Shrinkage, defect and membrane distillation performance of composite PVDF membranes

Abstract

This study addresses the importance of non-woven support (NWS) during the preparation of polyvinylidene fluoride (PVDF) membranes for membrane distillation (MD). Five different NWSs were investigated as substrate materials for composite membranes. Additionally, a series of unsupported membranes were also tested. Unsupported membranes were found vulnerable to shrinkage that constricted and diminished their pores, rendering them unsuitable for MD. The NWS provides sufficient strength for a composite membrane, thanks to its stiffness, thus limiting membrane shrinkage which occurred severely in unsupported membranes. However, some composite membranes were also found vulnerable to shrinkage and defect when cast on a weak NWS. Shrinkage level worsened at higher casting thicknesses, higher polymer concentrations and when the NWS was pre-wetted before casting. Overall, our findings emphasize the importance of NWS selection and casting thickness optimization. High elastic modulus is the first criteria for NWS selection to avoid membrane shrinkage and defects. Optimizing the casting thickness is also vital for obtaining defect free composite membranes with high MD flux. Since MD flux is inversely proportional to net membrane thickness, low casting thickness increases the flux. However, thin casting imposes vulnerability with respect to membrane defect which can lead to membrane wetting.