Structural development of asymmetric cellulose acetate microfiltration membranes prepared by a single-layer dry-casting method

Asymmetric membrane structures provide greater permeability due to reduced flow resistance and more efficient separation. The aim of this study was to produce a cellulose acetate (CA) microfiltration membrane with an asymmetric structure that combines a prefilter layer and selective layer in one membrane and, secondarily, to investigate how the new CA-based solvent system for manufacturing membranes affects membrane morphology. Asymmetric CA microfiltration membranes were prepared by a single-layer dry-casting process using a pseudo-ternary mixture of CA, methyl formate (MF), 2-propanol and water. The resulting membranes consisted of two isotropic regions with almost uniform pore size covered by a protective skin. Unlike other asymmetric membranes, the selective layer of these developed CA microfiltration membranes faced the solid–liquid interface, while the upper region facing the liquid–gas interface exhibited larger pore sizes. The top layer acted like a prefilter, and the denser bottom layer like a selective layer. Structural studies of the membranes were conducted by scanning electron microscopy. The membrane structures identified are discussed in detail and a theory about the events taking place in the casting solution during membrane formation.