PHYSICAL AND CHEMICAL MODIFICATION OF POLYMER TO IMPROVE HYDROPHOBICTY IN MEMBRANE DISTILLATION

Abstract

As a water treatment technology, the membrane distillation (MD) method which can operate at lower temperature than reverse osmosis (RO) and can recover concentrated water generated from RO with high recovery rate has been studied. Membrane distillation is a technology that allows the vaporized water pass through membrane pores and collect pure water vapor so that many types of research has been studied. In this study, a hollow fiber was made using Poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE), not usually used material, and compared with Poly(vinylidene fluoride) (PVDF). There is four type of membrane distillation methods. Among them, the vacuum membrane distillation method which has the highest flux method was studied in this research. In the case of PVDF-CTFE, it was confirmed that it has a higher flux because it has a macrovoids at lumen side than PVDF. However, since the mechanical strength is weak, there is breaking problem when operating long term experiment or high-pressure experiment. To solve the breaking problem, the mechanical strength was increased by using thermally induced phase separation (TIPs) method, which is a high-concentration spinning method, or by spinning hollow fibers into a dual-layer structure in which PVDF is spun out as a support layer. In the case of TIPS, the mechanical strength was increased, but the flux was found to be low. As can be seen from the cross-sectional images of the hollow fiber, in the case of the dual-layer membrane, the sponge-like structure at the middle was eliminated, and the flux was improved. Furthermore, to prevent the wetting phenomenon of membranes, physical and chemical modification experiments were carried out to increase hydrophobicity. First, a hydrophobic hollow fiber was prepared by physically blending with Polytetrafluoroethylene (PTFE), which is a highly hydrophobic material, in a dope solution, and physical properties were confirmed. PTFE was added as an additive to evaluate the structural change and performance. As a method of increasing the hydrophobicity using the chemical grafting method, attaching a hydrophobic chemical to the surface of the polymer using the atom transfer radical polymerization (ATRP) method was used. The chemical method was used to evaluate the performance by confirming the difference according to the length of the material with different reaction time. Both methods were confirmed physical property changes via FTIR and confirmed the increase in hydrophobicity through contact angle and liquid entry pressure (LEP). Through the physical blending method, it was confirmed that the contact angle was improved by the addition of the hydrophobic additive. Also, the cross-section of the hollow fiber was confirmed by SEM, confirming that the pores of the membrane became larger at lumen side. However, if more than 15 wt% of PTFE is contained, the sponge-like structure is formed again in the middle of the hollow fiber, and it becomes thicker depending on the PTFE content. As a result of this structural change, the flux was affected, and the flux was improved up to 10 wt%. Through ATRP, which is a chemical modification method, it was confirmed that a new peak of FTIR and XPS appeared on the polymer by grafting hydrophobic material. After grafting, membrane structural change was checked by SEM that there was no change of pore, the hydrophobicity was confirmed by the contact angle that there was increasing contact angle value, the flux was confirmed through the membrane distillation method that was no improvement. Increasing of hydrophobicity contributes to preventing the wetting problem that checked through LEP. With increasing reaction time, LEP value increased until 5 bars. Finally, the performance evaluation was carried out through the hollow fiber obtained by combining the two methods. The 10 wt% PTFE blended hollow fiber with the highest flux was prepared, and ATRP was carried out for 25 hours. In the case of the hollow fiber obtained, the flux is maintained as in the case of blending 10 wt% of PTFE, and the LEP is further increased which can be confirmed that the wetting phenomenon can be prevented.