Microfiltration track-etched

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

New, alternative methods for membrane preparation are also reported in the literature. For example, the membrane microsieves with a very regular pore structure (pores from several micrometers down to 0.1-0.2 pm) are prepared via photolithography [101,102]. These very thin silicon nitride membranes combine a very high porosity with extremely high permeabilities, much larger than the track-etched or other microfiltration membranes with similar pore size. 

Hydrophilic MF membranes can be made by the dry-wet phase inversion technique, which can also be used to make PVDF membranes. On the other hand, other hydrophobic microflltration membranes are made by the thermally induced phase separation technique. In particular, semicrystalline PE, PP, and PTFE are stretched parallel to the direction of film extrusion so that the crystalline regions are aligned in the direction of stretch, while the noncrystalline region is ruptured, forming long and narrow pores. Hydrophobic membranes do not allow penetration of water into the pore until the transmembrane pressure drop reaches a threshold called the liquid entry pressure of water (LEPw). These membranes can therefore be used for membrane distillation. The track-etching method is applied to make microfiltration membranes from PC. 

As will be described in chapter III, quite a number of techniques exist for preparing microfiltration membranes, i.e. sintering, stretching, track-etching and phase inversion. These techniques are not generally used to prepare ultrafiltrarion membranes, because the pore sizes obtained are only in... 

Ultrafiltration membranes are also porous, and it is therefore surprising at first sight that polymeric materials of a different type are used to that employed in microfiltraiion. A number of microfiltration membranes are prepared by techniques such as sintering, track-etching and stretching which lead to pores with a minimum size of... 

Figure VI - 4. Polymeric microfiltration membranes (a) phase inversion (b) stretching and (c) track etching...

Microfiltration and ultrafiltration membranes can be made from organic polymers or inorganic materials such as ceramic, glass, or metal or organic polymers. Materials used in MF and UF membrane fabrication are shown in Table 6.1. A number of different techniques are employed to prepare synthetic MF/UF membranes the most important are phase inversion, coating, sintering, and track etching. 

We have used a polymer track membrane (PTM) [64, 65] as a support for reagent immobilization. Such membranes are used for microfiltration in medicine, electronics, biotechnology etc. PTM is made by irradiation of a polymer with high energy ions. The irradiated film is etched and in this way the tracks after passing ion beams are removed. Various polymers can be used as a base film for fabrication of PTM (e.g. polypropylene, polyamide, polycarbonate etc.). The diameter of the pores obtained depends on the type of ions used and on their energy. 

---