Patent GB-920815-A (GB)
Title: Process for the production of membrane filters
IPC: C08J9/28
Filed: 1960-04-06
Abstract: Membrane filters are made by dissolving a water-soluble organic macro-molecular material in a solvent, adding to the solution so made an additive effecting enlargement of pore-size selected from the group consisting of watermiscible organic solvents and water-soluble salts, casting a thin layer of the solution, allowing it to gelatinize to form a membrane, stabilizing the pore size of the membrane by means of a pore-size stabilizing agent selected from the aforesaid group and drying the membrane. The stabilization may be effected by adding such a stabilizing agent to the solution prior to casting, and this method is preferred where the solvent is an organic solvent immiscible with water.  Thus a methyl cellulose membrane may be made using a chlorinated hydrocarbon solvent, especially methylene chloride, to which methyl alcohol has been added as pore-size enlarging additive and ethyl acetate as the pore-size stabilizing agent. The layer may be cast by spreading the solution of methyl cellulose or other macro-molecular material in the water-immiscible organic solvent, containing also the pore-size enlarging additive and pore-size stabilizing agent, uniformly on a flat support and allowing the solvent to evaporate freely into the atmosphere.  Alternatively, stabilization of pore-size may be effected by treating the cast layer with a pore-size stabilizing agent which is a liquid miscible with the solvent and hence capable of replacing solvent in the membrane, and this method is preferred where the solvent is water.  Where water is the solvent, there may be added to the aqueous solution of the macro-molecular material, the pore-size enlargement additive, and a gelatinizationpromoting additive.  Both additives are preferably water-miscible organic solvents or watersoluble salts, and the pore-size enlargement additive may also function as the gelatinizationpromoting agent.  Stabilizing agents suitable for this method are water miscible organic solvents, e.g. methyl and ethyl alcohols, acetone, and methyl acetate, and aqueous salt solutions, e.g. aqueous sodium phosphate.  Thus a gelatine membrane may be cast using water as solvent, and ethyl alcohol as pore-size enlargement additive and gelatinization-promoting agent.  A polyvinyl alcohol membrane may be cast using water as solvent, and aqueous sodium sulphate as pore-size enlargement additive and gelatinization-promoting agent.  Methyl acetate may be used as pore-size stabilizing agent for both gelatine and polyvinyl alcohol films made in this way.  A methyl cellulose membrane may be cast using water as solvent and aqueous sodium phosphate as pore-size enlargement additive, gelatinization-promoting additives and stabilizing agent.  The cast layer is immersed on its support, in a bath of the pore-size stabilizing agent, and is then dried in air at normal or elevated temperature.  The casting support may advantageously be coated with a water repellent substance, e.g. wax or oleaginous material, to facilitate stripping.  Nutrients for micro-organisms, dyestuffs and other chemical substances may be added to the solution of macro-molecular material prior to casting, and the cast membrane may be after-treated to inhibit its water-absorption properties, e.g. by heating or treatment with cross-linking agents, and may be sterilized, e.g. with gaseous ethylene oxide.  Membrane filters so made are permeable to air, can be used for the culture of bacteria and for filtering bacteria from fluids and are soluble in aqueous physiologically tolerable liquids.  In examples, a gelatine filter of pore diameter 0,5-0,7n  has an air permeability of 30-50 1/min., a polyvinyl alcohol filter of pore diameter 2-3n  has an air permeability of 300 1/min., and methyl cellulose filters of pore size 5-10m  have air permeabilities of 300-700 1/min.ALSO:Membrane filters permeable to liquids which can be used for the filtration of bacteria from fluids, and for bacterial culture, and which are soluble in aqueous physiologically tolerable liquids, are described.  In examples, a gelatine filter of pore diameter 0,5-0,7 m  has an air permeability of 30-50 1 min, a polyvinyl alcohol filter of pore diameter 2-3m  has an air permeability of 300 1 min. and methyl cellulose filters of pore size 5-10m  have air permeabilities of 300-700 1 min.  For the method of making the filters, (see Group IV(a)).