Saptarshi Chattopadhyay, PhD

Postdoctoral Associate, Rutledge Research group, MIT

Dr. Saptarshi Chattopadhyay (Sap) is currently working as Postdoctoral Associate in Rutledge Research group at MIT.

Dr. Chattopadhyay holds a PhD from IIT Bombay (India), where he worked on aerosol properties of nanometer sized liposome targeted for pulmonary drug delivery. Since, October 2012 his research focus is to develop electrospun polymeric nanofibers as adsorbents in air filter media.

His current research interests include Air Filtration, Gas Adsorption, Nonwoven and Cellulose

Cellulose Acetate Micro-and Nanofibers as Particle Filter Media

Abstract: Filtration of airborne particulate matter is essential in air purifiers, cigarette filters, respiratory protections and air samplers. Among various air filtration technologies, nanofibrous media are of interest, since these materials demonstrate high efficiencies to remove sub-micrometer aerosols. In general, studies have shown that nanofibers can exhibit higher filtration efficiencies and lower maximum penetrating particle size, or MPPS, while microfibers have the merit of low pressure drop and good mechanical integrity. Some commercial air filters are composed of superficial layers of nanofibers on a microfibrous support or have wide distributions of fiber diameter so as to enjoy the advantages pertinent to both types of fibers present in the filter medium. Although studies have been reported regarding the capabilities of such composite filters, little has been done to advance our understanding of the interaction of aerosol with nanofibers and microfibers in filter media.

In this study, 13-mm filter discs comprising nanofibers (NF) were prepared using a commercial Nanospider_ unit by Elmarco, Inc, which is based on free surface electrospinning. Three different precursor solutions having polymer concentrations of 11 wt% cellulose acetate (CA), 15 wt% CA and 17 wt% CA were used to generate fibers with median diameters of 0.1, 0.33 and 0.5 _m, respectively. For microfiber filter media, a commercially available cellulose acetate tow was compressed into sheets (denoted _CFc_), and 13-mm filter discs were punched out. The tow fibers had a trilobular geometry, with each lobe extending 10-15 _m from the fiber axis. Composite filter sheets (denoted _CFcm_) were made using NF (15 wt% CA) deposited directly onto CFc. Three types of CFcm designs were tested, which differed by either variation in the thickness of NF deposition or symmetry of deposition on the CFc. The NF thickness was manipulated by varying the electrospinning time. Symmetry was varied by coating NF on either one or both sides of the microfiber (CFc) filter disc. A commercial glass fiber filter and CFc, having known filtration properties, were used as controls. Aerosol tests were performed using atomized polydispersed salt particles as the challenge aerosol, and real time measurement of particle number concentration and pressure drop using a scanning mobility spectrometer and pressure transducer, respectively. All measurements were made using a commercial 13 mm filter holder that was maintained at room temperature and RH < 5%. A constant face velocity of 45 cm/s was applied for all tests, and filtration performances were evaluated in terms of percentage particle penetration and pressure drop across the filters.
Results show that the greatest particle penetration was recorded for CFc (60_11%), while the least penetrations (0.06-0.01%) were observed for GF and 11 wt% CA, with the latter being slightly less. Increase in particle penetration was observed with NF fiber diameter, which is consistent with aerosol filtration theory. The 15 wt% CA and 17 wt% CA had 0.2 and 2% penetration respectively. A reduction in MPPS was observed from 70 to 50 nm with a decrease in fiber diameter, which is also in agreement with calculated prediction. For CFcm, the layer of nanofibers resulted in a reduction of both particle penetration (from 60 to 30%) and MPPS (from 300 to 100 nm). It was evident that the composite filter had significant advantage due to the presence of nanofibers, even if the thickness of the nanofiber deposit was negligible compared to the total thickness of clean microfibers. The pressure drop for CFc and CFcm were 0.2 and 0.35 kPa, respectively; thus, elevation in pressure drop due to superficial nanofiber layer was small. The pressure drops ranged from 5 down to 1.5 kPa for NFs and GF. Although the measured solidities of CFc and CFcm were 3-4 times larger than NFs, the lower fiber diameter showed higher pressure drop due to the large specific surface area available for air drag. This study will further address the particle clogging in different types of filter media.