A team at Stanford has developed high-efficiency (>99.5%) polyimide-nanofiber air filters for the removal of PM2.5 from exhaust streams. In a paper published in the ACS journal Nano Letters, the researchers report that the new polyimide nanofibers exhibit high thermal stability. The PM2.5 removal efficiency was kept unchanged when temperature ranged from 25–370 °C.
The filters feature high air flux with very low pressure drop. A field-test showed that the new nanofibers could effectively remove >99.5% PM particles from car exhaust at high temperature. Some versions of the filters removed PM2.5 with efficiency higher than 99.98%—the standard of HEPA filters defined as filters with filtration efficiency >99.97% for 0.3 μm airborne particles.
… the control and removal of PM, especially PM2.5, remains a great challenge because of its small size, complex composition, sources, and evolution processes. Two types of air filters have been in common use. One is a porous membrane filter based on size exclusion filtration. The other type is thick fibrous air filter of diverse diameters from several microns to tens of microns. However, both types of filters have significant air pressure drop, and the removal efficiency of PM2.5 is limited.
Recently, our group developed a transparent nanofiber air filter for the high efficiency PM2.5 removal, which can be used for personal and building protection. Different from the existing filters based on nonpolar polymers, we found that the polar chemical functional groups (i.e., in polyacrylonitrile (PAN) polymer) are important to have strong binding affinity with PM2.5. Some other recent developments on using carbon nanotubes and polymer nanofibers are notable examples toward PM2.5 filtration. However, to eliminate the emission of PM into the air, the ultimate solution would be to remove PM from the sources often with high temperature. This calls for a new technology capable of high temperature air filtration.
… It is of great significance to develop high-efficiency filters with low flow resistance for the PM2.5 removal at high temperature. Here, we demonstrate a novel high-temperature polyimide (PI) nanofiber air filter, which has attractive attributes of high thermal stability (stable up to 370 °C), high PM2.5 removal efficiency, low resistance to air flow, lightweight, and long working lifetime.—Zhang et al.
The researchers said they chose PI as the high temperature air filter material because of its excellent thermal stability at high temperatures; they also believed that the PI has the right polar groups for binding PM.
They fabricated the PI nanofiber air filters by electrospinning a PI−dimethylformamide solution. They suggested that the high efficiency of the filters results from the polar chemical functional groups in PI molecules in conjunction with static charge deposited into nanofiber during electrospinning, which can attract and bind strongly with PM2.5. The temperature stability comes from the intrinsic molecular structure of PI polymer.
A long-term performance test showed that the PI air filters have a high PM particle removal efficiency and a long lifetime. The PI filters can effectively remove almost all the PM particles from the car exhaust at high temperature.
|Removal efficiency of PM particles from car exhaust gas. Zhang et al. Click to enlarge.|
Rufan Zhang, Chong Liu, Po-Chun Hsu, Chaofan Zhang, Nian Liu, Jinsong Zhang, Hye Ryoung Lee, Yingying Lu, Yongcai Qiu, Steven Chu, and Yi Cui (2016) “Nanofiber Air Filters with High-Temperature Stability for Efficient PM2.5 Removal from the Pollution Sources” Nano Letters doi: 10.1021/acs.nanolett.6b00771