Polymeric membranes
Disclosed is a method and apparatus for manufacturing a continuous web of polymeric membrane and for continuous downstream processing of said web. The apparatus ( 10 ) comprises: a casting station ( 20 ) for casting the continuous web (M); a carrier ( 24 ) for carrying the web downstream; a membrane drier ( 30 ) downstream of the carrier, for drying the web; and a brushing station ( 40 ) downstream of the drier for brushing the web. Said drier is located immediately downstream of the carrier, and upstream of said brushing station. The apparatus ( 10 ) further includes an additional drying station ( 50 ) downstream of the brushing station ( 40 ). Brushing after drying retains more surfactant in the membrane which is useful for certain applications. In addition, initial drying eliminates virtually all solvents from the membrane, but leaves some non-solvent (e.g. water) within it, which in turn fixes the surfactant on the nitrocellulose fibers, which improves significantly the consistency and reproducibility of the membrane.
1. A method for manufacturing a nitrocellulose membrane comprising the steps of:
a) providing a membrane casting mixture containing at least a nitrocellulose polymer, a polymer solvent, and a surfactant;
b) casting said mixture on a carrier;
c) initially drying said mixture to allow at least about 98% by weight of the polymer solvent content of the mixture to evaporate during said initial drying thereby fixing said surfactant in the membrane to thereby form a dried porous nitrocellulose polymeric membrane;
d) removing particles of dust on the surface of said dried membrane; and
e) following step d) further drying the membrane, wherein the dried membrane has a surfactant content of at least 0.5% by weight.
2. The method of claim 1 , wherein said removal of dust particles is by means of physical contact between the membrane and a dust removal member, by means of brushing, wiping or rubbing, or is by means of a flow of liquid.
3. The method of claim 1 , wherein said initial drying is performed by means of a heated drum, over which the membrane passes, said drum being heated to at least 50 degrees Celsius.
4. The method of claim 2 , wherein said dust removal is by means of brushing and includes rotating a cylindrical brush in a partially filled bath of fluid, and guiding the membrane to make contact with, and pass over an upper part of the rotating brush.
5. The method of claim 1 , wherein said further drying includes drying by means of a set of drum dryers.
6. The method of claim 5 , further including the later step of rolling said membrane to form a roll.
7. The method of claim 3 , wherein said drum is heated to between 50 and 100 degrees Celsius.
8. The method of claim 3 , wherein said drum is heated to between 68 and 84 degrees Celsius.