Optimizing stretching conditions in fabrication of PTFE hollow fiber membrane for performance improvement in membrane distillation

PTFE is one of the most approved hydrophobic materials for MD membranes. However, PTFE membrane was researched and applied on MD insufficiently due to its complex fabrication process. In this work, stretching conditions were comprehensively investigated to form a membrane with high flux and salt rejection. It was found that stretching led to increased cracks at an early stage, and turned to lead to the elongation of fibrils and physical dimension shrinkage of hollow fibers. A porosity prediction model was established to evaluate the improvement of porosity. It was found that a low stretching temperature was helpful for pore size control and hence a relative high stretching ratio can be carried out to achieve a higher porosity. Therefore high flux and rejection was achieved due to the higher porosity and well controlled pore size. Stretching rate of 30%/s gave rise to the most concentrated pore size distribution. NaCl rejection rate was high up to 99.99% for the membrane fabricated at a stretching ratio of 2.4 and a stretching temperature of 40 degrees C.