Dewatering of Micro- and Nanofibrillated Cellulose for Membrane Production

Cellulose-based membranes have tremendous potential to improve the sustainability and performance of high value applications, such as filters and energy devices, particularly as fluorinated compounds are becoming more regulated. Yet, a deeper understanding of how cellulose films are formed and their structure, in both the wet and dry state, is needed to meet application specific demands and scale-up. We investigated cellulose dewatering using dead-end filtration and the effect of particle size, pressure, temperature, ionic strength, and pH were explored. Dewatering times, filtration cake resistance and compressibility of microfibrillated celluloses (MFCs) and cellulose nanofibrils (CNFs), (and a combination thereof) were measured to understand the role of fibrillation and intermolecular forces during dewatering and forming of membranes. In this fundamental work, dewatering behavior was well described by conventional filtration theory and increasing the pressure from 1 to 4 bar reduced dewatering times by one-half with no significant impact on the mechanical properties. Cake compressibility was found to be directly related to particle size and degree of fibrillation, indicating that finer grades of MFCs and CNFs could be more effectively dewatered at higher pressures. Adjusting pH and ionic strength of cellulose dispersions could similarly reduce dewatering times, yet impacted the wet and dry mechanical properties. This work serves as a basis to better understand the structure–property relationships that develop during dewatering of MFCs and CNFs.