Preparation of synthetic membranes (1)

-A number of different techniques are available to prepare synthetic membrane. Some of these techniques can be used to prepare polymeric as well as inorganic membranes.

-The most important techniques are sintering, stretching, track-etching, phase inversion, sol-gel process, vapour deposition and solution coating.

Sintering

-sintering is quite a simple technique allowing porous membranes to be obtained from organic as well as from inorganic materials.

-The method involves compressing a powder consisting of particles of a given size and sintering at elevated temperatures.

-The required temperature depends on the material used. During sintering the ‘interfaces’ between the contacting particles disappears.

-A wide range of different materials can be used such as powders of polymers(polyethylene, polytetrafluoroethylene, polypropylene), metals(stainless, tungsten), ceramics(aluminium oxide, zirconium oxide), graphite(carbon) and glass(silicates).

-The pore size of the resulting membranes is determined by the particle size and particle size distribution of the powder. The narrower the particle size distribution the narrower the pore size distribution in the resulting membrane.

¨-This technique allows pore sizes of about 0.1 to 10μm to be obtained. The lower limit is determined by the minimum particle size. ¨-Only microfiltration membranes can be prepared via sintering, however. The porosity of porous polymeric membranes is generally low, normally in the range of 10 to 20% or sometimes a little higher.

Stretching

In this method an extruded film or foil made from a partially crystalline polymeric material (polyethylene, polytetrafluoroethylene, polypropylene) is stretched perpendicular to the direction of the extrusion, so that the crystalline regions are located parallel to the extrusion direction.

When a mechanical stress is applied ,small ruptures occur and a porous structure is obtained with pore sizes of about o.1μm minimum to about 3μm maximum. ¨Only(semi)crystalline polymeric materials can be used for this technique. ¨The porosity of these membranes is much higher than that of the membranes obtained by sintering, and values up to 90% can be obtained.

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