(817b) Development of Carbon Nanofibers By Electrospinning for Aerosol Filtration

Among various air filtration technologies, nanofibrous media are of interest, since theoretical calculations predict that these materials have high efficiencies to remove sub-micrometer aerosol particles with relatively lower energy consumption. Conventional fibrous filters made of nanofibers exhibit a local minimum of fractional collection efficiency, which is called the most penetrating particle size (MPPS). This is usually observed for particles 0.1-0.5 µm in diameter. Increasing filtration efficiency in this critical size range could be targeted by altering fiber composition, size and morphology. A method called “electrospinning” to prepare fibers with sub-micrometer diameters has received considerable attention in aerosol filtration research due to the reduction in filter mass required to obtain nanoparticle filtration efficiencies comparable to currently available commercial filters. Recently, electrospun carbon fibers prepared by carbonization of precursors like polyacrylonitrile, polycarbosilane, polyvinylidene fluoride and cellulose acetate fibers have been reported for various applications. During carbonization, the fiber structure remained unaffected but the mean diameter decreased by a factor of about 3-4. Additionally, properties like high electrical conductivity, wettability and specific surface area observed in such studies may offer other advantages for air filtration performance.

To this end, we report the development of continuous cellulose acetate and carbon fibers with diameters in the range of 0.1-1 µm and large surface areas (300-3000 m²/g) as high efficiency particle filtration media. Important filter properties such as aerosol removal efficiency at low pressure drop are studied vis-à-vis electrospinning and carbonization parameters. For the current study, engineered nanoparticles comprising monodisperse oil droplets of di-2-ethyl hexyl sebacate (0.09 to 0.35 µm in physical diameter), are generated using a condensation monodisperse aerosol generator (CMAG). This aerosolizer generates a high number concentration (10⁷ counts/cc) of spherical and charge-neutral particles by condensing oil vapor onto sodium chloride nuclei, based on the Sinclair-LaMer principle. The size distribution, number concentration, relative humidity and temperature of the aerosol were customized in an aerosol diluter before exposure to the test filter, which is housed in a standard 13-mm aerosol filter holder. The face velocity is regulated (1-100 cm/s) by a sump operating downstream of the filter assembly, and the pressure drop across the filter is monitored using a pressure transducer. The real time number concentrations of aerosol are measured using a scanning mobility particle sizer (SMPS), which measures the electrical mobility of aerosol in the size range 0.02-0.8 µm. The ratio of particle concentration, with and without the filter material and the geometric mean diameter of the particles that pass through the filter material are defined as ‘particle penetration’ and MPPS, respectively. Initial results indicate that the diameter (0.1-1 µm) and cross-section shape (ribbon and cylindrical) of electrospun cellulose acetate fibers can be controlled through selection of polymer solution and electrospinning parameters. The measured filtration efficiency for fibers having ribbon and cylindrical cross sections were found to be comparable, but the filters of cylindrical fibers were easier to handle and compact. The percentage retention by these cylindrical fiber filters was altered when solutions having different concentrations of cellulose acetate were used. Solutions with lower polymer concentration (15 and 17 wt %), which are anticipated to generate smaller diameter fibers, had 1.3 times higher percentage retention than counterparts formed from solutions of 19 wt % polymer. Likewise, the MPPS of filters decreased (from 90 nm to 77 nm) with decreasing solution concentration. In general, this seems to corroborate the understanding that smaller fiber diameter increases the filtration efficiency and decreases the MPPS. However, the anticipated decrease in fiber diameter was not evident through scanning electron micrographs, which showed that the fibers vary from 0.1-0.5 µm in diameter. Studies are underway to investigate other properties like surface area and solidity that could affect filtration properties. In addition, the filtration capabilities of carbonized polymeric electrospun will be reported for their possible application in high efficiency aerosol filters.