1. Introduction
Electrets are charged dielectrics, and they are generally formed through mobilizing the polar groups and trapping the charge carriers at above the glass transition temperature (T
g) [
1,
2,
3]. In this process, the material in an intense electric field acquires charges, then the charges are frozen at cooling [
4]. The electric field in electrets is sourced from the internal polarization of a dielectric or the externally accumulated electric charges [
1,
4,
5,
6]. Another simple classification of electrets is hetero-electrets and homo-electrets; where homo-electret mostly occurs by excess charge injection into nonpolar materials, and hetero-electret occurs through the internal charging by orientation of dipoles [
3]. Generally, a polymer with high dielectric constant (ε
r) polarizes more in the electric field, giving a high level of initial charges, while its loss with time is another issue [
1,
5,
7].
Commonly available commercial electret filter media is made by corona-charging. Rather recently, electrospun webs have been regarded as potential electret filter material, while its scalability is yet challenging [
8,
9,
10,
11,
12]. In most cases, both hetero- and homo-charges exist simultaneously, and the dominant type of charges determine the character of the electret [
3]. In practical application, charge retention capacity of electret material is important, as an electret filter exposed to high temperature and humidity during its shelf-life can lose charges with time [
10,
11]. Moreover, the tendency to respond to such environmental conditions is affected by the material properties. 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].
Generally, homo-electrets made from nonpolar materials hold stable charges, due to low electrical conductivity and low water absorption [
3,
19]. Thus, nonpolar polymers such as polyolefins can be good candidates of electret materials [
20,
21]. One of the common applications of polymeric electret is air filtering materials. The electret filter as opposed to the mechanical filter display enhanced ability of capturing airborne particles due to the benefit of coulombic attraction and induced polarization. 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. Moreover, the dielectric constant (ε
r) of material has rarely been associated with the charge decay phenomenon in experimental investigation [
1,
27], while there are implications that ε
r is related with the charge stability and conductivity [
7,
27,
28].
The purpose of this work is to investigate the polymeric attributes that influence the charge retention capacity of electret filters. In this study, polymers with three different dielectric constants—polypropylene (PP), polyvinylidene fluoride (PVDF), and polyacrylonitrile (PAN)—were compared for their charge decay phenomena with varied aging conditions. The material properties such as surface energy, moisture regain, ε
r, and crystallinity were analyzed, and their influence on charge decay and filtration performance were discussed. Ultimately, this study intends to provide informative discussion on the polymeric properties that affect charge retention capability, for the design of robust filter material of which charges sustain longer in harsh environmental conditions.
Figure 1 demonstrates the concept of this study.
With the growing environmental concern with the airborne particulate matters (PM), various filter materials are being explored in search of adsorbate specificity, high filtration performance, environmental sustainability, etc. [
10,
11,
22,
25]. Most of those studies focus on achieving the high filtration efficiency, but lack the understanding of material properties that make the robust electret filters in consideration of charge retention. This study will broadly impact the robust filter development, by guiding the material selection for the superior electrostatic filtration.
2. Materials and Methods
2.1. Materials
Polyacrylonitrile (PAN, Mw = 160,000 g·mol−1) and polyvinylidene fluoride (PVDF, Mw = 234,000 g·mol−1) were purchased from Sigma-Aldrich (Saint louis, MO, USA), and were used to make electrospun filter webs. N, N-dimethylformamide (DMF), and acetone were purchased from Fisher Scientific (Ottawa, ON, Canada) and Daejung Chemicals & Metals Co. (Siheung, Korea), respectively. Two types of commercially available polypropylene meltblown webs were obtained from CHL Korea (Seoul, Korea). Thicknesses of web materials including meltblown and electrospun webs were measured using a thickness gauge (Mitutoyo 7050 Dial Upright Gauge, MITUTOYO, Kawasaki, Japan) within ± 0.01 mm error range.
2.2. Electrospinning
A 20 wt.% of PAN prespinning solution was prepared by adding 1.058 g PAN in 10 mL DMF, and stirring for 24 h at 750 rpm and 40 °C. The electrospinning was carried through a 23-gauge needle at the feeding rate of 3 mL/h. The fibers were collected on a drum collector rotating at 100 rpm covered with polypropylene spunbond web, maintaining the tip to collector distance (TCD) to be 12 cm. The applied voltage was set at 15 kV.
For PVDF electrospinning, a 21 wt.% PVDF/DMF/acetone solution was prepared by adding 1.305 g PVDF in 3 mL DMF and 3 mL acetone, then the solution was stirred for 24 h at 850 rpm and 85 °C. The solution was fed through a 21-guage tip, at the feeding rate of 5 mL/h and 22 kV applied voltage. The TCD was set at 12 cm. After the spinning, electrospun web was placed in the hood for 24 h to evaporate the solvent residue.
2.3. Accelerated Environmental Aging
As filter media layers, the PAN electrospun web, PVDF electrospun web, and PP meltblown web, respectively, were layered with polypropylene spunbond webs at the top and the bottom. The electric field and the filtration performance of the respective filter media layers were examined before and after aging of filters at different conditions. All samples were conditioned at 20 °C, 65% RH for 24 h before aging. Aging conditions included thermal aging at 120 °C and moisture-aging at 25 °C, 90% RH for 48 h. All conditioning and aging were conducted using a climate chamber (PL-3KPH, ESPEC Corp., Osaka, Japan).
2.4. Evaluation of Filtration Performance
The instantaneous filtration performance was examined by a filter tester (TSI 8130, TSI Inc., Shoreview, MN, USA). The test was carried out using NaCl particles in the count median diameter of 0.075 ± 0.02 μm, where the particle charges were neutralized in Boltzmann distribution. The filter media of 40 cm
2 area was exposed to the NaCl aerosol in an average mass concentration of 25 ± 0.2 g/L. The percentage of particle penetration and the pressure drop of the filter media were examined at the face velocity of 12.5 cm/s (flow rate of 30 LPM). To account the relative efficiency to a unit pressure drop, the quality factor (QF) was calculated as follows.
2.5. Material Characterization
2.5.1. Moisture Regain
The moisture regains of the conditioned filter media (PAN electrospun web, PVDF electrospun web, PP meltblown web) at 20 °C, 65% RH for 24 h were examined. The weight of the dried sample was obtained by drying the sample at 105 °C for 2 h. Moisture regain was calculated by the following equation.
where
Wcond = weight of the conditioned sample at 20 °C, 65% RH,
Wdry = weight of the dry sample.
2.5.2. Contact Angle Measurement
The wetting property of filter materials was examined by measuring water contact angle (CA) on filter surfaces, using a contact angle analyzer (SmartDrop Lab, Femtobiomed Inc, Seongnam, Korea). To measure the contact angle, water was used as a liquid. A fixed steel needle supplied a water drop of 3.0 ± 0.4 μL onto the surface of the fabric sample to be investigated. The image of the drops was captured. The data that resulted from processing the images used specific programs to fit the profile with the Young equation. At least five different points on each sample were considered.
2.5.3. Measurement of Electric Potential
The electric field caused by the surface charges of filter media, or the electric potential of filter media, was measured using an electrostatic field meter (Simco-Model FMX-003, Simco ION, Hatfield, PA, USA). For measurement, the filter sample in 10 × 10 cm was hung in the air with a paper board placed behind the sample. The electric potential (static voltage) was measured from 2.54 cm away from the sample surface, and a total 64 measurements were done for a sample.
2.5.4. Scanning Electron Microscopy
The morphology of sample was characterized using a field-emission scanning electron microscopy (FE-SEM, JSM-7800F, JEOL Ltd., Akishima, Tokyo, Japan), with prior Pt coating on fibrous samples for 70 s at 20 mA, using a sputter coater (108 auto, Cressington Scientific Inc., Watford, Hertfordshire, UK). The fiber diameter of all samples was measured by selecting 20 fibers randomly from SEM images.
2.5.5. X-Ray Diffraction Analysis
The X-ray diffraction (XRD) of filter sample was analyzed using a powder X-ray diffractometer (SmartLab, Rigaku Corp., Tokyo, Japan). The obtained XRD data was used to calculate the change of crystallinity during heating by the XRD deconvolution method [
29,
30]. The deconvolution patterns of X-ray diffraction were analyzed using Gaussian function for each peak using ORIGIN PRO 8.5 software. The degree of crystallinity was calculated by the following in the range of 2θ = 10–30°.
where
WC = degree of crystallinity,
IC = integrated area of crystalline peaks,
IA = integrated area of all peaks.
2.5.6. Analysis of Pore Size Distribution
A 1100-AEHXL capillary flow porometer (Porous Media Inc., Ithaca, NY, USA) was used to analyze the pore size distribution of filter webs. For measurement, the web samples in 3 × 3 cm was placed in the instrument, and soaked with the Galwick liquid (Porous Media Inc., Ithaca, NY, USA) having a low surface tension (15.9 dyn/cm). The air flow was blown to obtain wet and dry curves for analyzing the pore size distribution.
4. Conclusions
Electret filters as opposed to mechanical filters have the benefit of electrostatic filtration, but this benefit can be limited when the electret charges are deteriorated by the environmental aging. The objective of this work is to investigate the polymeric parameters that influence the charge retention capacity of electret filters. Electrets with three different dielectric constants—PP, PAN, PVDF—were compared for their charge decay phenomena, and their impacts on the electrostatic filtration performances were investigated, with the accelerated aging conditions under high temperature (120 °C) or high humidity (25 °C, 90% RH) for 48 h. The effect of moisture-aging on charge decay was highest in the order of PAN > PVDF > PP, corresponding to the order of surface energy. The deterioration of filtration performance, in terms of QF, also agreed with the charge decay. The effect of thermal aging on charge decay appeared largest for PVDF that has the highest εr, followed by PAN and then PP. With thermal aging, the material with a high εr was more vulnerable to charge loss, leading to the deterioration of filtration performance. The effect of thermal aging on the filtration performance appeared significantly large than the effect of humidity aging for all filter samples. For all polymers, the crystallinity decreased after the thermal aging. PVDF, which had the highest crystallinity, showed the largest extent of decrease in crystallinity. Moreover, the material with the highest crystallinity, PVDF, showed the largest impact on charge decay and filtration performance with thermal aging.
Ultimately, this study intends to provide information on the polymeric attributes that affect the charge retention capability, for design of robust filter material of which charges sustain in harsh environmental conditions. The results of this study implicate that εr and wettability are important parameters influencing the electric conductivity and chain mobility, and they can be used as a convenient predictor for the charge retention capacity. This study will broadly impact the development of robust filters, by guiding the material selection for the superior electrostatic filtration.