Green preparation of polyvinylidene fluoride microfiltration membranes via thermally indueced phase separation method using diphenyl carbonate and diphenyl ketone as diluents

Diphenyl carbonate (DPC) and diphenyl ketone (DPK) were used as diluents to prepare microporous polyvinylidene fluoride (PVDF) membranes via thermally induced phase-separation (TIPS) method. The liquid - liquid phase-separation phenomena were found in PVDF/DPC and PVDF/DPK systems during the thermally induced phase-separation processes. The monotectic points of PVDF/DPC and PVDF/DPK systems appeared at PVDF concentration approximately 30 and 56 %(wt), respectively. This suggested that PVDF membrane with a bicontinuous or cellular structure could be obtained when the PVDF concentration was less than the point values. The effects of polymer concentration and quenching temperature on the membrane structure, porosity, and tensile strength were also investigated. The results showed that low polymer concentration and high quenching temperature led to a large pore size membrane. The tensile strength of the membrane increased with the increase of the polymer concentration, but the porosity of the membrane decreased. On condition that the PVDF concentration was higher than monotectic points, only polymer crystallization occurred and a typical compact spherulitic structures were obtained. Moreover, a binary diluent of DPK and secondary solvent such as 1,2-propylene glycol (PG) was used to replace the single diluent in TIPS process. The phase diagram for the PVDF/binary diluent of DPK and PG system was measured in the range of the PG/DPK mass ratio changing from 0 to 2/3. Then the effects of the PG/DPK mass ratio and the PVDF concentration on membrane cross-section structures and tensile strength were also investigated. The results showed that the addition of PG brought about a shift of the cloud point curve to a higher temperature and the extension of the liquid-liquid phase separation region to a higher polymer concentration. Therefore a bicontinuous cross-section structure was obtained when the PG/DPK mass ratio was 3/7 and the polymer concentration was 30 wt%. As an increase of the PG/DPK mass ratio, the tensile strength increased gradually at a fixed PVDF concentration. Moreover, for the same PG/DPK mass ratio, the cross-section microstructure changed from a bicontinuous or a cellular structure to a spherulitic structure, and the tensile strength increased drastically as the polymer concentration increased from 20 wt% to 50 wt%. According to the properties of diluents, the industrial scale production line with a twin-screw extruder of microporous PVDF hollow fiber membranes was established and a series of different type hollow fiber membranes were prepared for the corresponding applications of pressure-driven modulus, submersed modulus and membrane bio-reactor modulus. At the same time, the full recycle of diluents was almostly actualized by using a series of unit opertaions such as extraction, distillation, delamination, sedimentation and drying processes. It means there is no waste emission in the preparation of PVDF membranes or it is a green process.