A reusable, isoporous through-hole membrane filter for airborne particulate matter removal

Abstract

This work introduces a new cleanable (and thus reusable) membrane air filter. The membrane filters were fabricated using a soft-lithographic technique, and they are isoporous having through-hole pores. The soft-lithographic technique can be applied to the fabrication of the membranes on various supporting structures such as meshes, or even on freestanding isoporous membranes in an additive fashion leading to bi- or tri-layered membranes. When freestanding single layer membrane filters are used for the removal of PM2.5 contained in the air stream, isoporous through-hole membranes show 60–80% (depending on pore size) filtration efficiency with low pressure drop (less than 40 Pa), which is quite contrary to the common belief that the filtration mechanism would be size sieving. Using a bi-layer configurations of 800 nm isopore membrane layers on top of 5, 10, or 25 μm membranes, these filters’ removal efficiency is >96%, but with a bit high (200–300 Pa) pressure drop. To mitigate the trade-off between efficiency and pressure drop in single and bi-layer isoporous membrane filters, multiple 5 μm-pore membranes with a varying separation gap of 3–15 mm have shown an efficiency of up to 98% with pressure a drop of less than 100 Pa. The main filtration mechanisms, inertial impaction and interception, were identified based on experimental data and finite element analysis. Importantly, the membrane filters can easily be cleaned by wiping with wet tissue or by spraying them with water/ethanol (9:1) mixture, which enables many subsequent re-uses. In addition, the thermoset nature of the membrane material shows high temperature stability, exhibiting ~5% shrinkage at 350 °C and ~15% shrinkage at 400 °C. The membrane can also endure very low temperatures (−80 °C via dry ice), which makes the operating temperature range of the membranes very wide. Due to their simple soft-lithographic fabrication process and their good efficiency and reasonable pressure drop, the isoporous through-hole membrane filters described here present compelling alternative to more conventional filters used for the removal of airborne particulate contaminants.