Air permeability is the most important characteristic for Nonwoven fabric used in the manufacture of filters. The air permeability is determined by measuring the air flow rate through the sample under a certain pressure difference between the surface of the fabric. In this research sixteen different types of needle punched nonwoven fabrics were produced by varying the number of layer and varying the punch density during the needle punching process. Subsequently, the structure parameters (thickness, area weight, porosity, and fiber diameter of the specimens were measured or calculated. The pore size and its distribution in the nonwoven fabrics were measured by image analysis. The assessment of air permeability and its relationship with punch density have been reported. The results showed that with the increase in needling density the air permeability decreases .However, punch density have no the significance effect on air permeability. Nonwoven filter media are porous media and the theories concerning flow through porous media can be applied. Darcy's law is one of the fundamental equations concerning flow through a porous medium. For predicting the air permeability of needle-punched nonwoven fabric, modified Darcy's law was used. A comparison between theoretical and experimental results showed a good agreement. This equation can be used to porosities up to 0.90 The image analysis techniques can be used to identify the pore characterization of porous materials. Area (A) and Perimeter (P) are two basic size descriptors of a pore. Hydrodynamic Diameter (D h ) is defined: D h =4A/P. The results of this work showed that the punch density have significance effect on surface pore diameters. Image analysis was used for measurement of three dimensional porosity. Air permeability showed a good linear correlation with the volume porosity of the fabric with a R-square of 0.88, Thus, this method can be used for estimation of the porosity of the nonwovens with an acceptable accuracy . Key Words Needle-punched nonwoven, Air permeability, Pore diameter distribution, Darcy's law, Image analysis.
