Casting Processes
Millipore uses three different methods for making membranes, and all three are controlled precipitation processes:
Air casting: The dissolved polymer solution passes under a series of air flow ducts that control the evaporation of the solvents in a very slow manner.
Solvent or emersion casting: The dissolved polymer is spread onto a moving belt and run through a bath of liquid; the liquid in the bath exchanges with the solvent in the lacquer and causes the formation of the pores.
Thermal casting: Heat is used to drive the solubility of the polymer in a given solvent system. The lacquer is then cast out onto a moving belt that is being cooled. Quenching the heat in the lacquer causes precipitation to start and the pores to form.
Surface Modification Methods
Polymer selection and solvent/polymer compatibility determine which casting process is employed in the creation of the membrane. Since most polymers used in membrane filtration are being designed for process streams that tend to be chemically aggressive, manufacturers select polymers that are essentially inert. Identifying solvents in which these polymers dissolve is difficult.Membranes are often treated chemically after they are cast. Except for nylon, the polymers used to produce membranes are hydrophobic, which means they are water-repellent. There are many applications that require the filtration of aqueous liquids through the membrane, thus requiring conversion of the surface of the membrane from a hydrophobic to a hydrophilic, or water-wettable, state. Millipore accomplishes this by either modifying the surface chemistry of the membrane or applying a secondary chemistry coating over the base polymer.
To ensure product purity and consistency of performance, Millipore implements rigorous validation and qualification testing both during and after the manufacturing process.
Learn about validation and qualification testing