Electrospinning of several polyamides, PA6, PA66, PA612, PA614, PA1012, and PA1014, having different chain compositions and lengths of diacid and diamine segments, was demonstrated. Electrospinnability and fiber production rate of these polyamides were evaluated. Electrospun fibers were characterized with regard to their use in air filtration by measuring fiber diameter and filtration efficiency of fiber coating. Longer nonpolar chain segments of higher polyamides could in theory indicate higher dielectricity compared to PA6 and PA66, which would be an advantage in filtration applications. The solubility in polar formic acid and electrospinnability of higher polyamides, on the other hand, were clearly impaired with increased length of chain segments. Hence, PA66 is our best choice, and PA612 and PA6 our second options for commercial filtration applications if fiber electrospinnability, production rate, fiber diameter, and its distribution are concerned. Filtration efficiency of more than 95% of the particles having a diameter of 0.16 lm and above was achieved with 0.5 g/m 2 coating of PA66 nanofibers. Further increase in coating weight mainly increased the pressure drop to an unusable range without a significant further improvement of filtration efficiency.Like reviewed earlier some simple polyamides, such as PA6 and PA12, and some copolyamides such as PA66 and PA46 are verifiably applicable in electrospinning. Still, we can suppose that the structure of the polyamide FIG. 2. Electrospun PA66 fibers produced from solution having concentrations of (a) 6%, (b) 9%, (c) 12%, and (d) 15%. FIG. 4. Fiber diameters with standard deviation and fiber production rates of different polyamides. FIG. 5. X-ray diffraction spectrums of (a) newly electrospun PA6 sample and (b) after 2 months ageing.

Electrospinning was used to produce lightweight coatings of polyamide-66 nanofibers on different fibrous substrates. Filtration efficiency of coated and uncoated samples and strain capacity of the nanofiber layer were tested. Filtration efficiency clearly increased even with the lowest amount of nanofibers (0.02 g/m2) when compared to uncoated samples, and with the highest nanofiber coating weight (0.5 g/m2) the filtration efficiency of submicron aerosol particles was over 90%. Elongation capacity of the polyamide nanofiber layer was found to be insufficient to be used with elastic materials. The polyamide nanofiber layer is, therefore, suitable to be used with relatively rigid substrate e.g. spunbond nonwoven and at least thicker nanofiber layers are advisable to be protected with cover layer, because the nanofiber layer also delaminates easily from the textile surface.

Near traffic routes and urban areas, the outdoor air particle number concentration is typically dominated by ultrafine particles. These particles can enter into the nearby buildings affecting the human exposure on ultrafine particles indoors. In this study, we demonstrate an aerosol generation system which mimics the characteristic traffic related aerosol. The aerosol generation system was used to determine the size-resolved particle filtration efficiencies of five typical commercial filters in the particle diameter range of 1.3-240 nm. Two different HEPA filters were observed to be efficient in all particle sizes. A fibrous filter (F7) was efficient at small particle sizes representing the nucleation mode of traffic related aerosol, but its efficiency decreased down to 60% with the increasing particle size. In contrast, the filtration efficiency of an electrostatic precipitator (ESP) increased as a function of the particle size, being more efficient for the soot mode of traffic related aerosol than for the nucleation mode. An electret filter with a charger was relatively efficient (filtration efficiency >85%) at all the observed particle sizes. The HEPA, F7 and electret filters were found to practically remove the particles/nanoclusters smaller than 3 nm. All in all, the filtration efficiencies were observed to be strongly dependent on the particle size and significant differences were found between different filters. Based on these results, we suggest that the particulate filter test standards should be extended to cover the ultrafine particles, which dominate the particle concentrations in outdoor air and are hazardous for public health. EDITOR Jing WangCONTACT Panu Karjalainen

No abstract

Pressure drop evolution during filtration of bimodal aerosol was studied experimentally. A low-pressure drop pre-filter upstream of the actual collection filter was demonstrated to significantly reduce the pressure drop growth rate, when the aerosol is dominated by coarse particles. The pressure drop evolution during depthfiltration, that took place mainly in a pre-filter, could be predicted by adding up the separately measured contributions from the unimodal fine and coarse aerosols. However, the cake filtration (prefilter was not used) of coarse particles alone resulted in a faster clogging rate as compared to the same amount of coarse particles accompanied with fine particles (mass ratio coarse:fine 3:1). Apparently, fine particles deposited on coarse particles affect their surface properties and thus the porosity of the cake formed.