Material Properties Influencing the Charge Decay of Electret Filters and their Impact on Filtration Performance

Abstract

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 (εr) 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 εr 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 εr 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.

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“…Charge retention ability is reported to depend on the electrical properties of polymers such as conductivity and dielectric constants [ 37 , 39 , 44 ]. From a study by Lee and Kim [ 45 ], PP, poly(acrylonitrile) (PAN), and poly(vinylidene fluoride) (PVDF) which have different dielectric constants and moisture absorption properties were compared for the charge stability and its impact on filtration performance, when treated under high temperature (120 °C) or high humidity (25 °C, 90% RH). The effect of moisture-aging on charge decay was highest in the order of PAN > PVDF > PP, corresponding to the order of moisture absorption capacity.…”

“…For thermal treatment, charge decay appeared larger in the order of PVDF > PAN > PP, showing that the material with higher dielectric constant was more vulnerable to charge loss, leading to deterioration of filtration performance. The effect of thermal aging on the filtration performance appeared significantly larger than the effect of humidity aging for all filter materials from this study [ 45 ].…”

“…The charge retention capacity of a dielectric polymer is generally associated with the electrical conductivity and the energy level of localized trap sites [ 45 , 46 , 47 , 48 , 49 , 50 ]. The dielectric property is affected by the electron hopping at each energy level, where the band gap energy becomes important [ 49 , 50 , 51 ].…”

“…From this perspective, a nonpolar polymer would be a good candidate of electret material due to its low electrical conductivity [ 52 , 53 ]. The material with a high trap energy can hinder the charge transport to the conduction band, demoting the charge decay [ 45 , 46 ].…”

“…Efforts have been made to increase the surface potential of electret filter materials and to enhance the long-term service performance. PVDF, as a dipolar material, has been studied to apply as an effective electret filter [ 41 , 42 , 45 , 62 , 63 ]. It was expected that, when PVDF was electrospun, the crystal phase of PVDF molecules would be transformed from the nonpolar α-phase to the polar β-phase due to the spontaneous polarization upon electrospinning, and this β-phase conversion would maximize the dipole moment [ 41 , 42 ].…”