Synthetic Membranes: State of the art

Abstract

Synthetic membranes are used for a great variety of separation processes such as dialysis, electrodialysis, hemodialysis and hemofiltration, ultra-and hyperfiltration, and pervaporation, listing the most usual ones, where the process is generally designated according to the applied driving force such as a concentration, electrical potential, or pressure difference, respectively, across the membrane. Rather common requirements on the membranes used for separation processes are high flux, excellent permselectivity, chemical and partially temperature resistivity as well as high durability (long life time), to name the most important ones out of additional distinct qualifications which a membrane has to meet regarding an individual application. Therefore, a great deal of work was spent during the last 25 years for the development of the best possible membranes using almost every available polymeric material. At the beginning of membrane research, natural polymers such as cellulose and cellulose derivatives were essentially employed for the preparation of appropriate membranes such as collodion and cellulose acetate, for instance. These polymeric materials became rediscovered for the preparation of desalination membranes installed into hyperfiltration modules for the production of fresh water from brackish and seawater. In this connection, new membrane structures and configurations were designed besides the common homogeneous flat sheet membrane in order to meet the demands of high flux and high packaging density of a corresponding membrane module. With respect to an improved packaging density, spiral-wound, tubular, capillary, and hollow-fine fiber modules were designed in addition to the classical plate and frame module. Moreover, asymmetric and so-called composite membranes were developed to generate high flux membranes possessing the same selectivity as the corresponding homogeneous ones. Concurrently to the development of new membrane structures and configurations, new polymeric materials such as polyamides, polyimides, polysulfone, polypropylene, polycarbonate, polyurethane, copolyacryl nitriles, silicone, polytetrafluor ethylene (PTFE), and polybenzimidazolone, for example, were cast into membranes of improved transport properties and chemical resistivity. The different membranes together with their structure and field of application as well as their advantages and disadvantages will be discussed.