Influences Of Ultrasonic Cavitation On The Nonsolvent Phase Inversion Membrane

In recent years, the membrane industry has developed rapidly. On the market, there is a large demand for high-performance PVDF membrane. To this end, we need to find easy and quick preparation method. The most mature method to prepare PVDF membrane is nonsolvent phase inversion method. And a variety of factors that influence the structure and properties of the membrane have already been studied extensively. It is a new way to prepare membrane by introducing a specific field (such as ultrasonic field, electric field, magnetic field, etc.) as necessary in the coagulation bath. By controlling the intensity of external field, we can control system kinetics and thermodynamics rates of phase transformation to get the desired asymmetric membrane morphology. In particular, the introduction of ultrasonic field in the coagulation bath has not been studied. This paper discusses the regulation mechanism of ultrasonic cavitation mechanical effect on the membrane structure by phase inversion process. 1. For PVDF/H2O/solvent system using four different polar aprotic solvents, it can successfully obtain some perfect flat PVDF membranes by nonsolvent phase inversion method in the ultrasonic baths. Because ultrasonic vibration can control the exchange speed between nonsolvent and solvent, it can be used to control the phase transformation rate by ultrasound. Meanwhile PVDF membrane morphology and crystal structures can be also adjusted. An uniform transparent sponge-like layer, connectivity macro-porous sublayer and a thin dense top-surface of membrane can be prepared. The results show that the largest values of RBH2O flux and water flux were respectively84.5L-m-2.h-1and129.8L.m-2.h-1to PVDF/NMP/H2O system. It is significantly improved compared with pristine membrane (58.2L.m-2.h-1and80.6L.m-2.h-1), while the rejection could be kept at a high level (80.0%). Besides, it was proved that the β crystalline phase was partly converted to a crystalline phase in the ultrasound assisted phase inversion process.2. Different contents of Potassium iodide (KI) sensitive to ultrasonic cavitation were added in the casting solution. This paper studied the exchange laws of KI/solvent/nonsolvent under different ultrasonic fields. The nonsolvent phase inversion rate can be controlled by KI contents and ultrasonic intensity. Membrane structures were studied by scanning electron microscope (SEM). The result showed that more pores appeared on the bottom surface with increasing the KI contents or ultrasonic intensity. At the same time, more homogeneous long and straight fiingerlike structures were found throughout the entire cross-sections. The permeability and rejection of PVDF membrane were also studied. It showed that the flux of membrane can be largely improved by single or double-frequency ultrasound. Adding more KI, membrane RB-H2O flux and rejection to RB could be changed more significantly with increasing ultrasonic power. After adding11.5wt%KI, the a crystalline phase is completely converted to β crystalline phase.