 5% after 5-days’ use in this storage condition, losing a considerable extent of efficiency. In such a condition, the users may not recognize the deteriorated performance nor the breathing resistance when they use filters repeatedly.

With the repeated use with 90°C storage condition, the penetration went up over 5% in 5 days with a considerable build-up of pressure drop. In this case, users may replace the filter as they sense the significantly increased breathing resistance compared to the initial resistance. At 90°C recurrent storage condition, the pressure drop was drastically reduced on 21^st day because of the air leakage through the torn part of the filter. The mechanical property of PP filter media was deteriorated with the repeated thermal storage at 90°C, and at 21^st day, the filter media could not withstand the mass of accumulated particles, being torn. This result implies that the facepiece user needs to be careful about storing the facepiece under high temperature.

For the repeated use with 40°C dry storage, the filter performance was effective (penetration <5%) until day 7, but the build-up of pressure drop was a dominant factor influencing the useful lifetime. For the repeated use with recurrent storages, the deposited mass of aerosol was not directly related with the effective lifetime; instead, storage condition, particularly humidity, played an important role affecting the performance and effective lifetime. If the masks were not timely replaced, contaminants may unexpectedly penetrate in quite large quantities that may threaten the user health. Therefore, understanding the filtration behavior with actual use scenarios is important for a proper protection.