Characterization of Polymeric Nonwovens Using Porosimetry, Porometry and X-Ray Computed Tomography

The filtration efficiency of microfiltration (MF) and ultrafiltration (UF) processes is largely dependent on the properties of the filtration medium used. Membranes, more specifically, nonwovens are commonly used as filters in MF and UF processes. Performance of nonwovens as filters is dependent on the properties of their pore structure (i.e. pore diameter, constricted pore size, and porosity). Currently, the characterization tools available for determining these structural metrics are underdeveloped for soft materials.  In this study, several methods for evaluating pore size, pore diameter distribution, and porosity using polymeric nonwovens are investigated. Using both commercial polyester (PET) nonwovens and electrospun nanofibers, pore diameter distributions of nonwovens have been determined using two porosimetry techniques: mercury intrusion porosimetry (MIP) and liquid extrusion porosimetry (LEP).  Analytical results from porosimetry were compared to porosity estimates obtained from x-ray computed tomography (XCT) images and to predictions using a widely-adopted model. Constricted pore size, or pore throat diameter, was also measured using porometry.  In general, LEP resulted in higher measured porosities than MIP, likely due to compaction of the nonwoven in the latter.  Electrospun materials exhibited similar or slightly lower porosities when compared to PET nonwovens, but their constricted pore diameters were orders of magnitude smaller due to their smaller fiber sizes. The use of XCT was limited to the PET nonwovens due to limited resolution but it was shown to have potential for visualizing the 3-D structures of polymeric materials.