Ultrasonic detection of defects in highly porous polymeric membranes

Microfiltration (MF) and ultrafiltration (UF) membranes are used extensively for sterile filtration and virus retention applications that necessitate stringent defect control during their manufacture. Current protocols used to detect structural defects during membrane fabrication are operator-dependent, resulting in a slow and subjective process with lowered yields. Moreover, several types of membrane defects are usually not identified until after the final membrane cartridge is assembled via expensive post-processing. We describe the use of an ultrasonic technique to non-invasively detect a variety of defects in a polymeric membrane including pinholes (fully and partially penetrating), sub-surface (macrovoids) and surface (scratches) defects. Ultrasonic techniques can be used to detect both through-thickness and partial penetration defects in polymeric membranes. Ultrasonic reflectometry (UR) has been employed to successfully detect defects ranging in size from ∼ 600 μm for relatively large manually generated pinholes to much smaller macrovoid defects (∼30 μm) created during membrane formation via the phase-inversion process. The experimental demonstration of the detectability of these defects is particularly important given the complexity of wave propagation in highly porous materials with interconnected pores.