Nanofibre filters in aerosol filtration

Performance of a fibrous filter is mainly dependent on the size of the aerosolparticles, the velocity of the airflow, the size of the fibres and the packing density of thefilter media. High-performance filters useful for micro- and nanoparticle separationrequire ultra-fine fibres with diameters less than one micrometer. The use of nanofibresimproves filtration performance, especially in HEPA (High Efficiency Particulate Air)and ULPA (Ultra Low Penetration Air) filtration, since the ability of the nanofibres tocapture sub-micron particles is better than that of the larger fibres used in conventionalfilters. Existing HEPA and ULPA filters are mostly made of glass fibre paper. Otherviable nanoporous materials are ceramic membranes, expanded polytetrafluoro ethylene,and, of course, polymeric nanofibres. The use of the polymeric nanofibres instead ofglass fibres has many advantages. Electrospinning is a method that can be used in the production of polymericnanofibres. The small fibre diameter, small pore size, and high surface area of theelectrospun nanofibre web are properties that are advantageous for filtration applications. Electrospun nanofibre webs are proven to be effective in stopping aerosol particles, andsimulation of unsteady-state filtration has endorsed the efficiency of nanofibre filtrationmedia. The dramatic increase in filtration efficiency due to a thin coating layer ofelectrospun fibres on conventional filter media can be seen with a relatively small oralmost immeasurable decrease in permeability. Another interesting feature of theelectrospun webs that are advantageous to filtration applications is the possibility to addfunctionality to electrospun fibres.This chapter is an abridged and revised version of the PhD thesis of Pirjo Heikkilä,"Nanostructured Fibre Composites and Materials for Air Filtration" [1]. The thesiscontains an extended summary that consists of a literature review and an experimentalsection, and it is based on six scientific papers published in refereed journals [2-7].