Preparation of synthetic membranes (3)

Coating

Dense polymeric membranes in which transport takes place by diffusion generally show low fluxes. To increase the flux through these membranes the effective membrane thickness must be reduced as much as possible. This may be achieved by preparing composite membranes. ¨

Such composite membranes consist of two different materials , with a very selective membrane material be deposited as a thin layer upon a more or less porous sublayer. The actual selectivity is determined by thin toplayer, whereas the porous sublayer merely serves as a support.

Several coating procedures can be used such as dip coating, plasma polymerization, interfacial polymerization, and in-situ polymerization to achieve these membranes. These techniques will be described in more detail later in this chapter.

Another type of coating techniques is also possible. Where the coating layer plugs the pores in the sublayer. In this case, the (intrinsic) properties of the sublayer rather than those of the coating layer mainly determine the overall properties.

Conclusion

With sintering, stretching, leaching out and track-etching techniques only porous membranes are obtained. These membranes can also be used as sublayer for composite membranes, so that their application can be extended to other areas. ¨

Through the use of phase inversion techniques it is possible to obtain open as well as dense structures.

Coating techniques are normally used to prepare thin but dense structures, possessing a high selectivity and a relatively high flux 

The membrane in syringe filters can be made by the aboved methods.