Microporous Filters
Microporous membranes are made out of standard materials or may be track-etched.
Standard Microporous Filters
Standard microporous filters are manufactured by the controlled precipitation of polymer from a solvent system. The rate of precipitation controls the pore size. These filters are made from several polymers:
- PVDF filters are made from polyvinylidene fluoride. They provide high flow rates and throughput, low extractables, and broad chemical compatibility.
- PES filters are made from polyethersulfone. While other polymers produce pore structures that are fairly uniform through the depth of the membrane, PES precipitation can be controlled to produce a range of pore sizes within a single membrane.
- MCE filters are made from mixed cellulose esters – cellulose acetate and nitrocellulose. These are among the first membrane filters ever produced.
- UPE filters are made from ultra-high molecular weight polyethylene. This polymer has very broad chemical compatibility and can be bonded to many types of plastic for device manufacture.
- Nylon filters are made from one of several types of nylon polymer. Membranes made from this polymer are unique in that they are naturally hydrophilic.
- PTFE filters are Teflon-based. They exhibit broad chemical compatibility and can be bonded to plastic supports to facilitate device fabrication.
Track-etched membranes
Track-etched membranes are very thin but highly controlled membranes traditionally used for high-specification filtration in laboratory applications. Track-etched membranes are useful for the filtration of particles ranging from approximately 0.1 to 10 µm in diameter. Manufacturing starts with a thin film of polycarbonate or polyethylene. The film is exposed to ionizing radiation or ion bombardment, which produces tracks in the film where the polymer strands have been chemically broken. The polymer strands are then digested away using controlled chemical conditions. The length of the chemical treatment determines the diameter of the pores. The pores have very regular geometries and extend as cylinders from one face of the film to the other. While track-etched membranes are typically used in the filtration of samples prior to critical processes such as high-pressure liquid chromatography (HPLC), there is an increasing number of other operations that can be performed as a result of the unique properties found in track-etched membranes.
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