The Study Of Properties Of Poly(Vinylidene Fluoride) Membrane Material Modified By Cellulose Nanofibrils

In this paper, cellulose nanofibrils (CNF) and silane coupling agent KH-540 modified cellulose nanofibrils (K-CNF) were prepared based on the material of cellulose pulp. Then CNF and K-CNF were blended with poly(vinylidene fluoride) (PVDF) respectively to prepare CNF/PVDF membrane and K-CNF/PVDF membrane using a Loeb-Sourirajan (L-S) phase inversion process in order to improve the permeability, hydrophilic property, antifouling property and mechanical property of PVDF membrane. The properties of CNF/PVDF membrane and K-CNF/PVDF membrane were investigated. The milk protein solution was ultrafiltrated by both of CNF/PVDF membrane and K-CNF/PVDF membrane. The optimum technological conditions of milk ultrafiltration were explored. The effects of several membrane cleaning methods were investigated. The membrane forming mechanism was explored from angles of thermoldynamics and kinetics. The rheological performances of casting solutions were characterized by rotational rheometer.CNF presents a rod-like structure. The diameter of CNF ranges from 20 to 50 nm and its width ranges from 200 to 500 nm. The crystallinity of CNF is higher than cellulose pulp. The CNF was blended with PVDF to prepare CNF/PVDF membrane. The optimum preparation conditions is:PVDF content is 16 wt%, CNF content is 0.5 wt%, the solvent is DMAc, pore-forming agent PVP K30 content is 3 wt%, the coagulation bath is deionized water. The membrane performances of CNF/PVDF membrane prepared under the optimum preparation conditions were all improved compared with the properties of pure PVDF membrane. Compared to the pure PVDF membrane, the permeability of CNF/PVDF membrane increased 72.0%, the tensile strength and elongation-at-break of CNF/PVDF membrane increased 18.2% and 36.4%, respectively, the contact angle of CNF/PVDF membrane decreased from 81.4° to 66.9°, attenuation coefficient of CNF/PVDF membrane decreased from 44.59% to 31.65%.K-CNF was prepared by using silane coupling agent KH-540 modified with CNF. The AFM characterization showed that K-CNF retains the rod-like structure of CNF. The diameter of K-CNF ranges from30 to 40 nm, and its width ranges from 200 to 500 nm. FTIR and XPS characterization proved the interaction between KH-540 and CNF. The XPS result showed that the number of hydroxyl group at K-CNF surface was less than the number of hydroxyl group at CNF surface. When the K-CNF content is 0.5 wt%, the properties of K-CNF/PVDF membrane achieves the optimum. The permeability, hydrophilic property and antifouling property of CNF/PVDF membrane prepared under the optimum preparation conditions were all improved compared with the properties of pure PVDF membrane. Compared to pure PVDF membrane, the tensile strength and elongation-at-break of K-CNF/PVDF membrane increased 36.77% and 61.18%, respectively. The mechanical properties of K-CNF/PVDF had been further improved compared with CNF/PVDF membrane.Thermoldynamics study showed that the introduction of CNF and K-CNF made the cloud point line of PVDF/DMAC/H2O system closing to the PVDF-DMAc axis.Owing to the existence of the CNF and K-CNF, the thermodynamic property of system became unstable, the water capacity of casting solution was reduced, the phase demixing process was promoted. Kinetics study showed that the gel rate of PVDF casting solution was accelerated by the introduction of CNF and K-CNF, membrane-forming process was facilitated.The non-newtonian fluid index of CNF/PVDF casting solution and K-CNF/PVDF casting solution was less than 1. As a result, CNF/PVDF casting solution and K-CNF/PVDF casting solution were all belonged to pseudoplastic fluid. When the CNF content, K-CNFcontent and PVDF content increased respectively, the non-newtonian fluid index of PVDF casting solution decreased, the constant shear viscosity, zero shear viscosity, flow activation energy and modulus of PVDF casting solution increased.The optimum technological conditions of milk ultrafiltration processed by CNF/PVDF membrane and K-CNF/PVDF membrane was:operating pressure was 0.1 MPa, the milk protein content was 1 g/L, the pH value of milk protein solution was 11, the temperature of milk protein solution was 40℃.The cleaning effects of several membrane cleaning methods were investigated. For both of CNF/PVDF membrane and K-CNF/PVDF membrane, NaOH cleaning+backwashing method was the most effective method among those of membrane cleaning methods.