Effect Of Surfactants On The Structure And Performance Of Flat-sheet PVDF Ultrafiltration Membranes

In this paper, flat-sheet poly(vinylidene fluoride) ultrafiltration membranes were prepared via nonsolvent induces phase separation (NIPS) method with mixtures of different surfactants and second additive (including H2O, LiCl and PVP) as mixed additives. The effects of surfactant structure and its HLB value on the structure and performance of PVDF membranes were studied. The most optimized surfactant was screened. Then the influence of surfactant concentration and second additives on the structure and performance of PVDF membranes was further studied. Finally, the formation mechanism of PVDF membrane structure was investigated.Firstly, surfactants with different structures were compared in the structure and performance of resultant PVDF membranes, surfactants used in this paper consist of three anionic surfactants, two ampholytic surfactants and eight nonionic ones, and the mixtures of surfactants and second additives (H2O, LiCl) were used as porogen in the preparation of PVDF membranes. The PVDF casting solution containing anionic surfactants was too unstable to have a practicability for prepared PVDF membranes. The Pure Water Flux (PWF) of PVDF membranes using ampholytic surfactant as additive was high, while the selectivity declined greatly, the combination performance didn’t improve obviously. For nonionic surfactants, Span series surfactant is lipophilic, and PVDF membranes containing them showed poor PWF and porosity. The performance improvement of hydrophilic surfactant on the PVDF membrane was the best:The performance of the PVDF membrane using AEO-9as additive is in intermediate position, while PVDF membranes containing OP-10and Tween series performed obvious macrovoid structure, high porosity and PWF and a better retention of dextran (100,000Mw).Secondly, OP-10and Tween80were selected to investigate the effect of their contents on PVDF membranes. The mixture of selected surfactant (OP-10or Tween80) and the second additive (H2O, LiCl, PVP or Blank) was added to the PVDF casting solution, PVDF membranes were prepared by NIPS and their structure and performance were studied by changing surfactant concentration.For all casting solutions, the viscosity increased and the surface tension dropped due to the addition of surfactant, and the delayed demixing process was changed into the instantaneous demixing process, the precipitation rate get improved. As for the structure and performance of PVDF membranes, the addition of OP-10or Tween80caused many changes:on one hand, the number and size of macrovoids in supporting layer would get improved; on the other hand, most pores on the membrane surface would be enlarged. Under the premise of maintaining a certain rejection, the PWF, hydrophilicity, porosity and maximum pore size of PVDF membrane were greatly improved, meanwhile, the mechanical property decreased and the membrane thickness increased because of density declining of PVDF membrane. Besides, if we compare PVDF membranes prepared from the same concentration of additive, the effect of Tween80and OP-10on the performance of PVDF membranes is more significant.When the concentration of OP-10or Tween80was2%, the performance change of PVDF is the most obvious, and OP-10is better than Tween80on the improvement. For example, in the PVDF/DMAc/OP-10/LiCl system membranes, compared with OP-10(1.5) system, The PWF of membrane containing OP-10(2) was improved by80%.Finally, PEGs of different WM were employed as second additives, and PVDF membranes prepared from PVDF/DMAc/OP-10/PEG system were investigated. The improvement effect of surfactant OP-10on the structure and performance of PVDF membranres got proved.