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Core–Shell Nanofibrous Materials with High Particulate Matter Removal Efficiencies and Thermally Triggered Flame Retardant Properties

Dust filtration is a crucial process for industrial waste gas treatment. Great efforts have been devoted to improve the performance of dust filtration filters both in industrial and fundamental research. Conventional air-filtering materials are limited by three key issues: (1) Low filtration efficiency, especially for particulate matter (PM) below 1 μm; (2) large air pressure drops across the filter, which require a high energy input to overcome; and (3) safety hazards such as dust explosions and fires. Here, we have developed a “smart” multifunctional material which can capture PM with high efficiency and an extremely low pressure drop, while possessing a flame retardant design. This multifunctionality is achieved through a core–shell nanofiber design with the polar polymer Nylon-6 as the shell and the flame retardant triphenyl phosphate (TPP) as the core. At 80% optical transmittance, the multifunctional materials showed capture efficiency of 99.00% for PM2.5 and >99.50% for PM10–2.5, with a pressure drop of only 0.25 kPa (0.2% of atmospheric pressure) at a flow rate of 0.5 m s–1. Moreover, during direct ignition tests, the multifunctional materials showed extraordinary flame retardation; the self-extinguishing time of the filtrate-contaminated filter is nearly instantaneous (0 s/g) compared to 150 s/g for unmodified Nylon-6.


Table of Contents
Page Experimental Procedures 2 Figure S1 3 Figure S2 4 Figure S3 4 Figure S4 4 Table S1 5 Figure S5 5 Figure  To fabricate the TPP@Nylon-6 fiber by electrospinning, TPP (Sigma-Aldrich) and Nylon-6 (Sigma-Aldrich) were dissolved in formic acid (Baler Analyzed) with a concentration of 10 wt% and 20 wt%, respectively. A transparent solution was obtained by vigorous stirring. Then the solution was placed into a syringe with a stainless-steel needle. We used a commercial highvoltage source (Gamma High Voltage Research, model ES-30P-5W) for electrospinning. The solution was pumped out of the needle tip using a syringe pump (KD Scientific) and ejected to the rough side of the commercial copper foil (Pred Materials), which was grounded to collect electrospun nanofibers. The applied potential was 15 kV, the pump rate was 0.06 ml/h and the needle-collector distance was 15 cm. Characterization.
SEM were taken on an FEI XL30 Sirion. X-ray photoelectron spectroscopy (XPS) was carried out on SSI S-Probe Monochromatized, Al Kα radiation at 1486 eV. TGA was performed on a TA Instrument Q500 with a heating rate of 5.00 °C/min. The measurement was performed under simulated air atmosphere (20% O2 + 80% Ar). DSC was measured on TA Instrument Q2000 with a heating rate of 5.00 °C/min under nitrogen.
Optical transmittance measurement. The transmittance was measured used a xenon lamp (69911, Newport) as the light source, coupled with a monochromator (74125, Newport) to control the wavelengths. An iris was used to trim the beam size to ~5 mm×5 mm before entering an integrating sphere (Newport) for the transmittance measurements. A photodetector (70356, Newport) was inserted into one of the ports of integrating sphere. The photodiode is connected to lock-in radiometry system (70100 Merlin, Newport) for photocurrent measurements. The samples were placed in front of the integrating sphere; therefore, both specular transmittance 3 and diffuse transmittance were included. For air filters composed of nanofiber film and mesh, a clean mesh with the same geometry was used as a reference. The transmittance spectrum was then weighted by AM1.5 solar spectrum from 400 to 800 nm to obtain the average transmittance.
PM generation and efficiency measurement. The PM particles were generated from by burning incense. The incense smoke PM particles have a wide size distribution from <300 nm to >10 μm, with the majority of particles being <1 μm. The inflow concentration was controlled by diluting the smoke PM by air to a hazardous pollution level equivalent to the PM2.5 index >300.
PM particle number concentration was detected with and without filters by a particle counter (CEM) and the removal efficiency was calculated by comparing the number concentration before and after filtration. The pressure drop was measured by a differential pressure gauge (EM201B, UEi test instrument). Unless mentioned, the wind velocity used in the efficiency test was 0.5 m/s and the humidity was 30%.
Flame retardant properties testing. For the testing of the flame retardant properties in Figure 4, the filters was wetted by preweighed vegetable oil (~0.2 g) and vertically mounted, then ignited by a lighter. The lighter was removed as the filter started to combust.

Figure S1
XPS of the TPP@Nylon-6 air filter before and after etching.
4 Figure S2 A digital photo of the TPP reagents, showing that the TPP are prone to form flakelike crystals.

Figure S3
Schematic showing the transfer of freestanding electrospun nanofiber film.

Figure S4
Schematic set-up for the measurement of the air pressure difference across the air filter, i. e. pressure drop (DP).

Table S1
Performance summary of TPP@Nylon-6 flame retardant air filters compared with commercial air filters.

Figure S5
SEM of the TPP@Nylon-6 air filter after long term filtration. Figure S6 DSC curves of the a) TPP and b) Nylon-6.