Porosity measurements by a gas penetration method and other techniques applied to membrane characterization

In this paper several methods that can be used to determine membrane porosities are studied. Apparent densities, water pycnometry, Hg porosimetry and the gas penetration technique can be considered direct methods since no specific pore geometry needs to be assumed, whereas computerized analysis of scanning electron microscopy images and air-liquid displacement techniques are indirect methods since pores are usually assumed to have a specific shape. Both direct and indirect methods for polymeric and very regular inorganic membranes, Cyclopore and Anopore filters respectively, are compared. Other porous films with more complex structures such as asymmetric or composite membranes are also analysed, namely polymeric ultrafiltration membranes made by Gelman and Desalination, and inorganic filters made by Tami industries. As a consequence of a detailed comparison of the porosities obtained, we conclude that the gas penetration volumetry is preferable when we aim to obtain the actual void volume fraction. This method is highly reproducible and gives precise results which allow us to detect very small pores without any interaction with the structure due to the inertness of He and the low pressures applied.